Corrosion of alloys in high temperature molten-salt heat transfer fluids with air as the cover gas

The state-of-the-art concentrated solar power tower technology requires novel heat transfer fluids which can operate at a temperature as high as 700 °C. Several molten eutectic salt mixtures are promising heat transfer fluid candidates because they are thermally stable at such high temperature and have good thermophysical properties. However, their corrosion properties to alloys are not investigated thoroughly in the literature. Molten salts usually have high corrosion to alloys, which is a critical issue since the piping and containing materials of the heat transfer fluid in a solar power plant are generally alloys. In this study, the corrosion of three eutectic mixtures including NaCl-KCl-ZnCl₂, Li₂CO₃-Na₂CO₃-K₂CO₃ and LiF-Na₂CO₃-K₂CO₃ to different alloys were experimentally studied at isothermal condition of 700 °C in contact with air. Alloys tested were stainless steel 316 (SS316), Hastelloy C276 (C276), Inconel 625 (In625) and 718 (In718). Scanning electron microscope (SEM), energy dispersive spectroscopy (EDS) and X-ray diffraction (XRD) techniques were employed to characterize the coupon surfaces after corrosion tests. Results show that all the alloys have corrosion rates higher than 450 μm/year immersed in the NaKZn chloride-salt. Therefore, the NaKZn chloride-salt is not applicable as a heat transfer fluid if air cannot be evacuated. C276 immersed in the LiNaK carbonate-salt and In718 immersed in the LiF-NaK carbonate-salt may form protective compact layers on the alloy surfaces to resist corrosion. However, the high cost of lithium salts is another major obstacle to commercialize heat transfer fluids.