Formation of black anodic films on aluminum in acid electrolytes containing titanium complex anion

Galvanostatic anodizing of aluminum in oxalic acid electrolytes containing sulfate and [TiO(C2O4)2]2− complex anions results in the formation of black porous anodic film. The initial voltage–time response in the present electrolytes is similar to that in usual acid electrolytes; after showing a maximum voltage, anodizing voltage becomes almost constant, i.e. 40–50 V. Then, the anodizing voltage increases almost linearly with time, and black coloring of the films takes place. The films become darker with an increase in anodizing voltage. In contrast, the black films are not formed in the oxalic acid–titanium complex anion mixed solutions without sulfate anions, although the similar voltage increase has been observed in the sulfate-free electrolyte. From morphological examinations, it has been found that the presence of sulfate ions in the electrolytes reduces the cell size of the porous films formed before the voltage increase. After the voltage increase, the cell structure in the inner part of the film becomes rather irregular and cell size becomes larger. In addition, the metal/film interface is extremely rough, and particles of metallic aluminum are incorporated in the anodic films near the metal/film interface. Glow discharge optical emission spectroscopy analysis has revealed that a small amount of titanium species is present in the inner part of the film. From these results, the mechanism of the black coloring of the anodic films has been discussed.