Real time mapping of corrosion activity under coatings

Current accelerated testing of aircraft coating systems for corrosion prote ction relies heavily on salt spray methods. Electrochemical techniques such as electrochemical impedance spectroscopy GETS) and electrochemical noise methods (ENM) provide insight into the global properties of a coating syste m, and bath techniques are being used on a limited basis. However, there is a need to investigate corrosion events with greater spatial resolution und er coatings at the metal/coating interface. Such corrosion activity may be related to coating defects and variations in the surface chemistry of the u nderlying metal. The scanning vibrating electrode technique (SVET) has, been developed to al low high spatial resolution investigation of localized corrosion activity t hat may be associated with coating defects or galvanic coupled regions of t he metal surface. The SVET offers high resolution in current measurements o f the order of 0.5 muA/cm(2) and is able to detect in-situ initiation and p rogress of corrosion activity under a protective coating. Using the SVET, m inute variations in d.c. current associated with localized corrosion activi ty are detected and used to map both anodic and cathodic corrosion activiti es in a localized area. The difference in initial corrosion activity under various coatings can be correlated to the performance Life of the coatings. The application of SVET to aircraft coatings and corrosion is reported to demonstrate the utility of this important new electrochemical tool. In the current study, the SVET was used to discriminate the corrosion prote ction performance of selected sol-gel based coating systems. Sol-gel based surface treatments are being developed as part of an environmentally compli ant coating system alternative to the currently used chromate-based systems , The SVET results are compared with data obtained from chromium inhibition coating systems. The SVET analyses are compared with electrochemical imped ance measurements. The comparison of such data will provide the basis to ad opt SVET measurements as an early performance discriminator for newly devel oped coating systems. (C) 2001 Elsevier Science B.V. All rights reserved.