National Bureau of Standards

Library, N.W. Bldg

BUILDING SCIENCE SERIES

I iUi t'l •-•i ;V, »^ L::!( not, to be

taken from, ilio library.

Weather Resistance of

Porcelain Enamels

Effect of exposure site and

other variables after 7 years

U.S. DEPARTMENT OF COMMERCE

National Bureau of Standards
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UNITED STATES DEPARTMENT OF COMMERCE • John T. Connor, Secretary

NATIONAL BUREAU OF STANDARDS • A. V. Astin, Director

Weather Resistance of Porcelain Enamels

Effect of Exposure Site and Other Variables After Seven Years

Margaret A. Rushmer and Milton D. Burdick

Building Research Division

Institute for Applied TechnologyNational Bureau of Standards

Washington, D.C.

Building Science Series 4

Issued May 2, 1966

For sale by the Superintendent of Documents, U.S. Government Printing Office Washington, D.C.» 20402 - Price 20 cents
CONTENTS

Page

1. Introduction 1 2. Description of enamels and specimens 8

3. Exposure sites 8

4. Results and discussion 9

4.1. Cleaning of specimens 9

4.2. Corrosion of the base metal 9

4.3. Changes in gloss and color 9

4.4. Changes in gloss and color with exposure time 9

4.5. Comparison of exposure sites 10

4.6. Comparison of enamel types 12

4.7. Correlation of acid resistance with weather resistance 12

4.8. Cupric sulfate test for predicting color retention of red and

yellow enamels 14

4.9. Comparison of present-day enamels with those produced before

World War II 15

5. Conclusions 15

6. Summary 16

7. References 16

Library of Congress Catalog Card Number : 65-62473
Weather Resistance of Porcelain Enamels

Effect of Exposure Site and Other Variables After Seven Years

Margaret A. Rushmer and Milton D. Burdick *

An exposure test of porcelain enamels at seven representative sites in the continental United States was initiated by the National Bureau of Standards and the Porcelain Enamel

Institute in 1956. After seven years all specimens were returned to the Bureau and the changes In gloss and color determined. These changes were found to be different at all exposure sites except Pittsburgh and New Orleans. The most severe changes occurred for specimens exposed at Kure Beach, 80 feet from the ocean, while the least change occurred

for specimens exposed at Los Angeles. The differences in behavior of the specimens cor- related with both the relative humidity and the pH of the suspended particulate matter

at the different sites. A direct relation existed between the acid resistance of the enamels and weather

resistance. However, enamels of different types, such as enamels on aluminum and steel, having the same acid resistance did not necessarily show the same weather resistgince.

Comparison with enamel specimens exposed for seven years in an earlier test showed

that porcelain f^namels produced after the end of World War II were equally resistant

to changes in g ss at the Washington, D.C., site as those produced before the war. As a group, the regular glossy acid-resistant enamels on steel showed the best weather

resistance of the various types tested. No corrosion of the base metal was noted for any

specimen on which the initial coverage was complete.

Key words : Acid resistance, color, gloss, pH, porcelain enamel, relative humidity,

weather resistance.

1. Introduction

The weather resistance of porcelain enamels is important to architects and engineers who have

the responsibility of selecting exterior finishes for

various types of buildings, as well as to owners who are interested in obtaining a long, maintenance-free life from the finish. That some porcelain enamels have excellent weather resistance is widely recognized from inspection of early installations. It is also recognized that not all enamels

have this durability. Exposure test data are needed on new enamel types as they are developed,

and, in addition, there is a need for reliable lab- oratory tests that will predict the weathering

behavior of new enamels.

The National Bureau of Standards first recog- nized these needs of the building industry in 1939.

An investigation started at that time resulted in a

series of reports issued after exposure periods of

1 [1],** 7 [2], and 15 [3] years. The principle

finding of that investigation of porcelain enamels on steel, was that the best enamels showed no objectionable changes in either gloss or color at any of

Junior and Senior Research Associate, respectively, for the Porcelain Enamel Institute at the National Bureau of Standards. •Figures in brackets indicate the literature references at the end of this report.

the four selected exposure sites after exposure for

15 years and, further^ that the enamels with poor

resistance could be eliminated from consideration

by relatively simple laboratory tests. After World War II, several new enamel types were introduced and it became important to determine if these new types followed the same pattern,

with respect to weathering, as those produced

earlier. Such information was of added importance because of the rapidly expanding use of porcelain enamel in modern architectural design,

especially in those buildings employing curtain

wall construction. Thus, in 1956, a second test was

initiated by the National Bureau of Standards and

the Porcelain Enamel Institute Research Associateship at the Bureau. Progress reports of the

test have been published after 1 [4] and 3 [5]

years' exposure.

