NATIONAL BUREAU OF STANDARDS REPORT

8535

PROGRESS REPORT

April 1 through June 30, 1964

Development of Methods of Test , For Quality Control of Porcelain Enamels

PORCELAIN ENAMEL INSTITUTE RESEARCH ASSOC IATESHIP

NATIONAL BUREAU OF STANDARDS

WASHINGTON, D. C.

U.S. DEPARTMENT OF COMMERCE

NATIONAL BUREAU OF STANDARDS ,
THE NATIONAL BUREAU OF STANDARDS

The National Bureau of Standards is a principal focal point in the Federal Government for assuring

maximum application of the physical and engineering sciences to the advancement of technology in industry and commerce. Its responsibilities include development and maintenance of the national stand- ards of measurement, and the provisions of means for making measurements consistent with those standards; determination of physical constants and properties of materials; development of methods

for testing materials, mechanisms, and structures, and making such tests as may be necessary, particularly for government agencies; cooperation in the establishment of standard practices for incorporation in codes and specifications; advisory service to government agencies on scientific and technical problems; invention and development of devices to serve special needs of the Government; assistance

to industry, business, and consumers in the development and acceptance of commercial standards and

simplified trade practice recommendations; administration of programs in cooperation with United

States business groups and standards organizations for the development of international standards of

practice; and maintenance of a clearinghouse for the collection and dissemination of scientific, technical, and engineering information. The scope of the Bureau’s activities is suggested in the following

listing of its four Institutes and their organizational units. Institute for Basic Standards. Electricity. Metrology. Heat. Radiation Physics. Mechanics. Ap- Astrophysics.* plied Mathematics. Atomic Physics. Physical Chemistry. Laboratory * Radio Standards Laboratory: Radio Standards Physics; Radio Standards Engineering.** Office of Standard Ref- erence Data. Institute for Materials Research. Analytical Chemistry. Polymers. Metallurgy. Inorganic Materials. Reactor Radiations. Cryogenics.** Office of Standard Reference Materials. Central Radio Propagation Laboratory.** Ionosphere Research and Propagation. Troposphere

and Space Telecommunications. Radio Systems. Upper Atmosphere and Space Physics.

Institute for Applied Technology. Textiles and Apparel Technology Center. Building Research.

Industrial Equipment. Information Technology. Performance Test Development. Instrumentation.

Transport Systems. Office of Technical Services. Office of Weights and Measures. Office of Engineering Standards. Office of Industrial Services.

* NBS Group, Joint Institute for Laboratory Astrophysics at the University of Colorado.

* * Located at Boulder, Colorado.
NATIONAL BUREAU OF STANDARDS REPORT

NBS PROJECT NBS REPORT

42108-11-4210180 8535

PROGRESS REPORT

April 1 through June 30, 1964

PORCELAIN ENAMEL INSTITUTE RESEARCH ASSOC IATESHIP

NATIONAL BUREAU OF STANDARDS

WASHINGTON, D. C.

IMPORTANT NOTICE NATIONAL BUREAU OF ST

for use within the Government, and review. For this reason, the whole or in part, is not author Bureau of Standards, Washingtoi

the Report has been specifically

Approved for public release by the

Director of the National Institute of

Standards and Technology (NIST)

on October 9, 2015.

s accounting documents intended

subjected to additional evaluation

listing of this Report, either in Office of the Director, National

i the Government agency for which

pies tor its own use.

U.S. DEPARTMENT OF COMMERCE

NATIONAL BUREAU OF STANDARDS

PROGRESS REPORT

April 1 through June 30, 1964

PORCELAIN ENAMEL INSTITUTE RESEARCH ASSOCIATESHIP

NATIONAL BUREAU OF STANDARDS

WASHINGTON, D. C.

SUMMARY

A modification of the P.E.I. Alkali Test Equipment was devised to

assure equality of alkali attack in any of the six specimen positions.

In addition, the reproducibility of three cleaning methods was evaluated and one was selected for recommendation as the Standard Test

Method. The recommended method will involve the exposure of a limited

area of either three or six porcelain enameled specimens to a five

percent solution of sodium pyrophosphate maintained at 96° ± 0.2°C for

six hours. Weight loss will serve as a measure of the alkali resistance .

An analysis was made of citric acid spot test grading procedures.

A questionnaire was sent to 44 appliance manufacturers. Replies indicated that the majority of inspectors used the architectural wetrubbing method for grading A and B enamels rather than the PEI T-21

test method. A laboratory study in which six individuals graded 18

enamels by both procedures showed that the architectural method

(complete removal of pencil marks from the treated area) gave better

reproducibility than the T-21 test procedure in which the end point

is the difference in ease of removal of pencil marks in the treated

and untreated areas

.
2

Final arrangements were made for the five exposure sites for the

weathering of product ion*type aluminum enamels. Procurement of the

necessary specimens and the exposure racks progressed on schedule.

Measurements of specimens exposed for seven years in the PEI-NBS

weathering test were completed. Reduction of these data was nearly

completed. Results are given for green enamels at all sites. Boiling

citric acid tests were made on storage specimens of several enaaiels.

The correlation with weather resistance was comparable to the correlation obtained by the citric acid spot test.

A round-robin test was made to determine the reproducibility of

boiling citric acid test which is used for architectural porcelain

enamels on aluminum, An analysis of variance showed that the data

obtained by the three laboratories did not differ significantly at

the 95% confidence level.

I. STANDARD ALKALI TEST

Intr odue t i on

Laboratory work was started during the previous quarter to improve the design and reliability of the Porcelain Enamel Institute

alkali test equipment . The capacity of the device was increased from

three to six specimens per test. A small but statistically significant difference wag found, at that time, in the severity of the alkali

attack between specimens facing the inside and those facing the outside

of the equipment

.
3

Results and Discussion

1. Increasing Specimen Capacity

The specimen holder boxes were altered by cutting an opening

in the back side of the box and inserting an "0" ring seal to permit

two specimens rather than one to be tested in each box. A simple modification has been devised during the present quarter to insure complete

equality of attack in any of the six specimen locations available.

