Dissolution behaviour of the barrier layer of porous oxide films on aluminum formed in phosphoric acid studied by a re-anodizing technique

Chemical dissolution of the barrier layer of porous oxide films formed on aluminum foil (99.5% purity) in the 4% phosphoric acid after immersion in 2 mol dm⁻³ sulphuric acid at 50 °C has been studied. The barrier layer thickness before and after dissolution was determined using a re-anodizing technique. A digital voltmeter with a computer system was used to record the change in the anode potential with re-anodizing time. It has been found that the barrier layer material may consist of two or three regions according to the dissolution rate. The barrier oxide contains two layers at 35 V: the outer layer with the highest dissolution rate and the inner layer with low dissolution rate. The barrier oxide contains three layers at 40 V and above it: the outer layer with high dissolution rate, the middle layer with the highest dissolution rate and the inner layer with low dissolution rate. It has been shown that there is a dependence of the dissolution rate on the surface charge of anodic oxide film. Annealing of porous alumina films for 1 h at 200 °C leads to disappearance of layers with different dissolution rates in the barrier oxide. We explained this phenomenon by the absence of the space charge in the barrier oxide of such films.