NATIONAL BUREAU OF STANDARDS

10028

REPO A ft

CORROSION RESEARCH

Progress Report

January 1 , 1969 - March 31, ] 969

OSW No. 14-01-0001-1091

By

H. C. Burnett

Project Coordinator

Metallurgy Division

To

Office of Saline Water

Department of Interior

U.S. DEPARTMENT OF CC. ZRCE

NATIONAL BUREAU OF STANDARDS

NATIONAL BUREAU OF- STANDARDS REPORT

ms PROJECT Nas REPORT

3120401 1002°

CORROSION RESEARCH

Progress Report

January 1, 1 969 - March 30 19^9

OSW No, 14-01-0001-1091

By

H» C„ Burnett

Project Coordinator

Metallurgy Division

To

Office of Saline Water

Department of Interior

IMPORTANT NOTICE NATIONAL BUREAU OF S

for use within the Government,

and review. For this reason, t whole or in part, is not auth< Bureau of Standards, Washing

the Report has been speciticall

Approved for public release by the

Director of the National Institute of

Standards and Technology (NIST)

on October 9, 2015.

;ress accounting documents intended

is subjected to additional evaluation

ire listing of this Report, either in the Office of the Director, National

r, by the Government agency for which

I copies for its own use. <NSS>

U.S. DEPARTMENT OF COMMERCE

NATIONAL BUREAU OF STANDARDS

CORROSION RESEARCH

Progress Report

January 1, 1969 - March 31, 1969

OSW No. 14-01-0001-1091

by

H. C. Burnett

Project Coordinator

Metallurgy Division

The Institute for Materials Research of the National Bureau of Standards

is engaged in a fundamental study of corrosion phenomena. The program is

being conducted by the Metallurgy Division and consists of five tasks. The

objective, progress and principal investigators for each of the tasks is

reported below.

Task Activities

Task 1 Polarization Studies and Cathodic Protection

W. J. Schweratfeger

Objecti ve : To measure by polarization techniques the instantaneous

rates of corrosion (computed weight losses) of a series of selected

alloys exposed in the laboratory to synthetic sea water. To compare

computed weight losses with actual weight losses. To note the

effects of such variables as corrodent temperature, aeration and pH.

To look for a possible relationship between computed weight and pit

depth based on relative apparent areas corroded or based on types of

corrosion control (cathodic or anodic).

Progress : All previous work on this project ' reported by the writer

was on experimental data in the temperature range 72° - 80°F. The

present data were obtained on iron (same as No. 31 of previous reports)

exposed to synthetic sea water at temperatures increased periodically

in 3 steps, from 72°F. to 300°F. and then decreased in 2 steps back to

72°F. The sea water was prepared with distilled water and salts added

in accordance with specification ASTM D-1141-52.

The iron specimen in the form of a rod (0.5 in. diam) was mounted

vertically in the cylindrical chamber of an autoclave. Exposure was

to 3000 ml of the corrodent with the waterline at 8 in. from the bottom

of the rod. A silver wire (0.05 in. diam) coated with silver chloride

served as a reference electrode. The cylindrical inner wall of the

stainless steel autoclave was used as the auxiliary electrode. The

specimen and reference electrode which protruded through the autoclave

head to the outside were insulated by teflon bushings.

The initial polarization measurements were made after 6 hours of

exposure and thereafter at times and conditions as tabulated in the

table. The air vent referred to in the table terminated in the air

space above the corrodent. Total exposure time was a continuous 11 days and 3 hours. The theoretical metal losses are calculated using

average corrosion currents for each period between polarization

measurements, for example, 52 mg at 96 hours is based on 2500 microamperes for 20 hours.

Upon opening the autoclave after 11 days, it was observed that oil

had leaked into the chamber from the fan shaft in the head. Except

for a few small pits at the bottom end and side of the specimen, most

corrosion (pitting) was immediately above the waterline and to a diminishing degree up to one inch above the waterline. Nevertheless,

it appears that this corrosion was measured, in view of the reasonable

agreement between cumulative calculated (252 mg) and actual (282 mg)

weight losses. At 300°F. the vapor pressure was about 50 psi. The

corrosion rates (mdd) in the table are based on the metal surface

exposed below the waterline. Actually, the corrosion rates in the area where most of the corrosion occurred were several times greater.

