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The built-in color of aluminum anodized in oxalic acid electrolyte depended upon the wave form of the current for electrolysis. The E.S.R. spectrum of anodic oxide film at room temperature showed the presence of free spin (unpaired electron) in the films, where the spin concentration was related to the deepness of the color of the film. The pulsating current superimposed on direct current caused coloring of deep brown on the film, which is similar to the color obtained from sulfosalicylic acid bath (Kalcolor) Ultraviolet and visible ray absorption spectra of the thin film at -186°C showed several discrete absorption bands, while those at room temperature are broad and non-discrete. From these results the following three mechanisms were proposed as the causes of built-in color of anodic oxide films: (1) Inclusion of organic radicals in glassy alumina. (2) Electrons or holes trapped at the defective sites of alumina crystallites. (3) Semi-carbonized organic substances accompanied with some radicals or imperfections in the molecule.

Effects of Current Form on Built-in Color of Anodized Aluminum and Its Coloring Mechanism