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While electrochemical impedance spectroscopy (EIS) can be a rapid and accurate method for measuring corrosion rates, the difficulties in interpreting EIS data points have overshadowed the numerous advantages offered by this technique. Since the charge transfer conductance across a corroding interface is proportional to the corrosion rate, it is the prime parameter sought by all electrochemical techniques. The simplest model representing such an interface must also include a capacitance that describes the capacitive charge acceptance of any metallic surface exposed to an electrolyte. The validation of this RC equivalent circuit model can be achieved by permuting the three data points selected to project the center of a circle in a Nyquist representation. This paper reviews some of the applications of the permutation technique that have been recently published and indicate how some of the apparent limitations of the technique can serve to reveal important information on the mechanistic behavior of a corroding interface. These studies of various alloys exposed to aqueous environments indicate that the non-adherence of the EIS measurements to a perfect RC model is a rich source of information concerning the types of corrosion processes occurring on a metallic surface.
electrochemical impedance spectroscopy (EIS), interface modeling, permutation technique, corrosion monitoring
Associate professor, Royal Military College, Kingston, Ontario