Carburization of metals by a chemical mechanism of carbon transport through molten fluoride salts

Abstract Carburization of structural alloys used in Fluoride Cooled High Temperature Reactors (FHR) can result from the interaction between the metal and graphite components and cause changes in alloy properties. However, carburization requires transport of carbon from graphite to alloy surfaces through the molten fluoride coolant. This work introduces an alternative to previously proposed carbon transport mechanisms. The new mechanism is based on the generation of carbonate ions (CO32−) by oxide impurities in the salt reacting with graphite. CO32− is then reduced on metal surfaces to deposit carbon. This work demonstrates the reduction of dissolved CO32− on pure Fe, Ni, Cr, Mo, and W surfaces without an applied potential in molten FLiNaK at 700 °C. We also demonstrate that the interaction between oxide and graphite in molten FLiNaK media produces CO32−.