: 4 BurssD of Standarci

f^OV Z 8 1977

QcloG Stress Corrosion Cracking

^^f^^^Control Measures

c^

' ^ B. F. Brown

Chemistry Department

The American University

Washington, D.C. 20016

Sponsored by the

Advanced Research Projects Agency

under ARPA Order No. 2616, through

contract No. N00014-68-A-0245-0007

of the Office of Naval Research.

U.S. DEPARTMENT OF COMMERCE, Juanita M. Kreps, Secretary

Dr. Sidney Harman, Under Secretary

(J , ^ , NATIONAL BUREAU OF STANDARDS, Ernest Ambler, Acting Director

Issued June 1977
Library of Congress Cataloging in Publication Data

Brown, Benjamin Floyd, 1920-

Stress corrosion cracking control measures. (NBS monograph ; 156)

Bibliography, p. Includes index. Supt. of Docs, no.: C 13.44; 156

1. Stress corrosion. I. Title. II. Series: United States. National Bureau of Standards. Monograph ; 156. QC100.U556 no. 156 [TA462] 602Ms [620.r6'23]

76-608306

National Bureau of Standards Monograph 156

Nat. Bur. Stand. (U.S.), Mono. 156, 81 pages (June 1977)

CODEN: NBSMA6

U.S. GOVERNMENT PRINTING OFFICE

WASHINGTON: 1977

For sale by the Superintendent of Documents, U.S. Government Printing Office, Washington, D.C. 20402

(Order by SD Catalog No. C13.44:156). Stock No. 003-003-01674-8 Price $4.50 cents. (Add 25 percent additional for other than U.S. mailing.)
Truth will sooner come out from error than from confusion.

Francis Bacon, 16th century

Were I to await perfection, my book would never be finished.

Tai K'ung, 13th century
SI Conversion Units

In view of present accepted practice in this technological area, U. S. customary units of

measurement have been used throughout this report. It should be noted that the U. S. is a signatory

to the General Conference on Weights and Measures which gave official status to the metric SI

system of SI units in 1960. Readers interested in making use of the coherent system Guide, ASTM

Designation E 380-76 (available from American Society for Testing and Materials, 1916 Race Street,

Philadelphia, Pennsylvania 19103). Conversion factors for units used in this paper are:

Length

1 in = 0.0254* meter

1 ft = 0.3048* meter

Area

1 in2 = 6.4516* x lO"" meter^

1 ft2 = 9.2903 X 10-2 meter^

Force

1 lb (Ibf) = 4.448 newton

1 kip = 4448 newton

Pressure, Stress

1 psi = 6895 pascal

1 psf - 47.88 pascal

Energy

1 ft-lbf = 1.3558 joule

Moment

1 Ibf-ft = 1.3558 newton meter

Temperature

toe = 5/9 (top - 32)

*Exact Value

iv
FOREWORD

An important responsibility of the |Vational Bureau of Standards is the dissemination of

information to the engineering public. Such information becomes especially significant when it helps prevent the failure of materials and thereby saves both hves and money. It is thus especially

gratifying that NBS has been given the opportunity to contribute towards diminishing the incidence

of stress corrosion cracking, an increasingly serious mode by which metals fail, by publishing this

monograph. Dr. B. Floyd Brown, its author and a former guest worker at NBS, has provided an

invaluable tool to the design engineer to help him in the proper selection of materials in order to minimize the danger of fracture by stress corrosion. Dr. Brown calls this a "missionary volume"

because its aim is to inculcate the fear of stress corrosion into those who in the past have not always

been aware of its possibility.

This long needed work would not have been possible without the support provided by the

Advanced Research Projects Agency through the Office of Naval Research.

We are also grateful to American University for providing Dr. Brown with the opportunity to write this monograph.

Jerome Kruger

Chief, Corrosion and Electrodisposition Section

National Bureau of Standards

V
PREFACE

This present volume is the first of a two part series on stress corrosion cracking planned by the

Advanced Research Projects Agency for the designer. This first part is introductory in nature, is largely non quantitative, covers only the major alloy families, and treats only stress corrosion

cracking. The second part, currently being written under the direction of Professor R. W. Staehle of

the Ohio State University and Dr. M. O. Speidel of Brown Boveri Co., will be an advanced treatment,

will be quantitative, will cover all alloy families, and will treat both stress corrosion cracking and

corrosion fatigue.

Stress corrosion cracking is a failure mode which has occurred with increasing frequency in

both the military and the civihan sectors as our technological way of life has increased in complexity.

Research on the problem has brought much new understanding in recent years, but the theory has

not been brought to the point of enabling us to avoid or control the problem, and there is no basis for

confidence that it will do so in the near future.

Meanwhile, if we are to reverse the trend and diminish the incidence of stress corrosion

failures, we need to assemble the available practical measures to avoid or minimize the problem and

present these measures in a form comprehensible to those persons responsible for the design,

fabrication and maintenance of new structures. The assembly of these measures and that sort of

presentation are the aims of the present volume. Needless to say, in any dynamic technology such

as stress corrosion cracking, any such assembly will require supplementation and modification as our knowledge and understanding of that subject increase.

It should be emphasized that neither the theory nor the technology of stress corrosion cracking

is either complete or probably without flaw, and we are without the capability of predicting

accurately the cracking hazards or lack thereof for fundamentally new combinations of alloys and

environments. If one is concerned with components or structures the failure of which cannot be

accepted, then one must obtain test data on the specific alloy in the specific environment of concern. In this connection the reader should absorb the warning implicit in the account (in chapter

5) of the catastrophes which occurred to titanium alloy pressure vessels when the nitrogen tetroxide

they contained was furnished in slightly higher purity then previously.

References to key literature are included in the text for the benefit of the reader who wishes to dig deeper on a given topic, but these references are kept to a minimum, and names of investigators

and other "trappings of scholarship" so customary in research literature are omitted entirely, in the

interest of ease of reading by the nonresearch oriented person.

The modus operandi of this project was to prepare drafts of generalizations in various specialty areas and submit these drafts to persons knowledgeable in these specialty areas for review and

comment. Many of these reviewers kindly provided invaluable contributions in such forms as unpublished data, obscure references, unwritten or unpublished practices, and various aids, often

provided at the expense of personal time despite the press of busy schedules. Without implying any

responsibility on their part for any or all shortcomings of this volume, the author would like to record

here his indebtedness and appreciation to these reviewers:

I. M. Bernstein, W. K. Boyd, H. S. Campbell, M. A. Cordovi, J. B. Cotton, R. L. Cowan, II, H. L. Craig, Jr., G. Danek, C. P. Dillion, N. C. Feige, S. Glorioso, 8. Goldberg, J. J. Heger, E. C. Hoxie,

R. W. Huber, H. H. Johnson, S. J. Ketcham, J. A. Kies, L. J. Korb, J. Kruger, B. F. Langer, R.

Maersch, M. McNeil, D. A. Meyn, F. Ogburn, H. W. Paxton, E. H. Phelps, J. M. Popplewell,

E. N. Pugh, P. P. Puzak, S. D. Reynolds, J. H. Smith, D. van Rooyen, F. N. Rhines, L. Schapiro,

L. R. Scharfstein, R. Schmidt, J. Sedricks, M. O. Speidel, D. O. Sprowls, R. W. Staehle,

M. Streicher, T. J. Summerson, D. H. Thompson, R. S. Treseder, J. H. Westbrook, and

H. W. Zoeller. Prof. R. T. Foley of The American University not only reviewed the sections but,

additionally, made numerous helpful constructive suggestions and merits special appreciation,

which is here recorded.

vi
The author also wishes to express his appreciation to Dr. Jerome Kruger of the National Bureau of

Standards, and to the other NBS personnel, for their kind assistance in the publication of this

volume.

Finally, the author would like to express his appreciation to Dr. E. van Reuth of the Advanced

Research Projects Agency for financial support of this writing, and to Dr. P. A. Clarkin of the Office of

Naval Research for the very considerate manner in which the supporting contract was administered.

B. F. Brown

Washington, D. C.

October 31, 1975

vii
Stress Corrosion Cracking Control Measures

B. F. Brown

Stress corrosion cracking is a failure mode which has occurred with increasing frequency in all sectors of technology. This publication attempts to diminish the incidence of stress corrosion

failures by assembling the available practical measures to avoid or minimize the problem and

present these measures in a form comprehensible to those persons responsible for the design,

fabrication and maintenance of new structures. The alloys covered are copper, aluminum, titanium

and nickel alloys and high strength and stainless steels.

Key words: Aluminum alloys; copper alloys; high strength steels; nickel alloys; stainless steels; stress corrosion cracking; titanium alloys.

/

viii
Contents

Page Foreword v Preface vi

Abstract viii Chapter 1. Placing the Problem in Perspective 1

I. Sustained load environmental cracking 1

II. Interaction between failure modes 2

III. Theory 2

IV. References 4

Chapter 2. Test methods and Interpretation of Test Data 5

I. Smooth specimen tests under static loads 5

II. Precracked (fracture mechanics) specimens under static loads 7

III. Constant strain rate tests 10

IV. Metallurgical considerations in tests 10

V. Interpretation of SCC test data 10

VI. References 12

Chapter 3. Copper Alloys 13

I. Alloy nomenclature 13

II. Ammoniacal SCC 13

A. Necessary conditions 13

B. Case Histories and other illustrations of principles 17

III. SCC in other environments 19

IV. Miscellaneous measures 20

V. Diagnosing SCC 20

VI. Theory 21

VII. Summary 22

VIII. References 22

Chapter 4. Aluminum Alloys 23

I. Necessary conditions for SCC 23

II. Kinetic factors 32

III. Inhibitors 32

IV. An example of applying the principles to avoid the problem 32

V. Nonaqueous environments 33

VI. Identifying SCC 33

VII. Theory 34

VIII. Summary 34

IX. References 34

Chapter 5. Titanium Alloys 35

I. Liquid environments (excluding metals) 35

II. Solid and liquid metals 38

III. Sustained load cracking in inert environments 38

IV. Diagnostic criteria 39

V. Theory 40

VI. Summary 40

VII. References 40

Chapter 6. High strength steels 43

I. Degree of susceptibility 44

II. Control measures 47

III. Examples of practices and failures illustrating principles 49

IV. Single-load-path designs 52

V. Diagnosing SCC 53

ix
Page , VI. Theory 53

VII. Summary 53

VIII. References 53

Chapter 7. Stainless Steels (Austenitic and Ferritic) 55

I. Marine environments 55

II. Other low temperature environments 57

III. Chloride cracking 57

IV. Caustic cracking 60

V. Oxygen cracking 60

VI. Protective measures illustrating principles 61

VII. Identifying SCC 62

VIII. Theory 62

IX. Summary 63

X. References 63

Chapter 8. Nickel Alloys 65

I. Chemical industry 65

II. Power Industry 65

III. Aerospace industry 66

IV. References 66

Appendix—Glossary of Terms 67

Index 69

X
Consequence ofstress corrosion cracking in a nonredundant load path structure: collapse of the "Silver Bridge" over the Ohio

River, which occurred on December 15, 1967, carrying 46 persons to their death. Photo courtesy of Mr. H. C. Burnett, National Bureau of Standards.

CHAPTER 1

PLACING THE PROBLEM IN PERSPECTIVE

Stress corrosion cracking (SCC) is one of 19 proc- esses which can affect the structural integrity or

limit the life of metallic structures. These processes

are the following:

Mechanical

(1) Yielding

(2) Brittle fracture

(3) Overload or ductile fracture

(4) High temperature creep, with or without

rupture

(5) Fatigue

(6) Wear

Mechanical plus Chemical

(7) Corrosion fatigue

(8) Hydrogen cracking (hydrogen embittlement)

(9) Liquid metal assisted cracking (liquid

metal embrittlement)

{10) Stress corrosion cracking

(11) Erosion corrosion

Chemical

(12) Uniform corrosion

(13) Crevice corrosion

(14) Pitting

(15) Dissimilar metal corrosion

(16) Intergranular corrosion

(17) Selective dissolution (as in the selective

corrosion of zinc in brass)

(18) Filiform corrosion (as in surface grooving of

steel under a lacquer coating)

(19) High temperature oxidation (including subscaling)

Designing against yielding is of course an old

technology, based on well standardized macroscopic

tensile tests. Designing against brittle fracture is based either on the transition temperature

phenomenon or on fracture mechanics characteristics, both of which require macroscopic test speci- mens. The principles of designing against overload

or ductile fracture are in a state of rapid research

development. Designing against high temperature

creep is a well developed technology, based on data

from macroscopic specimens. For designing against

fatigue, there are fatigue limit data and fatigue crack

growth behavioral rules which are of general

applicability. The principles of the transition temperature and the fracture mechanics approaches to the brittle fracture problem, and the principles of

fatigue cracking, are treated in reference [1]. For

maximum utility in a given technology, the special

rules applicable to that technology may be profitably

collected and presented, as in reference [2], which

summarizes the rules of thumb which have been

found useful in controlhng fatigue in Naval aircraft.

The technology for controlling wear is summarized

in handbooks. The principles of designing against

the processes necessarily involving corrosion (proc- esses 7 through 19 in the list above) are treated in the

standard texts (see for example refs. [3-5]). The

technology for designing against many of these forms

of corrosion is presented in numerous monographs.

The theory of stress corrosion cracking has been

treated in the proceedings of a number of recent

conferences (refs. [6-8]), but no comprehensive con- ference on the technology of SCC and its avoidance

has been held since the conference of 1944 (ref. [9]).

The aim of the present work is to fill this technological need. Rather than restricting the coverage of

SCC alone, portions of the subjects of hydrogen

cracking and liquid metal assisted cracking are included for reasons which are noted below. These

three cracking processes, which are italicized in the

list above, constitute a class of processes which may

be grouped under the term "sustained load environmental cracking," which is not to be confused with

"sustained load cracking," a phenomenon which oc- curs in some titanium alloys in the absence of a chemically reactive environment.

I. Sustained Load Environmental Cracking

Hydrogen cracking, liquid metal assisted cracking, and stress corrosion cracking have a strikingly

large number of common features:

(1) All three processes can produce macroscopically brittle fractures, even when they occur in metals which are ductile in a simple tensile test.

(2) All three processes are beheved to involve localized plastic flow.

(3) All three processes can occur at stresses

which may be far below those required for general

yielding.

(4) Only a tensile stress will produce this class of

cracking.

(5) Fracture occurs on a plane nominally perpendicular to the tensile axis, that is, it is an "opening

mode" fracture (as in opening a book). No analog to the shear lip has been noted to be generated by

sustained load environmental fracturing, although in

some instances what appears to be a purely mechanical shear lip may form as a border after the passage

of a stress corrosion crack.

(6) There is a wide range of susceptibility among

various combinations of alloys and environments.

This feature is related to the following one.

(7) There is an approximation of specificity about

those combinations of alloy and environment which

lead to the most severe cracking problems. In

SCC, in other words, generally only a few chemical

species cause serious cracking problems in a given

alloy.

1
Placing the Problem in Perspective

(8) The chemical species in the environment essential for the cracking process may not be needed

in either high concentrations or large quantities.

(9) The combinations of alloy and environment

which lead to the most severe cracking problems are usually those in which the alloy is almost, but not

quite, inert to the environment.

(10) The cracks may be either simple or branched.

(11) Although the cracks are macro scopically brittle, on a microscopic level the sustained load en- vironmental cracking processes involve localized

plasticity and may follow paths which are different

from that of purely mechanical brittle fracture; and

environmental cracking may select either intergranular or transgranular paths depending on seemingly minor changes in environment, composition of

alloy, stress level, or processing variables.

(12) In addition to these eleven generalizations,

which apply to all three failure modes, stress corrosion cracking appears to occur only when certain

electrochemical conditions are met, as will be discussed in detail in subsequent chapters.

A thirteenth generalization is often found in the

older literature, namely that only alloys experience

sec, and pure metals do not. From a practical

standpoint this rule has merit, but a few laboratory cases of SCC have been observed in copper of ex- tremely high purity, so that the rule may have to be

stricken when considering theory.

Liquid metal assisted cracking can be regarded as

sufficiently akin to SCC to be treated in the present

writing, and it is so included in the metal systems

where major problems have occurred. Some proc- esses which are labelled SCC may be, mechanistically, manifestations of hydrogen cracking; and in

such cases certain corrosion control measures such

as zinc electroplating of and cathodic protection of

high strength steel can cause problems of hydrogen

cracking as serious as or more so than the SCC they

may be intended to prevent. Therefore, although the

primary focus of this volume is on SCC, Uquid metal

assisted cracking (and related solid metal cracking)

will be included in important systems because the

general approach to controlling the problem is similar to the approaches for controlling SCC, and

treatment of a small portion of the vast subject of

hydrogen effects is included where it is intimately

related to SCC and its control.

II. Interaction between Failure Modes

There is extensive interaction between various of

the 19 failure modes, especially the modes involving

fracturing. It is common for SCC to be initiated by

corrosion pitting. It is also common for SCC to initi- ate fatigue cracking, and in fact premature initiation

of fatigue cracks by SCC in airframe components is an important reason for concern about SCC in that

technology. Brittle fracture may initiate from fatigue

cracks, stress corrosion cracks, hydrogen cracks,

and in brittle steels from even corrosion pits.

The "Silver Bridge" (frontispice) represents an

unusually complex chain of interactions between different failure modes: (1) A corrosion pit initiated

(2) SCC, which caused, (3) brittle fracture of one

half of the ring forming the eye of an eye bar. This

brittle fracture transferred such a high load to the

other half of the ring as to cause it to fail by (4)

overload fracture, setting in motion the progressive

collapse of the entire bridge.

The design of the "Silver Bridge" represents a generic design type which may be termed non- redundant load path, non-redundant in the sense

that if one load path (such as one half of the ring

which forms the eye of the eye bar) fails, the entire

structure is doomed. The implications of such designs to requirements for knowledge of stress corrosion cracking and brittle fracture are discussed in

chapter 6.

III. Theory

SCC involves the disciplines of metallurgy,

mechanics, and chemistry, and it should be not sur- prise that the theory has proven to be difficult to formulate even qualitatively. One might suspect that

if all SCC has 12 features in common (as hsted

above), then there must be a strong element common

to all cases of SCC. And indeed throughout the older

hterature on the subject there was an implied conviction that a unified theory of all SCC awaited the

diligent investigator. With the passage of years and

no single theory forthcoming to account for SCC in

all systems (combinations of alloys and environments), attempts to find a single theory have gradually given way to attempts to find any theory which

will account for all the observations on any one system.

It has long been recognized that SCC characteristics are controlled by microfeatures and microprocesses. In microfeatures we include grains, grain

boundaries, precipitate particles, etc. By microprocesses we include motion of dislocations, passing

of metalUc atoms into the ionized (corroded) state,

reduction and diffusion of hydrogen, etc. These features and processes are on a scale of roughly a micron to an angstrom. Most of the microfeatures

and microprocesses thought possibly involved in

SCC are summarized schematically in figure 1. A full discussion of figure 1 would require a large

book in itself, for research is continuously adding to our knowledge of these microfeatures and microprosses. From an analysis of such knowledge a variety of theories of SCC have emerged, which can be

classified in six groups as follows:

(1) Film rupture, with consequent crack propagation by dissolution at the crack tip. (See upper left area of fig. 1.)

(2) Stress accelerated dissolution.

(3) Hydrogen embrittlement (which itself is understood very imperfectly).

(4) Adsorption and interaction of environment

species with strained bonds. (See black "atoms"

2
Placing the Problem in Perspective

CO<

Q

UJ>

O

M

5

/

CO

o

a,2

c/2

O

3
Placing the Problem in Perspective

adsorbed or sorbed at strained and broken bonds,

lower left area of fig. 1.)

(5) Brittle film formation and rupture, repeated

cyclically. This model is discussed in detaU in chapter 3.

(6) Formation of rows of corrosion pits which

lengthen into an array of parallel corrosion tunnels,

with uncorroded walls between the tunnels ulti- mately repturing mechanically to produce crack ad- vance. Some evidence has been found for all these models, yet none has been established and universcdly

accepted for even one system. Furthermore none of

the information which has been derived from these

fine-scale studies has eliminated the SCC problem,

and we have no basis for confidence that it will do so

in the near future. Such studies are essential to provide the guidance for orderly development of al- loys, but even if any one of the above theories were

to be firmly established, we do not at this stage know

how to use such knowledge for practical avoidance

of SCC. Theoretical studies are required for understanding for the alloy development, but information on another scale is required for practical control measures. That is the subject of the next chapter.

The reader interested in more detailed discussion

of theory should consult references [6-8].

IV. References

[1] Tetelman, A. S. and McEvily, Jr., A. J., Fracture of Structural Materials, (John Wiley & Sons, New York, 1967).

[2] Grover, Horace J., Fatigue of Aircraft Structures (NAVAIR

Ol-lA-13), Government Printing Office, Washington

(1966).

[3] Fontana, Mars G. and Greene, Norbert D., Corrosion Engineering, (McGraw-Hill, New York, 1967).

[4] Uhlig, Herbert H., Corrosion and Corrosion control, (WileyInterscience, New York, 1971).

[5] Evans, Ulick R., The Corrosion and Oxidation of Metals,

(Arnold, London, 1960).

[6] Staehle, R. W., Fundamental aspects of stress corrosion

cracking, (R. W. Staehle, A. J. Forty and D. van Rooyen,

Eds.) National Association of Corrosion Engineers, Houston, Texas (1969), p. 4. [7] Scully, J. C, Ed., The theory of stress corrosion cracking in jjloys, NATO Scientific Affairs Division, Brussels (1972).

[8] Hochmann, J. and Staehle, R. W., Eds., Stress corrosion

cracking and hydrogen embrittlement of iron base alloys,

National Association of Corrosion Engineers, Houston,

Texas, in press. ******

The following are additionally recommended for further reading on many topics treated in this book.

[9] Anon., Symposium on stress corrosion cracking of metals,

American Society for Testing Materials, Philadelphia, PA

(1945).

[10] Robertson, W. D., Ed., Stress Corrosion Cracking and Embrittlement (John WUey & Sons, New York, 1956).

[11] Rhodin, Thor, Ed., Physical Metallurgy of Stress Corrosion

Fracture, (Interscience, New York, 1959).

[12] Logan, Hugh L., The Stress Corrosion of Metals (John Wiley

& Sons, New York, 1966).

[13] Anon., Stress corrosion cracking of titanium, STP 397,

American Society for Testing Materials, Philadelphia, PA

(1966).

[14] Anon., Stress corrosion testing, STP 425, American Society

for Testing Materials, Philadelphia, PA (1967).

[15] Craig, H. Lee, Ed., Stress corrosion cracking of metals - A

state of the art, STP 518, American Society for Testing

Materials, Philadelphia, PA (1972).

4
CHAPTER 2

TEST METHODS AND INTERPRETATION OF TEST DATA

Since the theory of SCC based on microfeatures

and microprocesses (ch. 1, fig. 1) has not been developed to the point of providing predictive capability with respect to SCC behavior in engineering

components and structures, another approach is re- quired until that capability can be developed. The

other approach is the use of macroscopic specimens,

and it is the one upon which we presently rely for

guidance in making materials selection decisions.

One might say that the macroscopic specimen under

test acts as an analog computer to sum up experimentally (and quantitatively) the effects of all the

microfeatures and microprocesses operative in a given system (combination of alloy and environment). The reader should have a clear view of the

roles of the two types of experimentation: The study

of microfeatures and microprocesses, often termed

"basic research", is required for understanding the

nature of SCC and to provide guidance for orderly

development of alloys. The testing of macroscopic

specimens is necessary for determination of SCC

characteristics, the knowledge of which is necessary

for materials selection. The two approaches are complementary. Neither will do the work of the

other, and both are essential. Except for ASTM

standards, reference [1] contains the most recent

compilation of macroscopic SCC test procedures.

There are three fundamentally different types of

macroscopic specimens and tests. One employs an

essentially smooth specimen under static load, the

second uses a precracked specimen (also called a fracture mechanics specimen) also under static load,

and the third employs a smooth specimen under a

rising load with a constant strain rate.

I. Smooth Specimen Tests under Static

Loads

There are engineering situations in which the full component is the logical choice for a specimen. If, for example, one wishes to determine whether a given annealing treatment is adequate to remove the

forming stress from a rolled brass plug-fuse shell

sufficiently to avoid SCC in an ammoniacal atmosphere, the obvious choice of specimen is one of the

shells. Particularly when theory is incomplete, or poorly established, we would prefer to conduct tests resembling in every way possible the practical situation, and the shell thus makes an ideal specimen. Such ideality of specimen as the fuse shell is of hmited availability in the engineering world. We

cannot afford to run SCC tests on a whole submarine

hull or even very many landing gears, so it is necessary to select smaller specimens which will afford

the needed predictive capability with respect to the

larger structures or components.

One such group of specimens in common use has

a spectrum of stresses in each specimen, usually

varying from beyond the yield strength to zero. A

widely used specimen in this category is the U-bend

specimen (see fig. 1), made by bending a strip of

metal into a U-shape and restraining the legs by a tie

rod or other means. If desired, the U-bend specimen

can be so dimensioned as to avoid plastic deformation and produce only a spectrum of elastic stresses.

Other specimens in this category are sheet or plate

specimens which may be stressed by being indented

by a Brinell hardness indenter or by an Erichsen cup

test plunger. These self-stressed specimens are

BRINELL ERICHSEN

IMPRESSION CUP

tr

U-BEND

2-POINT

3-POINT

4-POINT

BEAMS

Figure 1. Smooth SCC specimens.

TENSILE C-RING

5
Test Methods

suitable for insertion into chemical process streams,

but they do not give a very quantitative picture of the

stress dependency of cracking.

In order to determine this stress dependence, it is common practice to use a bent beam specimen, a C-ring specimen, or an uniaxial tensile specimen

which may be stressed in a stout test frame. These

specimens are shown schematically in figure 1. The U-bend specimen and test procedure have

been standardized as ASTM recommended practice

G30-72. The preparation and use of two-, three-, and

four-point loaded bent beam specimens have been

developed to ASTM Standard Method G39-73,

which includes procedures for calculating stress. C-ring specimens may be cut from tubing or machined from plate or bar stock. The making and

using of C-ring specimens have been developed to ASTM Standard Recommended Practice G38-73.

If the environment of interest is known, as, for

example, uncontaminated sea water, or a chemical

process stream of known composition, the test environment of choice will be the service environment.

But if the service environment is incompletely defined, for example that of an airplane, the selection

of a test environment poses problems which have not

been completely answered to everyone's satisfaction. Should one test in a "worst-case" environment

including worst temperature and worst electrochemical potential? There are two problems here, first that we don't know the worst-case environment for

all alloys, and second, testing of some materials in

worst-case environments (or reasonable approximation thereto) would probably lead to excessive rejection of alloys. Regardless of the possible merits of

using worst-case test environments, they are not in

widespread use even for those alloy families for

which we think we have identified something approaching the worst-case environment.

Instead of the worst-case approach for general

characterization of SCC properties the approach

most commonly used is to select either natural environments (e.g., atmosphere, sea coast, sea water), or

alternatively to use standardized laboratory solutions

or atmospheres. Among the standardized solutions

there are several ASTM standardized environments:

G36-73 for cracking of stainless steels in boiling

magnesium chloride solution; G35-73 for cracking of

stainless steels in polythionic acids; G37-73 for

cracking of copper-zinc alloys (brasses) in

Mattsson's solution; and G44^75 for alternate immersion testing in sodium chloride solution at room

temperature.

It is tacitly presumed that testing various stainless

steels in the standardized boiling magnesium

chloride solution will permit assigning an order of

merit rating valid for the most part for the cracking

of the same steels in dilute hot chloride solution

representative of slightly contaminated boiler water.

Likewise tests in Mattsson's solution presumably

afford an order of merit rating of copper-zinc alloys

which would more or less predict the relative performance of the same family of alloys in other ammoniacal environments, particularly the more dilute

ones. And so forth with the other standardized environments.

In the most general case, the sequence of events

in a stress corrosion cracking test on an initially

smooth specimen is as shown left to right in figure 2.

LOCALIZED BREAKDOWN OF

OXIDE FILM

^CORROSION

PIT FORMATION Y1

'INITIATION AND GROWTH

OF SCC

0=0 (STRESS)

terminal purely 'mechanical rupturing

Figure 2. Generalized sequence of events in SCC, left to right,

assuming nonrelaxation of stress. Some systems may exhibit SCC without a

visible pit. Some systems (notably tilanium/water) do not exhibit pitting, but SCC

readily initiates from a fatigue crack.

Most metals are covered with thin films, often

oxides, which may be protective against corrosion as long as they remain intact. The first step in the SCC

process in the breakdown of this film, which may

occur because of mechanical damage or because of

electrical currents, but the most common cause of

film breakdown in the action of certain chemical

species in the environment, particularly the chloride

ion. After the film is broken down, a corrosion pit

may form. In some combinations of environments

and alloys cracking may initiate without visible pitting, but in a large proportion of service failures a corrosion pit precedes the SCC. The pit acts as a

stress intensifier, but it serves a second and perhaps even more important function in some systems: the

corrodent within the pit tends to be different in nature from the bulk environment because of a class of

reactions known in chemistry as hydrolysis. One

such reaction involving one of the corrosion products

of stainless steel is as follows:

Cr+++ + 3H2O - Cr(0H)3 + 3H+

The hydrogen ions thus produced cause local acidity

within a corrosion pit, and this acidity may be at least as important as the stress intensification of the

pit in causing SCC initiation.

Returning to the sequence in figure 2, if the load is not relaxed as the stress corrosion crack grows,

eventually the stress on the uncracked ligament in- creases to the point that the Ugament tears apart

mechanically. This final rupture can be either ductile overload fracture or brittle fracture, depending

on factors such as the nature of the alloy, the

geometry of the specimen, and the temperature.

In a test on a smooth specimen, the parameter

commonly measured is the time from commencement of test to failure. Failure is sometimes defined

as the appearance of a recognizable crack. In that case the total time to failure represents (for the general case) the sum of the times for film breakdown,

6
Test Methods

pit initiation and growth, and SCC initiation and

growth to an arbitrary size crack.

If failure is defined as separation of the specimen,

a convenience in that the separation event can be

used to actuate a timer, then the nature of the terminal fracture can make important contributions in

determining the total time to failure. Let us consider

the case of a high strength steel in which the terminal fracture is a brittle fracture. If one heat treats the

steel specimens to several levels of toughness and

puts them into a stress corrosion cracking test, the

tougher specimens will require a longer stress corrosion crack before they undergo terminal fracture

than the more brittle specimens. The tough specimens would then show a longer time to failure than

the brittle ones even if their stress corrosion characteristics were identical.

For emphasis of the deficiencies of the total time

to failure as a SCC parameter, note that the magnitude of this time depends not only in SCC kinetics

but also on such non-SCC factors as specimen size,

specimen geometry, toughness of the metal, initial

stress level, and initiation behavior.

Despite such deficiencies in the total-time-tofailure parameter for initially smooth specimens,

this parameter, perhaps tempered in interpretation

by investigators who take into account non-SCC factors which influence the parameter, has been the

principal basis on which our SCC control technology

rests. This time-to-failure parameter, when plotted as a function of initial stress, typically shows what appears (fig. 3) to be a horizontal asymptote which is usually denoted the threshold stress cr^. This apparent threshold stress is important in determining

order of merit ratings of alloys or environments, as

will be seen in the chapters on copper alloys,

aluminum alloys and stainless steels.

