Effect of pre-oxidation on hot corrosion of Ni-Cr-Al alloy in molten Na2SO4-NaCl

The effect of pre-oxidation on hot corrosion for Ni-20 mass%Cr-5 mass%Al alloy in molten Na2SO4-25 mass%NaCl at 1173 K has been examined by the measurement of corrosion loss, the electrochemical measurement and the analysis of surface oxide film formed on the alloy by pre-oxidation. Particular attention was given to the influence of pre-oxidation temperature. The pre-oxidation at 1173 and 1273 K led to a stop of the vigorous corrosion for the alloy in the molten salt, whereas the pre-oxidation at 1373 K did not lead to an inhibition of the corrosion. The anodic polarization curve measured for the alloy after the pre-oxidation, which stopped the vigorous corrosion of the alloy, showed the spontaneous passivation behavior, while that measured for the alloy without the pre-oxidation showed a large current peak due to the active dissolution reaction. This suggests that the pre-oxidation treatment inhibits the electrochemical corrosion reaction for the alloy. Such an inhibition effect on the corrosion was investigated by the analysis of surface oxide film formed by the pre-oxidation. The formation of oxide films consisting mainly of Cr2O3 was observed for the alloy after the pre-oxidation at 1173 and 1273 K, while that consisting mainly of Al2O3 was observed for the alloy after the pre-oxidation at 1373 K. Consequently, it was found that the difference in inhibition effect on the hot corrosion due to the pre-oxidation temperature resulted from a difference in the kind of oxide constituting the oxide film formed by the pre-oxidation.