POTENTIAL-PH DIAGRAMS FOR SULFUR AND OXYGEN ADSORBED ON CHROMIUM IN WATER

The concept of equilibrium potential-pH diagrams (E-pH) for two-dimens ional species adsorbed on metals is applied to the case of sulfur and oxygen adsorbed on chromium in water. Standard Gibbs energies of forma tion for sulfur and oxygen adsorbed on chromium are calculated from li terature thermodynamic data for chemisorption from the gas phase. The E-pH relations associated with the equilibria between water, the disso lved sulfur species, and adsorbed sulfur and oxygen are given at 25 an d 300 degrees C. The corresponding E-pH diagrams are calculated for a sulfur activity of 10(-4) and are superimposed on the usual Pourbaix d iagrams for the S-Cr-H2O system. Formation of adsorbed sulfur monolaye rs on the metal surface is predicted in E-pH domains where the usual d iagrams do not predict the stability of a chromium sulfide. Prediction s of the corrosion risk for chromium in sulfur-containing aqueous envi ronments can be done using the reported E-pH diagrams.