TESTING RESISTANCE OF ENAMELED SURFACES TO SCRATCHING GOUGING,AND ABRASION*

Seven types of cover-coat enamels were tested for resistance to scratching, gouging, and abrasion by various methods. These methods were (a) P.E.I. abrasion test, (b) P.E.I. gouge test, (c) variations of the gouge test, (d) Taber Abraser, and (e) a scratch test. Chemical durability and reflectance were found to have little or no correlation with the resistance of the enamels to abrasion. The bubble structure of the enamel layer seemed to have the greatest effect on abrasion and scratch resistance of the enamel layer.     

THE RELATIONS BETWEEN COMPOSITION AND PROPERTIES OF ENAMELS FOR SHEET STEEL1

Very little has been reported on the relations existing between enamel composition and the properties except the resistance to acids. In this investigation a study was made of the relation between enamel composition and such properties of the enamel as expansion, strength and acid resistance, as well as the effect of the relative “fit” of the ground and cover coats on resistance of the enameled ware to impact and to thermal shock. The work was done on fourteen ground coats and white cover enamels applied to eight-inch dinner plates. The latter were tested for resistance to impact, to thermal shock, and to acetic acid. Determinations were made of the expansion and compression strength of the enamels before application to the steel. Conclusions.—1. The replacement of boric oxide by sodium oxide in the enamels increased the coefficient of expansion. Fish-scaling decreased with increase of ex- pansion, due to increases of sodium oxide. 2. The compressive strength of the ground coats appeared to decrease with re- placement of boric oxide by sodium oxide, although the reverse conditions occurred in the case of the cover enamels. 3. Impact on the parts of enameled ware not free to deflect, such as the curved corners at the bottom, gives a measure of the toughness of the enamel as applied on the steel. The resistance of the enamel on corners to impact apparently is a function of the inherent strength of the enamel rather than its “fit” on the ware. 4. For greatest resistance to thermal shock, the ware should be coated with a ground coat having a coefficient of expansion equal to or preferably greater than that for the cover enamel. 5. Acid resistance, while dependent on the enamel composition, is not affected in the same way for all enamels. For these particular cover enamels, resistance to acids decreased with replacements of boric oxide by sodium oxide.     

THE THERMAL EXPANSION OF SHEET-IRON GROUND-COAT ENAMELS*

The thermal expansions of sheet-iron ground-coat enamels were determined by use of the interferometer. The results showed that the calculated expansions based on the factors of Mayer and Havas were close to the actual expansions. It was also shown that variations of borax, feldspar, and quartz resulted in a fairly uniform change in the expansion. It was possible to interpolate between values for these constituents. The softening of the enamels in the interferometer test proved to be a characteristic change depending on the fusibility of the enamel. These data showed promise as a test for fusibility.     

RELATION OF COEFFICIENT OF EXPANSION TO IMPACT RESISTANCE OF PORCELAIN ENAMELS*

The tentative standard impact test procedure, adopted by the Porcelain Enamel Institute, was used to test and compare cylinders coated with several experimental and commercial enamels. The important facts noted were (1) the effect of cubic thermal expansion on the impact resistance, (2) the importance of the ground coat to the resistance of any one cover enamel, and (3) the wide variation between cover enamels in resistance to impact.     

RELATION OF COEFFICIENT OF EXPANSION TO CROSS-BENDING STRENGTH OF SHEET-STEEL ENAMELS1

Two series of enamels were made with calculated cubical coefficients of expansion varying from 361 to 306. In both series of enamels, the cross-bending values were inversely proportional to the coefficient of expansion. This is due to the fact that all enamels on sheet steel have lower coefficients of thermal expansion than steel. In cooling, after the enamel is fused to the metal, strains are set up in the enamel; the lower the expansion of the enamel the greater the strains. When the enameled piece is bent, the greater the stresses present, the farther the pieces can be bent before there is any real tensile strain on the enamel. High bending ability is dependent primarily on high compressive strains induced by lower coefficient of expansion of the enamel than of the metal base. Compressive strains cause warping, fishscaling (in ground coat), and chipping; therefore the bending strength should not be too high. In all control work both the minimum and the maximum bending strength should be specified.     

CHEMICAL DETERMINATION OF METALLIC PARTICLES AND OXIDES OF IRON, NICKEL, AND COBALT IN FIRED GROUND COATS*

Chemical methods were investigated and are described for studying the products formed in fired ground-coat enamels. The results obtained show that (1) the metallic particles are essentially iron with a trace of nickel, (2) a film of free FeO exists between glass and iron, (3) total FeO content increases with firing time, but the NiO and CoO content remains constant, and (4) no metallic cobalt and only a trace of metallic nickel is present on the iron surface on which ground coats have been fired and then removed.     

FACTORS GOVERNING ADHERENCE OF ENAMELS APPLIED TO SHEET IRON *

A series of tests were conducted to ascertain the effect of nickel deposition and firing variations upon the adherence of enamels applied to special enameling sheets. Limits were established for securing optimum adherence of zirconium-opacifid white, antimony-molybdenum-opacified white, and commercial blue ground-coat enamels.     

