jfltetieicu Bureau of standards

Library, S,W-; 8M«

/ 4 .ViN T7 1357

/

Fifteen-Year Exposure Test of Porcelain

Enamels

United States Department of Commerce

National Bureau of Standards

Building Materials and Structures Report 148
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UNITED STATES DEPARTMENT OF COMMERCE • Sinclair Weeks, Secretary NATIONAL BUREAU OF STANDARDS • A. V. Astin, Director

Fifteen- Year Exposure Test of Porcelain Enamels

Dwight G. Moore and William N. Harrison

Building Materials and Structures Report 148

Issued June 28, 1957

For sale by the Superintendent of Documents, U. S. Government Printing Office, Washington 25, D. C.

Price 15 cents
Contents

Page

1. Introduction - __ 1 2. Types and sources of enamels 1 3. Description of panels, method of mounting, and weather conditions at exposure sites 2

4. Results 3

4.1. Ease of cleaning __ 3

4.2. Corrosion protection 3

4.3. Gloss measurements.. 7

4.4. Color-difference measurements 8

4.5. Microstructure of surfaces 8

5. Effect of variation in weather conditions at four exposure sites _ 8

6. Laboratory tests for predicting weather resistance 10

7. Discussion 11

8. Recommendations 12

9. Summary 13

10.

References 13

n
Fifteen-Year Exposure Test of Porcelain Enamels

Dwight G. Moore and William N. Harrison

The weather resistance of 768 panels representing 14 types of enamel was evaluated

after 15 years of exposure at Washington, D. C., St. Louis, Mo., Lakeland, Fla., and Atlantic

City, X. J. Changes in gloss and color were taken as criteria of weather resistance. A

direct relation existed between acid resistance and weather resistance, except that a few

of the red enamels of good acid resistance showed a pronounced fading after exposure. A

modified acid-resistance test wr as devised that separated red enamels that showed pronounced

fading from those that were highly resistant to color change.

The ease of cleaning was related to the weather resistance, the enamels that lost most

of their initial gloss on weathering being more difficult to clean than those that showed

high gloss retention. All of the enamels protected the steel from corrosion when initial coverage was complete.

Except for a relatively few enamels, variations in climate at the four exposure sites had only a minor effect on their weathering behavior.

1. Introduction

Porcelain enamel is being used in increasing

volume as an exterior finish for such structures

, as office buildings, store fronts, and gasoline

t tilling stations

[1,2].*

Curtain-wall construction

methods together with the emphasis on color

in modern architecture have helped to popularize

this particular finish. In fact, probably because

i of this trend toward color, porcelain enamel has

! been used to a greater extent than either stainless

I steel or aluminum as an exterior facing for buildi ings [2]. The relative ease of decontaminating

an enamel surface in case of atomic attack is an additional consideration in some types of structures [1], Another reason for the increasing use of porcelain enamel is the excellent resistance to weatherj ing shown by some of the early installations.

For example, there are reliable reports of street

i and advertising signs and of building fronts, that

have been installed for 25 yr and longer without

noticeable deterioration. On the other hand,

there have been occasional installations where

both the gloss and color of the finish have changed

appreciably in as short a time as 10 yr. To

prevent future installations of enamels of poor

resistance, it is desirable to have data on the

weather resistance of various enamel types and

it is also important to devise a test, that will indicate weather resistance.

A test to obtain such information was begun

by the National Bureau of Standards in 1940.

A total of 864 panels, 1 ft square, and an equal

number of 4-by 6-in. laboratory specimens, were

prepared by 12 cooperating manufacturers. Of

the 864 large panels, 768 were exposed and 96 were placed in storage for use as reference stand

; ards. The base metal in each case was enameling

iron. The exposure sites selected were Washing-

! ton, D. C., St. Louis, Mo., Lakeland, Fla., and

Atlantic City, N. J. Fourteen types of enamel were included. These

types represented enamels that were in common

use from 1930 to 1940. Shortly after World

War II, enamels opacified with titanium dioxide

*Fieures in brackets indicate the literature references at the end of this report.

came into widespread use for white and pastel

finishes. These enamels are considerably different

from the acid-resistant white and buff compositions

included in the present investigation. However,

because one of the principal objectives of study was to correlate weather resistance with some more easily measured property of the enamel

finish, the obsolescence of some of the compositions does not necessarily detract from the value

of the data.

The present paper constitutes the third progress

report of the investigation. The first report was published in 1942 [3] and the second in 1949 [4], It was originally intended that the test would be terminated after 15 yr, but. because

many of the enamels showed only a minor deterioration after this testing period, it was decided

to expose the Washington panels until a testing

time of possibly 25 or 30 yr had been accumulated.

However, mostly because of difficulties in prodding proper maintenance, the panels from Lakeland, St. Louis, and Atlantic City were returned

to Washington and the testing at these locations was terminated.

2. Types and Sources of Enamel

The enameled panels were supplied by the 12 cooperating companies who were active in tin' field of architectural enamels in 1939. The frits and various mill additions 2 for preparing the

enamels were supplied by 4 frit companies. Each

of these 4 companies furnished the materials for each of the 14 enamel types to 1 or 2 of the 12 different fabricators, who then applied the enamels

to the specimens. By this arrangement, it was

possible to introduce the two variables ot frit source and fabricator. The frits were all proprietary products for which no chemical analyses were available. It is probable that there were

at least minor variations in composition tor any

1 enamel type as supplied by the 4 frit companies

1 Frit is the principal ingredient used in preparing porcelain enamels.

It is formed by melting suitable raw materials and then quenching the i mass, usually by pouring into cold water.

2 In the preparation of an enamel for application to sheet iron, the enamel

frit is ball-milled together with such ingredients as clay, opaciftor. electrolyte, and water. The materials added at the mill constitute the mill

additions.

I
Likewise, each fabricator produced an enamel of

slightly different properties from the same frit because of minor variations in the mill batch,

the milling procedure, and the firing conditions.

Thus, although the investigation included nominally only 14 types, there were, in effect, 96

enamels under study.

3. Description of Panels, Method of

Mounting, and Weather Conditions at Exposure Sites

The panels, which were 1 ft square, w ere fabricated of 16-gage iron and had 1-in. flanged edges.

The flange of the lower side had a y2-in. outward

extension parallel to the face of the panel. Two

clips made of 1-in. strap iron were welded to

Figure 1. Reverse side of an exposure panel.

C, Attachment clips; F, lower flange extension used for fastening panels

to racks.

the top flange so as to extend downward (see fig, 1). The clips and the lower flange extension » fitted into 18-gage galvanized-iron channels j, 'm which in turn were firmly attached to the sup| |k porting racks. The crevices between the specit >5] mens were not calked but were left open tc facilitate removal of the panels during periods oi inspection.

The supporting racks were constructed oi %6-in. angle iron and, after priming, were painted

periodically with aluminum, paint. Each racf was constructed to support 28 of the 1-ft-square

panels. Seven racks were required for each

location. At those locations where the paneld were exposed on flat roofs (see table 1), the racks were anchored with weights, and at the grounc

locations the racks were anchored to piers o concrete blocks. Figure 2 shows the installatioi at Washington, D. C.

I

I "

in

ud

Table 1. Exposure-test locations

City Exposure site Exposure conditions represented

Washington, D. C .

.

Roof, Industrial Bldg., National Bureau of Standards.

Temperate, residential.

St. Louis, Mo Roof, Union Electric Co. Warehouse.

Temperate, industrial. Lakeland, Fla. Ground Municipal Air- port

Semitropical, resi- dential. Atlantic City, N. J.. Ground, U. S. Coast Guard Station. Temperate, “salt

air.”

2
Table 1 lists the locations and general condiions of exposure. Table 2 gives weather data

for the actual period of exposure. At all four

locations the racks faced south, the panels being

exposed at 45° to the horizontal.

