## 3 Results and Discussion
### Characterization of Steel Reinforcement Passivation Based on Electrochemical Impedance Spectrum
In this circuit, R\({}_{\text{s}}\) represents the solution resistance, R\({}_{\text{f}}\) and the constant phase angle element (CPE\({}_{\text{f}}\)) are the passive film resistance and constant phase element of the passive film, respectively, R\({}_{\text{ct}}\) is the charge transfer resistance through the passive film, and the constant phase angle element (CPE\({}_{\text{dl}}\)) can be used to reveal the deviation of the ideal passivation film capacitor caused by surface roughness, in which the double-layer capacitance parameters on the working electrode surface are Q\({}_{\text{dl}}\) and n\({}_{\text{dl}}\). The use of ZView software allows for the fitting of experimental data curves and the extraction of the fitting values for each circuit component in the equivalent circuit model of the test system. This approach provides a more accurate and reliable means of analyzing EIS data and understanding the electrochemical behavior of carbon steel in simulated pore fluids of concrete. With the help of an equivalent circuit analysis, scholars and experts can gain insights into the passivation behavior of carbon steel and take appropriate measures to enhance its corrosion resistance and durability in concrete structures. As the steel electrode undergoes a soaking test in the concrete environmental solution, a dense oxide film gradually forms on the surface of the electrode. This oxide film plays a crucial role in protecting the steel from corrosion and is considered a passivation layer. From the perspective of the entire electrochemical system, the Helmholtz double layer, which is located between the passivation film and the electrolyte, is in contact with the surface of the oxide film. It is noteworthy that the thickness of this oxide film is significantly greater than that of the Helmholtz double layer (typically, the carbon steel oxide film thickness is about 1 nm, while the Helmholtz double layer thickness is about 0.1 nm [35]). This difference in thickness implies that during impedance spectroscopy testing, the potential signal is predominantly determined by the properties of the oxide film. In other words, the oxide film resistance (R\({}_{\text{f}}\)) contributes to almost all of the overall polarization resistance (R\({}_{\text{p}}\)). This observation is further supported by the experimental data obtained from the EIS testing during the passivation stage of carbon steel in the simulated pore fluids of concrete, which are summarized in Table 1. Table 1 presents a comprehensive analysis of the EIS test results, providing insights into the electrical and kinetic properties of the oxide film formed on the carbon steel during its passivation process in the simulated pore fluid environment. Figure 5: The electrochemical equivalent circuit diagram. By quantitatively analyzing these data, researchers can gain valuable information on the passivation behavior of carbon steel and take appropriate measures to enhance its corrosion resistance and durability in concrete structures. Based on the data presented in Table 1, it is evident that as the soaking test progresses, the working electrode experiences a significant increase in impedance, from approximately 40 k\(\Omega\cdot\)cm\({}^{2}\) on the first day to approximately 1000 k\(\Omega\cdot\)cm\({}^{2}\) on the fifth day. This increase is a direct result of the formation of a protective oxide film on the steel electrode surface, which serves to hinder the corrosion process. When the soaking period extends to the tenth day, the electrode resistance remains above 1000 k\(\Omega\cdot\)cm\({}^{2}\) and gradually increases slightly, indicating that the oxide film has achieved comprehensive coverage on the steel surface. This gradual increase in electrode resistance is attributed to the growth and development of the oxide film, which becomes thicker and more uniform over time.