High speed anodizing of aluminium to form functional composite oxide films

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A two-step process to form composite anodic oxide films on Al, that is, step I (formation of metal-oxyhydroxide film on Al by electrolysis or dip methods, then drying or heat treatment, if necessary) * Step II (formation of porous oxide film on Al by anodic oxidation in acid solutions), was established.For Step I,(a) electrolytic treatment in molybdate solutions, (b) chromate treatment, (c) chromate-phosphate treatment, (d) electrolytic treatment in Ti (IV) containing solutions, and related methods were carried out. For Step II,the high speed anodizing with high current densities was realized by using sulfuric acid solutions containing Mn (II) salts.The anodic oxide films obtained by the above process always showed higher hardness than the conventional films formed under similar conditions. Further, depending on the method used for Step I,method (a) gave black anodic oxide films having very high hardness and high wear-resistance, (b) and (c) gave the films markedly resistant to pitting corrosion. Method (d), which was attempted to increase dielectric constant of the film, was unsuccessful, but during the experiment it was found that boiling in solutions containing Ba (II) salt is effective to modify anodic oxide films and forms composite oxide containing crystalline BaSO_4 when it is applied to the process using sulfuric acid for Step II.The Mn (II) salt added sulfuric acid bath enabled to anodize Al at high speed, and was found also to be useful to anodize Al alloys on which formation of uniform anodic oxide film is difficult.