ELECTROCHEMICAL IMPEDANCE SPECTROSCOPY ON THERMAL AGEING EVALUATION OF EPOXY COATING CONTAINING ZINC RICH PRIMER

Abstract

This research concentrates on the thermal ageing of a full 3-coat system with sacrificial pigment (zin c rich) primer on mild steel where the temperature dependence test is conducted to explore the correlation between the coating resi stances with the corrosion rate underneath the coating. Electrochemical impedance spectroscopy (EIS) is introduced over a range of te mperature to extract activation energies for the rate of controlling pro cesses in the corrosion reactions. It is expected t hat the zinc rich primer does not form a barrier coating for the metal substrate rath er it will be the most active component of the subs trate in the electrochemistry. Full 3-coat system with zinc rich primer show the extrac ted activation energy from coating resistance is si gnificantly lower than the activation energy extracted from the charge transfe r resistance. This suggested that the coating resis tance from EIS cannot be controlling the corrosion reaction. The activation energies generated for the corrosion process here (kJmol -1 ) are very much higher than those of ion transport through the coat ing (19‐37 kJmol -1 ) during early immersion. Further interesting findi ngs come from the activation energy trends over time particu larly for the corrosion process which shown that th e value is decreasing where at the end of exposure, the activation energy values f or coating and charge transfer resistance become qu ite similar. It is suggested that at this stage ion transport in the coating might be controlling the corrosion process unlike at the be ginning; the activation energy is getting smaller due to coating degradation.