Oxidation and sulfidation of Ni3Al

Oxidation and sulfidation of Ni3Al has been studied in air and in 1% SO2/air gas mixture atmosphere at 605, 800 and 1000 °C. The experimental results show that Ni3Al appears to have a severe corrosion phenomena resulting from the formation of NiO for both oxidation environments. For the oxidation–sulfidation reaction mechanism of Ni3Al, it was found that the substitution of sulfur on oxygen sites in NiO tends to decrease the electron hole concentration in the valence band. With a constant of oxygen partial, a decrease in concentration of electron hole will result in an increase in Ni cation vacancies and hence there will be a greater flux Ni to the gas-oxide boundary; the rate of self-diffusion of the Ni2+ ions, which determines the rate of oxidation, would increase. And this would increase the rate of oxidation with low sulfur partial pressure in the atmosphere in this research work. When the specimen surface is exposed to the oxygen environment, a thin spinel NiAl2O4 layer is built up immediately; the spinel formation rate is initially faster than that of spinel decomposition, which is a constant value. Gradually, when a thick oxide scale is reached, a balance between the rate of decomposition and formation will be met. As the temperature increased the spinel formation rate was higher than the spinel decomposition rate. Therefore, at 1000 °C, it is possible to detect the existence of spinel. On the other hand, at 800 °C the spinel formed will be decomposed into NiO and Al2O3 (needle-like), and they are the only oxides found on the specimen surface.