The variation of the specimen potential with temperature is considerable. The iron surface exposed to the salt water was dark in appearance.

The apparent film formed from the oil may have been a contributing

factor in the marked rise in potential, although some rise because of

high temperature is to be expected.

Future experiments with iron and other metals or alloys will include

the effects on corrosion of changes in oxygen concentration at high

temperature.

During the quarter, a manuscript entitled, "Corrosion Rates of Metals,

Alloys and Galvanic Couples As Measured by Polarization Techniques",

based on O.S.W. project polarization data at room temperature was prepared for oral presentation at the Galvanic Corrosion Symposium,

ASTM Meeting at Atlantic City, N.J., June 24, 1969. The manuscript is

being offered to ASTM for publication.

- 2 -

Table 1 - Corrosion of iron in synthetic sea water (72 300°F)

Exposure

time,

hrs

.

Corrosion

Current

,

yA

Specimen-^

potential

,

V

Solution

temp. , °F

Corrosion - metal loss

between intervals, mg

Corrosion^

rate

mdd

!

Remarks

6 320 -0.704 72 2 9.8 Day of exposure

Air vent open.

! 26 240 - .714 72 6

.

7.3 Air vent open - closed at 28 hrs of

exposure.

46 160 - .728 72 4 5.0 Set temp, control

for 150° at 49 hrs.

52 900 - .780 150 3 28 Set temp, control

for 212° at 54 hrs.

76 3100 - .640 212 50 95 Set temp, control

for 300° at 78 hrs.

96 1900 - .164 300 52 58

• No change

195 400 - .152 300 113 12 Set temp, control

for 212° at 199 hrs.

242 200 - .437 212 15 6.0 Opened air vent at

243 hrs.

245 270 - .470 212 1 8.3 Set temp, control

for 72° at 246 hrs.

| 264 235 - .480 72 6 7.3 Removed specimen at

267 hrs.

— Reference electrode, Ag/AgCl . At temperatures above 72°F, the potentials are corrected for reference to 72°F. in accordance with work of S. Greeley, William T.

Smith, Jr., Raymond W. Stoughton and M. H. Lietzke - Electromotive force studies in

aqueous solutions at elevatd temperatures . I. The standard potential of the silver - silver chloride electrode, J. Physical Chem. 64, 652 (1960).

2 / — Based on breaks in cathodic and anodic polarization curves and Faraday's law. Used

electrochemical equivalent for Fe=2. 8938x10-4 g/C. Cumulative wt. loss = 252 mg.

Actual wt. loss = 282 mg.

3/ — Based on area of specimen exposed to salt water below the water line. Area = 12.7 in

(0.82 dm^)

.

as k 2 Initial Stages of Corrosion Attack

Or. A. Wo Ruff., Dr. A. Fraker^ Mr. D. B. Ballard

O b i active : The initial stages of corrosion attack will be

studied using electron microscopy and diffraction techniques.

Copper-nickel and aluminum-brass and titanium alloys will be

studied under conditions approximating use in desalination

equipment. Thin-films transmission methods and replica studies

will be conducted to determine the relation between sample

microstructure and behavior in corrosive media. Lifetime predictions may follow the extrapolation of short time tests.

Progress ; Microstructural observation of corroded titanium

alloy thin foils continued through this past quarter. Experiments

were conducted as described previously to determine the effect of

exposure to wt. °jo of NaCl water at increased temperatures and

pressures on the corrosion resistance of the materials. The

alloys testedand the results are given in Table 1. Electron

diffraction studies indicate that the initial oxide covering the

titanium surface is a TiO film that is only partially crystalline.

Crystals of Ti02 (anatase) 5 form as the temperature and corrosion

times are increased. Another oxide^ probably anosovite (TisOs or Ti 203 -Ti 0 )j is found deposited at surface edges and cracks. Less

of the deposited form is found as the temperature is increased.

The thin oxide film which initially covers the titanium alloys grows

somewhat at increased temperatures but begins disappearing as the

Ti02 (anatase) crystals form and grow. At the onset of the TiC>2 crystal formation,, their appearance is similar to the TiH2 particles

noted in some corroded and uncorroded specimens.

