Electrochemical Impedance Spectroscopy as a Method to Predict Delamination of Coated Steel in Cathodic Disbondment Tests

A study was undertaken using electrochemical impedance spectroscopy to measure delamination and compare that measurement, which can be performed on the sample repeatedly, with the accepted method of cathodic disbondment (CD) testing, the major drawback of which is that this measurement can only be done once on the sample; in reaching this goal, EIS could be used to predict delamination based on electrochemical values and the relative effects of temperature and pH under these same conditions could also be predicted. Poor prediction capabilities and the variety of testing parameters are the problems with the present cathodic disbondment test method; no analytical correlation of these factors has been reported. These factors will be addressed in this document. Epoxy coated samples were run at a temperature of 65 OC at -3V vs. SCE polarization in a 3 wt% sodium chloride solution for a period of 20 days. Based on relatively constant increases in capacitance with delamination, a correlation between capacitance and delamination was established. Capacitance values were then estimated for times greater than 4 days; using these capacitance values, predicted delamination values were obtained and compared to observed delaminations. These values are comparable between Day 6 and Day 14. At Day 18 and Day 20, delamination predictions differ from measured values by approximately 35%; however this error decreases when the variation in observed delamination is considered. Therefore, it can be concluded EIS can be used as an effective prediction tool for delamination. Temperature and pH effects on delamination were also studied to determine if predictions could be made through scaling factors for various temperatures and pHs. Although the temperature observations show a good fit to an Arhennius behavior, the calculated activation energy of 330 Joules/mole is far lower then expected for a chemical reaction. This most probably indicates that a more complex delamination mechanism than the one considered in this study exists. A 2.5 fold decrease in delamination was observed in samples tested at an electrolyte pH of 1 when compared to the ambient electrolyte pH of 8. Because the electrochemistry drives an increase in the local pH, predictions in delamination can not be made. However, this study has shown that delamination is reproducible at pH 1 and 8 Thesis Supervisor: Dr. Ronald M. Latanision Title: Professor of Materials Science and Engineering