REFERENCE

UNITED STATES PARTMENT OF OMMERCE

IJBLICATION

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NBS TECHNICAL NOTE 707

Three-Year Inspection

of Nature-Tone

Porcelain Enamels

on Steel

U.S.

PARTMENT

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NATIONAL BUREAU OF STANDARDS

The National Bureau of Standards1 was established by an act of Congress March 3,

1 901 . The Bureau's overall goal is to strengthen and advance the Nation's science and

technology and facilitate their effective application for public benefit. To this end, the Bureau conducts research and provides: (1) a basis for the Nation's physical measure- ment system, (2) scientific and technological services for industry and government, (3) a technical basis for equity in trade, and (4) technical services to promote public safety. The Bureau consists of the Institute for Basic Standards, the Institute for Materials Research, the Institute for Applied Technology, the Center for Computer Sciences and

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THE OFFICE FOR INFORMATION PROGRAMS promotes optimum dissemination and accessibility of scientific information generated within NBS and other agencies of

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1 Headquarters and Laboratories at Gaithersburg, Maryland, unless otherwise noted; mailing address Washington, DC. 20234.

2 Part of the Center for Radiation Research.

3 Located at Boulder, Colorado 80302.
«. BUREAU OF STANDARDS JAN 1 9 1972 UNITED STATES DEPARTMENT OF COMMERCE

Maurice H. Stans, Secretary

(t)C \Of ^.^, NATIONAL BUREAU OF STANDARDS • Lewis M. Branscomb, Director

a®. 70^

tin \ NBS TECHNICAL NOTE 707

ISSUED DECEMBER 1971

Nat. Bur. Stand. (U.S.), Tech. Note 707, 16 pages (Dec. 1971) CODEN: NBTNA

Three-Year Inspection of Nature-Tone

Porcelain Enamels on Steel

M. A. Baker

Institute for Applied Technology

National Bureau of Standards

Washington, D.C. 20234

#*« <" c„

NBS Technical Notes are designed to supplement the Bureau's regular publications program. They provide a means for making available scientific data that are of transient or limited interest. Technical Notes may be

listed or referred to in the open literature.

For sale by the Superintendent of Documents, U.S. Government Printing Office, Washington, D.C, 20402.

(Order by SD Catalog No. C 13.46:707). Price 30 cents.

Table of Contents

Introduction Page

1

1.1 Description of Enamels and Specimens 1

1.2 Exposure Sites 3

1.3 Gleaning of Specimens „ 3

1.4 Enamel Thickness 3

1.5 Continuity of Coating 4

1.6 Color o o 4

1.7 Gloss 4

1.8 Acid Resistance 5

2. Results and Discussion 5

2.1 Color Retention 5

2.2 Gloss Retention „ 5

2.3 Corrosion of the Base Metal ...... o 8

2.4 Comparison of Exposure Sites 12

2.5 Comparison with Enamels in the 1956 Exposure Test . . 12

3. Plans for Future Exposure Testing .......... 13

4» Summary ...» 13

5. References 14

lii

THREE-YEAR INSPECTION OF NATURE-TONE PORCELAIN ENAMELS ON STEEL

M. A. Baker*

A weather exposure test of nature-tone porcelain enamels on steel was initiated by the National Bureau of Standards and the Porcelain Enamel Institute in 1966. Laboratory measurements for changes In gloss and color were made after the enamels had been exposed

for 0.5, 1 and 3 years at Kure Beach, North Carolina, Miami, Florida, and Gaithersburg,

Maryland. The average gloss and color retained for all 450 specimens exposed at 3 sites for 3 years were 90.5 and 99.1 respectively. There was a tendency for the enamels exposed

at the marine sites to rust around pinhole-type defects. Scanning electron microscope

studies of these defects showed a layer of readily corrodible enamel on the bottom of the defects. A high-voltage test for continuity of coating was effective in detecting specimens

that were apt to rust during the first year's exposure.

Key words: Acid resistance; color; continuity of coating; porcelain enamel; weather

resistance.

