Mechanism of cathodic polarization-induced passive film reconstruction of steel bar: From CaCO3 electrodeposition to CaFe2O4-assisted repair

Passive films (PF) were pre-formed on HRB400 steel bar in cement supernatant solution, and the interfacial evolution behavior and Cl - resistance of the PF under different cathodic protection (CP) potentials were systematically investigated through electrochemical measurements and microstructural characterizations. The results showed that the critical chloride concentration (Cl - crit ) for depassivation exhibited a pronounced non-monotonic dependence on CP potential. Compared with the unprotected condition (Cl - crit = 0.46 mol/L), Cl - crit decreased to 0.36 mol/L at −800 mV, increased to 0.52 mol/L at −900 mV, and sharply dropped to 0.02 mol/L at −1000 mV. This difference was mainly attributed to CP-induced interfacial phase reconstruction within the PF. Moderate cathodic polarization promoted the formation of CaFe 2 O 4 , which effectively filled defects generated by the dissolution of Fe 3 O 4 , thereby suppressing Cl - transport and enhancing the depassivation resistance of the PF. In contrast, excessive polarization caused severe degradation of the PF structure. Although a continuous CaCO 3 deposit layer formed on the steel bar surface and exhibited a relatively high apparent impedance, it failed to effectively prevent Cl - penetration to the steel bar substrate, resulting in rapid depassivation. These findings contribute to a better understanding of the stability evolution of PF under CP conditions and provide guidance for optimizing CP parameters in reinforced concrete structures.