The present paper is a progress report after 7

years' exposure. For ease of comparison, the same

general format is followed as was used in the 3- year report [5]. Also, as an aid to the reader, the

description of specimens and exposure sites is re- peated in this paper.

The next inspection of these enamels is tentatively planned after 15 years" exposure.

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2. Description of Enamels and Specimens

The 28 types of enamel included in this test were

furnished to 21 producers of architectural enamel

parts by seven companies who manufacture enamel

frits.^ The producers then applied the enamel to the test specimens. Table 1 not only includes

coded identification of the frit suppliers and fabricators, but also lists the types of enamels in- cluded. Minor variations in composition of the

frit for enamels of the same type, as supplied by

different manufacturers, probably occurred.

Likewise, in many cases, different producers applying the same enamel may not have achieved an

identical finish, because of minor variations in

milling and firing. If these variations are taken

into consideration, there were, in effect, 94 enamels

under study; of which 80 were applied to sheet

steel and 14 to sheet aluminum. After 3 years'

exposure [5], three of the sccreening paste enamels ^ had become so badly weathered that they were withdrawn from further testing. The 91 remaining enamels are evaluated in this report.

The 41/2-inch-square specimens with flanged

edges were made from 18 gage sheet metal. A

small hanging hole was punched in one corner of

each specimen. Each enamel was applied to about 60 specimens; of which 21 were used for

exposure testing (3 at each site), 2 were used as reference panels, and the remainder were kept in

dry storage to be used in the development of

accelerated weathering tests.

3. Exposure Sites

The seven exposure-test locations and the general exposure conditions that each site represents

are given in table 2. The sites were selected as being representative of the various exposure conditions in different parts of the United States. The racks at five sites were located in commercial

city areas. The remaining two were in a rural

seacoast location.

Table 3 summarizes both the weather and the

air quality data for each of the sites for the actual

period of exposure.

The specimens were loosely mounted in ceramic

insulators, which were fastened to the metal

stretchers of supporting racks. The supporting

racks were constructed of aluminum alloy, except

those at the two Kure Beach locations which were made of Monel metal. All specimens were exTable 2.

Exposure test locations

City Exposure site Exposure conditions represented

Kure Beach, N.C.

Kure Beach, N.C.

Washington, D.C__

Pittsburgh, Pa New Orleans, La... Dallas, Tex ...

Ground, 80 ft from ocean.. Ground, 800 ft from ocean.

Roof, Industrial Bldg., National Bureau of Standards.

Roof, U.S. Post Office Roof, U.S. Post Office Roof, U.S. Post Office Roof, U.S. Post Office

Temperate, sea spray. Temperate, sea air. Temperate, commercial.

Temperate, commercial.

Semitropical, commercial.

Texas, commercial. Southern Calif., commercial. Los Angeles, Calif.

^ Frit is the principal ingredient used in preparing porcelain enamel. It is formed by melting suitable glass-forming raw

materials and then quenching the molten mass, usually in water.

" Screening paste enamels are usually strongly colored enamels

of a consistancy which permits their application to a previously

fired enamel surface by the silk screen process.

posed at 45 degrees and faced south, except at the

Kure Beach-80 ft station where they faced the ocean at east-southeast. Figure 1 shows a typical

installation.

Figure 1. Exposure test site on the roof of the Industrial

Building, National Bureau of Standards, Washington,

D.C.

8
Table 3.

Weather data and air quality measurements for the 1-yr exposure "period

Exposure site

Kure Beach <• Washington..

Pittsburgh..

New Orleans. Dallas Los Angeles..

6.7

7.2

7.3

6.4

Ave. ann."

relative humidity

Percent

77

67

68

74

63

62

Ave. arm." temperature

ieq F

Tot. aim." sunshine

Hours

2669

2576

2202 2744

2911 3284

Tot. ann.'

precipi- tation

Inches

Sus- pended « particulate matter

31 108

1608995

169

» Averages from Air Sampling Network of the Department of Health, Education, and Welfare.

•> Measured for solutions prepared by refluxing an 8% aliquot of particulate matter from the atmosphere with 50 ml of distilled water and diluting to 80 ml. Values are averages of measurements made at approximately biweekly intervals.

° Averages from the U.S. Weather Bureau Records.

<i Data from Cape Hatteras, N.C., rather than Kure Beach, for which no data were available.

