Protective coatings for high temperature molten salt heat storage systems in solar concentration power plants

Three Cr ferritic/martensitic containing-steels (P91, VM12 and MarBN) and one carbon steel (A516) were exposed to a mixture of molten nitrates salt (Solar Salt) at 580°C. P91 and MarBN were also exposed to a newly developed ternary carbonate eutectic salt mixture at 650°C under static conditions. In contact with molten nitrates, the uncoated substrates developed a complex, fast growing multilayered oxide scale, which includes NaFeO₂. Significant spallation took place for all substrates. However, A516 exhibited lower corrosion rates and developed a more compact Fe₂O₃/Fe₃O₄ slower growing scale. Substrate nitriding occurred on the ferritic steels. On molten carbonates at 650°C the uncoated steels also corroded heavily and the formed oxide contained LiFeO₂. Cr was found in the salts exposed to the Cr containing uncoated steels, which likely implies chromate dissolution in the melts as a result of a basic fluxing mechanism, typical of molten salt corrosion. Al slurry coated samples were also tested by immersion in both salts. All the tested coated samples performed well as no evidence of significant weight variation or substrate attack could be observed. The protective nature of these coatings may be attributed to the formation of NaAlO₂. Some coating-substrate interdiffusion occurred, to a higher extent on aluminized A516. The Al richest phases (FeAl₃, Fe₂Al₅) transformed into the lower Al containing phase FeAl.