The pit morphology on copper in chloride- and sulphate-containing solutions

Abstract

For a better of understanding of the behaviour of copper towards pitting germination in chloride- and/or sulphate-containing borate-buffered solutions and more precisely of the synergetic effect of these two aggressive ions, the morphology of the pits appearing in these solutions has been examined in association with analysis of corrosion products by X-ray diffraction and analysis by X-ray photo-electron spectroscopy (XPS) of the material surface in attacked and unattacked zones. Pits generated in chloride-containing solutions are topped by an atacamite cap and the surface is covered with a layer of CuCl, while in sulphate containing solutions, copper is denuded under the caps which have a duplex structure of copper hydroxide and brochantite. The mechanism of pitting is explained by successive substitutions of the oxygen ions of the initial duplex structure of the passive film (CuO and Cu 2O) by the aggressive ions. In chloride- and sulphate-containing solutions, pit morphology and the mechanism of pitting depend on the relative concentrations of both ions. For low concentrations of chloride, sulphate ions are responsible for pitting. For high concentrations of chloride, chloride ions are responsible for pitting. For medium concentrations of chloride, there is competition between the aggressiveness of the sulphate ions (destruction of the oxide layers) and inhibition by the chloride ions (formation of a CuCl layer). The pit morphology in this case is a characteristic of both ions.