Influence of mill-scale integrity on corrosion behaviour of HPB300 rebars in simulated concrete pore solution

Potentiodynamic polarisation, electrochemical impedance spectroscopy, real-time video recording and microscopy were used to investigate the passivation and corrosion behaviour of HPB300 reinforcing steel in simulated concrete pore solution (SCPS). Three surface conditions were examined: polished rebar without mill scale (PR), as-received rebar with intact mill scale (AR) and rebar with defective mill scale (DR). The results show that PR more readily develops a dense, uniform passive film in SCPS, whereas the passive film formed on mill-scale-covered rebars is of lower quality. Despite its inferior passive film quality, intact mill scale provides additional physical shielding, delaying pitting initiation in low-chloride environments; below 0.6 mol/L Cl − , AR exhibited a pitting potential of approximately 0.8 V. However, once a local breakdown occurs, pitting preferentially initiates at weak regions of the mill scale. The resulting electrochemical heterogeneity and geometric discontinuities on the mill-scale surface further intensify localised attack, leading to larger pitting cavities and faster corrosion propagation than observed on PR. These findings elucidate the dual role of mill-scale integrity in chloride-bearing alkaline environments and provide mechanistic evidence to improve the durability of reinforcing steel.