Electrochemical study of the corrosion behaviour of steel rebar in a chloride-containing alkaline environment and the passive-film influence mechanism of Xanthium extract

Abstract

To develop a green corrosion inhibitor suitable for chloride-containing alkaline environments in reinforced concrete, Xanthium extract was investigated in a simulated concrete pore solution (saturated Ca(OH)₂ + 3.5 wt% NaCl). Open-circuit potential (OCP), electrochemical impedance spectroscopy (EIS), and potentiodynamic polarisation were employed to systematically evaluate its effect on the corrosion behaviour of carbon steel electrodes. The results show that the extract exhibits a pronounced concentration dependence. The solution containing 700 mg/L Xanthium extract shows the best performance in terms of overall impedance, Faradaic resistance, total electrode-process resistance, and long-term polarisation behaviour, followed by 1000 mg/L, whereas 200 mg/L provides the poorest overall protection. EIS analysis indicates that the extract influences steel corrosion by regulating the dielectric response of the composite film, the electrochemical reaction process, and the restricted transport process. Combined with the passive-film mechanism in alkaline media and the adsorption-film mechanism of inhibitor molecules, the effect of the extract is not simply to promote film thickening. Instead, at suitable concentrations, it stabilises the passive-film structure through surface adsorption, suppresses passive-film defect evolution, and reduces interfacial charge transfer, thereby enhancing the corrosion resistance of steel rebar in chloride-containing alkaline environments. This concentration-dependent synergy governs the final protection efficiency under the present conditions.