Application of the scanning electrochemical microscope to the examination of organic coatings on metallic substrates

The application of scanning electrochemical microscopy (SECM) for corrosion studies of organic coatings on reactive metals is presented. In this work, the SECM was used to monitor the electrochemical processes at painted galvanized steel substrates immersed in 0.1 M NaCl aqueous solution. The coatings were investigated both as obtained and scribed, to simulate a defect across the coating, down to the metal-substrate surface. The SECM was operated under either the feedback or a variation of the sample generation/tip collection modes. In the first case the variations in the feedback current measured at the SECM-tip when scanned at constant height over the sample allowed imaging of the topography of the coated surface with high local resolution. In the second case, chemical species participating in the corrosion reactions at the sample are detected at the SECM-tip by applying appropriate potential values to the microelectrode. Local concentration variations could be assessed in this way.