PITTING AND REPASSIVATION PROCESSES OF AL-ZN-MG ALLOYS IN CHLORIDE SOLUTIONS CONTAINING SULFATE

The mechanism of localized corrosion of Al-5.03%Zn-1.67%Mg-0.23%Cu alloy submitted to different heat treatments (annealed (A); cold-rolled (ST); quenched (F); quenched and aged (B); and quenched in two stages and aged (C)) in NaCl, Na2SO4 and in NaCl-Na2SO4 solutions has been studied by cyclic polarization (CP), open circuit potential measurements, chronocoulometry, SEM and EDX. The electrochemical results in NaCl were consistent with the assays of SCC resistance of alloys B and C previously reported in the literature. Sulphate addition to chloride solutions produced a considerable shift of the breakdown potential in the positive direction. However, the repassivation potential, much more reproducible than the latter, was only slightly shifted in the negative direction. The role of sulphate was related with the oxide presence, its incorporation in the film precluding significant chloride penetration. However, the film conductivity apparently increased with potential in the presence of sulphate, finally making pit propagation possible and resulting in a much more intense pitting attack.