Simultaneous sulphidation and oxidation of Fe-25 wt.%Cr and Silicon-containing Fe-25 wt.%Cr alloys in H 2H 2OH 2S gas mixtures at high temperatures

Abstract

The simultaneous sulphidation and oxidation of Fe-25wt.%Cr and silicon-containing Fe-25wt.%Cr alloys was studied at 1023, 1123 and 1223 K in H 2H 2OH 2S gas mixtures. The kinetic measurement showed a transition from oxide formation with slow mass gains at high P O 2 to sulphide formation with rapid mass gains at low P O 2 . The reaction rates usually decreased with increasing oxygen partial pressure at constant sulphur partial pressure. The critical oxygen partial pressure corresponding to the transition from oxide formation to sulhide formation increased with increasing silicon content of the alloys. The experimental results also showed that simultaneous sulphidation and oxidation appeared in a very large range of oxygen partial pressure. Metastable sulphides can coexist with oxides in the surface of the scale even when the oxygen partial pressure is much higher than the equilibrium pressure. Thermodynamic stability diagrams for (Fe, Cr, Si)SO systems were generated and were used to explain the experimental results. The critical oxygen partial pressures obtained by experiments are systematically higher than those calculated. The influence of silicon on the transition from oxide formation to sulphide formation is discussed and analysed on the basis of thermodynamics and kinetics.