Effect of SO42− ion impurity on stress corrosion behavior of Ni-16Mo-7Cr alloy in FLiNaK salt

In FLiNaK salt, acid radical ions impurities can cause aggravated corrosion for Ni-based alloys at elevated temperatures, and would result in deleterious effect on the SCC (stress corrosion cracking) resistance of structural alloy in the reactor. In this study, the corrosion behavior of Ni-16Mo-7Cr alloy under loading stress in FLiNaK salt with sulfate radical ion (SO42−) was investigated by the method of static immersion corrosion at 700°C for 400 h. The results show that the SO42- ion can remarkably accelerate the corrosion process of alloy. The corrosion products mainly consist of Cr2O3 and MnS, which are formed on and near the surface of the alloy. However, whether SO42- ion exists or not in the FLiNaK salt, the stress has little effect on the depth of Cr depletion along the grain boundary, and no SCC phenomenon appears in this alloy.