1. Introduction

Porcelain enamel has been used as an exterior finish for buildings ranging from gasoline filling

stations to high rise office buildings. The wide variety of colors available, the ease of cleaning and

the relative ease of installation are factors contributing to the use of porcelain enamel In architectural applications. The weatherability of porcelain enamels has been evaluated by exposure testing

[1,2,3] and these exposure test data have been correlated with accelerated laboratory test data.

These data provide manufacturers of porcelain enamels with laboratory tests to estimate the weatherability of their products.

As new enamel types are developed, it is necessary to subject them to exposure testing to determine whether the accelerated tests used in the past are still accurate estimators of the enamels weatherability. This report is a summary of the first three inspections of a new series of enamels called

nature-tones which are characterized by low gloss, muted, and earthy colors.

1.1 Description of enamels and specimens

There were twenty-five different enamels included in this program. Twenty of these enamels are

the standard nature-tone colors 101-120 [4] while the five other enamels 1,3,4,6, and 7 in table 1 are

a yellow, tan, orange-beige, green and grey-beige color respectively. Each enamel was commercially

applied to twenty 4x6 inch and twenty-eight 4-7/16 inch square panels of enameling grade steel.

Research Associate from the Porcelain Enamel Institute at the National Bureau of Standards,

Washington, D.C., 1964 to present.

Figures in brackets indicate the literature references at the end of this paper.

-1-
Table 1. Summary of Pre-Exposure Data for Enamels in the 1966 Exposure Test of Nature-Tone Enamels on Steel.

Enamel 45° Specular

Gloss

Acid

Spot

Rating

Acid

Solubility

(mg/in )

Thickness Continuity of Coating

(mils) Probe Voltage

(kV)

Overvoltage

(kV)

101 13.8 B 14.4 8.8 1.5 0.6

102 13.5 B 9.5 8.0 2.0 1.2 103 13.3 A 8.3 7.5 1.5 0.7

104 17.4 B 7.4 9.2 2.0 1.1

105 14.9 A 1.6 9.6 1.0 0.1

106 27.3 A 7.8 9.1 2.0 1.1

107 14.4 A 1.8 9.9 1.5 0.5

108 19.2 A 1.2 9.4 2.5 1.6 109 16.6 A 0.7 9.2 2.5 1.6 110 20.7 AA 0.8 9.0 2.0 1.1

111 18.5 A 1.0 10.8 2.5 1.5

112 13.5 A 0.9 10.7 2.0 1.0

113 10.1 A 1.0 10.8 2.5 1.5 114 15.0 A 1.0 8.8 2.0 1.1 115 9.3 A 0.9 10.6 2.5 1.5

116 16.8 A 0.9 9.9 2.5 1.5

117 20.7 A 2.4 8.7 2.0 1.1 118 14.7 A 2.4 10.2 2.5 1.6 119 27.7 A 3.9 9.4 2.5 1.6 120 21.2 A 0.7 9.2 2.0 1.1

1 24.4 A 2.6 9.0 2.0 1.1

3 20.5 A 3.0 9.6 2.5* 1.6

4 11.3 A 2.8 8.8 2.0 1.1

6 2.2 B 8.4 9.1 2.0 1.1

7 3.4 B 6.9 9.4 2.5 1.6

All of the specimens exposed at Miami and Gaithersburg passed the highvoltage probe at 2.5 kV, but the specimens exposed at Kure Beach passed

the high-voltage probe at 1.5 kV.

-2-
1.2 Exposure Sites

Six specimens of each enamel were exposed at each of three exposure locations: the exposure site

of the National Bureau of Standards, Gaithersburg, Maryland; the tidewater exposure site of the South

Florida Test Service, Miami, Florida; and the 80-foot exposure site of the International^ Nickel Company,

Kure Beach, North Carolina. The 4-7/16 inch square specimens were exposed at Miami and Gaithersburg

while the 4x6 inch specimens were exposed at Kure Beach. The specimens at Miami and Gaithersburg were

exposed at an angle of 45° to the horizontal and they faced south. These exposure parameters were

modified for the enamels exposed at Kure Beach where the enamels faced the ocean at east-southeast and

were exposed on the racks of the International Nickel Company which were at an angle of 30° to the

horizontal. In addition to the exposed specimens, three specimens of each enamel were kept in indoor

storage and were used as reference panels. The remaining specimens of each enamel were kept as laboratory

specimens to be used for routine testing and new test development.

