The formation of red blisters is an important mode of coating failure on epoxy coated aluminium alloy (AA) 2024–T3 exposed to chloride solutions. In order to achieve a better understanding of the mechanisms behind blister formation and growth and to understand the influence of the environment, impedance spectroscopy with and without microelectrodes was applied to the localised study of coating defects on AA 2024–T3. The results allowed features observed in regular impedance spectroscopy to be related to local phenomena and thereby allowed validation of the equivalent circuits used for data interpretation. In addition the electrochemical properties of smaller blisters and of uncorroded substrate areas could be determined separately on the same specimen. Such data are not accessible via regular electrochemical impedance spectroscopy experiments if a large blister is present. The data proved that the two semicircles observed at low frequencies in a regular impedance spectrum solely reflect the corrosion reactions taking place within the blister. Evidence for a strong local decrease in the coating resistance of the top of the blister was found. The coating resistance on areas not corroded appeared to decrease even after the end of the initial water uptake, although the impedance was still several orders of magnitude higher than on top of the coating blister. The passive dissolution rate on non-blistered substrate areas seemed to remain unaffected by the presence of the blister.
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