SOME SHEET-IRON GROUND-COAT REBOIL AND BOND STUDIES USING SEVERAL ENAMELING STOCKS*

This paper presents results and observations obtained from a study of metals and some metallic oxides as mill additions to eliminate or reduce reboiling of ground-coat enamels.     

RELATION OF ENAMELING IRON PREPARATION AND ACCESSORY CONDITIONS TO COPPERHEAD-TYPE DEFECTS*

The tendency of blue ground-coat enamels to copperhead is shown to be related to residual iron salts formed on the surfaces of enameling iron sheets in sulfuric acid pickling. Ferrous sulfate generated in the pickling oxidizes and hydrolyzes readily into compounds which are not soluble in cyanide neutralizer. These effects may be retarded by keeping the solutions between the sulfuric acid and the neutralizer slightly acid and by minimizing the time in these solutions and the exposure to air in transferring between them. The determination of these conditions and the establishment of satisfactory procedures in the production process are described. Some accessory conditions which affect copperheading are also included.     

RELATION OF METAL THICKNESS,ENAMEL THICKNESS,AND BOTTOM RADIUS TO IMPACT RESISTANCE OF PORCELAIN ENAMELED UTENSILS*

Three series of standard two-quart pudding pans of different metal thicknesses, coated with one, two, and three coats, respectively, of enamel were tested for impact resistance, using the Enameled Utensil Manufacturers' Council standard impact test. The impact resistance of the pans was increased with increase (1) in metal thickness of the pans, (2) of the enamel thickness, and (3) in the size of the bottom radius. The size of the chip formed on impact also increased with increase in enamel thickness. The literature dealing with the fracture of glass and enamels is reviewed, and the factors influencing spontaneous chipping and chipping caused by bending and impact are discussed.     

Recrystallization of Cerium Oxide in Porcelain Enamels

The effect of cerium oxide incorporated as the principal opacifying agent in the frit batch on the properties of white cover-coat porcelain enamels was studied. The effect of several variables on the opacity of the enamel was investigated as a function of firing temperature. Variables studied included (a) the amount of cerium oxide added, (b) the manner in which cerium oxide was incorporated into the enamel, i.e., mill addition, frit addition, or combination of both, (c) composition of the base glass, (d) weight of application, and (e) type of mill addition. Porcelain enamels were produced which had good acid resistance, high gloss, and a reflectance of 80% or more. Mill-added cerium oxide seemed to be less effective than that which was incorporated in the porcelain enamel as a component of the frit. The spectrophotometric analysis indicated good color stability of cerium oxide enamels.     

RELATION OF THICKNESS TO IMPACT RESISTANCE OF PORCELAIN ENAMEL*

Chipping of porcelain enamel on the flanges of table tops as a result of impact was studied and compared with the results obtained from the Porcelain Enamel Institute tentative standard impact test. A correlation was found to exist between the results using the P.E.I, test on standard 1-in. tubes and those obtained on the corner of a table-top flange. The impact resistance of enamels applied over a radius also increased as the thickness of the enamel was increased.     

A Study of Hairlining

A test for the hairlining tendencies of enamels is presented. A curved 4- by 8-in. piece of 18-gauge enameling iron is used as a test piece in which a simulated service condition of strain is produced by a 2- by 2-in. piece of 16-gauge enameling iron placed on the top center of the test piece. Before enamel application the specimen is curved lengthwise in a mold of 1 1/2-ft. radius of curvature with a resulting permanent distortion of approximately 1/8 in.; the strain condition is obtained by an unattached straining piece. These two conditions enable good reproducibility of the direction and magnitude of the distortion due to uneven heating during firing, and the resulting pattern of hairlines varies only as influenced by enameling variables. The stepwise process of hairline formation is described with the aid of photographs and photomicrographs of enamel surfaces and cross sections. The effects of ground-coat and cover-coat variables and the significance of proper firing schedules are presented and discussed.     

STUDY OF THE EFFECT OF SOLUBLE SALTS PRESENT IN ENAMEL MILL LIQUORS*

Studies were made of the solution of the frit during milling, the soluble salts derived from the frit were identified and studied, and some properties of the slips and mill liquors were investigated. The influence of some soluble salts present in cover-coat enamels on enamel film strength is shown, and a theory regarding the mechanism of tearing has been developed. The function of some salts, particularly sodium hydroxide and sodium nitrite, in eliminating tearing was studied, and the manner in which these salts improve film strength is reported.     

V2O5-O pacified White Enamels

Whiteness and opacity were developed in lead borosilicate enamels by additions of 3 to 8 weight % of V²O⁵. These enamels were characterized by their fusibility, maturing at 1125° to 1200°F. Compositions were adjusted to provide fit and adherence to both glass and aluminum. Normal methods of preparing, applying, and firing were employed. Some of the experimental enamels for glass had marginal resistance to hot alkali solutions. Others appeared to be suitable for general-purpose enamels, for which the chemical requirements are not so stringent. The enamels for aluminum, for which acid and weather resistance are desirable, were rated as having class A acid resistance, according to the classification system of the Porcelain Enamel Institute.     

