The program of research on building materials and structm-es, carried on by the

National Bureau of Standards, was imdertaken with the assistance of the Central Housing Committee, an informal organization of Government agencies concerned with

housing construction and finance, which is cooperating in the investigations through a

subcommittee of principal technical assistants.

CENTKAL HOUSING COMMITTEE

SUBCOMMITTEE ON TECHNICAL RESEARCH

Waltee Junge, Chairman. A. C. Shiee, Vice Chairman.

Federal Housing Administration. United States Housing Authority.

Steeling R. Maech, Secretary.

Albeet G. Beae,

Veterans' Administration.

PiEEEE Blouke,

Federal Home Loan Bank Board (Federal Loan Agency).

Caeeoll W. Chambeelain,

Public BuUdings Administration (Federal Works Agency).

Joseph M. DallaValle,

Public Health Service (Federal Security

Agency).

John Donovan,

Farm Seciu-ity Administration (Agriculture).

Geoege E. Knox,

Yards and Docks (Navy).

Vincent B. Phelan,

National Bureau of Standards (Commerce).

Edwaed a. Poynton,

Ofl&ce of Indian Affairs (Interior).

Geoege W. Teayee,

Forest Service (Agriculture).

Elsmeee J. Waltees,

Construction Division (War).

CHAIRMAN OF SECTIONS

Specifications Materials Maintenance

Caeeol W. Chambeelain Elsmeee J. Waltees John H. Schaefee

Mechanical Equipment Methods and Practices

ROBEET K. ThULMAN

NATIONAL BUREAU OF STANDARDS

STAFF COMMITTEE ON ADMINISTRATION AND COORDINATION

Hugh L. Deyden, Chairman

Mechanics and Sound

Phaon H. Bates, Gustav E. F. Lundell,

Clay and Silicate Products. Chemistry.

HoBAET C. Dickinson, Addams S. McAllister,

Heat and Power. Codes and Specifications.

Waeeen E. Emley, Henry S. Rawdon,

Organic and Fibrous Materials. Metallurgy.

The Forest Products Laboratory of the United States Department of Agriculture is cooperating with both committees on investigations of wood constructions.

[For list of BMS publications and how to purchase, see cover page III]
UNITED STATES DEPARTMENT OF COMMERCE • Harry L. 1 lopkins, Secretary

NATIONAL BUREAU OF STANDARDS • Lyman J. Bnggs, Director

BUILDING MATERIALS

and STRUCTURES

REPORT BMS44

Surface Treatment of Steel Prior to Painting

by ROLLA E. POLLARD and WILBUR C. PORTER

ISSUED APRIL 8, 1940

The National Bureau of Standards is a fact-finding organization;

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

UNITED

FOR SALE

STATES GOVERNMENT PRINTING OFFICE • WASHINGTON •

BY THE SUPERINTENDENT OF DOCUMENTS, WASHINGTON, D. C. • PRICE 10

1940

CENTS
Foreword

This report contains the results of a niunber of investigations covering accelerated

laboratory tests designed to deterniuae the relative effect of various surface-treatment

processes on plain or galvanized steel to prevent corrosion. The growing demand for structural sheet steel has indicated the need of better protection for the metal through

improvement of the durability of paint films to insure longer service life. A description of the tests and an explanation of their limitations were published in a previous report, Methods of Investigation of Surface Treatment for Corrosion Protection

of Steel, BMS8.

The present report is the first of a series in which the results of these exj)eriments

will be discussed. Comparison of results is limited to the tests themselves, and conclusions

are drawn on the basis of factors affected by conditions peculiar to the mdividual test. Lyman J. Briggs, Director.

[11]
Surface Treatment of Steel Prior to Painting

by ROLLA E. POLLARD and WILBUR C. PORTER

CONTENTS

Page

Foreword ii L Introduction 1 n. Scope 1 in. Pretreatments for galvanized-steel sur- faces 2 IV. Pretreatments for plain-steel surfaces 2

V. Methods of testing 3

Page VI. Results of tests 3 L Salt spray 3

2. Accelerated-weathering tests 10

3. Condensation corrosion tests 14 VII. Discussion of results 14

VIII. Summary and conclusions 17

1. Galvanized-steel panels 17

2. Plain-steel panels 17

ABSTRACT

A number of surface-treatment processes are described for both plain and galvanized steel. Among

such treatments subjected to accelerated weathering,

salt spray, and condensation corrosion tests, the hot-dip

phosphate treatments for both plain and galvanized

surfaces showed outstanding merit in improving the

protective value of paints. Particularly effective protection was obtained when such treatments were used,

under severely corrosive conditions, in combination

with a primer of the inhibitive type. A phosphate

cold wash for galvanized steel and two phosphateehromate cold-wash treatments for plain steel also appeared to improve paint protection.

