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

The oxyfuel process is a new technology in which coal combustion occurs in oxygen rather than air, producing a flue gas made up mainly of CO₂ and H₂O.  This makes CO₂ collection and sequestration feasible. The traditional low Cr martensitic steels used successfully in conventional power generation are vulnerable to corrosion in oxyfuel atmospheres. As a result, a higher Cr concentration is required to form a protective chromia scale. Because of flue gas recirculation and the lack of nitrogen dilution in the oxyfuel process, a relatively high content of sulphur is present in the gas. Although the effect of sulphur on Fe-Cr corrosion in CO₂-H₂O atmospheres is not clear, it is believed to be adsorbed on oxide surfaces and grain boundaries, affecting mass transport of carbon.

The present research investigates the effect of sulphur on the corrosion of Fe-20Cr, Fe-20Cr-2Mn and Fe-20Cr-0.5Si (all in wt%) alloys in Ar-20%CO₂-20%H₂O with and without additions of SO₂ at concentrations of  0.1%, 0.5%, and 1.0%, at 650°C. Corrosion kinetics were measured via weight gains in isothermal time lapse experiments. Corrosion products were characterized using X-ray diffraction, cross-section metallography, scanning electron microscopy and transmission electron microscopy together with energy dispersive spectroscopy.

In the sulphur-free gas, Fe-20Cr and Fe-20Cr-2Mn alloys experienced breakaway oxidation, forming a thick, iron-rich oxide scale, together with extensively precipitated intergranular and intragranular carbides within the alloys. Adding SO₂ to the gas significantly improved the corrosion resistance of Fe-20Cr and Fe-20Cr-2Mn alloys by forming thin protective Cr₂O₃/ MnCr₂O₄ scales.  The formation of carbides was also significantly retarded due to preferential adsorption of sulphur blocking the diffusion of carbon. The Fe-20Cr-0.5Si alloy developed a protective, thin oxide scale in both sulphur-free and sulphur-containing gases.  This is attributed to the formation of a thin silica layer at the chromia scale-alloy interface.  This layer slowed oxide scaling by impeding outward metal diffusion, and prevented internal carburization by blocking carbon entry.