Concentration‐Dependent Electrochemical Corrosion Inhibition and Interfacial Film Evolution of Nettle Extract on Carbon Steel in Chloride‐Containing Simulated Concrete Pore Solution

Natural nettle extract was evaluated as a corrosion inhibitor for carbon steel in chloride‐containing simulated concrete pore solution containing 3.5 wt% NaCl and saturated Ca(OH) 2 (pH 13.5) using time‐resolved OCP, EIS, potentiodynamic polarization, adsorption thermodynamics, FTIR, SEM/EDS, and XPS. The inhibition response was strongly concentration‐ and time‐dependent rather than monotonic. Low‐to‐intermediate concentrations initially suppressed corrosion but could interfere with passive‐film development, whereas 700 mg/L provided the best balance between molecular adsorption and passivation. This condition yielded the highest low‐frequency impedance and Faradaic resistance, the lowest long‐term interfacial activity, and the lowest corrosion current density after 7 d. Apparent kinetic analysis further showed rapid interfacial stabilization at 700 mg/L. Adsorption followed predominantly Langmuir behavior, with ≈−31 kJ mol −1 , indicating spontaneous mixed physical/chemical adsorption. SEM/EDS and XPS showed a more uniform protective surface, enhanced oxygen‐containing organic species, and reduced chloride accumulation at 700 mg/L. These results demonstrate that nettle extract protects carbon steel through adsorption‐assisted stabilization of the passive interface, with an optimum concentration arising from the competition and synergy between organic adsorption and passive‐film evolution.