THE ELECTROCHEMICAL IMPEDANCE RESPONSE OF FILM-COVERED MILD-STEEL IN NEUTRAL AERATED SOLUTIONS

Electrochemical Impedance Spectroscopy (EIS) has been widely used in the study of corrosion processes and data interpretation is often via simple equivalent circuits. However, such circuits often can not be used to model real EIS data obtained with film covered or corroded steel electrodes in near-neutral solutions, especially when interrelation between the various components of the equivalent circuit is allowed for. Electrochemical impedance spectra have been obtained at the corrosion potential for a mild steel rotating disc electrode (RDE) in chloride solutions with addition of zinc-polyphosphate inhibitor, which show clear evidence of mass transport through a porous layer. In addition, d. c. measurements show very low values of the steady state diffusion current, a significant effect of rotation speed of the RDE on the anodic reaction and evidence that the layer properties depend on the stirring conditions. Analysis of the impedance spectra with a general model previously proposed provides evidence that due to film-limited mass transfer the anodic reaction becomes reversible.