Role of diffusion-induced grain boundary migration and surface finish during molten salt corrosion of a Ni-30Cr alloy

Abstract

The response of Ni-Cr alloys to exposure to molten chloride and fluoride salts is typically characterized by Cr dealloying with the formation of a Cr-depleted porous subsurface layer. The exact mechanism behind Cr loss over distances unattainable by lattice diffusion alone is still debated. Here, two different surface finishes, namely electropolished and sanded, of a Ni-30Cr alloy were exposed to LiCl–KCl-2wt%EuCl3 eutectic salt at 500°C for 96 hours. In the absence of fast diffusion pathways, dissolution occurred layer-by-layer and was consistent with near congruent Ni and Cr removal being rate-limited by the slower-dissolving Ni, as observed over the grain interiors of the electropolished sample. Grain boundaries were subject to diffusion-induced grain boundary migration (DIGM), leading to the formation of Ni islands above grain boundaries. This overall behavior contrasted with the sanded surface response characterized by several micrometer deep interconnected porosity and complete Cr depletion. DIGM of the dense grain boundaries created by recrystallization of the sanded surface was responsible for the observed sub-surface microstructure. While the salt redox chemistry set by EuCl3 fixes the driving force and selectivity for dissolution, the evolving grain boundary microstructure controls the kinetics, depth, and morphology of the sub-surface layer.

The authors' abstract, as published at the source. Journal of Materials Research and Technology, 2026 · DOI ↗