Acoustic Emission Measurements to Evaluate the Degradation of Coating Films

Two types of acoustic emission (AE) have been observed during cathodic degradation of coating films. Under a cathodic polarization of very low overpotential, at which no hydrogen gas evolved, intermittent AE signals, with very long intervals, were observed. The ratio of ring down counts to AE events have sharp peaks at 10 and 20 for the cathodic delamination of coating films, and synchronized AE events with very small current spikes, several tens nA, were observed. This type of AE measured should be responsible for a chemical and/or physical breaking of the bond between the metal and the film. Therefore, the early stage evaluation of the coating films will be performed by this type of experiment. The second type of AE was measured under the higher overpotential. The AE events were counted continuously and the ratio of the ring down counts to AE events scattered more widely. The formation of hydrogen gas bubbles caused this type of AE. Thus, it is possible to detect the existence of the water layers or of the small blisters under the film by a cathodic polarization of coated steels and simultaneous AE measurement.