## 3 Results and Discussion
### Characterization of Steel Reinforcement Passivation Based on Electrochemical Impedance Spectrum  
Electrochemical impedance spectroscopy (EIS) is a powerful testing technique that has been widely used to evaluate the passivation behavior of steel in concrete. This method allows for the determination of the impedance spectrum of steel electrodes after soaking in concrete simulation solution for different times. By analyzing the changes in impedance, it is possible to assess the corrosion resistance and passive film stability of steel in concrete. Figure 4 presents the working electrode impedance spectra (Nyquist plots) after soaking for 0-10 days in the concrete simulation solution. Each spectrum represents the impedance response of the steel electrode at a given soaking time. The circular arc shape observed in the Nyquist plots indicates that the impedance response is dominated by a single time constant, which corresponds to the charge transfer process at the electrode/solution interface. The diameter of the arc represents the polarization resistance of the working electrode, which is a measure of its corrosion resistance. As the soaking time of the concrete simulation solution increases, the polarization resistance of the working electrode increases, indicating a more stable passive film. This observation suggests that the steel electrode becomes increasingly resistant to corrosion as it ages in the simulated concrete pore solution. After soaking in the concrete simulation solution for 1 day, a significant increase in the diameter of the arc is observed. This change indicates that the steel electrode has already undergone significant passivation in the early stages of soaking. The impedance spectrum capacitance arc of the working electrode gradually increases and shows significant changes in the first 5 days of soaking. This observation suggests that the formation and development of the passive film on the steel surface are dynamic processes that occur within this time frame. However, when soaked for more than 5 days, the capacitance arc diameter of the working electrode remains large, and no significant changes occur. This indicates that the carbon steel electrode undergoes passivation in the simulated concrete pore solution for 5 days. Beyond this time point, the passive film stabilizes, and further changes in the impedance spectrum are minimal. These findings have important implications for assessing the corrosion resistance and passive film stability of steel in concrete. For example, the alkalinity of concrete can be maintained by using cement with a higher alkalinity or by adding a certain proportion of alkaline additives to form a passivation film that protects the surface of steel bars. The use of EIS as a testing technique provides a convenient and  
Figure 4: The EIS curve during the passivation process of steel reinforcements in concrete. effective means to evaluate the passivation behavior of steel reinforcements in concrete structures. By analyzing the changes in impedance spectra, engineers can gain insights into the corrosion resistance of steel and ensure its long-term performance in concrete structures. To quantitatively analyze the results of EIS experiments and further investigate the passivation behavior of HRB400 carbon steel in simulated pore fluids of concrete, experts and scholars have proposed the use of an equivalent circuit method. By selecting appropriate equivalent circuit models, electrochemical kinetic processes and reaction mechanisms can be better characterized and quantified. During the passivation process, a capacitive circuit is formed on the surface of the steel electrode, as shown in Figure 5. An equivalent circuit with a single time constant can be used to fit all the EIS data.