US2076869A - Method of making enameled metal articles - Google Patents

Description

Patented Apr. I 13, 1937 UNITED STATES PATENT OFFICE- :METHOD OF MAKING ENAMIELED DIET ARTICLES Robert R. Tanner, Detroit, Mich., assignor tov Metal Finishing Research Corporation, Detroit.- Mich, a corporation of Michigan Application November 22, 1930, Serial No. 497,614

1 Claim. (Cl. 91-43) This invention relates to an enameled metal culties hitherto encountered in producing a proper bond between the vitreous enamel or glass and the metal to which it is applied.

In use it has been found that, where vitreous enamels have been applied to ordinary impure iron articles the enamel becomes discoloredby patches of material between the enamel and the metal. Presumably this is the result of reaction between the glass coating and the impurities in the metal. However this may be, it has been found necessary to use a particularly pure grade of iron as a base for glass enamels in order to avoid this defect, commonly referred to as ,fish scale.

It has been ordinarily considered necessary to use what is called a ground coat to properly unite the finishing coats of glass to the iron'article. This ground coathas been a difierent composition from the final coat and has been useful merely as a uniting coat between the metal and the finishing coator coats.

. high temperatures a decomposition takes place of the order of 2/1000ths of 1%, appears to be' more heat resistant than the usual phosphate coating; but even this phosphate coating produced with a trace of copper in the bath is disintegrated at high temperatures, and is not use-- fulr where subjected to temperatures above 1000" F. Y

u the glass coating.

Because of the disintegration by heat of phosphate coatings it was not supposed that such coatings could be employed successfully for forming a foundationfor vitreous or glass enamels, which are customarily baked on at a temperature from 1500 F. to 1700 R, which is far above the degree of heat which has been found to disintegrate the phospate coatings.

It has been discovered that when the phosphate coating ofthe character described above is protected from the air it. will withstand high temperatures without disintegration. In accordance with the present invention, this new discoverey is taken advantage of in order tb produce an iron article having thereon a glass enamel bonded to the article by a phosphate coating. Broadly speaking, this can be accomplished by producing on the article the phosphate coating in any well known or obvious manner and thereafter applying to this phosphate coating a glass enamel and baking it thereon in the ordinary way, the

' only point requiring care being that the enamel material shall be placed upon the phosphate coating in such a way that it protects the coating from the air before a temperature is reached at which the phosphate coating disintegrates in the open air.

It has been further discovered that where the iron metal is thus protected from direct contact with the glass by the bonding coating of phosphates, impurities in the iron do not result in the defect referred to above as "fish scale. For that reason ordinary ironcan be used as the base for porcelain or glass enamel instead of the high gradepure iron formerlyconsidered necessary.

In the ordinary method hitherto employed for preparing high grade iron to receive glass enamel, the iron article has been first treated in strong alkali for about 20 minutes and then pickled in a sulphuricacid bath for 15 or 20 minutes to remove all rust, grease, etc., and then treated with weak alkali to remove the residue of the acid and finally rinsed to remove the weak alkali. Thereafter a ground coating has been applied as referred to above, whereupon the article was prepared to receive the regular vitreous enamel or A satisfactory method of proceeding in accordance with the present invention is to employ an ordinary iron article instead of the high grade iron previously employed, and to treat it with alkalifor 5 minutesor merely wipe with naphtha in place of the elaborate cleaning operation hitherto employed. Thereafter the article may be immersed in a hot dilute solution of dihydrogen phosphate containing about 2/1000ths of 1% of copper in a soluble compound whereupon a sumcient foundation coating of substantially insoluble phosphates is formed in 5 to minutes.

Thereafter it may be briefly rinsed to remove the coating solution, and the regular glass enamel may be applied directly to this phosphate coating or, if a ground coat is employed, it may be made of a composition more nearly approaching the finishing coat than has been employed hitherto. A very satisfactory method is to employ a ground coating about half way between the composition of that customarily employed hitherto and the composition of the finishing coating customarily employed. In this Way a finish can be produced with the one coating of modified ground coat and one finishing coat of enamel which is sub-' stantially equivalent to the finish previously produced by the regular ground coat and two coats of enamel thereover. better bond produced by the phosphate coating renders such a finish as described more durable perature is reached which has an injurious effect upon the coating. The dry process may be used also, but in that case the glass powder must be coated over the phosphate coating before the article has been heated to a temperature which will injuriously affect the phosphate coating.

As a matter of fact the If particular conditions render it necessary the phosphate coating might be heated in a neutral atmosphere to a comparatively high temperature before receiving the glass powder, but under all ordinary circumstances this is entirely unnecessary as the vitreous or glass coat may be readily employed so as to exclude the air from the.

phosphate coating before a temperature is reached which is injurious to the phosphate coating.

When the phosphate coating is protected from the air during the heating process, the glass enamel may be baked thereon to a temperature from 1500 F. to 1700 F., as is customary, and no injurious defects are produced upon the phosphate coating. If the glass is dissolved from the phosphate coating by suitable chemicals, after having been baked on at this comparatively high temperature, the phosphate coating is found to be unchanged, thereby proving that the protection of the phosphate coat from the air during the heating thereof is efiective to prevent injury to the phosphate coating.

Those skilled in the art will be able to make numerous modifications Within the scope of the appended claim.

What I claim is:

A method of providing an iron article with a vitreous enamel coating which comprises providing the surface of the iron with a coating of substantially insoluble phosphates and baking upon the coating of phosphates a vitreous enamel while maintaining the phosphate coating, unchanged by protecting the phosphate coating from contact with air to form a vitreous enamel coat upon the phosphate coating.

ROBERT E. TANNER.