I. INTRODUCTION

The increasing use of sheet steel as a structural building material serves to emphasize the

need of improved methods for prolonging the

life of the material by suitable surface treat- ment. During the past 2 years the National

Bureau of Standards has been investigating

this subject as part of a general research pro- gram on building materials for low-cost house

construction. In a previous publication, report

BMS8, Methods of Investigation of Surface

Treatment for Corrosion Protection of Steel, the

testing program was outlined, the testing methods were described, and the pretreatment processes and paints being tested were listed. The testing program consisted of a number of

accelerated laboratory corrosion tests applied

to painted panels of plain and galvanized sheet

steel, supplemented by outdoor exposure tests

of similar materials at Washington, D. C.

Suflicient time has not elapsed as yet for the

outdoor exposure tests to produce significant

results. The condensation corrosion tests also have been rather slow, results to date being

limited largely to paints applied to untreated

galvanized-steel panels. However, results have

been obtained in a shorter time from the salt- spray and accelerated-weathering tests, and a

considerable amount of useful information has

been accumulated. It is believed that some of

these results may be discussed with profit if comparisons are limited to the tests themselves and conclusions are drawn on the basis

of factors aft'ected by conditions peculiar to the

individual test.

II. SCOPE

One of the chief objectives of the investigation has been to study the various methods

used in attempting to improve the protective

value of paints applied to steel. One such

method is to treat the surface prior to painting,

in order to provide an improved base for the

paint ; another method is to improve the pamt

itself. The present report will be concerned

chiefly with the first, or pretreatment, method

of improvement. This report is limited to the

description of the pretreatment processes tested,

and to the discussion of the relative merits of

such processes as measured by the protective

value of paints subsequently applied and then

tested in the salt-spray cabinet, acceleratedweathering machine, and condensation cor- rosion cabinet. Some of the factors affecting

[1]
the protective value of the paints themselves

will be considered, in a later publication.

The panels used in tests of the pretreatment

processes covered by the present report were of

plain hot-rolled annealed sheet steel or of hotdip zinc-coated (galvanized) sheet steel.

III. PRETREATMENTS FOR GALVANIZED-STEEL SURFACES

The object of pretreating galvanized steel is to improve the surface for painting. For ex- ample, paint adherence can be improved by

roughening the surface mechanically or chemically or by the deposition of a film, preferably

nonmetallic, which improves the bond between

paint and metal. It is desirable that the fihn

formed should be one that prevents reaction

between the zinc and the paint vehicle and re- tards corrosion by reason of its insolubility, its inhibitive action, or its reaction with corrosion

products to form an insoluble film. The following pretreatment processes for galvanized steel, designed to effect some of these

improvements, were included in the testing

program:

List 1.

Pretreatment processes for galvanized

steel

A. Untreated hot-dip galvanized surface.

B. Proprietary phosphate treatment. Cold

wash in aqueous zinc-phosphate solution containing free phosphoric acid and

an activating agent.

C. Etching solution. Water containing 8 oz. of copper sulfate per gallon.

D. Etching solution. Water containing 8 oz. of zinc sulfate per gallon. E. Etching solution. Alcohol, 60; toluol,

30 ; carbon tetrachloride, 5 ; and hydrochloric acid, 5 ; parts by volume.

F. Etching solution. Saturated aqueous so- lution of nickel sulfate, 10 parts by

weight, antimony-potassium tartrate,

0.5 part by weight, neutralized with

ammonium hydroxide, made slightly

acid with hydrochloric acid, and then

diluted to 100 parts by weight with

water.

G. Etching solution. Alcohol, 4 ; phosphoric

acid (85 percent), 1; part by volume.

H. Proprietary phosphate treatment. Hot

dip in aqueous zinc-phosphate solution

containing free phosphoric acid and an

activating agent.

/. Laboratory etching solution. Dilute acetic acid, 4 percent by volume.

J. Proprietary phosphate treatment. Hot

dip in aqueous zinc-phosphate solution

containing free phosphoric acid and an

activating agent.

K. Proprietary oxalate-phosphate treat- ment. Hot dip in an aqueous solution

containing an oxalate and phosphoric

acid. L. Proprietary dichromate treatment. Cold

immersion in a sodium-dichromate aqueous solution containing sulfuric acid.

IV. PRETREATMENTS FOR PLAINSTEEL SURFACES

The adherence of paint to a clean, dry steel surface is generally very good. An important

objective to be sought in any pretreatment

process, therefore, is to obtain a clean, dry

surface free of rust and loose scale, or to deposit a film having equal or superior adherence, which

will prevent or retard corrosion underneath the

paint.

The following pretreatment processes, which

were applied to plain sheet steel for the purpose

of attaining some of these objectives, were in- cluded in the testing program:

List 2.

