## 1 Introduction
examined the impact of the rust layer on the passivation and corrosion of Q235b carbon steel in a simulated concrete pore solution, revealing that pre-rusted steel can reach a passive state under high pH conditions without chloride ions but forms a poorer passive film compared to finely polished steel. The removal of the outer rust layer enhances the pitting corrosion potential. Rust layer reactions with hydrogen ions inhibit pitting initiation, while consuming corrosive agents retards pit area expansion [23]. [PERSON] et al. evaluated 304 types of stainless steel's natural passivation ability and chloride-induced corrosion behavior in alkaline concrete pore solutions containing red mud and fly ash. The synergistic effect of these solid wastes formed a passive film with high corrosion resistance to chloride attack. Chloride-induced corrosion mechanisms were proposed, offering insights into designing high-durable and low-carbon concrete in marine environments [24]. [PERSON] et al. characterized the passive film, critical chloride threshold, and degassivation mechanism of HP335 carbon steel in various solutions using a Mott-Schottky analysis, the open-circuit potential, the linear polarization resistance, and a point defect model (PDM). The results showed that the passive film quality and aggressive ion threshold were mainly influenced by pH, with a strong correlation between the resistance and chloride threshold. PDM effectively predicted carbon steel passivation and degassivation in highly alkaline solutions [25]. [PERSON] al. found that the passive film of B500 stirrups had a higher concentration of Fe\({}^{2+}\) oxides at its first 2 nm depth, resulting in a better protective property than Fe\({}^{3+}\) oxides and a higher chloride threshold value, making it more suitable for severe service environments than S275 stirrups [26]. [PERSON] et al. found that the primary passive film on 2507 super duplex stainless steel (SDSS) has superior corrosion resistance compared to the secondary passive film. This difference is due to the primary film having fewer defects, which results in a lower passive current density and higher polarization resistance [27]. [PERSON] et al. revealed that the addition of supplementary cementitious materials affected the Ca\({}^{2+}\) concentration and pH value of concrete pore solutions, influencing the passivation of galvanized steel bars. The passive film in the saturated Ca(OH)\({}_{2}\) solution showed the best protective properties, while no reaction occurred in solutions with lower Ca\({}^{2+}\) concentrations at a pH of 12.5 [28]. [PERSON] et al., who used EIS to study its passivation behavior in a simulated concrete pore solution (SCPS), found that the pitting corrosion resistance of passivated 00 Cr10 MoV steel is significantly higher than that of passivated 20 MnSiV steel due to the formation of a protective passive film and dense Cr-bearing corrosion products [29]. [PERSON] et al. found that the corrosion characteristics of HRB400 and HPB235 steel were exceedingly similar; the chloride threshold for HRB400 steel is smaller than that for HPB235 steel [30]. By providing a detailed analysis of these electrochemical methods and their application in characterizing the passivation process of steel reinforcement in concrete, the results could guide researchers and practitioners in selecting the appropriate techniques for their specific research or engineering applications. Furthermore, it highlights the need for further research in this field. For HRB400 reinforcing steel (i.e., the main reinforcing steel in China), the present study built an electrochemical method-based multi-parameter system to characterize the passivation process of steel reinforcement in concrete, which aims to provide a basis for optimizing the corrosion protection of steel reinforcements in concrete structures. The passivation behavior of the commonly used HRB400 steel reinforcement material in concrete was studied quantitatively using various electrochemical parameters.