First-Principles Investigation of Grain Boundary Effects on Fluorine-Induced Initial Corrosion of NiCr Alloys

TL;DR

This study uses density functional theory to analyze how grain boundaries in NiCr alloys influence fluorine adsorption and promote chromium depletion, shedding light on early-stage corrosion mechanisms in molten salt environments.

Contribution

It provides the first detailed atomic-level understanding of fluorine interactions at grain boundaries in NiCr alloys, highlighting their role in corrosion initiation.

Findings

Fluorine prefers binding at grain boundary sites.

Cr doping increases fluorine adsorption energy.

CrF₃ formation lowers chromium dissolution barriers.

Abstract

Chromium depletion at grain boundaries (GBs) due to selective attack is a critical issue in the molten salt corrosion of NiCr alloys. Despite the importance of GBs in this process from numerous experimental studies, most theoretical work has predominantly focused on fluorine interactions with idealized crystalline surfaces, neglecting the complexity of GB local environments. This study aims to bridge that gap by employing density functional theory (DFT) to investigate the atomic interactions and Cr dissolution mechanisms at GB in NiCr alloys under molten fluoride salt environments. Specifically, a $\Sigma$5(210)/(001) symmetrical tilt GB is constructed to explore the adsorption energies of fluorine on Ni(100) and Cr-doped Ni(100) surfaces. We find that fluorine exhibits a strong preference for binding at GB sites, with Cr doping amplifying this effect, leading to higher adsorption energies compared to bulk Ni surfaces. Fluorine bonding with Cr significantly alters the interaction between Cr-F complexes and Ni substrate, and the consequent dissolution barriers for Cr atoms; the formation of CrF$_3$ largely reduces the energy barrier for Cr dissolution. This work highlights the essential role of GBs in enhancing fluorine adsorption and accelerating Cr depletion, providing new insights into the mechanisms of early-stage corrosion in NiCr alloys.