Intergranular corrosion of 316H stainless steel induced by SO42− ions in MgCl2–KCl–NaCl molten salt

The application of molten chloride salts in next generation concentrated solar power plants is limited by their corrosiveness to structural materials. This work investigated the influence of SO42− ions on the corrosion of 316H stainless steel (SS) in MgCl2–KCl–NaCl (45.4-21.6-33 mol.%) molten salt using static immersion tests and microstructural analysis techniques. The results demonstrate that the presence of SO42− ions can significantly accelerate the intergranular corrosion of 316H SS by driving the dissolution of alloying elements Fe, Cr, and Mn. SO42− ions diffuse into the alloy bulk along the grain boundaries, resulting in the intergranular corrosion attack. The chemcial reaction rate of the metal atoms is directly proportional to the concentration of SO42− ions. The corrosion rate of 316H SS in salt with 1000 ppm SO42− (167 μm/year) was more than 10 times that in without SO42− (13.3 μm/year), increasing linearly with increasing SO42− concentration. Furthermore, MgCl2, the composition of the molten chloride salt, is involved in the corrosion reaction with SO42−, through Na2SO4 + 3Fe + MgCl2MgFe2O4 + 2NaCl + FeS (△G 700°C = −337 kJ/mol), leading to the severe dissolution of the Fe matrix element in 316H SS. Based on the corrosion mechanism of SO42− ions in chloride salts, several promising research directions are proposed for future studies. •The presence of SO42− ions in MgCl2–KCl– NaCl salt aggravated the intergranular corrosion of 316H SS.•SO42− ions affect the corrosion tendency of alloying elements in MgCl2-based salt.•MgCl2 is involved in the corrosion reaction of Fe in the presence of SO42−.•The corrosion rate of 316H SS is increased linearly with the concentration of SO42−.