US2138794A - Treatment of magnesium and its alloys - Google Patents

Description

Patented NOY- 29, 1938 a no Drawing. Application March 15, 1937,

a Se'rial-No.130,909

3 Claims. (01. ita -6)",

The invention relates to improvements in out substantially affecting its original dimensions methods of producing a protective and decorative or weight. The invention then consists of the coating upon articles fabricated of magnesium method hereinafter fully described and particuand its alloys. larly pointed out in the claims.

The type of treatment to which magnesium According to our invention, treating solutions 5 l alloy articles have heretofore been subjected to suitable for the purpose contain as efiective conproduce thereon a'conventional protective coatstituents from 1 to 20 per cent of an alkali ing generally consists in treating the article-with metal 'chromate, and from 0.5 to 10 per cent of an aqueous solution of a salt producing adchromic sulphate, based upon the dry weight 10 herent water insoluble deposit on the metal. Such of the salts, dissolved in water. A generally useb 10 coatings, however, have limitations which adful composition is made by dissolving in water versely affect their utility, so that they are sel- 9 per cent of sodium bichromate, Na2Cr2O7 and 1 dom used without, a further treatment, as by per cent of chromic sulphate, Cr2(SO4)3. P0- painting. One disadvantage is that they are tassium chromic sulphate (chrome alum) or amseldom uniform in appearance, the shade and monium chromic sulphate may be used, if de- 16 depth of color varying, we have found, with the sired, as the source of chromic sulphate. In carhomogeneity of the metal itself. The variations 'rying our invention into effect, the article to be in color and shade over the surface of a given treated is first cleaned, if n ss in a known treated article are particularly marked in cast manner, as by wire brushing or by pickling in 20 articles, such as die castings. Rolled and formed dilute nitric acid. The cleaned article is then articles also exhibit a certain amount of superimmersed in the treating solution, preferably at ficial variation of metal structure and as a the boiling temperature thereof, for from about. consequence develop non-uniform coatings when 0.5 to 60 minutes. until the desired coating is subjected to a conventional chemical treatment. obtained. After the coating has been produced,

This non-uniformity of the chemically produced the article is removed from the solution and 25 coatings on fabricated magnesium articles seems rinsed immediately, preferably in hot water, and to be due primarily to the different'rates of acthen air dried. I tion of the treating solution upon various por- The treating solution may contain other intions of the metal surface, and to the fact that gredients and may, after use, contain a minor the protective deposit does not possessa suflicient amount of products of solution of the articles 30 opacity to fully hide the underlying irregulari treated. It will eventually become exhausted or ties in the metal surface. Attempts to build up lean in active ingredients and require regenerathicker coatings, to mask the inhomogeneity of tion or renewal. Any solution preparable by disthe ordinary coatings, do not produce satisfacsolving in water the said active ingredients will I Y UNITED PATEN i o rlc-E tory results, because such coatings lack adequate function in accordance with our discovery. 35 mechanical strength. Such attempts also involve The following example is illustrative of a mode dissolving or etching away an excessive amount of of carrying out the invention:-- the underlying metal, thus materially affecting An article composed of a magnesium oy onthe dimensional accuracy of the article. taining 0 per cent A1, 0.1 per cent Mn, 0.5 per 40 a The principal object of the invention is, ac- Bent the ce being magnesium, W 0 cordingly, to provide a, method of producing a, mersed f0! 5 minutes at the boiling temperature corrosion-resistant coating upon magnesium and i a q s treating bath Containing D its alloys, without excessively attacking or discent of potassium chromic sulphate, K2Cr2(SO4)4 solving the underlying metal, which coating i and 9.0 per cent of sodium bichromate, NazCrzOr.

uniform in appearance, irrespective of any nor- The article was then withdrawn, rinsed, and 45 mal inhomogeneity of the structure of the metal dried. The treatment produced a uniform, black itself. Other objects and advantages will be adherent coating without material alteration of apparent as the description proceeds. the dimensions or weight of the article.

We have discovered that, by subjecting articles On subjecting the so treated article to the of magnesium and its alloys to the action of an action of a 3 per cent solution of sodium chloride 50 aqueous solution containing chromic sulphate by the usual alternate immersion test for resistand an alkali metal chromate or bichromate, a ance to corrosion, it was foundthat the coating uniform adherent corrosion-resistant coating is retained its original uniform black color for more produced upon the metal, regardless of any superthan two weeks of the test. Similar results are 65 ficial inhomogeneity of the metal itself and withobtained within the scope of our invention with other magnesium alloys, the depth of color varying somewhat with the composition of the alloy.

Among the advantages oi the invention are: that'a uniform appearing coating may be=v produced on articles 01' magnesium and its alloys in spite oi a iackvpt homogeneity in the; metal itself; the coatings are not only highly decorative, but also possess great resistance to corrosion.

Other modes of applying the principle of our invention may be employed instead of the one explained; change being made as regards the method herein disclosed, provided the steps stated by any of the following claims or the equivalent or such stated step-or steps be employed. l I

We therefore particularly point out and distinctly claim as our-invention:

1. In a method of articles of magnesium and its alloys, the step whichconsists in subjecting the athlete the step or? a about 1 to'20 producing a coating upon action ofan aqueous solution containing as e!- tective constituents chromic sulphate and an alkali metal chromate.

2. In a method of producing a coating upon articles ormagnesium and its alloys, the step which consists in subjecting the article to the action of ah aqueous solution containing'as effective constituents from about i to 20 per cent or chromlc sulphate and item about 0.5 to 20 per cent'ofan alkali metal chromate. v

In a method of producing a'coating upon articles of magnesium and its alloys, the step which consists iii-subjecting the article to the action of an aqueous solution containing from per cent of a chromic sulphate of the group consisting of sodium, potassium and boiling temperature.

blchromate at the CHARLES E. NELSON.

HERBERT DE LONG.-

- ammonium and from 0.5 to 20Lper centof sodium