pH-Cl− synergistic effect on passive film stability and stress corrosion cracking behavior of Q235 steel in simulated concrete pore solution

Purpose The purpose of this paper is to investigate the synergistic effects of pH reduction and chloride ions (Cl−) on the corrosion behavior, passive film stability and corrosion-assisted mechanical degradation of Q235 steel in simulated concrete pore solutions, providing theoretical support for the durability design and corrosion prevention of reinforced concrete structures in marine environments. Design/methodology/approach Electrochemical measurements, slow strain rate tensile (SSRT) tests, scanning electron microscope and energy-dispersive X-ray spectroscopy were used to study Q235 steel in simulated concrete pore solutions with different pH values (12.5, 11.5 and 10.5) and Cl− concentrations (0–1.0 mol·L −¹). Findings The results show that decreasing pH and increasing Cl− concentration synergistically accelerate passive film destabilization, corrosion development and mechanical degradation of Q235 steel. pH 11.5 represents the onset condition for significant Cl−induced passive film destabilization rather than complete failure. MnS inclusions act as preferential sites for pitting initiation. SSRT results indicate reduced tensile strength and ductility under low-pH and high-Cl− conditions. Research limitations/implications This study clarifies the synergistic mechanism of pH reduction and Cl− attack on passive film stability and corrosion-assisted mechanical degradation of Q235 steel. The findings provide useful reference for corrosion prevention, durability design and maintenance of Q235 steel-reinforced concrete structures in marine environments. Practical implications This study provides critical parameters (pH = 11.5, Cl− = 0.5 mol·L−¹) for passive film breakdown and corrosion initiation of Q235 steel in carbonated and chloride-contaminated concrete. The findings offer a scientific basis for durability design, corrosion monitoring and maintenance strategy optimization of marine reinforced concrete structures, helping to extend service life and reduce lifecycle costs. Social implications By elucidating the corrosion failure mechanism in marine environments, this research provides support for the safety and sustainability of large-scale marine infrastructure such as cross-sea bridges, ports and tunnels. It helps mitigate economic losses caused by corrosion and enhances public safety by improving the integrity and reliability of concrete structures in harsh environments. Originality/value This study systematically reveals the synergistic degradation mechanism of pH and Cl− on the stability of the passive film and the stress corrosion behavior of Q235 steel, addressing key gaps in existing research. By linking microscopic initiation sites (MnS inclusions) to macroscopic failure, it provides new insights for corrosion prediction and protection in marine engineering.