1.3 Cleaning of Specimens

The specimens were cleaned both before and after exposure by scouring 30 strokes with a sponge

that had been sprinkled with calcium carbonate and moistened with a one percent solution of trisodium

phosphate. Following scouring, the specimens were rinsed successively with tap water, distilled water

and alcohol. Before this method of scouring was selected for use, a glossy and a matte enamel were

scoured for 660 strokesby hand with the calcium carbonate abrasive on a cellulose sponge. The gloss

was measured after scouring for given intervals. The glossy enamel was not affected by scouring 660

strokes, while the matte enamel showed a 3 percent increase in gloss after scouring 260 strokes and

an 18 percent increase after scouring 660 strokes. It, therefore, appeared as though scouring might

have an effect on the enamel's gloss but this effect would be minimal through the first eight inspections if the enamels are scoured only 30 strokes per inspection as specified.

1.4 Enamel Thickness

The enamel thickness was measured, at one area on each of seven laboratory specimens of each

of the two sizes. These 14 measurements were averaged to give the thickness values reported in table 1.

The maximum standard deviation of the fourteen measured values was 1.0 mil among the 25 different

enamels.
1.5 Continuity of Coating

The high viscosity inherent with matte enamels makes it difficult for bubbles formed during the

fusing of the enamel to travel through the enamel and for the enamel to flow back together once the

bubble has burst. This may result in some discontinuities in the finished nature-tone specimens. In

order to insure that part of the porcelain enameled specimens placed on exposure were free from any

apparent discontinuities, one half of the specimens were placed on exposure on the condition that they

pass the high-voltage continuity of coating test [5 J without any discontinuities being located. The

other half of the exposure specimens were selected at random from the original lots of 48 specimens of

each enamel.

The probed specimens were tested at arbitrarily selected voltages of 2 or 2.5 kV dc. When too

many specimens failed at these voltages, and the possibility of running out of specimens before the

required number passed the continuity of coating test existed, the probe voltages were lowered to 1 or

2 1.5 kV dc. The actual probe voltages and overvoltages are presented in table 1.

1.6 Color

The three tristimulus color values [6] necessary to evaluate color change were measured with a

color difference meter both before and after exposure. One of the three storage specimens was used as

the color standard. The color change after exposure is reported as color retention which is 100 minus

3 the color change in NBS units .

1.7 Gloss

The 45° specular gloss was measured both before and after exposure at four orientations at 90°

to each other near the center of the specimen. Since these enamels have low gloss values, the gloss

meter was calibrated with a glazed ceramic tile having a specular gloss of 15. The initial specular

gloss values are presented in table 1. The gloss retained after exposure is reported as the percentage

of the initial gloss retained.

2 The term overvoltage is used to designate the difference between the applied test voltage and

the air gap voltage for an air gap whose length is estimated by the enamel's thickness.

3The color difference parameters, Rd (the 45°, 0° luminous daylight reflectance), a (red-green), and b (yellow-blue) were measured with a color difference meter. The color change in NBS units was calculated by the following formula:

I 2 2 Color Change (AE) =

J

A (10 JM) + Aa + Ab

2

-4-
1.8 Acid Resistance

The acid resistance of porcelain enamels has been used as an indicator of the enamels' weatherability [2,3]. The standard test used to determine the acid resistance is the citric acid spot test [7].

This test was performed on triplicate laboratory specimens of each enamel and these acid spot test ratings

are presented in table 1. All the enamels had acid spot test ratings of AA, A or B which is indicative

of good weather resistance even though enamels with AA and A acid spot test ratings are the only ones

recommended for architectural applications [8].

Since the citric acid spot test is difficult to evaluate on matte enamels, triplicate specimens

of these enamels were also subjected to the boiling citric acid solubility test [9], The results of this

test are presented in table 1 where it can again be seen that the enamels in this test have good acid

resistance as indicated by their low boiling-acid solubilities.

2. Results and Discussion

2.1 Color Retention

The color retention after 0.5, 1, and 3 years' exposure for these enamels is presented in table

2. These data indicate that the 450 enamels had an average color retention of 99.1, after 3 years'

exposure.

