Insights into protection mechanisms of organic coatings from thermal testing with EIS

The proposition that corrosion rate is limited by the ionic resistance of an organic coating has been tested. Mild steel panels coated with an epoxy-phenolic paint were exposed to 3% sodium chloride solution at 50 °C for different periods and characterized by electrochemical impedance spectroscopy (EIS) across a range of temperatures (25–50 °C). Changes in the film resistance and charge-transfer resistance with temperature were analysed to deduce activation energies for the processes involved. It was found that the calculated activation energy from coating resistance is significantly lower than the activation energy for the charge transfer resistance. This suggests that the ionic resistance of the coating, as apparent in an AC measurement, cannot be controlling the corrosion rate. Coating resistances for free films of the same coating show even higher activation energy values, so that the resistance of un-degraded areas of the coating within the current path could be controlling the current flow. Potentiostatic pulse tests on coated metal have enabled iR-corrected polarization curves to be plotted at different temperatures that gave high activation energies from the estimated corrosion currents. This provides two possible explanations that can account for the results.

► Effect of temperature on EIS response of painted steel has been measured.
► Activation energy for ion transport is different than activation energy for the corrosion process (from EIS).
► Therefore coating resistance as measured by EIS cannot be controlling the corrosion rate.
► Free film resistance (un-degraded coating) shows a higher activation energy.
► Kinetics of cathodic reaction beneath paint have been determined and might also be controlling the corrosion rate.