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

The Ni22Cr10Al1Y alloy was exposed in H₂/H₂S gas mixture under the sulphur pressure 10^–3 and 1 Pa as well as in SO₂ at 1173 and 1273 K. At p_s = 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_s = 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 H₂/H₂S atmospheres. The sulphide scales formed during sulphidation in H₂/H₂S had complex microstructures and compositions, with sulphospinel and sulphide phases being present, e.g. NiCr₂S₄, Ni₃S₂, Cr_xS_y. 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.