Photo-Acoustic Spectroscopy (PAS) and Acoustic Emission (AE) to Evaluate the Degradation of Coated Steels

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Photo-acoustics (PA), acoustic emission (AE), and interfacial pedance of coated steels were independently measured to evaluate the degradation of coating films. The PA signals induced by chopped LASER light decreased when the light beam pointed the blisters and/or delaminated area. So, a two-dimensional detection of degradation was performed by the scanning LASER method. Due to bradk out of bonds between the metal and films, AE signals were observed at the very initial stage of degradation. The rate of AE ements during degradation was found as a reliable measure for the delamination resistivity of coatings. Spectrum analysis of AE revealed differences of AE signals caused by the degradation and a hydrogen evolution. The break point frequency on a Bode diagram of the interfacial impedance corresponded very well to the area of the degradation. The study showed that these methods should be combined to establish a new evaluation technique.