pH-Sensitive Destabilization Behavior of Passive Films on HRB400 Steel in Low-Carbon Ferrite-Aluminate Cement Pore Solution

Carbonation-induced pH reduction is a key factor triggering steel depassivation and corrosion initiation in reinforced concrete. However, the influence of pore solution chemistry on passive film (PF) stability remains unclear. In this study, ordinary Portland cement simulated pore solution (OPC-SCP) and ferrite-aluminate cement simulated pore solution (FAC-SCP) were used to investigate the evolution of PF formed at pH 12.5 and subsequently exposed to pH 11.0 and 9.5 environments. Electrochemical and microscopic techniques were employed to investigate the degradation behavior of PF under reduced alkalinity. The results show that PF in both systems degraded with decreasing pH, but exhibited markedly different stability. In the OPC-SCP system, the PF resistance decreased slightly from 4.24 × 106 to 2.85 × 105 Ω·cm2, indicating that the steel remained in a highly passive state. In contrast, the PF resistance in the FAC-SCP system dropped significantly from 1.13 × 106 to 5.57 × 103 Ω·cm2. AFM and SEM observations further revealed greater surface roughness and more severe local damage in the FAC-SCP system. The superior stability of PF in OPC-SCP may be attributed to the higher Ca2+ concentration, which is likely beneficial for the formation of a relatively dense and protective film. Conversely, the higher SO42− concentration and lower Ca2+ content in FAC-SCP may facilitate defect growth and local dissolution, thereby contributing to depassivation. These findings highlight the critical role of pore solution chemistry in regulating PF stability under reduced alkalinity conditions.