Interfacial film evolution and corrosion protection of carbon steel by Ginkgo biloba leaf extract in simulated concrete pore solution

Ginkgo biloba leaf extract was evaluated as a natural corrosion inhibitor for carbon steel in a simulated concrete pore solution containing 3.5 wt% NaCl and saturated Ca(OH)2 at pH 13.5. Open-circuit potential, electrochemical impedance spectroscopy and potentiodynamic polarisation were used to clarify concentration-dependent interfacial evolution. The extract showed a distinct concentration effect. During early exposure, all extract-containing systems exhibited lower impedance than the blank, indicating initial disturbance of natural passivation. With prolonged immersion, the systems gradually diverged. In the 100-500 mg/L range, the adsorbed layer showed limited coverage and stabilisation, and the 300 and 500 mg/L systems were more prone to form loose, defect-rich composite films. The 700 and 1000 mg/L systems developed more stable inhibitor-passive composite films after interfacial rearrangement. Among them, 1000 mg/L showed the highest impedance, the highest film and total electrode-process resistances, the lowest equivalent film capacitance, and the best overall long-term inhibition performance.