Model Study Investigating the Role of Interfacial Factors in Electrochemical Impedance Spectroscopy Measurements

To observe the effect of interfacial factors on electrochemical impedance spectroscopy (EIS), model systems consisting of Parylene C (PC) film coated on Alclad 7075-T6 (UNS A97075) sheets and freestanding PC films were investigated. To modify the top surface of PC coating, the coating on Al alloy panels were treated with two plasmas: trimethylsilane (TMS) and a mixture of TMS + O₂. Plasma polymerization of TMS followed by hexafluoroethane (HFE) also was applied to the substrate surface prior to the deposition of PC film to improve the adhesion of the film to the surface. Coating performance was evaluated using EIS, and interfacial factors produced by plasma treatments were studied by applying suitable equivalent circuit models, which take into account all the interfacial parameters and bulk coating. Simulations performed with circuit models proposed in this work indicated that top surface modification with plasma treatment influenced the salt intrusion property of the top layer as well as of the bulk film itself; good adhesion of PC to the panel surface produced by the plasma pretreatment was reflected by the high resistance and low capacitance at the interface; and the reduction of the impedance modulus of a PC-coated Al alloy panel system with increased immersion time in salt solution was caused largely by microscopic scale delamination of the film from the panel surface.