The sulfidation of inconel 738 in H₂/H₂S/Ar gas mixture at 500-900°C

The sulfidation behavior of the cast nickel-base superalloy Inconel 738 (IN-738) was studied over the temperature range 500-900°C in three various ratios of H₂/H₂S/Ar atmospheres having sulfur partial pressures in the range 10^-4-10² Pa. The sulfidation kinetics followed the parabolic rate law in all cases, and the sulfidation rates increased with increasing temperature and sulfur partial pressure. In general, the scales formed were heterophasic and duplex, consisting of an outer layer composed mostly of nickel sulfides (mostly NiS and Ni₃S₂, and minor Ni₇S₆ detected at T>750°C) plus some CoS₂/Co₃S₄, CoMo₂S₄ NiCo₂S₄, and minor chromium sulfide (Cr₂S ₃/Cr₃S₄), while the inner layer contained a mixture of nickel sulfides and minor amounts of Al₂S₃ and Cr₂S₃/Cr₃S₄. The amount of NiS decreased and that of Ni₃S₂ increased with increasing temperature. The dependence of the sulfidation rate constants (K_p) and Ps₂(the n value) was significantly higher at T≥850°C as compared to that measured at lower temperatures (T ≤ 800°C).