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The polarization resistance and electrochemical impedance for a two-electrode cell consisting of a buried metallic structure and a small nonpolarizable disk electrode has been discussed considering the distribution of the signal current. Since interfacial impedance decreases with increasing ac frequency, the electrochemical impedances at high and low frequencies correspond to those for the primary and secondary current distributions respectively. The complex plane plot of the electrochemical impedance of a buried structure measured with the probe exhibits a semicircle or semi-ellipse depending on the structure, the intercept of which is not proportional on the real axis to the polarization resistance. The impedance diagrams for a buried structure in the presence of local corrosion deviate from those in the absence of local corrosion below a certain frequency, which is determined by the distance between the local corrosion site and the probe electrode. This threshold frequency serves as a reliable tool for detecting the location of the local corrosion of a buried structure.
buried structure, polarization resistance, primary current distribution, secondary current distribution, corrosion monitoring, measuring area, local corrosion, distributed parameter system, electrochemical impedance
Research manager, Electricity Technology Research and Development Center, Tohoku Electric Power Co., Inc., Miyagi,
President, Tokyo National College of Technology, Tokyo,