Table 2. General weather data for the 15-yr exposure

period (from U. S. Weather Bureau Records)

City Exposure Period Annual

rainfall >

Annual

sun- shine a Aver- age temperature a

in. hr o F

Washington, D. C Dec. 1939 through

Sept. 1955. 42.0 2, 584 57. 0

St. Louis, Mo Apr. 1940 through

Oct. 1955. 36.3 2,718 5S. 2

Lakeland, Fla July 1940 through Xov. 1955. 47.8 b 2. 878 72.8

Atlantic City, X. J__ Aug. 1910 through Xov. 1955. 40.0 2,675 55. G

a Average computed for actual period of exposure.

b Taken from Tampa, Fla., records. Average annual sunshine for Lake- and not available.

4. Results

4.1. Ease of Cleaning

At the beginning of the investigation it was

planned to clean the panels prior to making gloss

and color measurements by washing the test surface with a 1 -percent-by-weight solution of

trisodium phosphate. This cleaning procedure was satisfactory for the first year’s inspection [3], and at all locations except St. Louis for the

inspection after 7 yr [4], However, it was not

practicable to clean the panels between inspections, and, as a result, both the Lakeland and

Atlantic City panels, in addition to those from.

St. Louis, bad accumulated surface films after

the 15-yr exposure that could not be removed by

washing with the trisodium phosphate solution.

The film on some of the Lakeland panels was

chalky in nature, whereas rust stains were present on many of the Atlantic City panels. The film on

the St. Louis panels consisted of a fairly heavy

deposit of fly ash and soot bonded with a tar-like

substance. Gloss- and color-difference measure- ments made on such surfaces would be meaningi less ; therefore, all panels, including those at [Washington, were cleaned prior to the 15-yr

measurements by scrubbing with a commercial

scouring powder. It was believed that this cleaning procedure was justified inasmuch as it fol- lowed closely the practice of commercial signcleaning services. The procedure followed in the

cleaning was to continue the scouring until

further treatment caused no appreciable change

in the appearance of the enamel surface. In most

cases, it vr as found that the surfaces reached

substantially constant values of gloss arid color

by the time the cleaning was adjudged satisfactory

by visual examination.

The ease of cleaning varied with the acid resist- ance of the enamel, the surfaces of high acid re- sistance being easier to clean than those of poor

acid resistance. The semimat or satin-textured

enamels showed about the same cleaning behavior

as the glossy surfaces after weathering. On the

other hand, none of the full-mat enamels could be

cleaned satisfactorily even by a vigorous and pro- longed scouring treatment. 3

4.2. Corrosion Protection

All of the panels were inspected for evidence of

corrosion. Where the initial coverage w

T as complete on all parts of the panel no corrosion was

noted, irrespective of the type of enamel applied.

However, on many panels the under side was protected with only a single groundcoat application,

and good coverage of the metal wr as not always

achieved. Corrosion started at these areas of poor

coverage, and spalling of the enamel on the face

side occurred after the corrosion had progressed

only part way through the thickness of steel. Hydrogen originating from the action of con- densed moisture on the unprotected steel is believed responsible for the spalling. Norton [5] has

shown by a tracer technique that the hydrogen

generated from such a reaction will permeate the

steel structure at room temperature, and numerous

investigators, including Zappfe and Sims [6], have

demonstrated that whenever hydrogen diffuses

through steel, sufficient pressure can be generated

to rupture the enamel on the opposite face. Thus,

it appears entirely reasonable that the observed

spalling behavior could be caused by hydrogen

diffusion.

The surface spalls, which resembled large fish- scales, were noted on only a few of the panels at Atlantic City at the 7-yr inspection. However,

after 15-yr, the majority of the panels at that location showed one or more of these surface spalls.

Figure 3 is a comparison of panels having good

coverage on the under side (two or more enamel

applications) with similar panels that had poor

coverage (a single ground coat with local breaks in the coating). All four of these panels were from

the Atlantic City site, where corrosion on the unprotected metal was especially severe because of the “salt-air” conditions. Fortunately, the center area of the panels was free of spalls in almost all cases, and therefore they did not interfere with the

gloss and color measurements.

Surface spalls from poor coverage on the under

side were noted also on a few of the panels at Si. Louis and Lakeland. The following tabulation

3 Enamels with 45° specular-gloss readings of t.o or greater are refer as glossy, those with readings between 2.0 and 4.5 as semimat >r satin xtu and those with readings of less than 2.0 as full mat. I'he designations

glossy and semimat used in I iblo 3 ire those assigned by the manufacturers. Many of the specular-gloss readings do not conform with these d <

3
gives the prevalence of surface spalls at- each of the four locations:

Exposure site Percentage of penels

showing one or more surface spalls

St Louis 5 Lakeland 2 Washington 0

Noticeable edge corrosion 4 was observed only on the panels exposed to the salt-air conditions at Atlantic City. In no case, however, was this edge

corrosion considered to be serious. Numerous observations made during the 15-yr inspection indi- cated that whenever corrosion started at an edge

or at any area of pool’ coverage, there was very

little spreading of the corrosion under the adjacent

enamel.

Some of the top attachment clips (see fig. 1) showed serious corrosion in the salt air at At lantic

City. These clips had been spotwelded to the

panels prior to enameling and, in some cases, the

enamel coverage was poor where the clip joined

the panel. Corrosion occurred at these points,

the clips came loose from the panels, and, in a few

instances, the affected panels were blown away

4 Good coverage is difficult to achieve on the edge of a thin sheet and, hence, many of the panel edges were not properly covered with enamel.

Figure 3. Four 'panels after 15 years of exposure at Atlantic City.

Specimens on left had good enamel coverage, whereas the two on the righto show holes in the panel caused by rusting through from areas of incomplete) coverage on the back side.

from the rack during the periods of high wind

velocity. As a result, some of the data for Atlantic

City are missing from table 3. Table 3. Percentage of initial specular gloss retained, color-difference values, weather-resistance ratings, and acid resistancej of enamels of If types after 15 years of exposure at f sites

Initial gloss retained (Gr) and change in color (A E) at— c Fabrica- Average Acid re- Weather

tor of Frit initial sistance resist- Specimen identification

»

speci- sup- specular Washington Lakeland St. Louis Atlantic Citv (PEI ance mens plier gloss, b test) d rating® Gs

Gr AE Gr A E Gr A E Gr AE

WHITE, GLOSSY, ACID-RESISTANT ENAMEL % % % % %

A-l to 8 a 1 5. 70 83. 1 0.7 68.9 0.8 76. 6 0. 8 66. 7 1. 1 AA E A-U to 18 b 1 5. 74 86.9 1.0 88. 7 1.0 75.5 .9 89.6 2.2 AA E

A-21 to 28 a 2 6. 04 84. 2 0.6 68.4 0. 3 77.5 1.4 88.3 0.9 A A E

A-31 to 38 b 2 6. 01 76.6 . 5 66.6 .3 85.8 0.6 84.4 .2 AA E

A-41 to 48 a 3 5. 85 85.0 3.0 62.2 2.9 82.7 2. 1 81. 7 1.6 AA G

A-51 to 58 b 3 5. 81 85.4 1.2 48.8 2.4 85.6 1.0 46.8 2.2 AA F

A-61 to 68 a 4 6. 20 83.3 1. 1 66.0 3. 5 73. 6 1.0 72. 1 3.6 AA G

A-71 to 78 b 4 6.01 73.4 1.7 86.3 1.7 79.9 2.3 77.8 3.

1

AA G

Average - .... 82.2 1.2 69.5 1.6 79.6 1.3 75.9 1.9

WHITE, GLOSSY, NONACID-RESISTANT ENAMEL

B-l to 8 b 1 5. 26 18.0 2.9 62.8 1. 1 46.

1

8.6 (0 (

r) C P

B-21 to 28 b 2 5. 41 15.0 4.8 32.6 4. 6 41.3 6.3 19.0 2. 7 C P

B-41 to 48 b 3 5. 16 15.0 2. 7 38. 5 1.9 38.6 5.9 15.0 3.0 D P

B-61 to 68 b 4 5. 32 16.4 3.

1

56.0 0.5 35.5 6.9 18.7 1.3 C P

Average 16.

