Potential‐pH Diagrams for Sulfur and Oxygen Adsorbed on Chromium in Water

The concept of equilibrium potential‐pH diagrams ( E ‐pH) for two‐dimensional species adsorbed on metals is applied to the case of sulfur and oxygen adsorbed on chromium in water. Standard Gibbs energies of formation for sulfur and oxygen adsorbed on chromium are calculated from literature thermodynamic data for chemisorption from the gas phase. The E ‐pH relations associated with the equilibria between water, the dissolved sulfur species, and adsorbed sulfur and oxygen are given at 25 and 300°C. The corresponding E ‐pH diagrams are calculated for a sulfur activity of 10 − 4 and are superimposed on the usual Pourbaix diagrams for the S ‐ Cr ‐ H 2 O system. Formation of adsorbed sulfur monolayers on the metal surface is predicted in E ‐pH domains where the usual diagrams do not predict the stability of a chromium sulfide. Predictions of the corrosion risk for chromium in sulfur‐containing aqueous environments can be done using the reported E ‐pH diagrams.