The impedance characteristics of protective corrosion films on carbon steel in an alkaline sour medium

A physical model describing the impedance response of iron deficient (Fe_1-xS) iron sulfide films in alkaline sour solutions has been developed. It assumes that the faradaic impedance of the anodic reaction was dominated by the transport of cation vacancies, which are created at the film/solution interface and annihilated at the metal/film interface. Simulations showed that the model is sensitive, in the typical frequency range used in EIS measurements, to variations in the values of the diffusion coefficient of cation vacancies and the rate constants of the reaction at the metal/film interface. A typical impedance diagram for Fe_xS_y films in a 0.1 M (NH₄)₂S, 10 ppm CN^- solutions was compared to a calculated diagram using the model equations. Qualitatively, there is a good agreement between the model and experimental data, and the parameters make physical sense.