STRESS

THRESHOLD

TIME TO FAILURE

II. Precracked (Fracture Mechanics)

Specimens under Static Loads

The existence of a crack-like surface defect completely alters the analysis of the SCC behavior of an

alloy, for whether the defect extends as a stress corrosion crack depends not only on the nature of

the alloy and the environment and the magnitude of

stress but also on the depth of the initial crack-like

defect. In the presence of a crack-like defect or of a

stress corrosion crack, nominal stress is a fiction

which can be misleading, and at the present time the

response of a cracked body to loading can only be

treated quantitatively by fracture mechanics. Stress

corrosion cracks are brittle, that is, for the most part

they occur before the onset of general yielding, while

the bulk of the metal is still in the elastic state. Therefore it is appropriate to use the Irwin-Kies

Linear elastic fracture mechanics for SCC even in

metals too ductile for the practical measurement of

purely mechanical fracture toughness by fracture

mechanics methods.

When a piece of metal containing a crack is stressed

in such a fashion as to tend to open and extend

the crack, the nominal stress is intensified near the

crack tip. Elasticians have given us the means to quantify this stress intensity,^ which is designated

X

and which has the units ksiVirTor MNm"^'^.

Some of the various types of fracture mechanics

specimens which have been used in SCC studies are shown in figure 4. The equations for calculating K

values for various types of specimens are collected

in reference [2]. If one measures SCC growth rate as a function ofK,

general appearance is as shown in figure 5. Here it will be seen that in the area marked Region II the

crack growth rate is high and is insensitive toK. This

plateau is discussed further in chapter 4. At lower

levels oi K, in region I, the crack growth rate becomes very sensitive to K (note the log scale). At a

sufficiently low value of K there appears to be a threshold which has been designated Kjgcc- That

such a genuine threshold K exists has been clearly

proven for titanium alloys in salt water, and it appears probable in steels; but its existence is in question in some aluminum alloys. The value of such a parameter of Kigcc, where such a genuine threshold

exists, hes in one's ability to predict the combination

of remote (nominal) stress and size of crack-like

surface flaw through the use of the Irwin equation:

Figure 3. Schematic SCC data from smooth specimens. Existence of a

true horizontal asymptote threshold stress is not always readily provable.

^ Note that "stress intensity" as used throughout this volume is the fracture mechanics parameter. It is unrelated to "stress intensity" as used in the ASME code to designate the stress difference or two times the shear stress.

7
Test Methods

COMPACT

TENSION

DOUBLE

CANTILEVER

BEAM

CANTILEVER

BEAM CONSTANT K

Figure 4. Principal fracture mechanics type specimens which have been used in SCC studies.

CRACK-TIP STRESS INTENSITY

Figure 5. Effect of stress intensity on SCC Kinetics. Region III is of

little interest and is often missing. Region I is missing in some systems. Regions I and II

are not always straight lines in such a plot but may be strongly curved.

(1)

<^2-0.212|

fa

where a is the depth of the crack, cr is the stress, CTy

is the yield strength, and <^ is a factor for the shape of

the crack. If the length of the crack is 2b, then cp^ has

the values for various crack shapes listed in table 1. If one assumes a long, thin flaw and the existence of

yield-point stress, then from eq (1) stress corrosion

would be expected to propagate if the flaw depth

exceeded a^r, given by

= 0.2 Iscc

(Ty

(2)

The value of a^r may thus be regarded as a figure of

merit which incorporates both the SCC resistance

Kiscc and the contribution which yield strength stress levels can make to SCC hazard by virtue of residual

or fit-up stresses.

Table 1. Values of (p"^

a

b

0 (very long, thin crack)

.25

.5.75

1.0 (semicircular)

1.00 1.14

1.46

1.89 2.46

8
Test Methods

One can plot eq (2) for various values of a^r as shown in figure 6. This plot is a convenient

framework for displaying the SCC characteristics of

materials. If a given material was found to have a

Kjscc as indicated by point x, a surface crack 0.1 in

(2.5 mm) deep would be deeper than required to propagate a stress corrosion crack in the same environment used to determine the A^/sce- But a crack

0.01 in (0.25 mm) deep would not propagate a stress

corrosion crack in the same material in the same

environment.

A plot olKiscc data such as figure 6 can be used in a slightly different way: If one knows that he cannot

detect and remove crack-hke flaws less than, say

0.01 in (0.25 mm) deep, then if he has yield-point

stresses he must select a material having a A^/scc above the 0.01 in hne in figure 6. The Kigcc values for a number of commercial and

experimental titanium alloys are shown in figure 7 as

a function of yield strength. Lines corresponding to various critical crack depths (following eq (2) ) are included in this plot as an aid to interpretation of the

degree of susceptibility of various alloys. Another

way to regard Kiscc is to view it as linearly proportional to the load-carrying capability of a specimen

or component containing a standard size flaw.

YIELD STRENGTH

Figure 6. Plot of eq (2) for two assumed values of a^r, assuming

long surface flaws and yield strength stresses operating. A material having a K„„ value as indicated by X would be safe from SCC in the pertinent environment if one can be sure surface flaws will be no deeper than 0.01 in (0.25 mm): sec will occur if they are as deep as 0.1 in (2.5 mm).

CRITICAL

FLAW DEPTH

20 -

0.01'

0.001

TITAIMIUM ALLOY COMPOSITION • 7AI-2Cb-ITa 0 5AI-2y2Sn

o 8AI-2Cb-ITa <] 6AMSn-IV

A 6AI^V 6y2AI-5Zr-IV

A 8AI-M0-IV 01 13V-IICr-3AI B 10Mo-5.4Sn ^ 6AI-6Zr-IMo A 6AI-4V-2Sn 0 6AI-2MO

V 5AI-2Sn-2Mo -2V \/ 7AI-12Zr

6AI-3V-li\/lo © 7AI-liVlo-IV

6AI-2Sn-IMo- IV 0 11Mo-5Sn-5Zr

€ 6AI-2Sn-2IVIo -2V X 6AI^V-2IVIo

6AI-6V-2Sn- Cu-YiFe © 6AI-4Zr-2Mo

7AI-2y2Mo

7AI-3Cb-2Sn © 7AI-3MO

0 6AMZr-2Sn-y2Mo-y2V

C^NRL

100 110 120 130 140

STRENGTH (KSI)

150 160 170

Figure 7. Kiscc data on titanium alloys in sea water. Data of B. F. Brown, R. J. Goode, R. W. Judy, R. L. Newbegin. and L. J. Waldron. Figure kindly supplied by

R. W. Judy.

9
Test Methods

III. Constant Strain Rate Tests

A fundamentally different way to assess SCC

characteristics is to pull to fracture a series of small

smooth tensile specimens, each one at a different

but constant rate, and measure the reduction in area

as in a conventioned tensile test. (Other parameters

may be used as indices of SCC effects, such as elongation or fracture stress.) This series is done in an inert environment and again in the corrodent of

interest. If one or more stress corrosion cracks nucleate and grow during the test, that event decreases

the ductility to an extent related to the number and

size of the stress corrosion cracks. A schematic plot

of data representative of such tests appears in figure

8 and shows the behavior of an alloy in an environment which causes SCC compared with behavior in an inert environment. The extent of the maximum

separation between the two curves of figure 8 is an

indicator of the degree of susceptibility of the alloy in

the environment causing cracking (ref. [3]).

<

m

<

o

Q

LU

GC

pnNMENT.

CORROSIVE ENVIRONMENT

LOG STRAIN RATE

Figure 8. Plot of typical results in the Parkins constant strain

rate tests on one material in two environments, one inert and one causing SCC. After reference [3],

Constant strain rate test data might be used in the

following way. Let us suppose that unprotected steel

in (moist) soil would behave according to the sagging curve of figure 8. But if zinc anodes are connected to the steel the electrochemical potential of the steel can be altered with consequent altering of the corrosion behavior of the steel. Let us accept that the

electrochemical potential of this particular steel can be so altered that the steel would behave as if it were

inert (top curve of fig. 8). Such behavior, if it were

observed, might form the basis for specifying this

electrochemical treatment (cathodic protection).

It might be pointed out that such a procedure

would be precisely analogous to the standard method

for avoiding brittle fracture in ship plate. The absolute toughness of the ship plate steel is not measured, but rather the temperature of transition

between brittle behavior and tough behavior is determined, and the transition temperature is required

to be safely below any possible operating temperature.

IV. Metallurgical Considerations in Tests

Those unfamiliar with the internal structure of

metals and alloys might not know that their chemical

reactivity—their corrosion behavior—can be greatly

influenced not only by their nominal composition but

also by the way in which they are produced. The

chemical behavior of reagent grade sodium hydroxide is identical whether it comes out of a large

bottle or a small one, but the stress corrosion cracking behavior of a large forging may differ greatly

from that of the same metal rolled down to a thin

strip. Also, because of grain testure (analogous to the texture of wood) in certain alloys, notably

aluminum and some high alloy steels, the SCC

characteristics are directional. The stress required

to drive a stress corrosion crack across the grain

texture can be vastly greater than that required to drive it parallel to the grain texture (in the "weak"

direction). This "anisotropy" (as it is called) is also

true for other fracturing modes.

For these reasons, for critical components care must be exercised in interpreting SCC data. One

should not think of the SCC characteristics of an

alloy as having general applicability for all mill product forms. The careful worker would tend to report

SCC characteristics somewhat in the following pattern: Kiscc for 18 percent nickel maraging steel, vac- uum induction melted and vacuum arc remelted,

rolled to 1 in plate, heat treated to a yield strength of

190 ksi (1300 MNm"2) and tested in the weakest

direction in natural sea water, was found to be 90

ksi Vm (99 MNm-'*'^), based on tests of 1,000 h duration.

V. Interpretation of SCC Test Data

For the benefit of the person with little background in the subject, it should be admitted that a great deal of the published data on SCC are difficult to interpret in terms of practical materials engineering. Most of the extant data on the SCC of austenitic

stainless steels were taken in hot concentrated magnesium chloride solutions, solutions which are never encountered anywhere except in SCC testing chambers. We think that the order of merit of various

stainless steels determined in those solutions will

largely pertain also to the practical case of boiler

water containing only traces of chloride, just as we

think that the data taken on aluminum alloys in

alternate immersion tests in 3.5 percent sodium

chloride solution at room temperature are meaningful in predicting the performance of the same alloys

in aircraft. When such test data are interpreted by

someone with extensive field experience, the data

are meaningful, but the contribution of experience

not now expressable in quantitative terms is the key

here.

10
Test Methods

It is certain folly to design for a sustained working

stress equal to a measured SCC threshold stress,

however carefully that determination may have been

made, but rather a sizeable margin must be left. The

reason for this margin is that unknown stresses from

heat treatment, fit-up, welding, and thermal expansion are the causes of a majority of service failures,

and one does not want to run the risk that unknown

stresses from such causes may elevate his design

stress above the SCC threshold stress. An attempt has been made to harmonize the fracture mechanics term Kjscc and its implications to behavior of smooth specimens of the same alloy,

which was 7079-T651. Kjscc was estimated to be 4 ksi

Vm (4.4 MNm~^' For the same material, the

threshold stress was fond to be 8 ksi (55 MNm"^).

The yield strength of the material was 67 ksi (460

MNm~^). It is instructive to compare the implications

of these data graphically, as shown in figure 9. Inserting the data above in the Irwin equation (eq (1) above)

and rearranging and taking logarithms of both sides, one finds that the log of the stress in hnearly proportional to the log of the flaw depth a, as shown by the

sloping line in figure 9. One would suppose that this

hne should intersect the threshold stress line at some

value of flaw depth approximating either the initial

surface roughness of the specimen of else some value

of the flaw size commensurate with any corrosion

pits which may have formed. In fact, however, as may

be seen in figure 9, the intersection is at about 0.06 in

(1.5 mm). Either the estimate ofKjscc is too high, if the

concept ofKjscc is valid for this alloy, or some extrane- ous phenomenon is entering. In any event as matters

now stand, one would have to be governed by both the

smooth specimen threshold data and the fracture

mechanics data. The threshold data from smooth

specimens is incapable of taking account of the effects of flaws, and clearly from the sloping hne one

cannot be safe at the threshold stress level with flaws

present deeperthan about 0.06 in (1.5 mm), assuming

the customary surface flaw shape, 10 times as long as deep; likewise one is not safe using the fracture

mechanics guidance below that flaw size.

It seems to be the present status of SCC technology that one must act upon the most pessimistic

SCC characterization data, whether they be

1 1 1

1 I I I I

I 1—r~n

CO

to

LUocH

z

o

Lii W3to

toO

CCO

rm r

ctY.S. = 67 ksi

T—i i I I I I

I

CRITICAL FLAW FOR SCC

2c » a

K = 2y/ir- a = 4 ksi-yATrT.

"SAFE REGION"

ll I I I I ll I J L_L

0.001 0.01 0.1 FLAW DEPTH, a-in.

Figure 9. Flaw depth a versus section stress, showing threshold stress for aluminum alloy 7079—T651 rolled plate. (S-L cracks; see chapter 4 for meaning.) Sloping line is a plot of eq 1 on a log scale for this material. In triangular area to right marked "A", smooth specimen data would erroneously imply safety against SCC. In

triangular area "B" fracture mechanics data would erroneously fail to predict SCC. After reference [4].

11
Test Methods

threshold data from smooth specimens, fracture

mechanics data, or data from constant strain rate

tests, and in the absence of positive information to the contrary, it is a prudent rule to assume that

somewhere in the structure there will probably be

stresses equal to the yield strength.

The reader should appreciate that few laboratory

sec characterization tests are of durations comparable with the desired service life of most structures,

and added conservatism taking this state of affairs

into account is warranted in interpreting laboratory

sec data to engineering practice.

VI. References

[1] Craig, Jr., H. L., Sprowls, D. O., and Piper, D. E., Handbook on Corrosion Testing and Evaluation, William H.

Ailor, Ed. (John WUey & Sons, New York, 1971) p. 231.

[2] Smith, H. R. and Piper, D. E., Stress corrosion cracking in high strength steels and in titanium and aluminum alloys, B. F. Brown, Ed., Naval Research Laboratory, Washingtion, D.C., 18 (1972) p. 17.

[3] Parkins, R., Corrosion, Vol. 32, No. 5, p. 205 (1976).

[4] Sprowls, D. O., Shumaker, M. B., and Walsh, J. D., Evaluation of stress corrosion cracking susceptibility using frac- ture mechanics techniques. Final Report on Contract No. NAS 8-21487, May 31, 1973.

12
CHAPTER 3

COPPER ALLOYS

L AUoy Nomenclature

The atomic arrangement in crysttds of pure copper

is face-centered cubic, and as long as this arrangement is retained in copper alloys, they are designated alpha alloys, such as alpha brass. If a brass

contains more than about 35 percent zinc, grains of a second phase termed beta appear among the alpha

grains. The beta phase has the atoms arranged in a body-centered cubic lattice. Brasses containing both

phases are termed alpha-beta brasses. Above about

40 percent zinc the alloys consist of all beta grains

and are termed beta brasses. Beta brass alloys are

rare in commerce, but a few are produced in Europe.

Copper alloys are not strengthened by heat treat- ment, with few exceptions, notably beryllium copper. But they are commonly strengthened by cold

working. Ordering reactions occur during annealing

of some alloys, as will be noted, but these reactions

are not used for strengthening purposes.

The term bronze was once reserved for copper

alloys in which the principal alloy addition was tin,

but the term has been debased to the point that its use serves little more than to exclude the nearly

all-copper alloys and the nearly pure binary copperzinc alloys (brasses).

II. Ammoniacal SCC _

Ammoniacal compounds are generally believed to be the principal service environments causing SCC

in copper alloys, although these are known not to be

the only ones. SCC in the copper/ammoniacal system has often been termed "season cracking" by

leading authorities, but there is a lack of unanimity

in defining this term by various authors and nomenclature committees. Therefore, to avoid any possible

confusion, the term "ammoniacal SCC" will be used

here instead, though even so it should be understood

that the word "ammoniacal" is a convenience intended to include amines and all other species which

can react with copper to produce the cupricammonium complex ion or perhaps structurally similar complexes.

If one learns how to avoid ammoniacal SCC, a large proportion of the practical SCC threat to copper alloys can be prevented. Not only is ammoniacal

SCC a major hazard to copper alloys, but it is also

the SCC system for which we have by far the most

intercomparable laboratory data and field experi- ence. We will therefore treat ammoniacal SCC as the principal topic of this chapter, thereafter treating

the other alloy/environment systems which do not fit into the major group. These other systems, though

they have been responsible historically for fewer

SCC failures than ammoniacal SCC, can nevertheless be extremely troublesome, as will be seen. It should be especially noted that recent studies with

copper sulfate and copper nitrate solutions suggest

that some of the service failures surmised to have

been caused by ammonia may have been caused

rather by sulfate or nitrate ions under conditions

permitting the generation of concentrated copper

solutions. Also some of the SCC failures occurring in

the atmosphere and attributed to ammonia may have

been caused instead by sulfur dioxide, oxides of

nitrogen, or other contaminants, as discussed under

"SCC in Other Environments" below.

A. Necessary Conditions

There are five conditions which must be met before ammoniacal SCC occurs:

(1) Water

(2) Oxygen

(3) Tensile stress

(4) Alloy of susceptible composition and structure

(5) Ammonia

Considering these in turn:

(1) Water must be present, probably in the con- densed liquid state; a film of moisture on the metal

surface suffices to cause cracking if other conditions

are met. It appears possible that there are hygroscopic corrosion products which can produce the

aqueous film believed necessary in an atmosphere

well above the dew point. An example of cracking

attributed to the action of deliquescent contaminants and another example attributed to a film of

condensate caused by dew-point conditions will be

described.

(2) Oxygen or an oxidizing agent must be present,

unless there is an independent source of copper ions

in solution. An example will be given illustrating how

preventing access of oxygen is a practical strategy to avoid ammoniacal SCC in some situations. The

higher the oxygen content of the environment the

higher is the incidence of SCC (other necessary factors being supplied and kept constant), but quantitative- data are lacking.

As a corollary of (1) and (2), anhydrous ammonia

having neither water nor oxygen is quite compatible

with copper alloys, even those which are highly sus- ceptible to ammoniacal SCC. Under circumstances

where control measures may be so lax that one cannot be certain to exclude water contamination, it is of course unwise to employ brasses which are sus- ceptible to ammoniacal SCC. This is true of tankage

fittings in highway tankers used to transport agricultural ammonia in the summer and which upon

13
Copper Alloys

discharge may be purged with water in a manner

creating an SCC hazard possibly causing leaks. Not

only could the leaking ammonia be troublesome, but

in winter these tankers are used to transport low

pressure gas to utility plants, and if a cracked fitting

in such a tanker should leak gas and the gas ignite, a major catastrophe could ensue. Therefore, the Department of Transportation regulations prohibit the use of copper or any of its alloys in such tankers if these metals come into contact with the lading. Note

the conservatism here: As will be seen, pure copper

and several of its alloys are highly resistant to ammoniacal SCC. But in the transportation industry

maintenance personnel cannot be expected to know

this, so one simply excludes all yellow or red structural alloys.

(3) Tensile stress must be present, and the hazard- ous level may be only a small fraction of the yield

strength or higher depending upon the alloy and the

environment. This stress may be supplied by working stress (design stress), assembly stress, cold work

remaining from forming operations, or any combination of these stresses as long as their vector sum on

the surface is a tensile stress of sufficient magnitude. In structures or components Ukely to be

made of copper alloys it is seldom the design stresses

which are important to cracking, but rather the

stresses residual from cold forming (as by spinning,

drawing, sinking, roll-forming, stamping) or fit up or assembly stresses, including especially straightening or bending of pipe or tubing. As a corollary of this

generalization, thickening the structural elements is seldom effective in avoiding the cracking problem in

the appUcations for which these alloys are commonly

used. As another corollary, since castings are usually much freer from residual stresses than unannealed wrought products, they are not as frequently

beset by the cracking problem as wrought products.

Two apparently similar forming methods to produce

the same product may generate vastly different

residual stresses. For example forming tubing by

pulling it through a die without internal support

(sinking) leaves vastly higher residual stresses than

pulUng over an internal support or mandrel (drawing).

One way to remove the hazard of SCC caused by

cold work is to anneal the product, but the reader is warned that removing the cold work by annealing is no guarantee that stresses of hazardous magnitude

may not be re-introduced by even minor subsequent

deformation, even dropping the annealed article

from the bench to the floor!

If one decides the tactic to be used to avoid SCC

in a given product includes removal of cold-work

stresses, he may elect to specify either a full (recrystalHzation) anneal or alternatively a stress-relief anneal. Achievement of full annealing can be verified

by hardness measurements or (if feasible) by metallographic examination, or if the product is one whose

recrystalhzation characteristics are well known, as

for example brass condenser tubes, one may rely

upon a conservative specification based on experi- ence. If, however, one does not wish to sacrifice

strength and chooses to anneal only enough for stress relief, experimentation may be needed. Table

1 gives some guidance for such stress relief treat- ments and illustrates the wide range of annealing

temperatures which may be required. The temperatures in table 1 are typical values only. If for a given

cold-worked product avoiding SCC depends criti- cally on relief of cold-work stresses, the exact

minimum annealing temperature must be found by

trial, as it varies with such factors as composition,

degree of cold work, and microstructure. The procedure then is to try some annealing temperature near

the typical ones shown, and afterward to expose the

heat treated article to see whether it will crack in the

Table 1. Typical stress relief annealing temperature

Alloy name Alloy number '' Temperature

Commercial Bronze C22000 400 T 200 "C

Cartridge brass C26000 500 260

'

Muntz metal C28000 375 190

Admiralty brass C44300 575 300

Cupronickel (30%) C71500 800 425

Phosphor bronze (5%) C51000 375 190

Phosphor bronze (10%) C52400 375 190

Silicon bronze (3%) C65500 700 370

Aluminum bronze D C61400 1112 600

» After Metals Handbook, 8th ed., Vol. 1, p. 1001, ASM, Metals Park OH, 1961.

" ASTM-SAE Unified Numbering System for Metals and Alloys. The first three digits are the Copper Development Association

designations for the same alloys.

"Unified Numbering System for Metals and Alloys," published by the Society of Automotive Engineers (1975) shows the metals and

alloy numbers assigned through July 1975 together w^ith common designations for the same materials. Suggested annealing time for Muntz metal is Vz h; for cdl other alloys listed, 1 h.

14
Copper Alloys

most hazardous environment (from the point of view

of SCC) to be encountered in service. Only infrequently is this most hazardous environment

known; if it is not known then an arbitrary severe standard test environment must be used.

A quick test used to detect high and even moderate stresses in many copper alloys employs acidified

mercurous nitrate as the environment. This test has

been standardized as ASTM B154-71. In this test the article is acid-pickled to remove oxides, rinsed in

running water, immersed for 30 min in an aqueous

solution of 10 g Hg2(N03)2 (11.4 g if tetrahydrate)

plus 10 ml concentrated HNO3 per liter, and

examined for cracks. Note that this is not necessarily a test for susceptibility to SCC but rather a qualitative and potentially destructive test for residual

stresses in copper alloys. The mercurous nitrate en- vironment acts more rapidly than ammoniacal environments; but at low stresses the mercurous nitrate

may not crack an article which cracks (albeit slowly)

in a concentrated ammonia environment. Thus passing the mercurous nitrate test is no guarantee

against subsequent failure in an ammoniacal environment, and this fact has caused expensive fail- ures. In this respect the German standard DIN-1785

is slightly more conservative than customary U. S. standards, for DIN-1785 calls for testing for residual

stresses in condenser tubing by use of an ammonia

atmosphere when so agreed by the manufacturer

and the customer.

But note that some alloys (aluminum bronzes

C61900 and C63800, and aluminum brass C68800)

may, when annealed in certain temperature ranges,

undergo an ordering reaction which may increase

the degree of susceptibility to ammoniacal SCC even

though partial stress-relief has occurred. Hence the

mercurous nitrate test, which checks only for re- sidual stress, not susceptibility, is not as conservative a test for these alloys as an ammoniacal test. Note also that an article such as a condenser tube

may be adequately annealed or stress reheved only

to have stresses of dangerous level re-introduced by

straightening, denting, cutting and deburring, or improper installation. For these reasons conservative

practice does not rely solely on annealing to avoid

SCC.

Unfortunately to verify adequate annealing for stress relief there is no practical alternative to potentially destructive tests, though one may elect to over-anneal and thereby soften the alloy enough that

hardness or microstructure can be used as evidence

of the stress relief. Conservative practice hmits

straightening brass tubing after final annealing to 2-plane straightening and disallows rotary

straightening.

(4) An alloy of susceptible composition and struc- ture is a fourth (and obvious) condition which must

be met before ammoniacal SCC is a problem; the

degree of susceptibility differs enormously among

copper alloys. One measure of degree of susceptibility is the highest stress which the alloy will sustain in a specified environment without cracking. Data of

this sort from a "season cracker" environment are

shown in table 2 for various alloys in assorted degrees of cold work. The experiments from which this

table was constructed extended to about 1,000 h,;

which appears adequate to characterize a threshold

stress for this system. It would be very unwise to use

the stresses in table 2 as design working stresses

for critical parts, but when considered together with

other data and experience these stress levels provide

the basis for the very useful general classification of

the relative resistance of various classes of copper

alloys to season cracking shown in table 3. The

purist might wish for a more quantitative presentation than is shown in table 3, but the data are simply

iiot available for it, nor is it certain that the quantification would be interpretable in practice since in

many applications we do not know either the stress

in the completed and operating structure or the environment very well. Minor impurities in a highly sus- ceptible alloy such as cartridge brass (30% Zn) have

not been found to affect susceptibility. But note in

table 3 that the small amount of P in phosphorized

copper causes this alloy to be in a distinctly more

susceptible category than the phosphorus-free tough

pitch copper.

Other alloys have been reported susceptible to ammoniacal SCC, but the data are not easily compared with those from which table 3 was made.

These miscellaneous alloys are Cu-Si-Mn; Cu-Au;

Cu-Si; Cu-Ni-Si; Cu-Be; and Cu-Al-Fe.

(5) Ammonia in some form is the fifth (and also

obvious) condition necessary to cause ammoniacal

SCC. There are almost unbelievably diverse and

obliging sources of ammonia, including the decay of

all sorts of by-products of many hving organisms and

the decay of the organisms themselves, electrical

storms, spark discharges, some soldering fluxes

(NH4CI), and agricultural and industrial chemicals.

Three examples which have caused SCC in phosphorized copper tubing used in heating systems will

illustrate the diversity of ammonia sources: (a)

amines used as foaming agents in foamed concrete

used to insulate underground pipes, (b) latex adhesives used to lay vinyl tile on floors pierced by copper tubing, and (c) ammonia-containing agents used

to clean those floors. Not only can ammonia be produced by a wide variety of reactions, but additionally

there is some evidence that the ammonia so produced in dilute solution can be concentrated in

corrosion pits by an unexplained process. It is not

uncommon for evidence of ammonia surmised to have caused SCC to be missing at the time for

analyzing a failure, which may be done long after the

cracked item has been removed from the environment causing cracking and has been dried. More on

this below, (ref. [4])

Organic amines and hydrazine cause copper alloys

to crack (if oxygen is present), and it is sometimes

implied that they do so by first decomposing to ammonia. As an alternative and more reasonable possibility, perhaps they simply perform a complexing

role directly without the intermediate decomposition

(ref. [5]). In any event the presence around copper

alloys of any nitrogenous compound which has the

15
Copper Alloys

Table 2. Approximate sustained tensile stress for several copper alloys in an atomsphere of 16% NH^, 4% H2O, balance air, at 35 °C ^

Stress

Alloy name Alloy number Temper ksi MNm~'

Cu-30% Ni C71500 70% reduction 85 585

Silnic bronze C64700 75% reduction, aged 80 550

Cu-30% Ni C71500 Annealed 60 410

ffigh silicon bronze C65500 " 6 hard 50 345

Commercial bronze (10% Zn) C22000 4 hard 40 275

Electrolytic tough pitch copper CI 1000 Annealed 35 240

Low brass (20% Zn) C24000 4 hard 35 240

Silnic bronze C64700 Solution treated, aged 20 140

High silicon bronze C65500 Annealed 20 140

Admirahty brass — Annealed 10 70

Commercial bronze (10% Zn) C22000 Annealed 10 70

Low brass (20% Zn) C24000 Annealed 10 70

Antimonial Admiralty brass C44400 2 hard or annealed 5 35

Low silicon bronze C65100 Annealed 5 35

Cartridge brass C26000 Annealed or cold worked <5 <35

Muntz metal C28000 Annealed or cold worked <5 <35

Admiralty brass — 6 hard <5 <35

" After Whittaker (1), quoting data of Thompson and Tracy (2).

* "6 hard" etc.: An arbitrary copper industry scale of hardness due to cold work ranging from 1 hard (approx. Rockwell B68) to 8 hard

(about B97).

These stresses are not to be used in design. See text.

Table 3. Relative susceptibility of copper alloys to ammoniacal sec ^

Very low susceptibility:

Low susceptibility:

Intermediate susceptibility:

High susceptibihty:

Cupronickels,'' tough pitch copper, silicon bronze

Phosphorized copper

Brasses containing less than 20%

Zn, such as red brass, commercial bronze, aluminum bronze,

nickel silver, phosphor bronze,

and gilding metal.

Brass containing over 20% Zn,

with or without small amounts of Pb, Sn, Mn, or Al (such as leaded brass. Naval brass. Admiralty brass, manganese

bronze, or aluminum brass). The higher the Zn, the higher

the susceptibility.

^ After Thompson (3). For lowest susceptibility, the iron in 90-10 cupronickel should

be solutionized. But note that this alloy is highly susceptible to SCC in high

temperature steam.

capability of forming complexes resembling the

cupric-ammonium complex should be considered as

potentially causative of SCC as ammonia, unless

proven otherwise.

The effect of ammonia concentrations on cracking

kinetics in various alloys is not known quantitatively.

There is a recent adopted ASTM Standard Recommended Practice for ammoniacal SCC tests at one

constant ammonia concentration, designated ASTM

G 37-73, but this test is valid for binary Cu-Zn alloys

only. It is being modified to include Cu-Ni-Zn alloys

(nickel silvers).

Even if all five of the above conditions are met,

and if among these, stress, susceptibility, and ammonia concentration are at or near maximum

hazardous levels, ammoniacal SCC doesn't necessarily occur immediately. There is a finite period

available to take preventive measures, although

there are no quantitative rules available to fix the

length of the grace period. An example will be cited,

16
Copper Alloys

however, which will permit deducing something

about the length of this period for one set of conditions.

The presence of CO2 is not a necessary condition

for cracking, but it is observed to speed up the

process markedly. Corrosion pits or other stress

intensifiers also do not seem to be necessary conditions for initiating ammoniacal SCC, but some

service experience suggests that plug type dezincification may speed up the initiation. Corosion pits may

also act as ammonia concentrators and/or concentrators of other chemical species.

B. Case histories and other illustrations of

principles

(1) A case of liquid attributed to deliquescence.

Springs used in central office telephone equipment

were made of highly cold worked nickel silver (nickel

brass) (C75700) and installed in a nominally dry

space with provision for air cooUng from the outside

atmosphere, (ref. [6]) Some of the springs ultimately

failed by SCC. Let us check out our five necessary

conditions: Water was not expected to be present, at least in the condensed phase, but atmospheric

oxygen, high stress, and at least intermediate susceptibility were present, and one might assume the

possibility of ammonia unless conditions were such

as to afford confidence of its exclusion. The problem

was to explain how, in air of low humidity, the moisture requirement was met. Careful examination re- vealed the presence of particles of airborne solids,

including nitrates, which could deliquesce in air having a relative humidity as low as 55 percent. As a

result of laboratory tests with these solids, the con- densate so produced on the brass surface from air of

low humidity was concluded to account for the

cracking observed in service. Incidentally the airborne nitrate turned out to be ammonium nitrate

which is used as a fertilizer.

