The degradation of concrete and the corrosion of steel-reinforcing bar are serious problem in infrastructures. The electrochemical measurements are useful to evaluate the stability of passive film formed on steel rebar in concrete, which are mainly measurements of corrosion potential [1] and polarization curve. Dawson et al. [2] applied an electrochemical impedance spectroscopy (EIS) to the investigation of rebar corrosion in concrete because the charge transfer resistance at rebar/concrete interface can be determined from the impedance spectrum. Recently, the authors developed an electrode in Fig. 1 as a sensor probe to monitor concrete degradation and rebar corrosion. A couple of electrodes, which were made of same material as steel rebar, can be embedded in concrete structure and complex impedance can be measured continuously on the basis of two-electrode method.

The authors’ group has performed the following three types of impedance measurements in order to prove the of the developed sensor probe.

(1) Impedance between two electrodes in aqueous solution that simulates the water quality of transition zone (laboratory test).

(2) Impedance between two electrodes embedded in the concrete which was fabricated in the laboratory.

(3) Field test at bridge pier of the highway in coastal marine environment.

It is well known that the rebar corrosion in concrete is strongly influenced by the conditions of cover concrete. Thin water layer is formed at rebar/concrete interface due to water absorption in the concrete. The concrete is good health status and the passive film is formed on steel rebar when the pH of water layer on steel rebar is sufficiently high. The breakdown of passive film occurs by neutralization and enriched chloride ions in the water layer with the degradation of concrete. In this presentation, the authors investigate the critical values of pH and chloride concentration to maintain the passive film on steel rebar and discuss the depassivation mechanisms by the measurements of electrochemical impedance with the developed sensor probe.

References:

[1] ASTM C 876-91 (Reapproved 1999): Standard Test Method for Half-Cell Potentials of Uncoated Reinforcing Steel in Concrete, Annual Book of ASTM standards, Vol. 03.02, pp. 457-462, 1999.

[2] J. L. Dawson, L. M. Callow, K. Hladky and J. A. Richardson, Corrosion/78, No-125 (1978).