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The service life of an organic coating on a metal substrate is determined by variables associated with the environment, the coating, the substrate, and the interphasial chemistry that develops between the substrate and the coating. Although our basic understanding of the corrosion and materials issues within each of these regions is increasing, our understanding of the interactions among them and how these interactions lead to failure is limited. This gap in understanding is, in part, associated with the need to understand the origins of failure. Because corrosion failures on coated alloys initiate and grow from a local site, it is essential to know whether heterogeneities within the coating material, substrate alloy, or both are controlling the initiation of corrosion events. An understanding of the localized breakdown events of a coated alloy substrate will require a description of the local chemical and electrochemical events that take place within the coating and at the polymer/metal interface.Whereas the characterization of local electrochemical events on bare metal surfaces has been widely investigated, 1-7 the examination of local events on coated alloys has been less extensive. [8][9][10][11][12] It has been only recently that investigations of local defects of coated alloys has moved from the characterization of extrinsic (i.e., man-made) defects to the characterization of intrinsic 10 (i.e., naturally occurring) heterogeneities. The use of local electrochemical impedance methods has enabled the identification and monitoring of local breakdown processes of coatings on certain alloys. However, a more detailed understanding of defect origins and growth characteristics requires information on substrate microstructure and surface chemistry, as well as the underfilm solution chemistry that develops at a local site.Prior research on the analysis of underfilm chemistry has been limited to either pH determinations [13][14][15] or to more detailed chemical analyses of blisters on coated steel under cathodic conditions. [16][17][18][19][20] Acquiring information on the chemistry within these late stage defects represents an important step in the evolution of coatings research. Nonetheless, it would be of greater benefit to be able to sample the solution chemistry of an underfilm corrosion site in its early stages of development. Capillary electrophoresis (CE) is a technique that has shown feasibility of sampling and analyzing extremely small volumes of solutions (e.g., ca. 30 nL) from occluded corrosion sites to speciate and quantify the ions present. 21-23 Its potential application to analyzing the underfilm chemistry beneath early stage blisters has been explored in the present study.This investigation was an initial venture into understanding the breakdown of organic coatings on an aerospace alloy, AA2024-T3, through the characterization of the local electrochemical and chemical changes at the local sites. Information on the location and electrochemical nature of these defects along with the interfacial chemist...

The use of advanced graphite fiber polymeric matrix composites in multimaterial assemblies which must maintain mechanical integrity in aqueous environments has initiated interest in the galvanic degradation of these materials. Of recent concern is the galvanic degradation of bismaleimide (BMI)/graphite fiber (GE) composites. This study has used electrochemical impedance spectroscopy to monitor material interfacial changes in an 8 ply, 00, unidirectional BMI-GF composite subjected to cathodic and anodic polarization in NaC1 solution, exposure to caustic solutions, and galvanic coupling with various metals in NaC1 solution (Al, Fe, Cu, Ti). Cathodic polarization was found to produce porous electrode behavior which was attributed to breakdown of the fiber/matrix interface and subsequent moisture ingress. This is in contrast to caustic exposure which did not show a porous electrode response, presumably due to general BMI solvation. Results indicate that reaction intermediates generated during the oxygen reduction reaction, e.g., peroxide and superoxide radicals, and not OH-ions are the key damaging species. This will have important implications on the development of more damage resistant polymer chemistries. Couples to titanium, previously believed to be a benign couple, caused changes in the impedance spectra similar to those for low cathodic overpotentials. The long-term implications of these changes on composite durability are not known at this time, and warrant further evaluation. InfroductionThe high specific strength, specific modulus, and toughness of graphite fiber-polymer matrix composites (GFPMC) have made this material an important, if not optimum, candidate for certain structural applications in the aerospace, marine, and automotive industries. The increased use of PMCs as critical structural components

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