Understanding Coating and Substrate Heterogeneities using Electrochemical Impedance Methods
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alloys as occurring at local sites, particularly on multi-phase engineering alloys, clear demonstration of these events on contiguous coatings has been forthcoming. A method which could locate and evaluate these breakdown sites in situ could also be invaluable in understanding the mechanistic issues which control the life/death cycle of these defects, as well as the source of these defects. Questions remain as to whether the defects originate in the coating, the substrate, or whether they exist in both coating and substrate and must be juxtapositioned. Knowledge of the events at these sites will be important to understanding the parameters which control the performance of coated metal substrate. A method for mapping the local impedance of coated metal substrates was introduced by Lillard, Moran, and Isaacs [10] and later expanded upon in the investigation of a variety of intentional chemical and physical defects [11,12]. Although numerous electrochemical methods are available for the mapping of local electrochemical phenomenon [13-18], Local Electrochemical Impedance Spectroscopy (LEIS) is particularly well suited for the investigation of organic coated substrates due to the AC nature of the excitation which lowers the impedance of the dielectric interface. As mentioned above, this technique has been used thus far to investigate simulated chemical and physical defects on organic coated steels. The objective of the present research is to locate and characterize local electrochemical breakdown events that
35 Mat. Res. Soc. Symp. Proc. Vol. 500 ©1998 Materials Research Society
occur naturally on a coated aluminum alloy when exposed to an aqueous chloride environment. An understanding of the nature of these defects will eventually lead to an understanding of the factors which limit the service life of coated engineered products, and thus to ways for improvement. EXPERIMENTAL METHODS Local electrochemical impedance measurements were made with a five electrode arrangement shown in Figure 1 and discussed elsewhere [11,12]. The interface is excited potentiostatically using a conventional three electrode technique, however the local current is assessed by two vertically displaced micro-reference electrodes. The potential difference between these two electrodes is amplified, then converted into a local current
density knowing the solution conductivity and using Ohms law (see Figure 1), and compared to the excitation voltage via a Frequency Response Analyzer (FRA) to generate the local impedance. A 15 mV sine wave excitation at a fixed frequency (500 to 700 Hz) was used in the case of LEIM, and a swept frequency (from 1 Hz to 10 kHz) in the case of LEIS. The DC potential and AC excitation of the coated samples were established with a Solartron 1286 Electrochemical Interface and Solartron 1255 HF FRA all under computer control.
Figure 1. Schematic of five electrode
AgAgC, 3-
configuration used to acquire LEIS data. Note current lines (dashed) and local field lines (solid) associated with local defect.
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