Revisiting Chloride-Induced Rebar Passivity Breakdown: The Role of Passive Film Dissolution at the Critical Chloride Concentration

Critical chloride concentration ( C th ) for depassivation is an essential parameter for ensuring the durability of reinforced concrete structures, while the mechanism by which chlorides initiate the failure of the passive film at this threshold remains debated. C th required by passive film breakdown was determined by a cyclic potentiodynamic polarization test here. A combination of Mott-Schottky test and X-ray photoelectron spectroscopy was further applied to explore the semiconductive properties and chemical composition of the film under this critical environment. The results show that this C th is lower than that determined by a sharp change in corrosion potential, polarization resistance, or corrosion current density. Neither the pitting potential nor the cation vacancy concentration is sensitive to chloride near this threshold. The role of chloride is to accelerate the passive film dissolution, causing the film to thin and eventually break down. It does not penetrate the film or even reach the metal substrate.