The pit transition potential in the repassivation of aluminium alloys

This paper reports a systematic investigation of corrosion in 0.6 mol/l NaCl of freshly polished 1050-H24, 6082-T6, 2024-T3 and 7075-T6 Al alloys by single-cycle anodic polarization using different anodic current limits for scan reversal (irev). For all alloys but Al 1050-H24, an inflection point was repeatedly identified in the reverse curve in most experimental conditions. This feature was correlated with constricted localized attack other than classical pitting, as complemented by corrosion morphology analysis. Characteristic electrochemical parameters such as pitting (Epit) and protection (Eprot) potentials, as well as the potential at the inflection point (Eptp) and the corresponding current density (iptp), were determined with statistical significance. Empirical relationships were derived as a function of irev. Eptp was found to be independent of the surface state condition, changing little with the amount of corrosion, differently from Epit and Eprot. Conversely, iptp increases markedly with irev, which points to a transition related to repassivation kinetics. This was quantified as a function of corrosion extent by the relationship between the steepness of potential decrease with current below Eptp as a function of log iptp. The kinetics of the active/passive sequence in a given local environment is determined by the bulk properties of a given Al alloy.