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

Electrochemical Impedance Spectroscopy (EIS) has been widely used in t he study of corrosion processes and data interpretation is often via s imple equivalent circuits. However, such circuits often can not be use d to model real EIS data obtained with film covered or corroded steel electrodes in near-neutral solutions, especially when interrelation be tween 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 cl ear evidence of mass transport through a porous layer. In addition, d. c. measurements show very low values of the steady state diffusion cu rrent, a significant effect of rotation speed of the RDE on the anodic reaction and evidence that the layer properties depend on the stirrin g conditions. Analysis of the impedance spectra with a general model p reviously proposed provides evidence that due to film-limited mass tra nsfer the anodic reaction becomes reversible.