The tactic chosen to control the problem was to switch to a less susceptible alloy, cupronickel. High

stress and the same environment including presumably the same ammonia source were still present,

but substituting an alloy two categories lower in susceptibility was found to afford the necessary security. One might postulate alternative measures such

as filtering out the airborne particles, or zinc coating, about which more below.

This case illustrates an important concept: that

with water and oxygen requirements met, the probabihty of cracking is a function of the joint action of

the stress, susceptibility, and ammonia concentration. This extremely important practical relationship

is not quantifiable by existing data, and one is re- duced to using qualitative terms. In the nickel silver

instance the stress was certainly high, the susceptibihty intermediate, and the ammonia concentration

perhaps low (but perhaps high locally due to the

reduction of nitrate to form ammonia). Lowering the

susceptibility factor rather drastically by the substitution of cupronickel sufficed to avoid SCC even

though the other factors remained unchanged.

(2) Annealing usually avoids the problem but in one case was not done soon enough. High-zinc brass

is rolled to form the screw shells used as bases for

incandescent light bulbs and plug fuses. The stresses

after forming are high, the susceptibility of the material is in the highest category, and one could confidently predict that in service or in storage many of

the shells would eventually encounter enough water,

ammonia and oxygen to satisfy all remaining conditions for cracking. But ammoniacal SCC does not occur instantaneously, and there is usually ample

time to get the cold-formed shells into an annealing

furnace to remove the forming stresses. After proper

annealing the stress is sufficiently low to avoid trouble.

But there was trouble in one case described by

Thompson (ref. [3]): During a cold New England

winter one lot of shells cold-formed in one building of

a plant was trucked to another building to be an- nealed. If the shells had gone into the anneahng

furnace at once all would have been well, but this

particular lot arrived in the annealing building too

late on a Friday afternoon to be annealed that week,

and on the following Monday morning the shells

were found to be cracked by the thousands. Let us analyze this case by inventorying the necessary conditions for cracking. The stress and susceptibility were high, there was presumably at least a trace of

ammonia present in the annealing building perhaps

from chemical processes, and of course, atmospheric oxygen was present. But what about the

requirement for moisture? The annealing building

was warm, with air particularly humid from nearby

pickling and cleaning operations. The cold shells at lower than room temperature undoubtedly picked up

a thin film of moisture which along with even a trace

of ammonia would be sufficient to complete the

necessary conditions for cracking. If the lot had

been warmed and dried with hot air, or even more

simply if it had been left outside in the cold, dry air,

the cracking would probably have been avoided. Of

course, the safer procedure would have been to modify the schedule to ensure annealing immediately

after forming. The incident shows us that for these

conditions the time allowed to get the annealing

started before cracking starts was something less

than about 2V2 days.

(3) Sometimes materials of low susceptibility crack.

Phosphorized copper (C12200) is used for water pipe

in great quantities. The alloy has a low susceptibihty

to ammoniacal SCC and in straight lengths is commonly supplied in the fully hard condition. But the

stresses from cold forming might be expected to relax at least partially because of room temperature

creep, and in any event the experience is that ammoniacal SCC of this material is extremely rare. The

susceptibility is low, but the stresses as formed are

high. There is usually adequate oxygen and moisture, and enough ammonia is produced somewhere

to cause the rare failures. In one instance the cracking initiated from the outside and was attributed to ammonia produced by the decomposition of damp

animal hair in felt insulation surrounding the pipe. In
Copper Alloys

two other cases the ammonia was concluded to have

been formed by microbial decomposition in cooling

water. In another pair of cases involving residential

piping, the stresses were attributed to straightening

of coiled pipe, and the ammonia was surmised to have come from lawn fertilizer. To emphasize the

rarity of the occurrence of SCC in this material, it is instructive to note that the two cases cited last were

the only two failures reported out of 50,000 installations in the same area. Annealing is an obvious way

to reduce the hazard if a component is sufficiently

critical to justify the special treatment.

(4) Many highly susceptible alloys are used successfully. Many tube-and-shell heat exchangers are used both for main condensers and for auxiliaries in

various kinds of steam power plants. These may be

tubed with cupronickel, particularly in naval power

plants, but for economy reasons central station

power plants are largely tubed with aluminum brass

or with Admiralty brass, both highly susceptible to ammoniacal SCC. Although the brass tubing is cus- tomarily supplied in the fully annealed condition and

the cupronickel tubing (less susceptible) in the

stress-relieved condition, when tubing is expanded

to form the seal at the tube sheet dangerous stresses

may be introduced in the tube if the rolling tool is allowed to extend beyond the inner edge of the tube

sheet. In such circumstances the tube is expanded,

and stresses are produced in the outer surface of the

tube. As if this were not enough of a hazard, on the

steam (shell) side ammonia may be present due to decomposition of amines sometimes used to control

corrosion, or perhaps the amines may play the es- sential complexing role directly without need to be

converted to ammonia, as indicated above. Additionally, on the water side (tube interior) there is always the possibility of ammonia being produced

from decaying organic matter in the riverine, es- tuarine, or sea water widely used to cool the condensers. Let us see how, with stressed susceptible

alloys very probably in the presence of ammonia (or

adequate substitute), it is possible to control the

cracking problem well enough for these alloys to be

viable candidates as materials of construction.

In a well designed and well operated plant, the

oxygen level on the steam side outside the airremoval section is low, low enough to effectively

remove that one necessary condition for cracking in

all but exceptional cases. (In the air-removal section

where oxygen can be high and ammoniacal compounds are likely, alloys susceptible to SCC are

avoided). On the tube side the sluicing through of the

cooling water is almost always adequate to prevent

the build-up of dangerous levels of ammonia, and

failures originating on the water side are very rare. Note that the practical solution is to use two partial measures, (1) the tubing is stress relieved if cupronickel or fully annealed if brass, and care is taken not to introduce too much assembly stress

(difficult to avoid), and (2) the oxygen is kept low on

the steam side, and the ammonia is kept low on the

water side.

When the system is shut down, new hazards must

be considered. On the shell side, oxygen can leak in

and, if amines or residual ammonia and water are

also present, the conditions for cracking are fulfilled. On the tube side any ammonia produced by the

decomposition of organic matter is no longer being

diluted by incoming water, and dangerous levels of

ammonia may build up. If either side is drained,

there is a hazard that water and ammonia sources may remain in pocketed zones and in sagging tubes.

Two strategies are available to avoid the cracking

problem on each side of the condenser. If design

permits, the shell side can be filled with clean deaerated water, which should minimize in-leakage of

oxygen and also minimize the concentration of re- sidual ammonia; or it can be drained, thoroughly

dried, and blanketed with nitrogen for extra insur- ance. The tube side can likewise be fiUed with water

and left so; or it can be drained, dried, and possibly

additionally filled with nitrogen. The most hazardous

lay-up situation is to allow either side to remain partially covered with water. Thorough drying is the

preferred stabilizing procedure during a long lay-up.

It is a common occurrence for a completely fabricated condenser or other heat exchanger tubed with

aluminum brass or Admiralty brass to be dehvered to the site where a large fossil-fuel or nuclear power

plant is scheduled to be build three years or even more before the plant is finally completed. The con- denser already has the highly susceptible alloy condition met, frequently with stresses from tube

straightening and rolling. If on the construction site

rain or condensate furnishes water and the atmosphere provides the oxygen, only ammonia remains to be supplied to complete the conditions for cracking.

The addition of this last remaining condition has obviously happened in a number of recent plants, for units

intact at the time of shipment have been found

cracked at installation a few years later. Proper packaging to exclude water and also ammonia is an obvi- ous step to avoid this problem.

Incredible as it may seem there have been a few

cases in which completed condensers were leaktested on the plant site using ammonia as the pressurizing fluid! The other four conditions were

already met for cracking, and the ammonia factor

was now high. Needless to say the condensers were

ruined. One tube sheet was cracked completely

through its 2-in thickness, (ref. [7])

Admiralty brass heat exchanger tubing is used

successfully in the oil refining industry in process

streams containing ammonia. It has been generally

assumed that the prescence of hydrogen sulfide in

these streams contributes an inhibiting effect on SCC. This effect may be due to the ability of hydrogen sulfide to reduce oxygen.

The conservative approach to designing a heat

exchanger is to select a tubing material immune to SCC in all the environments which the component

will encounter. If this approach is not feasible, one

selects the best material which it is feasible to use,

but does not rely entirely upon stress relieving or annealing of the unassembled tubing: The tubes

are welded to the tube sheet, then rolled to fiU the

18
Copper Alloys

crevice between tube and tube sheet (to prevent

concentration of corrodents in the crevice); then the

entire exchanger is stress-relieved, (ref. [8])

(5) Household articles such as coffee urns and

metal goblets sometimes illustrate the SCC phenomenon. Cold-formed articles in this category are often

made of brass, plated (often one side only) with

chromium or silver. They may serve intact for a long

time and then exhibit rapid cracking. In some instances the onset of cracking has been definitely

associated with exposures to enhance levels of ammonia, such as the unfortunate use of a cleaning

compound high in ammonia, formulated for use on copper but not suited for brass. Since there is no

nondestructive way to ascertain the degree of stress

relief (if any) in these low-technology items, the only

prudent policy is to minimize access of ammonia.

in. sec in Other Environments

Industrial and marine environments. Moist air

containing as little as 5 ppm of sulfur dioxide can

cause SCC (ref. [9]). It is thus not surprising that a recent extended systematic test program (ref. [10])

showed that an industrial atmosphere (New Haven,

Connecticut), badly contaminated with sulfur

dioxide but essentially free from ammonia, caused

cracking of many alloys. By contrast only a few

alloys cracked in the marine atmosphere. The findings of the program, greatly condensed, are shown in

table 4. It is typical of the complexities of SCC that

at high concentrations of sulfur dioxide (>0.1% by

volume in air) copper alloys do not crack, but they

corrode (ref. [10]). Ammoniacal stress corrosion

cracks are tarnished, whereas sulfur dioxide cracks

are bright.

Table 4. SCC behavior of wrought alloys in industrial and marine

atmospheres.^

INDUSTRIAL Alloys which cracked: C26000 \ 353, 443,

672, 687, 688, 762, 766, 770, 782

Alloys which did not crack, 2.2 years or

longer: 110, 194, 195, 230, 405, 411, 422,

425, 510, 521, 619, 638, 706, 725, 752

MARINE Alloys which cracked: 672, 766, 770, 782

Alloys which did not crack, 2.2 years or

longer: All the alloys shown under IN- DUSTRIAL except these four above.

^ After reference [10]. Omitting thenceforward the prefix "C" and the suffix "00".

High tensile alpha-beta brass has been used for

fasteners in sea water, but because of cracking it is being replaced by either aluminum bronze or silicon

bronze (ref. [9]). Manganese propeller bronze

(C86100) has been observed to undergo SCC in sea

water, attributed by some to biologically produced

ammonia, but by some simply to water; this bronze

contains both alpha and beta phases, and heat

treatment plus care to keep the composition within

specified limits to reduce the amount of beta has

been reported effective in lowering the incidence of

SCC. Cracking of bronze propellers has also been

reported in some cases where cathodic protection

has been applied to the hull and therefore probably

to the propellers as well; perhaps this is a form of

hydrogen embrittlement.

Ship propellers are usually bronze castings for

which the following are the most representative al- loys: manganese bronze, nickel manganese bronze,

manganese nickel aluminum bronze, and nickel

aluminum bronze. The nickel aluminum bronze and

the manganese nickel aluminum bronze are practically immune to SCC in sea water. The other two

alloys are very susceptible to SCC, and if the castings are not cooled slowly in the mold, or if they are straightened or weld-repaired, they must be stress relieved to reduce residual stresses to safe levels

before the propellers are placed in service.

Stress relief temperatures, including those rec- ommended to follow straightening, are as follows for

the four alloys mentioned above (ref. [11]):

Manganese bronze

Nickel Manganese

bronze

Nickel aluminum

bronze

Manganese nickel

aluminum bronze

(after major

straightening only)

320-^30 °C (600—800 °F)

370-^30 °C (700—800 °F)

none

450-510 °C (850-950 T)

Miscellaneous environments. SCC has been observed in the field (and confirmed in the laboratory)

when nitric acid fumes react with stressed brass;

whether or not the ammonium ion, as a decomposition product of nitric acid, is involved does not seem

to have been established. SCC has been observed in

the laboratory when brasses are stressed statically in

the presence of aqueous solutions of sulfate, nitrate,

acetate, tartrate, or citrate which also contain copper ions. There is some evidence that oxides of

nitrogen (generating ammoniacal species?), carbonates, and phosphates can cause SCC of some copper alloys.

Common aluminum bronze D (C61400) is susceptible to SCC in plain live steam. Minor additions of

alloying elements, (especially about 0.2% Sn) as in a proprietary grade of the same basic bronze has been

reported helpful in avoiding the problem (ref. [12]).

Silicon bronze is also susceptible to cracking in

steam to the extent that one authority arbitrarily

places a temperature limit of 120 °C (230 °F) on the

steam if silicon bronze is to be used with it (ref. [9]).

Sulfamic acid in the concentrations used for cleaning

is capable of causing SCC in brass condenser and

heat exchanger tubes (ref. [13]).

None of these miscellaneous corrodents has re- ceived anything approaching the amount of systematic study that ammonia has received. The lesson

here is that if one is concerned about a fundamentally new combination of copper alloy and chemical

19
Copper Alloys

environment compared with any studied in the laboratory or experienced in service, there is no

adequate substitute for conducting a realistic test for

compatibiUty.

Liquid metal cracking. Certain liquid metals cause stressed copper alloys to crack. Mechanistically this process is certainly different from SCC in

aqueous solutions in at least some aspects and it is usually called liquid metal embrittlement , not SCC.

But from a practical standpoint it will be treated here

together with the other forms of sustained load

cracking which the design and maintenance en- gineer must control.

Mercury (Hg) is particularly troublesome to copper alloys, and neither metalhc Hg nor its compounds should be permitted to contact copper alloys.

Thompson illustrates the seriousness of the trouble

that can be caused when this stricture is violated:

A small brass heat exchanger had been manufactured.

It was vacuum tested using a Hg manometer to check the vacuum. A slug of Hg was accidentally allowed to enter

the heat exchanger. This was known and the heat ex- changer was opened and the Hg was cleaned out, pre- sumably perfectly. The heat exchanger was one of four

that were used as chillers in the air contitioning of a submarine. It was installed outboard of the keel on the lowest deck. After the vessel was completed and

launched, the system was pressure tested and it was

found that the chiller leaked. The only way to remove the

chiller was to cut holes through every deck from the top of the submarine to the bottom, remove and replace the

chiller, and repair weld all the decks. The chiller was, of course, the victim of liquid metal embrittlement from Hg.

The dire warning is never trust Hg in contact with any copper metal, (ref. [3])

Figure 1 shows a brass heat exchanger tube cracked

by Hg.

The difference between Hg cracking and ammoniacal SCC is emphasized by the experience that

cupronickel, highly resistant to ammoniacal SCC, is highly susceptible to cracking by Hg.

Molten solder, molten tin, and molten sodium

have also been observed to cause cracking of stressed

copper alloys.

Figure 1. Admiralty brass condenser tube failure induced by ac- cidental exposure to mercury in a marine condenser. Courtesy of

S. D. Reynolds, Jr.. and F. W. Pement, Westinghouse.

rV. Miscellaneous Measures

Coatings, of course, are not an attractive way to control the SCC problem on heat exchanger sur- faces, but organic coatings have been reported useful in preventing or delaying SCC in other situations,

such as phenolformaldehyde films on cartridge

brass. Systematic data in the open Uterature are

lacking. Chromium plating and nickel plating have

not been found effective in the situations in which it is practical to apply them.

Metalhc zinc coating has been reported to be

highly effective, on the other hand. The zinc may act partly as a barrier, but there is evidence that, additionally, cathodic protection from the zinc can prevent cracking at openings in the coating. If zinc

coating and cathodic protection can be proven dependable control measures, they may be found useful in many more situations than now employ them.

Benzotriazole either as a surface treatment or as an inhibitor in the environment may be useful in

preventing SCC in marginal situations.

V. Diagnosing SCC

Even in alloys highly ductile in a tensile fracture,

all stress corrosion fractures (and those produced by

hquid metal cracking) are macroscopically brittle.

Particularly when they are caused by complex re- sidual stress fields from forming operations the

cracks may be branched, but not necessarily so. Multiplicity of cracks, whether more or less parallel

to each other or diverging, is highly indicative of

SCC, but absence of multiplicity or of branching is by no means a basis for excluding SCC.

If a crack path is examined by standard optical

methods and found to be intergranular (fig. 2), either

SCC (including ammoniacal SCC) or hquid metal

cracking can be assigned as the cause. If the cracking is caused by Hg, it is inevitably intergranular,

and there may be enough Hg amalgamated with the

metal of the crack walls to be readily detected by

standard chemical analysis. Cracking by Hg in metals at high temperatures, however, may leave so

httle Hg that special spectrographic trace analysis

methods may be required to detect it.

Figure 2. Intergranular stress corrosion cracking of Admiralty

brass condenser tubing (500x). Courtesy of S. D. Reynolds, Jr., and F. W.

Pement, Westinghouse.

20
Copper Alloys

If the crack path is found to be transgranular,

sec can still be the cause (fig. 3). Whether the

crack path is intergranular or transgranular is a complex function of alloy composition and electrochemical factors, and few reliable simple rules are available to specify crack path choice. One such

rule, however, is that in alpha-beta alloys the SCC

path is intergranular between alpha grains and

transgranular across beta grains; in all-beta alloys

SCC is predominantly transgranular.

fatigue from SCC is available with electron fractography by which fatigue can be positively identified

by its characteristic striations. Striations from plastic cross slip are sometimes seen on SCC fracture

surfaces, but they lack the special features which

are conclusively diagnostic of fatigue striations. In summary, in a normally ductile copper alloy a brittle

crack lacking genuine fatigue striations may be

diagnosed as SCC unless Hg (or other metal liquid at some point in the history of the part) is present

indicating liquid metal cracking.

VI. Theory

Figure 3. Transgranular stress corrosion cracking of Admiralty

brass condenser tubing. Courtesy of S. D. Reynolds, Jr., and F W. Pement,

Westinghouse.

Fatigue cracks are also transgranular, whether

across alpha or beta grains. Transgranular SCC usually is branched, and there is usually some change in

direction when a crack crosses a grain boundary.

Except for aluminum brass, transgranular SCC is usually associated with some intergranular cracking

with some change in direction as a transgranular

crack crosses a grain boundary. One practical rule

for commencing the distinction between fatigue and

SCC from optical microscopy on a polished section

is as follows: If the crack is transgranular and blunt

ended, it is probably not SCC but fatigue. If it is transgranular but sharp ended, look further into the

failure conditions, such as the possibility of vibrations or other cause of reciprocating stress, before

ruling out fatigue. Although most transgranular SCC

is associated with some intergranular cracking (except for aluminum brass), fatigue cracks almost never propagate even partially along a grain boundary, and they tend not to deviate when passing from

one grain to another. A definitive distinction of

As in all other systems, SCC in copper alloys is caused by the strain induced activation of the metal

in the vicinity of the crack tip and the consequent

reaction of the active metal locally with the corro- dent within the crack. There is considerable (though

not universal) agreement about how the strain activates the corrosion process and how the consequent

reaction occurs for the case of alpha brasses in the

ammoniacal environments which tarnish them. The

model is basically as follows: Copper in the alloy

reacts first with ammonia to produce a complex ion,

then the copper in the complex ion reacts with hydroxyl ions to produce cuprous oxide. The cuprous

oxide grows into the metal from which the copper is derived and would stifle further reaction unless a separate process occurred. At this stage two views

have been advanced to account for crack growth.

One view is that the cuprous oxide film, which tends

to intrude along grain boundaries, is brittle and

eventually cracks to expose bare (reactive) metal at the end of the crack. A new layer of oxide forms at this point and eventually cracks also. This sequence

is shown in figure 4. Another view is that the dislocation arrays in the stressed metal are blocked at barriers and that the growth of the cuprous oxide at the

expense of underlying metal destroys some of the

blocking action, permitting highly localized plastic

flow of the metal to rupture the oxide layer and

expose bare metal for the beginning of another cycle.

Thus the one view is that the metal is exposed by the

cracking of a brittle oxide, the other is that dislocation unlocking by corrosion permits plastic flow of

the metal to punch through a (ductile) oxide film.

The chemical reactions may be written as follows

(ref. [14]):

Dissolution

reaction:

Cathodic

reaction:

Precipitation

reaction:

Cu -l-nNHs + -10,+ H2O

Cu (NHs)^ + 20HCu (NH3)r

+ 3NH,

+ e

2Cu (NH3)+2 + 20H2NH3 + H2O

Cu (NH3)+2

CU2O -I21
Copper Alloys

Figure 4. Schematic illustration of the tarnish-rupture

mechanism for SCC of alpha-brass in tarnishing solutions. The

tarnish is considered to grow preferentially along a grain boundary (b), until it undergoes

brittle fracture (c). The crack is arrested by slip in the substrate, exposing the metal to

the environment (d). Further intergranular penetration then occurs (e), leading to

further limited fracture (f). Crack propagation thus proceeds by repeated cycles, the

resulting fracture surface showing evidence of discontinuous fracture (g). (Pugh, ref.

[14].)

We can now account for the four factors (other

than inherent susceptibility) which combine to cause ammoniacal SCC: The ammonia is required to take

up the copper as a solution complex; the oxygen

generates the hydroxyl ions which precipitate the

cuprous oxide, and additionally oxygen is observed

to be required to dissolve copper initially; the water

serves both as a solvent for the copper ammonium

complex and is recycled between the dissolution

reaction and the precipitation reaction; and the

stress serves to bare metal by either cracking the

oxide film or by causing the metal to flow through

the film plastically, reinitiating the cycle of film

growth and rupture. Recent observations have

suggested that this theory may require modification.

The theory of SCC by other mechanisms has not

been worked out to the point of general acceptance

and will not be included here. Readers interested in

this aspect will find reference [14] a useful starting

point.

VII. Summary

Most service failures by SCC in copper alloys are caused by the conjoint action of five factors, (1)

water, (2) oxygen, (3) tensile stress, (4) alloy composition and structure, and (5) ammonia or perhaps al- ternatively ammonia-like complexing agents. Sulfur

dioxide can also be responsible for SCC in industrial

atmospheres.

Alloys have been classified in four categories of

susceptibility. Examples have been given illustrating

the roles of each of the five factors.

Although in principle one might think to avoid

SCC in a given case by decisively controlhng one of

these five factors, uncertainties about the precise

nature of the environment and about the stress in

assembled and functioning components in real life situations often lead to a strategy of a multiplicity of

control measures any one of which may not be

amenable to exact control.

There are believed to be several mechanisms of

SCC depending upon environment and alloy, but the

model widely believed responsible for most service

failures involves the periodic formation and rupture

of a film of CU2O.

Mercury and its compounds must be kept away

from copper alloys at all costs.

VIII. References

[1] Whittaker, J. A., A survey on the stress corrosion of copper based alloys. International Copper Research Association, Potters Bar, Herts., England (1966).

[2] Thompson, D. H. and Tracy, A. W., Trans. AIME 185 100

(1949).

[3] Thompson, D. H., Stress corrosion cracking of metals—

A

state of the art, (H. Lee Craig, Jr., Ed.), STP 518, Am.

Soc. -Testing Mat., Philadelphia (1972), p. 39.

[4] Popplewell, J. M., private communication, 1974.

[5] Foley, R. T., private communication, 1974.

[6] Schick, G., Manual of Industrial Corrosion Standards and

Control, STP 534 (F. A. Cocks, ed.) Am. Soc. Testing Mat.,

PhUadelphia (1973), p. 107.

[7] Thompson, D. H., personal communication, 1974.

[8] Dillon, C. P., private communication, 1975.

[9] Cotton, J. B., private communication, 1974.

[10] Popplewell, J.M. and Gearing, T. V., Corrosion, 31, No. 8, p. 279 (1975).

[11] Anon., Guidance Manual for Making Bronze Propeller Repairs, American Bureau of Shipping, New York, (1972).

[12] DilUon, C. P., Mater. Performance, 13, No. 2,49 (1974).

[13] Presley, Maurice, Proc. of the 18th Annual Water Conference, p. 79 (1957).

[14] Pugh, E. N., The theory of stress corrosion cracking in alloys

(J. D. Scully, ed.) NATO, Brussels, 418 (1971), (Available

from the editor at The University, Leeds LS 2 9JT).

22
CHAPTER 4

ALUMINUM ALLOYS

There are many commercial aluminum alloys having a wide variety of composition, but all of them are predominantly face-centered cubic in structure.

There may be particles of other phases present,

including impurity phases, but the matrix of all al- loys is face-centered cubic.

Aluminum alloys may be strengthened by cold

work (strain hardening). However, the highest

strength alloys are strengthened by heat treatment.

This consists of first heating to dissovle one or more

elements, rapidly cooling the alloy to retain these

solute elements in solution (solution heat treatment),

then producing controlled precipitation either by

"aging" naturally at room temperature or by artifi- cial aging at slightly elevated temperature (precipitation heat treatment). This heat treatment is termed

age hardening or precipitation hardening, and it

is the only process for strengthening metals not

known to the ancients. Age hardening may be

supplemented by cold work. The artificial aging

process may be deliberately carried past the point of

maximum strength to ensure stability, and the

treatment is described as overaging or stabilizing.

Certain strain hardened alloys may also be given

stabilizing anneals. These straining and thermal

treatments are termed "tempers."

The Aluminum Association system for alloy designation is the one in common use in the U. S. In this

system the basic composition of a wrought alloy is indicated by a four-digit number, and the temper is designated by an appended letter, usually plus one

or more numbers according to an arbitrary code.

Thus the letter O in the alloy designation 2024-O

indicated that that particular alloy is in the annealed

condition, 5456-H32 indicated another specific alloy

in a specified strain-hardened temper, and 7075-T6

indicates still another alloy which has been age hardened in a specific manner. The key to these compositions and tempers may be found in reference [1]. Cast alloys have their own special system for composition designation but have the same temper designation system as wrought alloys. The number of

commercial alloys and tempers in increasing at an

appreciable rate, and the reader may find it necessary to consult one of the major aluminum producers

for alloy information not found in the standard handbooks. In the Unified Numbering System for Metals

and Alloys, the Aluminum Association system has

been retained. For the wrought alloys the AA

number is preceded by A9, so that AA7075 becomes

UNS A97075. For cast alloys the letter if any is replaced by a number representing its alphabetical

position, and the decimal point is dropped. Thus AA

705.1 becomes UNS A07051, AA B358.2 becomes

UNS A23582, etc. The Unified Numbering System

for aluminum alloys is too new to have gotten into the

literature.

I. Necessary Conditions for SCC

Most of the SCC service failures in aluminum

alloys occur because of water, water vapor, or aque- ous solutions. Except for the cracking described in

the sections on Hquid metals and other nonaqueous

environments, then, SCC in this chapter is to be

understood to be aqueous SCC. Other constituents

of the environment such as sedts and acid fumes may

profoundly affect the kinetics, as will be discussed

below, but in most instances water is the essential

cracking ingredient in service failures. The necessary conditions for SCC in aluminum alloys are as follows

:

1. Water, aqueous solutions, or atmospheric

moisture

2. Alloy of susceptible composition and structure

3. Sustained tensile stress

In a subsequent section we will consider those factors affecting kinetics which are important to the

strategy of controlMng SCC in various engineering

situations, but let us first consider these three

necessary conditions in detail.

(1) Water. Cracking is observed not only in liquid

water and aqueous solutions, but also in air and inert

gases at relative humidity levels as low as 1 percent.

At such low humidities corrosion products are unlikely to produce liquid solutions by deliquescence,

therefore water in the hquid state is not considered a necessary condition for cracking. Oxygen is Ukewise

not needed. Water in the liquid state causes more

rapid cracking than does water vapor, and therefore one of the strategies used to mitigate SCC in

aluminum structures for atmospheric service such

as aircraft is to design so that water will not accumulate and stand in pockets and crevices. Not only is liquid water more aggressive than the usual atmospheric humidity, but also undrained pockets may

collect salts which concentrate during evaporation

and further accelerate SCC. Therefore, although

unnecessary machining of heat treated aluminum

parts is discouraged (see section on Stress below), in

some unfortunately designed structures some engineers would prefer to drill holes to provide drainage rather than to allow water to accumulate. One

model of aircraft was so entensively drilled for this reason that it was dubbed the "flying sieve," but the

tactic paid off. The annular space between the end of an aircraft

hydraulic tube and an aluminum connector fitting is an example of an area from which water must be

excluded. Not only can water collect in that crevice

and accelerate SCC of the high strength fitting if other necessary conditions are met, but also the

23
Aluminum Alloys

water tends to generate bulky corrosion products in

the confined space, stressing the fitting in tension

and causing SCC; hence, the mihtary specification

requires seahng such space by soldering, brazing, or

filling with an organic sealant.

The essential function which most paints perform

in mitigating SCC of aluminum alloys is one of re- stricting access of water to the metal surface. Good

quality paint systems are helpful in this respect even

though none of them totally excludes moisture; they

should always be considered one of the possible

supplemental measures in an SCC control plan even though such a measure operates only on a kinetic

factor and even though they may lose their effective- ness where damaged. Anodized films, which may be

specified for resistance to abrasion or wear or for

improved paint adhesion, are of Uttle or no direct

benefit in mitigating SCC, and may even slightly

promote it. Further remarks on coatings are included in the

sections on "Kinetic Factors," "Inhibitors" and "An

Example of Applying the Principles to Avoid the

Problem."

(2) Alloy susceptibility. The relative susceptibilities of the various alloys and of individual alloys

having various tempers (heat treatments and/or

mechanical treatments) may vary widely. Members

of the wrought 2000-series and 7000-series are particularly vulnerable in some tempers, as are some of

the more highly alloyed 5000-series in some strainhardened tempers. When these alloys are extruded,

rolled, forged or drawn, the grains are elongated in

the direction of maximum flow to produce a grain

structure or texture which is of major importance to the SCC behavior of these alloys, as the crack path

is invariably intergranular. The longest grain axis is designated the longitudinal direction, and fracture

properties (including SCC) determined by stressing

parallel to this longest dimension are designated

"longitudinal" properties, even though the fracture

propagates at least nominally in a transverse direction. Correspondingly the direction in which the

grain dimension is shortest is designated the short

transverse, and the direction of intermediate grain

dimension is designated the long transverse direction.

The directionality (anisotropy) of wrought

aluminum alloy products is so important to SCC that

the ASTM code for describing both the orientation of

the fracture plane with respect to texture and the

direction of propagation on that plane should be

learned. These designations are both precise and

brief, and they are expected to be in increasing

usage in the literature in the years ahead. This code

employs two letters, the first of which indicates the

stress axis and the second letter the direction of

propagation. For long plates the letters are L for

longitudinal, S for short transverse, and T for trans- verse (fig. 1).^ If one has not a plate but an extruded

' An earlier ASTM code used the letters R (for rolling direction). W (for width) and T

(for thickness) rather than the currect L, T and S. The crack orientation now given as S-L (note the hyphen) was given in older literature as TR (without the hyphen). A T-L

orientation was formerly written as WR.

or drawn cylinder, modified designations are re- quired, C for circumferential and R for radial (fig. 2).

Figure 1. Crack plane orientation identification code for rolled plate (ref. [2].)

Figure 2. Crack plane orientation identification code for drawn

bars {ref [2].)

It is of course the grain geometry rather than the

product geometry (the external geometry of the

piece) which is important to the relative susceptibility to SCC in various directions with respect to grain

texture, and L, T and S may be thought of in terms of

grain shape rather than product shape. This concept

becomes important in complicated forgings in which

external shape may give no clue to internal texture,

which may have to be determined by etching a section if it cannot be inferred from knowledge of forging practice.

