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TREATMENT OF ALUMINUM FOR CORROSION PREVENTION ARTHUR A. VERNON Northeastern University, Boston, Massachusetts
INTEE past twenty years over one hundred and fifty foreign and American patents have been issued concerning various methods of treatment of aluminum to produce a corrosion-resistant surface. Many more patents have been granted for methods of production of decorative or colored finishes. This paper is concerned with a brief review of the methods for and theories of the formation of corrosion-inhibiting surfaces.
layer of aluminum oxide, or (2) chemical attack. He points out that there are three possibilities of the first type: (a) Electrolysis in which little or no solvent action
takes place on the coating. This will produce a nonporous nonadsorptive film. (b) Electrolysis in which there is considerable film removal to ~roducea uorous and adsorutive surface. (c) Electrolysis in which the film is dissolved as rapidly as formed. This will cause brightening of the base metal.
METHODS
In general either an electrolytic or a chemical treatment is used. For both methods, i t is preferable to first clean the surface by brushmg, sandblasting, degreasing, or electrolytic polishing. Conditions of the treatment can be adjusted to give a transparent or gray surface of various shades. The film formed on the aluminum surface is usually porous and must be sealed by various means. Hydrolyzing in steam or dipping in chromate and silicate solutions are common after treatments, although many other solutions have been recommended. Usually the surface layer is harder than the base metal and, therefore, may crack if the article is bent; however, a considerable degree of rubbing resistance is obtained. Electrolytic Treatment. About one-third of the reported patents claim solutions for electrolytic treatment of aluminum anodes. The most widely described electrolytes are sulfuric acid, chromic acid, and oxalic acid. Practically all concentrations have been used a t a temperature in the neighborhood of 60 or 70' F. Many addition agents have been claimed to be useful, such as aluminum or copper sulfate, lead acetate, glucose, cresol, sodium acetate, to mention only a few. Other baths are described which contain solutions of sodium phosphate, fluorides, polyhydric alcohol, colloidal metal oxides, glycerol, sodium hydroxide, phosphoric acid, fluoborates, or sodium carbonate. Those most widely used commercially are the Bengough-Stuart method using the chromic acid, the Alumilite using sulfuric acid, and the Eloxal process using oxalic acid. Chemical. A large number of solutions for chemical treatment of aluminum have been reported. Among them are sodium silicate, alkaline metal chromates, and oxalates, sodium hydroxide, and sodium fluosilicate. The chemical methods may require some heat or further chemical treatment to produce an adherent film. DISCUSSION
Jernstedt (1) describes the action of film formation as due either to: (1) the formation of an amorphous
Jernstedt further points out that in chemical treatment there is probably a competition between chemical attack and film dissolution. For example, in a mixture of sodium carbonate and potassium dichromate (Alrok process) the carbonate promotes the attack and the chromate h i d e r s the action. It is obvious that certain balances between the two can be obtained to produce a film of the desired properties. Anderson (2) showed that with sulfuric acid and oxalic acid, film formation takes place in a dense layer of Alsoa next to the metal. He proposes that aluminum ions dissolve from the metal into the oxide layer and oxygen ions travel into the spaces thus created. There has always been much discussion as to whether the layer is amorphous or crystalline. Taylor, Tucker, and Edwards (3) showed that the layer has the structure of y alumina a t potentials above 100 volts; crystallme structure does not depend upon the electrolytic bath. At lower potentials the film is amorphous. The explanation for this behavior is not evident. Herenguel and Segond (4) found that the oxide film formed in anodic oxidation may be uniform or consist of two layers with differentstructures and hardness depending upon the action of the electrolyte. For example, with sulfuric acid a t low temperature the action is limited to the outside of the film and causes merely a decrease in thickness. At higher temperature the action takes place within the film to produce a more porous structure. Dankov, Kochetkov, and Shishakov (6) used electronic and x-ray methods to study the structure of oxides on aluminum surfaces and concluded that a complex relation exists between the metal and the oxide. The primary aluminum oxide film is amorphous. Keller and Edwards (6) studied the anodic oxide layer formed by electrolysis in sulfuric acid and showed that the film diiers in many respects from the natural 147
'am formed on aluminum. Coatings on pure aluminum are substantially continuous transparent films of aluminum oxide with a thickness of 0.0001 to 0.0006 inch-about 100 or 1000 times the natural film thickness. However, when impurities are present, as in commercial aluminum, the nature of the film is altered. The impurities may be unaffected? partially oxidized and remain in the surface, or anodically attacked and removed. Silicon is an example of the first possibility, FeA13 and MnAl, are examples of the second, while CuAL is dissolved more rapidly than aluminum itself. Studies reported by Kellcr and Edwards with the elcctron microscope showed that in a thick film the pores are about 0 . 0 6 ~apart a t the oxide metal surface but the outer edge has a sponge-like porous character. They propose that the base of the pores is covered by a layer
of oxide. There are about one hundred million pores per square inch, large enough to allow the entrance of a solution but no colloidal material. Sealing action by chromate is due to adsorption in the pores while steam or hot water converts the coating to a monohydrate of alumina. LITERATURE CITED (1) JERNSTEDT, C. W., A.S.T.M. Bulletin, No. 137.29 (1945). (2) ANDERSON, S. J., Applied Phys., 15, 477 (1945). (3) Taylor, C. S., C. M. Tucker, AND J. D. Edwards, Trans. Electrochen. Soc., 88reprint (1945). (4) J.., AND R. SEGOND. Rev. Met.. 42. 258 11943). . . HERENGUEL. (5) DANnov, P. D., A. A. KOCAETKOV, AND N. A. SHISAAKOV, Bull. Acad. Sci. U.S.S.R. Classe Sci. Chem.. 274 (1942) (English Summary). (6) KELLER, F., AND J. 0. EDWARDL~, Iron Age, 156, 75 (1945).
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