Abstract
Anodizing of aluminum in aluminate solutions was studied from the viewpoints of film structure and breakdown phenomena. It was shown by scanning electron microscopy and impedance measurements that the oxide galvanostatically formed in solutions more dilute than 0.1mol NaAlO2 consisted of two layers; an underlying barrier type oxide layer and a thick upper of aluminum hydroxide. The measurements of dependence of film impedance on frequency further showed the evidences of composite structure. The formation of the upper layer was explained in terms of the precipitation of aluminate anions caused by protons generated on the oxide/solution interface and field-assisted dehydration of the resultant hydroxide on the barrier layer, which was suggested from the impedance characteristics. It was demonstrated that aluminate solution was a very unique and useful forming solution for the barrier layer coating. The sparking voltages of the oxide film formed in aluminate were higher than those of the film in usual electrolytes such as borate, and the upper layer was considered to be a protective layer against breakdown. This remarkable effect was discussed in connection with the degree of dehydration of the upper layer. It was further observed that the thickness of barrier layer increased and that of upper layer decreased by the aging at a constant voltage for a few hours, which means as possibility that a part of the upper layer was converted into a barrier type oxide during the aging.
