The barrier property of a nanocomposite epoxy coating containing 1 or 1.25 vol% of carbon black (CB) applied on AA2024-T3 was investigated by using electrochemical impedance spectroscopy. Micro-electrochemical impedance spectroscopy and optical microscopy were also used to investigate the delamination of the nanocomposite coating containing 1.25 vol% carbon black (CB). A defect of about 500 µm in diameter was created in the composite coating as well as in the unfilled reference sample. The polarization resistance of the unfilled coating decreases 10 times faster than that of the composite. The blister size of the nanocomposite coating is also smaller after 200 h of immersion. It was concluded that the composite delaminates by a factor of ten slower compared to a similar unfilled coating. A protective mechanism was proposed in order to explain this observation. The presence of CB conductive pathways in the composite and the electrical contact between the coating and the substrate is helpful to delocalize electrons generated from the metallic part. After oxidation, electrons do not remain localized at the interface as they have the possibility to migrate into the coating through the CB percolation pathway. This reduces the rate of interfacial oxygen reduction and thus the rate of coating delamination.