EIS study of polyurethanes coatings on galvanized sheet by UV ageing

The aim of this study is to study the protection performance of polyurethanes coatings on galvanized sheets with electrochemical impedance spectra (EIS) and scanning electron microscopy (SEM) for UV ageing to simulate nature environment. The results indicated that impedance of the coatings acted as a capacitive character at the early UV ageing stage. The diffusion impedance was shown in low-frequency range, which was attributed to the self-healing effect of the chromate layer to the metal interface. After 2000 hours of ageing in the UV chamber, there is a low impedance values in the impedance spectra. Taking into account defects were found on the film surface on SEM picture, it is easy that the outside corrosive substances meet metal substrate through the coating at the defects.