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.
Experimental approaches for recording ac impedance data are discussed in this paper. Considerations of the high frequency performance of potentiostats and reference electrodes are given and experimental results are presented. Additional methods for analysis of experimental impedance data are discussed, with emphasis on the CIRFIT computer program in which a semicircle is fitted to the experimental data in the complex plane. The systems Cu/3.5% NaCl + BTA and Al 2024/3.5% NaCl are discussed in more detail. Pitting of the Al alloy in neutral NaCl leads to a second time constant at low frequencies. Results for iron and Al 2024 in tapwater show that the low conductivity of this test medium does not present particular problems with the recording of ac impedance data.
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