NATIONAL

BUREAU OF STANDARDS
The program of research on building materials and structures vmdertaken by the

National Bureau of Standards is planned with the assistance of the following advisory

committee designated by the Subcommittee on Design and Construction of the Central

Housing Committee.

TECHNICAL RESEARCH GROUP

Walter Junge, Federal Housing Administration, Acting Chairman

A. G. Bear,

Veterans' Administration.

Pierre Blouke,

Home Owners' Loan Corporation.

C. W. Chamberlain,

Procurement Division.

Jos. M. DallaValle,

Public Health Service.

Hugh L. Dryden,

National Bureau of Standards.

G. E. Knox,

Bureau of Yards and Docks, Navy

Department.

Charles E. Mayette,

United States Housing Authority.

Vincent B. Phelan,

National Bureau of Standards.

E. A. POYNTON,

Bureau of Lidian Affairs. John H. Schaefer,

Procurement Division.

A. C. Shire,

United States Housing Authority.

George W. Trayer,

Forest Service.

Elsmere J. Walters,

Quartermaster Corps, War Department.

Sterling R. March,

Secretary.

The program is administered and coordinated by the following staff committee of

the Bureau:

Hugh L. Dryden, Chairman

P. H. Bates W. E. Emley A. S. McAllister

H. C. Dickinson G. E. F. Lundell H. S. Rawdon

The Forest Products Laboratory of the Uniied States Department of Agriculture

is cooperating with the National Bureau of Standards in studies of wood constructions.

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UNITED STATES DEPARTMENT OF COMMERCE • Daniel C. Roper, Secretary

NATIONAL BUREAU OF STANDARDS • Lyman J. Briggs, Director

BUILDING MATERIALS

and STRUCTURES

REPORT BMS8

Methods of Investigation of

Surface Treatment for Corrosion

Protection of Steel

by ROLLA E. POLLARD and WILBUR C. PORTER

ISSUED OCTOBER 11, 1938

The National Bureau of Standards is a fact-finding organizat ion:

it does not "approve" any particular material or method of construction. The technical findings in this series of reports are to be construed accordingly.

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CENTS
Foreword

The Present Paper describes the methods used in an investigation now under way of

surface treatments for the protection of steel for low-cost housing construction against

corrosion. Accelerated laboratory corrosion tests of various pretreatment processes

and paints used for steel and galvanized surfaces form an important part of the investigation. Because of wide variations in service conditions, these tests cannot be ex- pected to produce results directly measurable in terms of years of service. However,

with the limitation of each type of tests in mind and with comparisons confined to the

conditions of testing, such tests form a valuable means of studying the factors affecting

the protective value of paints under particular conditions. In the present investigation

special attention is being given to the study of conditions affecting corrosion by con- densed moisture, such as might occur in the enclosed areas between walls. Lyman J. Bkiggs, Director.
Methods of Investigation of Surface Treatments for

Corrosion Protection of Steel

by ROLLA E. POLLARD and WILBUR C. PORTER

CONTENTS

Page

Foreword ii

I. Introduction 1

II. Factors affecting durability of paint on metal 2

III. Methods of testing , 3 IV. Testing program 5

1. Pretreatments for galvanized

steel 5

Page IV. Testing program—Continued

2. Surface conditions for plain sheet steel 8

3. Protective coatings 9

4. Status of investigation.-. 10

ABSTRACT

Some of the factors affecting the protective value of paints for steel and galvanized surfaces are being

studied by accelerated laboratory corrosion tests sup- plemented by outdoor exposure tests. Short descriptions are given of the various methods of testing. The

significance and the limitations of such tests as compared with actual service tests are briefly discussed and

the materials, pretreatment processes, and paints in- cluded in the testing program are outlined. Special

attention is being given to the severity of corrosion which may result from condensed moisture.

I. INTRODUCTION

As PART of a general research program on

building materials, the National Bureau of

Standards is conducting an investigation on

the surface treatment for corrosion protection

of steel used in the construction of low-cost

houses.

The use of steel as a building material is not

new, of course. Large amounts have been used

for many years as heavy structural members in

large buildings. Used in this manner, in heavy

sections, covered and protected to some extent by other building materials, the deterioration of

steel is not a serious problem, since small

amounts of corrosion do little damage, and appearance is not a factor. In recent years the use of light-gage sheet

steel as a building material has increased considerably. Structural members fabricated from

such steel are often used as joists or studs in the

framework of houses of conventional construction. The so-called "prefabricated" steel house

usually consists of wall, floor, ceiling, and roof

sections of standard size prefabricated in the

shop and joined together on the job by welding,

bolting, or interlocking joints. The sections or panels may consist of a steel frame work fabricated from sheet steel and covered on either side

by other materials, or may be made entirely of

sheet steel or galvanized sheet in cellular units.

