## 2 Materials and Methods
### Experimental Procedures  
This study utilizes electrochemical testing technology to investigate the passivation of steel reinforcements in concrete with a three-electrode system, consisting of a working electrode (WE), a reference electrode (RE), and a counter electrode (CE). This experiment selected the CS350H electrochemical workstation produced by Wuhan CORRTEST Co., Ltd. (Wuhan, China) for electrochemical testing. The experimental setup included a potentiostat/galvanostat, a three-electrode system, a stirrer, and a temperature controller. In this experiment, the WE was the HRB400 steel reinforcement, which was polished and cleaned before testing. The RE was the saturated calomel electrode (SCE), which was used to measure the potential of the working electrode accurately and stably. The CE was the platinum electrode, which was used to form a conductive circuit with the working electrode.  
The experimental process involved several steps. First, the steel reinforcement was polished and cleaned, then placed in the electrolyte. The reference electrode and counter electrode were also immersed in the electrolyte. Second, the potentiostat/galvanostat was connected to the three-electrode system, and the temperature controller was set to the desired temperature. Third, the experiment began with a certain potential applied to the working electrode, and the current density was recorded at regular intervals. Fourth, the  
Figure 1: The experimental materials and preparation process: (**a**) HRB400 working electrode; (**b**) concrete simulation solution.  
open-circuit potential (OCP), electrochemical impedance spectrum (EIS), and dynamic potential polarization curve (DPPC) methods were used to test the passivation process of the steel reinforcements. Finally, after a certain period of time, the experiment was terminated, and the steel reinforcement was removed from the electrolyte for further analysis. Figure 2 illustrates the electrochemical testing system and methods. It should be noted that the electrochemical testing system needed to be maintained with an alkaline solution during the long-term tests, in which the solution was refreshed periodically and covered with plastic wrap to protect it from interactions with the atmosphere such as carbonation and evaporation.  
The open-circuit potential (OCP) method involves measuring the potential of the working electrode in open-circuit conditions over time. For the OCP test, the test system should monitor the OCP for at least 30 min to ensure that the OCP remains at a relatively stable value, which is similar to the previous study [32]. Electrochemical impedance spectroscopy (EIS) is an important electrochemical method for studying electrode reaction kinetics, with a larger measurement frequency range. Following the previous study [33], in this study, based on the fact that a 10 mV sine wave signal and a 0.335 V potential difference were applied, the EIS test was carried out in a frequency range of \(10^{-2}\) Hz to \(10^{5}\) Hz and fitted by ZView software 3.0a version using the least-squares method. Then, the parameters of each equivalent component in the equivalent circuit diagram can be obtained. In addition, similar to the previous study [34], the potentiodynamic polarization method was conducted at a potential scan rate of 0.167 mV/s in a range of 0.25 V. The Tafel dynamic potential polarization curve method involves using the dynamic potential scanning approach to scan the Tafel region of the working electrode and obtain the Tafel polarization curve of that electrode.