Influence of different anions on the behaviour of aluminium in aqueous solutions

The influence of chloride, sulfate and perchlorate anions on the behaviour of native oxide layers on aluminium is investigated using electrochemical techniques. Due to its influence on the open circuit potential and the cathodic side of the polarization curve the oxygen concentration has been carefully controlled. Two kinds of attack on a commercially pure aluminium (99.5 wt %) have been observed. In all the investigated 0.5 M Cl−, 0.5 M ClO−4 and 0.5 M SO2−4 aqueous solutions the metal is corroded around the iron and silicon containing precipitates, but only in Cl− and ClO−4 solutions is crystallographic pitting observed. Comparison with high purity aluminium (99.99 wt %) shows that pitting corrosion is not influenced by the presence of impurities in the aluminium alloys, but by the presence of anions in solution. The pH and/or oxygen concentration determine whether or not the pitting potential coincides with the corrosion potential.