An interpretation of electrical impedance diagrams for painted galvanized steel

Abstract The physical significance of various arcs which appear on impedance diagrams for painted galvanized steel exposed to an aggressive medium is discussed. These diagrams undergo various changes during exposure, i.e. when the paint coating is still intact the response is almost purely capacitive, but as the coating degrades the impedance value decreases and the plots exhibit two arcs. Measurements with detached films confirm the relationship between the single arc appearing in the impedance diagrams during early stages of paint degradation and the intrinsic paint properties. However, it is more difficult to find an interpretation for the two-arc diagrams. The hypothesis that the low-frequency arc arises from effects associated with the double-layer capacitance and charge-transfer control at the pore base leads to unacceptable conclusions. Various arguments seem to support the view that the low-frequency arc is determined by a finite diffusion impedance.