Sheet steel also is used in considerable amounts

in window frames, metal cabinets, metal furniture, and the air ducts of air-conditioning equipment. Among the newer uses of sheet steel in

house construction, may be mentioned bath- rooms made as a separate unit complete and

ready for installation in the desired location and

requiring only connections for light, water, and

sewer. When used in the form of hght-gage sheet, the

protection of steel is a matter requiring serious

consideration. A corrosive attack that would

be relatively insignificant in a heavy steel beam

might perforate a light-gage sheet. In steel houses the initial protection of inaccessible sur- faces such as the interior of walls, is particularly

important since renewal after installation is not

practicable. Condensation within walls has

been greatly increased in modern houses by liigh

humidity caused by the use of winter air-con90695°—38 [1]
ditioning equipment. This is particularly true

in colder climates where parts of insulated walls

may be cooled below the temperature of the

moist inside air for long periods.

II. FACTORS AFFECTING DURABILITY

OF PAINT ON METAL

Painting is one of the cheapest and most

widely used methods for protecting plain steel. Galvanized steel surfaces are often painted also,

both for decoration and increased protection.

An important part of the present investigation

is a laboratory study of the factors affecting the

durability of paint films on steel and galvanized

surfaces under conditions somewhat similar to those which might be encountered in service.

The conditions under which house paints are expected to stand up vary considerably, not

only in different localities, but also in different

parts of the same house. Painted surfaces ex- posed outdoors, for instance, are affected by the

ultraviolet rays of sunlight, moisture in the

form of rain or dew, and the expansion and contraction resulting from temperature changes.

These factors vary according to climatic condi-

,tions. The corrosive action of rain or dew is (determined largely by the amount of impuri-

• ties, such as sulfur dioxide, with which the air may be polluted. The degree of pollution

varies considerably in different localities. In the interior of a house, painted surfaces are not exposed to ultraviolet radiation, but may be

subjected to rather wide variations in temperature and to corrosion by condensed moisture.

Recent tests at this Bureau have demonstrated

the severity of this type of corrosion on unprotected steel and even on galvanized sheet, and

have served to emphasize the need for surface

P'iGURE 1

.

AcceleraU'<l-irc(Ulurinij uppuralaa. The arc lamp has been raised out of the working position in the unit at the right. The cylindrical rack, with the specimens mounted thereon, makes a complete rotation once in 20 minutes. A fixed water spray (tap water) thoroughly wets each panel once every 20 minutes while the

light is continually in operation.

[2]
protection. The corrosive action of condensed

moisture, like that of rain, is influenced by the

impurities in the air from which it is condensed.

Corrosion may also be greatly accelerated by

accumulations of dust or other dirt.

III. METHODS OF TESTING

Some of the conditions mentioned can be simulated to some extent and with more or less accurate control in laboratory tests. In order

to get results within a reasonable length of time,

however, it is necessary to change the conditions

considerably. In the accelerated weathering,

for example, painted specimens are exposed

alternately wet and dry to the heat and the

ultraviolet light of a carbon-arc lamp. The

accelerating factors of this test as compared

with ordinary outdoor exposure are (a) almost

constant exposure to light containing ultraviolet

rays, (b) frequent wetting and drying, (c) fre- quent abrupt changes in temperature, and (d)

liigh humidity.

The experience of this Bureau in conducting

laboratory tests of painted metal panels extending over a period of years indicates that the

accelerated-weathering test often produces within a short time corrosion effects similar to those

produced by prolonged outdoor exposure. Tliis

is particularly true for conditions of exposure in

which the efi'ect of sunlight is the dominant

factor in determining the life of the paint. An

accelerated weathering apparatus similar to that shown in figure 1 is being extensively used

in the present investigation. Two other accel- erated laboratory tests in use are the moisturecondensation corrosion test and the salt-spray

test.The condensation corrosion test was designed

to simulate as nearly as possible the conditions

that may occur within a house, particularly in

enclosed spaces such as the interior of walls. The test consists in exposing steel panels which

have been painted or otherwise treated to the

corrosive action of moisture condensed from a controlled atmosphere. Photographs of the

condensation corrosion cabinet are shown in figures 2 and 3. The specimens rest in a nearly

horizontal position on the cooling tank in the

bottom of the insulated box through which air of controlled temperature and humidity is cir- culated. The degree of purity of the air is also controlled, at least so far as the amount of dust and degree of pollution with sulfur dioxide

are concerned. A test run consists in a series of cycles, each of which embraces a condensa-
Figure 3.