Pretreatment processes for plain-steel

surfaces

Fe. No treatment. Painted over mill scale. Fe-A. Pickled in hot aqueous solution of hydrochloric acid (20-percent con- centrated acid by volume).

Fe-B. Proprietary phosphate treatment. Hot dip in an aqueous zinc-phosphate solution containing free phosphoric acid and an activating agent.

Fe-C. Proprietary chromate treatment. Hot

dip in an aqueous chromic-acid

solution containing activating

agents.

Fe-D. Proprietary phosphate-chromate treat- ment. Cold wash in an aqueous

solution containing phosphoric acid,

phosphates, and cliromates.

[2]
Fe-E. Proprietary phosphate-clu'omate treat- ment. Cold wash in an aqueous

solution containing phosphoric acid,

phosphates, and chromates—generally used with an inhibitive primer.

Fe-H. Proprietary phosphate treatment. Hot dip in an aqueous zinc-phosphate solution containing free phosphoric acid and an activating agent.

Fe-K. Proprietary oxalate-pliosphate treat- ment. Hot dip in an aqueous solution containing an oxalate and

phosphoric acid.

Fe-M. Proprietary phosphate treatment. CoM wash in an aqueous solution of

phosphoric acid containing a wetting agent.

Fe-P. Piclded in hot aqueous solution of

phosphoric acid (10-percent sirupy

acid by volume)

Fe-S. Pickled in hot aqueous solution of

sidfuric acid (20-percent concentrated acid by vohmie)

V. METHODS OF TESTING

After pretreatment, the plain-steel or galvanized-steel panels were brushed, as a rule,

with two coats of primer, a week being allowed

for drying between application of coats. One

haK of the surface of each panel was then given a top coat of outside finish paint. Most of the

j painted panels were scratched diagonally before

'being tested. All panels were 3 by 6 inches

I except those for the condensation cabinet,

which were 5 by 7 inches.

Galvanized-steel panels are identified in this

•1 report by letters representing the surface treat- ments given in list 1. For the plain-steel

panels the letters are preceded by the symbol

\ Fe, as shown in list 2. Following each letter is a number identifying the primer with which the

panel is painted. For example, designation A-1

represents an untreated galvanized-steel panel

jcoated with primer 1 out of a group of some 80

rimers and paints. Similarly, panel Fe-A-1

represents a plain-steel sample pickled in hot

Lydrochloric acid and coated with primer 1. Panels representing aU the pretreatment

recesses Hsted above for galvanized-steel sur-

.ifaces have been tested by salt spray and by

accelerated weathering. A number have also

been tested in the condensation corrosion

cabinet. Tests of plain-steel panels have been more limited but include salt-spray and accelerated-weathering tests of samples I'epresenting

all the pretreatment processes listed.

VI. RESULTS OF TESTS

1. Salt Spray

The salt-spray test as used in this work

consisted in the continuous exposure of painted

panels to the fine mist or spray produced by

"atomizing" a 20-percent aqueous solution of

sodium chloride at 35° C.

The principal factors which accelerate breakdown of paint on galvanized steel in this test seem to be moistm-e penetration and action of

the chloride ion. The moisture, of course, causes corrosion of the metal and promotes

reaction between metal and paint or between

corrosion product and paint. These reactions

probably axe accelerated by the chloride ion.

The paints which would be expected to stand

up best in the salt spray are those which resist

moisture penetration or those which contain an

inhibitive pigment, such as zinc chromate.

The pretreatments for galvanized-steel surfaces which gave the greatest improvement to the protective value of paints subjected to salt- spray tests were the hot-dip phosphate treat- ments H and J. These treatments appeared to be about equal in value and were definitely

superior to all others tested. A panel which was given treatment J and then covered with

primer 8 is shown in J-8 of figure 1. The sur- face of the panel was in essentially perfect

condition after 365 days in the salt spray. The

phosphate cold-wash treatment B was foimd

to be slightly inferior to the hot-dip treatments.

Panel B-8, as shown in figure 1, was stiU in

excellent condition after 104 days in the spray. The acid-dichromate treatment L, probably be- cause of its inhibitive action, also gave very good

results in this test and was in many cases, almost

equal to treatments Hand J. As will be shown

later, however, this treatment has some dis- advantages in other tests and, therefore, is considered separately. The two left-hand panels in figure 2 show the relative protective value

of paint 1, after testing in the salt spray, when

applied to an untreated galvanized surface

[3]
[4]

(panel A-1) and when applied over treatment L

(panel L-1). It will be noted that panel L-1 was in much better condition than panel A-1

after exposures of 61 and 28 days, respectively.

The other pretreatments listed for galvanizedsteel surfaces were found to be of questionable

value for the improvement of the protective

value of paints in the salt-spray test. Treatment K, for example, appeared to improve or, rather, prolong the protective value of some of

the inhibitive primers but not of other types.