This was done by increasing the stirring speed from 1400 to 1800 r.p.m.

and, at the same time, placing an open-ended stainless steel cylinder,

3V in diam. by 4" high, around the outside of the heating element in

such a way that the lower open end of the cylinder was at the same

elevation as the bottom of the heater. With this arrangement, the

stirrer moves the test solution down through the inside of the stainless steel cylinder after which it flows upward around the specimen

holders. A series of alkali test results with this modified arrangement at both 96° and 99°C on a group of specimens of an alkaliresistant enamel is given in Table 1. These results show the uniformity of attack between specimens facing the center and those facing the

outer beaker wall. A statistical "t" test confirmed that the attack

was the same in either position. Further evidence on this point may

be seen in the three groups of alkali test data in the top half of

Table 2. The use of these data in connection with methods of cleaning

specimens will be discussed later. Within each of these three specimen

groups representing the three cleaning methods (top of Table 2) one

half of the specimens were exposed to the alkali solution facing the

center of the apparatus while the other half faced the outside.
- 4 -

(Of the method No. 1 specimens 3, 4, 7, 33, 35 and 37 faced the inside.) When the test results were grouped in this manner the following average values were obtained for weight-loss in mg/in.

Cleaning Cleaning Cleaning

Method #

1

Method #2 Method #3

Facing inside 7.11 8.33 8.32

Facing outside 7.17 8.20 CO co

An analysis of variance confirmed the equivalence of attack by

any method of cleaning in either position.

The data in Table 2 were also used to establish if alkali test

results were affected by the box into which the specimen was placed.

The determinations of alkali resistance * performed in box 1 were separated from those in box 2 and box 3. This separation for the test

equipment using brass boxes gave the following results :

Box Cleaning Cleaning Cleaning

No. Method #1 Method #2 Method #3

1 7.02 mg/in.3 8.25 mg/ in/ 8.22 mg/ii

2 7.20 8.35 8.40

3 7.21 8.20 8.40

Here again an analysis of variance confirmed what one can readily

see » that alkali resistance measured in weight, loss per unit area has

no significant dependence on the holder into which the specimen is

placed. Also, the data showed that uniform exposure to alkali solution

was obtained in all of the six possible positions in which the specimen

might be inserted for testing. Because of this finding all specimens

tested by a single cleaning method could justifiably be consolidated
5

as in Table 2

.

2. Selection of a Cleaning Method

In making a test of alkali resistance there are two specimencleaning steps which must be specified if gross errors resulting from

the cleaning method are to be eliminated. The specimen must be prepared for testing before the initial weight is obtained, and at the

conclusion of the treatment the corrosion products which cling to the

treated surface must be removed to a reproducible extent. It was

hoped to specify one cleaning method which would serve in both situations. The three methods selected for study represented a wide range

of severity. The first method required only rinsing with a fine jet

of distilled water from a wash bottle; the second method involved using a small amount of a 1% solution of trisodium phosphate on a soft

cellulose sponge and rubbing with a light pressure followed by a distilled water rinse; while the third involved rubbing with a nylonbristled hand-brush using a heavy pressure, followed by a distilled

water rinse.

An experimental study was designed to permit evaluation of the

cleaning methods and, at the same time, to compare results in the

equipment modified to accommodate two specimens per box with those

obtained in the stainless steel boxes containing only one specimen.

The latter equipment is similar to that now in use in industry.

Similar circular baffles were installed in both devices in the hope of

obtaining comparable results. A series of alkali tests was made on

comparable specimens of a single porcelain enamel. The results are

given in Table 2. Following the analysis of these data, method #2
6

(wet rubbing) was selected. This selection was based on the following

considerations

:

a. Wet rubbing exhibited the least scatter as indicated

by low coefficients of variations V,

b. Method No. 1 was eliminated from consideration because

it did not provide a sufficiently clean surface prior

to testing and it was not sufficiently severe to remove

"O'* ring marks which sometimes occurred on specimens after

the alkali treatment.

c. A nylon»bristled hand brush (Method No. 3), while not

an expensive item* is less likely to be found in Indus

trial testing laboratories than is a soft cellulose

sponge

.

d. The wet “-rub was apparently sufficiently severe to remove the major portion of the adhering corrosion product.

3. The Temperature Dependence of Alkali Resistance

Tests of alkali resistance of a single enamel were performed

at 960 C and at 99°G, It was desired to determine whether or not the

larger weight losses at 99° would contribute to increased reproducibility. It was found that the standard deviations were reduced from 6 to

2 percent at the higher temperature. However a test temperature of

99° is not recommended because crystalline deposits that form on the

exterior of the specimen boxes marking the solution level during the

test make it quite difficult to remove the boxes at the end of the

test period. The results in Table 1 allow one to calculate an increase

in weight-loss of 12 percent per degree Centigrade between 96° and
7

99°C, which exactly confirms the figure referred to in the preceding

report

4. Temperature Controller

Previous work with the thermistor-actuated, on-off controller

currently used with the equipment established short range control

(up to six hours) of plus or minus 0,15°C. A recent 20-hour test continuously recorded the e.m.f. of a copper-constantan thermocouple located in the mercury at the bottom of the thermometer well which was

immersed in the circulated alkali solution. The on-off cycling was

indicated by the thermometer to be of the order of ±0.15°C. This cycling was continuous and uniform during the twenty -hour period. No

larger temperature excursions were recorded and no long-time drift

could be seen. These results confirm the adequacy of this controller

for use in a standard test method.

Plans for- the Next Report Period

The laboratory work which was aimed at selection of a standard

procedure for alkali testing of porcelain enamels, is essentially complete. It is proposed that the standard procedure involve exposing a

limited area, on either three or six specimens, to a five percent solution of sodium pyrophosphate maintained at 96° ± 0.2°C for six hours.

