EIS investigation of cataphoretically electrodeposited epoxy coatings having different EEWs

Abstract

Epoxy based coatings having different EEWs (i.e. 400–450 and 850–950) were prepared by the reaction of epoxy resins with hexa methoxy methyl melamine. These were then electrodeposited at constant voltage on to steel substrates giving 20 μm thickness. The corrosion behavior of these coatings were investigated by the use of electrochemical impedance spectroscopy (EIS) during a period of 35 days exposure to a solution of 5 wt% NaCl in distilled water. Coating’s parameters such as the pore resistance and the coating’s capacitance measured by ac impedance technique showed three distinct stages of mechanistic corrosion behavior during the said exposure time. The first stage could be attributed to the absorption of water by the coating. The second stage could be related to the formation of pathways for the passage of electrolyte to the vicinity of the substrate. The third and the final stage could be envisaged as the start of the electrochemical corrosion reactions. The higher corrosion protection achieved by the coatings having higher EEWs could be attributed to the higher cross-linking densities obtained in such films. DSC studies were in complete agreement with the EIS results and confirmed conclusions reached.