Plasma coatings from hexamethyldisiloxane (HMDSO) have proven to be effective primer coatings for adhesion promotion to aluminium but the corrosion properties of these coatings have not been thoroughly assessed. In the present work, electrochemical impedance spectroscopy (EIS) was used to evaluate the corrosion behaviour of Alclad AA2024-T3 covered with a polymer film prepared by plasma polymerisation. using HMDSO as monomer. The present results indicate that, although the polymer film was mainly intended to provide a strong coupling between the metal substrate and the organic coating. the Film in itself confers protection against corrosion. Equivalent circuits are proposed for the immersion behaviour of uncoated and plasma-coated material, allowing the determination or the parameters relative to natural aluminium oxide. coating layer and pitting. An estimation of the water uptake during the immersion time from the values obtained for the coating capacitance was carried out. The evaluation of the corrosion performance of these plasma-coated materials by means of their water uptake behaviour may be useful. as the water volume fraction seems to be much more sensitive to the changes in the system due to delamination than the values obtained from the fitting of EIS data to an equivalent circuit.