In figure 3 the two left-hand panels show the

marked improvement obtained with inhibitive

primer 5 applied over treatment K (panel K-5)

as compared with the same primer applied to an untreated galvanized surface (panel A-5).

With noninhibitive primer 6 no improvement

was obtained, as shown by panels A~6 and K-6.

Treatments C, D, E, F, G, and /, classed to- gether as etching solutions, gave no evidence of

improvement over comparison panels of untreated material. An example of treatment E

of this group is shown in panel E-8 of figure 1 in comparison with the untreated panel A-8

after exposures of 123 days and 104 days,

respectively.

Salt-spray tests of plain-steel panels completed to date include at least two different

primers on all the surface treatments listed. In these tests the hot-dip phosphate treatments

Fe-B and Fe-Hha,Ye been found consistently the

best in improving the protective value of paints

applied over them. Figure 4 is an example of

treatment Fe-B (panel Fe-B-29) as it appeared after 202 days in the salt spray. It was removed at that time to be photographed

and was then returned to the spray. Since

there was no change in the appearance of the

specimen after the first 40 days of the original

exposure, it is probable that the test will continue for a considerable length of time. The

phosphate-chi-omate cold-wash treatments Fe-D

and Fe-E also have given good results in the

salt-spray test. Panel Fe-D-29, representative of these treatments, appears in figure 4. After the same period of exposure it was almost

as good as the panel {Fe-B-29) treated by the

hot-dip phosphate solution. With other paints,

however, this treatment gave slightly inferior

results. The hot-dip chromic-acid treatment

Fe-C showed considerable improvement over

the plain pickled surface treatment Fe~A, but was decidedly inferior to the treatments mentioned above, as shown by panels Fe~C-29 and

Fe-A-29 (fig. 4) as they appeared after 110 . and 48 days, respectively. The other treat- ments {Fe-K, Fe-P, Fe-M, and Fe-S) listed

'

for plain steel, showed no advantage over treat- ment Fe-A (pickled surface) in improving the

protective value of paint applied over them.

In testing the plain-steel panels comparisons were made with the plain piclded surface i (treatment Fe-A), because on the original hotrolled annealed sheet material there was con-

| siderable variation in the amount and distribution of the surface mill scale. On one side of

the sheet the scale appeared to be tight and

fairly uniform, whereas on the other side large areas appeared to have no scale at all. Test

results to date have been inconclusive in re- gard to the relative merit of paint applied over

mill scale as compared with a pickled surface.

However, in short-time tests under severely

corrosive conditions, such as the salt spray, it appears that there are no outstanding advantages in either of these conditions, as is shown

in figure 5. This figure shows the results of

salt-spray tests on selected panels which had

tight, uniform mill scale on the side marked h and

practically no mill scale on the side marked y, There appeared to be no particular advantage

in painting over mill scale (panel Fe-l-h) as compared with a sm-face practically free of

mill scale (panel Fe-1-^). Moreover, the hotdip phosphate treatment (Fe-H) appeared to be equally effective whether applied directly over mill scale (panel Fe-H-l-b) , on a surface

originally free of mill scale (panel Fe-H-l-y),

or on a pickled surface (panel Fe-A-H-1).

Other tests, in which direct comparisons of

paints on mill scale or scale-free surfaces are being made, have not been completed, and

definite conclusions cannot be drawn.

That the presence of initial surface rust has a much greater effect on paint than the presence

of mill scale is illustrated in figure 6. The two

right-hand panels of figure 6 were exposed out- doors until covered with rust. Panel R-Fe-5a

was then painted directly over mill scale and

rust. This panel was decidedly inferior in the

salt-spray test to panel Fe-5a, which was

painted over unrusted mill scale, and to panel

[6]

[8]
[9]
Fe-A-5a, painted on a pickled surface. The

other rusted panel, R-Fe-E-5a, was given the

phosphate-chromate cold-wash treatment Fe-E

prior to painting. The treatment evidently

removed some of the rust and improved the

surface for painting. However, the same treat- ment was much more effective when applied to a surface with unrusted mill scale (panel

Fe-E-5a). Superior results were also obtained with the hot-dip phosphate treatment on

a scale-free and rust-free surface (panel FeH-5a) . All panels in figure 6 were covered with an aluminum top coat over primer 5a. These

panels were not scratched.

2. Accelerated-Weathering Tests

In the accelerated-weathering test, painted

panels are exposed, alternately wet and dry, to the heat and light of a carbon arc lamp. During a 20-muiute cycle the panels are exposed to the light for more than 18 minutes and are heated to about 54° C. They are then suddenly wet by a water spray atl5°to21°C and

are quickly dried off when they come under the

light again to complete the cycle. Important

factors affecting the brealvdown of paint in this test are the action of light and the sudden

change in temperature. It is believed that

actual corrosion of metal due to moisture under

the paint film is relatively imimportant.

