Evaluation of chloride ion concentration and pH on depassivation of steel rebar in concrete investigated by electrochemical impedance measurement of probe electrodes

The relationship between chloride ion concentration and pH on de-passivation of steel rebar embedded in concrete was investigated by a two frequencies impedance measurement using probe electrodes. A saturated calcium hydroxide solution was used for the electrolyte solution as a simulated concrete pore solution. The pair of probe electrodes, which simulates the steel rebar, was immersed in the electrolyte solution prepared by an arbitrary concentration of chloride ions, and the two frequencies impedance measurement was performed. Air was supplied to the electrolyte solution to simulate the neutralization of concrete during the measurement. The charge transfer resistance related to the electrode/ solution interface, R_ct, was estimated from the two frequencies impedance measurement of probe electrodes in each electrolyte solution. This demonstrated that the R_ct was drastically reduced by decreasing the pH of the electrolyte solution, namely, the de-passivation of steel rebar. The corrosion conditions of steel rebar were categorized as “passive state,” “de-passivation,” and “corrosion” based on the changes in the monitoring of R_ct and pH in each electrolyte solution. These results were plotted on the two-dimensional diagram composed of chloride ion concentration and pH of the electrolyte solution. The conditions for the de-passivation of the passive film on steel rebar were discussed.