4. Results and Discussion

4.1. Cleaning of Specimens

All specimens were returned to the laboratory

for inspection after they had been exposed for 7

years. Upon arrival, each specimen was cleaned

with a soft cellulose sponge which had been dampened with a 1 percent, by weight, solution of tri- sodium phosphate in water. This method of cleaning was svifficient for the specimens exposed at all sites except Los Angeles and Pittsburgh. At Los

Angeles, a gumlike film had been deposited on the

specimens which was not removed by the trisodium

phosphate solution, but which was easily removed

with benzene. Hence, the Los Angeles specimens were cleaned first with benzene, and then with the

trisodium phosphate solution. The panels exposed

at Pittsburgh were covered with a dirt film that

appeared to consist mostly of soot and fly ash. In

addition, in the process of installing a new roof

covering on the Post Office Building in Pittsburgh, some of the specimens had been splattered with

small amounts of tar and cement mortar. The tar was removed with benzene, the cement was re- moved by heating at 200 °C for 24 hr followed

by brushing, and finally the specimens were

scrubbed with a commercial scouring powder. All

specimens, including those from Los Angeles and

Pittsburgh, were subjected to the trisodium phosphate solution cleaning process and to two rinses, one with distilled water and one with alcohol.

These rinses were required to prevent the formation of drying marks that might interfere with

later gloss and color measurements.

4.2. Corrosion of the Base Metal

After the specimens were cleaned, they were examined for corrosion of the base metal. None

of the enamels on steel at Washington, Dallas,

Pittsburgh, and Los Angeles showed any evidence

of corrosion of the base metal. One panel at New

Orleans, 10 at Kure Beach-800, and 29 at Kure

Beach-80 showed some rusting of the base metal.

The rusting was not confined to one enamel type

but was present on those particular specimens on

which either pinhole defects or blisters were pres- ent prior to exposure.

4.3. Changes in Gloss and Color

The degree of weathering was evaluated by

changes in both color and 45 deg specular gloss.

The gloss [6] is reported as the percentage retained

after exposure, and the color is reported as color

retention, which is 100 minus the color change in

NBS units [7].

The individual gloss and color values for each

enamel after 7 years' exposure are given in table

1. In later tables and figures, the white porcelain

enamels were not included when computing aver- age color changes, since white enamels normally

retain their initial color even after relatively se- vere surface attack. Also, screening paste enamels were excluded from the averages, because of their

abnormally large color changes.

4.4. Changes in Gloss and Color with

Exposure Time

The effect of exposure time on gloss and color

retention is shown in figures 2 and 3. Both the

gloss and color values of the enamels exposed at Pittsburgh and New Orleans have been averaged,

since there was no significant difference in the

degree of attack on the enamels after 7 years (see

sec. 4.5).

Figure 2 shows that the enamels at all the ex- posure sites lose their gloss most rapidly during

the first 3 years' exposure ; then, the gloss retained

remains nearly constant at the five milder exposure

sites, while it continues to decrease at the two

more severe sites at Kure Beach. The slight in- crease in gloss at Los Angeles after the second

year may possibly be caused by incomplete re- moval of the gumlike film. This would cause a doubly reflecting surface, which would increase

the gloss readings.

The color change was greatest during the first 2 years of exposure (fig. 3), after which the

enamels change in color at a nearly constant, but

9
ALOS ANGELES

DALLAS

30 ^ ^ 1 \ \ ^ I 0 1 2 3 4 5 6 7 EXPOSURE TIME, YEARS

Figure 2. Chcmge m average percentage of gloss retained with exposure time. Points are averages for all enamels except screening pastes.

different, rate at all seven sites. The lack of any

appreciable color change between 3 and. 7 years'

exposure of the enamels exposed at Pittsburgh and

Los Angeles suggests the possibility that adherent

films acted as protective coatings on the enamels

exposed at these sites.

4.5. Comparison of Exposure Sites

Table 4 gives the average percentage gloss re- tained and color retention for each exposure site. The gloss values are for all enamels tested, while

the color values are for all nonwhite enamels ex- cept the screening pastes.

The gloss data in table 1 were ranked to determine if there were significant differences between

sites. The analysis showed that such differences

(significant at the 5% level )^ existed between all sites except Pittsburgh and New Orleans. This

observation is unusual, since at the 3-year inspection the Pittsburgh site was appreciably more

severe than the New Orleans site [5]. The ex- planation is believed to be associated with the

scouring treatment that was necessary to clean the

Pittsburgh specimens. This treatment, because of

its polishing effect, could have raised the gloss

readings above those that would have been obtained had the specimens received only the normal

cleaning process.

