Electrochemical impedance spectroscopy (EIS) and direct current method (ASTM G8-96) have been utilized to characterize and monitor four types of polyamide-cured epoxy coatings including different extenders: CaCO₃, silica, clay, and talc. To investigate the corrosion phenomena of organic coatings on metal, SS 41 steel with epoxy coatings of thickness of 220 μm was prepared for the heavy duty industrial paint. The impedance variation and adhesion at the coating/steel interface were measured during 30 days exposure in 3 wt% NaCl. The degree of water absorption and the onset of the corrosion process at the interface were estimated from EIS parameters by analyzing EIS data using a conventional equivalent circuit model for organic coated metal substrate. In impedance and direct current methods, the decreasing order of corrosion resistance of the organic coating was clay > talc > silica > CaCO₃. Nearly constant impedance values for clay and talc, during the 30 days, were can be related to the plate-like structure of extender that contributed to the low moisture permeability due to the compliated diffusion path. It is shown that EIS technique can be use ful in obtaining information on the corrosion resistance and delamination at interface between metal and epoxy coatings including the different extenders.