Abstract: β-FeOOH, a key corrosion product formed in coastal atmospheric environments, plays a pivotal role in governing the corrosion kinetics of steel through its electrochemical reduction. However, the detailed electrochemical reduction pathways in environments containing both H⁺ and Cl^- ions remain insufficiently understood. Here, first-principles calculations both validate existing models and uncover previously unrecognized interactions in the cathodic reduction of β-FeOOH. The calculations reveal distinct bonding modes: H⁺ ions form covalent bonds with tunnel wall oxygen atoms, while Cl^- ions occupy tunnel centers and interact ionically with wall hydrogen atoms. The incorporation of [H⁺/Cl^-] ion pairs markedly enhances electrochemical stability and electron affinity, with β-FeOOH(H⁺, Cl^-)_0.167 showing maximum stabilization. Ion-migration analysis shows that Cl^- exhibits lower migration barriers than H⁺, leading to preferential Cl^- desorption under humid conditions. This forms localized β-FeOOH(H⁺) oxonium intermediates, which were previously unanticipated. Upon electron uptake, these oxonium species reduce Fe(III) to Fe(II), causing FeO₆ expansion and initiating dehydration reactions. Ultimately, phase transformation to Fe₃O₄ is achieved through the reorganization of the local coordination environment. These results provide mechanistic insights into the electrochemical reduction of β-FeOOH and reveal previously unanticipated electrochemical steps, offering guidance for predicting steel corrosion in marine environments and developing effective protection strategies.

Key words: β-FeOOH, Atmospheric corrosion, Electrochemical reduction, Tunnel structure, Ion migration, First-principles calculations