Moisture-driven chlorination and exfoliation corrosion of Inconel 625 in NaCl–MgCl2 molten salt: Effect of salt purification and surface roughness

Corrosion behavior of Inconel 625 was systematically evaluated in molten NaCl–MgCl 2 salt at 650 °C under an argon atmosphere for 300 hours. The effects of salt purification and surface roughness were examined using mass loss measurements, SEM-EDS, XRD, and ICP-OES analyses. Specimens immersed in salt with low H 2 O content exhibited significantly reduced corrosion rates—55.4 μm/year for polished and 79.6 μm/year for ground surfaces—compared to 288.3 μm/year in salt with high H 2 O content. Salt purification effectively suppressed the selective leaching of Cr and Fe and prevented the formation of surface oxides such as Cr 2 O 3 and MgO. In contrast, unpurified salt with residual moisture induced extensive intergranular corrosion (IGC), pitting, and the deposition of oxide layers due to hydrolysis-generated HCl and Cl 2 gases. Thermodynamic calculations using FactSage confirmed that corrosive species can be generated through the stepwise decomposition of MgCl 2 ·H 2 O. ICP-OES analysis revealed that dissolved Cr and Fe were detected in the purified molten salt, whereas in the salt with high H 2 O content these elements were not detected due to secondary reactions leading to the formation of volatile or solid corrosion products. These results clearly demonstrate that corrosion resistance in chloride-based molten salt environments is strongly governed by salt purity and surface condition. Accordingly, proper salt pretreatment and surface preparation are essential for the reliable deployment of Inconel 625 in molten salt reactor applications.