' Using tlie sign test for adjacent sites as listed in table 1 ; see, for example, NBS Handbook 91, Experimental Statistics, ch. 16.

DALLAS

0 12 3 4 5 6 EXPOSURE TIME, YEARS

Figure 3. Change in average color retention mth exposure

time. Points are averages for all nonwhite enamels except screening

pastes.

Table 4.Average color retention and percentage gloss retained after 7 years' exposure

Color Percentage Exposure site retention » gloss retained *

Eure Beach-80 — 91.4 36.4 Kure Beach-800 92.9 53.2 Washington --. 94.6 62.3 Pittsburgh-. - _____ 95.8 74.2 New Orleans - 94.8 70.5 Dallas __ -._-_- _ 97.1 82.0

97.1 86.4

• Color retention is given for all nonwhite enamels except screening pastes.

•> Percentage gloss retained is given for all enamels. The differences in the behavior of the enamels

exposed at the various sites were undoubtedly

caused by differences in the exposure conditions

existing at the respective sites. Air pollution by

acidic contaminants was found to have an influence on site severity at 3 years [5]. Figures 4 and 5 show that this same effect is active at 7 years. In

preparing these figures, gloss and color retention

values for the two Kure Beach sites were averaged,

since only one value for was available.* It can be seen that at the most severe sites the atmospheric particulates yield more acidic values,

while at the mild sites they give a nearly neutral

pH. The noncorrelation of the enamels exposed

at Pittsburgh and Los Angeles might be explained,

at least in part, by the protective action of the adherent films on the specimens exposed at these lo- cations.

Of the various weather parameters listed in

table 3, the one that showed the best correlation

with changes in both gloss and color was the rela-

* Data from Cape Hatteras, rather than Kure Beach for which no data are available.

10
100

90

80

70

60

50

40

1 1 1

L.A.

•

D.

•

P. •

N.O.

•

. W.

K.B.

•

1

-

1

1

64 68 72 76 AVERAGE RELATIVE HUMIDITY

80

FiGTJBE 6. Effect of average site humidity on the average

gloss retention values for all enamels.

Letters adjacent to points indicate exposure city ; D-Dallas, L.A.-Los Angeles, P-Pittsburgh, N.O.-New Orleans, W-WashIngton, and K.B.-Kure Beach.

4.5 5.5 6.5 7.5

SAMPLING NETWORK

Figure 4. Change in average gloss retention (all enamels)

with pH of particulate matter collected at the sites. Letters adjacent to points indicate the exposure city ; D-Dallas, L.A.-Los Angeles, P-Pittsburgh, N.O.-New Orleans, W-Washington, and K.B.-Kure Beach. ICQ

o

o

96

94

92

90

1 1 1

L.A. D.

• •

R _

•

N.O. W.

•

K.B.

•

1 1 1 4.5 5.5 6.5 7.5 AVERAGE pH FROM THE NATIONAL AIR SAMPLING NETWORK

FiGUKE 5. Change irt average color retention for all colored enamels (except screening pastes) with pH of particulate matter collected at the sites. Letters adjacent to points indicate exposure city ; D-Dallas, L.A.-Los Angeles, P-Pittsburgh, N.O.-New Orleans, W-Washington, and K.B.-Kure Beach.

60 64 68 72 76 AVERAGE RELATIVE HUMIDITY

Figure 7. Effect of average site humidity on average color retention for all nonwhite enamels except screening

pastes.

Letters adjacent to points indicate exposure city ; D-Dallas, L.A.-Los Angeles, P-Pittsburgh, N.O.-New Orleans, W-Washington, and K.B.-Kure Beach.

tive humidity. This is illustrated in figures 6 and

7. It will be noted that the mild sites, Dallas and

Los Angeles, have low relative humidities and the

severe sites at Kure Beach have high relative humidities. The averages for Washington and New

Orleans do not fall on the curves, but an overall

pattern of correlation is evident. A possible ex- planation of the correlation is that high humidity

permits condensation of moisture, as dew, which

in turn leaches salts from the particulate matter

deposited on the specimens, to give solutions of

varying acidities according to the nature of the

contaminants.

11
4.6. Comparison of Enamel Types

The average color and gloss retention at all sites for each enamel type is given in table 5. In this

table, enamel types have been ranked according

to their color retentions. The glossy acid-resisting

enamels on steel had the best color retentions,

while the screening paste enamels had the poorest.

