Theory of impedance for initial corrosion of metals under a thin electrolyte layer: a coupled charge transfer-diffusion model

Abstract: An impedance model for a metal surface corroding under a thin electrolyte layer is presented. The model describes the oxygen diffusion in the electrolyte, the cathodic current via the oxygen reduction reaction (ORR) reaction and the anodic current via metal dissolution reaction (MDR) at the metal/electrolyte interface under the pseudo-steady approximation. The results for the impedance are obtained in terms of the thickness of electrolyte, the diffusion coefficient of oxygen, the concentration of dissolved oxygen and the anodic/cathodic reaction rates. The impedance characteristic are analysed through Bode and Nyquist plots which unveils six distinctive frequency regimes viz., (i) purely oxygen diffusion controlled regime, (ii) electrolyte film thickness controlled regime, (iii) activation controlled regime, (iv) mixed diffusion-kinetic controlled regime, (v) capacitive electric double layer controlled regime and (vi) solution Ohmic controlled regime. The impedance response shows two asymmetrical depressed arc on the Nyquist plots indicating the Faradaic charge transfer controlled regime and purely electrolyte thickness diffusion controlled regime with an intervening straight Warburg line. The arc at low frequencies is strongly dependent on the concentration and diffusion coefficient of dissolved oxygen indicative of pseudo-steady state behaviour of interface whereas the high frequency arc represents Faradaic regimes due to MDR and ORR which is indicative of the dynamic nature of corrosion reaction rates at the interface. At thick electrolyte layer, the interface shows a mass transport controlled kinetic regime with a finite length Warburg type impedance whereas at thin electrolyte layer the interface is activation controlled with a finite diffusion Randles type impedance response. A comparison of the model with the experimental data of corroding metal shows reasonable agreement.