Electron transfer reactions on passive iron electrodes

The mechanism of electron transfer reactions (ETRs) on passive iron electrodes is studied using the redox system [Fe(CN)₆]^4-/ [Fe(CN)₆]^3- in a borate buffer at pH 8.4. Oxide films are prepared by potentiostatic polarisation at constant time, t_ox=300 s in a potential range, U_ox=0.45-1.4 V (SHE). The film stoichiometry is evaluated from XPS measurements. The current densities of ETRs are measured at constant film thickness in the potential range from 0 to 1 V with sweep rates up to 5 V/s. The constancy of the film properties is verified by cyclic voltammetry in redox free electrolyte before and after the ETR measurements. During all measurements the electrode capacity is recorded simultaneously. Current potential curves show various regions which are discussed separately. On the cathodic side a small influence of the film thickness is observed and the potential dependence of the ETR can be explained by a rate determining tunnel process from the lowest part of the conduction band of the film to the redox system in the electrolyte. On the anodic side the strong decrease of the current density with increasing film thickness in an inter- mediate potential range can be explained by a rate-determining Poole-Frenkel effect involving localized states within the film. For thick films at high potentials a contribution of low-lying states is indicated.