Mechanistic study of coating failure using local electrochemical methods

Although organic coatings play an important role in metallic corrosion protection, currently most of their applications are based mainly on empirical practice. The purpose of the current research is to identify the reasons behind organic coating degradation, and subsequent substrate corrosion in the absence of any physical coating defects. The studied system is a simplified epoxy-phenolic food can coating, deposited on common can substrates. Combining three powerful electrochemical techniques in the present work, has produced some insight to the changes taking place in the coating itself and also in the local environment before corrosion initiation. Electrochemical Impedance Spectroscopy (EIS) investigations have suggested that fundamental coating properties, such as coating capacitance can be used to monitor water absorption with time until failure. The coating thickness and the curing also appear to correlate well with these measurements. The coating barrier properties are strongly dependent on the coating curing, even though the derived absorbed water volumes are not significantly affected. Concurrent Local Electrochemical Impedance Spectroscopy (LEIS) measurements exhibited uniform impedance reduction with immersion, due to water absorption. The development of superficial coating swelling defects did not show substantial evidence of decreased coating performance, until a direct current connection was established with the metallic substrate. Last but not least, the Scanning Vibrating Electrode Technique (SVET) was employed, before and after corrosion initiation, to detect electrochemical activity in the electrolyte above the coating. Results under anodic polarisation, show localised anodic currents around the corrosion site, while cathodic activity spreads on the remaining coating surface. From these results it is believed that the formation of corrosion products blocks and partially protects the corrosion site.