In situ study of coatings using embedded electrodes for ENM measurements
By Chidambaram, Devicharan; Clayton, Clive R. & Dorfman, Mitchell R.
Published in Progress in Organic Coatings 2003
After preliminary studies attempting to use electrochemical impedance spectroscopy (EIS) as a method for in situ measurement of the corrosion protective properties of coatings failed, the use of electrochemical noise methods (ENMs) was examined for this purpose. It has been found that a modified version of the electrode configuration that is most commonly used for ENM in the study of coatings involving embedded wires in the coating film allows one to perform in situ measurements of the electrochemical properties of the film. These continuous measurements of electrochemical properties of coatings in exposure are the very first of this type of measurement ever accomplished. The results of our preliminary studies from ENM in an embedded two-electrode configuration are presented. These studies examined the use of embedded electrodes in coatings on exposure in a cyclic salt fog test chamber (Prohesionâ„¢ cycle). The test panels under examination undergo exposures that involve changes in temperature, humidity and electrolyte spraying, and are thus a good emulation of exterior exposure. Results are presented for several types of films along with in situ temperature measurements on the test panels under study. An analysis of this data is given which indicates that coating film noise resistance, Rn, decreases as moisture penetrates the film during the â€œspray cycleâ€?, then rises again during the â€œdry cycleâ€? in a manner that lags the temperature cycle, while the â€œlocalization indexâ€? of corrosion as defined by Eden goes through a maximum at the end of the â€œdry cycleâ€?. This would seem to indicate the internal increase in electrolyte concentration in the film that occurs during drying is one of the key drivers of the transition from low rate general corrosion to the onset of localized corrosion. Implications of the results and the possible uses of the sensing system are also discussed.