1

3.4 47. 5 2.0 40.4 6.9 17.6 2.3

WHITE SEMIMAT ACID-RESISTANT ENAMEL

C-ll to 18 c 1 4. 06 30.3 1. 5 54.4 1. 1 82.6 5.2 43.5 1.6 C P

C-31 to 38 c 2 5. 24 42. 1 2.3 40.8 1.7 92.3 0.9 46.6 1.5 A F

C—51 to 58 c 3 5. 16 76. 7 2.2 63.9 1.9 80. 7 3.8 55.6 2.2 A G

C-71 to 78 c 4 5. 32 54.8 1.3 33.0 1.9 86. 7 0.6 46.

1

0.8 A P

Average — 51.0 1.8 48.0 1.6 8.5.6 2.3 47.9 1.5

4
Table 3. Percentage of initial specular gloss retained, color-difference values, weather-resistance ratings, and, acid reactance

of enamels of 14 types after 15 years of exposure at 4 sites—Continued

Fabrica- Average

Initial gloss retained (Oh) and change in color (AE) at- ' Acid re- Weather

Specimen identifications

tor of speci- mensFrit supplierinitial specular

gloss, b Washington Lakeland St. Louis Atlantic City

sistance resist- (PEI ance

test)”1 rating' i Gs

Or A E On AE G„ AE Grt | A K

WHITE, SEMIMAT, NONACID-RESISTANT ENAMEL

D-l to 8. . - .. - c

c

c

c

12

3

4

5.69

5. 51

1.0 3.3

3.3 3.6

42AJ

29.4

3.5

2.9

0.9

3.0

53.9

45.0

9.2

2.4 5.2

16. 5

17. 17.7

2.6

2.9 3.9

4.4

D-21 to 28 D—tl to 48

15.0

15.0 D

O

P

P —

-

15.0 3.4 36.1 2.6 49.4 8.3 17.5 3.4

BUFF, GLOSSY, ACID-RESISTANT ENAMEL

E-ll to IS d i 5. 09 60.2 1.4 61.4 1.2 84.3 0.1 48.6 2.9 B F

E-31 to 38. . . - - . -- d 2 5. 43 54.8 1.8 76.3 1. 1 87. 6 1.7 76. 7 1.0 AA G

E-51 to 58 d 3 5. 44 90.0 0.4 63. 6 0.8 84.8 0.9 91. 0 0.9 AA G

E-71 to 78- . d 4 5. 35 76.0 1.6 50.8 2.6 78.6 1.2 82.5 1.7 AA G

Average .... 70.2 1.3 63.0 1.4 83.8 1.0 74.7 1.6

BUFF, GLOSSY, NONACID-RESISTANT ENAMEL

F-l to 8 d l 5. 15 24.3 7.8 22.4 3.3 58.9 3.0 18.4 8.4 r> • p

F 11 to 18 e 1 4. 87 35.6 4.4 21. 5 4. 1 49.0 3.0 19.8 6.4 D P

F-21 to 28 d 2 5. 02 33.2 3.8 67. 9 2.8 67.9 1.6 25.3 4.9 C

1 P

F-31 to 38 e 2 5. 56 44.3 2.9 33.

1

6.9 65.0 2.3 18.5 6.4 D P

F-41 to 48 rl 3 4.31 26.2 13.4 28.6 4.8 45.9 8.3 34.

1

19.7 D P

F-51 to 58 . . . ---- e 3 5. 66 27.6 5.3 47. 5 3- 1 29. 4 3.4 17.6 6. 7 D P

F-61 to 68 d 4 4.64 24.4 12.0 20.5 6.6 37.0 6.9 24.5 20.6 D P

F-71 to 78 e 4 5.26 23.7 12.

1

22.

1

4.8 31.0 9.2 34.3 16.4 D ! P

Average — —

-

29.9 7.7 32.9 4. 5 48.0 4.7 24.

1

11.2

BUFF, SEMIMAT, ACID-RESISTANT ENAMEL

H-l to 8__ __ e 1 3. 96 64.3 2.0 61.4 2.

1

62. 5 2. 1 76.6 1.8 A G

H-ll to 18 f 1 4.81 52.

1

0. 9 63.0 2. 7 71.3 1.3 72.8 1. 4 A G

H-91 to 28 e 2 5. 65 77.0 .8 58.2 1. 6 84.8 0.8 (0 (

f

) A G

I! 31 to 38 f 2 5. 45 74.3 .5 62.0 1.

7

88.6 .3 81.6 1.7 A G

H-41 to 48 e 3 4. 74 87.5 .6 95. 1 1. 4 92.0 .3 98.2 0.4 AA E

H-51 to .58 - - _ f 3 4. 85 76.8 1.2 85.9 1.

1

82.9 1. 5 87. 1 3. 1 AA G

H-61 to 68 e 4 5.51 83.9 4.3 68. 2 1.2 95.

8

0.8 87.5 0.3 AA G

H-.l to 78 f 4 5. 54 83.5 2.0 76.7 4.3 88.8 2. 7 90.2 1.7 AA G

Average — —

-

74,9 1. 5 71.3 2.0 83.3 1.2 84.9 1. 5 —

BUFF, SEMIMAT, NON ACID-RESISTANT ENAMEL

f 1 7. 4 2.0 3.7 K-ll to 18-. 1 7. 2 1. 5 6.9 s.o K-21 to 28 f 2 5.41 21.3 4.

1

38. 5 1. 1 52.9 2.4 19.5 6.0 D r K 31 to 38 g 2 5. 37 18.8 7.2 34.2 4.5 58.6 2.5 18. 4 6.9 D p K—11 to 48 f 3 5. 20 63.8 1.7 23.2 2.

1

69. 7 0.8 23.7 3.8 O p

K-51 to 58.- _ . _ g 3 5. 35 16.2 9.2 19.4 4.6 36. 1 2.9 19.2 6. 5 D F

K-61 to 68 f 4 6.9 12. 2 11. 1 10.

0

K-71 to 78__ -_ g 4 14.4 6.0 3.3 4.9 Average — —

-

30.0 7.3 28.8 4.2 54.3 4.2 20. 2 6.6

RED, GLOSSY, ACID-RESISTANT ENAMEL

L-l to 8. . g 1 5. 58 85. 5 3.8 91.2 3.5 67. 3 2.

1

S4. 1 0.4 AA G

L-ll‘to 18 h 1 5.61 80.4 0.8 85.9 2.3 84.8 1.5 (') 0 A A K

L-21 to 28 cr 2 5. 42 82.

1

2.2 73. 5 10. 6 SO . 8 0.5 (0 (0 B P

L-31 to 38 h 2 5.64 82.0 3. 1 62.5 12.

1

60. 7 4.5 84. 7 1.8 A P L—41 to 48 g 3 5.58 77.0 3.0 88.6 1.0 90. 5 1.6 SI. 1 0.4 AA G

L-51 to 58-. - _ h 3 5. 23 68.5 0.9 85.8 1.0 90. 7 2. 7 90.

2

1.9 A G

L-61 to 68 g 4 5. 30 80.8 2.3 93.2 1. 6 78. 1 0 0 91.0 5. 3 A F

L-71 to 78- . - - - h 4 4. 18 91.9 1.3 83.4 4.3 90.

3

2. 8 87. 9 4.4 A \. G

Average. . 81.0 2.2 83.0 4. 5 80.3 2. 2 86.

5

2.4
Table 3. Percentage of initial specular gloss retained, color-difference values, weather-resistance ratings, and acid resistance

\ of enamels of U+ types after 15 years of exposure at f sites—Continued

Specimen identification 8

Fabricator of speci- mens

Frit supplier

Average

initial specular

gloss, b Gs

Initial gloss r

Washington

etained (Gr) and

Lakeland

change in color (.

St. Louis

\E) at— c

Atlantic City

Acid re- sistance (PEI

test) d

Weather

resist- ance

rating® Gr A E Gr A E Gr A E Gr A E

RED GLOSSY, NON ACID-RESISTANT ENAMEL -

N-l to 8 h 1 5. 17 65. 8 2.5 37.2 6.3 65.4 2.

