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AC impedance measurements have been performed in 0.5 N NaCl for coated steel and aluminum alloys which had been subjected to different surface treatment procedures. These procedures included phosphating for steel and exposure to a conversion coating for the Al alloys. A polybutadiene coating of 8 ± 2 μm thichness was applied by spin coating. The AC impedance measurements made it possible to follow the penetration of electrolyte into the coating and to detect the initiation of corrosion at the metal/coating interface. A general model has been used to analyze the impedance data in terms of reactions occurring during the interaction of the coated metals with the environment. Based on this analysis the different pretreatment procedures have been ranked in terms of their efficiency in providing corrosion protection by the organic coating. A comparison between impedance results and visual observation over several days of the corroded and the delaminated areas under the coating confirm that AC impedance data can be used to characterize organic coating/metal systems.

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Electrochemical impedance spectroscopy (EIS) has been used to characterize the surfaces of anodized Al alloys during exposure to 0.5N normalNaCl . The EIS‐data show pronounced effects of anodizing procedure (sulfuric acid, chromic acid), sealing procedure (unsealed, hot water, dichromate), and alloy chemistry (Al 2024, Al 6061, Al 7075). Tests conducted after short exposure times (about 2h) can be used as a quality control test, while tests over longer exposure times give information concerning the corrosion resistance of the anodized surface. The high frequency impedance data show a pronounced difference for hot water and dichromate sealed surfaces which suggests different mechanisms of sealing. The corrosion resistance cannot be determined from the high frequency region in all cases, but becomes evident from the changes with exposure time of the low frequency region which is dominated by the properties of the barrier layer. A damage function based on the impedance at 0.1 Hz describes the corrosion behavior. EIS‐data for an anodized normalSiC/normalAl metal matrix composite are different from those obtained for the other Al alloys studied here. Models for the anodized Al surfaces discussed so far in the literature explain some of the features of the experimental EIS‐data, but modifications are needed to explain the spectra observed for dichromate sealing.

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