Table 6 lists the color retention of the best

enamel of each type at the various sites. This

table, in effect, gives the best color stability that

could be expected at each site if the most durable

enamel of each type was selected. Although the

gloss retained values are not given, these correlated

in most cases with the color change.

Inspection of table 6 and table 1 permits the fol- lowing observations to be made after 7 years'

exposure

(1) Some enamels on steel had excellent color

retentions, even when exposed to the salt-air conditions at Kure Beach.

(2) The best of the enamels on aluminum

showed only very small color changes at Dallas,

Los Angeles, and Pittsburgh. The color changes

for these same enamels at Washington were moderate, while fairly sizable color changes occurred

at the two Kure Beach sites and at New Orleans.

The cause of the unexpected severity of the New

Orleans atmospheric conditions on the aluminum

enamels has not been determined. It seems unlikely that the increased attack could be attributed

to salt particles carried from the Gulf of Mexico

or Lake Pontchartrain since the specimens were

exposed on a four-story building several miles

Table 5.

Average color retention and percentage gloss retained for the eight types of enamels

Enamel type '

Regular glossy steel, AR

1300 "F steel Mat steel Mat aluminum

Regular glossy steel, non-ARGlossy aluminum

1000° steel Screening paste steel

» Enamels with citric acid spot test ratings of AA, A, or B are designated

as AR (acid-resistant), while those with C and D ratings are designated as non-AR (nonacid-resistant).

from the nearest saltwater. The cause of the in- creased attack at Kure Beach was tentatively

ascribed to chlorides at the time of the 3-year

inspection [5].

(3) The color stability of the best of the 1000°

enamels on steel and the screening paste enamels was relatively poor at all sites. This is indicated

by their low color retentions.

4.7. Correlation of Acid Resistance with

Weather Resistance

As stated in the introduction, one of the goals

of the current investigation was to develop accel- erated tests to predict the weather resistance of

any given enamel. One test that is currently used

is the citric acid spot test [8]. This test assigns

the enamels alphabetical ratings of AA, A, B, C,

or D. In figure 8, it is evident that the AA and

A enamels, which are those recommended for

architectural applications [9], had good gloss re- tentions, while C and D enamels had lower values.

The weakness of the spot test is that the correlation existed only when averages were used, but

there was a great deal of scatter when individual

enamels were considered. This scatter is illus- trated in figure 9, which shows the color retentions

of the regular glossy enamels on steel exposed at Washington. The scatter is minimal for the AA

and A enamels, but it increases appreciably for

those with C and D ratings. Unfortunately, from

the correlation standpoint, some of the individual

C and D enamels showed as high color retentions

at Washington as the A and AA enamels.

Figure 10 illustrates that the scatter is even

greater when gloss is considered. This increased

scatter was especially noticeable for enamels with

AA and A ratings. Thus, when considering the

good correlation between gloss and acid spot test rating, as shown in figure 8, it must be remembered

that this correlation exists only when averages are

plotted and there are many individual exceptions

to the pattern.

Figures 11 and 12 are plots of color and gloss

retention, respectively, of these same enamels ex- posed at Washington, against another measure of

Table 6.Color retentions for the iest non-white enamel of each type after 7 years' exposure

Enamel type '

Regular glossy steel, AR

1300 °F steel Mat steel Mat aluminum

Regular glossy steel, non-AR

Glossy aluminum

1000° steel Screening paste steel

Kure

Beach,

80 ft

97.4

97.6

97.3 93.0

96.5

95.0

91.9

90. 6

Kure

Beach,

800 ft

99.0

98.7 97.9

97.7

98.3 95.5 93.9

90.5

Washington

99.2

99.3

98.3 96.9

98.8

97.4

91.

92.1

Pitts- burgh

98.7 90.5

92.

New

Orleans

98.7

99.0

97.8 93.9

98.1

95.

95.1 91.3

Dallas

98.0

99.2

98.8

98.1 99.3

98.1 97.5

95.0

Los

Angeles

9.4

9.2

99.2 99.4

97.6

89.3

» Enamels with citric acid spot test ratings of AA, A, or B are designated as AR (acid-resistant), while

those with C and D ratings are designated as non-AR (nonacid-resistant).

12
100

90

80

70

60

50

40

30

20

• KUR qKUR

aWASHINGTON

OPITTSBURGH

«NEW ORLEANS

aLOS ANGELES

DALLAS

I L

AA

CITRIC ACID SPOT TEST RATING

FiGTjBE 8. Correlation of citric acid spot test rating with

average of percentage gloss retained for all regular

glossy enamels on steel.

