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Composite oxide films were formed anodically on hydroxide-covered aluminum in a boric acid/borate solution at 40, 60, 80, and 95°C, and then relaxed by allowing water to penetrate into the voids in the oxide films. After relaxation, specimens were re-anodized with a low current to measure the time-variation in the anode potential, Eref. The void distribution in the composite oxide films was determined by analyzing the Eref vs. tref curve.It was found that the penetration of water started after an induction period, and that the apparent activation energy for the relaxation of composite oxide films was 69.5KJ/mol. Voids were observed in high concentrations at the inner layer/outer layer interphase, and at the center of the outer layer in the composite oxide film. The total volume of voids increased with anode potential, Ea, during anodizing at all temperatures except 95°C; at 95°C, the void volume increased with Ea up to Ea=300V, and then decreased to a small value at Ea=400V. The void volume showed a maximum at 60°C for all values of Ea.Film formation mechanisms are discussed to explain how void distribution is established.
Takahashi et al. (Sun,) studied this question.