| Anodic dissolution and passivation processes are important phenomena in electrochemical engineering, corrosion science, and the battery industry. The zinc/KOH system was chosen as a model system to investigate anodic dissolution and passivation processes. Electrochemical impedance spectroscopy, cyclic voltammetry, and chronocoulometry were used to investigate the system.; The essential features of the current-potential curve of zinc in 1 M KOH electrolyte were simulated based on a postulated reaction mechanism. The mathematical expressions of the electrochemical impedance were derived by simultaneously solving mass balance equations, diffusion equations, the charge balance equation, and Taylor expansions of the reaction rates. A general expression of the electrochemical impedance of the zinc/KOH system in the initial dissolution region was derived. In the passive region, with high rotation rates, the reaction rate was limited by mass transport across the passive film. A mathematical expression of the electrochemical impedance under these conditions was presented. This expression is the sum of charge transfer resistance and diffusion resistance across the passive film with a finite diffusion layer thickness. The film thickness rather than Nernst diffusion layer thickness was used in modeling and simulation of the electrochemical impedance spectra of the zinc/KOH system in the passive region. The essential features of the electrochemical impedance spectra of zinc in 1 M KOH electrolyte were simulated based on the reaction mechanism proposed by Prentice and Chang. Effects of mass transfer on the electrochemical impedance in the initial dissolution region were explained qualitatively in terms of the proposed model. Tafel slopes and flat-band potentials obtained agree with those in the literature. An algorithm to check the consistency of the electrochemical impedance data and to calculate polarization resistance with Kramers-Kronig transforms was presented.; The film thickness of the zinc/KOH system was measured with chronocoulometry. The estimated film thicknesses vary with the experimental conditions and are in general about 500 A. Film thickness values determined by chronocoulometry agree with those obtained with ellipsometry. |
