Interpretation of EIS data on passive steel surfaces in aqueous sulfuric acid solution in terms of carrier migration and recombination

A model is proposed for the description of the interplay of charge carrier migration, effected by external voltage and combination within very thin passive oxide layers. This theory is used to describe the electrochemical impedance spectra of passive stainless steel in sulfuric acid, a prototype of such surfaces. A simple Rs−Zmr||RpRs−Zmr||Rp circuit is considered with Zmr accounting for dispersion and Rp for the Faraday process. Zmr is based on a continuous time random walk (CTRW) expression and it is seen to be independent of voltage. Rp shows a strong voltage dependence which can be understood in the negative voltage range as a Butler-Volmer-Erdey-Grúz process, whereas in the positive range it can be described in terms of tunneling across the oxide. Curve fitting results in parameters characteristic to oxide structure and process kinetics. All fitted quantities are seen to be realistic with the exception of charge carrier density which turned out to be much too high, indicating that mobility might not obey the classical, frequency-independent law.