Effect of CO2 curing on the performance of the passivation film of steel bars in cement-based materials

Abstract CO2 curing can greatly enhance the properties of concrete while actively sequestering CO2. However, the influencing mechanisms of CO2 curing on the passivation film of steel bars in concrete remains unclear. In this study, the passivation and depassivation behaviors of steel bars in CO2-cured mortar were investigated via electrochemical measurements, and the microscopic morphology and chemical composition of the passivation film were examined using scanning electron microscopy (SEM) equipped with energy-dispersive spectroscopy (EDS), and X-ray photoelectron spectroscopy (XPS). The results demonstrate that CO2 curing can accelerate the passivation of steel bars, which can be attributed to the higher oxygen partial pressure around the steel bars when compared to standard curing. Although the thickness of passivation film on steel bars in CO2-cured specimens (4.06 nm) is less than that in standard-cured specimens (4.73 nm), the charge transfer resistance in CO2-cured specimens (458.54 kΩ·cm2) is higher than that in standard-cured specimens (384.49 kΩ·cm2). Specifically, the dense and ordered microstructure observed by SEM, together with the relatively high Fe2+/Fe3+ atomic ratio (0.90 vs. 0.63) of the passivation film detected by XPS, contributes to the enhanced electrochemical stability. In addition, it is found that CO2 curing significantly delays the depassivation onset of steel bars in mortar when subjected to chloride drying-wetting cycles, with the depassivation of standard-cured specimens initiating after 18 cycles and that of CO2-cured specimens being postponed to 30 cycles. Consequently, the protective performance of the passivation film in CO2-cured specimens surpasses that in standard-cured specimens despite the slightly thinner thickness.