Atomistic Mechanisms of Stress-Dependent Molten Salt Corrosion in NiCr Alloys

TL;DR

This study uses molecular dynamics simulations to reveal how tensile and compressive stresses differently influence grain boundary corrosion in NiCr alloys exposed to molten salt at high temperature.

Contribution

It provides the first atomistic understanding of how stress states modulate corrosion mechanisms in NiCr alloys in molten salt environments.

Findings

Tensile strain accelerates intergranular corrosion by increasing free volume.

Compressive strain suppresses corrosion by forming a protective ridge-like surface layer.

Stress state significantly alters corrosion behavior at the atomic level.

Abstract

Ni-based structural alloys in molten salt environments often experience simultaneous mechanical loading and corrosive attack, yet the mechanisms governing stress-corrosion interactions remain unclear. Prior studies largely emphasize tensile stress, while the role of compressive stress has received limited attention. Here, reactive molecular dynamics simulations are used to investigate the coupled effects of applied strain and corrosion in Ni$_{0.75}$Cr$_{0.25}$ exposed to molten FLiNaK at 800$^\circ$C. A $\Sigma5(210)$ grain boundary model is subjected to tensile (+4%) to compressive (-4%) uniaxial strains, and corrosion behavior is evaluated through fluorine adsorption, charge redistribution, and grain boundary evolution. Tensile strain accelerates intergranular corrosion by reducing local atomic packing through elastic dilation and increasing excess free volume at the grain boundary, which enhances atomic mobility and salt infiltration. In contrast, compressive strain suppresses corrosion by promoting the formation of a ridge-like surface layer along the grain boundary, limiting salt access to the underlying alloy. These results provide atomistic insight into how stress states influence grain boundary corrosion in molten salts.