Sulphide formation after pre-oxidation of chromia formers

Pure chromium and the alloys Fe-28Cr, Co-28Cr and Ni-28Cr (wt%) were pre-oxidized in CO-CO2-N-2 at 1173K and then reacted with CO-CO2-SO2-N-2 mixtures. All gases had the same oxygen and carbon activities. The SO2-containing gases had sulphur activities higher than the chromium-chromium sulphide equilibrium value, but Cr2O3 was always predicted to be stable with respect to sulphide. Pre-oxidation led to the growth of apparently compact, adherent Cr2O3 scales on all materials, plus sublayers containing carbide and nitride in the case of chromium. Addition of SO2 to the gas led to subsequent nucleation and growth of Cr3S4 on top of the pre-formed Cr2O3 in all cases where p(S-2) was relatively high. Sulphide formation beneath the oxide occurred only after extended periods of reaction. The formation of metastable sulphide in contact with the gas is attributed to preferential adsorption of sulphur onto the external surface. The presence of other alloy metals plays no significant role in the early stages of sulphide formation, and the rate of Cr3S4 growth is controlled by diffusion of chromium through the pre-formed oxide. Base metal sulphides develop at the scale exterior after longer periods. The gas permeability of pre-formed Cr2O3 on chromium was greater than for the alloy oxide scales. The addition of sulphur made the scale on chromium impermeable to nitrogen, but left it permeable to carbon. Sulphur made the preformed oxide on Fe-28Cr permeable to carbon (but not nitrogen), but did not affect the impermeability of Cr2O3 On Co-28Cr or Ni-28Cr for carbon and nitrogen, until mechanical disruption of the scale on Ni-28Cr resulted from extensive sulphidation. Sulphur adsorption on internal surfaces within the Cr2O3 is thought to block nitrogen access. Copyright (C) 1996 Elsevier Science Ltd