Abstract: | The sulfidation of Fe-20% Cr-30% Mn, Fe-25%Cr-20%Mn and Fe-25% Cr was studied at 700°C in H2-H2S and the oxidation and sulfidation in H2-H2O-H2S after preoxidation in H2-H2O. The sulfidation rate is strongly increased for the Mn-containing alloys, layers of (Mn,Cr)S and (Mn,Fe)Cr2S4 are formed. Also the oxidation rate is enhanced compared to Fe-25% Cr by formation of MnCr2O4 instead of Cr2O3. The sulfidation after preoxidation leads to internal and external sulfidation of the Mn-containing alloys. With increasing oxygen pressure p(O2) = 10?26…10?22 atm. of the H2-H2O-H2S mixtures the sulfidation is suppressed, for the higher oxygen pressure 10?23 and 10?22 atm. fast oxidation prevails under formation of MnCr2O4. Manganese cannot increase the sulfidation resistance of alloys, in spite of the stability and low degree of disorder of its sulfide, since the mixed sulfide (Mn,Cr)S is formed which has a high degree of disorder, high diffusivities and high growth rate according to the doping effect of trivalent Cr3+. |