Effect of Sulphur on Fe-20Cr-(Mn, Si) and Fe-20Ni-20Cr-(Mn, Si) Corrosion in CO2-H2O at 650°C

Model alloys Fe-20Cr and Fe-20Cr-20Ni (wt.%) and these alloys doped with 0.5% Si or 2% Mn were exposed to Ar-20%CO2-20%H2O-(0%, 0.1%, 0.5%, 1.0%)SO2 gas mixtures at 650 °C. Alloys of Fe-20Cr-(2Mn) and Fe-20Cr-20Ni-(0.5Si) underwent breakaway oxidation in sulphur-free gases, forming thick iron-rich oxide scales and internal carbides. SO2 additions promoted the maintenance of protective chromia scale on Fe-20Cr-(2Mn) and Fe-20Cr-20Ni-(0.5Si) alloys, but had no benefit for the already high corrosion resistance of Fe-20Cr-0.5Si and Fe-20Cr-20Ni-2Mn alloys. The role of sulphur in retarding carburization is discussed with reference to its strong adsorption on oxide grain boundaries.