The paints which stand up best under the

accelerated-weathering test are those which

have good adhesion and are opaque to light.

Good adhesion is particularly important when

primers and covering coats with different coefficients of expansion or difi^ercnt drying characteristics are used. A good test for adhesion

of a primer is to use a hard-drying top coat,

which, by drying at a rate different from that

of the primer, tends to loosen the priming coat from the metal.

Opacity to light is, of course, a property of

the paint itself. Adherence of the paint film,

however, is markedly affected by the condition

of the metal surface. Among the surface treat- ments listed for galvanized-steel panels, the

hot-dip phosphate treatments H and J stood up

best in accelerated-weathering tests, as shown

in figure 7 (panel H-5). It will be noted that

after 12 months' exposure most of the harddrying top coat on the top half of the panel had

chalked off, but the priming coat remained in- tact over the entire surface. In contrast, the same primer on the imtreated panel A~5 (fig. 7) was pulled off entirely by the top coat after 6

months and was also peeled on the bottom half

in large areas at the edges subjected to abrasion

by the vertical slots in which the panel rested

during testing. The cold-wash phosphate treat- ment B stood up very well in the acceleratedweathering test, as shown by the appearance of panel B-o in figure 7. After 6 months' exposure

the top coat pulled the primer off slightly along

the scratch, but all other areas were in good

condition. Panel D-5 (fig. 7) is representative

of a group of treatments (C, D, E, F, G, and K)

in which the improvement over an untreated

galvanized surface was very slight or entirely

negligible. Treatments / and L showed no improvement over untreated galvanized surfaces

in the accelerated-weathering tests. In figure 2

the two right-hand panels show primer 1 over

the acid-dichromate treatment, L, as compared

with the same primer on untreated galvanized

steel. Although a much softer black top coat was used on panel L-1, no improvement was

observed over the imtreated panel A-1 exposed

for a longer time.

Accelerated-weathering tests of steel panels

completed to date include only one series in which all the surface treatments listed have been

represented. The results of this test (of 9

months' duration) are illustrated in figure 8, in

which panel Fe-B-1 repres nts the hot-dip

phosphate treatments Fe-B and Fe-H, which

again stood up best. These treatments were

followed closely by the phosphate-chromate

cold washes Fe-D and Fe-E, represented in figure 8 by panel Fe~D-l . The hot-dip chromicacid treatment Fe-C gave slight improvement

over treatment Fe-A (panels Fe-C-1 and

Fe-A-1, fig. 8). The primer used on these

panels chalked badly in areas not covered with

top coat. The lower parts of the panels there- fore appear white except where the chalked

material has been rubbed off by handling. The

places which appear black should not be con- fused with the dark areas on the top half of the

panels representing spots where the top coat cracked and flaked ofi^ and took the priming coat with it. None of the other surface conditions listed

[10]


[13]
for steel panels (treatments Fe, Fe-K, Fe-P,

Fe-M, and Fe-S) has shown any improvement

over treatment Fe-A (acid piclde) in the one

accelerated-weathering test that has been completed. Other tests are in progress.

3. Condensation Corrosion Tests

The condensation corrosion test was designed

to simulate as nearly as possible the conditions

that may occur in enclosed spaces, such as the

interior of the walls of a house. The test consists essentially in exposing the prepared

panels to the corrosive action of moisture con- densed from a controlled atmosphere. The

panels rest in a nearly horizontal position on

the surface of a water tank forming the bottom

of an insulated box through which air of controlled temperature and humidity is circulated.

A test run includes a series of cycles, each of

which embraces a condensation period of

approximately 6 hours, an overnight period of 17 hours, during which the panels remain wet,

and a final 1-hour drying period. During the

condensation period the panels are maintained

at a temperature of 15° to 20° C by water

circulated through the tank, while the air circulated through the box is saturated at about 35° C. During the overnight period

circulation of air is stopped, but the cabinet

remains closed and the water tank is maintained

at a temperature varying between 10° and 25° C

by tap water. During the 1-hour drying

period hot water is circulated through the tank

and the panels are heated to about 40° C.

The principal factors causing deterioration

of paint coatings in the condensation corrosion

test are moisture penetration and rapid changes

in temperature. Paints that would be ex- pected to stand up best under these conditions

are those which resist moisture penetration or contain an inhibitive pigment and have good

adhesion. This has been confirmed by the

test results to date, in which failures have been

due either to corrosion under the paint or to flaking through loss of adhesion.