Figures 1 through 12 show the progress of corrosion on the Ti-0.15Pd

alloy as it is taken from the uncorroded state through several testing

conditions. Figure 1 is a low magnification electron micrograph showing the grain structure of uncorroded Ti-o.l5Pd. Figure 2 shows the same alloy at higher magnification. Figure 3 is a diffraction pattern

from this uncorroded alloy. Additional (small) diffraction spots result

from a hybride particle in the diffraction area. The absence of any

diffraction rings indicates that surface film formation is not advanced.

Figure 4 shows the microstructure after corroding for 1 hr. at 100°C

and Figures 5 and 6 show the alloy surface after corroding at 150°C and

175 °C respective 1 y . Figure 7 is an electron diffraction pattern taken

with perhaps some crystalline Ti02 (anatase) present. Figure 8 is taken

from a specimen corroded for 1 hr. at 200°C and Figure 9 shows the microstructure after corroding for 2 hrs. at 200°C. It is apparent that the

first film (TiO) is disappearing and that the final film and oxide

surface particles are Ti 02 (anatase) which give the crystallite diffraction pattern of Figure 10.

1 The most common titanium oxide., TiC>2 occurs in three forms; anatase;

brooklte and rutile. These are listed in order of increasing stability.,

increasing oxygen content and decreasing specific volume. Since rutile

is the most stable and adhering form it offers the most protection for

the base meta 1

.

-4 -

Figure 11 is an electron diffraction pattern showing the presence of

TiaOs and Figure 12 shows the morphology of this phase and the manner

in which it is deposited

.

This set of micrographs just described illustrates the general sequence

of corrosion product formation on all of the alloys. Only the rate at

which this process takes place appears to differ, varying with changes

in alloy composition . The fir t film,, TiO, occurs at a later corrosion

stage and it disappears sooner on the Ti-0.15Pd alloy than on the Ti-4A1-

IMo-lV and Ti-8A3 -i Mo-lV alloys. Electron diffraction patterns show

that the oxide covering the surface of all specimens after testing for

2 hours at 200°C in unaerated hot saline water is Ti02 (anatase) and not

Ti02 (rutile). Corrosion tests are in progress now on the Ti-2Mo alloy

and future testing will concern the Ti-2Ni and Ti-6A1 -k\J alloys.

Additional tests involving shorter and longer corrosion times at 100°C

and 200°C will be conducted in an effort to learn more about the corrosion

products and their relation to alloy composition and testing conditions.

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Fig. 1 X4 ,000

Ti-0.15Pd - uncorroded

Fig. 2 XI 8,000

Ti-0.15Pd - uncorroded

Fig. 3 Diffraction

pattern uncorroded

Ti -0. 1 5Pd and TiH

2

Fig. 4 X32 ,000

Ti-0.15Pd - corroded

1 hr. at 100°C

Fig. 5 X32 ,000

Ti-0.15Pd - corroded

1 hr. at 1 50°C

Fig. 6 X32 ,000

Ti-0.15 Pd - corroded 1 hr.

at 1 75°C.

Fig. 7 Diffraction

pattern Ti-0.15Pd and

TiO surface oxide of

Figs. 5 and 6

Fig. 8 X32 ,000

Ti -0 . 1 5Pd - corroded 1 hr.

at 200°C

Fig. 9 X32 ,000

Ti-0.15Pd - corroded

2 hrs . at 200°C

Fig. 10 Diffraction

pattern Ti-0.15Pd and

Ti02 (anatase) - corroded

2 hrs. at 200°C

Fig. 11 Diffraction

Ti-0.15Pd and Ti 3 05. deposit ~ corroded 1 hr.

at 1 75°C

Fig. 12 XI 2 ,000

Ti-0.15Pd with deposited

Ti 3 0r, -corroded 1 hr. at

1 75°C

Task 3 S pcctrometri c - El 1 i psometry o f Films Formed on Iro n in Aqueo us Solution

Dr. J. Kruger, Mrs. C. L. McLiee, Dr. H. P. Layer and John R. Ambrose

Objective : It is proposed to study the electronic properties of the

passive films formed on iron by determing their optical constants in the

visible region of the spectrum, to determine how these properties are altered by the introduction of chloride ions and other species into the

solution that cause passive film breakdown, and to determine how these

properties are changed by a change in the atomic arrangement of the metal

substrate.

