An impedance model for a metal surface corroding under a thin electrolyte layer is presented. Themodel 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 interfaceunder the pseudo-steady approximation. The results for the impedance are obtained in terms of the thicknessof electrolyte, the diffusion coefficient of oxygen, the concentration of dissolved oxygen and theanodic/cathodic reaction rates. The impedance characteristic are analysed through Bode and Nyquist plotswhich 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-kineticcontrolled regime, (v) capacitive electric double layer controlled regime and (vi) solution Ohmic controlledregime. The impedance response shows two asymmetrical depressed arc on the Nyquist plots indicating theFaradaic charge transfer controlled regime and purely electrolyte thickness diffusion controlled regime withan intervening straight Warburg line. The arc at low frequencies is strongly dependent on the concentrationand diffusion coefficient of dissolved oxygen indicative of pseudo-steady state behaviour of interface whereasthe high frequency arc represents Faradaic regimes due to MDR and ORR which is indicative of the dynamicnature of corrosion reaction rates at the interface. At thick electrolyte layer, the interface shows a masstransport controlled kinetic regime with a finite length Warburg type impedance whereas at thin electrolytelayer the interface is activation controlled with a finite diffusion Randles type impedance response. Acomparison of the model with the experimental data of corroding metal shows reasonable agreement.
Atmospheric corrosion of metal in contact with a thin electrolyte film via diffusion of dissolved oxygen coupled with oxygen reduction reaction and metal dissolution reaction in pseudosteady state.
Volume 134, 2022
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