Condensation corrosion tests so far have

been confined to painted galvanized-steel panels. These panels include representatives of

treatments A, H, J, and L. Of 14 different

primers on untreated panels (treatment A) all but two have failed after periods varying from

5 to 16 months. Of 14 similar primers applied over each of the hot-dip phosphate treatments

H and J, only one of treatment H and one of

treatment J have failed during this time.

Neither of two primers over the acid-dichromate

treatment L has failed after 16 months' ex- posure. In figure 9, primers 9 and 14 are shown on untreated galvanized-steel surfaces

(panels A-9 and A-14) as they appeared after exposures of 10 and 5 months, respectively.

Marked improvement was shown by the same

primers over the hot-dip phosphate treatment

H (panels H-9 and H-14, fig. 9) after exposures

of 16 and 15 months, respectively.

VII. DISCUSSION OF RESULTS

As far as the results of accelerated laboratory

corrosion tests are concerned, it is evident that

surface treatments for plain steel or galvanized

steel may be classified in several distinct groups

according to their effectiveness in improving

the protective value of paints applied over them.

For example, surface treatments for galvanized

steel (list 1 ) may be rated by groups in decreasing

order of merit as follows:

Group 1. Treatments H and J. Best in all tests to date; marked improvement over untreated galvanized

steel. Group 2. Treatment B. Marked improvement, but slightly less than

group 1 in all tests. Group 3. Treatments K and L. Improvement marked or negligible, depending on the type of paint

and particularly the conditions

of testing.

Group 4. Treatments C, D, E, F, and G. Improvement slight or negligible, depending on type of paint and conditions of testing.

Group 5. Treatments A and /. No improvement.

The hot-dip phosphate treatments H and J

(and cold treatment B in less degree) showed

outstanding merit in all the accelerated laboratory tests. Improvement in protective value of paints in the accelerated -weathering test may

be attributed chiefly to improved adherence.

Under severely corrosive conditions, such as the

salt spray, the phosphate film deposited by

[14]

i
[15]
these treatments apparently aids materially in

slowing down corrosion imderneath the paint.

This action is particularly effective when combined with that of an inhibitive pigment in the

priming coat. It is possible that the phosphate

film acts as a sort of anchor for the inhibitive

pigment, wliich, to be very effective, must be

slightly soluble.

The chromate film deposited by treatment L

apparently has some inhibitive action. Improvement in paint protection as a result of this

treatment is particularly marked with paints

that have little or no inhibitive value in themselves. However, this film seems to be slightly

soluble, and after an initial period of complete

protection the paints applied over it may fail rapidly at edges and scratches through dissolution of the chromate film and loss of adhesion.

A treatment of this type which is protective in

itself for considerable periods under severely

corrosive conditions would be very desirable if it could be achored more securely both to the

metal and to the applied paint. Some experiments along the line of combining inhibitive

action and adherence in films have been made,

but the results to date have not been very

promising.

In all the laboratory tests for galvanized steel discussed above, the smooth spangled surface

of hot-dip galvanized sheet was used. The

test results indicate that good treatments for

this material do more than merely roughen the

surface to hold paint. The best pretreatments

so change the surface chemically as to prevent

reaction between metal and paint, increase the

adhesion of the applied paint, and prevent or slow down corrosion under the paint film. It

is claimed that other types of metallic zinc

coatings—such as zinc-sprayed, electro-galvanized, or annealed hot-dip galvanized materials—have acceptable surface characteristics

and do not require special treatment prior to painting. Tests on these materials have not

progressed far enough as yet to warrant definite

conclusions being drawn.

According to the results of the accelerated

laboratory tests, surface treatments for plain

steel (list 2) prior to painting can be rated in

the following decreasing order of merit with

regard to effectiveness in improving the protective value:

Group 1 . Treatments Fe-B and Fe-H. Best

in aU tests to date; marked

improvement over plain pickled

surface.

Group 2. Treatments Fe-D and Fe-E. Improvement slightly less marked

than group 1 in all tests. Group 3. Treatment Fe-C. Improvement

slight or marked, depending on

conditions of testing.

Group 4. Treatments Fe, Fe-A, Fe-K, Fe-M,

Fe-P, and Fe-S. No improvement compared with plain pickled

surface.

These results are in agreement with those

obtained on galvanized-steel surfaces. The

best results to date, for example, have been

obtained with phosphate films produced by the

hot-dip processes Fe-B and Fe-H, closely

followed by the phosphate-chromate cold washes

Fe-D and Fe-E. Both types of treatment

appear to improve paint adhesion and to slow

clown corrosion under paint.

The chromate film deposited by treatment

Fe-C apparently has considerable inhibitive

value under severely corrosive conditions (saltspray test, fig. 4), but its effect on paint adhesion is not marked (accelerated-weathering

test, fig. 8).

The nature of the different acids used in the

pickling treatments so far tried (treatments

Fe-A, Fe-P, and Fe-S) does not seem to in- fluence the results; that is, the pickled surface

behaves the same regardless of the acid used.