Specimen weights before and after this treatment will permit calculation of loss-in-weight, expressed in mg/in. 2 of exposed surface, as a

measure of alkali resistance.

No further work on alkali test development is planned.
8

II. ANALYSIS OF SPOT -TEST GRADING PROCEDURES FOR

THE ACID RESISTANCE OF PORCELAIN ENAMELS

Introduction

The PEI spot test for acid resistance has enjoyed wide use in the

porcelain enamel industry ever since it was first promulgated in 1939.

The general consensus within the industry is that the test has been

reasonably satisfactory in predicting the degree to which the enamel

finishes on appliances, sanitary ware and even architectural panels

will resist the type of acid attack encountered in service.

Briefly, a small area of the enamel surface is exposed to 10%

citric acid for 15 min. at room temperature and then, after removal

of the acid, the treated area is evaluated for degree of etch. This

evaluation is made visually with the aid of such criteria as (1) Is

there a visible strain? (2) Does the etching cause blurring of a reflected image? and (3) Is a pencil mark placed across the tested area

of the specimen more easily removed from the treated than the untreated

area?

The evaluation procedure was believed to be reasonably well described in the original test pamphlet, but it was observed that some

operators deviated from the specified grading procedure, probably because of a misinterpretation. This was especially true for the wetrubbing test for removal of pencil, marks, which is used to separate

class A enamels from class B enamels. Many people that made the test

graded the enamel class A if the pencil marks could be removed by wet

rubbing rather than if there was no difference in the ease with which

the marks could be removed from the treated and untreated areas.
9

A somewhat better correlation was observed with weather resistance when

the "complete removal" grading system was used; hence, this type of

wet~rubbing test was included in the PEI Architectural Specification

S-100 (62).

In an effort to reduce the variation that apparently existed in

application of the earlier PEI T-7 test procedure, an ad hoc committee

was appointed to investigate and, if necessary rewrite the T-7 pamphlet to clarify the exact grading procedure to be followed. This was

done and the revised version was adopted by the Standards Committee in

1962 as PEI Standard Test T-21. Table 3 defines the differences in

the various grading systems, past and present.

At the Standards Committee meeting in February 1964 the grading

method for class A and class B enamels was again discussed. The point

was made at the meeting that a number of production enamels that have

in the past been graded as A are now graded as B by the new test procedure (T-21). These enamels have performed satisfactorily in service

yet they are no longer acceptable under most purchase specifications

because they require an acid resistance of class A or better. In the

ensuing discussion, the following questions were raised with regard to

selection of the most suitable method for differentiating between class

A and class B enamels;

(1) Which of the two grading methods (complete removal

or difference in ease of removal) is used most widely at

the present time in the appliance industry?

(2) Which method of grading gives the best reproducibility?

(3) What degree of temperature control is required?
10 -

It was decided that no action would be taken by the Committee on

any revision of T-21 until answers were obtained to these questions.

The progress that has been made towards obtaining these answers is

reported below.

Results and Discussion

X, Survey of Type of Wet-Rubbing Test Procedure Used by Appliance

Manufacturers

A questionnaire was sent to forty»four companies representing

the major manufacturers of porcelain enameled appliances and plumbing

fixtures. Thirty-eight replies were received from twenty of these companies . These replies were tabulated and the results are shown on the

sample questionnaire (Table 4).

The majority of the inspectors used the architectural wetrubbing method where the end point criterion is complete removal of

the pencil marks. Only three inspectors said they used the new standard PEI test T-21 , and even this number is questionable since no one

indicated that they had changed grading methods subsequent to promulgation of this new test.

Eleven replies stated that differences had arisen regarding

grading of the wet»rubbing test. Of these eleven* eight employed

either the ASTM (7) or the PEI (1) procedure for grading by the wetrubbing test, while only three used the architectural method. Thus,

it is indicated that fewer differences exist when complete removal is

used as an end point criterion than when. ease of removal is taken as

the end point

.
11

The tabulation also shows that nineteen of the inspectors used

a dry “tubbing test. While the questionnaire was not worded to reveal

what dry-rubbing method was used, it t& felt that the inspectors used

the same criteria as they reported using for the wet-rubbing test.

2. Laboratory Study of Reproducibility of Architectural and

T“21 Grading Procedures for Wet“Rubbtng Test

The reproducibility of the two wet»rubbing grading procedures

was investigated by having six impartial observers rate 18 specimens

of acid-resistant enamels. The specimens, which were 12-inches square,

were submitted by three different frit manufacturers. The acid-resistant enamels applied to the specimens were selected so as to include some

enamels that might be designated either A or B depending on the method

of grading by the wet-rubbing test.

All spots were applied at 80° ± 2®F by one individual in accordance with the standard procedure. A fresh spot was applied for each observer who rated the enamel after carefully reading the respective test

procedures. In the case of the architectural procedure, the enamel

received an A rating if the No. 3B pencil marks could be completely removed and a B rating if they could not be removed. If none of the observers could detect a visabie stain, the enamel was rated AA and not

considered in the analysis. Seven of the 18 enamels were so rated,

leaving only 11 enamels for study by the wet rubbing tests.

Observer No. 2 was unable to detect a stain on two enamels

where a stain was observed to be present by the other five, and similarly, observer No. 1 did not observe a stain on one enamel. For purposes

of the analysis the assigned rating in these three cases was class A
12

rather than AA since this part of the study was not designed to investigate any part of the grading system except the wet-rubbing test. Had

these AA ratings been included, the analysis would have been biased in

favor of the architectural procedure.

