Abstract
The mechanism for the occurrence of the burning of Al surface and the change of bath voltage during anodic oxidation were studied with a heat and mass transfer technique. The burning occurred at the point of the surface of the highest temperature rise before the start of the burning and at any point to which a concentrated oxalic acid solution from a small nozzle was injected. The analysis of the change in the bath voltage during anodizing and of the conductivity of the barrier layer showed that the concentration of oxalic acid near the barrier layer decreases with increasing thickness of the oxide film. The bath voltage calculated from a theoretical equation, derived from above analysis, agreed with the observed voltage. The experimental and theoretical considerations led to the following mechanism for the occurrence of the burning: At the final stage of anodizing before the burning, the concentration of the oxalic acid near the barrier layer becomes extremly low, and the formation of oxide film is controlled by the diffusion of oxalic acid through the porous layer. At this final stage, the anodizing current flows at the point of the AI surface of the highest temperature where the diffusion rate is maximum. This current concentration at that point causes the temperature rise, and this temperature rise promote the current concentration. The interactions of the temperature rise and the current concentrations finally lead to the burning.
