Abstract
The structure of anodic oxide films on aluminum formed in chromic acid solution at constant voltage, and under various electrical or electrolytic conditions, has been studied by electron microscopy. Electron micrographs of film section replica showed that pore branching and subsequent colony structure was formed during anodizing. With film growth, pore colony took hemispherical shape, which resembles to that of barrier layer bottom in typical porous type oxide film, at metal-oxide interface. These film structures were not in accord with vertical structure model proposed by Keller et al. A form of branching pore structure depended on anodizing voltage, the higher the applied voltage, the sooner the initiation of pore branching and colonial structure. The formation of branching colonial structure was strongly favored by following conditions. (1) Sudden voltage drop during anodizing (that is colled Recovery Effect). (2) Anodizing after formation of preliminary thick barrier layer, for instance, primary anodization was done in ammonium tartarate solution. The second anodizing voltage in chromic acid solution was lower than that of preliminary one. The facts suggest that the cause of pore branching is due to the film growth through thick barrier layer.
