POTENTIAL-PH DIAGRAMS FOR ADSORBED SPECIES - APPLICATION TO SULFUR ADSORBED ON IRON IN WATER AT 25-DEGREES-C AND 300-DEGREES-C

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 (Sads-Fe-H20), using thermodynamic data for sulfur chemisorption on iron. The resulting E-pH diagrams for the Sads“Fe-H2O 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. copyright. © 1990, The Electrochemical Society, Inc. All rights reserved.