EFFECT OF GROUND COATS ON REFLECTANCE OF COVER ENAMELS *

The problem of color variation in white cover-coat enamels has received considerable attention, but variations in color due to base coat have had little or no consideration. An attempt was made to determine the effect of the base coat by comparing color variations in opaque and superopaque enamels applied over blue, white, gray, red, yellow, and green base coats in weights ranging from 10 to 120 gm. per sq. ft. The resultant finishes were compared visually and with the aid of the Hunter multipurpose reflectometer. Over the entire range studied, the base coat was found to have a marked effect on the color characteristics of the finish coat. This is true particularly in the lower ranges of application including normal production application weights. Base coats that decreased the reflectance of the cover coat in the blue field, were found to give higher apparent opacity than those which maintained a relatively high blue reflectance.     

THE CAUSES AND CONTROL OF FISH SCALING OF ENAMELS FOR SHEET IRON AND STEEL1

Data obtained.—This paper presents measurements of the following factors and properties and their influences upon fish scaling in the case of typical single and three-coat enamels: time and temp. during melting of the frit; fusibility of the enamel; coeff. of expansion of the enamel, and of a variety of representative irons and steels up to 500°C (accuracy about 0.1%); annealing the enamel coat after firing; chem. comp. of the steels and irons; mechanical treatment (rolling, spinning, drawing, and so forth of the stock; microscopic surface structure of the stock; effect of chem. comp. upon thermal expansion of enamel. Incidentally it was found that the formulas of Mayer and Havas for computing the thermal expansion of enamel from its chem. comp. can not be relied upon: Conclusions.—On the basis of these exhaustive tests, the following conclusions have been drawn as to the causes and the control of fish scaling. (A) Causes.—1. The cause of fish scaling lies in the difference of the coefficients of expansion of the enamel and the stock, that for steel being higher than for enamels so that the latter are under a compressive stress. The factors influencing this phenomenon are as follows: (a) Composition of the enamel as affecting the coefficient of expansion of the enamel; (b) Overfiring—thus volatilizing those substances which tend to keep the coefficient high; (c) Lack of annealing enameled ware, the enamel coating of which is a glass and should logically be treated as such. 2. Fish scaling is due secondarily to a number of factors which may affect the strength of the enamel or its adherence to the metal. (a) The physical condition of the surface of the metal as influenced by drawing, spinning, cold rolling or other mechanical treatment; (b) Composition of the glass as affecting its elastic strength; (c) Underfiring, enamel not fused to metals; (d) Cleanliness of the surface as regards removal of drawing compounds, grease, etc. (B) Remedies.—1. (a) Adjusting the composition of the enamel so as to increase its coefficient, e. g., by decreasing the boric acid content; (b) Correct firing of the enamel to avoid unnecessary volatilization of such fluxes as soda and cryolite; 2. (a) Adjusting enamel composition to increase its strength; (b) Correct firing to give proper adherence. 3. Treatment of metal to give best adhesion by (a) cold rolling or other suitable mechanical treatment of the untreated steel; (b) thorough cleaning. (C) Summary.—Broadly stated, then, fish scaling can be absolutely controlled by one or all of the following methods: (1) By the development of an enamel with an expansion fitting the metal; (2) by annealing the enameled ware; (3) by treatment of the metal as cold rolling or some other method of working: (4) by developing an enamel with a wide range of compressional elasticity and applying this enamel to metal treated as indicated above.     

EFFECTS OF COMPOSITION ON THE PROPERTIES OF SHEET STEEL ENAMELS1

The effects produced by the substitution of feldspar for flint and flint for feldspar together with variations in other constituents commonly used in enamels were studied with reference to their resistance to mechanical shock, thermal shock and acid attack. Tests were made on three series of enamels of twenty each which were applied to 8- inch steel dinner plates over a standard ground coat. The enamels of the series containing both flint and feldspar were found to be most satisfactory for general use. Results indicate that enamels having certain excellent properties, making them more suitable for special purposes, may be found in the other two series.     

ABRASION RESISTANCE OF PORCELAIN ENAMELS*

The abrasion resistances of enamels, differing in composition and properties, were determined by the test for resistance of porcelain enamels to surface abrasion (a standard of the Porcelain Enamel Institute, March, 1942). Four to five classes of abrasion resistance were obtained, but for the most part, the results fell within relatively narrow limits. Differences in abrasion index, in general, were not distinguishable by visual inspection. No one class or kind of enamel was superior. While changes in abrasion index were affected by changes in frit formula, it seems that many compositions will give comparable results.     

RESISTANCE OF ENAMELS TO THERMAL SHOCK*

Thermal-shock resistance tests were made on enamels of varying thermal expansions applied on test pans. Increased weight of application and high expansion of the cover coat decreased the thermal-shock resistance, whereas high expansion of the ground coat increased the resistance.