Effect of chloride concentration, pH and dissolved oxygen, on the repassivation of 6082-T6 Al alloy

The effect of chloride concentration, pH and dissolved oxygen on the repassivation behaviour of 6082-T6 Al alloy, was investigated using single-cycle anodic polarization. Pitting taking the place of intergranular attack was obtained with increasing Cl− concentration and solution pH, as well as under continuous bubbling of O2. An inflection in the reverse curve at the pit transition potential Eptp shows correlation with pit bottom repassivation, leading to concentration gradients for mass transport and promoting further pit nucleation. The close relation between Epit and Eptp was confirmed, though the latter is likely characteristic of the acidified pit-like solution in contact with a freshly created new surface. The associated current density iptp is determined by the rate at which hydrolysis equilibrium is reached at a critical concentration of Al3+. The transition onset during repassivation was more marked for conditions favouring formation at the pit bottom of a solid layer, though poorly protective due to aluminium oxychloride intermediates. The steepness of potential decrease after the transition, related to a periodic active/passive sequence, is determined by Al dissolution controlled by proton reduction.