Corrosion inhibitors for TES material at high temperature for CSP plants
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Molten salt technology using nitrate salts as thermal energy storage material is the current state-of-the-art of concentrated solar power technology and power tower central receivers are currently limited by the maximum operating temperature of their working fluid. The limit of solar salt (60wt.%NaNO3 + 40wt.%KNO3) thermal stability is around 565°C with ambient air as the cover gas. In order to obtain higher efficiency goals using molten salt technologies working at higher temperatures (e.g., 650°C to 750°C), a different salt chemistry is required for new generation of CSP plants and chlorides molten salts could be a feasible option. Corrosion mitigation could solve the current issues related with the use of chloride salt at high temperature. In this work, the proposal of different corrosion mitigation strategies has been evaluated, as well as the identification of the most corrosive impurities present in the ternary chloride salt composed by MgCl2/KCl/NaCl (60/20/20 mol%). For this purpose, electrochemical impedance spectroscopy tests were carried out at 700°C on austenitic stainless steel (AISI 304) and a Ni base alloy (Inconel 702) during 100 hours. In this case, MgOHCl was identified as the main important corrosive impurity, present in the chloride salt, to control in the storage system. Regarding the materials tested, Ni base alloy (Inconel 702) showed the most promising results for their use as container material.