Electrochemical corrosion evaluation of structural materials in the blanket of Indian Molten Salt Reactor (lMSR) at 600-700 °C

The conceptual design of IMSR considers LiF-ThF₄ having 22.4 atom % ThF₄ as the blanket salt. A eutectic mixture of the blanket salt was prepared by repeated melting. Electrochemical measurements were performed inside an incoloy 800 H retort in the temperature range of 600 to 700°C, wherein the salt was placed inside a graphite crucible which was also the counter electrode. Platinum rod was used as pseudo reference. Hastelloy-N and Inconel722 were evaluated for corrosion in the molten fluoride salt under argon atmosphere with moisture below 5 ppm. These alloys have similar content of nickel which is primarily responsible for passivation in fluoride medium. The Tafel plots of the potentiodynamic anodic polarization studies for the two alloys at different temperatures are shown. The corrosion rates were calculated from these plots for both the alloys have shown increasing trend with temperature. The corrosion rates for Hastelloy-N increased from 5.5 to 26.1 mpy while the same for Inconel722 increased from 16.5 to 78.8 mpy. Evaluation of impedance data indicates that there is a transition from non active metal behaviour to active metal forming protective oxide behaviour for these alloys and the transition is occurring at lower temperature for Inconel722. (author)