The relationship was explored between nanoscale voids in anodic aluminum oxide films and the surface condition of aluminum samples prior to anodizing. Transmission electron microscopy ͑TEM͒ detected voids on the order of 10 nm in anodic films. Atomic force microscopy ͑AFM͒ of these films, obtained after partial oxide dissolution, revealed surface cavities; comparison of TEM and AFM suggested that the cavities were the oxide voids. AFM images after variable extents of oxide dissolution showed that the voids were distributed evenly through the inner 60% of the film thickness, indicating that they were formed at the metal-oxide interface during film growth. Both AFM and TEM showed that the void concentration in the film was sensitive to the extent of dissolution of the aluminum samples in NaOH prior to anodizing. Positron annihilation spectroscopy had previously detected voids in samples without anodic films, located in the metal near the oxide-metal interface; the quantity of these interfacial voids was controlled by NaOH dissolution. The void concentration in the inner part of the anodic films was proportional to the quantity of these pre-existing interfacial voids. It was inferred that the oxide voids were formed by incorporation, during anodizing, of interfacial metal voids into the oxide film. The uniform concentration of oxide voids in the inner film suggested that interfacial metal voids formed continuously during anodizing and that metal voids were generated repeatedly at specific interfacial sites during film growth.

The effect of impurities on formation of interfacial metallic voids, during uniform dissolution of aluminum in 1 M NaOH, was investigated. These voids are thought to act as initiation sites for pitting. Foils of three different bulk purities were used: 99.98% (3N), 99.997% (4N), and 99.9995% (5N). Positron Annihilation Spectroscopy (PAS) and Atomic Force Microscopy (AFM) revealed that nm-scale voids were formed by dissolution in each foil. The void volume fraction increased to a maximum during dissolution, at a time which increased with foil purity. The concurrent accumulation of near-surface Cu and Fe impurities during caustic etching was characterized using Rutherford backscattering spectrometry (RBS). For the three foils, a correlation of void volume fraction with Cu surface concentration was suggested. Processes involving Cu impurities may then at least partly control the formation of voids.

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