POTENTIAL PH DIAGRAMS FOR SULFUR AND OXYGEN ADSORBED ON NICKEL IN WATER AT 25-DEGREES-C AND 300-DEGREES-C

The principle of equilibrium potential-pH diagrams for two-dimensional species adsorbed on metals3 is applied to the case of sulfur and oxygen adsorbed on nickel in water. Standard Gibbs energies of formation for sulfur and oxygen adsorbed on nickel 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-degrees-C. The corresponding E-pH diagrams are drawn for two sulfur activities (10(-4) and 10(-6) and superimposed on the usual Pourbaix diagrams for the S-Ni-H2O 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 nickel sulfide. The E-pH diagrams for species adsorbed on metals allow one to make new predictions of the corrosion risk for metals in sulfur-containing aqueous environments.