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...

Local electrochemical impedance mapping (LEIM) represents a potentially important tool in the characterization of discrete electrochemical phenomenon on heterogeneous surfaces, such as the degradation of a coated aluminum alloy. Since LEIM is an emerging tool, it is essential to resolve between real material and interfacial changes and a possible sampling artifact to the measured LEIM response. To this end, both analytical and numerical models were developed to calculate the field above an equipotential disk to examine possible LEIM measurement artifacts. Several features in LEI maps which could be the result of an artifact in the measured field above a corroding region were addressed. These included changes in the field due to the following: (i) edge effects, (ii) two closely spaced corroding sites affecting spatial resolution, (iii) a change in radius of a corroding site, including the ability of LEI to resolve small sites. Numerical analysis was found to more realistically model the finite dimensions and discrete change in potential that were used in actual LEIM experiments. The calculated field above the equipotential disk was then compared to experimental LEI maps of gold disk microelectrodes embedded in SiO2. Edge effects were found to be an unlikely source for experimental observations of reduced admittance. The spatial resolution of the LEI probe was predicted through theoretical modeling and determined experimentally to be sufficient to resolve two disk separated by 35 μm. The probe was also predicted and shown experimentally to have the capability to measure a disk of radius 17.5 μm. The successful demonstration of this numerical model will allow the exploration of more complicated material interfaces, such as materials coated with a dielectric layer. © 2003 The Electrochemical Society. All rights reserved.

Local electrochemical impedance mapping ͑LEIM͒ was used to investigate local underfilm corrosion occurring on organic-coated AA2024-T3 exposed to chloride solution. Films ͑5-20 m thick͒ of neat epoxy, polyurethane, and vinyl resins were studied. Frequently observed features in the LEI maps included: (i) an unexpected decrease in admittance in early stages of development, (ii) trenching at the periphery of admittance peaks, (iii) both increases and decreases in peak height over time, (iv) changes in peak width over time, (v) different amplitudes in admittance over different regions of multilobed blisters, and (vi) different admittance magnitudes for different types of defect. These LEIM features could provide significant insight into and documentation of the local breakdown processes of coated metals, if they truly represent electrochemical phenomena associated with changes at the interface. These features were observed on each of the coating chemistries studied. Proposed origins and supportive evidence of these LEIM features is presented. The corrosion protection provided by an organic-coated metal is determined by variables associated with the coating, substrate, service environment, and polymer/metal interface. In order to fully appreciate how these variables interact to control service life, it is essential to know the origins and understand the growth mechanisms in early stages of underfilm corrosion. For example, it is essential to know whether corrosion of a coated alloy has initiated as a result of chemical heterogeneities within the coating ͑e.g., regions that might facilitate ion transport͒, heterogeneities within the substrate ͑e.g., microgalvanic couples created by intermetallic compounds͒, or whether corrosion initiates because of a juxtaposition of these two types of heterogeneities. A detailed delineation of the origin and ensuing events that control corrosion breakdown of a coated metal will require a description of the local chemical and electrochemical events associated with the polymer-coated metal. This paper examines an initial library of reproducible local electrochemical events.A wide variety of dc-and ac-based methods have been developed within different fields of science for the in situ examination of local electrochemical events. Some of these methods include the scanning reference electrode technique, 1-5 scanning vibrating probe, 6-8 scanning Kelvin probe, 9-12 electrochemical microscopy, 13-17 and scanning tunneling microscopy, 18 among others. 19 Each of these methods is useful depending on the interface under study, the type of information desired, and the measurement resolution required. However, when considering the characterization of local electrochemical events associated with a coated-metal substrate, local electrochemical impedance methods have been a logical option based on (i) the ability of an ac-based excitation to lower the impedance a dielectric interface at higher frequencies and (ii) the ability to more fully characterize a specific site by the application of a frequen...

This study examines natural breakdown events on organic coated AA 2024-T3 coated using Local Electrochemical Impedance (LEI) mapping (M) and spectroscopy (S). LEIM was able to identify not only different types of defects on this system, but also provided information about the kinetics and stages of development of these defects. Supportive evidence regarding the impedance characteristics of these defects was provided by Capillary Electrophoresis (CE). Data from early stages of defect development indicate that an increased impedance develops at the site. This is related to either the development of aggregated water or electrolyte, as well as corrosion product development. Direct evidence of defect healing is also provided.

Local electrochemical impedance mapping and spectroscopy (LEIM/S) have become important tools for the investigation of local electrochemical breakdown events associated with the degradation of organically coated metals in aqueous environments. LEIM/S of organic coated metal substrates has revealed local degradation events that are distributed spatially and temporally. These observations provide support to a number of long-standing theories, as well as provide new insight into the damage process. The local changes in impedance observed at early stages of immersion support the presence of virtual pores, while the metastability of impedance peaks representing the local changes provide evidence of healing via corrosion product formation. Each of these are long-standing theories used to explain global electrochemical impedance measurements. This paper will provide an overview of some of the events observed using LEIM and examine these results in the context of recent analytical and numerical models. Models used to predict the electric field above an equipotential disk electrode support the interpretation of most experimental LEI data as being representative of chemical and physical phenomenon and not a result of measurement artifact. However, certain features may be an artifact of the finite nature of the experimental process. The interpretation of LEIM events in view of current experimental and modeling results will be discussed.