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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...