A summary of the results is listed in Table 5. Perfect agreement between observers occurred for six of the 11 enamels when the architectural procedure for grading (complete removal) was used as

against three of the 11 enamels for the T-21 procedure (difference in

ease of removal). The total number of discrepancies (deviations from

the majority rating) was only six for the architectural procedure while

for the T-21 procedure it was three times as great (18). A point of

some interest was that one observer (obs . No. 4) arrived at exactly

the same ratings by both grading procedures for all 11 enamels. If

all observers were like No. 4 there would be no need for concern about

which wet-rubbing test to use.

Some variations in the appearance of the various stained areas

were observed on several of the specimens, indicating that there were

local variations in acid resistance across the surface. This effect

could account for some of the disagreement among observers but the effect should be present to the same extent on the spots used for the

T-21 grading as on the spots used for the architectural. A more important cause for the discrepancies is probably the interpretation

that is placed by the various observers on the written procedure. It

seems highly doubtful, with a test involving the judgement of an operator, that any better agreement could be achieved than is represented

by the architectural grading procedure.
13

The data in Table 5 seem clear in showing that there is considerably more disagreement among observers when using the T-21 procedure

than when using the architectural method, at least in the case of this

particular series of enamels. The survey of appliance manufacturers

also indicated more differences of opinion by inspectors when using

T-21 than when using the architectural procedure. Thus, because both

the laboratory analysis and the industry survey showed fewer disagreements among observers when complete removal of the pencil marks was

used as the criterion, it may be concluded that the architectural

grading method is a more reproducible test procedure for current production enamels.

Table 5 also shows that if the architectural method was incorporated into T-21, only two of the 11 enamels (Nos. 11 and 15) would be

raised from B to A when majority ratings were considered and even for

these two enamels two of the six observers graded the enamel A by the

T-21 method. Thus there does not appear to be any clear-cut upgrading

of B enamels when the architectural grading procedure is used.

Enamels nos. 4, 5 and 7 are of special interest in the comparison

of the two test procedures. Using the architectural method, all observers rated these three enamels A; yet by the T-21 procedure three observers assigned A ratings and three assigned B ratings to each of the

three enamels. This type of disagreement when using the T-21 method

could undoubtedly be lessened by a special training program for inspectors. However, special training of a test operator does not seem

to be a feasible approach for a standard test procedure.
14

3. Effect of Temperature on Spot “Test Rating

A preliminary attempt was made to determine the effect of

test temperature on the aeid°resi$tanee rating for this particular

series of 18 enamels. Test procedure T°21 was used. All of the testing and grading was done by operator No. 4.

Table 6 gives the results. Three enamels (Nos, 6, 7 and 13)

increased in rating with temperature. This is believed to be due to

local variations in acid resistance across the specimen surface which

might possibly be eliminated by applying and grading a larger number

of spots o

Of the remaining 15 enamels, eight showed a decrease in rating from 70° to 90° F, but the decrease was never greater than one AR

step. Thus, the tendency is for decreasing acid resistance rating

with increasing temperature. To obtain more meaningful results on the

temperature effect would require a much more comprehensive study in

which more than one observer would grade a minimum of three spots for

each of the test temperatures. There is some question whether such a

study would be worth the time since the present test procedure calls for

a temperature of 80° ± 2

e F. The only reason for considering it at all

is to determine whether or not this degree of temperature control is essential .

4, Reproducibility of Spot -Test Rating Methods

The same group of 18 enamels was also rated by the six observers with respect to the other criteria used for assigning ratings by

spot -‘test procedures. The results, which are summarized in Table 7,
15

show that the wet-rubbing test when made by the architectural method,

gives about the same degree of agreement among observers as the visible

stain and blurring highlight tests. However, when the wet-rubbing test

is made by the T-21 method, the degree of agreement drops markedly.

Hence, these data show that, for these enamels, the architectural wet

rub (but not the T-21 wet rub) is almost equally as reproducible a

test criterion as the visible spot and the blurring highlight tests.

All six observers passed the five stain-free enamels in the

dry-rubbing test irrespective of whether the testing was done by the

architectural or T-21 method. From the results with this series of

enamels, there would appear to be some question as to whether dry-rubbing is a worthwhile test to include in the standard test procedure

since all five enamels passed. However, the returns from the field

questionnaire showed that half of those replying used the dry-rubbing

test in their regular grading procedures. Unfortunately, the question

as to whether or not they ever observed a failure was not asked.

Plans for the Next Report Period

No further work is planned on the AR spot test.

III. EXPOSURE TEST OF ALUMINUM ENAMELS

Introduct i on

A separate exposure test of current production enamels for aluminum was approved early in the calendar year by the Aluminum Council of

the PEI. Activities since that time have been concerned with (a) selection of enamels, (b) selection and procurement of exposure sites, (c)

procurement of the metal sheet and (d) application of the selected

enamels to the sheets to provide the necessary specimens. Table 8
- 16

lists the enamels that will be included in the test.

Pr ogress _ Pur in& Pas t Quarter

1. Arrangements for five exposure sites were completed. These

five sites are Washington* Kure Beach* North Carolina, Los Angeles*

New York City and Montreal.

2, The racks for each site were finished and placed in packing

crates for shipment.

3 * A total of 205 - 3 x 5' sheets of 0.064" thick aluminum alloy

6061 (H-12 temper) was received. These sheets were first divided int

smaller lots and then the required number shipped to the ten cooperating fabricators.

4. Instruction sheets were sent to each fabricator at about the

same time that 3x5 ft. sheets were shipped. These sheets gave expli

cit instructions on a) enamel type, b) suggested mill batch, c) thickness of coating, and d) desired gloss and color.

Plans for the Next Report Period

The enameled sheets upon return from the fabricators will be cut

into 4§- x 4§-" specimens and the gloss, color, and thickness measured.

The target date for installation of the specimens at the 5 sites is

October of this year

.

