Modeling of electrochemical impedance data of a magnesium-rich primer

The application of Mg-rich primers (MRPs) for the protection of aluminum structures represents an attractive alternate to the environmentally unfriendly Cr-rich primers that are presently used. The protective modes of MRPs are similar to those of Zn-rich primers (ZRPs) on steel and include cathodic protection driven by the more active Mg particles compared to the Al substrate and a barrier-type protection due to the insulation of the substrate from the environment. Interpretation of ZRP electrochemical impedance spectroscopy (EIS) data has been accomplished using a transmission-line model that accounted for the contact impedance between the zinc particles, the impedance associated with the zinc dissolution, and the percolation resistance of the coating. EIS experiments results are presented here for an MRP on a gold substrate under immersion in dilute Harrison's solution. The data were analyzed using the measurement model technique to determine the consistency with respect to Kramer-Kronig relationships. The transmission-line model was used to analyze the data to demonstrate their applicability for analyzing the protection afforded by the MRP. Gold was used as a noble substrate and to promote that the electromotive degradation of the MRP as the mixed potential of an MPR/gold system was more positive than that of an MRP/Al system. (C) 2008 The Electrochemical Society.