Electrochemical impedance spectroscopy of metals covered with a thin electrolyte layer

Abstract

The impedance behavior of metals covered with a thin electrolyte layer has been investigated. A transmission line (TML) circuit type behavior, which was attributed to an uneven current distribution profile over a working electrode, was observed in a wider frequency range as the thickness and the concentration of the electrolyte were decreased. It was found from the theoretical analysis based on the TML model that the current distribution profile becomes uniform at least in the low frequency limit when the phase shift θ goes further than −45 ° on a plot of θ vs. log f( f = frequency). In such a condition, an accurate corrosion rate can be determined from the obtained polarization resistance R p . The EIS has been applied to investigate the dependence of the atmospheric corrosion of stainless steel and of copper on the thickness of the electrolyte layer. The corrosion rate of stainless steel covered with an NaCl solution layer was independent of the thickness at least down to the thickness of 10 μm because it was controlled by the anodic process. The corrosion rate of copper covered with a neutral Na 2SO 4 solution was independent of the thickness at least down to 10 μm, whereas the copper covered with an acidic solution of pH 3 showed a maximum at a thickness of several tens of micrometers.