DEGRADATION OF EPOXY COATINGS ON PHOSPHATIZED ZINC-ELECTROPLATED STEEL

The corrosion behavior of phosphatized zinc (Zn)-electroplated steel c oated with epoxy films of different thicknesses was studied using elec trochemical impedance spectroscopy (EIS), the breakpoint frequency met hod, potentiodynamic measurements, and the faradaic distortion method The trends with time of the coatings' electrical properties (resistanc e and capacitance) and of the corrosion current were recorded Coated s amples 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 defec t in the organic coating and to establish the substrate area in contac t with the electrolyte showed the breakpoint method failed to provide accurate information during a long initial period.