The synergetic effect of chloride and sulphate on pitting corrosion of copper

The pitting corrosion of copper in chloride and sulphate containing berate-buffered solutions has been investigated by statistical analysis of pit induction times. The pitting susceptibility is quantified by the pit generation rate (lambda(0)) deduced from the pit induction time distributions. Sulphate ions, like chloride ions, can induce copper pitting and the same pit induction time distribution laws are observed. However sulphate ions are more aggressive than chloride ions. In the presence of both species the variation of lambda(0) appears to be complex. Depending on the chloride content, three zones are revealed. In the first zone, corresponding to low chloride contents, the species responsible for copper pitting were the sulphate ions. Increasing the chloride content produces an increase of lambda(0) showing an accelerating effect of the chloride ions. In the second zone, for medium chloride contents, the sulphate ions were always the species which produced copper pitting but chloride ions had an inhibiting effect since lambda(0) decreased. A maximum of lambda(0) was then reached between these two first zones and the corresponding chloride content appeared to be independent of the sulphate content. The third zone corresponds to high contents of chloride ions which, here, were the species responsible for pitting. The rate lambda(0) increased rapidly with increasing chloride content. The minimum of lambda(0) corresponding to the transition between the second and third zones was always observed for the same chloride to sulphate ratio and thus resulted from competition between these two aggressive ions. Copyright (C) 1996 Elsevier Science Ltd