Evaluation Methods for Degradation and Adhesiveness of Coatings using Microscopic Photo-acoustic (MPA) Method

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New evaluation technique for degradation and adhesiveness of coatings on steels have been developed based on photo acoustic method and AC impedance method. Chopped infrared laser light or visible Ar laser light was led into a microscope focusing on a sample, then photo-acoustic signal was detected by piezo polymer sensor on the other side of the sample. The new instrument, based on microscopic photo-acoustic (MPA) method, indicates a map which denotes degraded area or less adhesive area of coatings.Using MPA method, degradation of organic and TiN ceramics coatings on a mild steel and a stainless steel were clearly detected in a 5 to 10 mum spatial resolution. It is found that for the organic coatings the sensitivity of MPA signal depends upon the corrosivity of the substrate. On the other hand, under film corrosion is detected for the ceramics coated steels. Wave length of the laser light has an influence on the detection of the corrosion products and under film corrosion. This effect is interpreted by the difference in the absorbency and reflectivity of the laser light for coatings and corrosion products.AC impedance analysis on coated steels with an artificial defects indicated that the current distribution has to be considered especially for narrow gap delamination of coatings. For very narrow gap between the coating film and the substrate, the electrical equivalent circuit is simulated by a transmission line model. Experimental results from impedance method are simulated by curve fitting method, and the area of the degradation is estimated from the values of cuircuite elements by fitting.