24
Aluminum Alloys

Practically all service SCC failures in aluminum

alloy components involve the short transverse properties, and the fractures would be designated S-L or S-T. A few service failures are T-L or T-S fractures.

Longitudinal (L-T or L-S) SCC cracks are essentially

unknown in practice. Figure 3 shows a cylindrical

landing gear forging which failed by SCC. Examination of an etched section of the forging would disclose that the failure is in the forging parting plane or "flash line." The grain texture on this plane is roughly equivalent to that of the short transverse

direction of a rolled plate, that is, having the poorest

resistance of SCC of any planes in the piece.

Figure 3. Stress corrosion crack in a nose landing gear assembly.

Cylinder is made of 7075-T6. After the stress corrosion crack had propagated about

SV4 in the remainder of the cylinder spUt by brittle fracture. Heat treatment to the -T73

temper would greatly reduce the probability of failures such as this. Photo courtesy of

S. J. Ketcham.

The relative susceptibility of different alloys and

of the same alloy in various tempers is currently

being assessed in two basically different ways. One

method employs smooth specimens which may be of

various shapes. These are placed under sustained

stresses of various levels, exposed to selected environments, and the total time for a specimen to crack

is observed. Maximum exposure times in laboratory

tests for specimens which do not crack are typically

a few months. Data from experiments with one alloy

in three orientations with respect to the texture are shown in figure 4. Two important features are apparent in this figure: First that there appears to be a threshold stress, designated cr,,j, below which SCC

appears not to initiate; and second, that the a,!, for

the longitudinal direction is much higher than for the

short transverse direction. Because it is the short

transverse cracking which constitutes the bulk of

service SCC problems, the cr,/, values for this orientation form the principal basis for ranking alloys and

tempers according to susceptibility.

The only formally standardized environment for

conducting susceptibility tests for aluminum alloys

is the 3.5 percent solution of sodium chloride in

water specified in FED-STD ASTM 044-75. This

method calls for repeated cycles of immersion for 10

min in the salt water followed by drying 50 min in air

(described as "alternate immersion"). This environment and test procedure have been adopted in

MIL-STD 1568 which calls for an exposure of 2000

cycles. Such laboratory tests are backed up with

correlative tests in natural environments such as seacoast and industrial atmospheres.

Some typical cr,/, data from alternate immersion

tests are shown in table 1, together with yield

strengths.

A second and fundamentally different method to rate the relative susceptibility of alloys and tempers

is to pre-crack fracture mechanics type specimens

and measure the plateau crack growth rate (Region

II, fig. 5, ch. 2); the apparent Kigcc can also be used

to rank alloys, but at the present time there is some

uncertainty whether there is an absolute threshold

below which SCC will not occur on some aluminum

alloys, many plots of K versus crack growth rate

show no clear sign of such a threshold. As with

smooth specimens there is no formally standardized

environment for ranking the relative susceptibilities

of aluminum alloys using fracture mechanics specimens, but aqueous 3.5 percent sodium chloride has

been used extensively. A more complex corrodent

the composition of which is shown in figure 5 is also

in current use. Some typical plateau and apparent

Kiscc values for several alloys may be observed in

figure 5.

(Tttt values such as those in table 1, plateau velocities and apparent K,scc data, together with service

experience, form the basis for arranging aUoys and

tempers in four arbitrary categories of relative susceptibility, as follows (after refs. [5, 6]):

(a) Very low susceptibility: No known instance of

SCC in service or in standard laboratory tests.

(b) Low susceptibility: No known instance of SCC

in service; cracking in standard tests only at high

stress (intensity) levels.

(c) Moderate susceptibility: No known SCC in

service. SCC not anticipated in service due to short

transverse tensile stress caused by heat treating and

quenching, or from design or assembly stress providing these are kept below 40-50 percent of the

guaranteed minimum yield strength. Higher sus- tained tensile stress from any source must be

avoided.

25
Aluminum Alloys

30 60 90 120 150 180

DAYS TO FAILURE

Figure 4. Effect of orientation of7075-T6 extruded section of a given thickness on resistance to SCC by alternate immersion in 3.5% NaCl

solution (lower bound data, after ref.

{d) Appreciable susceptibility: Appreciable SCC

in service; avoidance of SCC requires sophisticated use of several control measures, such as protection

of the end-grain texture from aqueous environments,

peening, and cathodic protection. SCC not likely if tensile stress is only longitudinal to the grain direction.

It is possible to characterize the categories chosen

above somev^hat quantitatively, as shown in table 2.

It is not now clear whether plateau velocities will

afford a positive measure of SCC characteristics,

but they are included here for possible usefulness in

comparing with plateau values (rapidly obtainable)

for future alloys and tempers.

Using these categories, the relative susceptibilities of various wrought alloys are shown in table

3, and those of cast alloys are shown in table 4.

(3) The tensile stress which causes SCC in

aluminum alloys may be provided by working stress (design stress), but a large majority of service fail- ures are caused by assembly stresses, or heat treat- ing stresses, or both.

Considering first the assembly stresses, the

sketch of figure 6 shows how a component,

machined in such a way as to expose the vulnerable

short transverse texture (or end grain), is assembled

by bolting to a rigid member with a slight angular

mismatch, so that there is sustained high tensile stress across the short transverse texture, a sure-fire

recipe for SCC failure if the alloy is susceptible. The

lesson of figure 6 should be learned thoroughly, for

the principles apply to many seemingly different

situations. For example hexagonal coupling nuts are often machined from extruded or rolled bar stock. In

service such a nut, when tightened, experiences circumferential (hoop) tensile stresses, and these are across the vulnerable short transverse texture of the

extruded material. When made of alloy 2024-T4,

which is appreciably susceptible to SCC, these nuts

fail by SCC from atmospheric moisture. When the

material specification is shifted to 6061-T6, which

has very low susceptibility to SCC, the problem is eliminated, even though qualitatively both alloys

have the same fabrication texture. (As a precautionary measure, however. Naval aircraft practice

prohibits use of rats machined from extruded rod

regardless of alloy.) Figure 7 shows an SCC failure in an extruded tube of 7075-T651, with the failure

originating at a hole which in service contained a tapered pin fastener. Note that once again this design causes sustained stresses (from the tapered pin) across the vulnerable short transverse texture of the

extrusion. The use of forced-fit inserts such as the

tapered pin or bearings in aluminum alloys susceptible to see is unwise, and when such designs must

be used, conservative practice dictates that the alloy

and temper be in the "low" or "very low" category

as these terms are used in table 3.

26
Aluminum Alloys

Table 1. Comparison of the resistance to stress corrosion of various aluminum alloys and products (ref. [4], change no. 2)

Estimate of highest sustained tension stress (ksi) at which test specimens of different orientations to the grain structure would not fail in the 3.5% NaCl alternate immersion test in 84 days ' (not for use in design). These parameters are commonly designated a-,^.

Alloy and

Type of Temper

Test

Direction ^ Plate

Rolled Rod

and Bar

Extruded Shapes

Section Thickness,

0.25-1 Inch

>l-2

Hand

2014-T6 L 45 45 50 45 30

T T 97 ocZi> ST "15 ^8

2219-T8 L <^40 = 35 "=35 = 38

T T oo C QCOO C 90OO ST ^38 "=35 = 38

2024-T3, T4 L 35 30 "=50 <=50

T T 9n Oi 1 o18 ST ^8 "io ^8

2024-T8 L SO ^^47 '60 = 60 = 43

I T aU DU 43

ST 30 "43 = 45 15

7075-T6 L 50 50 60 60 35

T T o\J QOoZ OCZo ST •^8 "is ^8 ^8

7075-T76 L ^49 "=52

T T C ACi ST 25 25

7075-T73 L '^SO 50 54 = 53 = 50

T T c /IQ TO AQ AQ C AO C AO48

ST "^•"43 =•"43 "=46 =•"46 =•"43

7079-T6 L <=55 60 = 60 = 50

LT 40 50 35 30

ST «8 ^8 ^8

7178-T6 L 55 65 65

LT 38 45 25

ST ''8 «8

7178-T76 L '=52 =55

LT ^52 •=52 ST 25 25

^ Lowest stress at which tests were conducted; failures were obtained.

" Ratings are for transverse specimens machined from round or square bar stock.

= Highest stress at which tests were conducted; no failure observed.

" These values will be lower for sections greater than 3 in thick, but will be at least 75% of the guaranteed yield strength.

= See section 9.5.2 for test method used to determine values.

' Tests performed at Alcoa Research Laboratories.

^ Tensile axis during test, L = Longitudinal, LT = Long-transverse and ST = Short-transverse.

Table 2. Approximate quantitative characteristics of SCC susceptibility categories used if this chapter (after ref. [5])

(Cracking in short tran sverse orientation)

Category of

susceptibility (o-,h/yield strength) x 100 {K,,cM X 100 plateau velocity

A. Very low >90% >95% <7 X 10-"m/s

B. Low 75-90% 80-95% 7 X 10"" - 3 X 10"'°

C. Moderate 40-75% 50-80% 3 X (10- '» - 10-')

D. Appreciable <40% <50% >3 X 10-9

27
Aluminum Alloys

12 16

10

-7

10-8

<X

o

DC^ 9 o

<o

10

-1 0

10"

20

5456-SEI\IS

24 MNm -3/2

7079-T651

2024-T351

2219-T37

2219-T87*

2021-T81*

r

7075-T7351

6061-T651 • •5456-H117

CORRODENT -0.6M NaCI +0.02IVI Na2Cr207 + 0.07M

NaC2H302 + HC2H3O2 TO pH 4 (2000hr)

*MECHANICAL FRACTURE - NOT SCC

APPRECIABLE

MODERATE

LOW

VERY LOW

QUALITATIVE

SUSCEPTIBILITY

IN TABLE 2

12 16

STRESS INTENSITY ksix^n"

20 24

Figure 5. K-SCC rate curves for several alloys showing various plateau levels and various apparent K]scc levels (after ref. [51).

Let us now consider heat treating stresses.

Aluminum alloys which are strengthened by age

hardening are first given a solution treatment at elevated temperature, quenched, and then are either

allowed to age at room temperature or are aged at an

elevated temperature. To estabhsh the right conditions for age hardening it is necessary to then cool a given alloy from the solution temperature faster than

some minimum rate, and for many alloys this requires quenching in cold water. Quenching usually

places the surface in compression, as shown in fig- ure 8. SCC does not initiate or propagate when the

alloy is in compression, therefore machining off the

compressive layer after quenching is undesirable

from the SCC viewpoint. Note in figure 8 that if machining should extend to the centerline, stresses

there if unaltered by the machining operation would

exceed the threshold stress for SCC for several al28
Aluminum Alloys

Table 3. Categories of aqueous susceptibility of commercial

wrought alloys in plate form—short transverse orientation {after ref m

^iic/^*ir^fir»ilit\7 ous^-cp iiuiuiy

Alloy ±1 pc7 mnf^ 111 ijcr r1

A. Very low 1100 all 3003, 3004, 3005 all 5000, 5050, 5052, 5154, all 5454, 6063

^5086 0, H32, H34

6061, 6262 0, T6

Alclad: 2014, 2219, 6061, all 7075

B. Low 2219 T6, T8

5086 H36

5083, 5456 controlled 6061 T4

6161, 5351 all 6066, 6070, 6071 T6

2021 T8

7049, 7050, 7075 T73

C. Moderate 2024, 2124 T8

7050, 7175 T736

7049, 7075, 7178 T6

D. Appreciable 2024, 2219 T3, T4

2014', 7075, 7079, 7178 T6

5083, 5086, 5456 sensitized 7005, 7039 T5, T6

See precautionary notes about some members of the 5000 series

alloys given in text.

without being stressed, perhaps because of precipitation during the quench. Therefore one follows

faithfully the heat treating procedure which has been

proven for each commercial alloy.

TRANSVERSE

DIRECTION

SHORT

TRANSVERSE

DIRECTION - VERTICAL

LONG

TRANSVERSE

DIRECTION - HORIZONTAL

SCHEMATIC OF GRAIN

ORIENTATION

LOCATIOW OF MACHINED ANGLE WITH RESPECT

TO TRANSVERSE GRAIN FLOW IN THICK TEE

RIGID

yiEMBER

ANGULAR MISMATCH (CAN

ALSO HAPPEN WITH

PARALLEL GAP)

HIGH ASSEMBLY STRESS

IN SHORT TRANSVERSE

DIRECTION

RIGID MEMBER

Table 4. Categories of aqueous SCC susceptibility of commercial

cast alloys " (after ref. [6])

Category Alloy Temper

A. Very low 514.0, 518.0, 535.0, A712.0 as cast 356.0, A357.0, 359.0, B358.0 all

B. Low 319.0, 333.0, 380.0 as cast 355.0, C355.0, 354.0 T6, T61

C. Moderate 520.0 T4

295.0, B295.0, 707.0 T6

D712.0 as cast

^ This table is based on far less experimental data than table 3.

loys, as shown in table 1. If the quenched piece is stretched mechanically, the quenching stresses are

largely removed (fig. 8). This mechanical relief is a standard mill procedure available for most product

forms and converts the — T6 temper to the — T651

temper.

Blind holes in pieces being quenched cause re- sidual tensile stresses in the poorly quenched re- gions of the hole unless special fixturing or sprays

are used. Quenching stresses may be reduced by

quenching in air or hot water providing the alloy,

part thickness, and mill product form are favorable.

But quenching some alloys in boiling water rather

than in cold water, such as 2024-T4, can render the

product highly susceptible to intergranular corrosion

LOCKED IN ASSEMBLY STRESSES

FROM MISMATCH

Figure 6. One method of generating high sustained stress across

the short transverse direction—an open invitation to SCC if alloy

is susceptible. Upper figure shows grain flow pattern in mill product. Lower figure shown how sustained stress can be generated by mismatched assembly, (ref. [6]).

Figure 7. SCC in end of extruded tube of 7075-T651, C-L crack plane. Terrestrial failure of Apollo Program, Lunar Excursion Module component,

stressed by taper pin which was driven into the hole shown. Cautious practice does not

permit such pins stressing the vulnerable texture unless the -T73 temper is used. Magnified 3X. Courtesy of Mr. C. D. Beacliem.

29
Aluminum Alloys

1 3/4" THICK PLATE

1 3/4"

+10 TENSION

Figure 8. (Above) Stresses in 7075—T6 plate given cold water quench and not stress relieved. (Below) Stresses in 7075-T651 plate stretched 2% after*cold water quench, (ref. [6])

Another tactic to minimize quenching stresses is to quench in water-glycol solutions of controlled

composition and temperature. These quenching

baths can be used for sheet material which it has

been proved that there is no loss in mechanical

properties compared with sheet quenched in cold

water, and for forgings where there is no loss in

mechanical properties compared with forgings

quenched in hot water.

Still another source of stress important in the SCC

of some aluminum alloys is the wedging action of

bulky corrosion products within the crack; in such

alloys a stress corrosion crack initiated at one end of

a long bar may propagate the entire length of the bar

because of this corrosion product wedging. Alloys

and tempers in which this action has been either

observed or surmised include 2014-T351, 2024-

T351, 2219-T37, 5456 senshized by heating, 7039-

T6351, 7075-T651 and 7079-T651. It may be noted

that most of these alloys are in the highest category

of susceptibihty in table 3.

It is good practice to use stress reheved parts

where possible, and to avoid residual tensile stress being introduced by plastic deformation of fully hardened alloys. Severe forming operations should be

performed with annealed metal and the part subsequently solution heat treated. Less severe forming can be done in the freshly quenched condition, preferably followed by mechanical stress relief. Cold shearing or stamping of susceptible alloys,

particularly those subject to corrosion product wedging noted above and particularly sections thicker

than about 1 mm {%2 in), can introduce enough re- sidual stress to initiate severe SCC problems in the

sheared edges. Cutting by other methods, such as milhng, is preferred.

It is possible to introduce compressive stresses on

the surface of a part by shot peening. If this treat- ment is one of thorough saturation everywhere, it is effective providing the surface compressive layers are not subsequently removed by corrosion, abrasion

or machining. But at sharp re-entrant angles as where a bolt head meets a flat surface, the peening

action may not extend far enough into the sharp

corner to prevent initiation of SCC there. Shot peening may thus be a practical measure for certain

situations, but where 100 percent coverage cannot

be conferred and maintained, this technique is of

marginal value. It is however considered sufficiently

worthwhile as a supplementary measure to be frequently specified for military aerospace structures,

particularly when the structure is given a subsequent coating system.

Following are some precautionary notes for

aluminum alloys exerpted from reference [4].

Alloys 5083, 5086, and 5456 should not be used

under high sustained stress at temperatures exceeding 65 °C (150 °F) because of the hazard of developing susceptibility to SCC (sensitization). In general

the H34 through H38 tempers of 5086, and the H32

through H38 tempers of 5083 and 5456 are not rec- ommended, because these tempers can become sus- ceptible to SCC.

In the recommended tempers, H113 and H321 for

sheet and plate, cold forming of 5083 and 5456

should be held to a minimum radius of five times the

thickness. Hot forming of the O temper of alloys

5083 and 5456 is recommended, and is preferred for

the H113 and H321 tempers in order to avoid excessive cold work and high residual stress. If the H113,

H321, H323, and H343 tempers are heated for hot

forming, a slight decrease in mechanical properties,

particularly yield strength, may result.

Where short times at elevated temperatures of

65-260 °C (150-500 °F) may be encountered, the

precipitation heat treated tempers of 2024 and 2219

are recommended over the naturally aged tempers.

Certain alloy-temper combinations, notably 2024

and 2219 in the T6 or T8-type tempers and 7049,

7075 and 7175 in the T73-type tempers are decidedly

more resistant and sustained tensile stresses of 50 to 75 percent of the minimum yield strength may be

permitted without concern about SCC. The T76

temper of 7075 and 7178 provides an intermediate

degree of resistance to SCC, i.e., superior to that of

the T6 temper, but not as good as that of the T73

temper of 7075.

30
Aluminum Alloys

The following guidelines are given in MIL-STD1568 "Material and Processes for Corrosion Prevention and Control in Aerospace Weapons Systems."

"Whenever the design requires the selection of

aluminum for structural components, maximum use

shall be made of alloys, heat treatments, and claddings which minimize susceptibility to pitting, intergranular and stress corrosion. The following are alloy/temper recommendations for resistance to . . . stress corrosion: 2024, 2124, and 2219 all artificially

aged; 7049, 7050, 7075, 7175, and 7475 all in the

T73XXX temper.

"In the event these alloys and tempers or other

approved alloys are not used, the susceptibility to stress corrosion cracking of the selected alloy will be

established for each application in accordance with

. . . (a detailed alternate immersion test in salt water).

"All aluminum sheets used in external environments and interior corrosive environments shall be

clad on both sides except where the design requires

surface metal removal by machining or chemical

milhng or where the design requires adhesive bonding or where the design uses alloys of the 5000 or 6000 series type . . . All mill product forms of

aluminum alloys 2020, 7079, and 7178 in all temper

conditions shall not be used for structural applications. The use of 7075-T6 shall be hmited to thicknesses not to exceed 0.125 in (3 mm) . . . Maximum metal removal from the surfaces of nonstress relieved structural parts after final heat

treatment shall not exceed 0.150 in (3.8 mm) unless

the final temper or condition has been demonstrated

to have a stress-corrosion resistance of 25 ksi (172

MN/m^) in the short transverse grain direction as determined by a 20-day alternate immersion test given in FED-STD-151, Method 823 .. . Maximum

metal removal requirements are not intended to apply to mechanically stress reUeved products be- cause of the low level of internal stresses resulting

from mechanical stress-relieving . . . "All surfaces of all structural forgings, where accessible, after final machining and heat treatment,

must be completely shot peened using a minimum of

two coverage passes or placed in compression by

other suitable means, except for forgings having a demonstrated stress corrosion resistance of 25 ksi

(172 MN/m^) or higher in the short transverse direction and web areas under 0.080 inches (2 mm) thick

where no short transverse grain is exposed by

machining . . ." NASA Marshall Space Fhght Center (MSFC) has

published further guidance for controlhng SCC in

aluminum alloys for space vehicles and associated

equipment in a document (ref [10]) based on laboratory tests in salt water at room temperature, on

marine and industrial atmosphere tests, and on service experience summarized (for aluminum alloys) as follows

:

The alloys shown in table 5 have been found to have high resistance to SCC. They are preferred

materials and may be selected without prior MSFC

approval. No other aluminum alloys may be selected

(for MSFC hardware) without a materials usage

agreement.

Table 5. Aluminum alloys with high resistance to stress corrosion

cracking (from ref. [10])

Aluminum alloys

Wrought[ a.b Cast

Alloy Condition Alloy f Condition

1000 Series All 319.0, A319.0 As Cast

2011 T8 333.0, A333.0 As Cast 2024 T6' T8* 355.0, C355.0 T6

2219 T6% T8 356.0, A356.0 AU

3000 Series All 357.0 All 4032 T6 B358.0 (Tens-50) AU

5000 Series AO- 359.0 AU

6000 Series AU 380.0, A380.0 As Cast

7075 T73 A612.0, C612.0 As Cast

7475 T73 514.0 (214) As Cast

518.0 (218) As Cast

535.0 (Almag 35) As Cast

^ Mechanically stress relieved (TX5X or TX5XX) where possible.

*' Including weldments of the weldable alloys.

Except plate and forgings which have low resistance. Except forgings which have low resistance and plate which has

moderate resistance.

^ High magnesium content alloys 5456, 5083, & 5086 should be

used only in controlled tempers for resistance to SCC and exfoliation; these alloys are not recommended for high temperature application (65 °C, 150 °F, and above).

' The former designation is shown in parenthesis when significantly different.

The alloys shown in table 6 are considered moderately resistant to SCC. MSFC approval must be se- cured before any alloy or weldment in table 6 can be

used. The materials in table 6 will not be approved

for applications involving high sustained installation

stresses, such as springs or fasteners.

Table 6. Aluminum alloys with moderate resistance to SCC ^'^ (from ref [10])

Alloy Condition

2024 Plate T8

2124 Plate T8

7001 T75, T76

7049 T73

7050 T736, T76

7075 T76

7175 T736

7475 T76

7178 T76

^ Mechanically stress relieved products (TX5X or TX5XX) should

be specified where possible.

Sheet, unmachined extrusions, and unmachined plate are the

least susceptible forms.

31
Aluminum Alloys

The alloys shown in table 7 are classed as highly

susceptible to SCC. They should be considered for use only where it can be demonstrated that SCC is not possible for environmental reasons, and even

then prior MSFC approval is required.

Table 7. Aluminum alloys with low resistance to SCC " (from ref. im

Wrought Alloys Casting

Alloy Condition Alloy Condition

2011 T3 295.0 (195) T6

2014 All B295.0 (B195) T6

2017 All 520.0 (220) T4

2024 T3, T4 707.0 (Temalloy 7) T6

2024 Plate & T6 D712.0 (40E, D612) As cast forgings

2024 Forgings T8

7001 T6

7039 All 7075 T6

7079 T6

7178 T6

7475 T6

" Mechanically stress relieved products (TX5X or TX5XX) should

be specified where possible.

Sheet, unmachined extrusions, and unmachined plate are the

least susceptible forms.

the "appreciable susceptibility" category to "very

low susceptibility." (See for example alloy 2014 in

table 3).

in. Inhibitors

Although paint coatings reduce the susceptibility

of an aluminum structure to SCC by restricting the

passage of moisture, such coatings are not completely impermeable to moisture, and SCC can nucleate and grow under a paint film, particularly under

a film which has become cracked by aging. For such

structures as aircraft it is possible to provide some

additional defense against SCC by incorporating an

inhibitor such as a chromate in the primer coat. One

such formulation is a polymer system containing

from 2 to 10 percent magnesium chromate, which is sufficiently soluble in water permeating the overlying paint coating to inhibit corrosion reactions on the

aluminum surface (7). Chromate inhibited primer

coats are called out in military specifications for all aluminum aerospace weapons systems, structures

and parts, even though this measure does not

guarantee freedom from SCC. The primer widely

used by the military (MIL-P-23377) employs 12 percent strontium chromate as the inhibitor, applied to a surface which has been either anodized or given a chromate conversion coating for paint adhesion purposes.

II. Kinetic Factors

In addition to stress intensity and alloy composition and heat treatment, the following factors affect

SCC kinetics to degrees which are of practical importance:

(a) Water concentration (in nonaqueous solutions)

and relative humidity (in the atmosphere)

(b) Chloride concentration

(c) Temperature

(d) Electrochemical potential

Increasing water concentration, whether in

nonaqueous solutions or as relative humidity in the

atmosphere, increases SCC kinetics appreciably. So

does increasing chloride concentration and increasing temperature—the plateau velocity in one alloy

was observed to increase about a factor of 10 between the ice point and the boiling point of water.

The effect of electrochemical potential is such that

cathodic protection methods are important measures where feasible. Coupling a susceptible alloy to zinc

in salt water, for example, greatly delays initiation of

stress corrosion cracks and decreases the crack velocity above Kig^c- Cathodic protection also greatly

reduces the tendency to form corrosion pits which

can stimulate the initiation of cracking. As a corollary to this cathodic effect, cladding a susceptible

alloy with a sacrificial coating (which can confer

cathodic protection to an area of the underlying alloy

laid bare by a deep scratch) can shift the alloy from

IV. An Example of Applying the Principles

to Avoid the Problem

The world's most capacious research submersible

is the Aluminaut (fig. 9). To meet the strength re- quirements for preventing collapse, the alloy 7079-

T6 was selected. This alloy is highly susceptible to SCC, in the short transverse direction, but the vehicle saw lengthy satisfactory service in a hostile environment. Let us see how this was done.

First, the problem of possible SCC was not

avoided by ignoring it until the structure was designed and fabricated: The control measures started

in the conceptual design state. The design called for a cylindrical hull with two hemispherical end clo- sures. The cylinder was formed by bolting together

rings which had been ring-forged in such a manner

that the most vulnerable end-grain area was folded

into the interior of the vehicle where it would experi- ence only atmospheric moisture rather than salt water (reducing both the water and chloride kinetic

factors). After being fabricated, the vehicle was

painted with six coats of polyurethane based paint;

each coat was of a different color from the preceding

in order to ensure full coverage by each coat, and

also in order to enable one to determine, in case of

scratch damage, how deep the scratch entended into

the paint system. Rather than attempt to clad the

ring forgings, the electrochemical equivalent was

provided by attaching galvanic (sacrificial) anodes of

a special aluminum alloy to the hull exterior, which

would minimize corrosion while the hull was in the

32
Aluminum Alloys

Figure 9. The deep submersible ALUMINAUT resting on the ocean floor near Himini. This vehicle, though made of a highly susceptible

alloy, perfortned successfully because of meticulous attention to redundant measures to avoid sec. Photo courtesy of Reynolds International, Inc.

sea. Finally, when the submersible was hauled out

after each cruise, it was washed down with fresh

water, minimizing chloride concentration on the hull

exterior between cruises.

One might be tempted to conclude that since the

working stresses in a submerged submersible are compressive, and since SCC cannot occur in compression, all these SCC control measures for

Aluminaut were superfluous. But most SCC is caused by fabrication stresses, not design stresses in

aluminum structures, and the fabrication stresses

could be high enough to cause the resolved surface

stresses to be tensile even during a dive. The actual

fact is that an entire class of foreign (steel) submarines had to be redesigned because of the appear- ance of SCC in the huUs because of fabrication

stresses, as noted in the chapter on high strength

steels.

In summary, design constraints for a submarine

hull forced the selection of a highly susceptible alloy,

and the stresses as fabricated could not be predicted

with high confidence. But by controlling kinetic factors (keeping the end-grain out of the sea water,

cathodicaUy protecting the hull exterior when in the

sea, removal of chloride by washing down after each

cruise), the susceptible alloy performed satisfactorily.

V. Nonaqueous Environments

Liquid metal cracking (liquid metal embrittlement). Mercury, gallium, indium, sodium, tin and

zinc in the molten state cause cracking of aluminum

alloys. Mercury is the most common cause of liquid

metal cracking of aluminum alloys in service, and it causes cracking at exceedingly low stresses. In addition to mercury, its compounds can also cause cracking. Mercury-bearing antifouling p£iints are still used

occasionally on boat hulls, and although a vinyl barrier coat six mils (0.15 mm) thick is sometimes

specified between the aluminum and the antifouling

paint to isolate (hopefully) the mercury compounds

from the metal, in point of fact this barrier is not

reliable. No waivers should be permitted from the

rule that mercury and its compounds should not be

allowed in proximity to aluminum alloys.

Other nonaqueous environments. Some alloys have

been found to crack in nitric acid, inhibited red

fuming nitric acid, nitrogen tetroxide, methanol, and

carbon tetrachloride, (ref. [8])

VI. Identifying SCC

Essentially all SCC service failures are in

aluminum alloys intergranular, sometimes with

patches of plasticity interspersed on a microscopic

scale, but sometimes devoid of apparent plasticity.

By searching on a metalographic plane normal to the

principal fracture plane one can usually find crack

branching, which may vary in degrees from incipient

to extended (fig. 10). A "mud crack" pattern (fig. 11) can usually be seen on fracture surfaces caused by

SCC in salt water, but electron fractography is re- quired to see the pattern in most cases. The pattern

is often missing in SCC due to atmospheric moisture, so that its absence does not rule out SCC. The

pattern may also be seen at the bottom of corrosion

pits, and hence its presence is not absolute proof of

SCC. The material in which this pattern forms

appears to be a noncrystalhne oxide or hydroxide which may be removed by repeated repUcation

of the surface (often practiced to clean a fracture

for study of the final rephca in the electron microscope); therefore an early replica should be ex- amined for this diagnostic evidence. Liquid metal

cracking is also usually intergranular, but can be

distinguished from aqueous SCC by identifying the

residual Uquid metal, supplemented by noting the

Figure 10. Intergranular stress corrosion cracks in alloy 5456,

showing extensive branching. Optical micrograph of polished section, magnified 7X. Photo courtesy of Dr. C. T. Fujii.

33
Aluminum Alloys

Figure 11. "Mud Crack" pattern on surface of stress corrosion crack in alloy 5456—H321, possibly sensitized in service. The features

of the mud crack pattern are unrelated to features of the underlying metal. Scanning

electron micrograph, magnification 1500X. Photo courtesy of Dr. C. T. Fujii. (From ref.

[9])

absence of the mud-crack pattern. Purely mechanical fractures in aluminum alloys, particularly of the

S-L orientation may also be largely intergranular in

the alloys of high strength, usually lacking both

crack branching and mud-crack pattern.

Although all service failures due to SCC are re- ported to be intergranular, laboratory failures of

7000-series alloys in the T73 temper have been re- ported to be transgranular. These latter require long

times at high stress levels compared to the T6

temper.

VII. Theory

The mechanism of aqueous SCC in aluminum al- loys is considered by some authorities to be one of

highly localized removal of metal by corrosion at the

crack tip. The remainder of the crack walls may be

passive, but the crack tip is kept chemically reactive

by localized straining, abetted if the environment is aqueous chloride solution by acidity in the solution

immediately adjacent to the crack tip. Alternatively

the cracking has been postualted as due to a lowering of the surface energy by an adsorbed layer of

hydrogen ions, which is more plausible for cracking

in air of only moderate humidity.

VIII. Summary

The most prevalent form of SCC service failures

in aluminum alloys is caused by a combination of

water, aqueous solutions or atmospheric moisture,

alloy of susceptible composition and structure, and

sustained tensile stresses, most often caused by heat

treatment or assembly.

The principal practical measures to control SCC

are the following:

(1) Select an alloy of minimum susceptibility (tables 3 and 4) permitted by other constraints.

