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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.

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The objective of the study is to correlate the effects of H<sub>2</sub>S, and Cl<sup>-</sup> concentration on the passivity limits and the onset of localized corrosion, in UNS S31603 stainless steel by evaluating the changes in the semiconducting behavior exhibited by the passive layer. The study is accomplished experimentally by using a combination of direct and alternate current electrochemical methods, to study the passive layer formed by the stainless steel, in equilibrium with a gas phase at 2.8 MPa (400 psi) containing up to 60% mol of H<sub>2</sub>S (bal. CO<sub>2</sub>) at 25○</sup>C. The results obtained using the Mott-Schottky analysis indicate that the decrease of the passive layer stability formed on the UNS S31603 stainless steel is consistent with the increase in the electron donor carrier density. This is observed as the consequence of the effect of Cl<sup>-</sup> and H<sub>2</sub>S. In this context, the Cl<sup>-</sup> content in the brine was found to exert a larger effect than the H<sub>2</sub>S activity. The correlation with the evaluation of the passive layer using the Point Defect model suggest that both the polarizability and the rate of annihilation of the cation vacancies at the metal / film interface increase with the H<sub>2</sub>S content in the environment. This behavior can explain the increased content of Cl<sup>-</sup> and sulfides as main electron donor species, also the observable increase in the passive layer susceptibility to both stable and metastable pitting.