When measuring OCP over an inert thin film coating (e.g. silica on steel) most people measure a positive shift in OCP. These coatings are seldom defect-free and from our understanding the OCP over the coating should be equivalent to the OCP of the substrate. Why does the OCP change over a coating system?
Mr. Felix, I totally agree with you that in case of inert thin film coating and these coatings are not totally defects-free, as these defects like connected pores and cracks should considered as pits and crevices within the coating matrix which represents the easy path for corrosive species to reach the substrate.
The OCP Vs. exposure time is a real function used for ranking the coating performance of different coating systems, this means that OCP is relative value followed the changes in the ratio between the available area for the anodic and cathodic reactions in terms of exposure time.
At zero time the measured potential has positive shift compared with that of substrate, but this value decrease by many function trends over minutes, hours or days and in most cases decrease to the OCP of the substrate before coating indicating that the area available for anodic reaction through pores and crevices become equivalent in terms of reaction rate intensity to the area of whole substrate before coating. This explains why you observed the OCP of substrate in presence of coating film after a certain time. In this respect, this case may be not found or found in much less degree with inert, thick, dense, adherent, coherent and defect free coatings.
Regards
I agree, I usually perform a 40 min test in OCP, then a quick polarization between finla value minus 5mV and plus 5 mV, and then again, 10 min in OCP condition, the final stationary value is which I take for the corrosin potential to begin the polarization test.
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