Understanding how dissolved impurities influence salt chemistry and corrosion is critical for predicting materials degradation in high-temperature molten salts. Fluoroacidity and redox potential are key salt properties controlling corrosion, yet how impurity speciation and impurity-induced structural changes affect these properties remains unclear. In this work, ab initio molecular dynamics simulations investigate the effects of carbon, silicon, chromium, and nickel impurities across multiple fluorination states on molten FLiBe (2LiF-BeF2) at elevated temperature. Structural analysis shows that all impurities perturb the Be-F network, promoting redistribution between isolated [BeF4]2− tetrahedra and extended Be-F clusters. The free fluoride population, determined by network connectivity and impurity coordination, shifts accordingly. Bader charge analysis reveals that impurity-induced electronic redistribution controls the balance between fluoride release and sequestration through network restructuring and impurity complexation. Thermodynamic analysis using a Nernst-based framework shows that impurity addition shifts the salt potential in the order C(IV)/C > Ni(II)/Ni > Cr(II)/Cr > Si(IV)/Si, corresponding to decreasing thermodynamic stability toward fluorination and dissolution, consistent with previous experimental and computational observations. Changes in fluoroacidity contribute only weakly to the shift in salt potential, with the dominant contribution arising from impurity redox chemistry. Therefore, accurate corrosion prediction requires explicit treatment of impurity redox chemistry.
In the International Thermonuclear Experimental Reactor (ITER), first mirrors (FMs) are directly exposed to the tokamak plasma environment, where impurities eroded from plasma-facing components can deposit on their surfaces. These deposits require periodic cleaning. However, low-energy ion irradiation during RF plasma cleaning can induce nanoscale roughening, thereby reducing specular reflectivity and degrading the performance of optical diagnostics. Understanding irradiation-driven surface pattern formation is therefore important for assessing long-term FM performance. In this work, a two-field continuum model is developed to investigate ion-induced surface-pattern formation in contamination-modified metallic first-mirror systems under normal-incidence ion bombardment and is applied to Rh-Al2O3 and Mo-Al2O3 mirror surfaces. The model couples surface height evolution with the composition of the irradiation-modified near-surface layer, and incorporates curvature-dependent sputtering, preferential sputtering, ion-induced mass redistribution, and composition-dependent transport and relaxation. Linear stability analysis is used to determine the unstable wavelength range and symmetry of the emerging structures, while their nonlinear evolution is simulated using a pseudo-spectral exponential time-differencing fourth-order (ETDRK4) method. The analysis identifies two effective control parameters governing morphology selection: the composition-height coupling strength S and the curvature anisotropy parameter Δ. Within the present model, Rh-Al2O3 is characterized by strong coupling and near-isotropic curvature response (S≈8, Δ≈0), develops mound-like nanostructures. In contrast, Mo-Al2O3 corresponds to weaker coupling and negative curvature anisotropy (S≈0.052, Δ<0), forms anisotropic wavy or ripple-like structures. These results show that the distinct morphologies arise from material-dependent sputtering response and near-surface transport.
