Transition from Dehydration to Rehydration of Chloride Salts near the Melting Point: Thermodynamic Origin and Implications for High-Temperature Corrosion

Molten chloride salts are promising functional media for high-temperature energy systems; however, trace water can accelerate corrosion of structural materials. We investigated changes in water concentration in six chloride salts—LiCl, NaCl, MgCl2, KCl, CaCl2, and ZnCl2—during heating across their solid-to-liquid phase transitions under a nominally inert Ar atmosphere. During solid-state heating, water concentration generally decreased, indicating dehydration. With further heating toward and through melting, however, this trend unexpectedly reversed in five salts except ZnCl2. As these salts entered the liquid phase, their water concentrations increased, marking a transition from dehydration to rehydration. Thermodynamic analysis showed that melting shifts the free energy balance toward water incorporation through enthalpic and entropic contributions. By comparing molten MgCl2 under conditions allowing or restricting entry of Ar gas, we identified trace water in the Ar atmosphere as the source of rehydration. Corrosion tests further showed rehydration substantially increased 316H stainless steel corrosion.