(2) For wrought alloy, take precautions not to impose high sustained stresses across the short trans- verse grain direction unless the alloy and temper

confer low susceptibility. Remember that stresses

other than design stresses are those which cause most service failures by SCC in aluminum. Avoid

machining after heat treatment if feasible.

(3) Keep water and water vapor away from the

metal surface by providing natural drainage and by

painting, with an inhibitor such as chromate incorporated in at least the primer coat.

(4) Minimize opportunities for chlorides to concentrate.

(5) When using a susceptible alloy and temper, if geometry is favorable, compressively stress the

metal surface by peening or rolUng followed by painting.

(6) Use cathodic protection if feasible.

(7) Do not permit mercury or its compounds in

close proximity to any aluminum structure, regardless of alloy or temper, and regardless of protective

coatings.

IX. References

[1] Metals Handbook, 8th ed.. Am. Soc. for Metals, Metals

Park, OH (1961). See also latest annual Aluminum Standards and Data, Aluminum Association, New York.

[2] 1973 Annual Book of ASTM Standards, Am. Soc. Testing &

Materials, Philadelphia, PA, (1973) 975-6. part 31.

[3] Sprowls, D. O. and Brown, R. H., Metal. Prog. 81 No. 4. 79

(1962); 81, No. 5, 77 (1962).

[4] MIL-HANDBOOK-5.

[5] Sprowls, D. O., Shumaker, M. B. and Walsh, J. D., Evaluation of stress-corrosion cracking susceptibility using frac- ture mechanics techniques, Marshall Space Flight Center

Contract No. NAS 8-21487, Final Report, Part I, May 31, 1973.

[6] Spuhler. E. H. and Burton. C. L., Avoiding stress corrosion

cracking in high strength aluminum alloy structures, ALCOA Green Letter, Aluminum Co. of America, New

Kensington, PA, April 1970.

[7] U. S. Patent 3.730,937, May 1, 1973 (Wilburn A. Boggs and

Robert N. Miller).

[8] Hyatt, M. V. and Speidel, M. O., Stress-corrosion cracking

in high strength steels and in titanium and aluminum al- loys, (Ed. by B. F. Brown), p. 147, Naval Research Laboratory, Washington, D. C, 1972 p. 147.

[9] Fujii, C. T., Beachem, C. D., Meyn, D. A. and Brown, B. F., NRL Memorandum Report 2422, Naval Research Lab.,

Washington, D. C, April 1972.

[10] Franklin, D. B., Design criteria for controlling stress corrosion cracking, Marshall Space Fhght Center Document

10M33107(B), August 20, 1975.

34
CHAPTER 5

TITANIUM ALLOYS

Unalloyed titanium at room temperature has a close-packed hexagonal crystal structure designated

alpha. The incidental presence of small amounts

of iron, manganese and sihcon or the addition of

certain alloying elements, particularly vanadium,

molybdenum, nickel or chromium, causes the appearance of grains of body-centered crystal structure designated beta. Alpha alloys have been

developed for sheet material that could be heli-arc

welded and retain sufficient ductility in the weld

zone to allow some forming without complicated heat

treatments. The other alloys which have become

technologically important have been those containing both alpha and beta phases. Alloys of pure beta

phase have been produced but have not yet found

appreciable technological application. The physical

metallurgy of titanium is discussed in detail in refer- ence [1].

Titanium and its alloys have a good record of

freedom from the SCC problem in service, but not a perfect one. The reasons for a good record include

(1) the low stresses which were characteristic of

many of the early applications, (2) the predominance

of unalloyed titanium, resistant to SCC in many en- vironments, in the early history of titanium technology, (3) the fact that many applications were in

technologically advanced areas involving personnel aware of published accounts of earlier failures,

and (4) the fact that some of the combinations of

environment and alloy which might have produced

widespread SCC in service were identified in the

laboratory before they could occur in the field. Consequently there have not been enough service fail- ures nor enough different alloys used to establish a

failure pattern, except that design stresses rather

than stresses caused by heat treatment or assembly

have been responsible for some of the better known

service failures. The SCC characteristics of titanium

alloys will be presented according to the more important environments which have caused cracking,

whether the cracking has been observed in service

or only in the laboratory. Note that some of the

environments which have been found capable of

causing SCC in laboratory tests have not been re- ported incompatible with titanium alloys in service,

(fig. 1)

One of the striking features about the environments which can cause SCC in titanium alloys is the

great fundamental diversity of their chemical nature.

Not all investigators are agreed that each of many

seemingly different environments is fundamentally

different as far as the cracking mechanism is concerned. For example, some investigators postulate

that the fundamental cracking agent in hydrogen

gas, water, methanol, hot salt, and carbon tet- rachloride is hydrogen, provided in the last two

Figure 1. The research submersible ALVIN, which is equipped

with a pressure hull made of Ti-6%Al-2%Cb-l %Ta-0.8%Mo and

with 15 buoyancy spheres made of low oxygen Ti-6%Al-A%V. It is possible to crack both these alloys in the laboratory at highK levels, but the design and

fabrication of the full and buoyancy spheres were such that the vehicle has performed

satisfactorily in the sea.

named substances by traces of moisture, so it is supposed. Such investigators might then tend to think of a number of seemingly different environments under a single heading "hydrogen-bearing en- vironments." Perhaps this approach will eventually

prove to be valid, and other groupings such as "chloride containing environments" may also prove

to be valid and useful. But at the present it seems

wiser to discuss SCC behavior largely according to individual nominal environments until we are surer about possible roles of trace impurities in both environment and metal.

I. Liquid Environments (Excluding Metals)

Water (sea water, tap water, distilled water, and

nearly neutral aqueous solutions, but apparently not

atmospheric water vapor.) All these environments

cause SCC in many titanium alloys in the presence

of pre-existing crack-like flaws, but susceptibility of

these alloys is not detected by smooth specimens

because these alloys do not undergo pitting attack,

the precursor to SCC in the absence of a pre-crack

in many systems. Therefore the significant characterization data are of the fracture mechanics type,

and a genuine threshold K,scc appears to exist. The

resistance of alloys to SCC in 3.5 percent sodium

chloride solution, which may be indicative of their

resistance to SCC in many neutral or near-neutral

aqueous environments, has been found to vary

enormously from alloy to alloy even at a given

strength level, as may be seen in figure 7 of chapter

2.

35
Titanium Alloys

Systematic study of data of the sort presented in

that figure has shown that high aluminum, high oxygen, and high tin contents in aluminum-bearing

titanium alloys are undesirable from the standpoint

of sec because each of these elements promotes the

formation of ordered domains of Ti3Al or Ti3(Al +

Sn) in the alpha phase, and the appearance of this

compound correlates with decreased resistance to sec. The upper limit for aluminum in order to have

a reasonable resistant alloy is about 6 percent, which

fortunately permits (albeit barely) the workhorse

alloy Ti-6%A1-4%V to he in what is considered ac- ceptable compositional territory. Even so, it is desirable to restrict oxygen to 1300 ppm, and even less is desirable. Iron causes a phase designated omega to precipitate in any beta phase grains which may be

present in the microstructure, making the beta

grains susceptible to SCC.

Ti-50A is a grade of unalloyed titanium low

enough in oxygen to be essentially immune to aque- ous SCC. Ti-75A and Ti-IOOA have higher oxygen

and iron content, so that some beta phase is present,

and depending on processing history they may have

enough omega phase precipitation in beta regions to make the alloys moderately susceptible to aqueous

SCC.

Ti-50A and low oxygen Ti-6%A1-4%V have sufficiently high resistance to aqueous SCC that these

alloys offer the engineering solution to avoiding the

problem. Note however that the Ti-6%A1-4%V alloy

may have widely varying properties depending upon

actual composition and processing, as may be seen

in figure 2, and until stricter specifications are developed and put into effect, it may be necessary to test the alloy lot by lot for critical applications such

as fracture-critical aircraft components. As other

alloys are developed, they can be readily characterized using one of the fracture mechanics type

tests.

160

STRESS CORROSION CRACKING RESIS140

TANCE IN SALT WATER

120

"to

100

80

'oo

CO M 60 ~ ^^^^^^^

—N^ 40

20

o.oiHiii;

0

: , 1 , 1 , 1 , I

I 1 1 1 1 1 I 1 I I 100 no 120 130 140 150

YIELD STRENGTH (ksi)

Figure 2. K,scc characteristics of various lots of Ti-6% Al-4%V at various strengths levels, in salt water. Oval encloses all known K„„ data for

this alloy in salt water.

Methanol. This Uquid was first found to cause SCC in titanium alloys when it was used as a supposedly inert pressurizing fluid to leak-test a fuel

tankage and associated system made of Ti-6%A1-

4%V. Extensive SCC of the tankage occurred during

the pressure hold. (fig. 3) Laboratory experiments

then confirmed that many titanium alloys, including

the most important commercial ones, are susceptible to cracking in both commercial and high purity

methanol, and without the need of a pre-existing

crack-like flaw. Unalloyed 50-A titanium cracks intergranularly even without stress. Laboratory tests also demonstrated that 1-5 percent of water added to the methanol inhibits the initiation of a crack from a smooth surface, but not if the solution is contaminated with about 10 ppm of chloride. Even in the

absence of chloride, water does not inhibit SCC

growth from a preexisting crack. Also, methanol at- tacks the grain boundaries of even unstressed

titanium alloys. The practical conclusion from these

laboratory observations is simply that one does not

expose titanium structures to methanol.

It is to be noted that methanol is a constituent of

many industrial substances such as marking fluids,

de-icing fluids, and paint removers. One would be ill advised to allow any of these substances to contact

stressed critical components made of titanium without direct experimental evidence of compatibility,

using precracked specimens. See figure 4 for an example of a failure which may have been due to such fluids. Ethanol and ethylene glycol have also

been observed to crack titanium alloys.

Hot Salt. On one occasion when some weldments

were heated for stress relief, they were found to experience SCC during the annealing operation in a

largely inert gas atmosphere. The cause of the

phenomenon, which is rarely encountered in practice, was traced to sodium chloride, and it was found

to occur only if the temperature exceeds about 275

°C (525 T) and only if water vapor and apparently

oxygen are present. The sodium chloride responsible for some instances of hot salt cracking has been

traced to fingerprints. Practical control measures

during annealing include chloride removal from and

white glove handhng of parts before annealing,

perhaps supplemented by the use of furnace atmospheres which are dry and nonoxidizing, such as dry

argon.

For parts which may experience both chloride

contamination and elevated service temperatures, a

barrier in the form of commercial nickel paint re- ported to be adherent at 540 °C (1000 °F) has been

suggested (ref. [2]).

All commercial titanium alloys are susceptible to hot salt cracking in laboratory tests, alpha alloys

being more susceptible than alpha-beta alloys. Unalloyed titanium appears not to be susceptible.

Somewhat surprisingly, titanium alloys have been

used for compressor blades in aircraft gas turbines

operating at temperatures which exceed the

minimum for hot salt cracking without a reported

instance of service failure, possible due to low

operating stress. MIL-STD 1568 notes that "All ap36
Titanium Alloys

Figure 4. Rocket motor case made ofTi-6%Al-4%V which failed

during hydrostatic testing, possibly due to marking fluid. Photo

courtesy of Mr, Carl Hartbower.

plications of titanium above 600 °F (315 °C) should

include considerations of the hot salt cracking

phenomenon."

Organic fluids. In addition to methanol, the fol- lowing organic compounds have been reported

to cause SCC in one or more titanium alloys:

Carbon tetrachloride, chlorinated diphenyl (at 315

°C, 600 °F), ethanol, monomethyl hydrazine, tri- chlorofluoromethane, trichlorotrifluoroethane, and

trichloroethane. None of these fluids poses as seri- ous a threat as does methanol. Although laboratory

tests indicate that Ti-6%A1-4%V is susceptible to cracking in monomethyl hydrazine, the susceptibility is sufficiently minor (according to NASA's

Johnson Space Center) that there has been a perfect

service performance record with this combination of

alloy and environment.

Some specifications for aircraft prohibit the use of

chlorinated hydrocarbons to degrease titanium components for fear of residues, particularly chlorides

and acids, which might give problems in service.

MIL-STD 1568 cautions that care be taken to ensure

that cleaning fluids and other chemicals used on

titanium alloys are not detrimental to them. Substances known to be capable of producing SCC include "hydrochloric acid, trichloroethylene, carbon

tetrachloride, chlorinated cutting oils, freons,

methyl alcohol, and all chlorides." British practice

reportedly permits the use of chlorinated hydrocarbons, including trichloroethylene, providing they

are inhibited by the addition of deactivating agents

such as butylene and acid absorbers such as triethylamine.

In the chemical industry some cracking has been

observed in titanium processing equipment attri- buted to reducing organic acids at elevated temperatures. Iron contamination of the surface such as by

wire brushing or by using steel forming tools plays a causative role, with high iron content of the alloy

perhaps playing a contributive role. Preventive

measures include avoiding contamination of the sur- face by iron, or, failing this, removing the iron before

heat, treatment or welding operations, which cause

the icon to diffuse into the titanium. A preventive

measure routinely practiced by one large chemical

industry consists of anodizing equipment after final

assembly. The anodizing removes any iron from the

surface and also generates a thick oxide which better

insulates the metal from the chemical environment

than does the naturally formed oxide, (ref. [3])

Nitrogen tetroxide (N^4). This hquid is an

oxidizer widely used in aerospace propulsion units,

and it had been contained successfully in pressure

vessels made of Ti-6%A1^%V until one such

pressure vessel shattered (fig. 5). Investigation ulti- mately disclosed that the N2O4 which caused cracking was purer than had been previously supplied, in

37
Titanium Alloys

Figure 5. Pressure vessel made of Ti-6%Al-4%V which cracked

because of N2 O4. Photo courtesy of Dr. L. J. Korb, Rockwell International Space Division.

particular being low in nitric oxide (NO) (a "no-NO"

situation, as it were.) The problem was brought

under control when the SCC inhibitive property of

the NO was identified and a specification was put

into effect (NASA PPD-2B) which calls for 0.60 -1

percent NO. The reason for the inhibitive action of

NO is unknown.

Red fuming nitric acid. This liquid has also been

widely used as an oxidizer in aerospace propulsion

units. It was found to cause SCC in, and pyrophoric

reaction with, titanium until the addition of water was

found to inhibit the attack. A military specification

was then written caUing for up to 14 percent NO2 and

2.5 percent H2O, balance HNO3.

Gases. Moist chlorine gas at temperatures of 290

°C (550 °F) and higher has been reported to crack

various titanium alloys. Hydrogen chloride has

caused cracking of the alloy Ti-8%Al-l%Mo-l%V.

Hydrogen gas causes slow crack growth of preexisting cracks in stressed titanium alloys but is of

little hazard to relatively unstressed titanium at room

temperature providing the protective oxide film is in

good condition and gross flaws are absent. Water

vapor does not appear to cause slow crack growth even in Ti-8%Al-l%Mo-l%V, which is very sensitive to aqueous SCC.

II. Solid and Liquid Metals

Mercury, gallium and molten cadmium cause

rapid cracking (liquid metal embrittlement) of

stressed titanium alloys. Mercury from a biocide in an elastromeric ring has been suspected to cause

cracking of some highly stressed aircraft parts.

Experience with assemblies which included steel

bolts plated with silver in contact with any of several

commercial titanium alloys demonstrated that above

343 °C (650 °F) silver caused the titanium alloys to crack, although far below the melting point of sliver.

Similar service experienced with cadmium plated

fasteners in intimate contact with various commercial titanium alloys at temperatures as low as 38 °C

(100 °F) indicates that solid cadmium can cause

cracking. Experiments at 150 °C (300 °F) showed that

cracks so formed are self-limiting in depth at about 1 mm (0.05 in). Cracks of such depth might not be of

much concern unless conditions should be such that

they might initiate fatigue or fast (brittle) fracture. It should be noted that this solid cadmium cracking of

titanium is not related to the cracking problem

caused by electroplating cadmium on high strength

steels.

As a consequence of the experience cited above

and other evidence, MIL-STD 1568 prohibits cadmium plating of titanium parts and their use in direct

contact with other cadmium plated parts or tools.

This Standard also calls for avoiding silver brazing of

titanium parts and avoiding silver plated fasteners

for elevated temperature service. MIL-S-5002C

prohibits silver plating on titanium or in contact with

titanium.

III. Sustained Load Cracking in Inert

Environments

Sustained load cracking (SLC) in inert environments (including air) is a type of slow crack growth of

practical importance to highly stressed critical components for contemporary military aircraft and possibly to other applications of advanced titanium

technology. It is similar to SCC except that it is much slower and occurs in the total absence of a reactive environment. SLC is caused by, or is enor- mously exacerbated by, hydrogen dissolved in the

titanium during processing, and the present military

specification of 125 ppm maximum hydrogen for

most titanium alloys is far too generous to prevent

this mode of cracking under high sustained stresses. Weldments are particularly vulnerable to SLC partly

due to the additional hydrogen which tends to be

added during the welding process and partly due to the residual stresses left in a weldment unless it can

be stress relieved. Vacuum annealing can reduce the

hydrogen level to less than 10 ppm, at which concentrations the threat of SLC is largely removed (ref.

[4]). Even as high as 125 ppm hydrogen appears not

to have caused SLC in the alloys and designs (including stress levels) common in the chemiceil industry.

38
Titanium Alloys

IV. Diagnostic Criteria

The purely mechanical overload fracture surfaces

of commercial titanium alloys are composed of dimples (fig. 6). The SCC fracture paths may be cleavage (fig. 7) in alpha alloys, or in beta alloys, or in the

alpha phase of alpha-beta alloys; in the latter case,

the beta phase may part by a microvoid coalescence

process. Cleavage SCC fractures may or may not

exhibit branching. SCC in these alloys may also be

intergranular.

Whether the crack path is cleavage or intergranular, or whether it is branched or not, does not appear

to be diagnostic of just what sort of environment

caused the cracking. These choices of crack path

appear rather to be related to the region in the stress

intensity (/C)-cracking rate curve in which the cracking was produced, in a manner as shown in figure 8.

Figure 6. Dimples ofvariegated size produced by purely mechanical "plane strain" fracturing of Ti-8%Al-l%Mo-l%V. Electron

fractograph, magnification 6000X. Courtesy of Mr. D. A. Meyn.

Figure 7. Intergranular cleavage-like SCC fracture of Ti-8%All%Mo-l%V in salt water. Note local plasticity toward right side. Electron

fractograph, magnification 6000X. Courtesy of Mr. D. A. Meyn.

CRACK-TIP STRESS INTENSITY

Figure 8. Appearance ofstress corrosion cracks in titanium alloys

is generally relatable to cracking kinetic behavior: Region I crack- ing tends to be intergranular, region II is clevage-like plus dimples

which increase in size and proportion with increasing crack tip

stress intensity, becoming entirely dimples in region III ( approaching Kjc conditions). See Reference [1] for details.

In region I the cracks are intergranular, with some

cleavage which increases in proportion as theK level

rises toward the plateau, and there may also be

incipient branching on a microscopic scale. On the

plateau the fractures are cleavage plus dimples (microvoids) which increase in proportion as the K level

rises; macro-branching may appear in the plateau

region. The foregoing is to be taken as a generalization from which there may be exceptions, but it holds for many combinations of alloys and environments, including hquid metals such as mercury. One

known exception is carbon tetrachloride, for which

no intergranular SCC has been reported even though

clearcut region I behavior is exhibited.

Although many SCC fractures are cleavage

(transgranular), some low-amplitude fatigue fractures and sustained-load fractures may be so similar

in appearance that these three fracture modes may

not be distinguishable by even skillful electron fractography.

There is one environment, gaseous hydrogen,

which produces what appears to be a fracture topography differing enough from other fracture mode

topography to be diagnostic, and that topography is terraced cleavage, as shown in figure 9.

39
Titanium Alloys

Figure 9. Terraced Cleavage in Ti-8%Al-l%Mo-l%V cracked in gaseous hydrogen. Electron fractograph, magnification 3000X.

Courtesy of Mr. D. A. Meyn.

V. Theory

One would not expect to find a single mechanism

to account for SCC in titanium alloys in all the di- verse environments in which it is observed. One

mechanism which has been proposed for several

environments, however, involves hydrogen which is envisioned as sorbing at crack growth sites and re- ducing the energy for separation. It is known that

hydrogen in the alloy, inherited from processing, can

cause sustained load cracking in titanium alloys, and

it is entirely credible that during the SCC process in

titanium alloys in sea water, distilled water, and

methanol, hydrogen can be reduced from the environment, enter the metal, and cause cracking by the

same ultimate mechanism as dissolved hydrogen or perhaps also gaseous hydrogen.

Titanium when stressed in salt water is ordinarily

kept passive by reaction with the water to form a

tight adherent layer of Ti02, which insulates the

underlying metal from the environment and stifles

further attack. But inside a crack the reaction of

titanium with water to produce Ti02 (Ti -I- 2H2O =

Ti02 + 4H"'" -I- 4e) also produces excess hydrogen ions

which cause local acidity. This acidity builds up to the point that Ti02 is not stable. Therefore the crack

tip can not go passive but continues to react to produce hydrogen, some of which enters the metal

and the rest of which is available to maintain the

local acidity.

With methanol, titanium reacts to form an organic

compound which is soluble in methanol and which

therefore does not form a protective film; the reaction also produces hydrogen which can enter the

titanium metal to participate in the cracking process. In methanol no generation of acid is therefore

required to prevent titanium from passivating—the

solubility of the metallic corrosion product obviates

that need. Hence, in methanol the crack can initiate

from a smooth surface.

Hot salt cracking appears to occur as follows:

sodium choloride produces hydrochloric acid locally

by reactions involving pyrohydrolysis and the acid

penetrates the otherwise protective oxide film. The

acid then attacks the underlying metal, producing

hydrogen. The hydrogen then enters the metal and

assists the sustained stress to produce a fracture

which is typically intergranular near the surface and

partially cleavage away from the surface. The hydrogen is viewed as reducing the energy for fracture

by sorbing, as noted above.

The mechanism of cracking in N2O4 may require

the periodic rupture of the oxide at the crack

tip, followed by the oxidation of the bared metal,

then repetition of the cycle, somewhat Uke the

mechanism postulate for brass in tarnishing solutions.

VI. Summary

The practical SCC hazards to titanium involve a wide range of environments.

For some groups of environments, such as natural

waters and nearly neutral aqueous solutions, the

cracking occurs only in the presence of a preexisting crack-like flaw, and fracture mechanics

type tests readily characterize the SCC behavior of

alloys. Unalloyed titanium and the widely used Ti6%A1-4%V alloy are reasonably resistant to SCC in

these environments. The alloys Ti-8%Mn, Ti4%Al-l%Mn, Ti-8%Al-l%Mo-l%V, Ti-7 or 8%A1-

2%Cb-l%Ta, Ti-5%Al-5%Zr-5%Sn, Ti-5%A1-1.4%

Cr-1.5%Fe-1.2%Mo, Ti-5%Al-2.27%Cr-1.25% Fe,

Ti-5%Al-2 l/2%Sn, Ti-13%V-ll%Cr-3%Al, TilOOA and Ti-5%A1^% (Fe-I-Cr) should be avoided

unless specific SCC tests using precracked specimens demonstrate compatibility with the service en- vironment. Most of these alloys are no longer in

production.

For two chemical oxidizers, N2O4 and red fuming

nitric acid, practical control of the problem is available through controlUng the composition of the

oxidizers, for which standard specifications are

available.

For the hot salt cracking problem during heat

treatment, meticulous cleanliness of the titanium,

backed up preferably by excluding moisture and

oxygen from the heat treating atmosphere, serves to control the problem. Though untried in practice, a nickel barrier paint may be useful when contamination at elevated temperatures cannot be avoided in

service; the alternative is to design for low operating

stresses, but these have not been quantified.

For some environments, such as methanol, mercury, cadmium or silver, the necessary strategy is simply to exclude them from titanium surfaces.

VII. References

[1] Blackburn, M. J., Smyrl, W. H. and Feeney, J. A., Stress

corrosion cracking in high strength steels and in titanium

and aluminum alloys. (B. F. Brown, ed.). Naval Research

Lab., Washington, D. C, 1972, p. 245.

[2] Feige, N. G. and Covington, L. C, STP 518 (H. Lee Craig,

Jr.. Ed.), Am. Soc. for Testing and Mat., Philadelphia,

PA, (1972), p. 119.

40
Titanium Alloys

[3] Cotton, J. B., The science, technology and application of titanium, (Robert I. Jaffee and N. E. Promisel, Eds.),

(Pergamon, London, 1970), p. 156.

[4] Meyn, D. A., Effect of hydrogen on fracture and inert- environment sustained load cracking resistance of alpha- beta titanium alloys. Metallurgical Transactions, Vol. 5, No. 11, p. 2405 (1974).

41

CHAPTER 6

HIGH STRENGTH STEELS

The term "high strength steel" as used in this

work refers to a steel having a yield strength of 130

ksi (900 MNm~2) or greater, whether the strength is developed by heat treatment of any sort or by cold

working, and whether the steel is stainless or not. Two cases involving lower strength steel are included because generically they belong to the high

strength family and they illustrate important points.

The factors involved in most service SCC are the

following:

(a) Water, aqueous solutions, atmospheric water

vapor, H2S and H2

(b) Sustained tensile stress

(c) Degree of susceptibility

(d) Electrochemical potential

For the most part, the environments which are of

major concern to the cracking of high strength steels are natural waters—rain water, sea water, and at- mospheric moisture. Any of these may of course be

contaminated to produce environments significantly

more likely to cause SCC than the uncontaminated

ones. Contamination with H2S is especially serious,

and in fact the presence of H2S in high concentrations in salt water associated with certain deep oil wells (termed "sour crude" wells) places an upper

Hmit (roughly 90 ksi, 600 MNm-^) on the yield

strength which can be tolerated in stressed steel in

such environments without causing cracking. This

experience, together with experience with sour gas

condensate wells, has given rise to a "sour" test solution of water plus 0.5 percent acetic acid, the

whole periodically saturated with H2S; this solution

may additionally contain either 3.5 or 6 percent

NaCl. Such a solution may constitute something of a worst case for high strength steels, but most present

day investigators consider the sour test solution as being unrealistically agressive to give useful ranking

of the various steels for service not involving H2S.

Those investigators who are engaged in comparing

the SCC properties of various steels usually employ

water containing about 3.5 percent NaCl, artificial sea water, the marine atmosphere, or (rarely) natural sea water.

For many small components such as bolts and

springs made of high strength heat treated steels for

stresses causing service SCC failures are the design

stresses, but in large forgings the heat treating stresses are the most important cause of failures. In other

cases, assembly stresses can be responsible, for

example in certain rocket motor cases made of hard

rolled AISI 301 stainless steel sheets assembled by

spot welding; residual stresses associated with these

spot welds have caused some service failures. Although the term "sustained" stress is almost inevitably used as one of the necessary conditions for

SCC, note that repetitive loads between two levels in

tension, or from tension to zero stress, can constitute

the necessary and sufficient stress conditions for

SCC, even though it may be "cyclic SCC," and such

failures have been noted in high strength steel fasteners in vibrating equipment.

By way of introduction let us briefly review the

physical metallurgy and nomenclature of the various

classes of high strength steels. The martensitic

steels, also known as quenched and tempered or Q

& T steels, have been known since antiquity, but the

technology of present day low alloy martensitic

steels is a 20th century development, and current

premium melting technologies are space age developments. Some of the martensitic steels, typified

by AISI type 410, contain enough chromium to qualify as stainless steels. The martensitic steels are heat treated by being first heated into the range of

very roughly 840 - 1000 °C (1550 - 1830 °F), at which

temperature the crystal structure is face centered

cubic and is designated austenitic. The carbon dissolves in this austenite, which upon being quenched

or otherwise sufficiently rapidly cooled, transforms

to a metastable body-centered tetragonal structure

designated martensite. The maximum hardness and

strength which this martensite can possess is determined largely by the amount of carbon originally

dissolved in the austenite, but in commercial steels

this maximum strength is usually accompanied by

brittleness too severe to be tolerated, so that the

strength of the as-quenched martensite is sacrificed

in a tempering operation to improve the toughness to an acceptable level.

The precipitation hardening steels are also hardened by heat treatment, in this case consisting of

solution treatment at high temperature followed by

rapid cooling which may or may not form martensite,

depending on composition. Even if martensite is formed, it is not hard like the martensite of AISI

4340 steel, for example, and the strengthening of the

alloy is achieved by subsequent precipitation (or aging) reactions at elevated temperature. Underaging

at lower temperatures and overaging at higher temperature produce less than the maximum strength

with compensating changes in brittleness and SCC

characteristics. Most of the precipitation hardening

steels are stainless.

The maraging steels—the name represents an

abbreviated combination of "martensitic" and

"aging"—constitute the newest class of heat treated

high strength steels. After being solution treated

(annealed) at high temperature (815 °C, 1500 °F) and

cooled, they are martensitic, but the martensite contains extremely low carbon and is of low strength

until after being aged (480 °C, 900 °F). One of the

43
High Strength Steels

interesting possibilities of the maraging steels is the

ability to make very large structures which require

only the low temperature aging treatment to achieve

very high strength. Annealed plates are welded, and

the as-welded weld areas are in the annealed condition, ready for both prime plate and weld areas to be

strengthened by the low temperature aging treat- ment. We will see an example of a structure—

a

research submersible—too large to be readily heat

treated if made of a martensitic steel but successfully fabricated of the 18 percent nickel grade maraging steel. Stainless grades of maraging steels have

been developed.

One may distinguish three classes of steels whose

high strength is due to cold work. (1) One is unalloyed carbon steel such as is used in making wire

rope (cable) or piano wire. The steel used extensively

for bridge cable has a yield strength typically 180 ksi

(1250 MNm~^); this steel consists of crystals of ferrite (body centered cubic iron) plus aggregates of

ferrite plus iron carbide (the aggregate termed pearlite), all highly deformed and, incidentially, ferromagnetic. (2) AISI type 301 EFH (extra fuU hard) is a stainless steel having a yield strength which can exceed 230 ksi (1600 MNm~^); the deformation process produces some martensite, and the alloy is weakly magnetic because of this martensite. 301

EFH is used for some rocket cases, e.g., those rockets used for the launching of communication satellites. (3) Finally there is a class of high strength cold

worked steels containing a high percent of manganese (e.g., 18% Mn) but which are fuUy austenitic

and are therefore nonmagnetic. Because of their

high strength (e.g., yield strength 175 ksi, 1200

MNm~^) and nonmagnetic character, they are used

for retainer rings for generator rotors. This class of

steel can be made stainless if desired.

Table 1 below summarizes the various groups of

high strength steels and gives examples of prominent

types in each group.

Table 1. Summary of types of high strength steels

General group Examples of types

Heat treated

Low alloy martensitic AISI 4130, AISI 4330, AISI 4340, 300M

Stainless martensitic AISI 403, AISI 410, AISI 416, AISI 431

Precipitation hardening AM-350, Almar362, 17^PH, PH13-8-

Mo

Maraging steels 18% nickel grades 200, 250, & 300

Cold worked

Unalloyed Piano wire, wire rope (cable, "improved

plow steel")

Austenitic 18% manganese

Stainless AISI 301

As in other alloy families, the Unified Numbering

System for steels consists of a letter followed by five

numbers. The letter may be G for AISI and SAE

carbon and nonstainless alloy steels, H for steels of

controlled response to heat treatment (hardenability), K for miscellaneous alloy and specialty steels

(such as valve steels), S for stainless steels, and T for

tool steels. The five numbers usually incorporate the

numbers of the most commonly known designation.

