Potential–pH diagrams for sulfur and hydroxyl adsorbed on copper surfaces in water containing sulfides, sulfites or thiosulfates

Abstract

The principle of equilibrium potential–pH (Pourbaix) diagrams can be extended to the case of bidimensional layers of species adsorbed on metal surfaces. It allows us to predict the E–pH conditions of adsorption of sulfur and hydroxyl on copper surfaces immersed in water containing dissolved sulfur species such as sulfides, sulfites or thiosulfates. The E–pH relations associated with the equilibria between water and adsorbed hydroxyl, and between dissolved sulfur species and adsorbed sulfur are given at 25 °C. The E–pH adsorption diagrams are calculated for a sulfur concentration of 10 −4 mol kg −1 and superimposed to the usual Pourbaix diagrams for the S–Cu–H 2O system. It is shown that an adsorbed sulfur monolayer may form on the copper surfaces in E–pH domains in which usual Pourbaix diagrams predict no stability of the bulk copper sulfide. The calculation of stability domains of adsorbed sulfur allows prediction of the corrosion risk for copper-based materials in sulfur-containing aqueous environments.