Electrochemical impedance of anodic films formed on Fe 80B 20 glassy metal in borate buffered and acid sulfate solutions

Abstract

Anodic films formed by electrochemical passivation of Fe 80B 20 ribbons in borate-buffer and in acid sulfate solutions, were studied using impedance spectroscopy, in a broad range of frequencies, at potentials within a range of constant passive currents. The impedance data were analysed according to the general, multiparameter electrode admittance equation, which takes into account at the most, four relaxation processes. An insulator or n-type semiconductor barrier layer impedance determines the total electrode impedance of the passive film formed in the borate buffer solution. In acid sulfate solutions, the total electrode impedance is determined by both an ion-conducting inner layer and a precipitated outer layer. The impedance of the outer layer is characteristic for diffusion in a finite length region, coupled with an ion accumulation hindered kinetics at the inner/outer layer boundary at moderate frequencies, and modulation of the outer layer thickness at very low frequencies. In the borate buffer solution, there is negligible influence of boron on the corrosion rate. Films formed in acid sulfate solutions show a pronounced effect of boron, through formation of a more ion conductive inner layer. Higher corrosion rate of the substrate is caused by a higher dissolution rate of the anodic film.