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Abstract

A method of enhancing naturally developed color of an anodic oxidation coat on Al or Al alloy wherein the coat is formed by an electrolysis employing a warmed electrolyte containing at least sulfuric acid and an electric current of which the polarity is periodically reversed to be negative for a period less than 50% of each cycle while being applied to Al or Al alloy dipped in the electrolyte as anode for 20 to 40 minutes and the thus coated Al or Al alloy is subjected to a heat treatment in a boiling sealing liquid or a boiling metallic salt solution for 20 minutes. Preferably, the electrolysis is carried out for most of the electrolysis time with the current reversed for a small period of about 5% each cycle and during the terminating stage with the current reversed for a larger period of 30 to 35% each cycle. A sulfur compound is accumulated in the coat during such electrolysis and is combined with an added alloy element of the Al alloy during the heat treatment to further develop the color of the coat.

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