Abstract
An explanation of the origin of integral color anodizing of aluminum has been proposed in terms of the ratio of the cell size to the pore diameter of the oxide layer, which is characteristic of bath voltage and the electrolyte used. The P ratio, the ratio of cell radius to pore radius, was found to play an essential role on the current density at the pore bottom. For semi-spherical current flow at the pore bottom, the theoretical ratio of the current density at the pore bottom to the density at the metaloxide interface was proportional to the P2. Electronmicroscopic observations of the cross section and/or transmission photographs of the porous oxide layer indicated that the integral color anodizing is probably obtained under such a condition that the P ratio is more than 5. In such a case, concentration of electrolytic current at the pore bottom induces an extremely high field strength (1×109V.m-1), by which electrons are injected into the oxide layer and further accelerated to gain an energy enough for breaking or for exciting bonds of organic compounds in the oxide film. Since the incorporation or inclusion of organic anions in the oxide film is always detected irrespective of the coloration, the incorporation of the organic anions itself is not the origin of the color, but the break or excitation of bonds of organic anions by high energy electrons should, at least, take part in the formation of the colored oxide film.