1

38.4 8.6 C P

N-ll to IS k 1 5. 05 63. S 2.1 38.9 4.3 80.2 3.3 19.0 8.8 c P

N-21 to 28 h 2 5.12 67.3 1.9 19.6 10.3 54.4 3.2 38.4 12.3 D P

N-31 to 38 k 2 4.55 62.3 2.7 40.0 6.9 80.7 2.1 19.0 8.8 D P

N-41 to 4S h 3 5. 17 63.2 3.3 39.2 7.4 57. 6 8.2 (

f ) (0 C P

N-51 to 58 k 3 4.68 49. 7 2. 7 48.4 3.6 80.0 3.6 38.5 9.3 c P

N-61 to 68-.. h 4 4.71 33.8 4.3 21.2 4.9 31.6 24.5 23.7 15.

1

D P

3NT-71 to 78 k 4 5.44 28.7 22.0 46.0 4.4 29.7 22.5 19.6 13.1 D P

Average — .... 54.3 5.2 36.3 6.0 59.9 8.7 28.1 10.8

RED, SEMIMAT ACID-RESISTANT ENAMEL

P-1 to 8 k 1 5. 54 86.3 3.8 84.3 3.5 90.3 3.4 68.0 2.8 AA G

P-11 to 18 1 1 24. 7 17. 1 26.0 35.3 P-21 to 28 k 2 38.0 29.4 46.2 41.2 P-31 to 38 l 2 3. 99 48.6 6.2 26.9 10.8 73.9 1.1 55.6 8.7 C P

P-41 to 48 k 3 4. 95 70.6 2.6 91. 1 4.2 71. 1 2.9 85.8 4.4 A G

P-51 to 58.. 1 3 3. 34 84.2 5.9 79.0 12.9 94.4 10. 1 94.2 10.9 A P

P-61 to 68 k 4 4. 97 83.5 2. 7 89.8 2.4 72.6 4.6 94.4 4.0 AA G

P-71 to 78 . 1 4 5. 54 89.7 5.3 85. 1 6.

1

93.8 6.3 87.8 1.4 AA F

77.

1

11. 1 76.0 10.8 82. 7 13.2 81.0 14.3

RED, SEMI MAT, NONACID-RESISTANT ENAMEL

S-l to 8. _ 1 1 24.3 14.6 28.0 40.6 S-ll to 18 1 15. 6 4.

1

8.8 22. 6 S-21 to 28 1 2 4. 51 76.9 2.9 30.6 5.6 80.3 1.0 76.9 4.3 B P

| S-31 to 38 m 2 4.51 63.8 6.7 36.3 12.4 63.5 5.4 43.6 9.7 C P

S-41 to 48 i 3 3. 50 61.9 2.6 29.3 5.6 70.5 3.5 41.9 7.4 A P

| S-51 to 58 m 3 3. 95 89.4 1.0 63.0 6.0 90.0 1.8 82.2 1.9 A F

S-61 to 68 i 4 4. 50 38.8 14.3 23.

1

7.9 48.7 12.2 24.3 21.4 C P

S-71 to 7S m 4 3. 57 50. C 18.6 35.7 7.9 61.0 12.7 27.9 9.9 C P

Average — .... 63.5 10.7 36.3 8.0 69.0 9.2 49.5 14.7

BLACK, GLOSSY, ACID-RESISTANT ENAMEL

T-l to 8 m 1 7.30 57.9 0.8 71.8 0 78.7 0.8 69.0 0.8 AA G

T-ll t.0 IS n 1 6. 60 73.4 0.4 88.9 .8 82.5 .8 80.4 0 AA E

T-21 to 28 m 2 5. 38 87.8 1.4 97.8 . 7 95.6 .6 86.4 7 AA E

T-31 to 3S n 2 5. 55 79.

1

0.8 93.9 0 89.3 .8 (0 (0 AA E

T-41 to 48 m 3 6.09 64.2 0 92.6 0 88.

1

. 6 79.4 0 AA E

T-51 to 58 n 3 5.90 85.4 1. 5 93.6 0 81.6 1.6 80.6 0 A E

T-61 to 68 m 4 6. 55 66.7 1. 1 68.2 .6 86.8 2.0 69.0 . 1 AA E

T-71 to 78 n 4 6. 56 57.6 0.8 65. 7 .6 86.2 1.4 67.9 .6 AA G

Average — .... 71.5 0.8 84.1 .3 86.1 i.i 76.1 .3

BLACK, GLOSSY, NONACID -RESISTANT ENAMEL

V-l to 8 n 1 5.52 61.5 19.9 69.0 11.5 74.2 15.8 53.3 20.5 C P

V-ll to IS a 1 5. 62 34.5 41.3 46.7 13.2 17.8 40.5 23.8 32.6 C P

V-21 to 28 n 2 5. 76 64.0 1.4 53. 7 4.6 86.7 2.6 34.4 7.9 C P

V-31 to 38 a 2 5. 60 41.

1

4.0 42. 5 6.4 58.5 5.8 55.3 5.6 C F

V-41 to 48 n 3 5. 30 66.3 4.6 76.8 5.2 81.8 4.6 (0 (0 C F

V-51 to 58 a 3 5. 49 58.2 12.3 75.6 6.9 82.3 9.0 60.9 7.2 C P

V-61 to 68 n 4 4. 67 54.1 13.4 59.0 13.1 74.7 5.1 29.9 17.4 C P

V-71to78.. a 4 5. 45 61.7 6.4 62.5 16.3 56.1 4.5 65.0 6.4 C P

Average — .... 55.2 12.9 60.7 9.6 66.5 11.0 46.

1

13.9

a Groups of S panels exposed, 2 at each location. A ninth panel was kept

in storage.

b Figures reported are for percentage of light incident at 45° that was specularly reflected. Enamels with 45° specular-gloss readings of 4.5 or greater are

referred to as glossy, those with readings between 2.0 and 4.5 as semimat or

satin-textured, and. those with readings of less than 2.0 as full mat. The

designations of glossy and semimat used in the headings are those assigned by

the manufacturers. Many of the initial specular-gloss readings do not con- form with these designations. The full-mat enamels having initial gloss too low for measurement are represented by blank spaces in columns.

c Percentage of gloss retained, Gr, was computed from 45° specular-gloss readings. Color change, AE, is in NBS units as computed from colordifference measurements. Values in both cases are average of 2 panels with

2 readings on each panel.

d Test made on storage panel in accordance with Test for Acid Resistance

of Porcelain Enamels, part I, Flatware. Issued by the Procelain Enamel

Institute, 1145 19th St., X. W., Washington 6, D. C. In this test the degree

of attack is evaluated after treatment with citric acid. Class A A shows no

visible effect from the treatment and is the most resistant, with class A, class; B, class C, and class D following in that order. Enamels falling in the latter two classes are not considered acid resistant.

e E, excellent; G, good; F, fair; and P, poor. See text for procedure used in assigning ratings.

f Panels lost to study because of corrosion of attachment lugs.

6
4.3. Gloss Measurements

Specular-gloss measurements were made on

each panel with the Hunter Multipurpose Reflectometer [7] adjusted for a 45° angle of incidence.

The gloss scale of the instrument was calibrated

against liquid films [8]. Measurements were made at two fixed locations at the center of each

panel and compared with similar measurements

made at the start of the investigation. The

data were expressed as the percentage of initial specular gloss retained.

Table 3 includes the gloss data for each enamel

at the four exposure sites.5 Comparison with the

results of the 7-yr inspection [4] shows that the

percentage of gloss retained in many cases was

higher after 15 yr than it was after 7 yr. This

effect is due to the difference in cleaning treatment,

the panels at 7 yr having been washed with

trisodium phosphate solution, whereas the scouring treatment was required to clean the specimens

for the 15-vr measurements.

Except for a few isolated cases, the percentage

of gloss retained was considerably higher for enamels of high acid resistance than for the nonacid-resistant types. The enamel showing the

least change in gloss for all four locations after

15 yr was an acid-resistant black (T-21 to 28),

which retained an average of 91.9 percent of its initial gloss. Among the poorest was a nonacidresistant composition (K-51 to 58), which had

an average percentage of gloss retained for the

four locations of only 22.7. In general it was noted that the specular gloss

changed at a faster rate in the earlier stages of

exposure than later. Figure 4 illustrates this

effect for five panels at Washington, D. C., whose

surfaces had been washed with trisodium phosphate solution. These same panels, along with

all others from Washington, were later cleaned

by scouring so as to conform to the cleaning

procedure used at the other three locations.

