Structure Change and Deveropment of Alkali Resistance of Aluminum Anodic Oxide Film by Hydrothermal Treatment

It is investigated that the effect of anodizing times and dipping times of the aluminum in a 4 mass% oxalic solution bath at 313 K and that of the hydrothermal temperature (373K, 423K, 473K) on the alkali resistance time (ART) and film structure of treated films. As the temperature of the hydrothermal treatment increased, the ART also increased considerably, The SEM observation of the cross sections of hydrothermally sealed anodized films revealed that, they were, in general, composed of an outer layer consisting of cube-shaped hydrate crystals (outer hydrate layer), an middle layer having no distinct morphological features (inner hydrate layer), and an inner oxide layer containing the original anodized oxide film (oxide layer). The actual microstructures of sealed films could be classified into three groups corresponding to three regions of anodizing times. In the case of short anodizing times (region I), the oxide layer was absent. In the case of long anodizing times (region III), the thickness of the oxide layer was constant. In region II, intermediate region between region I and region III, the thickness of the oxide layer increased as the anodizing time increased. The thickness of outer hydrate layer was about 0.5μm within all region. In addition, if the sum of the anodizing time and dipping time was the same, the thickness of the inner hydrate layer was also the same. And, the ART of the hydrothermally sealed anodized films was affected by the inner hydrate layer thickness and it's characteristics.