The Effect of Temperature on Dealloying Mechanisms in Molten Salt Corrosion

Abstract

The mechanism of molten salt corrosion of Ni− and Fe-based model alloys is studied at different homologous temperatures relevant to molten salt nuclear reactor application. Dealloying of Fe and Cr occurs in molten chloride salts in the range of 350 °C–700 °C and the dealloying parting limit depends on temperature. At 350 °C, molten salt dealloying is similar to aqueous systems; surface diffusion of elemental Ni at the solid/electrolyte interface is the governing transport mechanism, and the microporous ligaments have an isotropic morphology. The high surface mobility of Ni blurs the ordinary parting limit concept, but such a limit is still present. Above 500 °C, grain boundary dealloying is prevalent; the governing mechanism is interface-controlled, but a transitional morphology evolves, signaling a role of lattice diffusion. When the temperature exceeds 600 °C, the crystal orientation of dealloyed substrates is no longer that of their parent grain, and the fairly isotropic nature of dealloying shifts to a more one-dimensional corrosion ahead of the dealloying front that indicates some kind of hybrid mechanism. At 700 °C, the dealloying threshold approaches below 22 at%, accompanied by rapid coarsening and densification of the dealloyed material due to strong influence of lattice diffusion of alloying elements.