Electrochemical study of the effects of green tea water extract on the passivation behavior and corrosion inhibition performance of carbon steel in an alkaline chloride-containing environment

Green tea hot-water extract was used as a natural corrosion inhibitor, and its effects on the passivation behavior and corrosion resistance of carbon steel in a saturated Ca(OH) 2 solution containing 3.5 wt.% NaCl (pH 13.5) were investigated by open-circuit potential, electrochemical impedance spectroscopy (EIS), and potentiodynamic polarization. Results show that the extract significantly changes the interfacial state of carbon steel, with clear concentration and time dependence. EIS reveals that the extract simultaneously influences the dielectric response of the inhibition–passivation composite film, the electrochemical reaction beneath the film, and the finite-layer transport process. At the initial exposure stage, 100 mg/L gives the highest low-frequency impedance, the lowest corrosion current density, and the highest polarization resistance, indicating that a low concentration favors rapid establishment of the initial protective film. At intermediate and later stages, 500 mg/L shows the highest overall impedance and total resistance, suggesting formation of a passivation film with higher integrity and fewer defects. The 300 mg/L group exhibits the best overall stability and performance balance during the whole exposure period. Open-circuit potential and polarization results further suggest that the inhibition behavior cannot be attributed solely to adsorption shielding, but may involve a synergistic interaction between molecular adsorption and passive-film regulation, thereby affecting film growth, stability, and subsequent corrosion evolution.