The electrical breakdown during anodizing of high purity aluminium in borate solutions

High resolution scanning electron microscopy of the surfaces of aluminium specimens supporting barrier-type anodic films and transmission electron microscopy of the stripped films have been employed to gain further insight into the mechanism of the electrical breakdown during anodizing of aluminium in borate solutions. Particular attention has been paid to the following aspects of the phenomenon: (1) the sites at which breakdown initiates, (2) the development of breakdown and (3) the structure and morphology of the films after the breakdown events. The observations indicate that the breakdown is possibly of a thermal nature and is triggered by local heating effects caused by highly localized processes taking place at flaws during anodizing, including the possibility of initial electron avalanching.