Effect of sulfide on corrosion behavior of stainless steel 316SS and Hastelloy C276 in sub/supercritical water

Stainless steel 316SS and Hastelloy C276, as the representatives of iron-based and nickel-based alloy, respectively, were employed to explore the corrosion properties under reducing subcritical and supercritical water containing sulfide. Experiments were executed at a pressure of 25 MPa, temperatures of 350 °C–520 °C, and sulfur concentrations of 1000 and 5000 ppm for 80 h. An isothermal equilibrium phase diagram involving the oxidation/sulfidation products of Fe, Cr, and Ni, was established by theoretical calculation in supercritical water system at 520 °C, in order to predict the corresponding products under various conditions and assist the discussion on corrosion mechanism. The results show that whether in subcritical water or in supercritical water, 316SS always exhibited better corrosion resistance relative to C276. In subcritical water at 350 °C, a portion of corrosion film peeled off from 316SS specimen, while numerous pores or cracks appeared on the surface of scale for C276. Under supercritical water at 520 °C, a compact scale grown on 316SS sample surface was composed of Fe₃O₄, FeCr₂O₄, and FeS. For C276, a duplex-layer scale formed on alloy surface. However, due to the higher content of Ni in C276, Ni-sulfide channels through the inner layer were developed, accelerating the sulfidation corrosion of alloys. Overall, the high-temperature alloys with high Cr content and low Ni/Cr ratio can be considered as the candidate material of equipment in supercritical water gasification of sulfur-containing coal.