抄録
This paper describes the mechanism of built-in color of anodic oxide films formed in various electrolytes in terms of the crystal structure of the alumina films. The film color seems to be dependent on the co-ordination number of oxide ion to Al3+ ion. The co-ordination number was determined either by chemical shift of the fluorescent X-ray spectoscopy or by radial distribution method. The co-ordination number of γ-Al2O3 is 5.3, whereas that of the colorless anodic oxide films is 4.7, those of sulfosalicylic acid film and oxalic acid film being 5.8 and 5.9 respectively. The film formed in mixed electrolyte of oxalic-sulfuric acid bath showed the co-ordination number of 5.3. In conclusion, the oxide film having the coordination number of less than 5.3 is colorless, and one having the number of more than 5.3 shows built-in color.
