A Reactive Force Field Approach to Modeling Corrosion of NiCr Alloys in Molten FLiNaK Salts

TL;DR

This study develops a reactive force field for NiCr alloys in molten FLiNaK salts, enabling atomic-level simulations of corrosion processes and revealing how salt composition influences corrosion rates and mitigation strategies.

Contribution

We created a new ReaxFF parameter set for Ni/Cr/F/Li/Na/K, allowing detailed molecular dynamics simulations of corrosion in fluoride salts, which was previously unfeasible.

Findings

ReaxFF accurately models FLiNaK structure and corrosion behavior.

Cr dissolution depends on alloy and salt composition.

Increasing Li concentration reduces Cr dissolution by forming a compact double layer.

Abstract

The interface between NiCr alloys and FLiNaK molten salt exhibits complex corrosion behavior, mainly driven by intricate chemical interactions involving Cr and F$\mathrm{^-}$ ions. Understanding these dynamic reactions is crucial for developing effective corrosion mitigation strategies to ensure the long-term durability of Ni-based alloy components in molten salt technologies. However, obtaining molecular-level understanding through experiments is challenging. To address this, we utilize reactive molecular dynamics simulations enabled by a reactive force field, ReaxFF, to investigate detailed reaction dynamics at the atomic level. Since there is currently no available force field involving fluoride salt and Ni-based alloys, we first present the development of the ReaxFF parameter set for Ni/Cr/F/Li/Na/K based on extensive first-principles calculations. With this force field, we achieve a strong agreement for the structure of FLiNaK molten salt by comparing the pair distribution functions with experimental and simulation results. Furthermore, it successfully reproduces the experimental phenomenon of Cr dissolution in fluoride salt, with the corrosion rate depending on the alloy and salt compositions. Particularly, it reveals that increasing the concentration of Li can enhance the formation of a compact double layer, mitigating Cr dissolution. This work enables a fundamental understanding of the interfacial behavior between fluoride salt and NiCr alloys.