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

TL;DR

This study reveals that diffusion-induced grain boundary migration (DIGM) significantly influences the corrosion behavior of Ni-30Cr alloys in molten salt environments, with surface microstructure playing a crucial role.

Contribution

It demonstrates for the first time that DIGM is a key mechanism in alloy molten salt corrosion, linking microstructure to corrosion response.

Findings

DIGM causes Ni island formation above grain boundaries.

Surface finish affects corrosion microstructure and Cr depletion.

Recrystallized grain boundaries lead to microstructure-dependent corrosion patterns.

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 bi-continuous porous subsurface layer. The exact mechanism behind the loss of Cr over distances unattainable by lattice diffusion alone is still debated. To address this question, two different surface finishes, namely electropolished and sanded, of a Ni-30Cr alloy were exposed to LiCl-KCl-2wt% EuCl3 eutectic salt at 500 {\deg}C for 96 hours. In the absence of fast diffusion pathways, dissolution occurred layer by layer and was kinetically controlled by Ni dissolution, 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 pure Ni islands above grain boundaries. This overall behavior contrasted with the sanded surface response that was 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. This work unequivocally establishes DIGM as a key mechanism in alloy molten salt corrosion, and microstructure as a decisive contributor to an alloy's corrosion response.