This work investigated the protection of mechanism of organic coatings on steel exposed to 3% sodium chloride solution at 50oC, coupled with the use of electrochemical impedance spectroscopy (EIS) to monitor progress of corrosion and degradation of coating. Unlike Walter[1], EIS measurement was conducted at 50oC as well as after cooling, and measurements at intermediate temperatures have been used to characterize the dependence of the process involved.
The proposition that corrosion rate is controlled by the ionic resistance of an organic coating has been tested. EIS results were fitted to a model circuit and changes in the film resistance and charge-transfer resistance with temperature were analyzed to deduce activation energies for the processes involved. Surprisingly, the calculated activation energy for coating resistance is significantly lower than the activation energy for the charge transfer resistance. This suggests that ion conduction in the coating, as apparent in an AC measurement, cannot be controlling the corrosion rate.
Potentiostatic pulse tests on coated metal enable iR-corrected polarization curves to be plotted at different temperatures. From this, the activation energy determined from the corrosion currents also higher matches the higher activation energy value calculated from the charge transfer resistance. However, measurements of coating resistance on free films of the same coating also generate higher activation energy values, leaving two possible models that can account for the results.
[1] G.W. Walter, Corrosion Science, 32, 10, 1085