9 90 -

.ACID RESISTANT ENAMELS oNON-ACID RESISTANT ENAMELS

AA A B C D

CITRIC ACID SPOT TEST RATING

Figure 9. Color retentions at Washington for regular

glossy enamels on steel with different spot test ratings.

60

30

• ACID-RESISTANT ENAMELS

O NON- ACID-RESISTANT ENAMELS

CITRIC ABC ACID SPOT TEST RATING

Figure 10. Gloss retentions at Washington for regular

glossy enamels on steel with different spot test ratings.

• ACID-RESISTANT ENAMELS O NON- ACID-RESISTANT ENAMELS

_L L

16 24 32 40 48

BOILING ACID SOLUBILITY, mg/m.^

56

Figure. 11. Color retentions at Washington for regular

glossy enamels on steel plotted against their acid

solubilities.

acid resistance, the boiling acid solubility [10].

This test consists of determining the weight loss, per unit area of specimen, after exposure to a

boiling, 6 percent by weight, solution of citric acid

for 21/2 hours. Since the boiling acid solubility

provides a more continuous measure of acid resist13
50

40

30

• ACID-RESISTANT ENAMELS O NON- ACID- RESISTANT ENAMELS

J I I L

16 24 32 40 48

BOILING ACID SOLUBILITY , mg / in.^ 56

FiGTJEE 12. Gloss retentions at Washington for regular

glossy enamels on steel plotted against their acid

soluMlities.

ance, the shape of the fitted curves is different

from the comparable cun-es for the acid spot test. The boiling acid solubility appears to provide

a somewhat better correlation with color retention

(only 5 enamels more than 2.5 color retention units

from the curve) than does the citric acid spot test (11 enamels more than 2.5 units from the curve).

Some of the C and D enamels with high color

retentions had acid solubilities under 10 mg/in.^

as do all the AA and A enamels on steel. Although the gloss retention of these same C and D

enamels was relatively low (see fig. 12), it was not

lower than some of the AA and A enamels which

are now considered as acceptable architectural

enamels.

The two gloss curves (figs. 10 and 12) were

fitted without considering enamels C-1, C-2, C-3,

and (table 1), because they were of such poor

acid resistance and showed such poor gloss reten- tions that they could not be plotted on a reasonable curve. An inspecition of figure 10 will reveal

that there were only two C and D enamels that

had gloss retentions lower than the AA and A

enamels. The wide scatter in gloss retention for

the individual acid-resistant enamels indicates

that acid resistance, however measured, is not a

reliable indicator of gloss retention during weathering. However, from the appearance standpoint,

changes in gloss were not as serious as changes in

color. One advantage of the boiling acid solubility test

is that, since it works for mat enamels, it over- comes the difficulties that are almost always en- countered in assigning spot test ratings to enamels

of this type. Figure 13 shows the dependence of

color retention on the acid solubility of both mat

and glossy enamels on steel which were exposed

.ACID RESISTANT GLOSSY ENAMELS

oNON-ACID RESISTANT GLOSSY ENAMELS iMAT ENAMELS

J I I I I I L

16 24 32 40 48

BOILING ACID SOLUBILITY, mg/in.^

FiGUKE 13. Color retentions at Washington of regular mat

and glossy enamels on steel plotted against their 'boiling acid soluMlities.

at Washington. Comparison of this figure with

figure 11, which shows this same dependence for

the glossy enamels only, reveals no large change

in either the scatter or in the slope of the fitted curve. The improved correlation between color reten- tion of individual enamels and boiling acid solubility and the ease with which the test can be

performed on mat enamels indicates that the boil- ing acid solubility of enamels should be considered

in future evaluations of architectural enamels.

Although figures 9 through 13 illustrate the

relationship between acid resistance and gloss and

color retention only for the enamels exposed at Washington, similar relationships exist for the

enamels exposed at the other six sites. The 7-year exposure data indicate that whether

the citric acid spot test or the boiling acid test is used as a measure of acid resistance, enamels of

different types, such as enamels on steel and

enamels on aluminum, with the same acid resist- ance, will not necessarily have the same weather

resistance.

4.8. Cupric Sulfate Test for Predicting

Color Retention of Red and Yellow Enamels

One accelerated test that was developed after

the first-year inspection is the cupric sulfate test [11] for enamels pigmented with cadmiumselenium-sulfur complexes. This test has been employed to predict the oftentimes poor color

retention of the acid-resistant red, yellow, and

orange enamels that contain the cadmiumselenium-sulfide pigments. Enamels that passed

this test showed average color retentions of 98.2 or better after 3 years. At 7 years, however, the

cupric sulfate test is not as reliable. Although the

average color retention of the enamels that passed

this test was 95.6, the two glossy red acid-resistant

14
enamels on steel mentioned earlier (D-3 and D-4,

in table 1) showed sizable color changes at New

Orleans, yet both of these enamels passed the

cupric sulfate test. The test has a certain useful- ness but, unfortunately, it cannot be considered as completely reliable for all enamels at all sites.

