Corrosion mechanism and intergranular attack of 316 stainless steel in NaCl-MgCl2 molten salts: The critical role of SO42− impurities for solar thermal storage

Effect of SO 4 2 ‐ in NaCl-MgCl 2 on corrosion of 316SS was first studied at 600 °C. • Intergranular corrosion became more obvious with increased SO 4 2 ‐ concentration. • Corrosive of 316SS increased continuously with SO 4 2 ‐ concentration increased. • SO 4 2 ‐ concentration in NaCl-MgCl 2 salt should be controlled within 0.05% in Ar. NaCl-MgCl 2 molten salts are promising heat transfer fluids and thermal energy storage media for next-generation concentrating solar power (CSP) systems. However, the corrosion of the stainless steel in NaCl-MgCl 2 molten salt containing sulfate impurities remains a significant challenge for practical application. In this work, the corrosion behavior of 316 stainless steel (316SS) was systematically investigated in NaCl-MgCl 2 (58.0-42.0 mol%) eutectic molten salts containing 0.005-5.0 wt% SO 4 2− at 600 °C for 240 h under Ar. The corrosion of 316SS samples increased markedly with SO 4 2− concentration. At low impurity levels, only slight surface degradation and limited elemental depletion were observed. In contrast, elevated SO 4 2− concentrations caused severe corrosion-layer degradation, accelerated selective dissolution of Cr and Fe, and pronounced localized and intergranular corrosion. A critical impurity threshold of 0.05 wt% SO 4 2− was identified, beyond which the weight loss increased sharply to 1.96 mg cm −2 and significant intergranular attack developed. Furthermore, increasing the SO 4 2− concentration from 1.0 to 5.0 wt% promoted the transition from localized corrosion to extensive grain-boundary penetration. Mechanistic analysis indicates that sulfur-induced precipitation of Mo–S- and Mn–S-containing species, together with selective alloy dissolution, destabilizes the protective corrosion layer through a synergistic sulfidation-oxidation mechanism. These findings establish a practical sulfate impurity limit for NaCl-MgCl 2 salts and provide important guidance for molten salt purification, impurity control, and structural material selection in CSP thermal energy storage systems.