The effect of the scouring treatment in raising

the percentage of gloss retained by the five panels

is shown by the following tabulation:

Percentage of gloss retained

Ac-id resist- Panel ance ("PEI

test) Cleaned with W ashed with scouring

NasPOr powder

E—71 AA 64. 3 76. 0 P-41 A 56. 3 71. 7 E—12 B 48. 0 53. 1 C-l 1 C 27. 2 28. 6 B-41 D 16. 1 15. 0

3 The initial gloss values, G-. are expressed as the percentage of incident

light that is specularly reflected. This conforms to the practice used in the

earlier papers. Recent practice in the industry, however, is to express the values in “gloss units,’' defined as 10 X 0«.

0 2 4 6 8 10 IP. 14 16 18 EXPOSURE TIME, YEARS

Figure 4. Selected data showing change in gloss with ex- posure time for five panels at the Washington, D. C., si/e.

Letters following panel numbers indicate acid-resistance ratings by citric acid spot test.

ACID RESISTANCE -PE I TEST

Figure 5. Percentage of gloss retained for • classes of acid resistance after exposure for Id .

' s Lakeland, Fla.

Dashed line is drawn through average value for each class of aci l resistant

420965—57
The curves plotted in figure 4 represent selected

data, in that these particular five panels were the

ones that best illustrated the usual weathering

behavior of enamels having different degrees of acid resistance when measured by the citric acid

spot test of the Porcelain Enamel Institute. 6 Actually, table 3 shows appreciable differences

in the percentage of gloss retained for enamels

of the same acid resistance. This high scatter

for the Lakeland panels is illustrated in figure 5.

4.4. Color-Difference Measurements

The color changes that occurred during the

1 5-yr exposure period were measured with a Hunter Color-Difference Meter [9]. In each case, the exposed panel was compared with the storage

panel, and the difference between the two ex- pressed in |NBS units [10], Diffuse reflectance

measurements were made at the beginning of the

investigation, and these indicated excellent color

match between the storage panels and the ones

selected for exposure. One NBS unit is about

five times the smallest difference perceptible

to the eye under the best experimental conditions

[11], It is the opinion of the authors that five NBS units might be considered objectionable by

a user, especially if two panels showing such a color difference were placed side by side in a structure. A change of 10 units due to weathering

would probably be objectionable in most instal lations where retention of a specified color is important, for example in signs and structures where the color has been specially selected as an

identifying characteristic.

Table 3 shows that color changes were minor for the A\diite enamels, except for the St. Louis panels

of poor acid resistance. These are the same

enamels that became etched during weathering.

Dirt and grime in the atmosphere at the St. Louis

site entered the pores created by the etching

action, and these dirt particles could not be re- moved completely by the cleaning treatment. The presence of the dirt particles retained after cleaning made the panels appear off-white to the

eye and darker to the color-difference meter. At

the other sites, there was less pollution of the

atmosphere, and dirt retention after cleaning of the nonacid-resistant enamels had a much smaller

effect on the color-difference measurements.

The colored enamels of poor Aveather resistance

became lighter and Aveaker in color with exposure,

which gave them a faded appearance. The

greatest color change aahs obseiwed for the full-mat

enamels, P-11 to 18, P-21 to 28, S-l to 8, and

S-ll to 18 in table 3; in addition, these were the

compositions that AA'ere the most difficult to clean.

6 Test, tor Acid Resistance of Porcelain Enamels, part I, Flatware. Issued by the Porcelain Enamel Institute, 1145 19th St., N. W., AAr ashington 6, D. C. In the commercial test, which separates enamels according to classes, a small

pool of 10-percent citric acid is placed on the specimen for 15 min at 80° F. The degree of attack is then evaluated visually bv observing such characteristics as staining, blurring of image, and ease' of removal of a pencil mark.

Class AA shows no visible effect from the treatment and is most resistant, with Class A, Class B, Class C, and Class D following in that order. Enamels falling in the latter two classes are not considered acid resistant.

4.5. Microstructure of Surfaces

The surfaces of the enamels that were examined

with the metallographic microscope at the 7-yr

inspection were reexamined after 15 yr of exposure. In general, the appearances of the surfaces Avere

( unchanged. The enamels showing little or no

change in surface microstructure at 7 yr still showed little or no change at 15 yr. The photomicrographs shoAvn in the earlier report [4] of enamels of poor acid resistance are representative

also of the 15-yr appearance. A gel-type layer was present on enamels of this type, and the , thickness of the layer Avas found to increase with

exposure time, but this increase occurred Avith- out much change in surface appearance. 7 The

thickness of the layer varied Avith enamel composition. On a black nonacid-resistant enamel,

represented by panels V-ll to V-18, the thickness

had increased from an average of 0.001 in. at 7 yr

to a thickness of about 0.0025 in. at 15 yr. The

film still maintained a glossy surface appearance,

but was soft enough to be scraped away Avith a knife blade. Table 3 shows the average color , change of this enamel at the four locations to be

31.9 NBS units. Similar gel-like films of measurable thickness

had formed on some of the other enamels of poor

acid resistance after the 15-yr exposure period.

For example, panel P-27 (Atlantic City) shoAved a

film thickness of 0.0005 in., V-63 (Lakeland) a

thickness of 0.0004 in., and V-67 (Atlantic City)

a thickness of 0.001 in. It should be pointed out that only those enamels

of very poor Aveather resistance showed layers of 1 this type. The acid-resistant compositions may

have had very thin films of a gel-like nature

present on their surfaces, but, if so, the films were

too thin to have any marked effect on gloss or

color.

5. Effect of Variation in Weather Conditions at Four Exposure Sites

Tables 4 and 5 are summaries of the data on

gloss and color difference, respectively, at the four

exposure sites. For convenience, the enamels

have been grouped according to their acid resist- ance by the Porcelain Enamel Institute Spot Test

for the Acid Resistance of Flatware. In the table

giving the gloss summaries (table 4) the grouping

is made without considering color. The initial color, hoAveAmr, had a definite effect on the magnitude of the color difference after exposure. Therefore the enamels in table 5 are grouped according ; to both color and acid resistance.

Standard deviations are listed in both tables. In practically all cases these values are high, indi- cating poor agreement among panels in any one

group. Nevertheless a trend appears to be

7 The scouring treatment used for cleaning removed the thinner gel-like ^ layers, but cn panels having layers 0.001 in. on heavier, some of the film remained even after prolonged scouring. 8
Table 4. Average percentages of initial specular gloss retained for enamels of various classes of acid resistance after Id gr of weathering at J+ exposure sites

Number of enamels Aeid-re- averaged » sistance

class b

Percentage of initial specular gloss retained *

Washington Lakeland St. Louis Atlantic City All sites

Avg SD Avg SD Avg SD Avg SD Avg SD

29 AA 79.0 9.9 77 7 13.3 84.5 7.2 80.6 10.7 80.4 10.6

15 A 71.

1

13.3 65.3 19.8 80.3 10.4 73.6 12.4 72.8 14.4

3 B 73.

1

9.3 55. 2 18.

1

81.8 1.8 43.1 5. 5 63.3 10.6

21 C 47. 5 16.9 50.3 15. 5 63.6 17.6 37.6 14.9 49.7 16.3

18 D 31.1 12.0 30.5 9.5 48.2 8.9 22.6 6.4 33.

1

11.1

- Each enamel represented by two panels at each location.

t From spot tests made on 12- by 12-in. storage panels, using the standard test of the Porcelain Enamel Institute.

- Measurements made on panel surfaces cleaned with a commercial scouring powder; SD denotes the standard deviation of the individual rcadim- • ju* the average.

