Molten chloride salts for next generation concentrated solar power plants: Mitigation strategies against corrosion of structural materials

Molten chloride salts are promising advanced high-temperature (400-800 degrees C) thermal energy storage (TES) and heat transfer fluid (HTF) materials in next generation concentrated solar power (CSP) plants for higher energy conversion efficiencies. However, severe corrosion of structural materials in contact with molten chloride salts is one of the most critical challenges limiting their applications at elevated temperatures. In this work, two corrosion mitigation strategies are investigated to alleviate the hot corrosion of structural materials in molten chloride salts: (1) adding corrosion inhibitor and (2) using a Fe-Cr-Al alloy with a protective alumina layer on the surface after pre-oxidation. Three commercial high temperature Fe-Cr-Ni alloys (SS 310, Incoloys (R) 800 H and Hastelloy (R) C-276) were exposed to molten MgCl2-NaCl-KCl (60-20-20 mol%) mixed salts with 1 wt% Mg as corrosion inhibitor, for 500 h at 700 degrees C under inert atmosphere. By addition of the Mg inhibitor, the corrosion rates of the studied alloys were found to be significantly reduced, more precisely by similar to 83% for SS 310, similar to 70% for In 800 H and similar to 94% for Ha C-276 compared with the exposure tests without Mg addition. The corrosion mitigation mechanism of Fe-Cr-Ni based alloys in molten chloride salts by adding Mg is discussed based on corrosion thermodynamics. To assess the second mitigation strategy two pre-oxidized alumina forming Fe-Cr-Al alloys were exposed to the same molten chloride salts without Mg corrosion inhibitor under the same conditions. It is observed that the adherent alumina scales can effectively inhibit the dissolution of Cr and Fe and the bulk penetration of corrosive impurities. Overall, both strategies offer enormous potential for enhancing the expected lifetime of commercial alloys in molten chloride salts.