IV. PEI-NBS WEATHERING TEST

I ntroduction

The PEI-NBS exposure test is now entering its eighth year. At

the end of seven years all specimens were returned to Washington and

their gloss and color measured. These 7»yr data are now in process

of being reduced and analysed preparatory to writing a 7-yr report.
17

This does not constitute termination of the test but it appears likely

that there will be no further complete inspections of specimens until

1971 .

Results and Discussion

1. Correlation of Two Color Difference Meters

In February 1963 the National Bureau of Standards traded in

their original Hunter Color Difference Meter in the purchase of the

latest model of the instrument. The operating characteristics of the

new instrument were checked when it was first received and they were

believed to be satisfactory. Recently, however, when measuring the

last two sets of specimens from the Kure Beach sites, it was discovered

that the two color difference meters did not give the same readings on

all enamels. Consultation with the Bureau's Colorimetry Section indicated that this is common in color difference meters produced at different times because they do not have the same f liter-photocell response.

This difference in the filter-photocell response is not noticeable if

the "standard" has spectral characteristics close to those of the color

being measured. This was true for the whites, blacks, and grey-blues

in the test but the greens, reds, yellows and bright blues differed

significantly from the NBS standards, which had been used as the

"standard" for the color difference meter.

The difference between the NBS standard and the specimen color

resulted in a measured color difference of as much as four NBS units

between six and seven year®' exposure on enamels that showed no appreciable color change between three and six years' exposure. Correction

curves were established to eliminate these differences. These curves
18 -

were based on the Rd s a, and b readings of the reference enamels after

they had bean stored for five and seven years. These enamels had spectral characteristics close to those of the exposed specimens and should

have shown no color change between five and seven years

1 storage if the

color difference meter had not been changed. Knowing this, the individ

ual Rd, a, and b values for the reference enamels for five years

(old meter) were plotted against those for seven years (new meter) for

each color standard used in the test, A least squares line was then

fitted through these points. The difference between this fitted line

and a 45 degree line (see Figure 1) is the difference in the filterphotocell response of the two color difference meters. All data taken

with the new instrument were corrected through use of curves of this

type

.

2. Effect of Exposure time

The gloss retention and the color stability index of all the

green enamels exposed at all seven sites are given in Tables 9 and 10.

The enamels exposed at Dallas, Washington and Kure Beach - 80 ft sites

were chosen to represent the mild, moderate and severe sites as re27

ported in 1963“° . It can be seen (Figure 2) that the gloss retention

at the mild site, Dallas, and the moderate site, Washington,- remains

nearly constant after three years

8 exposure, while the gloss at the

most severe site, Kure Beach - 80 ft, continues to decrease with exposure time.

The color continues to change slightly during all seven years'

exposure with the greatest color change occurring at Kure Beach - 80 ft

and the least change at Dallas (see Figure 3).
19 -

3. Correlation Between Acid Resistance and Weather Resistance

The average color stability index, gloss retention and acid

resistance for the mat steel enamels and the mat aluminum enamels are

given in Table 11. These are the only two types of enamels evaluated

in this section because they are the only types completely represented

by green enamels (see Table 12). It can be seen from the data in Table

11 that the gloss retention does not show good correlation with the

acid spot rating for the mat steel enamels at any of the exposure sites

and it correlates for only four of the seven sites for the mat aluminum

3/

enamels. These results are similar to those reported by Potter after

these enamels had been exposed for one 3rear,

In the case of the color stability index, the correlation is

somewhat better; correlation being noted for three of the seven exposure sites for the mat steel enamels, while the mat aluminum enamels

showed a correlation for all seven exposure sites.

The boiling acid solubility had also been determined on the

mat aluminum enamels to determine whether the weathering characteristics would correlate better with this measure of acid resistance than

with the spot test rating. The results obtained from the boiling acid

tests correlated with the gloss retention for only two of the seven

sites and with the color stability index for six of the seven sites.

Thus, for the limited number of enamels in the test, the spot test rating seems to correlate better with the weathering characteristics of

the mat aluminum enamels than does the boiling acid solubility test.
- 20 -

4. Comparison of Acid Spot Test and Acid Solubility with the

Weather Resistance of Steel Enamels after Five Years'

Exposure at Washington

The acid resistance of all the enamels in the test has been de»

4 / termined by the acid spot test method"” . This method of rating the

2 5/ enamels places them into five distinct classes. It has been shown*”1-”*

that the degree of weather resistance of porcelain enamels on steel is

indicated by the acid spot test ratings only when averages are con6 / sidered . In 1960, Sopp , et alv noted that the weather resistance of

porcelain enamels on aluminum correlated better with a boiling acid solubility test than with the spot test ratings. This improved correlation

resulted from rating the enamels on a continuous weight loss scale.

During this quarter the acid solubility was determined on

twelve steel enamels in the weathering test. Six of these enamels were

selected because their acid spot ratings correlated with their weather

resistance, particularly their color stability index, while the other

six were selected because they lacked any apparent correlation between

weather resistance and acid spot rating. The results of the acid solubility and acid spot test are given in Table 13 and are plotted against

gloss retention and color stability index after five years 8 exposure

at Washington in Figures 4 and 5, respectively.

It can be seen in Table 13 that the acid solubility correlates

with the spot test ratings for the enamels whose weathering characteristics correlated with the spot test rating. This indicates that the

acid spot test and boiling acid solubility measure the same property

of the enamel and those enamels whose weathering characteristics do not

correlate with one method of determining acid resistance will not
- 21

correlate when the other method is used.

A curve was fitted through the acid solubility values (forall 12 enamels) while the spot test values were averaged. When a

straight line is fitted through the average spot test values, it be

comes evident that the same trend is present whether the acid spot test

or the boiling acid solubility is compared with weather resistance.

It can be seen in Figure 4 that the individual acid solubility points

scatter about the fitted curve in the same manner as the individual

acid spot test points scatter about their average values. However the

color stability values shown in Figure 5 correlate somewhat better with

the acid solubility than with the average spot test ratings. It is

difficult with this limited data to say which method of measuring acid

resistance correlates better with weather resistance.

