Electrochemical response and mechanism of steel rebar passivation in simulated concrete pore solutions with different pH values

Reinforced concrete is widely used in various large-scale and coastal structures due to its unique strength, wide availability, and low cost. However, many concrete structures suffer from corrosion damage due to the presence of chloride ions and other corrosive agents. The optimal alkaline conditions for the formation of passive films on low-carbon steel, as well as the passivation mechanism, remain unclear and require further study to elucidate the passivation mechanism and corrosion resistance strategies of low-carbon steel. In this study, open circuit potential (OCP) and polarization resistance methods are employed to investigate the passivation mechanism of low-carbon steel.

Electrochemical methods were used to study the passivation time and passive film thickness of low-carbon steel. The electrochemical measurements were conducted using a classical three-electrode corrosion cell configuration. The corrosion potential measurements reveal the corrosion potential (Ecorr) trends of ordinary low-carbon steel during the passivation phase in CH, LC, and ST solutions with different pH values over 10 days. This allows for the determination of the passivation effectiveness and passivation time of low-carbon steel in the three solutions. Polarization resistance and corrosion current density data were collected over the first 10 days, and the variation of passivation performance with pH value was derived from the rise and fall of the curves. Electrochemical impedance spectroscopy (EIS) tests were performed on the steel specimens to obtain the passivation time, thereby verifying the correctness of the corrosion potential results. ToF-SIMS analysis was conducted using the TOF.SIMS 5-100 system from ION-TOF GmbH, Germany, with the vacuum pressure maintained below 1×10⁻⁹ Torr throughout the analysis. ToF-SIMS was used to determine changes in the passive film thickness, and the conclusions drawn were compared with those obtained through electrochemical methods.

The passivation time and passive film thickness of low-carbon steel can be obtained from several aspects of electrochemical methods:1.Corrosion potential testing indicates that the passivation performance of the steel's passive film is best in the ST solution with the highest pH value, followed by the slightly higher pH LC solution, while the CH solution with the lowest pH exhibits the poorest passivation performance.2.In the CH solution, the polarization resistance stabilizes within a certain range after four days, while the samples in the ST solution show fluctuations in polarization resistance within a certain range after six days. This indicates that the stability of the passive film is related to the pH value—the higher the pH, the more stable and protective the passive film, which has a significant impact on the film’s effectiveness.3.EIS results show that the passivation time in the CH simulated solution is about 9 days. The ordinary low-carbon steel specimens can passivate in the LC simulated solution, with a passivation process taking 3 days, while the passivation time in the ST simulated solution is only 1 day. 4.ToF-SIMS sputtering removed the outer passive film, revealing an inner layer rich in iron oxides. The increase in Fe⁻ indicates that the passive film has a bilayer structure. After 1 day of passivation, the passive film thickness was approximately 2.81 nm, and after 7 days, it was about 3.75 nm. Figure 6 shows that the film thickness in the ST solution increased from 3.35 nm to 4.35 nm between 1 and 7 days, indicating that the film thickness increases over time. The composition mainly consists of iron oxides, and under the same passivation time, the passive film thickness increases with higher pH values. Finally, the passivation mechanism of low-carbon steel was concluded: The initial stage of passive film formation primarily involves the synthesis of Fe(OH)₂ adsorbed on the reinforced substrate surface. As the reaction progresses, Fe(OH)₂ transforms into FeO, FeOOH, Fe(OH)₃, and Fe₂O₃. The outer layer is rich in iron compounds, while the inner layer contains ferrous compounds. Ferrous compounds are denser than iron compounds, making the inner layer of the passive film more compact than the outer layer. As the pH increases, the OH⁻ concentration rises, enhancing the formation of Fe(OH)₂ and leading to the development of a thicker Fe(OH)₂ film. Subsequent reactions of Fe(OH)₂ result in the formation of FeO, FeOOH, Fe(OH)₃, and Fe₂O₃, ultimately forming a more stable passive film. Thus, an increase in the pH of the simulated solution correlates with increased stability of the passive film on the surface of low-carbon steel.Conclusions:This study explored the effects of pore solution pH on the passivation of ordinary low-carbon steel and elucidated the passivation behavior and mechanisms. Low-carbon steel can spontaneously form a passive film under different pH conditions, with the electrochemical performance of the passive film improving as the passivation process progresses. Additionally, the performance of the passive film shows a positive correlation with pH. The pH value drives the passivation of iron-based materials and can directly influence the passivation outcome. The higher the pH, the better the passivation effect. In a high-pH environment, the passivation tendency of low-carbon steel is more pronounced, and under different pH conditions, the passive film on the surface of low-carbon steel undergoes a rapid initial growth phase, followed by gradual stabilization over time.