Observation of Anodic Oxide Films on Aluminium and Mechanism of Pore Closure during Hydrothermal Treatment

摘要

The transmission electron microscopy characterization of anodic films on aluminium is reported. Anodizing was conducted in an aggressive solution of chromic acid. A thick and porous film was formed. This treatment was followed by hydrothermal sealing. Film cross sections showed the duplex microstructure of the amorphous alumina coating (thick irregular branching porous layer and thin barrier layer). Cellular growth took place at the oxide/metal interface. At the beginning of the sealing process, the hydration of alumina produced a thickness increase of the pore walls. The pores maintained roughly a tubular geometry. For higher sealing ratios, the wall irregularities induce the formation of bumps leading to a cellular structure of the sealing product which plugs the pores thus limiting the penetration of primers, paints or adhesives. This microstructural study of the sealing process explains why a partial sealing gives an optimal configuration to promote both corrosion protection and adhesion. In contrast, the unsealed and totally sealed films offer less effective corrosion protection and adhesion respectively.