Effects of Mo and Al additions on the sulfidation behavior of 310 stainless steel

The high-temperature sulfidation behavior of 310 stainless steel (310SS) with Mo and Al additions (up to 10 at.%) was studied over the temperature range 700-900°C in pure-sulfur vapor over the range of 10^-3 to 10^-1 atm. The corrosion kinetics followed the parabolic rate law in all cases and the sulfidation rates increased with increasing temperature and sulfur pressure. The sulfidation rates decreased with increasing Mo and Al contents and it was found that the addition of 10 at.% Mo resulted in the most pronounced reduction among the alloys studied. The scales formed on 310SS with Mo additions were complex, consisting of an outer layer of iron sulfide (with dissolved Cr), (Fe, Ni)₉S₈, and Cr₂S₃/Cr₃S₄ (with dissolved Fe), and an inner heterophasic layer of Fe_1-xS, Cri₂S₃/Cr₃S₄, NiCr₂S₄ Fe_1.25Mo₆S_7.7, FeMo₂S₄, and MoS₂. The scales formed on 310SS with Mo and Al additions had a similar mixture as above, except that Al_0.55Mo₂S₄ was also observed in the inner layer. The formation of MoS₂ and Al_0.55Mo₂S₄, partly blocked the transport of cations through the inner scale, resulting in the reduction of the sulfidation rates compared to 310SS.