Sulphidation and oxidation of the Ni22Cr10Al1Y alloy in H2/H2S and SO2 atmospheres at high temperatures

The Ni22Cr10Al1Y alloy was exposed in H-2/H2S gas mixture under the sulphur pressure 10(-3) and 1 Pa as well as in SO2 at 1173 and 1273 K. At p(s2) = 1 Pa the sulphidation rate was relatively high and the reaction obeyed the linear rate law. Under these conditions a nickel/nickel sulphide eutectic was formed. At p(s2) = 10(-3) Pa nickel sulphides became unstable and the sulphidation rate was significantly lower. The reaction obeyed the parabolic rate law. The oxidation rate of the alloy in SO2 was lower than that in any of the H-2/H2S atmospheres. The sulphide scales formed during sulphidation in H-2/H2S had complex microstructures and compositions, with sulphospinel and sulphide phases being present, e.g. NiCr2S4, Ni3S2, CrxSy. As the temperature increased and the sulphur pressure decreased, these phases were replaced by the chromium-rich sulphide phase. Various oxides formed during oxidation of the alloy in SO2. (C) 2000 Kluwer Academic Publishers.