The promotion of the anodic dissolution of polycrystalline iron surfaces by adsorbed sulfur: A UHV-electrochemical study

Abstract

The effect of systematically increasing surface coverage of sulfur on the electrochemical behavior of polycrystalline iron in a mildly alkaline (pH 8.4) buffer has been examined by combined ultrahigh vacuum/ electrochemistry methodology. Polycrystalline Fe surfaces were cleaned and characterized under UHV conditions, then exposed to H 2S gas prior to immersion in the electrolyte solutions. Adsorbed sulfur is found to enhance anodic dissolution, though passivation is ultimately observed at all levels of surface sulfur saturation. The iron dissolution current is found to vary in a non-linear manner with S coverage. XPS data reveal that at sulfur levels exceeding 64% of saturation coverage, there is an abrupt change in the anodic dissolution current and in the surface composition ratio of Fe 2+ to Fe 3+. An increase in the ratio of hydroxide to oxide is also observed. An abrupt change in overall overlayer 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 surface films that are formed in the presence of S contain significantly higher proportions of Fe 3+ and OH −.