Valve Metals Can Be Coated Uniformly by Anodization
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Valve Metals Can Be Coated Uniformly by Anodization In the article, "Coatings for TiAl" (October MRS Bulletin, page 31), Shigeji Taniguchi describes several thermal processes for coating titanium aluminide materials with oxide surface films for the purpose of enhancing high-temperature oxidation resistance. The author describes the difficulty, for a variety of kinetic reasons, in obtaining uniform thermal oxide films. I would like to point out that a wide variety of metals, including titanium and its alloys and intermetallic compounds with aluminum, fall into the broad category of materials known as "valve" metals. These materials form uniform oxide films via anodization in appropriate electrolytes. When anodization occurs under conditions resulting in the formation of dielectric-quality films, the oxide thickness is reproducible within a few tens of angstroms.
Titanium and other valve metals, alloys, and intermetallics may be readily coated with dielectric-quality films by anodization in polar, aprotic solvent solutions of phosphoric acid/soluble phosphates, as described in my British Patent application, GB 2,168,383, filed December 6, 1985. While the organic electrolyte anodizing process was developed with the electronics industry in mind, it is applicable also to the production of barrier oxide films for use as thermal oxidation barriers, prosthetic implant corrosion barriers, etc. Coatings may be produced to voltages well in excess of 500 volts, giving film thicknesses of over 10,000 angstroms.
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