Assessment of organic coating degradation via local impedance imaging

The paper presents a new approach to organic coating condition evaluation at micrometer scale using localized impedance measurements. It is based on atomic force microscopy (AFM) in contact mode. Impedance is measured between conductive AFM tip and metal substrate covered with organic coating. A single-frequency voltage perturbation signal is applied between the electrodes and current response signal is registered. As the tip is scanned over the surface of the specimen a localized impedance characteristics of the material is obtained. In this way it is possible to map impedance of the scanned area along with other surface features available via classical AFM measurements such as height profile.Degradation of thin acrylic coating was induced by exposure to 3% sodium chloride solution. Localized impedance measurements were performed periodically along with classical ones carried out on macroscopic scale for comparison. Localized impedance data revealed good correlation with the height profile of the coating at each stage of the investigation. Spots of coating degradation were identified and localized. Their presence was confirmed by DC scanning spreading resistance measurements. The proposed method allows to overcome a shortage of the classical approach by provision of spatially resolved image of coating condition instead of an averaged one.