Abstract
Electrochemical monitoring of corrosion rate is usually carried out by continuous measurement of polarization resistance, the reciprocal of which is theoretically proportional to the corrosion current density in some cases. In Ac impedance method, which is often employed for corrosion monitoring, impedance at low frequency limit is defined as the polarization resistance. Physical meanings of the polarization resistance depend on the corrosion mechanism. In the first half of this paper, the impedance characteristics of three different corrosion systems under charge transfer control, under diffusion control and under mixed charge transfer-diffusion control and the physical meanings of polarization resistance obtained from the low frequency limit are described.
In the monitoring of atmospheric corrosion, problems of high ohmic drop and uneven current distribution over working electrode are often encountered because it proceeds under a very thin electrolyte layer. Electrochemical impedance spectroscopy (EIS) is very effective in the atmospheric corrosion monitoring. Because the solution resistance can be determined from high frequency impedance and the ohmic drop can be compensated by subtracting the solution resistance from polarization resistance obtained from low frequency limit. The EIS enable us to clarify how degree the applied voltage (response current) distributes over working electrode. The details of impedance characteristics under a thin electrolyte layer are interpreted in the latter half of this paper.
