Reinforced concrete structures have been widely used for the social infrastructures because the rebar embedded in concrete shows high corrosion resistance due to the formation of passive film formed on the rebar. The monitoring of the conditions of the passive film formed on the rebar is important to ensure the proper maintenance and management of reinforced concrete structures. An electrochemical impedance spectroscopy (EIS) is a useful method to evaluate the corrosion state of reinforced concrete nondestructively^{1, 2)}. In the present study, the stability of passive film formed on the rebar was evaluated in the concrete simulated environment by the EIS.

A saturated calcium hydroxide solution was used to simulate the environment in concrete. The chloride ion concentration was adjusted to 3.8 mmol dm^-3 and 16 mmol dm^-3 by adding sodium chloride to a saturated aqueous solution of calcium hydroxide. The electrochemical impedance measurement was carried out by a three electrode system. A rod electrode made of cylindrical carbon steel (SR345) that has the same composition as rebar SD345, as is often used in actual structures, was adopted as a working electrode. A saturated KCl silver / silver chloride electrode and a platinum wire were used as a reference electrode and a counter electrode respectively. The electrochemical impedance was measured every 12 hours during immersion. The 3D impedance was measured in order to investigate the time-dependent change of the thickness of the passive film. An electrochemical measurement system (HZ-7000, Hokuto Denko Corporation) was used as the measurement device. The measurement frequency range was 10 kHz to 10 mHz with 5 points per logarithmic sweep.

By plotting the admittance and capacitance calculated from the impedance of carbon steel on the complex plane, the resistance and capacitance of the passive film formed on the carbon steel were evaluated. The changes of thickness of passive film formed on the carbon steel with time were discussed.

References

1) J. L. Dawson et al., Proceedings of NACE Corrosion / 78, Paper No. 125 (1978).

2) Yoshinao Hoshi et al., Electrochemistry, 87, 78-83, (2019)

3) Masayuki Itagaki et al., Electrochemistry, 75, 649-655, (2007).