The color retention data are also plotted as a function of exposure time in figure 1. These

data indicate that the color change is greatest during the first six months' exposure after which it

tends to reach a constant rate of change. If the color continues to change at the current rate, these

enamels will change color on the average of 0.2 NBS units per year.

2.2 Gloss Retention

The gloss retained after 0.5, 1 and 3 years' exposure is presented in table 2. These data

indicate that enamels with higher initial gloss tend to decrease in gloss during exposure while the

enamels with the lower initial glosses tend to increase in gloss. This apparent increase in gloss is

probably caused by a combination of three effects: 1) the natural degradation of the enamel caused by

exposure to the weather, 2) the polishing action of the scouring given these specimens during cleaning

and 3) slight errors in measuring the gloss of enamels with extremely low gloss values. Therefore, it

should be kept in mind that these gloss data are probably less representative of the effect of weathering

than the color data.

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-7-
2.3 Corrosion of the Base Metal

One of the major reasons for applying porcelain enamel is to protect the base metal from corrosion. Therefore, it is of interest to determine whether newly developed enamels also provide this

protection. After cleaning, the enamels were visually inspected for signs of corrosion of the base metal.

This corrosion was noted by rust around pinhole-type defects or by pinhead-sized iridescent spots. The

iridescent spots were thought to be forerunners of the rust occurring around pinhole-type defects. The

total number of specimens with these two types of defects is presented in table 3. The data in table 3

indicate that the high-voltage continuity of coating test was fairly efficient in predicting the performance of enamels with discontinuities that became evident after 6-months' and 1-year's exposure. However, this difference between selected and unselected enamels did not continue after 3 years' exposure.

It was hypothesized that the apparent failure of the test to detect enamels that are apt to rust after

longer periods of exposure was caused by the existence, at the bottom of the pinhole-type defects, of

a thin layer of glass that is less corrosion resistant than the bulk of the enamels. This glass layer

would protect the base metal for a short period but would corrode itself after longer exposure periods.

The existence of a thin layer of glass would increase the voltage required to locate a discontinuity.

Figures 3,4, and 5 are scanning electron micrographs of three of these pinhole-type defects.

Figure 3 confirms the existence of a thin layer of apparently less corrosion-resistant glass on the

bottom of a pinhole-type defect. The glass is beginning to crack, and bits of corrosion products can

be seen collecting on the cracked glass surface. Once the glass has cracked, the corrosion continues

as evidenced in figure 4 which shows the bottom of another pinhole-type defect completely covered with

the corrosion product. Figure 5 shows another defect where the corrosion has evidently progressed

further. The corrosion products apparently collected and built up enough pressure to crack the enamel

surface and open up the defect thus allowing much of the corrosion product to be removed.

These figures verify the existence of glass at the bottom of pinhole-type defects that rust after

exposure. This indicates that higher overvoltages may be needed when testing for continuity of coating to insure that the enamels will not rust when placed on exposure.

No evidence has been found as yet to verify that the iridescent areas are indeed forerunners

of rust. Therefore, the specimens with rust spots appearing on the enamel's surface after three years'

exposure are presented in table 4. Again there is no significant difference between the enamels that

passed the continuity of coating test and those that did not in regards to rust after three years'

exposure.

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-9-
Figure 3. Scanning electronmicrograph of the bottom of a pinhole-type defect that rusted after being exposed for three years at Kure

Beach. Note the cracks and corrosion products

at the bottom of the defect. Original magnification 1150 times.

Figure 4. Scanning electronmicrograph of the bottom of a pinhole-type defect that rusted afte after being exposed for three years at Kure

Beach. Note the build up of corrosion products

on the bottom of the defect. Original magnification 700 times.

Figure 5. Scanning electronmicrograph of the bottom of a pinhole-type defect that rusted after being exposed for three years at Kure

Beach. Note the cracks in the enamel near

the pinhole opening and the relative lack of corrosion products inside the pinhole.

Original magnification 560 times.

-10-
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-11-
2.4 Comparison of Exposure Sites

The three-year color and gloss data in table 2 were subjected to a two sided sign test [10 ] at

the 95 percent confidence level to determine whether the atmospheric conditions at the three sites

caused the enamels to change color and gloss at significantly different rates. These analyses indicated

that, even though the color changes are extremely small at all sites, the color changed significantly

more at Kure Beach than at either Miami or Gaithersburg, while it did not differ significantly at the

latter two sites.