4.9. Comparison of Present-Day Enamels

With Those Produced Before World War II Table 7 compares the gloss retention values of

the regular mat and glossy enamels on steel ex- posed 7 years at Washington, in both the present

test and in a test initiated in 1939 [3] . This comparison shows that there is only a two-percent difference in the average gloss retained between the

enamels in the two tests. Thus it may be con- cluded that the enamel types produced after the

end of World War II, most of which are opacified

with titania, are equally resistant to weathering

(as gaged by gloss retention) as those produced

before the war. For the most part, these earlier

enamels were opacified with either antimony or zirconium oxide.

A similar comparison based on color retentions

could not be made, since no color measurements

were made at 7 years in the earlier study.

Table 7.

Comparison of 7-year average gloss retentions

at Washington, D.C., for regular glossy enamels on

steel produced in 1939 with those produced in 1956

Citric acid spot test rating

1939 test1966 test i>

Gloss re- tention Number of enamels

Gloss re- tention Number of enamels

Percent Percent AA 73.7 29 11. b s A 69.7 15 76.3 12 B- 70.7 3 65.0 11 C 64.9 24 56.1 17 D - 44.4 16 52,9 9 Average for all enamels. _

62.5 87 64.6 58

« Data obtained from ref. 3. ^ Data from present study.

5. Conclusions

The results of the 7-year inspection of porcelain

enamels exposed at seven sites indicate that the

regular and low-temperature enamels weather at

different rates. The regular enamels include all the enamels on steel, except the 1000° enamels,

while the low-temperature enamels include the

enamels on aluminum, both mat and glossy, and

the 1000° enamels on steel. The regular enamels were found to follow the same pattern, with respect

to weathering, as those reported by Moore and

Harrison [3] . The weatherability of these enamels

can be predicted reasonably well by two relatively

simple laboratory tests: The citric acid spot test (or the boiling acid test) and the cupric sulfate

test. Although the citric acid spot test has proven

successful in eliminating enamels with poor

weather resistance, the improved correlation between color retention and boiling acid solubility,

and the ease with which the acid solubility can be

determined on mat enamels, may well lead to this

test replacing the acid spot test as a method of

predicting weather resistance.

The two acid tests and the cupric sulfate test do not appear to be reliable criteria of weather

resistance in the case of the low-temperature

enamels, since many of the low-temperature enamels with acid ratings of AA, A, or B (boiling acid

solubilities of 22 mg/in.^ and lower) showed rela- tively poor color retentions at most sites. In fact,

the only low-temperature specimens that showed

reasonably high color retentions were the mat

enamels on aluminum exposed at Pittsburgh, Los

Angeles, and Dallas. This failure of either of the

acid tests (or the cupric sulfate test) to predict

with any degree of reliability the weather resist- ance of the low-temperature enamels means that

exposure testing of new enamels of this type will

be needed until such time as a more suitable accel- erated test can be devised.

The 7-year results confirm the 3-year findings

that acidic pollutants and moisture in the atmosphere affect the rate at which an enamel weathers.

However, it must be recognized that these are not

the only atmospheric conditions that affect the

rate of weathering. Undoubtedly, all of the parameters given in table 2 have at least a minor

effect. Perhaps it will be possible at some future

date to devise a single equation that will, when

solved, predict the rate at which an enamel will

weather at any given site. This would be invaluable information, not only to architects and engineers, but also to the fabricators, who oftentimes

are required to guarantee an installation for prolonged periods.

Although it cannot be said with absolute cer- tainty that enamels that show good resistance to weathering after 7 years will still be good after

15 or 20 years, a comparison of the results at 1, 3, and 7 years shows that the rate of change of both

gloss and color is decreasing, at least for the more

resistant enamels. This indicates that further

changes will be moderate and that further inspections can be scheduled at much longer time in- tervals. The next inspection is tentatively planned

after the enamels have been exposed 15 years.

15
6. Summary

After 7 years' exposure at Los Angeles, Calif.

;

Dallas, Texas ; New Orleans, La. ; Pittsburgh, Pa.

