Potential‐pH Diagrams for Adsorbed Species: Application to Sulfur Adsorbed on Iron in Water at 25° and 300°C

Abstract

The principle of equilibrium potential‐pH diagrams (Pourbaix diagrams) is extended to the case of two‐dimensional species adsorbed on metals. Up to now, the only solid compounds taken into account in these diagrams have been three‐dimensional compounds. However, species may be adsorbed in E‐pH domains where three‐dimensional compounds are unstable. The principle of the calculation of the E‐pH relations existing at equilibrium in aqueous solution between the dissolved species and the species adsorbed on a metal is presented. The calculation is made for sulfur adsorbed on iron in water , using thermodynamic data for sulfur chemisorption on iron. The resulting E‐pH diagrams for the system at 25° and 300°C are shown. Formation of adsorbed monolayers of sulfur is predicted in E‐pH domains where a classical diagram does not predict the formation of sulfide. Direct applications of such diagrams in corrosion, e.g., in water at 300°C, are emphasized.