Effect of H2S on metal dusting of iron

Abstract

AbstractA review is given on the effect of H2S on metal dusting of iron which has been studied by gas carburisation in CO‐H2‐H2O‐H2S and CH4‐H2‐H2S mixtures at 500 and 700°C.The presence of H2S in carburising gas atmospheres leads to sulphur adsorption on the iron surface, which retards carbon transfer. Segregation experiments and surface analyses have shown that sulphur segregates (and thus adsorbs) on cementite surfaces as well as on iron surfaces. The adsorbed sulphur also suppresses graphite nucleation and thus can stop the reaction sequence of metal dusting.Experiments by thermogravimetric analysis (TGA) have shown that the extent of retardation of metal dusting depends on temperature, carbon activity and H2S content. The higher the carbon activity, the higher is the H2S content required for suppression of metal dusting.At carbon activities aC > aC(Fe/Fe3C) the metastable iron carbide, cementite (Fe3C), occurs as an intermediate phase during metal dusting. Carburisation experiments in CO‐H2‐H2O‐H2S mixtures at 500°C and X‐ray diffraction analysis (XRD) of carburised samples have revealed that at very high carbon activities a second iron carbide, Hägg carbide (Fe5C2), forms on the cementite surface. Microstructural investigations have shown that both metastable carbides decompose during metal dusting.Metal dusting experiments on iron at 700°C have been performed in CH4‐H2‐H2S gas mixtures. By adding 15 ppm H2S to the CH4‐H2 atmosphere the onset of metal dusting can be retarded for more than 350 hours. By means of Auger electron spectroscopy (AES), scanning electron microscopy (SEM) and energy dispersive X‐ray analysis (EDX) it was shown that coke contains graphite, cementite and iron particles with adsorbed sulphur.