USE OF ELECTROCHEMICAL IMPEDANCE SPECTROSCOPY FOR THE STUDY OF CORROSION PROTECTION BY POLYMER-COATINGS

A discussion of the requirements for hardware and software necessary for collection and analysis of electrochemical impedance spectroscopy data for polymer coated metals is presented. Most authors agree that a simple model can describe the frequency dependence of impedance spectra for polymer coated metals exposed to corrosive environments. The water uptake of the coating can be estimated from the time dependence of the coating capacitance C-c. The pore resistance R(po) depends both on the resistivity p of the coating and the disbonded area A(d). The polarization resistance R(p) of the corroding area under the coating and the corresponding capacitance C-dl both depend on A(d) The breakpoint frequency method is discussed in detail and the dependence of the breakpoint frequency f(b) on p and A(d) is derived. In addition to f(b) other parameters can be obtained which depend on the ratio A(d)/p or only on A(d) or P. Since these parameters can be obtained at frequencies exceeding 1 Hz without the need for an analysis of the impedance spectra in the entire frequency region, this approach is considered especially useful for corrosion monitoring. The concepts proposed for the analysis and interpretation of EIS data for polymer coated metals are illustrated using data for Al alloys, Mg and steel exposed to NaCl. For an alkyd coating on cold rolled steel the time dependence of A(d) and p during exposure to 0.5 M NaCl has been determined qualitatively using the modified breakpoint frequency method.