EIS of cathodically deposited wet paint films prior to the stoving process

Cathodic deposition of paint is industrially exploited for the application of the polymer primer to metal mass ware surfaces. EIS was systematically employed for the first time for the characterization of the wet paint film prior to stoving on Al and Fe. A distinct advantage is the lower impedance in comparison to stoved paints. The Bode diagrams show two well-separated processes in most cases. The low frequency part is attributed to the electrochemical impedance at the metal surface. A low double layer capacity of 0.1-1 mu F/cm(-2) was found due to the very low ion concentration. The charge transfer resistance was as high as about 1-5 M Ohm cm(2). The high frequency part is due to the layer impedance. Interlayers such as AlOOH from cathodic corrosion of Al and lead deposits from lead silicate in the pigment systems are the origin of characteristic features in the impedance spectra. Surface treatments as zinc phosphatation are also of some influence on the EIS results. A quantitative fitting of the Bode plots was possible after the introduction of constant phase elements (cpe).