Thus S41000 is AISI type 410 stainless steel, G43400

is AISI type 4340, and S17400 is 17-4PH. But the

tool steel designated AISI H-11 is T20811. Some of

the important high strength steels such as the maraging steels have not yet been assigned a UNS designation. All the UNS designations for steels have appeared too recently to have been incorporated in the

literature, and for the most part only the older designations wUl be used here.

I. Degree of Susceptibility

There seems to be httle doubt that in salt water

and fresh water there exists a true threshold Kjgcc for

all the high strength steels which have been tested

using fracture mechanics methods, and this appears

to be a useful way to characterize their SCC behavior. The best available summary of SCC characteristics of heat treated high strength steels is given

in reference [1], from which figure 1 is taken. In this

figure the data were taken in salt water or sea water

environments in the freely corroding condition. For

modem clean commercial steels the SCC characteristics show little anisotropy, but even so they are customarily tested in the most vulnerable direction,

S-L. (See ch. 4, figs. 1 and 2 for the meaning of this

designation.) The graphs of figure 1 show Kjscc plotted against yield strength, but rather than individual

points, envelopes are drawn enclosing all known

vaUd data for the various steels. The steel featured

in each individual graph is represented by the en- velope drawn with a heavy line which can be compared with characteristics available in other steels

represented by the Hght envelopes. See chapter 2 for

further explanation of the diagrams of figure 1. From data such as those from which figure 1 was

derived and other studies the following generalizations have been derived. The most important single

factor in determining the SCC characteristics of

steels is strength level. Yet at any given strength

level there are obviously other factors which can combine to make large differences in characteristics. High manganese content, (e.g., 2 percent; used

in part to counter the embrittling effect of sulfur) is detrimental to the SCC characteristics of martensitic steels. High purity in high strength steels, such as one can achieve using premium melting stock

vacuum-induction melted and vacuum arc remelted

(VIM-VAR), is associated with superior SCC properties, possibly attributable to low sulfur and oxides.

(Note for example HP 9^20 and the upper bound

for maraging steel in fig. 1.) In martensitic steels at a given strength level there appears to be an advan44
High Strength Steels

tage to having low carbon contents, e.g., 0.2 percent

or less. There are other elements the presence of

which affects SCC characteristics, but these have

not been identified. One structural feature has been

identified as being very detrimental to SCC properties of martensitic steels, and that is the structure

which the metallographer terms "twinned martensite."

o

o

JCl L»b

CRITICAL FLAW FOR S.C.C.

20 40 60 80 20 40 60 80 20 40

100 200 300

YIELD STRENGTH (ksi)

20 40 60 80 20 40 60 80 20 40

100 200 300

YIELD STRENGTH (ksi)

o

o

V.s.

CRITICAL FLAW FOR S.C.C.

-Lcr - a y.s. L>>b

CRITICAL FLAW FOR S.C.C.

20 40 60 80 20 40 60 80 20 40

100 200 300

YIELD STRENGTH (ksi)

20 40 60 80 20 40 60 80 20 40

100 200 300

YIELD STRENGTH (ksi)

Figure 1. Envelopes for Kiscc data for individual steels (heavy line) compared with similar envelopes for other steels (light line). Oval

envelope at far right is for strain-aged experimental alloy AF-77; its interesting properties may he due to cold work. From Sandoz (ref. (11).

45
High Strength Steels

The high chromium content which confers stainlessness in the ordinary sense does not confer immunity to sec, as the data for 17-4PH steel clearly

show. In point of fact, the data shown for this alloy

may be grossly optimistic, as may similar data taken

on all other stainless high strength steels in salt water. The reason is that the saline test solution is nearly neutral, whereas in sheltered areas, such as

in crevices, the salt water tends to become ex- tremely acid, the pH about 1, and this acid solution

CRITICAL FLAW FOR S.C.C.

± 0= a„

CRITICAL FLAW FOR S.C.C. 10NI-2Cr-1IVlo-8Co

20 40 60 80 20 40 60 80 20 40

100 200 300

YIELD STRENGTH (ksi)

20 40 60 80 20 40 60 80

1 00 200

YIELD STRENGTH (ksi)

20 40

300

240 -

220 -

200 -

180 -

160 -

140 -

120 -

100 -

80 -

60

40

20

0

1 CO

L >>b

CRITICAL FLAW FOR S.C.C.

L»b

CRITICAL FLAW FOR S.C.C.

20 40 60 80 20 40 60 80 20 40

200 300

YIELD STRENGTH (ksi)

20 40 60 80 20 40 60 80 20 40

100 200 300 YIELD STRENGTH (ksi)

Figure 1. (continued)

46
High Strength Steels

can be extremely effective in initiating SCC. The

reason for this acidity is understood by the corrosion

scientist as a consequence of a chemical reaction

termed the hydrolysis of chromium corrosion products. This problem is discussed further under the

topic of cathodic protection.

The acidity within corroding crevices in stainless

CRITICAL FLAW FOR S.C.C.

20 40 60 80 20 40 60 80 20 40

100 200 300 YIELD STRENGTH (ksi)

Figure 1. (continued)

steels mentioned above represents an extreme situation. Variations in bulk pH from about 3 to 10 have

httle effect on cracking behavior. In the few

comparisons which have been made, Kjgcc values

determined using 3.5 percent NaCl solution have not

usually been found different from those determined

using natural sea water.

Figure 1 includes only data on heat treated steels.

The SCC characteristics of cold drawn wire such as

is used for wire rope (cable) and bridges have not

been well established by quantitative methods. AISI

type 301 is one of the few nominally austenitic stainless steels which crack at room temperature in the

atmosphere, but little can be said quantitatively

about its characteristics. (In the usual cold-worked

state, 301 contains much martensite). The K^f.^ of

the austenitic 18 percent manganese steel cold

worked to produce a yield strength of 175 ksi (1200

MNm~^) for generator retaining rings, as determined

in distilled water, was found to be about 7.5 ksi Vm (8

MNm"^'^). Such a low value indicates that a scratch

would suffice to set off SCC at stresses approaching

yield, (ref. [2]) A related steel was used for the hull of

a comatant submarine which will be mentioned

further below.

Note in the graph for HP 9-4-20 steel in figure 1 that the weld metal has a somewhat lower resistance

to SCC than does the prime plate. The limited ex- perience to date suggests that this is usually the case

with heat treated weldments, the weld centerline

being often the most vulnerable location.

II. Control Measures

A primary measure to avoid SCC should be the

selection of a steel with good SCC properties, in

particular selecting one with no higher strength than

is needed. In this regard the practice of specifying

minimum strengths but not maximum strengths is a

faulty one, not only with respect to SCC but with

respect to brittle fracture as well.

The MarshaU Space Fhght Center (MSEC)

guidelines (ref. [7]) permit the use of the steels in

table 1 for space vehicles and associated equipment

without prior approval. It should be understood that

the information in tables 1, 2 and 3 apply only to room temperature service in the atmosphere and do

not pertain to elevated temperatures or to chemical

environments, for which special information must be

obtained.

The steels in table 2 may be used for aerospace

equipment for MSFC only after MSFC approval.

Proposed use of materials in this table for application involving high sustained tensile stress (such as springs and fasteners will not be approved.

The steels in table 3 have been found to be highly

susceptible to SCC and MSFC permits their use only

after it has been demonstrated conclusively that

SCC is not possible for environmental reasons. After a steel and its strength level have been

selected, additional measures to mitigate SCC may

be considered, as below.

47
High Strength Steels

Table 1. High strength steels with high resistance to stress corro- sion cracking (after ref. [71)

Steel Condition

AM350 stainless steel AM355 stainless steel Almar 362 stainless steel Custom 455 stainless steel PH 13-8 Mo stainless steel 15-5 PH stainless steel 17-4 PH stainless steel PH 14-8 Mo stainless steel PH 15-7 Mo stainless steel 17-7 PH stainless steel Carbon steel (1000 Series)

Low alloy steel (4130, 4340,

D6AC, etc.) Music wire (ASTM 228)

SCT 1000 and above

SCT 1000 and above

HIOOO and above

HI000 and above

HIOOO and above

HIOOO and above

HIOOO and above

CH900 and SRH950 and above

CH900

CH900

Below 180 ksi YS (1250 MNm-^)

Below 180 ksi YS (1250 MNm-^)

Cold drawn

Table 2. High strength steels with moderate resistance to stress corrosion cracking (after ref. [71)

Steel Condition

Carbon steel (1000 series) 180 to 200 ksi Y.S.' (1250-1380 MNm-2)

Low alloy steel (4130, 4340, 180 to 200 ksi Y.S. D6AC, etc.) (1250-1380 MNm-2)

PH13-8 Mo stainless steel Below HIOOO

15-5PH stainless steel Below HIOOO

17-4PH stainless steel Below HIOOO

Table 3. High strength steels with low resistance to stress corro- sion cracking (after ref. [7])

Steel Condition

Carbon steel (1000 Series)

Low alloy steel (4130, 4340,

D6AC, etc.)

H-11 steel

440 stainless steel 18 Ni maraging steel, 200 grade

18 Ni maraging steel, 250 grade

18 Ni maraging steel, 300 grade

18 Ni maraging steel, 350 grade

AM 350 stainless steel AM 355 stainless steel Custom 455 stainless steel PH 15-7 Mo stainless steel 17-7PH stainless steel

Above 200 ksi Y.S. (1380

MNr

Above 200 ksi Y.S. (1380

MNm-2)

Above 200 ksi Y.S. (1380

MNr

All Aged at 900

Aged at 900

Aged at 900

Aged at 900

Below SCT 1000

Below SCT 1000

Below HIOOO

All except CH900

All except CH900

'F (480 °C)

'F (480 °C)

F (480 °C)

F (480 °C)

Inhibitors are available which are very effective in

preventing the growth of corrosion pits which are the

common forerunners of SCC in steels in natural

waters, and they should be considered for closed

systems. Sodium chromate was at one time a common inhibitor, but its disposal now presents a problem because of environmental protection regulations. Effective as it was in preventing pitting, it has not been found to be effective in preventing the

propagation of existing cracks. It is possible though

by no means certain that future research may disclose inhibitors which are practical and effective

against crack growth.

Cathodic protection is a measure which, where

feasible, is also very effective in preventing pitting

corrosion in steels. Its use on high strength steels

must be done with care to prevent hydrogen damage

and hydrogen cracking which can be caused by

"overprotection." Protection and "overprotection"

are best discussed in terms of the electrochemical

potential of the steel surface compared to that of a standard reference electrode, such as the saturated

calomel electrode (SCE). The potential of freely cor- roding low alloy steel such as AISI type 4340 in sea water is about -0.6 - 0.7V (SCE) and that of high

alloy steel such as 17-4pH is about —0.4V, with the

potentials of the nonstainless maraging steels in between these potentials. The potential of unalloyed

steel is also about —0.6 - 0.7V, that of cadmium about

—0.75V, and that of zinc is —1.03V. The potentials

of freely corroding aluminum and its alloys in sea water are apt to vary considerably with time as the

protective films break down and re-form. It is common practice to design cathodic protection systems

for ship hulls to produce a potential of —0.85V

(SCE), which prevents both pitting and general rusting. Cathodic protection inadequate to prevent general rusting may still be adequate to prevent pitting.

As an example of this pattern, we see that unalloyed

steel coupled to maraging steel in sea water does not

prevent rusting of the maraging steel but does prevent its pitting. In most cases in which the effect of

cathodic protection on Kjgcc has been measured, Uttle or no effect has been observed between the potential of the freely corroding high strength steel and — 0.85V (SCE). For the case of 18 percent Ni maraging steel, however, cathodic protection increased

Kiscc- But at potentials more negative than —0.85V,

the /C/scc values for 17^pH fall off rapidly because of

hydrogen discharged on the high strength steel.

If one stresses high strength steel somewhat above

Kjscci holds the K level nearly constant, and varies

the potential from the freely corroding potential

toward more negative potentials, one observes decreasing cracking rates as one approaches the potential which is that of normal designs, but at somewhat

more negative potentials (overprotection region)

there is a sharp rise in cracking rates. This behavior

is illustrated by the data in figure 2. In this figure the

ordinate, labeled "dial deflection rate," is a measure

of cracking rate. This experiment was conducted in

3.5 percent NaCl solution, and the potential was

measured not against a calomel electrode but against

a silver-silver chloride electrode, which however in

this solution has essentially the same potential as the

calomel electrode. In this figure the left-most point

corresponds to the freely corroding potential of a given steel, and the points to the the right of this

point represent varying degrees of cathodic protection extending to overprotection. Note that despite

the large differences in the several steels, not only in

composition but in strengthening mechanisms as

well, all show a minimum in cracking rates, and all at substantially the same potential.

Stainless high strength steels in aqueous salt environments pose the special problem of crevice corro48
High Strength Steels

EFFECT OF ELECTROCHEMICAL POLARIZATION

ON CRACKING RATES

0.4 -0.6 -0.8 -1.0 -1.2 POTENTIAL. VOLTS (Ag/AgCI)

-1.4

Figure 2. Effect of varying degrees of cathodic protection on SCC

rate (DIAL DEFLECTION RATE) at nearly constant K. Left-most

point on each curve represents freely corroding potential. Visible hydrogen evolution appears on right-hand branch.

sion and attendant acidity noted above, and cathodic

protection is a very effective measure to mitigate this

form of attack. Without this protection, the acid

solution in crevices (or pits) can cause SCC not

predictable from present fracture mechanics data.

Summarizing the effects of cathodic protection, a moderate degree is effctive in mitigating pitting in all steels and crevice corrosion in stainless steels. This

same moderate degree of cathodic protection has

little detrimental effect on Kjscc and in some steels

may increase K/gcc- Cathodic protection also de- creases crack growth rate above A^/scc- Excessive

protection, vi^hich may be defined as conditions producing potentials more negative than about —0.85V

(SCE), confers no additional benefit, can be seriously detrimental in decreasing /C/scc drastically, and

in the situation of potentials more negative than

about —IV may appreciably accelerate cracking

rates. Organic coatings are not reliable means for preventing SCC in high strength steels, but even poor

coatings improve the ability of a cathodic protection

system to perform its function. Coating with cadmium is beneficial in two ways: It acts as a barrier to corrodents, and even when the steel is bared by

scratches or other damage to the cadmium coat, the

adjacent cadmium tends to protect the nearby steel

without danger of overprotection. (Note the potential

of cadmium in the discussion of Cathodic Protection

above.) Zinc coating is hazardous to heat treated

high strength steel because of the possibility of overprotecting at breaks in the coating. But steels which

are strengthened by cold work are much less susceptible to hydrogen enbrittlement, hence galvanized

cold drawn bridge wires perform well.

III. Examples of Practices and Failures

Illustrating Principles

The rocket motor case illustrated in figure 3 was

made of a martensitic steel designated AMS 6434

and was heat treated to a yield strength slightly over 200 ksi (1400 MNm"^). It was pressure tested with

tap water, and before the specified 15-min hold was

completed, the tank shattered. Examination of the

fracture showed that most of it was purely mechanical brittle fracture, with shear lips bordering the flat fracture whose chevron markings (characteristic of

brittle fracture) pointed back to a thumbnail-shaped

origin (fig. 4). The electron microscope fractographic

Figure 3. Brittle fracture of high strength steel rocket motor case

triggered by stress corrosion cracking. Photo courtesy of the late Mr. J. A.

Kies.

Figure 4. Origin of failure of figure 3, enlarged about , I5X. Thumbnail-shaped area at bottom is stress corrosion cracking.

Brittle fracture chevrons are beginning to develop, radiating from

the thumbnail area. Gray bands at top and bottom are shear lips. Dark areas on brittle fracture are rust. Photo by D. Price.

49
High Strength Steels

features of this thumbnail shaped area (intergranular

fracture) matched those of a stress corrosion crack

generated in the same steel in the laboratory, also

using tap water, and accordingly the origin was con- cluded to have been caused by SCC. Let us look at the problem using fracture mechanics. The Kjscc of

this steel in tap water would have been about 10 ksi

Virr(ll MNm~^'^). In the era of the manufacture of

this motor case, a certain amount of out-ofroundness was inevitable, so that although the aver- age stress during the pressure test was somewhat

less than yield, some out-of-round areas would be

at or above yield strength. Therefore we are justified

in applying the simplified version of the Irwin equation (eq (2), ch. 2) for a surface crack, and we find

that the critical flaw depth needed to initiate SCC is 0.2(10/200)2 = 0.0005 in or about 0.01 mm! Such a small flaw could be provided easily by a grain boundary intrusion of oxide, a common occurrence in the

heat treating atmospheres of large furnaces in use at that time, and in fact such intrusions were noted in

this particular motor case. Adding a corrosion inhibitor would prevent pitting, but the fact that the

failure occurred in less than 15 min indicated that no

pitting was neeed to initiate the SCC, and as we have

noted earlier, inhibitors tested to date have not been

found effective in preventing growth of existing

cracks, which the oxide intrusions could have provided. Cathodic protection would not be practical in an environment of high resitivity such as the tap

water, and, even in a highly conducting electrolyte,

cathodic protection would at best simply slow the rate of growth of the stress corrosion cracks from the

oxide intrusion. Though coating the interior of the case would have been possible, it would have been

impractical. Therefore the solution chosen was to replace the tap water with an inert oil as the pressurizing fluid. In addition, new specifications were

written for the steel to have lower levels of impurities

(sulfur and phosphorus) and slightly lower strength

(190 ksi, 1300 MNm-^'), which would raise K„cc

somewhat and improve fracture toughness markedly, so that a much larger stress corrosion flaw

would have to grow (assuming SCC could occur by

contamination of the oil) before brittle fracture

would occur. With these measures the problem was

brought under control.

The Silver Bridge (frontispiece) was constructed of a steel of strength too low (80 ksi, 550 MNm"^) to qualify as a high strength steel as defined above, but

its SCC and fracture toughness characteristics make

it very suitable for treatment with the high strength

steels. The bridge collapsed on December 15, 1967,

and a study of the cause pointed to an eye of one of

the eye-bars of which the bridge was constructed,

(ref. [3]) A portion of one of the fractures of the eye is shown in figure 5, with the inside of the eye at the

top in this figure. Note the Ught colored area in the

upper right corner, which is obviously the origin of

the brittle fracture. This area was concluded to have

been caused by stress corrosion cracking. In salt water_containing H2S the Kjscc was found to be 15

ksiVin (16 MNm"^'^). The design allowable stress used in the shank of the eye-bar was 50 ksi (350

Figure 5. Portion offracture ofeye-bar ofthe "Silver Bridge". Eye is

at top. SCC area (lighter color, upper right) extended downward about 0.12 in {3 mm).

Brittle fracture spread down and toward left. Photo courtesy of Mr. H. C. Burnett. National Bureau of Standards.

MNm~2), which because of the stress raising properties of the eye placed the material in the region of

the fracture at just over the yield strength, and we

are therefore again justified in using the simplified

form of the Irwin equation. The size flaw needed to

initiate the stress corrosion crack would be 0.2(15/

80)2 = 0.007 in (about 0.2 mm), a flaw which a tiny

corrosion pit probably supplied. The fracture tough- ness (Kjc) of the steel was found to be 46 ksiVirr(50

MNm~^' 2). The flaw required to achieve Kjc would be

given approximately by 0.2(46/80)^ = 0.066 in (1.67

mm). The actual stress corrosion crack in figure 5 is 0.12 in (3 mm) deep, not a really bad agreement

considering that the steel varied in yield strength

from 70 to 86 ksi (480 to 590 MNm'^), that the shape

of the SCC area is not "long and thin" as the

simplified equation assumes, and that the brittle

fracture is propagating at an angle to the minor axis

of the elliptical SCC area. This failure will be discussed further in the section

below entitled "Single-Load-Path Designs."

The General U. S. Grant bridge over the Ohio

River at Portsmouth, Ohio, was constructed of cold

drawn wire having a yield strength of about 180 ksi

(1250 MNm~2). The wires were ungalvanized, and

after about 12 years of service a number of breaks were found and led to the replacement of the af- fected cables with cables made of galvanized wire.

The breaks were intergranular and could be duplicat-ed in the laboratory by stressing the wire immersed in dilute (0.01 N) ammonium nitrate or sodium nitrate, and the service failures were then

diagnosed as SCC (ref. [4]). Zinc coating (and

perhaps the organic coating over the zinc) solved the

problem. Note that this is cold drawn steel, not heat

treated, and the cold drawn steel is much more

resistant than heat treated steels to any hydrogen

embrittlement which the zinc might threaten at a break in the coating because of the electrochemical

potential of zinc (see Cathodic Protection above).

50
High Strength Steels

The Mount Hope bridge in Rhode Island was constructed using quenched and tempered steel wire of

about the same strength as that of the U. S. Grant

bridge, and cracking failures occurred a few months

after the wire was spun, well before the bridge had

been completed. This wire was hot dip galvanized,

but unlike the cold drawn wire the martensitic wire

was highly susceptible to hydrogen cracking, which

probably occurred at breaks in the zinc coating. The

solution adopted was to replace the martensitic wire

with galvanized cold drawn wire. (ref. [4])

The Lockheed submersible Deep Quest (fig. 6) was

made of slightly modified 18 percent nickel 200

grade maraging steel, actually heat treated to only

about 190 ksi (1300 MNm^^) yield strength for en- hanced fracture toughness. The aging treatment

Figure 6. Submersible DEEP QUEST aboard support ship. Al- though the steel of the hull can be made to undergo SCC at high stress intensities in the

laboratory, conservative material specification, design, fabrication and maintenance added up to satisfactory performance. Photo courtesy of Lockheed Missile and Space Company.

which confers high strength was given only after

welding together the spun hemispheres to form the

spherical hull. The K,scc of the welds and hull plate

after aging were in the range of 50-60 ksi Vm (55-65

MNm~^'^), so that the critical flaw size for SCC

would be about 0.01 in (0.25 mm) assuming yield

stresses. But since the steel was welded in the weak

condition, the residual stresses would not likely be

as high as the yield strength of the aged

material, and the critical size flaw for existing stresses

was probably very much larger. It is common practice in such structures to grind the welds which both

reduces the stress raising capability of the weld crowns and enhances the detectability of any cracks

present. The hull was given an organic coating system (carefully maintained at approximately semiannual inspections) plus additional insurance in the

form of cathodic protection. In this case the cathodic

protection system chosen consisted of carbon steel

sacrificial anodes, which as noted in the section

above on cathodic protection would tend to prevent

pitting of the maraging steel without any possible

hazard from overprotection. Thus the vehicle was

made of a high strength steel capable of being

cracked in the laboratory using sea water, but the

combination of several measures added up to a successful vehicle—simplicity of form, conservatism in

strength level, selection of maraging steel having

superior SCC properties (compared with alternative

materials at the same strength level), a carefully

maintained coating, and moderate cathodic protection. Another example of successfully pyramiding

control measures. A stainless steel component made of the stainless

precipitation hardening steel designated UNS

S35500 for which the heat treatment is unavailable

but which probably left the steel lower in strength

than that of Deep Quest was put in service in the

highly corrosive marine atmosphere of Cape Canaveral. The component experienced SCC from assembly stresses (fig. 7). One solution to the problem

Figure 7. Fitting of high strength precipitation hardening stainless steel designated UNS S35500 which cracked, because ofassembly stress, in the extremely corrosive marine atmosphere at Cape

Kennedy, Florida. Magnification 5X. Courtesy of Mr. J. D. Morrison, NASA.

would be overaging at a higher temperature. Another

solution might be to switch to a nonferrous alloy

resistant to both cracking and other forms of cprrosion in the marine atmosphere. MIL STANDARD

1568 (Material and Processes for Corrosion Prevention and Control in Aerospace Weapons Systems)

prohibits the use of precipitation hardening stainless

steels heat treated to their maximum strength

(H900).

A class of European combatant submarines was

designed using an 18 percent Mn 5 percent Cr steel

related to the nonmagnetic steel used for generator

rotor retaining rings described earlier and for which

d K,scc of 7.5 KsiVm (8 MNm"^'^) has been measured. The SCC properties of the submarine hull

steel were probably similar to those of the generator

steel. We know from the popular press that there were service failures, and the corrective measure

51
High Strength Steels

was to scrap the design. Note here that the submarine hull was probably of complex design compared with Deep Quest and hence probably had high

residual stress from assembly as well as being made

of probably a much more susceptible steel.

Aircraft bolts are made of high strength martensitic steel, and where they are under sustained tension

in service, they would be subject to inevitable cracking if they should react with water, including atmospheric moisture. They are therefore coated with

cadmium, commonly by electrodeposition. Such

electroplating with cadmium hazards the steel with

embrittlement by hydrogen co-deposited with the

cadmium and concentrated in the steel immediately

beneath the cadmium plate, which seals the hydrogen in the steel. But baking of the steel after plating

(according to MIL-S-5002 23 h at 350 T, 175 °C, a treatment which may not be adequate for critical

items) redistributes the hydrogen sufficiently to bring the maximum concentration down to a tolerable level. If the cadmium coating should be abraded

or scratched deep enough to expose bare steel, the

cadmium would tend to protect the adjacent steel

from pitting corrosion with a minimal hazard to hydrogen discharge and without hazard of overprotection, as discussed in the section on Cathodic

Protection. Cadmium plating is not suitable for applications involving sliding or wear. Also it is not rec- ommended for service above 230 °C (450 °F) because

of the possibihty of cracking because of the cadmium

itself (akin to liquid metal embrittlement). Zinc plating is not desirable because of the hazard of overprotection, among other reasons. The Canadian

Forces high speed prototype hydrofoil Bras (TOr has a 250 grade maraging steel main high speed foil which

is hollow with the inside coated with an inorganic

zinc-rich paint; a crack admitted sea water, and

subsequent extension of the crack was attributed to hydrogen discharged because of the overprotection

from zinc. (ref. [5])

An aircraft landing gear is typically constructed of

martensitic steel which may be heat treated to as much as 240 ksi (1650 MNm-^) yield strength. AISI

4340 and, more recently, 300M steels have been

used, and at such high strength levels they are of course highly susceptible to SCC. They are, however, cadmium plated, carefully monitored for initiating cracks, and the sustained stress (when

parked) is kept much lower than maximum design

landing stress, perhaps under 100 ksi (700 MNm"^).

Once again a combination of measures, no one of

which might be adequate, adds up to a largely successful technological solution.

A disk in a low-pressure turbine of the Hinkley

Point "A" power plant (Britain) disintegrated causing catastrophic failure of one 87 MW turbine and

damage to two identical neighboring turbines. Investigation showed that brittle fracture of the martensitic steel disk had originated from an intergranular

stress corrosion crack located in a keyway. The

keyway probably elevated the stress in that crack

region to something approximating the yield strength

of the steel. The corrodent responsible for the cracking was presumed to be the steam condensate, with

caustic entering with carryover and possibly concentrated by alternate wetting and drying during

turbine operations. The measured varied from

about 36 to 80 ksiVIi^ (40-90 MNm-^i^), the spread

being due to inhomogeneity. It is interesting that the

size of the stress corrosion crack (Vig in. 1.6 mm)

enables us to make an independent estimate of the

fracture toughness of the steel. Assuming yield point

(107 ksi, 740 MNrn'^) stresses and the simpUfied

Irwin equation, 0.0625 - 0.2 [KJIQIY, Ky. = 60

ksiVm (65 MNm"^'^), near the middle of the range

measured directly. Corrective measures were (1) to redesign to ehminate the keyways in the highly

stressed bore region, (2) to avoid construction which

exposed highly stressed regions to wet steam, and (3) to use steel of higher fracture toughness, (ref. [6])

IV. Single-Load-Path Designs

High strength steels are not the only constructional materials which are potentially vulnerable to the consequences of single-load-path designs. But

they can be highly vulnerable. Two examples have

appeared in the illustrations used above, and hence

this appears to be an appropriate place to call attention to the hazard.

In the rocket motor case of figure 3, once the

stress corrosion crack has grown to a size critical for

brittle fracture, the structure is doomed. Likewise

the vast structure of the Silver Bridge (frontispiece) was doomed when one stress corrosion crack had

propagated 0.12 in (3 mm). The rotor disk of the

Hinkley Point "A" steam turbine disintegrated when

a stress corrosion crack grew to a depth of Vie (1-6

mm). The corrodents responsible for the SCC in

these three cases were respectively tap water, at- mospheric moisture (probably contaminated), and

steam condensate, perhaps concentrated. All were

operating at high nominal stresses, and in each case

the monimal stress was elevated by stress raisers:

out-of-round areas in the motor case, the eye of the

eye-bar, and the bore of the disk plus the keyway in

the case of the turbine. Let us then summarize the

elements of these generically related failures: (1)

SCC (2) in relatively mild corrodents (3) in singleload-path design structures (4) fabricated of steel

having low fracture toughness (5) with high design

stresses (6) elevated by stress raisers.

The lesson is abundantly clear: If the conse- quences of a failure cannot be tolerated, the. designer is denied the option of single-load-path design

unless he has thorough knowledge of both the stress corrosion possibilities and the fracture toughness of

the steel. Note that the SCC failures in the General

U. S. Grant bridge did not lead to a catastrophe

because unlike the Silver Bridge, the Grant bridge

was a multiple-load-path structure, with redundancy

in the multiple paths.

At the time of material selections for the Silver

Bridge and the Hinkley Point turbine it was not

suspected that SCC could occur in those steels in

their intended environments, and as fracture

52
High Strength Steels

mechanics had not yet been developed, the conse- quences of having a sharp crack (such as a stress

corrosion crack) in a stressed steel component having what we now know as low fracture toughness were unknown.

V. Diagnosing SCC

sec in the high strength steels can be intergranular (along the grain boundaries of the prior austenite

grains, fig. 8), quasi-cleavage or transgranular with

respect to the prior austenite grains, (fig. 9), or it can

occur as microvoid coalescence, resembling figure 6

of chapter 5, or by a combination of all three modes.

The factor which determines which of these three

modes is operative appears to be the K level. At low

levels of ^, intergranular cracking tends to predominate, at intermediate values, quasi-cleavage, and at high levels, microvoid coalescence. If a given steel

has a low toughness, it may not be possible for the K

level to be raised high enough to cause microvoid

Figure 8. Intergranular SCC in martensitic steel. Replication electron fractograph, magnification 2250X. Courtesy of Mr. C. D. Beachem.

Figure 9. Quasi-cleavage SCC facets in precipitation hardening

steel. Replication electron fractograph, magnification 2400X. Courtesy of Mr. D. A. Meyn

coalescence SCC before brittle fracture occurs.

Also, if the steel has high resistance to SCC, it may

not be possible to propagate a stress corrosion crack

at K levels low enough to permit the grain boundary

mode to operate. Thus the cracking mode is not

indicative of corrodent. Neither does the cracking

mode permit one to distinguish between SCC and

hydrogen embrittlement (see Theory below). Finally,

cyclic stresses between two levels of tension have

been observed capable of producing intergranular

cracking indistinguishable from sustained load SCC.

VI. Theory

It is known that all high strength steels are susceptible to cracking by hydrogen. In many circumstances it is known that hydrogen is produced

deep within the growing stress corrosion crack, and

in many additional circumstances it is known that

the thermodynamic conditions there are favorable

for hydrogen ion reduction. The general macroscopic appearance of stress corrosion cracks in high

strength steels is the same as hydrogen embrittlement cracks (except that SCC may leave corrosion

products on the fracture surfaces), and the details of

SCC fractures and hydrogen embrittlement fractures as viewed by high resolution electron fractography are indistinguishable. It is probable that

many, perhaps even most, of the service failures

involving high strength steels in aqueous or other

hydrogen-bearing environments are due to hydrogen.

