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 all oy submitted to different heat treatments (annealed (A); cold-rolled ( ST); quenched (F); quenched and aged (B); and quenched in two stages a nd aged (C)) in NaCl, Na2SO4 and in NaCl-Na2SO4 solutions has been stu died by cyclic polarization (CP), open circuit potential measurements, chronocoulometry, SEM and EDX. The electrochemical results in NaCl we re consistent with the assays of SCC resistance of alloys B and C prev iously reported in the literature. Sulphate addition to chloride solut ions produced a considerable shift of the breakdown potential in the p ositive direction. However, the repassivation potential, much more rep roducible than the latter, was only slightly shifted in the negative d irection. The role of sulphate was related with the oxide presence, it s incorporation in the film precluding significant chloride penetratio n. However, the film conductivity apparently increased with potential in the presence of sulphate, finally making pit propagation possible a nd resulting in a much more intense pitting attack.