Abstract
Optical and electron microscope examinations have been carried out on anodic oxide films formed on aluminium in H 3BO 3/Na 2B 4O 7 solutions under various temperature and formation voltage conditions. Whereas at low temperatures and voltages a barrier type of film was produced, an increase in either parameter encouraged crystallization and porosity. The crystalline phase appeared as agglomerates in an amorphous matrix. Electron and x-ray diffraction showed that the crystalline phase was not homogeneous, consisting of γ′-Al 2O 3, γ-Al 2O 3, and at least one of the compounds boehmite or α-Al 2O 3. A model is proposed for the interrelation between the amorphous and the crystalline phases in the film. This model is aimed to explain discrepancies in the literature.
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