The scaling of a Fe-20Cr alloy in H2-H2O-H2S mixtures

The scaling of an Fe-20Cr alloy has been studied in H₂-H₂O-H₂S mixtures between 973 and 1223 K. According to a simplified phase diagram, Cr₂O₃ and FeS should be the thermodynamically stable compounds in the gas mixtures chosen. The reaction followed a mixed rate law between linear and parabolic, indicating that the reaction rate at the scale-gas interface was comparable with the diffusion rate in the scale. At a constant H₂/H₂S ratio the scaling rate initially decreased slightly with increasing water-vapor pressure. A further increase of the water-vapor pressure resulted in localized sulfide formation, while the other parts of the surface were covered with a Cr₂O₃ film. Only Cr₂O₃ formed above a critical water-vapor content. Three zones could be distinguished when a sulfide scale is formed. The outer zone consisted of practically pure FeS; the intermediate zone was a solid solution of (Fe,Cr)S, partially decomposed to FeCr₂S₄ and metal during cooling; and the inner zone contained small Cr₂O₃ inclusions in an (Fe,Cr)S matrix.