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This paper examines the mechanism of pore formation in anodic films on aluminium. For this purpose, the dimensional changes of specimens during growth of porous films on aluminium in phosphoric and sulphuric acid electrolytes are examined using transmission and scanning electron microscopies. Further, the compositions of films and the efficiencies of anodizing are determined by Rutherford backscattering spectroscopy and nuclear reaction analysis. Significantly, the efficiency of anodizing is about 60%, while the surface of the anodic film is located above the original aluminium surface, i.e. before anodizing. The ratio of the thickness of the anodic film relative to the thickness of the consumed aluminium is about 1.35 for the selected conditions of anodizing. The behaviour runs counter to the widely accepted mechanism of pore formation by field-assisted dissolution of alumina. It is explained by the high plasticity of the anodic alumina in the barrier region in the presence of ionic transport, with film growth stresses displacing material from the barrier layer towards the cell wall region during anodizing. The response of the film to volume constraints on growth indicates a major role of stress and stress-relief processes in determining the morphology and self-regulating organization of pores.

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The growth of an anodic oxide film on a specimen consisting of a layer of aluminum deposited upon a layer of tungsten was examined by transmission electron microscopy. The findings reveal initial growth of anodic alumina, followed by penetration of the alumina by fingers of tungsten oxide once the metal/film interface reaches the tungsten layer. The fingers are approximately cylindrical or lanceolate, with width in the range 1-15 nm and aspect ratios up to about 40. The current flows preferentially to regions of tungsten oxide rather than the alumina due to the lower electric field for growth of the former oxide. The penetration of the alumina is assisted by the relatively high Pilling-Bedworth ratio for W/WO 3 and the inherent plasticity in amorphous anodic oxides. The tungsten oxide of the fingers appears to be of higher density than that formed at the metal/film interface after the aluminum has been fully oxidized, which may be due to the penetration of the alumina layer by a displacement process.