Controlling molten salt corrosion in NiCr alloys: From interfacial chemistry to facet-dependent attack

Molten fluoride salts are attractive for advanced energy technologies because they combine high thermal stability with excellent heat-transfer properties. Their corrosivity, however, remains a major barrier for structural materials. For NiCr alloys in FLiNaK, chromium is the weak link: fluorine-rich salt environments selectively destabilize Cr at the interface, initiating dealloying and long-term surface degradation. Experiments capture the macroscopic outcome, but the atomistic pathway from fluoride adsorption to pit formation is much harder to resolve directly.

Using reactive molecular dynamics in AMS with ReaxFF, Penn State University researchers built an atomistic picture of how molten FLiNaK corrodes NiCr alloys and how that process depends on both alloy composition and crystallographic orientation. They first developed and validated a Ni/Cr/F/Li/Na/K ReaxFF force field and showed that the simulations reproduce key structural features of molten FLiNaK as well as the experimentally observed preference for Cr dissolution. The simulations revealed that stronger Cr-F interactions increase fluoride coverage at the alloy surface and drive selective Cr loss into the salt, while Li-rich interfacial environments can compact the double layer and suppress corrosion. This provided the mechanistic basis needed to move from composition effects to a more detailed understanding of where and how corrosion starts.

The follow-up study extends that picture by asking a practical materials question: which NiCr surface is most vulnerable in molten FLiNaK, and can corrosion be controlled? Comparing Ni₀.₇₅Cr₀.₂₅ (100), (110), and (111) surfaces from 600 to 800 °C, the simulations show a clear orientation dependence. The (110) surface is the most corrosion-prone, with stronger fluoride interaction, faster Cr dissolution, more pronounced near-surface disruption, and deeper pitting-like damage. The (111) surface is the most resistant, while (100) shows intermediate behavior. Rather than proceeding through slow bulk Cr transport, corrosion is governed by kinetically controlled near-surface events: Cr dissolves first, vacancies form, and coupled Ni/Cr counter-diffusion helps the corrosion front advance into a progressively restructured surface region.

This mechanistic detail is exactly where modeling adds value beyond experiment alone. The simulations connect interfacial ion distributions, atom mobility, morphology evolution, and activation barriers into one consistent picture. Arrhenius analysis gives low effective barriers for Cr dissolution, much smaller than the reported bulk diffusion barrier, reinforcing that molten salt corrosion in this system is controlled by local interface chemistry and surface diffusion rather than long-range transport through an intact solid.

The study also points to a possible control strategy. External electric fields perpendicular to the interface alter local fluoride concentration and strongly affect corrosion kinetics. A positive field enhances fluoride accumulation and accelerates Cr loss, while a negative field suppresses fluoride adsorption and mitigates corrosion.

Together, these two papers show how reactive MD simulations with ReaxFF in the Amsterdam Modeling Suite can move corrosion research from descriptive trends toward actionable design rules. Alloy composition, surface orientation, and interfacial electrostatics all matter, and all can in principle be engineered to improve durability in molten salt environments.