The gloss data for the enamels exposed at these three sites indicated that the greatest gloss

changes occurred at Gaithersburg, followed by those exposed at Kure Beach and Miami.

These data indicate that the color and gloss change at different rates. This apparent difference

is probably caused by the increase in gloss of many of the enamels with very low initial glosses. It is

probable that the ranking of sites based on color change is more reliable than the rankings based on

gloss retention.

The two marine exposure sites, Kure Beach and Miami, were included in this test to determine

whether the atmosphere at Miami, an exposure site operated by a commercial testing laboratory, was as

severe as that at Kure Beach, a privately owned exposure site not normally available to the general

public. There are three indicators of an exposure site's severity: changes in color; gloss retention;

and the number of specimens placed on exposure which had rust occurring on the enamel surface. Since

it was shown above that color change is a better indicator of the site severity than gloss retention,

only the color change and the number of specimens with rust on the surface will be considered when

determining whether Miami could replace Kure Beach as a severe marine environment. These two parameters indicate that the environment at Miami is not quite as severe as that at Kure Beach. However,

it appears to be much more severe than Gaithersburg when the corrosion of the base metal is considered.

The Miami site could, therefore, be used by frit companies or enamelers for the relatively rapid (2-3 yr)

evaluation of new enamels as they are developed. Once it was determined that the Miami site was suitable

for the rapid evaluation of new enamels, exposure testing at Miami was discontinued.

2.5 Comparison with Enamels in the 1956 Exposure Test

The color retention data for the enamels exposed three years at Kure Beach and Gaithersburg are

presented in table 5 together with the color retention data for glossy acid resistant enamels exposed

at Kure Beach and Washington in the 1956 exposure test [11]. (The Washington and Gaithersburg exposure

sites are only 20 miles apart and are thought to be similar enough exposure conditions for direct

comparison. These data indicate that the nature-tone enamels have better color retentions than the

glossy acid-resistant enamels on steel tested previously [ll]. Therefore, if the rusting tendency of

the nature-tone enamels can be overcome, they may have better weather resistance than conventional

glossy porcelain enamels on steel.

-12-
Table 5. Comparison of nature-tone enamels with the acid-resistant enamels in the 1956 exposure test after three years' exposure.

Exposure

Test

Enamel Type Average Color Retention for Enamels Exposed at: Kure Beach Washington-Gaithersburg

1956

1966

Acid-Resistant on Steel

Nature-tone on Steel

97.9

98.9

98.2

99.1

3. Plans for Future Exposure Testing

The enamels at Kure Beach and Gaithersburg were returned to the exposure racks for additional

weathering. The next inspection of these enamels is tentatively scheduled for 1974 after seven years'

exposure.

4. Summary

An examination of 450 nature-tone porcelain enamels on steel was made after 0.5, 1 and 3 years'

exposure at Kure Beach, Miami, and Gaithersburg. A summary of the findings follows:

1. Enamels with good acid resistance hadaverage color retentions of 98.9, 99.3, and 99.1 at

Kure Beach, Miami, and Gaithersburg respectively.

2. Enamels with good acid resistance had average gloss retentions of 84.9, 95.5 and 84.9 percent

at Kure Beach, Miami, and Gaithersburg respectively.

3. Some enamels may have pinhole-type defects which permit early corrosion of the base metal,

particularly at the severe exposure sites such as Kure Beach and Miami.

4. Many of the enamels with pinhole-type discontinuities that rusted after 0.5, to 1 year's

exposure could be detected with the high voltage continuity of coating test at overvoltages

ranging form 0.1 to 1.6 kV dc.

-13-
5. References

1. Moore, D.G. and Harrison, W.N., Fifteen Year Exposure Test of Porcelain Enamels, Nat. Bur. Stand.

(U.S.) Bldg. Matls. and Structures Rept. 148 June 1957.

2. Rushmer, M.A., and Burdick, M.D., Weather Resistance of Porcelain Enamels; Effect of Exposure Site

and Other Variables after Seven Years, Nat. Bur. Stand. (U.S.), Bldg. Sci. Ser. 4, 16 Pages (May

1966).

