THE PROMOTION OF THE ANODIC-DISSOLUTION OF POLYCRYSTALLINE IRON SURFACES BY ADSORBED SULFUR - A UHV-ELECTROCHEMICAL STUDY

The effect of systematically increasing surface coverage of sulfur on the electrochemical behavior of polycrystalline iron in a mildly alkal ine (pH 8.4) buffer has been examined by combined ultrahigh vacuum/ele ctrochemistry methodology. Polycrystalline Fe surfaces were cleaned an d characterized under UHV conditions, then exposed to H2S gas prior to immersion in the electrolyte solutions. Adsorbed sulfur is found to e nhance anodic dissolution, though passivation is ultimately observed a t all levels of surface sulfur saturation. The iron dissolution curren t is found to vary in a non-linear manner with S coverage. XPS data re veal that at sulfur levels exceeding 64% of saturation coverage, there is an abrupt change in the anodic dissolution current and in the surf ace composition ratio of Fe2+ to Fe3+. An increase in the ratio of hyd roxide to oxide is also observed. An abrupt change in overall overlaye r thickness is noted above this level of S coverage. The data indicate that when compared to passive films formed in the absence of S, the s urface films that are formed in the presence of S contain significantl y higher proportions of Fe3+ and OH-. (C) 1997 Elsevier Science Ltd.