Plans for the Next Report Period

The rest of the seven-year data will be reduced and a paper will

be prepared for presentation at the Ceramic-Metal Systems Division

meeting in September and at the PEI Shop Practice Forum in October.

V. ROUND-ROBIN TEST ON THE ACID RESISTANCE OF

ALUMINUM ENAMELS BY THE BOILING-ACID TEST

Late in the report period, the Research Associateshxp cooperated

in a round-robin test to explore the reproducibility of the ASTM C-283

boiling acid test. Three laboratories took part in the test: a frit

manufacturer , a fabricator, and the Research Associates of the Porcelain Enamel Institute.

The results obtained are given in Table 14. The differences in

results obtained between the most divergent laboratories represented
- 22

approximately 17 percent of the total weight loss. These differences

included equipment and operator variables, barometric pressure and

elevations above sea level, interpretation of and technique in maintaining a "rolling boil" as well as the ever present sampling error

among specimens

.

It is known that not unreasonable differences in the rate of boil

alone can effect the boiling acid weight-loss results by several milligrams per square inch.

An analysis of variance was made on the first three specimens

tested by each laboratory. This analysis showed that the three laboratories did not differ significantly (95 percent confidence level).

Hence, it is believed that reasonably good agreement between laboratories was obtained.

VI. STANDARD REFERENCE MATERIALS

A new shipment of 4 | by 4 f by ^ inch plate glass specimens was

received and calibrated during this report period. The new glass

plates did not differ significantly (at the 95% confidence level) from

those calibrated in January 1963. However, the surface abrasion factor

of 50 ± 0.5% (shown on p. 5, section II-C-4, PEI Bulletin T=2) will be

changed to 49.2 ± 0.5%, and the subsurface abrasion factor of 5.00 (shown

on p. 8, section IV-C-7, PEI Bulletin T-2) will be changed to 5.058.

These changes are identical to those used with the 1963 standards and

an information sheet giving these changes will be sent with the new

standard glass plates .
- 23

The following stock of standards was on hand as of July 1, 1964.

Corundum Abrasive, March 1960 issue, for subsurface

abrasion test 324 lbs - 81 jars

Standard Pennsylvania Glass Sand for surface abrasion,

July 1963 issue 336 lbs - 112 jars

Distinctness of image gloss standards - 20 sets

Calibrated glass plates for abrasion testing - 39 dozen*

Respectfully submitted,

M. D. Burdick

M. A. Rushraer

Research Associates
REFERENCES

1* M. D. Burdick and M. A. Rushraer, " N»B a S s Progress Report No. 8347,

Jan. 8, 1964 - Mar. 31, 1964, National Bureau of Standards,

Washington, D. C. 20234.

2. D. G. Moore and Alan Potter, "Effect of Exposure Site on Weather

Resistance of Porcelain Enamels Exposed for Three Years", N.B.S.

Monograph 44 (1962) U. S, Government Printing Office, Washington,

20234

3. Alan Potter, "MBS Progress Report No. 6346, Nov. 16, 1958 - Feb, 15, 1959, National Bureau of Standards, Washington, D. C. 20234.

4. "Test for Acid Resistance of Porcelain Enamels, Pt . 1, Flatware",

Issued by the Porcelain Enamel Institute, Inc., 1145 19th St. N. W. Washington, D. C. 20036.

5. D. G. Moore and W. N. Harrison, "Fifteen-Year Exposure Test of

Porcelain Enamels", National Bureau of Standards Building Materials

and Structures Report 148, June 1957, U. S. Govt. Printing Office,

Washington, D. C. 20234.

6. A. L. Sopp, P, F. Wallace, and R. W. Ricker, "Chemical and Weather

Resistance of Porcelain Enamels on Aluminum, Ceramic Age, 75 (66)

1960.
Table 1. Dependence of Alkali Resistance on Specimen Position

Tests at

Facing Inside Facing Outs ide

Specimen Weight Specimen Weight

No

.

Loss No

»

Loss

mg/ in2 mg /in2

A~70 2.6 A-71 2.7

A“73 2.6 A“72 2.4

A=73 2.7 A-74 2.6

A-77 2.8 A-76 2.7

A-79 2.6 A“78 2.6

A”81 3.0 A-80 2.7

A -8 3 2.8 A-82 2.9

A-85 2.7 A-84 2.8

Average 2.72 2.68

Std. Dev. 0.1643 0.1761

V, Percent 6.0 6 .

6

Tests at 99 C,, 6 Hours, 57» Pyro

A=86 3.7 A-87 3.6

A»88 3.6 A-89 3.6

A“90 3.6 A-91 3.7

A-93 3.6 A-92 3.6

A-95 3.7 A“94 3.8

A-97 3.8 A-96 3.7

Average 3.67 3.67

Std. Dev. 0.0812 0.0812

V, Percent 2.2 2.2
Table

2.

Specimen

Cleaning

Procedures

in

Alkali

Testing

ms i©

GO 5*M

H0 U lA lA CM

eem

98

a9) O

MS CM OK ao a- CM HD Ch Mf lA 10 fH MS

© lA 00 a>. o a. ee <f

H Mf LA lA •4- lA <t LA 6© LA LA o CM 00 00 pH S' ee

© os

© «3 Id sH »H H 44 98 o 98 eo in s ±J 05 . *4 eo LA A.

43 Cfl

d LA e© pH A« LA vH 0s 5=4

m Ol lA c© CA CM CO CM 06 (*» lA GO lD <t in

eo

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O CA 00 0s eo e© 00 a. 00

© LA

08 60

Is 0 0A% £3 1

fid

d

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Table 3. Procedural Differences Between Three Standard Methods

for Determining Citric Acid Spot Test Ratings.

