Fundamentals of high temperature molten salt corrosion relevant to future nuclear reactors

Abstract

For molten salt reactor application, changes in the mechanical stability of structural materials exposed to molten salts have the highest importance. This work investigates corrosion mechanisms of Fe-(Cr)-Ni and Ni-Cr model alloys in molten chloride salts at 350-700 °C; and determines dealloying mechanisms, alloying effects, and critical alloy compositions at different homologous temperatures. It is found that dealloying of electrochemically reactive elements (Cr and Fe) are the dominant form of corrosion. At low homologous temperature, dealloying is similar to the type observed in aqueous solutions with porosity formation and parting limits (critical atomic fraction of reactive elements), and the dealloying mechanism is mediated by surface diffusion of Ni, but the parting limit for electrolytic dissolution of reactive elements (55-60 at.% in aqueous systems) drops by several percent due to the very fast surface diffusion of elemental Ni in molten Cl salt media. It is also observed that at higher temperatures, grain boundary dealloying prevails. Between 500-700 °C, the governing mechanism is still the interface-controlled process, but a transitional morphology evolves indicating the role of lattice diffusion. At 700 °C, the dealloying parting limit is decreased to a value close to the fcc site percolation threshold, 20 at.%.