Progress : The objectives of this task encompass the whole gamut of

protective film breakdown starting with spectral studies of the nature of

the protective passive film, then kinetic and spectral studies of the

beginnings of breakdown and finally, holographic studies of the end of

breakdown process, pitting. The results obtained on each of these

objectives during the last quarter were as follows:

(1) Passive Film Spectra : As explained in earlier reports the formidible

part of this study is to obtain unambiguous spectra. Work has continued

on a new approach to this problem of multiple values of refractive indices

for single wavelengths. The goal is to eliminate those values which may

be a result of the computational processes, leaving only single values for

each wavelength which may be attributed to the conditions on the specimen

surface.

The latest approach to this problem has concerned itself with thickness

experiments, as described in the last report. Films are grown at various

potentials producing oxide films of increasing thickness as the potential

is increased. It is hoped that the various thickness films would have only

one set of refractive indices in common, thus eliminating the other multiple

values associated with each A. A number of such experiments have been

attempted so far. Growth at potentials o volts, +0.3V, +0.6V and +0.75V

produced films of from 5A to 5GA. The spectra for these experiments, however,

did not coincide as closely as would be desirable. The reason for this

behavior has not been established as yet, but may lie in the treatment given

the surface during the thickness experiments. An oxide film is grown for

1 hr. at a set potential, readings are taken, then the film is reduced back

to the substrate values and another film is grown, etc. It was found that

the specimen so treated did not produce the same kind of overnight passive

spectrum (at +0.75v) as did a specimen that was just reduced to a clean

substrate once and then passivated overnight. Figure 1 illustrates this

situation using plots of the relative phase retardation, a. Thus, it may be

possible that the growth and reduction cycles are producing some change on the

surface. It will be necessary to produce a surface that does not change

before thickness experiments can be successfully utilized to combat the

multiple value problem. Since thickness experiments can only be carried out

reasonably for short growth times, it is important to determine whether a film

grown for 1 hr. (or x hours) at a certain potential would be the same as a film grown for a longer period of time under the same conditions. In Figure 2

a spectrum of the relative phase retardation, a for a film grown for 1 hr. at

+0.3v is compared with that for a film grown overnight at +0.3V. The

- 9

-

correlation is quite good and indicates that data taken for 1 hr. growth

experiments would reasonably be applied to spectra for films grown for

>20 hrs.

Further research aimed at interpreting the positive spectra has been

carried out in the attempt to choose the correct optical constants for the

passive film by comparison with bulk oxide spectra. A crystal of the Fe 304

(artifically grown) was heated for 1 week at 240°C in the presence of air

and water vapor. These conditions should oxidize the Fe304 to y-Fe 203 . X-ray

data is being taken to determine whether this is the case. If so, a spectrum

for y-Fe 203 could then be obtained.

In another experiment attempting to classify one of the possible oxides of

the passive film (y-Fe2<33 and Fe 304 ) 5 a specimen was held at +1 .'25V in the

transpassive region for 1 hr. According to previous electron diffraction

data the film formed under these conditions would probably be Fe 304 - Difficulty with gas evolution from the surface prevented readings below

4300A and above 5900$ where the reflections are least intense. Figure 3

shows the absorption coefficient spectrum for the transpassive film, for

d = 68A. Multiple values for single x‘s are connected vertically. This

spectrum does not duplicate the spectrum for bulk FegCk reported previously.

This fact may indicate that the films grown in solution are actually not

comparable to bulk oxide spectra. Further investigations will be needed,

however, to determine whether this is the case.

(2) Kinetics and Spectral Studies of Breakdown by Chloride : Previous

results of kinetic studies of breakdown by chloride showed the existence of

two induction times corresponding to breakdown by chloride.

O They also showed no breakdown occurred below a potential labeled E

a

. During

this period both kinetic and spectral studies of films grown below E

a ^ with

chloride present were carried out to see if the growth as well as the breakdown

process were not affected by chloride below E a 2 .

Figure 4 shows that the growth process of the passive film was the same regardless of whether or not chloride ions were present as long as growth

occurred at potentials below the potential called E

a

^. This is the potential

observed for the passivation of iron below which ferrous ions were observed

in solution while above it no appreciable ferrous ion concentration was observed. Figure 4 also shows that when a film was grown above this potential

there was a marked difference in the growth process as compared to that of a film formed in the presence of chloride. The film continued to grow to much

greater thicknesses than the limiting thickness observed when chloride was The chloride produces a different film, which has been shown in the

:ontain yFeOOH as well as a cubic oxide. Moreover, it looks as chloride present the deposition of ferrous ions is taking place

absent,

past to

if with

continuously with the production of thick films (>100X).