;

Washington, D.C. ; and two sites at Kure Beach,

N.C., the changes in gloss, color, and general sur- face conditions of triplicate specimens of 91

porcelain enamels were determined. A summary

of the more important findings follows

(1) All specimens could be cleaned easily, ex- cept those exposed at Pittsburgh and Los Angeles.

(2) No metal corrosion was noted, except for 1 specimen at New Orleans, 10 at Kure Beach-800,

and 29 at Kure Beach-80. In most cases, this cor- rosion occurred on enamels having pinhole defects

prior to exposure.

(3) An adherent film on the enamels exposed at Pittsburgh and Los Angeles may have partially

protected the enamels from weathering action.

(4) The severity of the exposure conditions on

the enamel specimens was found to dilfer at the

various sites, except at Pittsburgh and New

Orleans. The sites from mildest to most severe were: Los Angeles, Dallas, New Orleans- Pittsburgh, Washington, Kure Beach-800, and Kure

Beach—80. (5) A good correlation was observed between

relative humidity and the gloss and color changes.

The gloss and color were also affected by the pH of

the suspended particulate matter.

(6) The glossy acid-resistant enamels on steel were found to have the best weather resistance of

the seven types of enamels included in the test.

(7) For enamels of similar types, a direct rela- tion existed between the acid resistance, as meas- ured either by the acid spot test or by boiling acid

solubility test, and the weather resistance as meas- ured by changes in gloss and color. However, this

correlation with the citric acid spot test was evi- dent only when averages were considered.

(8) The boiling acid solubility test was found

to be more reliable in predicting the color reten- tion of the regular enamels on steel than was the

citric acid spot test.

( 9 ) Comparison with enamel specimens exposed

for 7 years in an earlier test showed that porcelain

enamels produced after the end of World War II were equally resistant to changes in gloss as those

produced before the war.

The authors thank Dr. T. P. May, Harbor Island

(Kure Beach) Corrosion Laboratory, International Nickel Company, for his cooperation in making

available the two sites at Kure Beach for the PEINBS Exposure Test. Grateful appreciation is also extended to previous research associates and

their assistants, and to D. G. Moore for his guid- ance throughout the investigation and for his

valuable assistance in the preparation of the manuscript.

7. References

[1] W. N. Harrison and D. G. Moore, Weather resistance

of porcelain-enameled iron structural units, J. Res. NBS 28, 735 (1942) RP1476.

[2] W. N. Harrison and D. G. Moore, Weather resistance of porcelain enamels exposed for seven years, J. Res. NBS 42, 43 (1949) RP1949.

[3] D. G. Moore and W. N. Harrison, Fifteen-year ex- posure test of porcelain enamels, NBS Building

Materials and Structures Report 148 (1957).

Supt. of Documents, U.S. Govt. Printing Office, Washington, D.G., 20402.

[4] Alan Potter, First-year data on the weather resist- ance of screening-paste enamels, Proc. Porcelain

Enamel Inst. Forum 19, 131 (1957). Porcelain

Enamel Inst, Inc., 1900 L St. NW., Washington,

D.C., 20036.

[5] D. G. Moore and Alan Potter, Effect of exposure site on weather resistance of porcelain enamels ex- posed for three years, NBS Monograph 44 (1962).

Supt. of Documents, U.S. Govt. Printing Office, Washington, D.C., 20402.

[6] Standard method of test for 45-deg specular gloss of ceramic materials, ASTM designation : C346-59, 1964 Book of ASTM Standards, pt. 13, p. 3^, Am.

Soc. for Testing and Materials, 1916 Race St., Philadelphia, Pa., 19103.

[7] R. S. Hunter, Photoelectric tristimulus colorimetry

with three filters, NBS Circ. 429 (1942).

[8] Test for acid resistance of porcelain enamels, pt. 1, Flatware. Issued by the Porcelain Enamel Inst., Inc., 1900 L St. NW., Washington, D.C., 20036.

[9] Specification for architectural porcelain enamel on

steel for exterior use. Porcelain Enamel Inst.

S-100(62), Issued by the Porcelain Enamel Inst,

Inc., 1900 L St. NW., Washington, D.C., 2(X>36. [10] Standard method of test for resistance of porcelain

enameled utensils to boiling acid, ASTM designation : 0283-54, 1964 Book of ASTM Standards, pt

13, p. 324, Am. Soc. for Testing and Materials, 1916 Race St., Philadelphia, Pa., 19103.

[11] Cupric sulfate test for color retention. Issued by the Porcelain Enamel Inst., Inc., 1900 L St. NW.,

Washington, D.C, 20036.

16

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