Interior view

of chamber shoiving speci- mens in position.

Note how the specimens have

been intentionally scratched for the test.

tion periotl, a poriotl in which the panels staiul wet, and a drying period. Since tlie composition of the air and the temperatures are such

as might be encountered in service, the only

accelerating factors involved in the test are the

anu)unt of condensed moisture, the length of

time wet, and the comparative frequency and

suddenness of the temperature changes.

The salt-spray test consists in conthuious

exposure to the fine mist or spray produced

from a 20-percent solution of sodium chloride.

The important principles of the testing- apparatus are shown in the drawhigs of figure 4. This type of test, because of its simplicity and

speedy results, is witlely used in laboratory cor- rosion testing, although it has been repeatedly

shown that the results are comparable to those

produced in service only when the conditions

of service include frequent wettuig by salt water

or prolonged exposure to salt spray. Salt- spray tests were included in the present investigation not so nuich for direct comparison with

the corrosive conditions of marine atmospheres,

but more as a comparatively quick and simple means of studymg some of the factors afiecting

the protective value of pamt films, such as

]HM ineability to moisture and inhibitive action

of pigments.

The accelcratctl-weatheruig tests mentioned

above are being supplemented b,y ordinary out- door exposure tests at Washmgton. Part of the exposure test racks are shown in figure 5. Special exposure tests are being made in which

specimens are exposed in ventilated enclosures,

the specimens being shielded from the sun and

direct rainfall but open to the aii" through

louvres and exposed to the condensation of moisture. This test should be similar to the

h-VENT

WATER.-N|

TO COUNTER WEIGHTS

WATER. SEAL

SUSPENDED COVER.-^ ^WINDOW

SATURATING TOWER.^

CLEANING

. TOWER.

LONGITUDINAL

DR-AlM SECTION THR.OUGH SALT- SPRAY 50X Figure 4.

Diagram showing the principle of flic salt- spraij test. A stream of air which has been cleaned and saturated with moisture "atomizes" the test solution (sodium chloride solution) and produces

a mist which completely fills the interior of the cabinet where the speci- mens are exposed.
Figure 5.

Part aj cxpusurc leslfi of painted metal panels on roof of Chemistii/ BuUduiy, National Bureau of Standards.

condensation corrosion test previously described, but probably considerably slower.

The various tests described above should

yield useful information on the protective

value of various pretreatment processes and

paints used for steel and galvanized surfaces. Some of the tests have been used for a number of

years and their limitations, as compared with

actual service conditions, are well understood.

Service conditions vary so widely that actual

service tests in different localities may not

produce comparable results. It is not to be

expected, therefore, that laboratory tests in

which similar conditions are changed to speed

up the action will produce results directly

measurable in term of years of service. However, in the laboratory the conditions of testing

are under control and by varying the conditions

in different tests, it will be possible to study

the factors affecting the protective value of

paints under particular conditions.

Most of the panels are 3 by 6 inches. In the

condensation chamber, larger panels 5 by 7

inches are being used. After pretreatment the

panels are brushed with two coats of primer with

1 week drying time between coats. One-half

of the surface is then given one topcoat of an

outside finish paint. Two diagonal scratches

are made across the front of each specimen. In

most cases test panels are prepared in duplicate.

IV. TESTING PROGRAM

1. Pretreatments for Galvanized Steel

Most of the tests to date have been made on

painted panels of galvanized steel. The painting of a fresh galvanized surface has always

been a difficult problem because of the poor

adherence of paint to the smooth surface and

because of possible reactions between the zinc

and the pigment or veliicle of the paint. Both

difficulties may be overcome to some extent by preliminary weathering, which roughens

the surface and changes it chemically so that

reactions with the paint do not occur so readily.

However, weathering takes at least 6 months

[5]
in most cases, and numerous pretreatments

have been proposed and used for modifying the

surface so that painting may be done at once. The pretreatments so far included in the

present testing program are:

(a) Common hxboratory etching solutions,

including:

(1) Copper sulfate (8 oz per gal);

(2) Zinc sulfate (8 oz per gal);

(3) Alcohol (60%), toluol (30%), car- bon tetracliloride) (5%), hydrochloric acid (5%) by volume;

(4) Saturated solution of nickel sulfate

(10 parts by weight), antimonypotassium tartrate (0.5 parts by

weight), neutralized with ammoPanel A^. Panel B^. Figure 6.

