Three different coating conditions influencing the quality of silicon polyester galvanized steel system were investigated by means of EIS. The first aim was to investigate the influence of different types of pores or discontinuities on the corrosion resistance of the above-mentioned coating system. A second aim was to study the influence of mechanical deformation find damage of the coating on the corrosion behaviour of the coating system. Finally, degradation due to ultraviolet exposure was investigated. In each of these investigations it was possible to distinguish different time constants for different electrochemical events in the Bode phase diagram. Since the difference in magnitude of the values of the resistances was large, they could not be distinguished in the Nyquist plots. Therefore, this type of plot was not used in this work. From the build-up of the coating system and the number of time constants deduced from the EIS measurements, it was possible to model the system. From the model, the physical find electrochemical phenomena occurring during the service life of an imperfect organic coating system was explained.