Evaluation of Chloride Ion Concentration and pH on Depassivation of Steel Rebar in Concrete Investigated by Electrochemical Impedance Measurement of Probe Electrodes

Abstract

Relationship between chloride ion concentration and pH on depassivation 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 simulated concrete pore solution. The pair of probe electrodes, which simulates the steel rebar, was immersed in the electrolyte solution prepared by arbitrary concentration of chloride ions, and the two frequencies impedance measurement was performed. Air was supplied to the electrolyte solution in order to simulate the neutralization of concrete during the measurement. The charge transfer resistance related to electrode/solution interface, R_ct, were estimated from the two frequencies impedance measurement of probe electrodes in each electrolyte solution, demonstrating that the R_ct was drastically decreased with decreasing the pH of electrolyte solution, namely, the depassivation on steel rebar. The corrosion conditions of steel rebar were categorized as “passive state”, “depassivation”, and “corrosion” based on the changes of monitoring of R_ct and pH in each electrolyte solution, and these results were plotted on the two dimensional diagram composed of chloride ion concentration and pH of electrolyte solution. The conditions for depassivation of passive film on steel rebar were discussed.