IntroductionAs the issue of steel corrosion in reinforced concrete structures becomes increasingly severe, enhancing their durability has become crucial. Therefore, the application of highly corrosion resistant steel, such as stainless steel and corrosion resistant alloy steel, is an effective countermeasure. However, the durability of steel in concrete is influenced by numerous factors, and its corrosion mechanisms exhibit long term and complex characteristics. The use of simulated concrete pore solution methods in research simplifies influencing factors and shortens the research cycle, thus being widely applied in academic studies. In modern construction practices, to meet low carbon and environmental requirements, a large amount of mineral admixtures is often added to cement. However, the use of these admixtures can cause fluctuations in the pH value of the concrete pore solution, thereby affecting the stability of the passivation film on the steel surface and increasing the risk of steel corrosion. Therefore, this study aims to investigate the passivation behavior and evolution of Cr10Mo alloy steel in simulated concrete pore solutions with different pH values.MethodsSteel was machined using a controlled lathe to produce rebar electrode discs with a diameter of 16 mm and a thickness of 10 mm. The bottom surface of the rebar was used as the working surface, with a working area of 2.01 cm². All other non-working surfaces were sealed with epoxy resin. Each experiment utilized three rebar electrode discs as parallel samples. The working surfaces were gradually polished to a mirror finish using 400 #, 800 #, 1200 # and 2000 # silicon carbide sandpaper, then cleaned with deionized water, followed by an alcohol wash to remove grease. After drying, the discs were immediately placed into the standard corrosion cell. This procedure was repeated to polish a total of nine rebar electrode discs.The electrochemical measurements were conducted using a classical three electrode system. The rebar electrode served as the working electrode, the saturated calomel electrode was used as the reference electrode, and a platinum electrode was employed as the auxiliary electrode. All measurements were performed at a laboratory temperature of 20 ℃. The parameters selected for the linear polarization resistance test were as follows: the scanning potential was ±10 mV vs. OCP, and the scanning rate was 10 mV/min. The EIS test parameters were as follows: the perturbation voltage was a sine wave signal with an amplitude of 10 mV, and the scanning frequency range was from 10 mHz to 100 kHz.Results and discussionThe OCP and LPR test results indicate that in the CH simulated solution with the lowest pH value, alloy steel exhibits the lowest corrosion tendency after passivation. In the higher pH LC solution, corrosion tendency slightly increases, while in the highest pH ST solution, it is the highest. The R_p and I_corr values are as follows: CH solution at 1100 kΩ·cm² and 0.047 μA/cm², LC solution at 806 kΩ·cm² and 0.064 μA/cm², and ST solution at 534 kΩ·cm² and 0.097 μA/cm². Thus, the passivation performance ranks as CH > LC > ST, indicating that lower pH conditions lead to a more stable passivation film, effectively preventing further corrosion. EIS and equivalent circuit fitting results show that the CH simulation has the best passivation effect, with its α value and R_ct value surpassing those of the LC and ST simulated solutions, highlighting its superior passivation performance. Additionally, the electrochemical behavior of the rebar samples in different environments reveals that the integrity and electrochemical protection capability of the passivation film are closely related to the pH value. Comprehensive investigations, including Pourbaix diagrams, Gibbs free energy analysis, and the passivation reaction process of chromium, indicate that the selective transformation of Cr(OH)₃ to Cr₂O₃ or CrO2– 4 in the passivation film influences its stability at lower pH values. Specifically, at lower pH values, Cr(OH)₃ tends to convert to Cr₂O₃, forming a denser passivation film, thereby enhancing the passivation performance of the alloy steel. Conversely, under higher pH conditions, some Cr(OH)₃ may convert to the less stable CrO2– 4, leading to a decrease in the stability of the passivation film and weakening its protective effect.ConclusionsRegardless of the pH conditions, alloy steel can spontaneously form a gradient passivation film consisting of an inner layer of Cr₂O₃ and an outer layer of Fe₂O₃. As the solution pH decreases, the content of Cr oxides/hydroxides in the passivation film increases, resulting in a denser and more stable film structure. The passivation performance of the alloy steel improves with decreasing pH value, primarily due to the selective transformation of Cr(OH)₃ to Cr₂O₃ or CrO2– 4 in different pH environments. At lower pH values, the I_corr of alloy steel significantly decreases, and R_p significantly increases, indicating better formation and stability of the passivation film. In different pH simulated solutions, the passivation performance of the alloy steel follows the order: pH 12.4(CH) > pH 12.9(LC) > pH 13.5(ST).