Chromate conversion layers, formed on Alclad 2024-T3 aluminium alloy after different surface preparation procedures, were investigated by means of EIS measurements performed in aggressive and middle aggressive electrolytes. The morphology of the chromate layer varies depending on the type of solution and on the time of duration due to deposition of corrosion products inside the defects and deterioration of the layer. Therefore, three slightly different equivalent circuits have been used to model the chromate film in the two electrolytes and for different times of exposure. The proposed equivalent circuits enable the study of the effect of variation in the aluminium surface microstructure, caused by different surface pretreatment, on the performance of the deposited chromate conversion coating. The removal of galvanic couples, mainly caused by precipitates, from the aluminium surface appears to be a necessary condition for the formation of a chromate layer with good corrosion resistance. Besides, the properties of the surface of the aluminium substrate largely influence the morphology and composition not only of the chromate film but also of the corrosion products formed at weak locations, which seem to play an important role in the protection provided by the chromate conversion coating.