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 oxyg en adsorbed on nickel in water. Standard Gibbs energies of formation f or sulfur and oxygen adsorbed on nickel are calculated from literature thermodynamic data for chemisorption from the gas phase. The E-pH rel ations associated with the equilibria between water, the dissolved sul fur species, and adsorbed sulfur and oxygen are given at 25 and 300-de grees-C. The corresponding E-pH diagrams are drawn for two sulfur acti vities (10(-4) and 10(-6) and superimposed on the usual Pourbaix diagr ams for the S-Ni-H2O system. Formation of adsorbed sulfur monolayers o n the metal surface is predicted in E-pH domains where the usual diagr ams do not predict the stability of a nickel sulfide. The E-pH diagram s for species adsorbed on metals allow one to make new predictions of the corrosion risk for metals in sulfur-containing aqueous environment s.