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Anodic oxidation film of aluminium was prepared in sulfuric acid (19.8wt%) with the pulse method. The anodic film (approximately 0.1mm thick) was removed from the substrate by applying the reverse voltage on the substrate. On heating (10°C/min), dehydration of the film occurred at the temperature range of 100° to 400°C and the original semi-transparent gray color of the film became more transparent and less dark. At approximately 950°C, an endothermic decomposition reaction was observed which was followed immediately by a sharp exothermic change. The amorphous- to γ-Al2O3 crystallization took place at this temperature and the specific surface area increased sharply. Large part of the sulfur in the film was eliminated in this stage. At approximately 1150°C, the rest of the sulfur was eliminated. The γ-Al2O3 began to transform exothermically into α-Al2O3, and the film became white and opaque. The specific surface area decreased with increasing temperature. The transformation to α-Al2O3 was completed at approximately 1300°C.
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