Anodizing of aluminium under applied electrode temperature: Process evaluation and elimination of burning at high current densities

Using a temperature controlling electrode holder anodizing of aluminium is studied under conditions of applied electrode temperature. At different current densities the influence of the electrode temperature on the anode potential and oxide morphology is evaluated. Additionally, the results are compared to those of experiments performed under conditions of uncontrolled electrode temperatures at various electrolyte temperatures. By increasing the electrode temperature the anode potential decreases at all considered current densities, this even at a constant electrolyte temperature. The stationary, as well as the initial maximum potential decline; though, at high electrode temperatures the latter is more sensitive to temperature variations. Due to the observed large decline of the maximum potential at high electrode temperature, the possibility to grow oxide layers at very high current densities without encountering burning is evaluated at low electrolyte temperatures. At high electrode temperatures of 65°C the initial maximum potential is reduced to such an extent that at an electrolyte temperature of 25°C anodizing can be performed up to 25A/dm2 without observing burning. By increasing the electrolyte temperature up to 45°C, even at 30A/dm2 burning does not occur during anodizing at applied electrode temperature of 65°C. This is not possible under conditions of uncontrolled electrode temperature, which require higher electrolyte temperatures to grow oxide layers at very high current densities without encountering burning.