Early stages of sulfidation of iron and iron-chromium alloy in hydrogen sulfide

Abstract

<p>Sulfidation of polycrystalline iron and an Fe-25 wt% Cr alloy at various partial pressures of H(,2)S in the temperature range of -170 C to 500 C was studied in situ in an ESCA spectrometer, and the sulfidized layers were investigated by depth profiling in an Auger spectrometer. The role of surface oxygen during sulfidation was investigated. The mode of sulfidation of sputter-clean iron was found to involve several stages: associative adsorption of H(,2)S at low temperatures, dissociative adsorption of H(,2)S at around ambient temperature, and the formation of first a sulfur-deficient iron sulfide and then stoichiometric FeS at slightly elevated temperatures as the H(,2)S pressure or the time of exposure was increased. Iron sulfides are more readily formed on the oxidized iron surface than on the sputter-clean surface. Chromia (Cr(,2)O(,3)) was found to be stable in an H(,2)S atmosphere at T (LESSTHEQ) 250 C, but was converted to chromium sulfide at 300 C. Chromium sulfide is more readily formed on the clean alloy surface than on the oxidized surface. During annealing in vacuum, there is surface chromium enrichment on the FeCr alloy, and this is enhanced in the presence of oxygen or sulfur on the surface. Sulfidation proceeds primarily by cation diffusion, and the growth of the sulfide layers was found to be more dependent on temperature than on time of exposure. Iron was found to be more mobile than chromium in the FeCr alloys.</p>