Macrocell corrosion is a frequent and detrimental problem plaguing reinforced concrete structures exposed to chloride environments. This paper investigates the rate-limiting step(s) of macrocell corrosion of reinforcing steel bars in concrete by quantifying the respective contributions of anodic polarization, cathodic polarization, and ohmic subprocesses to the overall macrocell corrosion. Experiments were carried out to study the impact of the distance between active and passive steels, the active-to-passive steel area ratio (A/P), and the microcell corrosion rate of active steel. Numerical simulations were further performed to extrapolate experimental results to large-scale concrete structures. Results demonstrated that with a decreasing A/P, the contribution of the anodic polarization subprocess was increased, while that of the cathodic polarization subprocess was decreased. The macrocell effect that aggravated the total corrosion rate of active steel was weakened as the microcell corrosion activity increased or with increasing the relative humidity.

This paper aimed to investigate the corrosion mechanisms of stirrups in concrete, considering the macrocell current between stirrup and longitudinal rebar. The impacts of relative humidity (RH) and area ratio of stirrup to longitudinal rebar were studied. To avoid mechanical damage like bending on the corrosion activity of stirrups, straight stirrups were used in concrete specimens. Corrosion experiments of concrete were carried out under a simulated marine atmospheric environment. Measurements of polarization curves and macrocell currents were performed to calculate the increase of the total anodic current of stirrups due to the macrocell current by means of curve-fitting analysis. The results showed that the microcell corrosion is the major mechanism in the corrosion of stirrups under different RH conditions. Results also indicated that as RH increased, microcell and macrocell corrosion current densities increased, whereas the galvanic coupling effect that was used to quantify the increase of the total current density of stirrups decreased.