Table 5. Average color differences between exposed panels and storage panels for enamels of two classes of arid resistanrr

after 15 yr of weathering at f exposure sites

Xumber

Color difference in XBS units c at

Enamel color

of en- amels

averAcid re- sistance b Washington Lakeland St. Louis Atlantic City All sites

aged a Avg SD Avg SD Avg SD Avg SD Avg SD

White f 11 AR 1.4 0.8 1. 7 1.0 1. 5 0.9 1.8 1.0 1.6 0.9

1 7 Xon-AR 2. 7 1.4 1.9 1.3 5.7 1.8 2.6 0.9 3.2 1.2 Buff r 12 AR 1.4 1.2 1.8 0.9 1.

1

0.7 1.2 .8 1.4 0.9

\ 12 N'on-AR 7.0 3.8 4.0 1.6 3.8 2.6 9.4 O. { 6.

1

3.8 Red r i6 AR 2.8 2.3 5.2 3.6 3.5 2.6 4. 1 3.0 3.9 2.9

1 12 X" on-A R 7.3 6.7 7. 2 2.8 8.4 7. 7 11.4 3.8 8.6 5. 6 Black / 8 AR 0.9 0.4 0. 7 0.1 1.

1

0.5 0.5 0.3 0.8 0.4

l 8 Xon-AR 12.9 12.2 9. 7 4. 1 11.0 11.8 14.0 9.4 11.9 9.9 White d __ _ 9 2.3 0.8 3.2 0.3 12. 8 3.4 3.5 0.9 1.8 Colored d _ . __ .. 8 18.3 10.4 10.0 9.2 16.8 14.2 23.2 14.7 16.8 12.4

a Each enamel represented by two panels at each location.

- Grouped according to spot "test of Porcelain Enamel Institute; AR includes class AA, A, and B enamels, Xon-AR includes class C and I).

• Measurements made on panels cleaned with a commercial scouring powder; SD denotes the standard deviation of the individual readings about 'he average.

d Full-mat enamels; all others are glossy or semimat.

present, the conditions at Lakeland being- more

severe on the acid-resistant panels (class AA, A,

and B ) and the conditions at Atlantic City giving

the most attack on the nonacid-resistant (class C

and D) compositions. This order of severity is a reversal of the trend noted in the 7-yr inspection [4]. The cause of the reversal is believed

to be the difference in the cleaning procedures.

The scorning treatment used at the 15-yr inspection removed surface films more effectively than

the washing treatment used at the 7-yr inspection,

and, in addition, scouring corresponds to commercial cleaning practice. Therefore, it is sug- gested that the 15-yr results give a more realistic picture of the relative severity of the exposure

conditions at the four sites than do the 7-yr data.

The greater severity of conditions at Lakeland

on acid-resistant compositions is especially noticeable for some of the red enamels. For example,

table 3 shows a color change at Lakeland of 10.6 XBS units for the enamel represented by panels

L-21 to L-28, and 12.1 units for L-31 to L-38,

yet the greatest color change for these same two

compositions at any of the other sites was only 4.5

units. The cause of the greater changes in color

and gloss for acid-resistant enamels at Lakeland

has not been determined, but it could be associated

with (a) the higher values for annual sunshine,

rainfall, and temperature, as given in table 2. or

(b) the attack by organic acids produced by algae

and fungi that were found to be attached on the

surfaces of all panels at the Lakeland exposure

site, and that were described in the earlier paper [4]. Similar growths have been found by

Jones [12] to cause etching of certain optical

glasses used in the tropics.

The sulfur dioxide content of the atmosphere

at the St. Louis site was found to be about 45

times as great as in Washington [3], yet table 4 shows that, on the average, the Washington panels were affected more by the 15-yr exposure than

those at St. Louis. This could be caused by the

heavier rainfall in Washington see table 2 . but

a more likely explanation is that the heavy ac- cumulation of dirt and grime on the panel surt'ae - at St. Louis acted as a protective layer. Had

the St. Louis panels been thoroughly cleaned at periodic intervals, the attack at St. Louis migl

well have been greatly increased.

A statistical analysis, performed b\ W . J. Youden of the Applied Mathematics Division at the National Bureau of Standards, showed a
the observed differences in behavior at the four

sites were statistically significant. The same

analysis showed the presence of small but real

differences in frits of the same type made by

different manufacturers; however, there was no

evidence of significant differences between enamels

made with the same frit by two fabricators.

6. Laboratory Tests for Predicting

Weather Resistance

An acceptable test for predicting weather

resistance should give a definite separation between the enamels that showed only minor

deterioration after 15 yr of exposure and those

that showed objectionable fading and loss of

gloss. To determine whether or not the citric acid spot test would give such a separation, all of the enamels except the full mats were first given ratings of excellent, good, fair, or poor, depending oil a combination of their measured

gloss and color-change values. These ratings were then compared with the results of the citric acid spot test. The adjective ratings for the comparison were

assigned in accordance with the following arbitrarily chosen criteria:

Rating Percentage of

gloss retained Color change in NBS units

Excellent 65 to 100 0 to 2.5 Good _ 50 to 65 2.5 to 5.0 Fair__ 40 to 50 5.0 to 7.5 Poor 15 to 40 >7.5

Enamels with values of gloss retained between

65 and 100 percent at all four sites were rated

excellent only if the maximum color change did

not exceed 2.5 NBS units at any of the locations.

If the values for either gloss or color change fell outside of these limits at any one of the four sites,

the enamel was given the lowest applicable rating.

For example, in table 3 the enamel represented by

panels P-51 to 58 would be rated excellent from

the standpoint of gloss retained (lowest value

79.0), but the maximum color change at one of the test sites was 12.9 units. On the basis of color change alone this particular enamel is rated

poor, in spite of its excellent gloss retention.

Conversely, the white enamel represented by

panels C-71 to 78 shows a maximum color change

of only 1.9 units; yet, because the percentage of gloss retained at Lakeland was as low as 33.0,

the enamel received a rating of poor. The effectiveness of the spot test in separating

enamels of varying weather resistance when rated

in accordance with the aforementioned procedure

is given in table 6. The figures in parentheses

give the number of red enamels included in the

Table 6. Number of enamels having weather-resistance

ratings of excellent, good, fair, and poor when grouped

according to the citric acid test

Acid resistance (PEI test) Number

of enamels tested

Number of enamels with weatherresistance rating of

Excellent 11 Good a Fair a Poor a

AA 29 11(1) 16(5) 2(1) 0 A 15 1 7(2) 3(2) 4(3)

li 3 0 0 1 2(2) C . 21 0 0 2 19(8) D 18 0 0 0 18(4)

a Figures in parentheses are number of red enamels included in the total. See text for procedure used in assigning ratings.

total. From these figures it can be seen that if the red enamels are eliminated from consideration,

there are only three remaining cases in which an

acid-resistance rating of A or AA did not correctly

indicate a weather-resistance rating of good or

better. Two of these three had fair weather

resistance.

The poor ratings of most of the red enamels of

class AA, A, and B acid resistance is a result of excessive color change. It should be pointed out

that red enamels are pigmented with the so-called cadmium sulfo-selenide complexes. When such

pigments are subjected to strongly oxidizing conditions, the}r are known to change color. Thus, if the surface of a red enamel were to become slightly

etched during weathering, the exposed pigment

particles would be expected to change in color if oxidizing influences were present. When the

vitreous phase of the enamel initially surrounds

and covers the particles of pigment completely,

and is not subsequently etched away, the pigment

is protected against the color change associated

with oxidation. Correspondingly, a laboratory

test, to correlate well with performance data, must

provide a commensurate degree of etching, and for pigments that are highly sensitive to oxidation,

also a commensurate degree of oxidizing potential.

In this connection, McDonald [13] found that cer- tain red screening enamels faded noticeably in 1 to 4 years, although they showed class AA acid

resistance by the citric acid spot test. He also found that a spot test with concentrated nitric acid, which is more corrosive and highly oxidizing,

would distinguish the red enamels showing poor

weather resistance from those that were resistant

to fading.

