Development of porous anodic films on aluminium

Abstract

Development of porous anodic films on aluminium has been investigated by transmission electron microscopy of ultramicrotomed sections, with particular attention directed to the location and origin of the non-uniform thickening of the initial barrier oxide. Preparation of thin ultramicrotomed sections, less that 10 nm thick, allows new information to be obtained on the local morphology of thicker oxide regions. Based on the new refined data, and also on the recent advances in understanding of the atomic migrations proceeding during barrier oxide growth, a model is proposed to explain the observed features of porous anodic film growth on aluminium. Key features of the model are the inward and outward migration of O^2- and Al³⁺ ions across the thickening barrier oxide with only the former contributing to film growth at the metal/film interface. Under such conditions the initial barrier oxide is associated with tensile stresses, which contribute to local cracking of the film above pre-existing ridges on the metal surface. Repair of the cracked regions gives rise to non-uniform film growth. Current is subsequently concentrated into the thin film regions between protuberance of locally thicker film which become the preferred regions for pore development.