Nanoscale Mechanisms of Corrosion Threshold: Effects of Passivation Film Crystal Structures

The corrosion resistance of passive films on reinforcing steel is fundamentally governed by the depassivation stability of iron oxide polymorphs under chloride-containing environments. In this study, the electrochemical behavior and atomistic depassivation mechanisms of γ-, β-, and α-Fe2O3 passive films were systematically investigated through combined electrochemical measurements, structural characterization, metadyn2amics, and reactive force field (ReaxFF) molecular dynamics simulations. Electrochemical results demonstrate that β-Fe2O3 exhibits the lowest corrosion current density, the highest impedance stability, and the strongest resistance against chloride-induced depassivation. Across 0-5 wt % NaCl, the β-Fe2O3-dominant coating exhibited the lowest mean corrosion-current response and a comparatively persistent impedance response. At 5 wt % NaCl, the corrosion current density was (5.46 ± 0.65) × 10-5 A for the β-dominant coating, compared with (7.11 ± 0.19) × 10-5 A for the γ-dominant in situ film and (3.45 ± 1.24) × 10-4 A for the α coating (mean ± SD, n = 3). Metadynamics simulations reveal that β-Fe2O3 maintains the highest free-energy barrier for Fe atom exfoliation even at elevated NaCl concentrations, indicating intrinsically superior thermodynamic stability. At 5 wt % NaCl, the calculated Fe-exfoliation barriers were approximately 55, 21, and 10 kJ mol-1 for the β-, γ-, and α-Fe2O3 models, respectively. Atomic-scale analyses further show that the β-phase suppresses chloride penetration, limits interfacial hydration accessibility, and preserves stable Fe-Os coordination networks. In contrast, γ- and α-Fe2O3 undergo significant hydration-assisted coordination exchange and lattice destabilization under chloride exposure. The results establish that the macroscopic corrosion threshold is partly associated with polymorph-dependent coordination stability and interfacial depassivation energetics. The results indicate that polymorph-dependent coordination stability and interfacial depassivation energetics contribute to the macroscopic corrosion threshold, although they do not fully account for the complexity of corrosion initiation in real systems.