Molten salt corrosion evaluation for solar power plants

Molten salts can be widely used for concentrated solar power (CSP) technologies, either as heat transfer fluids or as thermal energy storage materials. Because molten salts are corrosive at high temperatures, the corrosion behaviour of metallic components to be used in such environments must be carefully evaluated. Within this objective isothermal and stagnant gravimetric corrosion tests were conducted with preselected alloys in two molten salt media: the 54%KNO₃/46%NaNO₃ and the 68%ZnCl2/32%NaCl eutectic compositions. In order to increase the thermal conductivity of the salts, expanded natural graphite was added to both systems in concentrations up to 5%wt. For the nitrate media the unalloyed carbon steel St52.3 and the 316L stainless steel were selected, whereas 316L steel and Incoloy 825 were chosen for the chloride mixture. Corrosion tests were conducted for more than 2600 hours at a temperature of 340°C. Corrosion rates were determined by chemically descaling coupons. Post corrosion test studies included SEM/EDS and XRD analysis. Obtained results indicate that for the KNO₃/NaNO₃ salt media ST52.3 carbon steel and 316L stainless steel could be considered technically acceptable. Corrosion rates of 10 microns/year were observed for ST52.3 steel, and less than 2 microns/year for 316L steel. Passive "non-soluble" films were observed for both steels. For the ZnCl₂NaCl media, Incoloy 825 showed metal losses up to 13 microns/year whereas 316L steel coupons were totally destroyed after 1500 hours. For the stagnant gravimetric tests general corrosion was the only corrosion phenomenon observed. However tests performed with TIG sealed 316L steel capsules filled with the nitrate composite, showed secondary cracking when testing at 340°C, thus indicating SCC sensitivity. This behaviour was mainly attributed to the high chloride content of the nitrate salt (up to 0.5%wt). As a conclusion, the SCC behaviour of metallic alloys should be evaluated for stressed components. In addition, the chloride content of nitrate salts should be kept as low as possible.