Hot corrosion of different alloys in chloride and carbonate molten-salt mixtures under argon atmosphere

Molten eutectic salt mixtures can be used as thermal energy storage materials or heat transfer fluids at concentrated solar power plants. The targeted operating temperature for the next generation concentrated solar power plants is 700 °C. Although chloride and carbonate eutectic salt mixtures with high thermally stable temperatures were reported to have great thermophysical properties, their corrosion to alloys at high temperature has not been thoroughly studied. Corrosion of the thermal energy storage materials and heat transfer fluids to alloys is critical, and oxygen in air is confirmed to accelerate the corrosion significantly. In the present study, three eutectic mixtures of NaCl-KCl-ZnCl₂, Li₂CO₃-Na₂CO₃-K₂CO₃ and LiF-Na₂CO₃-K₂CO₃ were considered as high temperature thermal energy storage materials or heat transfer fluids. Corrosion of stainless steel 316, Hastelloy C276, Inconel 625 and 718 immersed in the three salt mixtures up to 21 days were experimentally investigated at 700 °C under argon atmosphere. The specimens after corrosion tests were characterized by scanning electron microscope, energy dispersive spectroscopy and X-ray diffraction. The results show that SS316 is only compatible with the LiNaK carbonate-salt and the corresponding corrosion rate is lower than 95 μm/year. Both C276 and In625 have relatively good corrosion resistance for the NaKZn chloride-salt and LiF-NaK carbonate-salt, and the corrosion rates are lower than 78 μm/year. The highest corrosion rate over 1700 μm/year is detected for In625 immersed in the LiNaK carbonate-salt. In addition, In718 shows excellent corrosion resistance less than 10 μm/year immersed in all the three salt mixtures.