- 10-

An experiment was carried out to characterize the behavior of films grown

below the potential E-2, i.e. around -0.35V. (vs. SCE) First a film was grown at this potential in the borate solution. A spectrum of a is shown

for this growth of 1 hr. in Figure 5, curve A. Curve B in the figure

indicates a passive film decayed to -0.3V in a previous experiment. The

correspondence between the two cases is good. Figure 6, curve C, indicates

ahe effect of a concnetration of (.005N) chloride ion present during the

1 hr. growth period, at -0.35V. The only significant change occurs at

5900a as shown. For comparison, curve A again shows the growth without

chloride present.

(3) Holographic Studies of Pitting : Plans for applying the holographic

technique described in the last report to a pitting study were initiated

during this quarter. A simple inexpensive potentiostat and a cell were

designed which will enable studies of aluminum alloys above and below the

critical potential. By means of this potentiostat the potential can be

controlled within 1-2 mV. The inception and rate of pitting for long times

(up to 1 year) growth will then be studied .using the holographic interferometric studies described in earlier reports.

WAVELENGTH, A

Fig. 1 Comparison of spectra of for passive

films grown overnight on surfaces which

have undergone thickness experiments

with a surface that has just undergone

reduction once.

Fig. 2

Spectrum for a film grown just 1 hr. at

+0.3V in borate solution compared with a film grown 24 hrs. under the same conditions.

- 12 -

n nr 4000 4400 5200 e 000

WAVELENGTH. A

Fig. 3

The absorption coefficient, H , spectrum for a film

grown in the transpassive region at +1.25V.

Fig- 4

The effect of potential above t (+0.8V SCE) and

below E (-0.35 V SCE) on film growth in the absence

and presence or chloride ion ( 1

0

_

) in boric acid - sodium tetraborate buffer solution (pH = 8.4). Film

thickness was measured by el 1 i psometry

.

- 13 -

WAVELENGTH,A

Fig. 5

The spectrum for 1 hr. growth at -0.35V

(near E^) is shown in curve A to be almost

the sami as a spectrum for a decayed passive

film, decayed to -0.3V, curve B.

Fig. 6

The spectrum for 1 hr. growth at -0.35V, curve A, in borate is shown in comparison

with a 1 hr. growth spectrum for the same potential, with 0.005N chloride ion present

in the solution.

1

1

,

Task 4 Structure and Growth Kinetics of Oxide Films on Iron

Dr. A. J. Melmed, and Mr. J. J. Carroll

Objective : Significant progress in understanding surface phenomena can only be expected as the result of several types of investigation on a given

problem. Often this is made difficult or uncertain by the fact that each

investigation may be done under different experimental conditions. We propose to avoid these uncertainties by combining three surface-study tools into

one research instrument (ELF), specifically an ultra-high vacuum instrument

which will permit the simultaneous observation of a metal surface by

ellipsometry and low-energy-electron-diffraction. The instrument will also

have a field-electron-emission microscope built into the specimen region.

It is proposed to study simultaneously, during the oxidation of metals by

gaseous oxygen, the atomic structure, by the technique of low-energy electron

diffraction, and growth kinetics, by the optical technique of el 1 i psometry

.

Progress : The ELF vacuum system is now functioning properly. We have

completed a prototype experiment using an evaporated metal film on a (Oil) - orientated si ngl e-crystal tungsten substrate. The tungsten specimen had some carbon contamination as determined by LEED pattern observations . However,

n and k substrate values remained reproducable after repeated high temperature

heat treatments of the specimen. The metal was barium*, for which optical

constants data of evaporated films are available in the technical literature.

Our results are shown in Figures 1 and 2. Starting with the tungsten substrate

heated to above 2000K in ultra-high vacuum, barium was deposited by evaporation

of sucessive doses from a heated iron clad barium wire source, and ellipsometric

measurements were made after each dose. Figure 1 shows n and «, the

effective optical constants of the Ba film plus tungsten substrate, as a function of the number of Ba doses (which were not equal doses). Figure 2

shows the optical constants as a function of computed film thickness, assuming

that the film optical constants are equal to the limiting values found in

Figure 1. It can be seen that the optical constants of the film-substrate com- bination become approximately constant for film thickness greater than about

400A, in agreement with the range of thickness values found for other metal

films by previous workers. Effective optical constants obtained at high barium

film thicknesses appear in good agreement with O'Bryan's work.