VII. Summary

Most SCC service failures in high strength steel

structures are due to a combination of design plus

assembly loads, aqueous environments (which may

include contaminants important to the failures), and

susceptible alloys. The most important single factor

in determining the degree of susceptibility is strength level, which for SCC resistance should be

as low as other considerations permit. Premium

melting stock and premium melting practice to minimize sulfur, phosphorus and oxides are significantly beneficial to SCC resistance. The combination of composition and heat treatment which

produces twinned martensite should be avoided.

Cadmium plating is beneficial for atmospheric service, and cathodic protection (but not overprotection)

is beneficial where feasible. Successful structures of

non-redundant load path design place special re- quirements on knowledge and comprehension of

Sec and brittle fracture technology.

VIII. References

[1] Sandoz, G., Stress corrosion cracking in high strength steels and in titanium and cJuminum alloys (B. F. Brown, Ed.),

Naval Research Lab., Washington, D. C, (1972) p. 80.

[2] Speidel, Markus O., Stress corrosion cracking in Fe-Mn-Cr

alloys, Corrosion/75 Paper No. 110, Nat'l. Assn. of Corrosion Engrs., Houston, (1975).
High Strength Steels

[3] Bennett, J. A. and Mindlin, Harold J., Testing and Evaluation, Vol. 1, No. 2 (1973), p. 152.

[4] Pollard, R. E., Symposium on stress corrosion cracking of metals, A. S. T. M., Philadelphia, 1945, p. 437.

[5] Carson, J. A. H., Barer, R. D. and Peters, B. F., Fourth

inter-naval conference on marine corrosion. Extended

Abstracts of Papers, Naval Research Lab., Washington,

D. C, 1972, p. 1.

[6] Kalderon, D., Proc. institution of mechanical engineers. Vol. 186 31/72, (1972) p. 341.

[7] Franklin, D. B., Design criteria for controlling stress corro^

sion cracking, Marshall Space Flight Center Document

No. 10M33107 (B), 1975.

54
CHAPTER 7

STAINLESS STEELS

(Austenitic and Ferritic)

Stainless steels owe their stainlessness primarily

to the presence of chromium, 12 percent or more of

which in a steel is generally considered sufficient to qualify the steel to be called stainless. The stainless

steels considered here are the low strength grades in

which the crystal structure is either face-centered

cubic (austenitic, which is nonmagnetic) or body

centered cubic (ferritic, which is strongly magnetic).

Some of the steels which are nominally austenitic

may contain enough of either ferrite or martensite

(particularly after being cold worked) to be weakly

magnetic. AISI type 301 has a high proportion of

austenite, but after being cold worked it contains

much martensite and can be made strong enough to qualify as a high strength steel. It is discussed with

those steels in chapter 6. It is also mentioned briefly

below. There are martensitic precipitation hardening, and maraging stainless steels which are discussed exclusively in the chapter on high strength

steels. The physical metallurgy of all stainless steels

is discussed in great detail in reference [1].

As a rule, the carbon content of the austenitic and

ferritic stainless steels is kept as low as economics

will allow, but there is in almost all the commercial

steels enough carbon so that if these steels are held

long enough in a broad elevated temperature zone,

carbides rich in chromium will precipitate, impoverishing the adjacent regions in chromium and

thereby altering their corrosion behavior. This precipitation occurs preferentially in the grain boundaries and is called sensitization, and it is of great

importance to the corrosion technology of the stainless steels. It can be removed by "annealing", which

in these alloys means extended heating at about 1065

°C (1950 °F) for austenitic steels or 790 °C (1450 °F)

for ferritic steels, followed by quenching or rapid

cooling to room temperature to keep the carbon in

solution. Alternatively the sensitized steel (if austenitic) may be heated for a few hours at about 840

°C (1550 °F) to homogenize the chromium; this practice is too risky to be a preferred measure. Even after the carbides are dissolved and held in

solution by rapid cooling, the annealing treatment

does not confer permanent insurance against re- sensitization, which may occur in the heat affected zone of a weld, made after annealing, for example.

Stainless steel components are sometimes welded to a non-stainless carbon steel pressure vessel, and

pursuant to the pressure vessel fabrication code, the

pressure vessel (with any stainless steel attachments) is heated to 620 °C (1150 °F) to reheve fabrication stresses to avoid brittle fracture of the carbon

steel. During this stress relieving anneal, the stainless steel generally becomes severely sensitized, and

in this situation, usually irreversibly, as it is not

feasible to heat large pressure vessels to 1065 °C and

then cool them sufficiently rapidly.

It is possible to add a strong carbide former such

as columbium (niobium) (in type 347) or titanium (in

type 321) which "stabilizes" the steel by tying up

most of the carbon in carbides more stable than the

chromium containing carbides. Low carbon and

extra low carbon grades are available in some types

and are effective in minimizing sensitization at welds. Low carbon stainless steels are usually indicated by the suffix "L" after the grade number.

The stainlessness of these steels is due to the

formation of a tight, adherent oxide skin on the sur- face which tends to isolate the steel from chemical

environments. The stainlessness is thus not an intrinsic property of the metal, and when the oxide

film is broken, as can be done mechanically or chemically (especially by chloride ions), the underlying

metal may form a new oxide film (repassivate). Alternately, some form of corrosion, including SCC,

may ensue. In some technologies it is common practice to "passivate" stainless steel components in

nitric acid with or without sodium dichromate before

they are placed in service. A flash "pickle" in HNO3

+ HF is also used. Such passivation treatments are

beneficial in that they tend to clean the surface, but

they do not confer permanent inertness. Federal

Specification QQ-P-35 is often followed to passivate

stainless steel components after fabrication. The

important property of the nitric acid in conferring

passivity is its oxidizing power, not its acidity.

The Unified Numbering System designations for

all stainless steels, including austenitic and ferritic

grades, consist of the letter "S" followed by five

numbers which usually incorporate the numbers of

the more common alternative designations. Thus

S30400 is the austenitic stainless steel commonly

designated AISI type 304, and S44600 is the ferritic

steel AISI type 446. The UNS designations have not

commenced to appear in the technical literature at this writing, therefore the commonly used designations will be used here.

The general approach for avoiding the SCC problem in (low strength) stainless steels depends upon

the environments which the various technological

needs impose. This chapter is accordingly organized

according to environment categories.

I. Marine Environments

In sea water at or near room temperature the

commercial austenitic and ferritic stainless steels do

not experience SCC. They are, however, subject to crevice corrosion or pitting to the extent that they

55
Stainless Steels

are not suitable for prolonged service in sea water

unless special precautions are taken. One such precaution is cathodic protection. If the ferritic steels are overprotected (see Cathodic Protection in ch. 6),

they are apt to bUster or crack because of the hydrogen discharged by the cathodic protection action.

Hence it is probably unwise to attempt to protect

such steels to a potential more negative than about — 0.85 V (SCE). There are some non-U. S. austenitic

stainless steels having carbon contents of 0.2 percent (higher than the U. S. grades) which, when

sensitized, cracked in sea water at room temperature, and even higher carbon aUoys (0.5 percent C)

cracked in sea water without being sensitized. The

high strength stainless steels crack readily in sea water at room temperature, as do the low alloy high

strength steels, as discussed in chapter 6. The tubing used to heat liquid bulk cargoes in

merchant ships may be made of austenitic stainless

steel, typically AISI type 316 or 316L. These heating

coils, when cold after discharging a particular cargo,

may be safely washed with sea water, but they

should be rinsed immediately with fresh water to avoid crevice corrosion, and especially they must be

so rinsed before steam is admitted, for in heated sea

water, chloride SCC (discussed below) is a high

hazard to such steels. For the same reason

avoiding hot chloride SCC—condensers using austenitic stainless steel tubes must be so designed and

maintained as to prevent hot sea water from contacting the steel.

In the marine atmosphere the temperature of the

surface of steel in sunlight, particularly in tropical

and subtropical climates, may range from near at- mospheric temperature to one substantially higher.

Although SCC is seldom seen in austenitic or ferritic

stainless steels in other atmospheric conditions, in

the marine atmosphere SCC has been seen occasionally, particularly in the tropics, particularly (but

not only) in parts heated by solar radiation, in the

following AISI types: 201 (sensitized), 202 (sensitized), 301 (see also ch. 6), 302 (sensitized), 303 and

sensitized 304 (plastically strained), all of which are

austenitic, and 430F, which is ferritic. (See figs. 1 and 2). Types 302, 303 and 430F contain especially

high levels of sulfur or selenium to make them

free-machining, and the resulting sulfides or

selenides are probably responsible for accelerated

SCC initiation and perhaps propagation in these

steels. Note the word "occasionally" above is emphasized. Large numbers of stainless steel structures serve successfully in the marine atmosphere,

and the failures represent exceptions. Type 304 L

might be worth the premium if one is concerned with

a critical component which will be unavoidably sensitized in manufacture.

Periodic cleansing to remove accumulated salt,

particularly if the situation permits a follow-up application of a water-displacing oil spray, is helpful in

mitigating SCC in the marine atmosphere in structures made of the steels mentioned above. For criti- cal items, types 304 or 316 might be preferred over

the vulnerable types cited if the mechanical property

requirements can be met. Type 316 has one qualitycontrol advantage: This steel contains 2-3 percent

molybdenum, the presence of which is readily detected by a simple spot test. Of the vulnerable steels

Figure 1. SCC in AISI type 303 stainless steel sleeve in service in the marine atmosphere near windward surf at Cape Kennedy.

Magnified 3X. This sort of cracking of austenitic stainless steel at room temperature is rare. A Similar steel but without the extra sulfur or selenium of 303 would probably not have cracked. Courtesy of Mr. J. D. Morrison, NASA.

Figure 2. SCC in free machiningferritic stainless steel, AISI type

430F, in service, in the marine atmosphere near windward surf at Cape Kennedy. Magnified 3X. Cracking of this steel in the temperature at room

temperature is rare. A similar steel but without the extra sulfur or selenium of 430F would

probably not have cracked. Courtesy of Mr. J. D. Morrison, NASA.

56
Stainless Steels

mentioned above, only 303 contains molybdenum,

and that at a sustantially lower level (0.6 percent

maximum).

II. Other Low Temperature Environments

Although the commercial U. S. austenitic stainless steels do not crack in sea water at room temperatures, there are three groups of other aqueous

chemical environments which can cause SCC at room temperature, two of them of practical importance, and the third an interesting laboratory test environment: (1) Polythionic acids cause SCC of

sensitized steels at room temperature. These acids

may be found in petroleum refinery components

under certain conditions, and further discussion of

them will be deferred to the "Refinery Applications"

section. (2) Dilute fluoride solution has been blamed

for causing SCC in sensitized type 304 stainless steel

in newly constructed nuclear power components

having high residual tensile stresses. Cold worked

surface areas, crevices, elevated temperatures and

visible oxide films (as may appear near welds) seem

to promote this type of cracking, which is invariably

intergranular. Concentrations of F~ as low as Ippm

sufficed to produce cracking in laboratory tests, but

cracking is thought unhkely in more dilute solutions.

The source of fluoride in practical cases is presumably weld flux. Phosphate or hydrazine additions

served to prevent the cracking in laboratory tests

(refs. [2] and [3]). The preferred remedial measure is probably meticulous clean-up and packaging immediately following fabrication and proof testing. It

is possible that factors in addition to fluoride may be

important to this form of cracking as some attempts

to reproduce the failures in the laboratory have not

been successful. The marine atmosphere has been

! blamed for cracking in heavily sensitized type 304

steel. (3) Certain strong acid solutions containing

chloride, of interest primarily as laboratory test environments, have been found capable of causing SCC

in austenitic stainless steels at room temperature.

These have included the following: 5A'^H2S04-I- O.SA'^ NaCl (which produced both intergranular and transgranular cracking); 3M HCIO4+O.5M NaCl; and

0.5M — l.OAf HCl. Any strong acid containing a

trace of chloride may cause cracking at room temperature. Perhaps the cracking in these environments is akin to that in hot chloride environments.

Tap water plus 1 percent HNO3 has also been re- ported to cause cracking of type 304 steel at room

temperature with or without added chlorides. Sulfurous acid (~6%H2S03) also can cause SCC in sensitized stainless steel at or near room temperature.

Heavy slabs of type 304 are sensitized due to the

slow cooling from the hot rolhng temperature. After

being heavily ground (to remove surface imperfections) they may crack intergranularly if stored outdoors in industrial atmospheres for several months.

The solution to this problem is to regrind to remove

incipient cracks in any slabs that have been so stored over three months, (ref. [4])

III. Chloride Cracking

The first SCC seen in stainless steels was aqueous

chloride cracking, which remains by far the most

prevalent form of SCC in stainless steels today. It has been observed in austenitic stainless steels of

various types in steam power plants (fossil fuel and

nuclear), chemical plants, petroleum refineries, the

food processing industry, the pulp and paper industry, the metal processing industry, and others. An

example of SCC in an AISI type 347 boiler superheater type is shown in figure 3. Table I illustrates

the widespread nature of the problem in various

technologies.

Figure 3. SCC in 1 in diameter superheater tube made of AISI

type 347 stainless steel. The corrodent may have been chloride in "carry-over", concentrated by evaporation. The stress responsible for cracking is presumably due to a bending moment generated by thermal stresses and the support shoe (bottom) frozen in place by fly ash. Eighteen out of forty-five such superheater tubes failed thus in ninety steaming hours. Careful attention to boiler water chemistry to diminish carry-over solved

the problem.

The primary causative factors in the problem are

as follows:

(1) Chloride ions in aqueous solutions or in solutions containing water or perhaps other electrolyte

(2) Sustained tensile stress

(3) A susceptible steel

(4) Oxygen or other oxidizer (except note test below)

(5) Elevated temperature (except as noted earlier

in this chapter)

(6) Favorable electrochemical potential

(7) Opportunity to develop local acidity

(8) Opportunity for local breakdown of passivity.

We will consider these factors in the order given.

The chloride ions can be suppHed by an almost

incredible diversity of sources: sea water, the

marine atmosphere, river water (with a chloride concentrating mechanism operative), sweaty hands,

human waste, salted foodstuffs, soluble contam57
Stainless Steels

Table 1. Some illustrative service SCC failures of austenitic stainless steels, (after ref [5'\)

Corrodent L. lemperature (t) Remarks

316L Toluene diisocyanate ? SCC in 6 months.

300-series Cycled cooling water ctg. Cl~ 60— 70 (140—160) Cracked.

316 Acetic acid ctg. Cl~ 115—120 (240—250) Cracked in 4 months.

316, 316L Crude amines 160 (320) Cracked.

304, 316 Wash water + CI" 88— 99 (190—210) Cracked.

316L Trichloroethylene + HCl + CI" 100—120 (200—250) Cracked in 6 months.

304 Hydrocarbon + Cl~ 149 (300) Cracked, 1 year. 304 Condensate + CI" 60— 82 (140—180) Cracked during down time. 316L Cooling water + 200ppm CI" under 150 (302) Cracked in 3 months.

304 Brackish cooling water 135 (275) Cracked.

316 Granular NaCl being dried 135 (275) Cracked in 6 months.

304L Sodium sulfite + 4000ppm Cl~ boiUng Cracked in 6 weeks.

316 Hypochlorite bleach salt mixture 27 (80) Cracked.

316 Detergent ingredients 9 Cracked.

304, 316 Tomato soup (salted) (hot) Cracked in 3—4 months.

inants in thermal insulation, HCl and other chloridecontaining contaminants, lubricants, sealants, decomposition products of polyvinyl chloride (PVC)

components in fires—the list is endless. The concentration of chloride ions of practical importance to SCC depends upon the condition of the other factors

listed above. The level of concern may be a few

thousand ppm (parts per million) in a component with

low fabrication stresses, made of a steel with high

resistance, with low oxygen, near room temperature, at the freely coroding potential, in an alkaline environment. In contrast, the level of concern may be less than

1 ppm of chloride in structures involving cold worked

components of a susceptible steel (such as type 304)

with a few ppm of oxygen at high temperature (e.g., 250

''C, 480 °F) in the near neutral range, also at the freely

corroding potential. The more concentrated the

chloride, the more probable SCC becomes. Therefore one designs to avoid concentrating mechanisms, such

as wetting and drying cycles.

The stresses responsible for chloride cracking in

service are almost always those caused by fabrication, fit-up, welding, or differential heating rather

than either design stresses or stresses due to heat

treatment. The stress responsible for the cracking in

figure 3 was probably caused by bending due to the

support shoe which was probably immobilized by

being prevented from sliding or tilting by furnace

slag. Stresses even lower than 10 ksi (70 MNm~^) can

cause cracking in a steel having an ultimate tensile

strength of 90 ksi (630 MNm"^). The stresses which

can cause cracking are in fact so low that rehance for

control of chloride cracking is largely placed in the

control of one or more of the other factors. Attempts

to counteract other stresses by shot peening have not

been very successful.

The relative susceptibilities of the major austenitic

stainless steels have been evaluated in one study in

which homogenous data (rare in this field) were developed using boiling magnesium chloride, about

which more later. The findings of this investigation

are shown in figure 4, which is taken from reference

[6].

At zero nickel content (and with certain other conditions discussed below met) the steels are ferritic and have high resistance to chloride cracking. With

increasing nickel content, starting as low as 0.1 percent Ni (in hot magnesium chloride solutions) stainless steels become rapidly more susceptible to SCC,

reaching a maximum vulnerability at about 8-12

percent nickel, the range which includes the most

commonly used austenitic types. With still higher

nickel contents the resistance to cracking in magnesium chloride increases until at about 42 percent

or higher the alloys appear to be immune to cracking

in this test. There is some evidence for a similar

behavior in other chloride test environments. In- creasing silicon is beneficial in magnesium chloride

but not in sodium chloride solutions. Nitrogen is detrimental, and the high nitrogen contents used in

some grades to reduce the need for nickel (needed to produce the austenitic crystal lattice) cause the steel

(e.g., type 205) to be quite susceptible. Phosphorus

is detrimental. Sensitization promotes pitting or

grain boundary grooving, both of which establish

local acidity and can act as stimuH to initiate SCC.

Therefore low carbon grades (e.g., 304L) or a

stabilized grade (e.g., type 347) is preferred if welding is to be done, as the welding process otherwise is certain to cause sensitization in the heat affected zone. In the austenitic steels molybdenum is detrimental as far as transgranular chloride cracking is concerned. If sensitization can be avoided, keeping

the carbon on the high side is generally thought to be

advantageous.

Summarizing the susceptibilities of the austenitic

stainless steels, all are susceptible to chloride cracking and there is not a tremendous difference between the resistance of the least resistant and the

most resistant, but even so, one might recognize four

non-quantitative categories of relative susceptibility, as follows:

58
Stainless Steels

o

o

to

COm

ocH

COQ

T

1

1 I I I I I 500 1000

FRACTURE TIME - HOURS

Figure 4. Relative SCC behavior of major types of austenitic stainless steel in boiling magnesium chloride. (E. E. Denhard, ref. [6]).

Highest

susceptibility: e.g., sulfur bearing 303; 301;

and sensitized 304

Intermediate

susceptibility: e.g., nonsensitized 304; 304L

Lower

susceptibility: e.g., 316; 316L; 347; 347L

Lowest

susceptibility: e.g., 310; 314; USS 18-18-2.

There are stainless alloys (e.g., Carpenter 20Cb-3)

which have even lower susceptibility to chloride

cracking.

In recent years there have been important ad- vances made in the technology of ferritic stainless

steels which make some of them suitable or even

preferred substitutes for austenitic stainless steels as far as chloride SCC is concerned. These developments have included the discovery that hmiting

the nickel and copper contents to easily achievable

levels and keeping the nitrogen and carbon ("interstitials") to very low levels confer on these steels

superior resistance to chloride cracking, though not

total immunity. One such alloy containing 26 percent

Cr and 1 percent Mo is commercially available and is designated ASTM XM-27 (ref. [7]). Another low interstitial ferritic stainless steel developed to be resistant to corrosion pitting as well as cracking contains

28 percent Cr and 4 percent Mo (ref. [8]). A third

steel is high purity 18 percent Cr—2 percent Mo.

Welding procedures are available for these alloys

but care is required to exclude oxygen and nitrogen

which would embrittle the weld area. There are two

other embrittlement phenomena which must also be

avoided. One of these phenomena is commonly

known in the U. S. as "885 embrittlement" and

occurs if service temperatures approach 885 °F (475

°C). The other is sigma phase embrittlement at somewhat higher temperatures. Additionally, as in

all ferritic steels, there is a ductile-to-brittle transition which can be an important consideration regarding structural integrity at thicknesses exceeding

about 3 mm (Vtth in) (ref. [7]).

In addition to austenitic and ferritic stainless

steels, there are stainless steels which are a mixture

of austenite and ferrite. These are called "duplex"

steels some of which may have interesting SCC

properties. The value of having several per-cent of

ferrite in otherwise austenitic weld metal to avoid

SCC in austenitic steel weldments is well recognized, and is possibly related to the performance of

duplex steels.

59
Stainless Steels

Oxygen or an oxidizer is an important factor in

chloride cracking in neutral solutions, and as a con- sequence it will be seen that control of oxygen level

is an important measure in avoiding SCC in pressurized water reactors. It is somewhat surprising to note in table 1 that cracking occurred in a sulfite

solution, which would be reducing. Oxygen is not

necessary for cracking in slightly acid solutions,

such as the (deaerated) boiling magnesium chloride

solution extensively used in laboratory studies of

SCC in stainless steel. The sulfite solution mentioned above may have been acidic, or it may have

permitted local acidification in corrosion pits.

Temperature is an important factor in chloride

SCC. Except for a few chemical environments cited

earlier, SCC of austenitic stainless steels seldom

occurs except at elevated temperatures, above about

60 °C (140 °F). Nevertheless, one must be prepared

for ugly and usually unexplained surprises in this

regard. This is the reason for including in table I the case of type 316 steel being cracked by hypochlorite

bleach salt mixture essentially at room temperature.

There appears to be an electrochemical potential

below which chloride cracking does not occur in

austenitic stainless steels. In the research Uterature

this potential is sometimes called the critical potential for cracking. In environments having sufficient

conductivity, this phenomenon can be exploited to prevent cracking by coupling a more anodic material

such as carbon steel, aluminum, or even lead, which

would cause the potential of the stainless steel to become lower than the critical cracking potential in

many environments. This technique is an example of

cathodic protection.

The factors of local breakdown of passivity and

local acidification are interrelated. In the presence

of chloride ions the passive oxide film on stainless

steel tends to break down, and by hydrolysis, the

chloride, chromium and water cause local generation of hydrochloric acid. If this acid is sufficiently

concentrated, it is difficult for the steel to re-grow a good passive film, and a corrosion pit may form,

often the precursor of SCC. The liquid near the tip of

a growing crack remains acid as long as the crack is growing. Cathodic protection tends to prevent the

acidification from occurring in the first place, and is also capable of neutralizing the acid solution and

therefore permitting repassivation. The development of local acidification tends to be promoted by a

shelter such as a crevice or wet thermal insulation,

the possibility of which would be best avoided in

design. Local acidification is also favored by shelters

such as those under sand or rust deposits, which

contribute what is commonly termed a "poultice ef- fect." Hence meticulous cleaning and passivating

are useful practices. Sometimes pickling of stainless

steels causes incipient grain boundary corrosion

which tends to act as a crevice, and incomplete

removal of acid can give chloride acidification a head

start. Alkaline rinsing of pickled components is therefore helpful.

IV. Caustic Cracking

Hot caustic solution (sodium hydroxide or potassium hydroxide) over a wide range of concentrations,

from one percent to 100 percent, can cause cracking

of stressed stainless steel without the requirement of

oxygen and, as far as we know, without the requirement of any other chemical species.

One current reason for interest in caustic cracking

of stainless steels Ues in their use for sodium cooled

reactors from which presumably the sodium may be

drained on occasions, and if incompletely, there is the possibility of residual sodium reacting with moisture to generate the caustic. Of even more immediate interest, where heat transfer surfaces are in

contact with water containing even small amounts of

free caustic, there is the possibility of high rates of

local boiling in stagnant areas, with resulting in- crease in causticity. Such caustic solutions, based

either on sodium or potassium, cause cracking of

austenitic stainless steels. Caustic cracking has

been a problem in the chemical and paper industries, and sodium freed from ion exchange resins has

been blamed for caustic cracking in steam generating equipment. By contrast with the subject of

chloride cracking, caustic cracking of austenitic

stainless steels has received only limited study.

Studies to date indicate that there is little differ- ence in the susceptibilities of the various austenitic

steel types studies, and that 304, 304L, 316, 316L,

347, and USS 18-18-2 all crack rapidly in solutions

of 10 to 50 percent NaOH and at temperatures of

150-370 °C (300-700 °F). Carbon content and degree

of sensitization have no effect on the relative susceptibility in this temperature range; at somewhat lower

temperatures sensitization can have a somewhat ad- verse effect. Whatever the mechanism of caustic

cracking of austenitic stainless steels, it is not the

same as the caustic embrittlement of carbon steel

such as in boiler desuperheater tubes, in which hydrogen generated by caustic corrosion of the steel

reacts with carbides inside the steel to form

methane, which in turn produces internal cracks.

See reference [9] for a recent major study of caustic cracking of austenitic stainless steels.

V. Oxygen Cracking

There have been a limited number of incidents of

SCC in sensitized type 304 stainless steel in high

temperature high purity water which at present do

not appear to fit the pattern for either chloride cracking or caustic cracking. Since this cracking tends to occur in systems in which the oxygen content of the

water is 0.2 ppm or higher, it has come to be known

as oxygen SCC. There is a tendency for this kind of

cracking to occur preferentially in type 304 which is sensitized and in areas of high stress, perhaps ex- ceeding yield.

60
Stainless Steels

See references [10] and [11] for recent major

studies on the subject, which is discussed further

below.

VI. Protective Measures Illustrating

Principles

Chemical industry. One important measure is to avoid conditions by which chloride can be leached

from thermal insulation and then become concentrated by drying on underlying hot stainless steel

pipe. One way to achieve this is to use insulation low

in chloride,^ another is to waterproof the insulation

thoroughly. Still another measure is to coat piping

prior to insulating. Inhibited asbestos or calcium

silicate insulation is now available as a substitute for

chloride-contaminated insulation and either provides another satisfactory way to avoid the SCC

problem on the insulated side of stainless steel

equipment.

One seeks to avoid conditions in which leaks of even low chloride water can drip on hot stainless

steel and concentrate chlorides by evaporation.

(Such concentration may also occur at a waterline.)

A further measure is to avoid insulating the bottom

several inches of a storage tank, which is the zone where leached chlorides would accumulate and

where the insulation would provide a "poultice" effect, permitting the concentration of damaging

species. In heat exchangers be sure the design is such that there is proper circulation of cooling water

and that there are no zones of stagnation or air

pockets, so that temperatures remain as low as possible and there is no opportunity for chlorides to concentrate by splashing. Another measure is to avoid designs with crevices where possible. In a

vertical heat exchanger, coating the water side of the

top tube section by oxyacetylene spraying of a lead

alloy (2 percent tin, 2 percent antimony) physically

protects the warmest (most vulnerable) ends of the

tubes and electrochemicaUy protects (by cathodic

protection) the bare stainless steel for as long a distance as the conductivity of the cooling water af- fords. In caustic solutions contaminated with

chloride, minor additions of lead nitrate (presumably

tying up the chloride as insoluble lead chloride) have

been reported to permit the safe use of caustic for

cleaning stainless steel without risk of either

chloride or caustic cracking. And one final use of

lead: considerable cracking of flanges has been at- tributed to chloride leached from asbestos gaskets; an emulsion of a light oil and htharge applied to the

gasket surfaces has been reported effective in preventing such cracking.^ (Largely summarized from

ref. [12])

^ ASTM standard "wick" test C692-71 can be used to evaluate the chloride cracking

characteristics of thermal insulation, to which silicate inhibitor is sometimes added to reduce risk of cracking.

^ Caution: Note in chapter 8 that lead in high temperature water systems can cause

cracking of certain nickel-base alloys.

Power industry. SCC of austenitic stainless steel

is not observed in completely dry steam; the design

and operation of stainless steel superheating tubes in

fossil fuel power plants therefore aim to avoid the

carrying over of water containing either chloride or

caustic from the steam drum into the main steam

line and the turbine. Free caustic can cause cracking

if carry-over occurs and if the boiler water is maintained alkaline by free hydroxide. By use of the

conventional phosphate treatment to maintain alkalinity, the hazard of caustic cracking is minimal,

according to laboratory tests. SCC has been observed in the nuclear power industry, where stainless steel is used extensively in

contact with water as well as steam. Eighty-eight

such failures have been collected and analyzed in

reference [13]. Residual stresses from fabrication,

and sensitization during stress rehef annealing of

carbon steel components, are recurring themes in

this account. The fluorinated and/or chlorinated

hydrocarbons used for lubricants, gaskets, and

leak-checking gases are rather stable at room temperature, but at the temperatures of pressurized

water reactors (PWR's) and boiling water reactors

(BWR's) they tend to decompose to Uberate fluorides

and chlorides to which are attributed a significant

number of failures. Leachable chloride in asbestos

gasketing has been a cause of cracking of austenitic

stainless steel.

The trend to meticulous cleanliness in fabrication

and maintenance, avoidance of sensitization, care to minimize fabrication stress, and care to avoid crevices and other means by which either caustic or

chlorides can concentrate (as by wet-and-dry cycles,

for example), have gone a long way in reducing the

incidence of SCC failures in the power industry.

Maintaining low chloride levels in nuclear power

plants (e.g., 50 parts per billion maximum chloride,

less than 0.3 parts per million oxygen in BWR's and

much lower in PWR's) have been important operational measures in controlhng the SCC problem.

Overpressuring PWR's with hydrogen is believed

important in minimizing SCC in these power plans

by keeping the oxygen low.

Water chemistry in PWR's is specified by AEC

RDT Standard Al-IT; the corresponding document

for BWR's is RDT Al-2.

Refinery applications. In addition to the hazards

from hot chloride cracking and caustic cracking, the

petrochemical industry has an additional threat in

the form of polythionic acid cracking, which can

occur after shut down of equipment due to the formation of this complex acid by decomposition of

sulfides on metal walls in the presence of oxygen and

water. This form of cracking occurs only in sensitized steels, such as may be found in heat-affected zones of welds and in components which have operated in the sensitizing range—above 425 °C (800 °F).

Different steels have different degrees of susceptibility to this form of cracking, which can be assessed

by an ASTM recommended practice (G35-73).

61
Stainless Steels

The preferred way to avoid polythionic acid cracking is to use only unsensitized steel. But even an

initially unsensitized steel, such as unsensitized type

304, may become susceptible to polythionic acid

cracking after prolonged service above about 425 °C

(800 °F). For such service mill stabilized type 321 or 347 is preferred.

When equipment containing welded or otherwise

sensitized steel and in which polythionic acid can form is shut down, there are a number of available

tactics which can be used, depending upon conditions, to alkalize, deny oxygen, or avoid a liquid

phase. These tactics are collected in NACE Recommended Practice RP-01-70 (Protection of Austenitic Stainless Steel in Refineries against SCC by

use of Neutralizing Solutions During Shutdown.)

If equipment is not to be opened, once the equipment is depressurized but before metal surfaces

have reached the dew point, it should be purged with an inert gas containing a minimum of 5000 ppm of

ammonia, after taking care that any brass be isolated. This alkalizing and exclusion of air prevents

polythionic acid cracking. If the equipment is opened, the most effective way to neutralize is to wash all stainless steel surfaces with a 1.5—2 percent solution of soda ash (sodium carbonate) in water; the addition of 0.5 percent sodium nitrate is recommended to reduce the probability of chloride

cracking. The Recommended Practice may be consulted for many practical and detailed suggestions.

A review of refinery experience in SCC is given in

references [14] and [15].

