Motoi Hara1, Toshio Hisaichi2, Koichi Itoh3 and Yutaka Shinata1
1Department of Materials Engineering and Applied Chemistry, Mining College, Akita University, Akita
In order to investigate the effect of gas atmosphere on the hot corrosion of nickel and Ni-Cr alloys in molten Na2SO4, the corrosion behavior of nickel and Ni-Cr alloys in molten Na2SO4 at 1173 K under (SO3 + SO2 + O2) mixture and pure SO2 and O2 atmospheres were evaluated by the measurement of corrosion loss, the observation of corrosion morphology and the electrochemical measurement. The corrosion loss of nickel and Ni-Cr alloys measured under the mixed atmosphere containing SO3 was larger than those measured under the pure SO2 and O2 atmospheres. Under the mixed atmosphere containing SO3, the corrosion loss of Ni-Cr alloys was smaller that that of nickel. It was found from the observation of corrosion morphology that the corrosion loss corresponded to the thickness of the oxide layers formed on the specimens. The cathodic polarization curves of Ni-Cr alloys measured in molten Na2SO4 indicated that the limiting current density from the corrosion potential observed under the mixed atmosphere containing SO3 was higher than those observed under the pure SO2 and O2 atmospheres. This suggests that SO3 can be readily reduced in molten Na2SO4 in comparison with SO2 and O2. The dependence of the corrosion behavior in molten Na2SO4 upon the gas atmosphere for nickel and Ni-Cr alloys could be explained by the partial polarization curves presumed from the electrochemical results. Consequently, it was thought that the large corrosion losses observed for nickel and Ni-Cr alloys under the atmosphere containing SO3 were attributed to the fact that SO3 strongly acted as an oxidizing agent for the corrosion process in molten Na2SO4.
hot corrosion, molten salt, sodium sulfate, gas atmosphere, nickel, nickel-chromium alloy, polarization curves
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