Painted galvanized steel panels after the six-months' accelerated-weathering test.

Panel A^, no treatment prior to painting. Panel B2, surface-treated prior to painting. Two coats of the same primer were used on each panel; a single top coat was applied to the upper half of the panels.

[6]
Panel A9. Panel B9.

Figure 7.

Painted galvanized steel panels after the six-months' accelerated weathering test.

Panel A9, no treatment prior to painting. Panel B9, surface-treated prior to painting. Two coats of the same primer were used on each panel; a single top coat was applied to the upper half of the panels.

nium hydroxide, made slightly

acid with hydrocliloric acid,

and then diluted to 100 parts by

weight with water

;

(5) Phosphoric acid (1 part), alcohol (4 parts), by volume; and

(6) Dilute acetic acid (4 %) by volume.

(b) Proprietary pretreatment processes, in- cluding : (1) Zinc phosphate (hot dip and cold

wash)

;

(2) Iron-zinc phosphate (hot dip);

(3) Barium phosphate (hot dip); and

(4) Acid dichromate (hot dip).

[7]
2. Surface Conditions for Plain Sheet

Steel

The surface conditions of plain sheet steel prior to painting also has a considerable influ- ence on the durability of the paint coatings

applied. Among the surface conditions in- cluded in the testing program are the following:

(a) As rolled (hot-rolled annealed sheet)

;

(b) Acid pickle, including:

(1) Hot hydrochloric acid (20% by

volume)

;

(2) Hot sulfuric acid (10% by volume)

;

and

(3) Hot phosphoric acid (20% by

volume).

(c) Proprietary pretreatment processes, in- cluding:

Panel B2. Figure 8,

Painted galvanized steel panels after the salt-spray test.

Panel AS (4 weeks in spray), no treatment prior to painting; panel BS (6 weeks in spray), surface-treated prior to painting. Two coats of the same

primer were applied to each panel.
Panel A5. Panel Bo. Figure 9.

Painted galvanized steel panels after 18 weeks in the salt-spray test.

Panel AS, no treatment prior to painting. Panel B5, surface-treated prior to painting. Two coats of the same primer were applied to each panel.

(1) Zinc phosphate (hot dip);

(2) Iron-zinc phosphate (hot dip)

;

(3) Barium phosphate (hot dip);

(4) Phosphate cold wash—inhibitive

pigment—aluminum-paint top

coat;

(5) Phosphate-chromate cold wash;

and

(6) Hot dip chromic acid treatment.

3. Protective Coatings

A total of 60 priming paints for steel or galvanized surfaces are being included in the

tests. A small number of different types of

top coats are also being tested.

(a) Pigments used in the priming paints and

top coats mclude the following:

(1) Lead pigments (metaUic lead, lead

[9]
oxides, lead carbonate, chromate,

and sulfate;

(2) Zinc pigments (metallic zinc, zinc

oxide, zinc chromate)

;

(3) Aluminum;

(4) Litliopone (barium sulfate-zinc sulfide);

(5) Carbon pigments; and

(6) Iron oxide.

(b) Paint vehicles include representatives of

the following classes:

(1) Drying oils;

(2) Synthetic varnishes (mostly oleoresinous, phenolic, and alkyd types);

(3) Asphalt varnishes; and

(4) Natural and chlorinated rubber vehicles. o

4. Status of Investigation

A considerable number of accelerated-weathering tests and salt-spray tests have been made

on painted galvanized panels. Outdoor ex- posure tests have been in progress for periods

ranging up to 8 months, while condensation

tests have been running about 2 months.

Photographs of some of the panels after being

tested are shown in figures 6 to 9, inclusive.

The test program has not been completed, and

in the absence of completed results from all types of tests it is not considered advisable to draw definite conclusions at the present time.

It may be stated, however, that some of the

pretreatment processes show considerable promise of increasing the adherence of paints to galvanized surfaces, and that some of the newer

paints may, under favorable condition, give

good protection even on imtreated surfaces.

Washington, August 8, 1938.

[10]
The National Bureau of Standards was established by act of Congress, approved

March 3, 1901, continuing the duties of the old Office of Standard Weights and Measures

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