Method

Procedures

PEI T-7

ASTM C-282

PEI T-2I Architectural

Specification

S-100 (62)

Marking Pencil No. 1 3B No. 1

Temperature of

Test 80 ± 10° F 80 ± 2°F 80° ± 10°F

Wet and Dry

Rubbing Endpoint 1/ 2/ 3/

Rubbing Pressure Not

Specified

4/ Vigorous

1/ "If the marks are retained more tenaciously on the treated area than on the protected area the specimen fails the test."

2/ "If the marks are removed from either of the two areas more readily than from the other the specimen fails the test."

3 l "If the marks are removed completely by this wet -rubbing treatment the specimen passes the test."

4 / "Starting with gentle pressure and then applying gradually increasing pressure rub repeatedly across the lines... in such a manner that the marks in the treated and protected areas are rubbed at the same time."
Table 4. Questionnaire and Tabulation of Replies

On the Grading of Porcelain Enamel Using the

a/ Citric Acid Spot Test—.

1. l)o you use the dry-rubbing test on enamels that show no visible stain?

Yea 14 No Ig

2. In the wet^rubbing test, do you, after applying the pencil marks,

grade the enamels as class R if % (architectural method) a _2 1 pencil marks In the treated

area cannot be completely

removed by rubbing,

(T“2X method) b , 3 pencil marks are removed from

either of the two areas more readily than from the other area,

(ASTH method) e v 14 _ pencil marks over the treated area

are definitely more difficult to remove than those over the profcected area.

3. Have you changed your method of grading A and B enamels in the past year?

In the past five years?

4. If your answer to question

did you previously use for

5. Have you graded enamels where a difference of opinion arose between

different people as to whether the enamel should be rated '“A" or "B"?

Yes 11 No 26 (1 did not answer)

6. If yes, please describe how these differences are resolved. (Reply summary

Differences arise on whether or not pencil marks must be completely removed

and amount of pressure and time used in wet rubbing test. Differences re- solved by re-examination, by referral to supervisor, or by customer's

decision.

)

7. Have you encountered many field complaints directly attributable to lack of

adequate acid resistance?

(Only two affirmative answers, one being a sixigle case, and the other involving Government specifications.)

COMPANY _ ADDRESS

jV This is a sample questionnaire that was sent to the manufacturers.

The numbers indicate the repiie* received.

Ttes ...O.— Ho 38

Yes 0 No_ 36 (2 did not answer)

3 is yes, which of the methods in question 2 the wet -rubbing test?

2a NA

2 g^ NA *> No answer

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Ena- No

1

a

3

4

5

6

7

8

9

10

H

12

13

14

13

16

17.

18

AE Eatings by Observer No. 4 when Using T-21 Grading

Procedure On 18 Enamels at Three Test Temperatures.

Test Temperature

7 0° F 80° F 90° F

AA AA AA

AA AA A

AA A A

A A A

A A A

A A AA

AA A AA

AA AA AA

B B B

A B B

A B B

A B B

A A AA

AA AA A

A A A

AA AA AA

A A B

B B C
Table

7.

Results

by

Six

Observers

for

18

Enamels

When

Using

Various

Spot

-Test

Criteria.

CO CQ l/J CO Cd to

y\

CMMy

0)

fa. faW0) <A

fa. CMMy

(fl CO CO CO CO 00 CO

« 1* 00 CO to CO eo (0 eo to t» CO CO CO CO CO o 68 cd td cd cfl eg eg 68 eg eg eg eg cd cd cd is cu 04 04 CXi CM ftl Gj Gj G< G* 04 cu 04

V«*4 co to CO CO CO CO CO CO

fa, fa

CO ISB co

faMy

CO CO CO CO CO CO

y*\ CMwCO CO CO (0 CO CO CO CO CO <0 to CO 60 m CO CO (0 (0 £0 o eg ed td td td C3 eg eg eg tg eg eg cd eg cd cd cd cd 25 Gj 04 04 04 04 Gj CL) Gm Gj & IXi fa 04 fa 04 04 04 04

m eo

l CO

m co

fa oo

y*s. fa. fa fa >*M fa. y"S 1

v~4 CM to eo fa eo CM ¥*-4 CM eM

v^y w* My w vy My vy w My 'w' v«y fa jS co CO CO CO w co CO fa 4*4 CO CO ¥“4 CO CO ¥**4 «M fa CO CO CO (0 co CO CO co •H fa fa CO CO •H CO CO *»«4

• eu cd «g td eg 68 eg eg eg cd eg eg cd eg cd Cd cd cd cd H as fa fa fa &4 fa fa fa fa 04 fa fa 04 fa 04 04 04 04 01

3

fatfa 6M! 43 Si CO CO 09 CO to CO co CO i—

i

I—

1

eo fa co CO CO CO CO ¥“4 o u CO to co CO to m as co fa fa co fa co CO CO CO CO u <u «d eg «a cd eg «8 eg eg eg 08 «g eg eg eg cd cd cd cd < a 04 fa fa 04 fa fa fa fa fa fa fa fa fa fa 04 04 04 04

CO

C0 CO

CO CM

i—i r..

