The repassivation response from single cycle anodic polarization : The case study of a sensitized Al-Mg alloy

The repassivation behaviour of artificially aged 5083-H111 Al-Mg alloy in near neutral NaCl solutions was investigated by means of single cycle anodic polarization. Artificial aging was carried out at 150 degrees C during different times up to 360 h. The characterization of the microstructure and composition by XRD, SEM/EDS and metallographic analyses pointed out most favoured grain-boundary beta(Al3Mg2) phase precipitation for 168 h of isothermal treatment time. Nitric acid mass loss test (NAMLT) and microhardness measurements indicated the highest degree of sensitization (DoS) and developed strength for 168 h as well. For all the aging conditions, the potential drop at high currents during the reverse scan of the cyclic polarization was detected at the pit transition potential E-ptp with similar but higher values than the corrosion potential of beta phase (E-corr approximate to -900 mV vs SCE). The associated current density i(ptp) and the steepness of the potential drop increase as the amount and contiguity of b phase precipitates along grain boundaries. The onset and sustenance of metastable conditions that limit the simultaneous repassivation of all the corroded surfaces are driven by the anodic dissolution of beta(Al3Mg2). Metastable processes that limit the hydration of metal ions while producing high local hydrogen concentration are more likely to involve the formation/decomposition of reactive hydride intermediates. E-ptp corresponds to the mixed electrode potential at which both metal dissolution and hydrogen evolution occur beyond some distance into the cavity, while iptp evaluates the catalytic activity of the corroding surfaces. The effective anodic charge transfer coefficient alpha(eff) determined from the steepness of the potential drop estimates the contribution of Cl- electromigration in response to local electrodissolution processes.