STTR Phase II: Corrosion Inhibition of Stainless Steel Alloys in High Temperature Chloride Salts for Concentrated Solar Power Applications

This STTR Phase II project will focus on the development of cost effective, high temperature molten salt heat transfer fluids for third generation concentrated solar power (CSP) plants with operating temperatures >700?C. In the Phase I of this research, an additive package was developed that formed an adherent ceramic oxide coating on 316/316L stainless steel at the temperatures >700?C when added to specific chloride salt blends and inhibit corrosion. The utilization of the inhibitor package strategy will enable the utilization of economical stainless-steel alloys in the CSP plants and would reduce the infra-structure cost. Such solutions will have a direct impact on the renewable energy market which would help to lower the Levelized Cost of Energy (LCOE) for thermal solar from the current cost of 12?/kwh to the target 3?/kwh by 2030. The scientific and technological insight gained in this project could be beneficial to many other applications, such as molten carbonate fuel cells, thermal energy storage systems, nuclear molten fluoride/chloride reactors, and other high-temperature systems that are susceptible to aggressive corrosion. Incorporation of the inhibited molten chloride salt in the CSP plants will also provide sustainable green energy, reduce water usage, create more jobs and offer U.S. energy independence and security. On a societal level, impact will be achieved by the continued delivery and development of educational activities based on the research topics embodied by this work. The core innovation in this proposed program is the development of a chemical mixture, an additive package that would minimize the corrosion of stainless steel induced by molten chloride salts at temperatures >700?C. The additive package reacts with the stainless steel and forms a corrosion inhibiting ceramic oxide coating in-situ when added to the molten chlorides at operating temperatures>700?C. The chloride salts are proposed to be the heat transfer fluids for third generation CSP plants. The feasibility of the inhibitor package strategy in a real world CSP plant will be critically assessed in this proposed research. A prototype bench scale molten test loop will be designed and built to mimic the flow conditions in a CSP plant. The effect of prolonged exposure to high temperature molten salts on the mechanical properties of the base metal under dynamic conditions will be studied with attention to the molten salt induced degradation of strength and toughness at elevated temperatures. The degradation behavior of the salt with the inhibitor package will be studied as well. Additional research will be performed to gain fundamental insight on the nucleation and growth of the ceramic oxide coating and its thermal and mechanical stability in dynamic flow conditions. This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.