Electrochemical Impedance Spectroscopic Study of Oxide Scales on 316L Stainless Steel in Molten FLiNaK Salt

Molten fluoride salts have been proposed to be excellent candidates as primary reactor coolant and liquid fuel in a Molten Salt Reactor (MSR) due to their advantages of high thermal conductivities, low viscosities, high boiling points and high specific heats, etc. However, the corrosion of structural materials in molten fluoride salts at high temperature is a great challenge that prevents the successful fruition of MSR. In most corrosive environments, materials derive their corrosion resistance by the formation of a protective oxide scales and/or passive films. However, these oxide films are chemically unstable in high temperature molten fluoride salts and cannot provide protection of materials from corrosion. An understanding of in-situ dissolution rate and corrosion mechanism of oxide film in fluoride salt is significantly important for the development of MSR.<br/>In this work, coupons of 316L stainless steel (SS) were pre-oxidized in hot air and then immersed in molten FLiNaK salt at 700 ^oC for electrochemical experiments. The impedance of oxide films on 316L SS as a function of exposure time were investigated using in-situ electrochemical impedance spectroscopy (EIS). The oxide resistance and capacitance under different exposure time were compared. The results show that the resistance of the oxide film decreases with increasing the exposure time, due to the dissolution of the Fe and Cr elements into the molten FLiNaK salt. After experiments, the structures of the oxide film were characterized and compared.