Degradation of Epoxy Coatings on Phosphatized Zinc-Electroplated Steel

The corrosion behavior of phosphatized zinc (Zn)-electroplated steel coated with epoxy films of different thicknesses was studied using electrochemical impedance spectroscopy (EIS), the breakpoint frequency method, potentiodynamic measurements, and the faradaic distortion method. The trends with time of the coatings’ electrical properties (resistance and capacitance) and of the corrosion current were recorded. Coated samples were immersed in 5% sodium chloride (NaCl) in distilled water. To study the delamination tendency of the epoxy coatings, a small hole of 0.1 mm diam was drilled through the coatings to the metal-polymer interface. Comparison of the methods to evaluate the area of the defect in the organic coating and to establish the substrate area in contact with the electrolyte showed the breakpoint method failed to provide accurate information during a long initial period.