1

?fa Si CO eo co co m

60 6M 44 CO to co co co & II <U eg eg 1 i i i eg i t ii t i eg 1 eg

°tr=4 H JS fa fa fa fa fa St

Si3

fa

• 13

>> • O

h 43 43 CO CO CO eo eo Q O 4J CO eo co eo co U <D eg eg * i i t eg t i » t • eg i eg < 5S fa fa fa fa fa

uo o

o

m o

o

cm

CM CO co v~4 eo fa fa fa fa fa CO fa m f-4

68

1

• to CO •H •H ‘H »h4 co fa fa fa fa fa co fa to «»*4 *H

*J 9 eg eg <d 68 eg eg td 68 eg eg eg eg 8) eg eg td td cd O

&

25 a, fa fa fa fa 04 fa fa fa fa fa fa fa fa 04 04 0. &0

<D yM

fa «-4 y\

St My <M CM

fa My v-y W fa (0 co v*4 fa H CO fa fa fa fa fa co *-i m ¥=—1 fa CO eo “H ‘r-J fa *r4 co fa fa fa fa fa eo fa 06 -H > O «g eg <d eg eg eg cd eg eg eg eg eg eg eg eg cd cd <d 25 fa fa 04 fa fa fa 04 fa fa fa fa fa fa fa 04 04 04

fa

(D8

• *9 o

s 25 p-4 CM <T) m i£» rv. 00 eri o fa tM 60 m v£> r^- 00

BeJ fa fa fa fa <r*“4 fa H ¥-4 r-4

cm rfa CO

60 CM

i fa i fM. a) cu

ai Q) Gi u

CO 60 60

fa <g to eg

a; co ?fa CO 8 fa 60 cU fa § VI £ CO 8

5

VJ G O >4

OJ o S 0) a o

£ eg > a) £

fa 6 co O 43 fa 0) fa 43 fa

• fa £ CO o fa £

O fa QJ s fa CU 25 3 8 0 s-s 3 8 a/

Figures

in

parenthesis

indicate

number

of

observers

that

did

not

agree

with

majority.
Table 8. Enamels in PEI-NBS Aluminum Weathering Test

Enamel Color

White

White

White

White

White.

Black

Black

Black

Red

Dark green

Pastel green

Pastel green

Pastel blue

Pastel 8rey

Pastel brown

Pastel yellow

Enamel Gloss Number

Coat

Full gloss One

Full gloss Two

Full gloss Two

Semi gloss One

Semi gloss Two

Full gloss One

Semi gloss One

Mat One

Full gloss One

Full gloss One

Semi gloss Two

Mat Two

Semi gloss Two

Semi gloss Two

Semi gloss Two

Semi gloss Two

Enamel Thickne

(mils

)

2.5 - 3.5

4 - 6

under 3

2.5 - 3.5

4 “ 6

2.5 - 3.5

2.5 - 3.5

2.5 - 3.5

2.5 - 3.5

2.5 - 3.5

4 - 6

4 - 6

4 - 6

4 - 6

4-6

4 _ 6
Table

9.

Gloss

Retentions

For

Green

to

<11 2d

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m n* yo c© 03

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Enamels
Table 12. Enamels in the PEI - NBS Weathering Test

Type of Enamel Number of Enamels Number of Green

in Test Enamels in Test

Regular Glossy Steel

Acid-Resistant 24 0

Mat Aluminum 4 4

Mat Steel 18 18

Regular Glossy Steel

Non-Ac id -Res is ting 16 0

Glossy Aluminum 10 4

1000°F Steel 2 1

1300°F Steel 4 2

Red and Yellow

Screening Pastes 13 0

Total Number of Enamels 91 29
Table 13. Comparison of Boiling Acid Solubility with the Acid Spot

Test Ratings of Enamels Exposed Five Years at Washington

A) Enamels whose acid spot test ratings correlated with

their weather resistance

Enamel Acid Spot

Test Rating

Boiling

Acid Solubility

G los s Retention

Color Stability

Index

(mg /in.2

) (%)

A4 AA 0.6 81.3 99.3

H4 A 2.1 79.8 98.9

Ml A 2.7 78.2 98.3

Bl B 13.4 43.0 98.5

N1 C 21.1 67.5 92.1

K4 D 63.9 59.6 83.3

B) Enamels whose acid spot test ratings

with weather resistance

did not correlate

Enamel Acid Spot Boiling

Acid Solubility

Gloss

Retention

Color Stability

Index

(mg/in.2

) (7.)

D3 AA 3.4 73.2 98.4

D4 AA 4.0 63.4 98.5

L3 A 3.2 73.4 99.1

N4 C 41.5 73.7 91.1

K1 C 20.9 70.0 98.8

E4 D 22.3 71.6 99.0
Table 14. Results of Round-Robin Bolling Acid Test

(ASTM C«283) on Two Aluminum Enamels

A) Light Gray Enamel B) Light Brown Enamel

Test Fabricator Frit Mfger

.

PEI Fabricator Frit Mfger. PEI

No. mg /in . 2 mg/ in „ s mg /in.2 mg/ in. 2 mg /in . 2 mg /in.2

1 10.9 10.9 12.0 11.1 15.7 10.1

2 12 .3 14.9 10.7 10.2 14.1 15.8

3 14.1 13.7 10.4 10.3 13.9 15.0

4 11.8 - - 9.9 -

5 12 .2 - - 10.4 - -

6 12.7 - 10.8 - -

7 - - - 10.9 - -

8 - - 12.3 “ -

9 » - 11.8 - -

10 - - - 12.1 - -

11 11.4 • *

Average 12.3 13.2 11.0 11.9 14.6 13.6
(H3J.3W (no) SHV3J. £ - S3A1VA Q.

Figure

1.

Correction

curve

for

b

when

NBS

standard

number

SKC-15

is

used.
100

95

90

85

80

75

TO

65

6o

55

50

45

4o

35 01234567

TIME IN YEARS

ure 2, The effect of time on the gloss

retention of all green enamels

exposed at Dallas, Washington and

Kure Beach - 80 ft.
COLOR

STABILITY

INDEX

100

TIME IN YEARS

Figure 3. The effect of time on the color

stability index of all green enamels

exposed at Dallas, Washington, and

Kure Beach-80 ft.
o

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co o

t- o

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Figure

4.

Comparison

of

boiling

acid

solubility

and

acid

spot

test

rating

with

gloss

retention

of

12

enamels

exposed 5

year

at

Washington.
100

X3GNI AimaViS aoioo

Figure

5.

Comparison

of

boiling

acid

solubility

and

acid

spot

test

rating

vith

the

color

stability

index

of

12

enamels

exposed

5

years

at

Washington.