A variation of the phosphoric acid pickle, by

which a phosphate film is said to be deposited,

is being tested. This treatment, recommended

by The Corrosion Committee of the British

Iron and Steel Institute (5th Report), consists

of an initial pickle in sulfuric acid to remove

scale, followed by treatment in hot dilute

phosphoric acid containing dissolved iron.

The relative effect of scratches and edges is noteworthy in the tests on both plain- and

galvanized-steel panels. Defects, such as staining and blistering, developed along and spread

from the scratches. In many cases, however,

actual failures, such as peeling or disintegration

of paint, seem to have been influenced more by

the edges than by the scratches. In view of

the fact that no precautions were taken against

[16]
I edge effect other than to apply approximately

I the same amoimt of paint as in other areas, the

results obtained by some combmations of pretreatments and priming paints are considered

remarkable for both plain- and galvanized-steel

' surfaces.

VIII. SUMMARY AND CONCLUSIONS

1. Galvanized-Steel Panels

A number of accelerated-weathering, salt- spray, and condensation corrosion tests have

been made on painted galvanized sheet-steel

panels. The results of these tests, insofar as

j they are affected by surface treatment of the

I material, may be briefly summarized as follows:

I (a) The hot-dip phosphate treatments (H

I and J) and the cold-wash phosphate treatment

j (5) improved both the adhesion and the corroj sion protection of paints under widely varyuag

conditions. The hot-dip treatments apparently were slightly superior to the cold wash.

(b) The acid-dichromate treatment (L) apparently had considerable protective value in

itself and had an inhibitive action under paints

I but did not improve their adherence,

i (c) The oxalate-phosphate treatment (K)

prolonged the protective value of inhibitive

paints under severely corrosive conditions (salt

i spray) but had little effect on other types, and

did not improve adhesion to any appreciable

degree.

(d) All other pretreatments tested were found

to have slight or negligible effect in improving

the protective value of paints.

(e) Tests of other types of zinc-coated material are being conducted but have not progressed

far enough to warrant conclusions being drawn.

These materials include zinc-plated, zincsprayed, and aimealed hot-dip galvanized

surfaces.

2. Plain-Steel Panels

Salt-spray tests of painted steel panels include

at least two types of paint for all surface conditions listed. Accelerated-weathering tests in- clude at least one paint for all pretreatments.

The results of these tests may be briefly svun- marized as follows:

(a) The hot-dip phosphate treatments {Fe-B

and Fe-H) markedly improved the protective

value of paints in both tests. (b) The phosphate-chromate cold-wash treat- ments {Fe-E and Fe-D) were only slightly

inferior to the hot-dip phosphate treatments in

improvement of paint protection.

(c) The hot-dip chromic-acid treatment

(Fe-O) improved paint protection as compared

with a plain pickled surface, but the improvement was not as marked as that obtained with

the treatments mentioned above.

(d) All other treatments tested were found to have slight or negligible effect in improving the

the protective value of paints. This includes

various types of plain pickled surfaces but does

not include a type of pickling which is said to

result in the deposition of a phosphate film.

(e) Tests to determine the effect of mill scale on the protective value of paints in short-time

tests have not been completed. However, some

of the pretreatments, particularly those mentioned under (a) and (b), have given good

results either over mill scale or on scale-free surfaces.

(f) Under severely corrosive conditions (saltspray test) the protective value of paints applied on rusted surfaces was decidedly inferior to that of paints applied on plain pickled surfaces.

Some of the pretreatments apparently were able

to remove light rust. However, they improved

paint protection much more effectively when

used on a rust-free surface.

Washington, October 2, 1939.

o

[17]

BUILDING MATERIALS AND STRUCTURES REPORTS

On request, the Superintendent of Documents, U. S. Government Printing Office, Washington, D. C, will place your name on a special mailing list to receive notices of new reports in this series as soon as they are issued. There will be no charge for receiving such notices. An alternative method is to deposit with the Superintendent of Documents the sum of $5,

with the request that the reports be sent to you as soon as issued, and that the cost thereof be

charged against your deposit. This will provide for the mailing of the publications without

delay. You will be notified when the amount of your deposit has become exhausted.

If 100 copies or more of any paper are ordered at one time, a discount of 25 percent is allowed. Send all orders and remittances to the Superintendent of Documents, U. S. Government Printing

Office, Washington, D. C.