We will next repeat this experiment for evaporated iron films, and proceed

to investigate the oxidation of such films.

We have experimentally determined that either one or both of the fused quartz

ellipsometry windows in the ELF tube are strained. The amount of strain is

such that a change of about 10% in the measured optical constants for tungsten

occurs as a function of the light beam entry and exit positions. This

effects the absolute values of our measured optical constants for (110)

tungsten, but does not effect the measurements of relative changes in

el 1 ipsonieter parameters resulting from reaction of the specimen with oxygen

or other gases. We have also tested a new set of fused quartz windows for the new ELF tube being constructed, and found no detectable strain.

- 15 -

An iron crystal of low carbon content was obtained from Corrosion Section

personnel. It has been x-ray oriented and is presently being cut by sparkerosion so that specimens can be prepared for our oxidation investigation.

This work will be done in the new ELF tube and should proceed in parallel

with the work on evaporated iron films in the original ELF tube.

-1

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- 17 -

Tungsten

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- 18 -

Barium

Film

Thickness

(A)

Figure

2

Task 5 Relation of Corrosion to the Electronic Structure of Alloys

Dr. L. H. Bennett and Dr. L. J. Swartzendruber

Obj ective : It has been observed that the corrosion rate in sea water of

copper-nickel alloys is reduced by the addition of small quantities of iron

or manganese. An understanding of this effect, which is as yet unexplained,

could lead to the development of more corrosion resistant materials. It is

suggested that the effect of iron or manganese on copper-nickel alloys may

be to produce a so-called "local moment". It is proposed to investigate

this possibility experimentally.

Progress : Further Mossbauer effect studies have been undertaken in the Cu-Ni-Fe

system with progress being made on two fronts - in elucidating the constitution

as well as the magnetic structure of these alloys. A manuscript "On the Interpretation of Mossbauer Effect Spectra as Related to the Constitution of Cu-Ni-Fe

Alloys" has been prepared. In this paper we have identified, for the first time,

six different Mossbauer patterns corresponding to iron in six different atomic

environments. These include isolated Fe atoms in solid solution in Cu, Fe-Fe

and Fe-Ni complexes in solid solution in Cu and Cu-Ni matrices, as well as Fe

in precipitates and preprecipitates (Guinier-Preston zones) . The effects of

annealing, quenching from the solid state, and quenching from the liquid state

("splat-cooled") are shown. Since the presence of different phases with different electrochemical potentials can have important effects on the sea water

corrosion of these alloys (see, for example, our earlier paper "The Effect of

Fe on the Corrosion Rate of Copper-rich Cu-Ni Alloys", Scripta Met. 2_, 93 [1968]),

these identifications from the Mossbauer spectra are important.

As noted above in Obj ective , we have proposed that "local moments" may bear re- lation to the corrosion of Cu-Ni-Fe alloys. Small amounts of nickel apparently

do not form local moments in Cu, but we suspected that the Fe would do so. We

have now confirmed this initial suspicion. We have obtained Mossbauer evidence

that Fe forms a local magnetic moment, and in fact causes the neighboring Ni

atoms to form substantial moments as well. Data as a function of temperature and

magnetic field for a Fe source in Cu-30 Ni shows the existence of magnetic

clusters of about 5 Bohr magnetons per Fe atom. These studies are being extended

to Cu-10 Ni and Cu-20 Ni as well.

A furnace has been put into operation to take Mossbauer spectra above room

temperature. Spectra have been obtained to 580°C in a number of samples.

Plans are being formulated to set up magnetic susceptibility apparatus to investigate other magnetic impurities in Cu-Ni alloys.

A copper-5% palladium specimen was corroded in salt water, to compare with the

20% palladium result. Although Cu-20% Pd appeared passive, the Cu-5% Pd corroded

very rapidly especially near the edges, where an embrittlement (hydrogen?)

occurred. Metallographic and microprobe measurements are underway on these

alloys, and new specimens with other compositions are in preparation.

- 19 -