Abstract
Crystallinity of barrier oxide films formed, by anodic oxidation, on aluminum in an aqueous solution of 1 g/l ammonium dihydrogen phosphate at 85°C was found to be strongly dependent, among other factors, on the immersion time of aluminum specimens in the anodizing bath prior to anodic oxidation. When electropolished aluminum specimens were anodized immediately after they had been introduced in the anodizing bath, normal amorphous film formation was observed. However, when the immersion time was about 30 sec or longer, the resulting films consisted of both amorphous and crystalline γ'-alumina. Extensive examinations of the surface of aluminum specimens after immersion in the anodizing bath have shown the presence of trace amounts of hydrous oxide along the grain boundaries or cell boundaries of the aluminum surface. Good correlations were found between the manner of formation and distribution of hydrous oxide and the growth of γ'-alumina.within the barrier oxide. The development, within the barrier oxide, of γ'-alumina can be explained reasonably if it is assumed that the trace amounts of hydrous oxide present along the grain boundaries or cell boundaries are incorporated within the growing barrier oxide and subsequently dehydrate to fine crystallites of γ'-alumina which act as nuclei for. amorphous to γ'-alumina transformation.
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