Corrosion of 316L Stainless Steel Alloy and Hastelloy-N Superalloy in Molten Eutectic LiF-NaF-KF Salt and Interaction with Graphite

Molten FLiNaK salt [46.5%LiF-11.5%NaF-42%KF (mol%)] has been proposed for use as a secondary reactor coolant and medium for transfer of high-temperature process heat from nuclear reactors to chemical plants. Two alloys—Hastelloy-N superalloy (Hastelloy-N) and Type 316L stainless steel alloy (316L steel)—were exposed to molten FLiNaK salt in a 316L steel crucible under argon cover gas at 850°C for 1000 h. Graphite was also introduced into the test with the goal of studying the corrosion behavior of relevant reactor material combinations. The results show that corrosion of 316L steel occurred primarily through surface depletion of Cr. Contrarily, Hastelloy-N experienced weight gain due to the electrochemical plating of corrosion products, Fe and Cr, derived from the 316L steel crucible. The graphite sample enhanced the corrosion of the 316L steel sample and crucible, which induced the formation of (Cr,Fe)₇C₃ and (Mo,Cr,Fe)₂C carbides on the surface of graphite. These carbide formations were attributed to the nonelectric transfer between 316L steel and graphite. Besides reducing the availability of chromium to plate, the presence of graphite did not change the basic corrosion of the 316L steel and plating process of Hastelloy-N.