Understanding the effects of applied cathodic protection potential and environmental conditions on the rate of cathodic disbondment of coatings by means of local electrochemical measurements on a multi-electrode array

The effect of cathodic protection (CP) potential on cathodic disbondment of a defective coating has been investigated by monitoring the coating disbondment processes during the exposure of coated electrodes to a corrosive solution. The monitoring of coating disbondment was achieved by in situ measurement of local electrochemical impedance and direct current distributions over a multi-electrode array surface under various levels of CP potentials as well as under open circuit potential (OCP). The effects of CP potential and the environmental conditions on the initiation and propagation of coating disbondment have been quantified based on changes in local electrochemical impedance and current distribution maps. The rates of cathodic disbondment have been found to be highly dependent upon the applied CP potential. At OCP and low CP potentials of −760 mVAg/AgCl and −950 mVAg/AgCl, little or only slight coating disbondment occurred. When the CP potential reached hydrogen evolution potential levels, such as −1100, −1200 mVAg/AgCl, and −1400 mVAg/AgCl, the rates of disbondment increased significantly. This result indicates that the evolution of hydrogen likely played an important role in accelerating cathodic disbondment of the coating, and that the 'critical potential' at which the hydrogen evolution becomes a significant and dominant electrochemical reaction is important for cathodic disbondment of coatings. It was also found that once the cathodic disbondment initiated under very negative CP potentials, the disbonded distance would spread at an almost constant rate. These results suggest that a practical method of controlling coating disbondment is to ensure that the applied CP potential is below the 'critical potential'.