3. Baker, M.A., 1964 Exposure Test of Porcelain Enamels on Aluminum: Three Year Inspection, Nat. Bur.

Stand. (U.S.), Bldg. Sci. Ser. 29, 11 Pages (April 1970).

4. Colors for Architecture, AIA File No. 15-M-l, Porcelain Enamel Institute, 1900 L Street, N.W.,

Washington, D.C.

5. Test for Continuity of Porcelain Enamel Coatings, PEI Bulletin T-26, Porcelain Enamel Institute, 190C

L Street, N.W., Washington, D.C.

6. R.S. Hunter, Photoelectric Tristimulus Colorimetry with Three Filters, NBS Circ. 429 (1942).

7. Acid Resistance of Porcelain Enamels (Citric Acid Spot Test), C-282 ASTM Book of Standards, Pt. 13.

(1968).

8. Specification for Architectural Porcelain Enamel on Steel for Exterior Use, PEI; S-100 (1959).

Issued by the Porcelain Enamel Institute, 1900 L Street, N.W„ Washington, D.C.

9. Resistance of Porcelain Enameled Utensils to Boiling Acid, C-283 ASTM Book of Standards, Pt. 13,

(1968).

10. Natrella, M.G., Some Tests Which are Independent of Form of Distribution, Chapter 16, Experimental

Statistics, Nat. Bur. of Stand. (U.S.) Handbook 91, 1962.

11. Moore, D.G. and Potter, A., Effect of Exposure Site on Weather Resistance of Porcelain Enamels

Exposed for Three Years, Nat. Bur. Stand. (U.S.) Monograph 44, 13 pages, (April 1962).

-14-
FORM NBS-114A (1-71) U.S. DEPT. OF COMM. BIBLIOGRAPHIC DATA

SHEET

1. PUBLICATION OR REPORT NO. NBS-TN 707

2. Gov't Accession No.3. Recipient's Accession No.

4. TITLE AND SUBTITLE Three- Year Inspection of Nature-Tone Porcelain Enamels

on Steel

5. Publication Date December, 1971

6. Performing Organization Code

7. AUTHOR(S) 8. Performing Organization Mo A. Baker

10. Project/Task/Work Unit No. 4216571

9. PERFORMING ORGANIZATION NAME AND ADDRESS NATIONAL BUREAU OF STANDARDS DEPARTMENT OF COMMERCE

WASHINGTON, D.C. 20234

11. Contract/Grant No.

12. Sponsoring Organization Name and Address

Porcelain Enamel Institute

1900 L. Street, N.W.

Washington. D.C. 20036

13. Type of Report & Period Covered Interim

14. Sponsoring Agency Code

15. SUPPLEMENTARY NOTES

16. ABSTRACT (A 200-word or less factual summary of most significant infotmation. If document includes a significant bibliogtaphy or literature survey, mention it here.) A weather exposure test on nature-tone porcelain enamels on steel was

initiated by the National Bureau of Standards and the Porcelain Enamel

Institute in 1966. Laboratory measurements for changes in gloss and

color were made after the enamels had been exposed for 0.5, 1 and 3 years at Kure Beach, North Carolina, Miami, Florida and Gaithersburg,

Maryland. The average gloss and color retained for all 450 specimens

exposed at 3 sites for 3 years were 90.5 and 99.1 respectively. There was a tendency for the enamels exposed at the marine sites to rust around

pinhole-type defects. Scanning electron microscope studies of these

defects showed a layer of readily corrodible enamel on the bottom of the

defects. A high-voltage test for continuity of coating was effective in

detecting specimens that were apt to rust during the first year's exposure.

17. KEY WORDS (Alphabetical order, separated by semicolons) Acid resistance; color; continuity Of coating; porcelain enamel; weather

resistance.

18. AVAILABILITY STATEMENT

[*1 UNLIMITED.

I I FOR OFFICIAL DISTRIBUTION. DO NOT RELEASE

TO NTIS.

19. SECURITY CLASS

(THIS REPORT)

UNCLASSIFIED

20. SECURITY CLASS

(THIS PAGE)

UNCLASSIFIED

21. NO. OF PAGES

16

22. Price

30

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