Electrochemical Impedance Spectroscopy Optimization on Passive Metals

Through the imposition of various assumptions that have been validated using numerical analysis, a greatly simplified method has been developed for expressing the impedance of a passive metal upon the basis of the Point Defect Model. The approach has greatly decreased the dimensionality of the problem, by, for example, showing that the relaxation of the barrier layer thickness can be neglected. Clear criteria have been formulated for identifying the circumstances when the faradaic impedance of the barrier layer does not depend on frequency. In many practical systems, the last simplification can be used when the frequency of the applied, modulated voltage is greater than 10−3–10−2 Hz. Application of the revised model is illustrated by optimization of model parameters based upon experimental EIS data for carbon steel in borate buffer solutions, for nickel in chloride-containing solutions, and for Type 316 SS in H2S and CO2 environment. It is shown that, in the cases of Fe and Ni, the simplest equivalent electrical equivalent circuit (parallel resistance and capacitor) adequately describes the EIS behavior of passive metals, but in the case of Alloy 316 a constant phase element must be used instead of the capacitor in this equivalent circuit.