Structures of typical oxide films formed on aluminum, and the structural changes with a combination of treatments are reviewed.
Oxide films on Al are classified into four typical groups by structure : porous oxide films, barrier oxide films, thermal oxide films, and hydroxide films. The porous oxide films are formed anodically in acid solutions and barrier oxide films in neutral solution and the thermal oxide films are formed by heat treatments and hydroxide film by hydrothermal treatments. The morphology and formation behavior of these oxide films are described briefly.
Three characteristic phenomena observed with the combination of treatments are described : pore-filling, sealing, and formation of composite oxide films. Pore-filling, where pores of porous oxide films are filled with new oxides, takes place by anodizing porous oxide-covered Al in a neutral solution, and is useful for determining the porosity of the oxide film. Sealing of pores in oxide films is attained by hydration of oxide exposed to hot water, and the dissolution rate of the sealed oxide film is much lower than that of unsealed film. Composite oxide films are formed by anodizing hydroxide-covered Al in neutral solutions, and the growth of the oxide film proceeds by dehydration of hydroxide at the hydroxide/oxide interphase and formation of oxide at the oxide/metal interphase.
