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

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-H₂O 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.