Effect of annealing on the pitting corrosion resistance of anodized aluminum-magnesium alloy processed by severe plastic deformation

The effect of annealing on the pitting corrosion resistance of anodized Al-Mg alloy processed by equal-channel angular pressing (ECAP) was investigated by electrochemical techniques in a solution containing 0.2 mol/L of AlCl3 and also by surface analysis. The degree of internal stress generated in anodic oxide films during anodization was evaluated with a strain gauge. The ECAP decreased the pitting corrosion resistance of anodized Al-Mg alloy. However, the pitting corrosion resistance was improved by annealing after the ECAP. The internal stress present in the anodic oxide films was compressive, and the stress was higher in the alloys with ECAP than without. The compressive internal stress gradually decreased with increasing annealing temperature. Cracks occurred in the anodic oxide film on Al-Mg alloy during initial corrosion. The ECAP produces high internal stresses in the Al-Mg alloy; the stresses remain in the anodic oxide films, increasing the likelihood of cracks. It is assumed that the pitting corrosion is promoted by these cracks as a result of the higher internal stress resulting from the ECAP. The improvement in the pitting corrosion resistance of anodized Al-Mg alloy by annealing appears to be attributable to a decrease in the internal stresses in anodic oxide films.