US2261960A - Method of producing corrosion resistant coatings on magnesium - Google Patents

Description

Patented Nov. 11, 1941 UNITED STATES PATENT OFFICE METHOD OF PRODUCING CORROSiON RE SISTANT COATINGS ON MAGNESIUM Robert W. Buzzard, Kensington, Md.

No Drawing. Application October 14, 1937, Serial No. 168,931

(Granted under the act 'of March 3, 1883, as

amended April 30, 1928; 370 0. G. 757) 4 Claims.

The invention relates to the treatment of corrosion resistant coatings on light metals and their alloys, especially magnesium, and one of the objects is to increase the durability and the protective characteristics of the coatings on the metal.

The method specifically relates to subjecting protective coatings on light metals or alloys ,thereof to electrolytic treatment so as to form a seal that not only increases the durability of the metal to wear but also improves its resistance to corrosion.

The metal to be treated according to this in vention is washed free of the primary coatingsolution, which may be applied by an electrolytic treatment or by the dip or immersion method, and is then subjected to an electrolytic treatment in a solution containing one or more chemicals which attack the metal through the porous openings of the primary coating, and fixes the coating in a more insoluble form thereby reducing the porosity of the protective coating on the metal.

I have found that a very good and uniform seal for protective coatings on light metals can be obtained by anodizing the coated metal in a solution containing one or more substances selected from the groups comprising chromium, borates, fluorides, sulphates, manganese and the compounds of these substances. Either the sealing solution or the method of application should difier from the solution or method used toform the coating on the metal. I The concentrations of the substance or substances selected to make ;up the anodic sealing bath is limited only by the solubility of the substance and may vary from -1 gram per liter solution up to the saturation point of the particular substance. The temperature of the solution may vary from normal room temperature to 90 C. and the current density may also vary from to 100 amperes per square .foot. The time of treatment depends upon the strength of the solution, the current density, and thequalityof seal desired. Good seals of coated metal have been obtained by subjecting the coated metal to. treatment for periods varying from 5 minutes to one hour.

Specifically, as an example, if an anodic coating be formed on magnesium or its alloys in a bath of per cent dichromate and 2 per cent phosphate or if a dip, or immersion coating be formed on the metal in a bath of sodium bichromate and nitric acid, and the resultant coating has been washed free of solution, its cormetal is then subjected to anodic treatment in a solution containing per cent dichromate.

As an example of a change in method of applying the protective coat and my improved seal on metal, I formed a coating on two identical magnesium specimens by immersing the metal in a bath of 10 percent sodium dichromate and 2 per cent monosodlum phosphate heated to approximately at or above the boiling point of water and then followed this treatment by subjecting one of the coated specimens to anodic treatment in the same bath. In tests the sealed specimen proved far superior to the unsealed,

specimen.

-'It should be understood that the protection obtained by my treatment is dependent on the presence of a coating, either by an electrolytic or dip method, on the metal and is not a prime surface treatment.

As further examples of proportions of some of (g) 10 percent dichromate and 2.to percent phosphate. The coated metal may also be given two or more sealing treatments, provided, the solutions in the coating treatment and.the subsequent sealing treatments difier from each other, however, at least one of the sealing treatments shall be anodically applied iii-accordance with my invention herein described. As examples, magnesium coated in a bath of 10 percent dichromate and 2 percent phosphate may be first immersed in a solution of dichromate and chromium fluoride followed by an electrolytic treatment in a solution of- 25 percent by weight dichromate;

: or, coated magnesium may be first anodically rosion resistance will greatly increased ifthe sealed in a solution of 5 percent borate followed by a second anodical sealing treatment in a solution of 25 percent dichromate.

The above mentioned substances used in my improved sealing treatments are mentioned by way of illustration and not limitation, since the invention comprehends treatments of the character indicated broadly above and is not to be limited except by the appended claims.

The invention described herein may be manu factured and used by or for the Government oi the United States of America for governmental purposes without the payment of any royalties thereon or therefor.

What I claim is;

1. A method of producing corrosion resistant coatings on magnesium or its alloys, comprisingv forming a protective coating on the metal in an aqueous bath containing water soluble salts of dichromate, and phosphate, then immersing the coated metal in an aqueous solution of dichromate and chrome fluoride: and then subjecting the treated and coated metal to anodic treatment in an aqueous solution containing from 20 percent to 75 percent by weight dichromate.

2. A method of producing corrosion resistant coatings on magnesium or its alloys, which comprises subjecting the metal to an anodic treatprimary coating on the metal by anodically treating in an aqueous bath containing at least one water-soluble salt selected from the group consisting, of the chromates, dichromates, fluorides, and borates 4. A method of producing corrosion resistant coatings on magnesium or its alloys, which comprises forming a protective coating on .the metal ment in an aqueous bath containing a watersoluble dichromate and phosphate, and then in an aqueous bath containing a water-soluble dichromate and nitric acid, and then sealing the primary coating on the metal by anodically treating in an aqueous bath containing a watersoluble dichromate.

ROBERT W. BUZZARD.