The Use of Electrochemistry and Ellipsometry for Identifying and Evaluating Corrosion on Aircraft

Electrochemical corrosion testing using AC impedance measurement and ellipsometry, has been performed on aircraft aluminum 2024-T3 samples in order to assess organic coating degradation, as part of an Air Force Office of Scientific Research (AFOSR) SBIR Phase I project. The AC Impedance technique has been used by a number of investigators to assess coating deterioration. The AC Impedance technique was used to acquire a precise low-frequency impedance signature which is currently being used to develop a simple accurate sensor for the early detection and measurement of corrosion processes on aircraft. Laboratory tests show that the painted aircraft aluminum 2024-T3 does degrade with a distinct signature having well defined regions of water uptake, incubation, and intense corrosion activity. The use of ellipsometry and DC electrochemical techniques have verified that the low-frequency impedance spectrum obtained from the AC Impedance technique is representative of the corrosion process for painted aircraft aluminum 2024-T3. X-ray Photoelectron Spectroscopy was used to characterize the constituents of the in ethane topcoat of the test samples. Based on these findings, several versions of prototype electrode sensors were developed to obtain this signature utilizing a two-electrode approach. Our two-electrode approach eliminates the need for an electrolyte used in conventional three-electrode AC Impedance testing and enables in-situ analyses of aircraft metal/coating systems. These in-situ electrode sensors were extensively tested using AC Impedance, electrochemistry, and ellipsometry. Results indicate the ability of the electrode sensors to monitor the corrosion process on aircraft aluminum 2024-T3 and to accurately indicate the extent of concision. In addition, the sensor has the ability to withstand harsh operating environments. This paper discusses the design and testing of the in-situ electrode sensors.