Electrochemical impedance spectroscopy (EIS) is often used to obtain corrosion-related information when knowledge of the corrosion mechanism and the environment is incomplete. This dilemma often forces the corrosion practitioner to resort to phenomenological circuit models to make the required prediction from the experimental information. The recent proliferation of “all-purpose” computer programs that can be used to estimate values for linear circuit analogues from the spectra is making such modeling a routine exercise.1-4 The tendency might be to assume that good agreement between the measured impedance spectrum and that calculated from the modeling circuit means that the model used is the best representation of the corrosion mechanism and provides an explanation for it. However, one cannot assume the uniqueness of a circuit model merely on the basis of a good fit to the observed spectrum.
Impedance spectra having two relaxation time constants are commonly encountered when examining corrosion under organic...
