Abstract
Thermal expansion of the anodic oxide layers of aluminum formed in certain aqueous electrolytes, such as sulfuric acid, oxalic acid, sulfuric-oxalic acid mixture, and sulfosalicylic acid has been investigated for each type of the films on the assumption that this phenomenon is different between inorganic acid and organic acid films. An attempt has been made to explain these phenomena in terms of the film growth in four different electrolytes. In a case of using inorganic acid, thermal shrinkage of films occurs up to 200°C, but in another case of using organic acid, it occurs up to 900°C. And the phenomenon has shown a reversible change within the temperature range from 25°C to 700°C. The possible causes for the thermal shrinkage investigated here may be due to the difference of thermal expansion coefficient between film surface area and film-aluminum interface area or at least in the film structures.
