Structure and Formation Mechanism of Composite Oxide Films on Aluminum.

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Composite oxide films formed by galvanostatic anodizing of hydroxide-covered aluminum in a neutral borate solution have been investigated by electron microscopy, chemical analysis, gravimetry, XPS, and electrochemical measurements. The major results obtained in the present investigation are as follows:(1) Composite oxide films generally consist of two layers: An outer crystalline oxide layer and an inner amorphous oxide layer. At the oxide formation, i) the inner layer is produced by the inward transport of <O^(2-)> ions at the metal/oxide interphase, ii) the outer layer is produced by the electric field-assisted dehydration of hydroxide at the hydroxide/oxide interphase, and iii) the crystallization of amorphous oxide proceeds at the inner layer/outer layer interphase. The rates of reactions i) and ii) are constant and independent of anodizing temperature. However, the rate of reaction iii) increases with increasing anodizing time and becomes higher at higher temperatures.(2) The composite oxide film contains a large number of voids and cracks in the outer layer, and the voids distribute at a high concentration in the region close to the outer layer/inner layer interphase. The total volume of voids, <alpha_v> , shows a maximum for the composite oxide formed at 60 ゜C and it increases considerably by hydroxide-aging prior to anodizing. Aging of hydroxide at higher temperatures leads to lower values of <alpha_v> .