Many infrastructures such as expressways and buildings were constructed in Japan between the 1950s and the 1970s. There is greatly influence of airborne sea salt on corrosion in Japan surrounded by the sea, and reinforcement corrosion is accelerated by rainfall, dew condensation and water absorption of airborne sea salt. Therefore, many of these infrastructures are in a condition requiring repair or renewal in the present. In this study, electrochemical measurements of carbon steel were conducted in an aqueous solution simulated the inside of concrete, in order to investigate corrosion behavior of reinforcing bars in concrete. In addition, the influence of surface condition of the reinforcing bars before corrosion on their corrosion resistance was examined by the corrosion monitoring using an electrochemical impedance method.

Carbon steel and 0.1% Ca(OH)₂ solution were used as the sample for the reinforcing bar in concrete and as the test solution simulating the internal environment of the concrete, respectively. The corrosion resistance of passive film in concrete was investigated by anodic polarization measurement and electrochemical impedance measurement. The passive film region was observed regardless of concentration of chloride ion in the test solution. The current density in the test solution containing 0M and 0.01M of chloride ion did not abruptly increase in the passive state region and local corrosion such as pitting corrosion did not occur on the steel surface. A sharp rise in current density was observed in the test solution containing 0.1M or more of chloride ion.

The anodic polarization curves of carbon steel with passive film formed by immersion in the test solution was measured, in order to investigate the influence of the film formation period on the properties of passive film. A passive region was observed in the anodic polarization curves of all samples. The passive current density decreased with an increase of immersion time, indicating that the corrosion resistance of the passive film improved as the immersion time increased. Furthermore, the electrochemical impedance measurements of carbon steel passivated in the test solution were carried out. The impedance characteristic can be represented by a typical Randles type equivalent circuit, and the diameter of the semicircle corresponds to the film resistance. The film resistance increased in proportion to the immersion time in the test solution. This result is consistent with the result of the anodic polarization measurement.