US1946151A

US1946151A - Protecting aluminum from corrosion

Info

Publication number: US1946151A
Application number: US668628A
Authority: US
Inventors: Junius D Edwards
Original assignee: Aluminum Company of America
Current assignee: Howmet Aerospace Inc
Priority date: 1931-06-20
Filing date: 1933-04-29
Publication date: 1934-02-06
Anticipated expiration: 1951-02-06

Description

Patented Feb. 6, 1934 UNITED S-TA PROTECTING AL COBRO SION Junius 1).

Aluminum Company of Edwards, Oakmont, Pa., aaaignor to America, Pittsburgh,

Pa., a corporation of Pennsylvania No Drawing. Application April 29, 1988, Serial No. 668,828, and

This invention relates to the covering of aluminum and aluminum alloys with a coating capable of resisting the action of corrosive agencies. The invention particularly contemplates the produc- 5 tion on such metal surfaces of a corrosion resistant coating of the type known in the art as oxide coatings.

These oxide coatings, as they will be termed in the following description and claims. may be 0 produced by several methods, which methods, with few exceptio comprise a reaction, either chemical or electro-chemical, between the aluminum or aluminum alloy and a solution of a chemically active substance or substances. The coatingsproduced on aluminum by these various and well known methods have ascribed to them certain properties and advantages, one of which is alleged to be the protection against corrosive agencies afforded to the coated metal. However, only a few of the oxide coatings so produced afford any substantial protection to the aluminum on which they are formed and even the best of such coatings leave much to be desired in their "protective effects.

It is accordingly an object of this invention to provide certain methods of covering aluminum and its alloys with protective coatings having a high resistance to the action of corrosive substances and particularly to salt solutions. A further object of this invention is the provision of an oxide coated aluminum article of good corrosion resistance and having other desirable and novel properties as will hereinafter. appear.

For the purpose of this specification and the appended claims, aluminum or aluminum alloys regardless of the purity of the metal or the nature of the alloying element, will be inclusively described by the generic term aluminum".

l. have discovered as the result of an extensive so investigation that an adsorbent oxide coating when formed on aluminum may be thereafter treated by a simple process which greatly increases the emciency of the oxide coating as a protective covering against corrosive agencies. An important advantage of the process is that the corrosion resistance is not less in one area or part of the coating than another but is substantially equal over the entire coated surface. 50 My novel method comprises, as a first and neces= sary step, the formation on the aluminum of an oxide coating having adsorptive properties and, as a second step, the treatment of the coated metal with a solution or a soluble substance, 55 namely chromic acid, inhibitory to corrosive tack by chromic acid, and

to strip the coating.

. little attack on the in Canada June so. 193 3 Claims. (01. 1486) agencies and capable of being uniformly adsorbed in and on the adsorptive oxide coating.

I have found, however, that oxide coatings on aluminum, however produced, are subject to atmay be virtually if not M completely destroyed thereby, especially when the acid solution is hot. For example, a 0.3 mil oxide coating on duralumin may be completely stripped by immersing the coated metal in a 5 per cent solution of chromic acid at C. for 5 about 6 minutes, whereas 20, 45 and minute immersions are required in 5 per cent chromic acid solutions at 40, 30 and 20 C., respectively, On the other hand, with 1,

5 and 10 percent solutions of chromic acid at 1 40 C., the times req ed to dissolve a 0.3 mil coating on duralumin are about 25, 20 and 17 minutes, respectively. Concentrations as low as 0.1 per cent are effective for my process and cause film. In cases where the oxide 75 film is not completely stripped from the metal, the surface of what is left of the film may be rendered soft and powdery by the use of a chromic acid solution at too high a concentration or temperature. By adjustment of concentration, temso perature and time of treatment, the impregnating treatment can be effected without material impairment of the film. In general I prefer a concentration of not more than about 10 per cent, and a temperature not higher than about. 35 35 C., as giving satisfactory results in practice with an immersion of not more than about 20 minutes. By a few trials at given concentration or temperature a suitable time of treatment can be readily found. In some cases a time which no will not impair the film may be too short to produce' the desired impregnation and in such cases the temperature or concentration, or both, can

be adlusted.