Sweo [14] noted that a red semimat enamel of

class A acid resistance showed poor weatherability

when exposed for 10 months. This same enamel

gave a weight loss from 40 to 200 times greater

than class A enamels of good weather resistance

when the specimens were subjected to a boilingsolution of 6-percent (by weight) citric acid for

2 y2 hr. Therefore, Sweo suggested that the boiling

citric acid test might be useful in predicting the

suitability of enamels for exposure applications.

10
Botli the nitric acid spot test [13] and t lie boilingcitric acid test [14] were used in the present in- vestigation on storage panels of several of the

red enamels. The correlation was somewhat

better than the citric acid spot test, but neither

test gave results that would predict the rather

large color change found for enamels L-21 to 28

and L-31 to 38 at Lakeland. On the other hand,

a treatment for 2b hr in a boiling aqueous solution

of 10-percent (by weight) nitric acid did dis- tinguish these two enamels from the acid-resistant

reds that gave satisfactory weather resistance at

all sites. Therefore, because of the promise shown

by the boiling nitric acid test, all of the red enamels were subjected to this treatment. The test was made on the 4- by 6-in. laboratory

specimens. Each specimen was weighed to the

closest milligram and then clamped to the ground

face of a Pyrex-brand glass tube of 2%-in. inside

diameter, a rubber gasket being used to prevent

leakage. A reflux condenser was attached to the

opposite end of the tube, and 70 ml of the acid

solution was added. The assembly was next placed on a hotplate with the back of the specimen

in contact with the hot surface and the solution

allowed to boil for 2 }{ hr. After removal from the

hotplate, the specimen was cleaned by rubbing

with a sponge wetted with a 1 -percent (by weight)

solution of trisodium phosphate, rinsed in ethyl

alcohol, air dried, and then reweighed, the change

in weight being expressed as milligrams per square

centimeter of exposed surface. In addition, the

difference in color between the exposed and un- exposed areas was measured, and the results expressed in XBS units.

Table 7 lists the results of the boiling nitric

acid test for the 28 glossy and semimat red enamels

included in the study. The enamels are arranged

in groups according to their weather resistance

rating. The acid-resistance class by the citric acid spot test is given for purposes of comparison.

It can be seen from this table that there are 5 enamels of class A and B acid resistance by the

citric acid spot test that show unsatisfactory

weathering behavior at 1 or more of the 4: exposure

sites. These 5 enamels all gave weight losses greater than 1.0 mg/cm2

. The 4 class A finishes

with passable weather resistance (excellent, good,

or fair) and all of the class AA enamels show

weight losses of less than 1.0 mg/cm2

. Thus, for the red enamels included in this investigation,

the weight change after 2% hr of boiling in 10- percent-by-weight nitric acid gave the desired

separation between enamels of poor weather re- sistance and those that were considered passable.

The data in table 7 show that the measured

color change after test is not as reliable a criterion

as weight loss, because one enamel of good weather

resistance (P-1 to 8) shows a fairly high color

change (5.3 NBS units) after the nitric acid

treatment, whereas one of the enamels of poor

weather resistance (P-51 to 58) shows a color

change of only 3.6 XBS units.

Table 7. Citric acid spot lent and boiling nitric arid, tee 1 results for red enamels having excellent, good, fair, anil poor

weather resistance

j Specimen identifies- Weather resist- Acid re- sistance

Boiling nitric acid

test*

tion anee rating PJ l test) Weight

loss Color change

L-ll to 18 Excellent. __> J AA

mo/cm1 0.29 NBS

unit*

2.3 L-l to 8 AA 26 1.6

l. 11 to 48 AA .30 1.8 P-l to 8 A A .44 5.3 L-71 to 78 IGood. . _ AA .58 2.5 P-61 to 68 A A .88 2.6 L-51 to 58 A .63 1.9 P-41 to 48 A .30 0.4 P-71 to 78 1 ( AA . 53 2.5 L-61 to 68 >Fair. A .30 1. ] S-51 to 58 1 l A .93 9.3

S-41 to 48 . 1.30 7 7

1 P-51 to 58 A 1.37 3.6 L-31 to 38 A 1.62 10.9 L-21 to 28 B 2.35 9.6 S-21 to 28 B 3.50 23.7

S 31 to 38 C 4.98 33.

5

P-31 to 38 c 1.89 6.8 N-41 to 48 C 2.60 17. 1 N-5I to 58 >Poorl>- __ c 3.22 14. 1 N-l to 8 . . c 17.73 51.9 N-ll to 18 c 5. 46 50.1 S-61 to 68 c 34. 52 .50.0 S-71 to 78__. c 12.84 46.0 N-21 to 28 D 24.38 56. 7 X-31 to 38 D 23.00 60.0 N-61 to 68 D 89.59 37. 5

IN -71 to 78 _ ... D 22. 59 39.7

a Measurements made on one specimen from each group.

b Poor ratings of two of the three A and one of the two B enamels were

assigned because of the large color changes noted at the Lakeland site.

7. Discussion

The four most important requirements of a porcelain enamel finish that is to be used for outside exposure are (1) corrosion protection. (2) color stability, (3) ease of cleaning, and (4 absence of major changes in initial gloss.

The results of the 15-yr study show that cor- rosion protection on a steel base can be obtained

with any of the enamels included if proper attention is given to initial coverage. Two coats of enamel should be applied to the back surfaces of

all panels and to attachment lugs, except possibly

for those installations where absence of moisture on the back surface of the panels can be assured

for the life of the installation.

From observations made during the test, and

from a study of the literature, the change in color

with exposure is believed caused by three effects. The first and most important effect is the slow

leaching of soluble constituents from the structural network of the enamel glass, leaving behind

a gel layer rich in silica. The hydrated layer

usually assumes a lighter color than the original

surface and gives a faded appearance to the panel.

'Phe layer is probably present on all enamels after long exposure, but only on the enamels of poor

acid resistance does it become sufficiently thick

after 15 yr to give objectionable changes in color
and gloss. It is significant that glass on long ex- posure shows this same type of gel formation at the surface [15, 16]. The second effect that can change the color of enamel glasses is solar radiation. Parmalee and

Badger [17] found a distinct darkening of a cer- tain whiteware glaze when exposed for 2 months

to sunlight through a window, or when exposed

for a few hours to radiation from either a quartz

mercury arc or a carbon arc. Tests made early

in the present study [3] showed that there was no

detectable change in color for any of the 1 4 types

of enamel after 500 hr of exposure to carbon arc

radiation. Nevertheless, solar radiation striking

the panels over a 15-yr period could conceivably

have an unfavorable effect. In fact, the greater

color change for the acid-resistant red enamels at Lakeland, which had the highest annual sunshine,

might have been caused, in part, by the effects of solar radiation.

The third factor that might affect color change

is the oxidation of the pigment used for coloration.

Most ceramic pigments are mixtures of oxides that

are known to be chemically stable. The red and

orange enamels; on the other hand, are normally

pigmented with complexes formed from mixtures

of cadmium, selenium, and sulfur, whereas yellow

enamels normally contain cadmium sulfide. Such

compounds are sensitive to strong oxidizing

agents. It is conceivable that a slow oxidation

of these compounds could occur during exposure

and thereby cause a color change. This effect would be most noticeable in enamels with high

pigment concentrations, and especially in those

cases where the particles of pigment at the sur- face were not properly covered by the enamel

glass.

The ease of cleaning of enamel surfaces after long exposure is one feature of an enamel finish that helps to make it popular both for signs and

for architectural installations. All enamels, with

the exception of those with a full-mat finish (see

footnote 3), clean easily when first installed. As

exposure time increases, however, the finishes of poor acid resistance become etched and cleaning

becomes more difficult. After 15 yr of exposure,

wide differences in cleanability were noted between acid-resistant and nonacid-resistant compositions, the surfaces with poor acid resistance

being more difficult to clean. At the two sites where there was little atmospheric contamination

by soot particles (Washington and Lakeland),

cleaning was not a problem even on those finishes

that had become etched during weathering.

The four exposure sites selected for the tests have fairly high annual rainfalls (see table 2). Moisture is essential to the formation of the gel

layer mentioned earlier; therefore, in a dry

climate, weathering would be expected to proceed

at a slower rate than at Washington, Lakeland,

St. Louis, or Atlantic City. Likewise, in an area

of extremely heavy rainfall the weathering action

would undoubtedly be accelerated.

