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 O-2. An inflection in the reverse curve at the pit transition potential E-ptp shows correlation with pit bottom repassivation, leading to concentration gradients for mass transport and promoting further pit nucleation. The close relation between E-pit and E-ptp 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 i(ptp) 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.