Influence of chloride and sulfate on steel corrosion in simulated concrete pore solutions

Through electrochemical tests, XRD analysis, and DFT calculations, this study investigated the individual and combined effects of Cl⁻ and $\text{SO}_{\text{4}}^{{2-}} $ ions on the corrosion behavior of steel reinforcement in simulated concrete pore solutions at different pH values (12.4, 12.9 and 13.5). The results reveal that in the lower pH CH solution, even at a Cl⁻ concentration of only 0.02 mol/L, the open circuit potential (OCP) drops sharply from −375 mV to −575 mV, indicating significant acceleration of corrosion by Cl⁻ at low concentrations. Conversely, in the higher pH ST solution, as the concentration of corrosion ions increases gradually from 0.01 mol/L to 0.4 mol/L, the steel’s polarization resistance (R_p) stabilizes and decreases from approximately 100 kΩ·cm² to 5 kΩ·cm², demonstrating superior overall corrosion resistance compared to lower pH conditions, highlighting the effective inhibition of corrosion at higher pH value. Furthermore, in the presence of both Cl⁻ and $\text{SO}_{\text{4}}^{{2-}} $ ions, the overall corrosion rate lies between the rates observed when each ion is present individually, due to the competitive adsorption mechanism. The presence of $\text{SO}_{\text{4}}^{{2-}} $ mitigates to some extent the corrosion initiated by Cl⁻. Based on these findings, the study proposes a two-stage competitive adsorption-catalytic corrosion reaction model, elucidating in detail the corrosion behavior under their combined influence: Cl⁻ accelerates corrosion by disrupting the passive film, while $\text{SO}_{\text{4}}^{{2-}} $ participates in the corrosion process by influencing the stability and distribution of corrosion products.