I have further found that even though an adsorbent oxide coating be impregnated with chromic acid without being seriously attacked, and be then washed, the retained chromic acid which the coating had taken up readily washes out; but that if after removal from the impregnating solution the coating is thoroughly dried without washing a better retention of the acid is obtained. The protectiveefiect of the coating is thus substantially increased, even under such severe conditions as exposure to salt spray for extended periods. I am unable to say what stabilizing change is caused or brought about by the drying, but it effects a marked improvement in corrosion resistance. In preparingoxide coated aluminum articles in mi accordance with my invention it is desirable that the coating be comparatively heavy or thick, but a comparatively thin coating may be satisfactory, especially' if its adsorptive power is high. ,To the preparation of such oxide coatings certain methods are peculiarly adapted. My preferred method consists in making the aluminum the anode of.

an electrolytic cell, the cathode of which may be aluminum, lead or other metal, and using as the electrolyte in such cell an aqueous solution of sulphuric acidr The aluminum anode and the cathode'are immersed in such an electrolyte and external electrical energy is impressed upon the electrodes with the result that a layer of oxide is formed on the aluminum anode surface which is adherent, hard and dense and possesses remarkable adsorptive powers. The concentration of sulphuric acid may vary within very wide limits, such as 1 to per cent, and good results will be obtained, but I have preferred to use solutions containing between -5 to 10 per cent of sulphuric acid. I also prefer to use a current density of about 0.1 to 0.4 amperes per square inch of anode surface. Other methods of forming these oxide'coatings are well known, and may be used, but I prefer the method outlined above.

The coating must be firm, adherent and resistant to abrasion, and hence such coatings as are produced by water or by atmospheric oxygen are excluded when I speak of oxide coatings.

The aluminum, having been provided with an adsorptive oxide coating, is now preferably washed with water to remove residual solution remaining from the coating treatment, and is then treated, in accordance with my invention, with the chromic acid solution. This treatment may take various forms but I prefer immersion of the oxide-coated aluminum article in an aqueous solution of the acid, after which the article is dried without washing.

As a specific example of my method of coating aluminum with a corrosion resistant coating and the resultsobtained thereby, there may be cited the treatment of aluminum spandrels which are designed for use on the outer walls of buildings. Cast articles of this nature made of an aluminum base alloy containing about 5 perIcent of silicon were made theanode of "a cell the electrolye of which was a solution of 7 per cent sulphuric acid. In 20 minutes a. highly adsorbent oxide coating of gray color was formed on the aluminum alloy castings. The coated castings were then washed and dried, and immersed in a 4 per cent chromic acid solution at room temperature (20 C.) for -.a period of 10 minutes. At the end of this period they were removed from the solution and dried without washing. The-resulting coated articles were then tested by immersion for a period of 72 hours in a highly corrosive solution containing about 7.5 per cent of sodium chloride and about 3 per cent of hydrogen peroxide. An inspection of the castings at the end of that period revealed that the coating on their surfaces had completely resisted the corrosive action of the sodium' chloride-hydrogen peroxide solution. At the end of a further period of 72 hours in a corrosive solution of the same composition the same articles were only slightly affected. Similar oxide coated aluminum castings treated in accordance with my invention exhibited no signs of corrosion after an exposure of about 5 months to the atmosphere.

This application is a continuation in part of my copending application Serial No. 472,744, filed August 2, 1930.

I claim:

- 1. A method of protecting aluminum surfaces against corrosion, comprising depositing on the aluminum surface an adsorptive oxide coating, thereafter impregnating said oxide coating with a chromic acid solution, and drying the impregnated coating without washing.

2. A method of protecting aluminum surfaces against corrosion, comprising forming on the aluminum surface an adsorptive oxide coating,

thereafter treating said oxide-coated surface 3. A method of protecting aluminum surfaces against corrosion, comprising forming on the aluminum surface an adsorptive oxide coating; thereafter impregnating said oxide coating sur- ;face with a chromic acid solution of not more i .20 than 10'per cent strength at a temperature not higher than 35. C. and'for a period of not more than about 20 minutes; and drying the impregnated coating without washing.

JUNIUS D. EDWARDS.