Mechanism of he development of .GAMMA.'-alumina within barrier oxide films on aluminum during their growth.

The development of γ'-alumina within the growing barrier oxide films on aluminum has been examined from the microscopic point of view. Under the present situation, where the structure of amorphous anodic alumina remains uncertain, it was assumed that the arrangement of oxygen ions in the amorphous alumina can be represented approximately by DRP model proposed by Bernal. An close examination of oxygen ion migration at the amorphous/γ'-crystalline alumina interface has shown that the migration of oxygen across the interface is accompanied by the loss of excess volume associated with oxygen ions in the amorphous phase. The excess volumes eventually agglomerate to form voids at the amorphous/γ'-crystalline alumina interface. It has been shown clearly that the amount of excess volume released at the amorphous/γ'-crystalline interface is considerably greater at the interface near to the oxide/electrolyte interface than at the interface near to the metal/oxide interface, implying preferential formation of void at the former interface. On the basis of DRP model, an explanation is also given to the origin of "highly ordered" amorphous alumina surroundingγ'-alumina crystals.