VII. Identifying SCC

Chloride cracking of austenitic stainless steel is typically transgranular and extensively branched

except in sensitized steel in which it can be (but is not always) intergranular. (figs. 5 and 6) Chloride

cracking of ferritic stainless steel is almost always

intergranular in service failures and in laboratory

specimens in hot sodium chloride solutions, but

transgranular cases have been seen in laboratory

specimens cracked in magnesium chloride solutions.

The crack path in austenitic steels in caustic solutions may vary with the concentration of solution.

One study (ref. [8]) indicated branching intergranular cracking at low concentrations and branching

transgranular cracking at high concentrations. The

polythionic acid cracking of (sensitized) austenitic

stainless steel is invariably intergranular. The occasional cracking of highly stressed sensitized type 304

stainless steels in BWR's occurring in water ex- tremely low in chloride but containing about 0.2 ppm

oxygen or more is invariably intergranular.

VIII. Theory

No system has received more theoretical study

than the austenitic stainless steel/chloride system.

Many theories have been proposed, but none has

succeeded in being generally accepted. One theory

Figure 5. Chloride SCC, service failure in AISI type 304 stainless

steel showing typical branching cracks. Magnification 250X. Photo courtesy

of Mr. W. Gerhold, National Bureau of Standards.

Figure 6. High magnification (900X) photomicrograph of trans- granular SCC produced in the laboratory. AISI type 304 stainless steel. Courtesy of Dr. J. Kruger, National Bureau of Standards.

which accounts for many observations is termed the

"slip dissolution model" and pictures the tip of'the

advancing crack remaining chemically active (and

dissolving) due to the continual arrival of fresh metal

surface by creep (plastic flow). The model is discussed in detail in reference [16]. Arguments have

been put forward that hydrogen plays a causative

role. These arguments are summarized in reference

[17].

62
Stainless Steels

The theory of the other cracking processes in

stainless steels is even more incompletely developed.

IX. Summary

sec at room temperature is rarely a problem with

austenitic and ferritic stainless steels except in the

petrochemical industry, for which detailed practical

guidance is available in the form of an NACE Recommended Practice. (Warning: High strength stainless steels can be highly susceptible to cracking at room temperature. See ch. 6.)

The most serious SCC problem with austenitic

stainless steel is caused by chlorides. Although it isn't feasible to totally eliminate chlorides, meticulous care to minimize their introduction is both effective and essential. Avoiding thermal insulation and

gasketing material high in chlorides, avoiding elastomers, sealants, lubricants and other materials containing halogens, avoiding sensitizing, minimizing

fabrication stress, in particular avoiding severe cold

work, and practicing meticulous cleanliness have

combined to permit extensive satisfactory use of

stainless steels.

X. References

[1] Gordon, G. M., Stress corrosion cracking and hydrogen embrittlement of iron base alloys, (J. Hochmann and R. W.

Staehle, Eds.), National Association of Corrosion Engineers, Houston, Texas, in press.

[2] Ward, C. T., Mathis, D. L. and Staehle, R. W., Corrosion,

Vol. 25, No. 9, p. 394 (1969).

[3] Theus, G. J. and Cels, J. R., Corrosion problems in energy

conversion and generation, (Craig S. Tedmon, Jr., Ed.), Electrochemical Society, Princeton, New Jersey, 1974,

p. 384.

[4] Heger, J. J., private communication to the author, 1975.

[5] E-BRITE 26-1, Second Performance Rept., (November

1974. Pubhshed by AIRCO Vacuum Metals. Other compilations of SCC failures in stainless steels are given in A.S.T.M. STP 264 (1960) and in paper by A. W. Loginow

et al.. Materials Performance, Vol. 11, No. 5. p. 35 (May

1972).

[6] Denhard, E., Corrosion, Vol. 16, No. 7, p. 131 (1960).

[7] Dillon, C. P., Materials Performance, Vol. 14, No. 8, p. 36

(1975).

[8] Streicher, Michael A., Stress corrosion of ferritic steels. Paper No. 68, Corrosion/75, NACE, Houston, Texas.

[9] Wilson, I. L., Pement, F. W. and Aspden, R. G., Corrosion,

Vol. 30, No. 4, p. 139 (1974).

[10] Clarke, W. L. and Gordon, G. M., Corrosion, Vol. 29, No. 1, p. 1 (1973).

[11] Cowan, R. L. and Gordon, G. M., Stress corrosion cracking

and hydrogen embrittlement of iron base alloys

(J. Hochmann and R. W. Staehle, Eds.), National Association of Corrosion Engineers, Houston, Texas, in press.

[12] van Droffelaar, H., Rapport technique 172, CEBELCOR,

Brussels, (January 1970.)

[13] Bush, S. H. and Dillon, R. L., Stress corrosion cracking and

hydrogen embrittlement of iron base alloys (J. Hochmann

and R. W. Staehle, Eds.), National Association of Corrosion Engineers, Houston, Texas, in press.

[14] Couper, A. S. and McConomy, H. F., Proc. API Div of Refining, Vol. 46, p. 321 (1966).

[15] Manuel, R. W., Proc. API Div. of Refining, Vol. 52, p. 476,

1972.

[16] Staehle, R. W., The theory of stress corrosion cracking in alloys (J. C. Scully, Ed.), NATO, Brussels, 1971, p. 223.

[17] Theus, G. J. and Staehle, R. W., Stress corrosion cracking

and hydrogen embrittlement of iron base alloys, (J. Hochmann and R. W. Staehle, Eds.), National Association

of Corrosion Engineers, Houston, Texas, in press.

63

CHAPTER 8

NICKEL ALLOYS

Nickel and the alloys treated in this chapter have

the face centered cubic crystal structure. With rare exceptions, stress corrosion cracks in these materials are intergranular, often with extensive branching.

The Unified Numbering system for this family of

materials consists of the letter N followed by the

customary five numbers, which usually include the

numbers of traditional designations. Thus Alloy 400

(Monel* 400) is designated N04400, Alloy 600 is N06600, Alloy 800 is N08800, etc. The UNS designations have not yet appeared in the technical literature, therefore the traditional designations are used

here.

L Chemical Industry

Pure nickel is used to contain hot caustic solutions

in the chemical industry, all the way up to pure fused

caustic. There have been cases of grain boundary

cracking when the caustic was contaminated with

mercury. Above about 315 °C (600 °F) there have

been instances of cracking along precipitated grain

boundary graphite in nickel 200, but this cracking can be avoided by using the low carbon nickel 201.

(ref. [1])

Cold drawn Alloy 400 has been found cracked by

warm hydrofluosilic acid, mercury, and mercury

salts, (ref. [2]). Alloy 800 has been observed to crack

in polythionic acids, but only when sensitized, (ref.

[3])

Sensitization causes Hastelloy* C to be susceptible to sec in hot chloride solutions, including 10

percent ferric chloride at temperatures as low as 50

°C (120 °F). Removal of the sensitization by a solution anneal removes the susceptibility. HasteUoy

C^, having low carbon and containing the carbide

stabilizer titanium, appears to be immune even when

given what would otherwise be a sensitizing anneal,

(ref. [4]).

II. Power Industry

The high nickel alloys play a critical role as materials for steam generating tubes in nuclear power

plants and as materials for other components in both

nuclear and fossil fuel power plants. The corrosion

performance of a given steam generator depends not

only on the alloy used and the water chemistry but

also on design (particularly thermohydraulic characteristics and the presence or absence of crevices),

fabrication (including cleanliness and magnitude of

residual stresses) and operation (including especially

the integrity of the condenser.)

* In no case does identification of certain materials by their trade names Imply recommendation or endorsement by the National Bureau of Standards, nor does it imply

that the material identified is necessarily the best available for the purpose.

Alloy 600, which has been in commercial use for

about 40 years, has been used world-wide in nuclear

steam generators. The performance record for this

alloy has been impressive, but there have been some

sec service failures the recognition of which offers

guidance for better future performance. Only one of

these failures has been reported in initiate from the

primary side. In this instance it cannot be excluded

that the pickling process which was applied to the

primary side during the final stages of fabrication,

and the water chemistry during the early days of

operation, have influenced or accelerated the corrosion (ref. [5]).

The remaining SCC service failures of alloy 600 in

steam generators have initiated on the secondary side

usually near the tube sheet. This is a region which

may have poor circulation, crevices, sludge, and high

residual stresses. There is evidence that the solution

within the crevices associated with some of the

cracked tubes was strongly caustic. The free caustic

condition can be avoided by using a water treatment

of hydrazine to control oxygen, ammonia to control

pH, and care to avoid inleakage of minerals from the

condenser cooling water. This all-volatile treatment

has largely replaced the phosphate treatment formerly

used in many PWR steam generators.

Laboratory tests of alloy 600 at high stress and

temperature have demonstrated cracking in high

purity water as well as in the presence of caustic or

lead compounds. Lead contamination produces

intergranular cracking in annealed material but

(surprisingly) transgranular cracking in sensitized

material.

The nature of the oxide film on the alloy, whether

produced in the mill or formed by reaction with

dissolved oxygen in service, may have an important

effect on cracking behavior by moving the electrochemical potential into a range favorable for SCC.

Over-pressurizing with hydrogen (practiced in

PWR's) keeps oxygen levels low.

Extended laboratory tests (22,000 h) in uncontaminated deaerated water at 300 °C (570 °F) showed shght

intergranular penetration but no cracking, (ref. [6])

Tests at higher temperature, 330 °C (625 °F), run to 16,000 h in uncontaminated water have led to the

conclusion that nonpickled alloy 600 having commercially important microconstructures does not suffer

intergranular SCC under those conditions, providing

the material is not excessively strained, (ref. [7])

A metallurgical reaction of unknown significance

has been reported in alloy 600 in steam generators,

(refs. [5] and [8])

The experience with SCC of alloy 600 in the nuclear power industry has been reviewed in refer- ences [9] to [13].

65
Nickel Alloys

It is to be expected that as the roles of design,

fabrication, operation, alloy composition, microstructure and processing are better understood, the

performance of alloy 600 in steam generators will be even more impressive than at present. Avoidance of

high stress is essential.

AUoy 800 has recently been selected for use in

several European steam generators.

III. Aerospace Industry

Marshall Space Fhght Center (MSFC) has published guidelines for controlling SCC in space vehicles and associated equipment for service in space

or in the atmosphere essentially at room temperature

(ref. [14]). These guidelines do not pertain to special

chemical environments such as may be associated

with propulsion and power devices.

The following nickel alloys are deemed by MSFC

to have high resistance to atmospheric SCC and may

be selected by their contractors in all conditions

without prior approval by MSFC: Hastelloy

C—Hastelloy X—Incoloy 901—Incoloy 903—

Inconel 718—Inconel X-750—Ni-Span-C 902—

Rene' 41—Unitemp 212 and Waspaloy.

IV. References

[1] Corrosion resistance of nickel and nickel containing alloys in caustic soda and other alkalies, Corrosion Engineering

Bulletin CEB-2, International Nickel Co., New York

(1973).

[2] Fraser, O. B. J., Symposium on Stress Corrosion Cracking of Metals, American Society for Testing Materials, Philadelphia, PA, (1945) p. 458.

[3] Boyd, W. K. and Berry, W. E., Stress corrosion cracking of metals—A state of the art (H. Lee Craig, Jr. Ed.) STP 518,

American Society for Testing Materials, Philadelphia, PA

(1972) p. 58.

[4] Streicher, Michael A., The effect of composition and struc-' ture on crevice, intergranular and stress corrosion of some

wrought Ni-Cr-Mo alloys, submitted to Corrosion, August

1975.

[5] Schenk, H. J., Tube defects in KWO steam generators and

results of their investigation, NACE Preprint, Corrosion/

75. See also Materials Performance, Vol. 15, No. 3, p. 25,

(1976).

[6] Flint, G. N. and Weldon, B. A., Some investigations into the stress corrosion behavior of Fe-Ni-Cr alloys in high

temperature water. Preprint for conference on Water

Chemistry and Aqueous Corrosion in Steam Generators,

Ermenonville, France, March 13-17, 1972.

[7] Pement, F. W. and Graham, N. A., Corrosion problems in energy conversion and generation (Craig S. Tedmon, Jr., Ed.), The Electrochemical Society, Princeton, NJ, (1974),

p. 423.

[8] Weber, J. and Sury, P., Materials Performance, Vol. 15, No. 2, p. 34, (1976).

[9] van Rooyen, D., Corrosion, Vol. 31, No. 9. p. 237 (1975).

[10] Bush, S. H. and Dillon, R. L., Stress corrosion in nuclear

systems. Proceedings of International Conference on SCC

and HE of Iron-Base Alloys, R. W. Staehle et al., NACE,

Houston, TX, in press.

[11] Cordovi, M. A. Corrosion considerations in light water

cooled nuclear power plants, paper presented at the Inter- national Nickel Power Conference, Kyoto, Japan, 1972.

[12] Fletcher, W. D. and Mahnowski, D. D., Operating experi- ence with Westinghouse steam generators, paper pre- sented at the International Conference on Materials for Nuclear Steam Generators, Gathnburg, Tennessee, (Sept.

1975).

[13] Berry, Warren E., Materials Performance, Vol. 15, No. 1, p. 7 (Jan. 1976).

[14] Franklin, D. B., Design criteria for controlling stress corrosion cracking, Marshall Space Fhght Center Document

10M33107, 1975.

66
APPENDIX

GLOSSARY OF TERMS

**alclad. Composite sheet produced by bonding

either corrosion-resistant aluminum alloy or aluminum of high purity to base metal of structurally stronger aluminum alloy. (The cladding

is usually anodic to the base alloy.) ** alloy. A substance having metallic properties

and being composed of two or more chemical

elements of which at least one is an elemental

metal.

** anion. A negatively charged ion.

**anisotropy. The characteristic of exhibiting different values of a property in different directions with respect to a fixed reference system in

the material.

* anode. The electrode of an electrolytic cell at which oxidation is the principal reaction. (Electrons flow away from the anode in the external

circuit. It is usually the electrode where corrosion occurs and metal ions enter solution.)

**au8tenite. A solid solution of one or more elements in face-centered cubic iron. Unless

otherwise designated (such as nickel austenite),

the solute is generally assumed to be carbon.

**au8tenitic steel. An alloy steel whose structure is normally austenitic at room temperature. (In the

present book austenitic steels owe their structure primarily to the addition of nickel, or in a few cases, to manganese.)

**bra88. An alloy consisting mainly of copper (over

50%) and zinc, to which smaller amounts of

other elements may be added.

**Brinell hardness test. A test for determining the

hardness of a material by forcing a hard steel or carbide ball of specified diameter into in

under a specified load. The result is expressed

as the Brinell hardness number, which is the

value obtained by dividing the applied load in

kilograms by the surface area of the resulting

impression in square millimeters. (A Brinell impression may be used to produce sustained

stresses in a metal for SCC testing.)

* cathode. The electrode of an electrolytic cell at which reduction is the principal reaction. (Electrons flow toward the cathode in the external

circuit.)

*cathodic protection. A technique to reduce the

corrdsion rate of a metal surface by making it the cathode of an electrochemical cell,

corrosion. The chemical reaction between a metal or aUoy and its environment.

** corrosion fatigue. A cracking process caused by

the combination of repetitive stress and corrosion. (In some situations, corrosion fatigue can

include cyclic stress corrosion cracking.)

*Definitions marked with asterisk are verbatim ASTM definitions (G15-74). Tutorial material in parentheses is by the author.

**Derinitions marked with two asterisks are from Metals Handbook, Vol. 1, Amer. Soc. forMetals, Metals Park, OH, 1961. Tutorial material in parentheses is by the author.

** crevice corrosion. A type of concentration-cell

corrosion; corrosion of a metal that is caused by

the concentration of dissolved salts, metal ions,

oxygen or other gases, and such, in crevices or pockets remote from the principal fluid stream,

with a resultant building up of differential ceUs

that ultimately cause deep pitting,

critical cracking potential. The electrochemical

potential above which SCC occurs but below

which it is not observed.

* electrode potential. The potential of an electrode in an electrolyte as measured against a reference electrode. The electrode potential

does not include any resistance losses in potential in either the solution or external circuit. It represents the reversible work to move a unit

charge from the electrode surface through the

solution to the reference electrode. (In this volume the sign of the potential follows the Pourbaix (Stockholm) convention.)

**Erich8en test. A cupping test in which a piece of

sheet metal, restrained except at the center, is deformed by a cone-shaped spherical-end

plunger until fracture occurs. The height of the

cup in millimeters at fracture is a measure of

the ductility. (Deformation in the plastic range

but short of fracture is sometimes used to produce residual stresses in sheet metal for SCC

testing.)

**erosion. Destruction of metals or other materials

by the abrasive action of moving fluids, usually

accelerated by the presence of solid particles or matter in suspension. When corrosion occurs

simultaneously, the term erosion-corrosion is often used.

**fatigue. The phenomenon leading to fracture

under repeated or fluctuating stresses having a maximum value less than the tensile strength of

the material. Fatigue fractures are progressive,

beginning as minute cracks that grow under the

action of the fluctuating stress. **ferrite. A solid solution of one or more elements

in body-centered cubic iron. Unless otherwise

designated (for instance, as chromium ferrite),

the solute is generally assumed to be carbon.

On some equilibrium diagrams there are two

ferrite regions separated by an austenite area. The lower area is alpha ferrite; the upper, delta

ferrite. If there is no designation, alpha ferrite is assumed.

* filiform corrosion. Corrosion that occurs under

some coatings in the form of randomly distributed threadlike filaments.

Fracture mechanics. The science of the cracking behavior of a cracked body under stress. In

this volume the term is restricted to the linear

elastic fracture mechanics of Irwin and Kies.

**hydrogen embrittlement. A condition of low

67
ductility in metals resulting from the absorption

of hydrogen,

inhibitor. A chemical substance or combination

of substances that, when present in the proper

concentration and forms in the environment,

prevents or reduces corrosion,

intergranular corrosion. Preferential corrosion

at or adjacent to the grain boundaries of a metal

or alloy.

K/^cc. The lowest stress intensity at which

stress corrosion cracking has been observed for a given alloy in a given environment. The units are stress times the square root of length, as ksiVm or MNm"^'^. See "stress intensity" below.

martensite. (1) In an alloy, a metastable transitional structure intermediate between two allo- trophic modifications whose abilities to dissolve a given solute differ considerably, the hightemperature phase having the greater solubility.

The amount of the high-temperature phase

transformed to martensite depends to a large

extent upon the temperature attained in cooling,

there being a rather distinct beginning temperature. (2) A metastable phase of steel, formed by

a transformation of austenite below the Mg (or

"Ar") temperature. It is an interstitial supersaturated solid solution of carbon in iron having

a body-centered tetragonal lattice. Its microstructure is characterized by an acicular, or needle-like, pattern. (Martensite is the struc- ture responsible for the hardness and strength

of the traditional quenched and tempered

steels.)

passive. The state of the metal surface characterized by low corrosion rates in a potential re- gion that is strongly oxidizing for the metal.

Passivity is generally due to the formation of

thin, tight oxide film, usually invisible to the

naked eye.

Spitting. Corrosion of a metal surface, confined to a point or small area, that takes the form of

cavities.

poultice effect. The concentration of corrosive

chemical species under debris or other sheltering material.

** precipitation hardening. Hardening caused by

the precipitation of a constituent from a supersaturated solid solution.

sensitizing. A heat treatment, whether accidental, intentional, or incidental (as during welding)

which causes carbide precipitation at grain

boundaries, often causing the alloy to be sus- ceptible to sec.

SCE. Saturated calomel electrode, a standard

reference electrode for measurements of

electrochemical potentials.

Shear lip. The border of a brittle fracture, sloping nominally 45 degrees to the plane of the

brittle fracture.

* stress-corrosion cracking. A cracking process

that requires the simultaneous action of a cor- rodent and sustained tensile stress. This

excludes corrosion-reduced sections which fail by fast fracture. It also excludes intercrystalHne or transcrystalUne corrosion which

can disintegrate an alloy without either applied

or residual stress. (The term as used in this

volume is not restricted to any mechanism.)

Stress intensity (or stress intensity parameter). A quantitative continuum mechanics

descriptor of the stress field around the tip of a crack in a stressed body. It is designated K but

is not to be confused with the Neuber K. The

units are length times the square root of stress. Note that "stress intensity" as used throughout

this volume is the fracture mechanics parameter. It is unrelated to "stress intensity" as used

in the ASME code to designate the stress differ- ence or two times the shear stress.

68
INDEX*

A

Aerospace industry

aluminum components 25(F),26,30,31>32

landing gear 52

nickel alloys 66

steel bolts 52

titanium components 36,37,37(F),38

Aircraft bolts 52

Aircraft landing gear 52 Aluminaut (submersible) 32 Aluminum alloys 23

cathodic protection 32

extruded rod and tube 26

identifying SCC in 33

inhibitors 32

fracture mechanics SCC of some alloys 28

K vs. cracking rates, various alloys 28(F)

liquid metal cracking 33

machining effects 28,31

quenching stresses 28

relative resistance to SCC 27(T),29(T) shot peening 30

theory of SCC in 34

Aluminum alloys (specific alloys) cast alloys (characteristics of 19 alloys) 29(T)

wrought alloys (characteristics of 49 alloys

and tempers) 29(T). See also 31-32

2020 31 2014 27(T),36

2024 26,27(T),30,31

2124 31 2219 .^^.'

' ^^^^^^^^^^^^^^ /27(T),3^^

5083 30

5086 30

5456 30,33(F),34(F)

7039 30

7049 30,31

7050 31 7075 25(F),26,26(F),27(T),29(F),

30,30(F),31

7079 11(F),27(T),30,31

7175 31 7178 27(T),30,31

7475 31 Ammonia (brass) 13 anhydrous 13

refineries 18 Austenitic high strength steel 47,51

Austenitic stainless steel (see Stainless Steels)

B

Bolts (aircraft) 52 Bridges

General U.S. Grant 50

Mount Hope 51

"Silver Bridge" xi,2,50

c

Cadmium (titanium alloys) 38

Carbon tetrachloride aluminum aUoys 33

titanium alloys 37

Cathodic protection

aluminum alloys 32

copper alloys 20

high strength steels 48

stainless steels 60,61

Caustic cracking (stainless steel) 60

*F following a page number indicates a figure on thai page, T indicates one or more

tables.

Chemical industry

nickel alloys 65

stainless steels 61 Chloride

stainless steel 57 standard test in 6 Chlorine 38

Chlorofluoromethane 37

Cold drawn (bridge) wire 50

Condensers (see heat exchangers) Constant strain rate test 10 Copper alloys 13

agricultural and anhydrous ammonia 13 coatings 20

diagnosing SCC in 20

heat exchangers 18 household articles 19

liquid metals 20

mercury 20

sea water 19 sulfamic acid 19 theory of SCC in 21

relative susceptibihty of in ammonia 16(T)

Copper alloys (specific alloys)

admiralty brass 14(T),16(T),18

aluminum brass 15,16(T),18,21

aluminum bronze 14(T),15,19

antimonial admiralty 16(T),20,21

beryllium copper (Cu-Be) 15 commercial bronze 14(T),16(T)

cupronickel (10%) 16(T), 17,20

cupronickel (30%) 14(T),16(T)

gilding metal 16(T)

leaded brass 16(T)

low brass 16(T)

manganese bronze 16(T),19

Muntz metal 14(T),16(T)

naval brass 16(T)

nickel bronzes 19 nickel silver 16(T),17

phosphor bronze (5%) 14(T),16(T)

phosphor bronze (10%) 14(T)

phosphorized copper 17 red brass 16(T)

silicon bronze (low) 16(T)

silicon bronze (3%) 14(T),16(T),19

silnic bronze 16(T)

tough pitch copper 16(T)

D

Deliquescence 17 Diphenyl 37

F

Fracture plane (ASTM designations) 24

Fracture mechanics 7,67

G

Gallium 33,38

General U.S. Grant Bridge 50

Glossary of terms 67

H

Heat exchangers 18,19,20,56,61

High strength steels 43

austenitic 47

cathodic protection 48

diagnosing SCC in 53

relative susceptibility to SCC 43,45-47(F)

69
stainless grades are susceptible to SCC 46

theory of SCC in 53 High strength steels (specific alloys)

AISI type 410 43

AlSl type 4340 43,48,52

AMS 6434 49

cold drawn bridge wire 50

HP 9-^20 44,47

maraging-200 51

various steels (characteristics in salt water) 45-47(F). See also 48(T)

17-4PH 46,48,51

18% manganese 47,51

300M 52 Hinkley Point turbine 52 Hot salt cracking of titanium 36 Hydrazine 37 Hydrogen chloride 38

Hydrogen embrittlement 1,38

I

Inhibitors aluminum alloys 32

high strength steels 48

Indium (liquid) 33

Industrial environments

copper alloys 19

stainless steel 57

Insulation (thermal, and stainless steel) 61 K

defined 68

interpretation 9

Landing gear 52 Liquid metal embrittlement

in aluminum alloys 33

in copper alloys 20

in titanium alloys 38

M

Marine atmosphere 56 Magnesium chloride

stainless steels 58,59(T)

standard test 6

Manganese austenitic high strength steel 47

Maraging steels 43

relative susceptibilities 45-47(F)

Mercurous nitrate test 15 Mercury

aluminum alloys 33

copper alloys 20

titanium alloys 38

Methanol

aluminum alloys 33

titanium alloys 36 Mount Hope Bridge 51 "Mud crack" pattern 33

N

Nickel and nickel alloys 65 aerospace industry 66

chemical industry 65

lead 65

power industry 65

Nitric acid aluminum 33

titanium 38

Nitrogen tetroxide (N2O4)

aluminum 33

titanium 37 Non-redundant load path designs 52

Polythionic acids

refineries 62

stainless steel 57

standard test 6

Polyvinyl chloride 58

Precipitation hardening steels 43

relative susceptibilities 45-47(F)

Propellers (marine) 19 PVC 58

R

Refinery applications 18,61

Rocket motor case

high strength steel 49

titanium 37

SCC features 1 SCC standard test solutions alternate immersion (NaCl) 6 magnesium chloride 6 Mattsson's solution 6

polythionic acids 6 SCC test methods 5 constant strain rate 10

fracture mechanics 7 precracked specimens 7 U-bend 6 SCC theory

tduminum alloys 34

copper alloys 21 general 1,5 high strength steels 53

stainless steels 62

titanium alloys 40

Sea water/salt water aluminum alloys 29(T),31,32

bronze 19 high strength steels 45-47(F)

stainless steels 55

titanium 9(F)

Season cracking 13

Sensitization (stainless steels) 55,58,61

Sensitized stainless steels 56,57,59,60,61,62

Silver (and Ti) 38

"Silver Bridge" xi,2,50

Single-load-path designs 52 Sodium (liquid) 20,33

Solder (molten) 20

Stainless steels (austenitic and ferritic; see "High

Strength Steels" for high strength stainless steels) 55

caustic cracking 60

chemical industry 61

chloride 57 duplex 59

identifying SCC in 62

miscellaneous service environments 58(T)

oxygen cracking 60

polythionic acids 57 power industry 61

refinery applications 61

relative susceptibilities of austenitic steels

to chloride SCC 59(T)

theory 62 thermal insulation 61

Stainless steels (austenitic or ferritic), (specific

alloys)

AISI 201 56

202 56

70
301 56,59

302 56

303 56,56(F),59

304 56,57,58(T),59,60,62,62(F)

304L 58,58(T),59,60

310 59

314 59

316 56,58(T),59,60

316L 56,58(T),59,60

347 57(F),59,60

347L 59 430F 56,56(F)

Carpenter 20 59

USS 18-18-2 59,60

various alloys, characteristics in MgClz 59(F,T)

Steam (silicon bronze) 19 Stress relief annealing temperatures,

copper alloys 14(T)

Submarines (submersibles)

"Aluminaut" 33

"Alvin" 35

"Deep Quest" 51 nonmagnetic steel 51 Sulfamic acid (brass) 19 Sustained load cracking (Ti alloys) 38

hot salt 36

hydrazine 37

K/,^^ of Ti-6% A1-4%V in salt water 36(F)

K/^„ of various alloys in salt vtater 9(F)

liquid and solid metals 38

methanol 36 N2O4 37 sec paths 39

theory of SCC in 40

trichloroethane 37

trichlorotrifluoroethane 37

Titanium alloys (specific alloys)

50A 36

75A 36

lOOA 36

6-4 35,36(F),37

6-2-I-O.8M0 35

8-1-1 38,39(F),40

various alloys characteristics in sea water 9(F)

various alloys, mostly obsolete 40

Trichloroethane 37

Trichlorotrifluoroethane 37

Turbine (Hinkley Point) 52

u

U.S. Grant Bridge. 50 Theory of SCC

aluminum alloys 34

copper alloys 21 general 1,5

high strength steels 53

stainless steel 62

titanium alloys 40

Tin (liquid) 20,33

Titanium alloys 35

chlorinated diphenyl 37

chlorofluoromethane 37

diagnosing SCC in 39

w

Water

aluminum alloys 23

copper alloys 13 titanium alloys 35

Zinc (liquid) 33

6U.S. GOVERNMENT PRINTING OFFICE: 1977 0—220-030

71
1*6S-114A (REV. 7-73)

1 U.S. DEPT. OF COMM.

BIBLIOGRAPHIC DATA

SHEET

I* PL) b Lie A 1 1(JJ\ (JK KhFUKl NU.

NBS Monogr. 15 6

i.. Oov t Accession

No.

J. Kecipient s Accession No.

4. TITLE AND SUBTITLE

oxress uorrobion uracKiny i./Untrui ricaburcb

5. Publication Date June 1977

6. Performing Organization Code

7. AUTHOR(S)

B. F. Brown

8. Performing Organ. Report No.

9. PERFORMING ORGANIZATION NAME AND ADDRESS

NATIONAL BUREAU OF STANDARDS

DEPARTMENT OF COMMERCE

WASHINGTON, D.C. 20234

10. Project/Task/Work Unit No.

11. Contract/Grant No. N00014-68-A-0245-0007

12. Sponsoring Organization Name and Complete Address (Street, City, State, ZIP)

Prepared with the support of the Advanced Research Projects

Agency under ARPA Order No. 2616, through Contract No. N00014-

68-A-0245-0007 of the Office of Naval Research

13. Type of Report & Period

Covered

Final

14. Sponsoring Agency Code

15. SUPPLEMENTARY NOTES

Work carried out at America University under contract from AEPA.

Library of Congress Catalog Card Number: 76-6O83O6.

16. ABSTRACT {A 200-word or less {actual summary of most significant information. If document includes a significant

bibliography or literature survey, mention it here.)

Stress corrosion cracking is a failure mode which has occurred with increasing

frequency in all sectors of technology. This publication attempts to diminish the

incidence of stress corrosion failures by assembling the available practical measures

to avoid or minimize the problem and present these measures in a form comprehensible

to those persons responsible for the design, fabrication and maintenance of new

structures. The alloys covered are copper, aluminum, titanium and nickel alloys and

high strength and stainless steels.

17. KEY WORDS (six to twelve entries; alphabetical order; capitalize only the first letter of the first key word unless a proper name; separated by semicolons

)

Aluminum alloys; Copper alloys; High strength steels; Nickel alloys; Stainless

steels; Stress corrosion cracking; Titanium alloys

18. AVAILABILITY [X] Unlimited

1 For Official Distribution. Do Not Release to NTIS i

kTJ Order From Sup. of Doc, U.S. Government Printing Pffice ^

Washington, D.C. 20402. SD Cat. No. C13./l.4: 156

1 1 Order From National Technical Information Service (NTIS)

Springfield, Virginia 22151

19. SECURITY CLASS

(THIS REPORT)

UNCL ASSIFIED

21. NO. OF PAGES

81

20. SECURITY CLASS

(THIS PAGE)

UNCLASSIFIED

22. Price

$4.50

USCOMM-DC 29042-P74