The direct relation between the acid resistance

of an enamel finish and its resistance to weathering

is evident in tables 3, 4, and 5. These results are in agreement with the findings of the earlier inspections on the same panels [3, 4]; also with

conclusions of other investigators [14, 18, 19,

20 , 21 , 22], The citric acid spot test might be considered

as a reasonably satisfactory laboratory test for gaging weather resistance if the correlation was

as good for the red enamels as for the other types.

However, the fading of the reel finishes cannot

be predicted with certainty by the citric acid spot- test ; hence for red enamels it should be supplemented with a second treatment that provides

strongly oxidizing conditions. The boiling 10- percent nitric acid test appears to fulfill this

requirement

.

In the present study, all panels were exposed

at a 45° angle. This type of exposure would be

expected to give a more rapid weathering of the

enamel than it the same enamel were exposed on

a sheltered vertical wall. It is significant that even under the severe conditions of 45° exposure,

tiie enamels with class AA acid resistance by the

PEI test showed only small changes in gloss and

color after the 15-yr exposure period.

8. Recommendations

On the basis of the 15-yr data the authors feel justified in making the following recommendations:

1 . When the enamel is applied to an iron or low-carbon-steel base, at least two coats of enamel

should be applied to the back surface so as to insure good coverage of the metal and prevent

damage from corrosion. Good coverage on the

face side is an obvious requirement.

2. Only enamels of class A and class AA acid

resistance (PEI test) should be used in any

architectural installation where general appearance, absence of fading, and ease of cleaning are important. In addition, when the color of the

enamel is red, orange, or yellow, only those

enamels should be selected that will give a weight

loss of less than 1.0 mg/cm2 when subjected to a

solution of boiling 10-percent nitric acid for 2% hr.

3. Where appearance is an important factor,

full-mat enamels of the type included in this investigation should not be used for outside

installations, as they tend to accumulate and

retain a dirt film. In addition, the colored full- mat enamels fade after short exposure periods.

4. New enamel types, and especially those

prepared from screening pastes, should be tested

for weather resistance as they are developed to determine whether or not they show the same

correlation between acid resistance and weather

resistance that was shown by the enamels included

in this study. This correlation could be established in periods of considerably less than 15 yr. 12
9. Summary

An examination of 784 1-ft-square porcelain

enameled panels of varying types was made after approximately 15 yr of exposure at Washington,

D. C.. Lakeland, Fla., St. Louis, Mo., and Atlantic

City. X. J. The observations made after this examination may he summarized as follows.

1. Ease of cleaning was related to weather re- sistance. the enamels showing large losses in gloss

being more difficult to clean than those that showed

only minor losses.

2. All enamels successfully protected the steel base from corrosion when initial coverage was

complete.

3. Poor coverage on the backs of panels re- sulted in spalls on the face surface at 3 of the 4 locations after 15 yr of exposure.

4. Based on gloss and color-difference measure- ments on panels cleaned by scouring, the exposure

conditions at Lakeland were found to be some- what more severe on the enamels of high acid re- sistance whereas the conditions at Atlantic City were slightly more severe on the nonacid-resistant

compositions.

5. A direct relationship was found between acid

resistance as measured by the citric acid spot test and weather resistance as measured by changes

in gloss and color, except that some of the red

enamels of good acid resistance showed excessive

fading after 15 yr of exposure at Lakeland.

6. A test that involved exposure of red enamels

to a boiling solution of 10-percent nitric acid for 2k hr was found to give a separation of the compositions that faded from those that had superior

resistance to fading.

7. The best of the enamels showed no objectionable changes in either color or gloss at any of the four exposure sites.

The assistance of the Porcelain Enamel Insti- tute in the financing of the installation of the

panels is gratefully acknowledged; also, the help

of Thomas Robusto, Charles Brubaker, and John

Anderson of the Bureau staff, who assisted in making the gloss- and color-difference measurements, and of Gerald Galler, who performed the

tests with boiling nitric acid.

The investigation was made possible by the co- operation of the following companies, which supplied the necessary enamel frits and specimens:

Baltimore Enamel & Novelty Co. W. A. Barrows Porcelain Enamel Co. Chicago Vitreous Corporation

Davidson Enamel Products Co. Erie Enameling Co. Ferro Enamel Corp. General Porcelain Enameling & Mfg. Co. Ingram-Richardson Mfg. Co.

Pemco Corp.

Porcelain Metals Corp.

Porcelain Products Co. Seaporcel Corp.

.1. M. Seasholtz & Sons

Texlite, Inc. Toledo Porcelain Enamel Products Co. Wolverine Porcelain Enameling Co.

10. References

[1] Proceedings of Conference on Porcelain Enamel in the Building Industry, Research Conf. Report No.

6 of Building Research and Advisory Board, National Academy of Sciences (National Research

Council, 2101 Constitution Ave., Washington 25, D. C., March 1954).

[2] Proceedings of Conference on Architectual Porcelain Enamel (Porcelain Enamel Institute, 1145 19th

St., N. W., Washington 6, D. C., March 1955 ). [3] W. N. Harrison and D. G. Moore, Weather resistance

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

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

of porcelain enamels exposed for seven years, J. Research NBS 42, 43 (1949) RP1949.

[5] F. J. Norton, Diffusion of D2 from D2 0 through steel.

J. Appl. Phys. 24, 499 (1953).

[6] C. A. Zapffe and C. E. Sims, Relation of defect in enamel coatings to hvdrogen in steel, J. Am. Ceram.

Soc. 23, 192 (1940).

[7] R. S. Hunter, A multipurpose photoelectric reflec- tometer, ,T. Research NBS 25, 581 (1940) RP1345.

[8] D. G. Moore and R. S. Hunter, Use of liquid surfaces

as standards of specular gloss, J. Am. Ceram. Soc. 24, 167 (1941).

[9] R. S. Hunter, Photoelectric color-difference meter,

J. Opt. Soc. Amer. 38 , 661 (1948).

[10] R. S. Hunter, Photoelectric tristimulus colorimetrv with three filters, NBS Circ. 429 (1942).

[11] Deane B. Judd, Color in business, science and industry, John Wilev & Sons, Inc., New York, N. Y.

(1952).

[12] F. L. Jones, Deterioration of glass in tropical use,

J. Am. Ceram. Soc. 28, 32 (1945).

[13] E. F. McDonald, Comments on paper on weather

resistance, Proceedings of Conference on Architectural Porcelain Enamel, p. 44 (Porcelain Enamel

Institute, 1145 19th St., N. W., Washington 6. D. C., March 1955).

[14] B. J. Sweo, Correlation of weather resistance of porcelain enamels with chemical test data, J. Am.

Ceram. Soc. 32 , 333 (1949).

[15] A. W. Laubengayer, The weathering and iridescence

of ancient Roman glass found in Cyprus, J. Am.

Ceram. Soc. 14 , 833 (1931).

[16] J. W. Mellor, The durability of glazes, glasses and

enamels in service, Trans. Eng. Ceram. Soc. 34 , 113 (1934).

[17] C. W. Parmelee and A. E. Badger, Notes on the col- oration of glazes bv light, J. Am. Ceram. Soc. 13 , 1 (1934).

[18] M. G. Ammon, Exposure tests on architectural porcelain enamels, Enamelist 16 , 7 1936'

.

[19] B. J. Sweo, Progress report on weathering tests of architectural enamels, Enamelist 18 , 13 1940

[20] J. C. Hudson and T. A. Banfield. The protection of

steel bv various non-metallic coatings, .1. Iron Steel

Inst. 158 , 99 (1948).

[21] Anonymous, Weather resistance of enamel- . G , Email- Keramo-Tech 6 , 238 1955 . [22] R. Marker, Principles for testing weather resistance

of enamels, Silikattech 4 . 343 1953V

Washington, Xovember 21, 1950.

13

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[Continued from cover page n]

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6

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[Continued from cover page hi] Recommended Building Code Requirements for New Dwelling Construction With

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