The Effect of Temperature on Dealloying Mechanisms in Molten Salt Corrosion

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 degrees C-700 degrees C and the dealloying parting limit depends on temperature. At 350 degrees 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 degrees 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 degrees 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 degrees 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.

Automated summary

The mechanism of molten salt corrosion of Ni- and Fe-based model alloys at different temperatures relevant to molten salt nuclear reactors has been studied. It was found that Fe and Cr alloy elements separate in molten chloride salts, which depends on temperature. At lower temperatures, the corrosion mechanism is similar to aqueous systems, while at higher temperatures, grain boundary corrosion becomes prevalent and lattice diffusion plays an important role. When the temperature exceeds 600 degrees C, the crystal orientation of the corroded material is no longer the same as its parent grain, and the corrosion becomes more one-dimensional.