AN APPLICATION OF ELECTROCHEMICAL IMPEDANCE SPECTROSCOPY TO ATMOSPHERIC CORROSION STUDY

An electrochemical impedance spectroscopy (EIS) technique has been applied to estimate the corrosion rates of metals covered with a thin electrolyte layer. A two electrode cell system, which consists of a pair of identical metal electrodes embedded in parallel in epoxy resin, was used for measuring the corrosion rates. The impedance measurements for Type 304 stainless steels covered with an NaCl solution layer and ordinary carbon steels with an H2SO4 solution layer, 10-1000 mu m in thickness, were carried out in the frequency range of 10 mHz-10 kHz to determine the equivalent circuit of a metal-thin electrolyte layer interface and the influence of current (potential) distribution over the electrode surface on the EIS data. It was found that the obtained EIS data can be described by a transmission Line (TML) model, in which the current distribution over the electrode surface is considered. The analysis of current distribution based on a TML model demonstrated that ELS data give information on the current distribution under the application of AC voltage. If the phase shift theta goes further than -45 degrees on a plot of theta vs log f(f-frequency) the current distribution becomes uniform at least in the low frequency limit, and consequently an accurate corrosion rate can be determined from the obtained polarization resistance. The impedance of copper covered with an Na2SO4 solution was measured to investigate the influence of the thickness of the electrolyte layer on the atmospheric corrosion rate. For copper covered with a neutral Na2SO4 solution of pH 6, the corrosion rate was independent of the thickness at least down to the thickness of 10 mu m, whereas the corrosion rate of copper covered with acidic solutions of pH 3 indicated a maximum at a thickness of several 10s of micrometers.