The following publications in this series are now available by purchase from the Superinandent of Documents at the prices indicated:

BMSl Research on Building Material^ and Structures for Use in Low-Cost Housing lOift BMS2 Methods of Determining the Structural Properties of Low-Cost House Constructions 10(4 BMS3 Suitability of Fiber Insulating Lath as a Plaster Base 100 BMS4 Accelerated Aging of Fiber Building Boards 100 BMS5 Structural Properties of Six Masonry WaU Constructions 150 BMS6 Survey of Roofing Materials in the Southeastern States 150 BMS7 Water Permeability of Masonry Walls 100 BMS8 Methods of Investigation of Surface Treatment for Corrosion Protection of Steel 100 BMS9 Structural Properties of the Insulated Steel Construction Company's "Frameless-Steel" Constructions for Walls, Partitions, Floors, and Roofs 100 BMSIO Structural Properties of One of the "Keystone Beam Steel Floor" Constructions Sponsored by the H. H. Robertson Company 100 BMSll Structural Properties of the Curren Fabrihome Corporation's "Fabrihome" Constructions for Walls

and Partitions 100 BMS12 Structural Properties of "Steelox" Constructions for Walls, Partitions, Floors, and Roofs Sponsored

by Steel Buildings, Inc 150 BMS13 Properties of Some Fiber Building Boards of Current Manufacture 100 BMS14 Indentation and Recovery of Low-Cost Floor Coverings 100 BMS15 Structural Properties of "Wheeling Long-Span Steel Floor" Construction Sponsored by Wheeling

Corrugating Company 100 BMS16 Structural Properties of a "Tilecrete" Floor Construction Sponsored by Tilecrete Floors, Inc 100 BMS17 Sound Insulation of Wall and Floor Constructions 100 BMS18 Structural Properties of "Pre-Fab" Constructions for Walls, Partitions, and Floors, Sponsored by the Harnischfeger Corporation 100 BMS19 Preparation and Revision of Building Codes 150 BMS20 Structural Properties of "Twachtman" Constructions for Walls and Floors Sponsored by Connecticut

Pre-Cast Buildings Corporation 100 BMS21 Structural Properties of a Concrete-Block Cavity-Wall Construction Sponsored by the National Concrete Masonry Association 100 BMS22 Structural Properties of "Dun-Ti-Stone" Wall Construction Sponsored by the W. E. Dunn Manufacturing Company 100 BMS23 Structural Properties of a Brick Cavity-Wall Construction Sponsored by the Brick Manufacturers

Association of New York, Inc 100 BMS24 Structural Properties of a Reinforced-Brick Wall Construction and a Brick-Tile Cavity-Wall Construction Sponsored by the Structural Clay Products Institute 100 BMS25 Structural Properties of Conventional Wood-Frame Constructions for Walls, Partitions, Floors,

and Roofs 150 6MS26 Structural Properties of "Nelson Pre-Cast Concrete Foundation" Wall Construction Sponsored by

the Nelson Cement Stone Company, Inc 100 BMS27 Structural Properties of "Bender Steel Home" Wall Construction Sponsored by The Bender Body

Company 100 BMS28 Backflow Prevention in Over-Rim Water Supplies 100 BMS29 Survey of Roofing Materials in the Northeastern States 100 BMS30 Structural Properties of a Wood-Frame Wall Construction Sponsored by the Douglas Fir Plywood

Association 100 BMS31 Structural Properties of "Insulite" WaU and "Insulite" Partition Constructions, Sponsored by The

Insulite Company 150 BMS32 Structural Properties of Two Brick-Concrete-Block Wall Constructions and a Concrete-Block Wall

Construction Sponsored by the National Concrete Masonry Association 100

(List continued on back page of cover)
BMS33 Plastic Calking Materials 10(1 BMS34 Performance Test of Floor Coverings for Low-Cost Housing: Parti IC

* BMS35 Stability of Sheathing Papers as Determined by Accelerated Aging 1( BMS36 Structural Properties of Wood-Frame Wall, Partition, Floor, and Roof Construction with "Red

Stripe" Lath Sponsored by The Weston Paper and Manufacturing Co 10j5 BMS37 Structural Properties of "Palisade Homes" Constructions for WaUs, Partitions, and Floors, Sponsored

by Palisade Homes 10^ BMS38 Structural Properties of Two "Dunstone" Wall Constructions Sponsored by the W. E. Dunn Manufacturing Co 10^ BMS39 Structural Properties of a Wall Construction of "Pfeifer Units" Sponsored by the Wisconsin Units Co.

BMS40 Structural Properties of a Wall Construction of "Knap Concrete WaU Units" Sponsored by Knap M

America, Inc 1(m

BMS41 Effect of Heating and Cooling on the Permeability of Masonry Walls IQm

BMS42 Structural Properties of Wood-Frame WaU and Partition Constructions With "Celotex" Insulating > Boards Sponsored by the Celotex Corporation lOji: BMS43 Performance Test of Floor Coverings for Use in Low-Cost Housing: Part 2 lO^i BMS44 Surface Treatment of Steel Prior to Painting lOjJ,'