Formation of Anodic Coatings on Aluminum

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Abstract
The coatings formed on aluminum by anodic oxidation in dilute solutions of sulfuric, oxalic, chromic and boric acids are, as far as can be determined by x‐ray and electron diffraction methods, amorphous. They are comprised chiefly of aluminum oxide but also contain substances adsorbed from the electrolyte. Coatings formed in boric acid electrolyte are substantially impervious and non‐adsorptive; whereas those formed in sulfuric, oxalic and chromic acid electrolytes are porous and adsorptive. The porosity in the latter type of coating is attributed to the solvent action of the electrolyte and this porosity is thought to be responsible for the adsorptive characteristics of these coatings. The presence of pores permits the continuous passage of current; consequently, coatings made in these electrolytes can be produced with a substantial thickness. For the electrolytes investigated, the anodic oxidation process is about 100% efficient as far as the conversion of aluminum to oxide is concerned. As a result of the solvent action of the electrolyte on the oxide, the apparent current efficiency in terms of oxide remaining as coating is shown to be as low as 50% or less. Microscopic examination of especially prepared cross‐sections of anodically coated aluminum shows evidence that the oxide has a cellular structure. The fact that the oxide occupies a larger volume than the metal from which it is formed is demonstrated visually. The increase in thickness resulting from the application of an anodic coating must be considered where close fits are required between coated parts.

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