Atomic-scale insights into halide anion size effects on iron pitting susceptibility

The pitting susceptibility of iron and steels in halide-containing solutions is widely observed to depend on the halide species, a phenomenon commonly referred to as the anion size effect. The aggressiveness of halide ions follows the order Cl− > Br− > I− > F−, with Cl− exhibiting the strongest ability to induce passive film breakdown. Although this behavior has been reported previously, the distinct adsorption characteristics of different halide ions and their influence on interfacial reaction kinetics have not been explicitly addressed. In this work, a fundamental understanding of the anion size effect in iron is established by combining an extended Point Defect Model with first-principles calculations from both thermodynamic and kinetic perspectives. The hydration, adsorption, insertion energies of halide ions, as well as their effects on the activation energies for metal cation ejection and passive film dissolution, are quantitatively evaluated and compared. This developed framework successfully rationalizes the dependence of critical breakdown potential, pitting incubation time, and steady-state passive film thickness on halide species, providing a unified mechanistic understanding of halide-induced passivity breakdown in iron.