First-principles investigation of grain boundary effects on fluorine-induced initial corrosion of NiCr alloys

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 Σ5(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 adsorption energies increasing by approximately 5%–10% compared to bulk Ni surfaces. Cr doping further strengthens fluorine binding, leading to adsorption energies as high as -3.78 eV. Fluorine bonding with Cr significantly alters the interaction between Cr-F complexes and Ni substrate, reducing the energy barrier of Cr dissolution from 2.95 eV for individual Cr to 1.39 eV for CrF₃ complex at the GB. Notably, energy calculations indicate that Cr can dissolve into the fluoride salt either as adatoms from the bulk or directly from GBs, with similar dissolution energy barriers for both mechanisms. Given that GBs are fast diffusion pathways, this suggests that intergranular corrosion may progress through Cr dissolution as CrF_x species at the GB or via Cr transport along the interface to locations with high local F ion concentrations that facilitate CrF_x formation and dissolution. This work details the unique role of GBs in accelerating Cr depletion in NiCr alloys from the first principles.