VOLTAMMETRIC STUDY OF THE LOCALIZED CORROSION OF AL-ZN-MG ALLOYS CONTAINING CR AND NB IN CHLORIDE SOLUTIONS

The resistance to localized corrosion of Al-5%Zn-1.7%Mg-0.23%Cu (alloy H) containing 0.14% Cr (alloy L), 0.053% Nb (alloy J) or both, 0.14% Cr and 0.053% Nb (alloy O), annealed (A), cold-rolled (ST), quenched ( F), quenched and aged (B) and quenched in two steps and aged (C), has been studied by means of cv, optical microscopy, SEM, EDX and XPS. The cyclic voltammograms obtained at different sweep rates in NaCl soluti ons at concentrations in the range 0.1-1 mol dm(-3) showed the same ch aracteristics as those obtained previously for alloy H. One breakdown potential was found for the specimens F, which consisted of a supersat urated phase. One breakdown potential was also found for the specimens A and ST although they consisted of two phases: the matrix solid solu tion and big MgZn2 precipitates (0.2-0.4 mu m in length). It was shown that the MgZn2 precipitates disappeared from the alloy surface from p otentials well before the breakdown potential. In contrast, an anodic maximum and a further breakdown potential were obtained for the age-ha rdened specimens (B and C) which consisted of the matrix solid solutio n and small MgZn2 precipitates (about 0.02 mu m in length). The anodic maximum was explained by a surface dealloying affecting all the alloy surface. Pit propagation was not detected. The breakdown potential af ter the anodic maximum was related with the further pit propagation in pits nucleated at random. The voltammograms of the specimens B and C corresponded to Al-alloys resistant to SCC, in agreement with previous experiments using the cut-edge method and breaking stresses measureme nts. The repassivation potentials were shown to characterize well the pitting corrosion resistance of the alloys, which was increased by the addition of Cr and Nb, independently or both at the same time. The ef fect of Cr and Nb was related with their good distribution in the allo y and in the oxide film. The repassivation potentials increased in the sequence H < J < L < O for a given heat treatment and in the sequence F < A approximate to ST < B approximate to C for a given composition, in agreement with previous corrosion potential measurements in chlori de solutions containing hydrogen peroxide. Copyright (C) 1996 Elsevier Science Ltd.