The susceptibility of alloy 8090 T8171 to exfoliation corrosion has been analysed using the electrochemical impedance spectroscopy technique (EIS). Corrosion susceptibility was determined in a standard medium (EXCO tests). This acid solution creates a very severe attack in a short period of time and makes it possible to distinguish the different stages of the corrosion process. The EIS technique enables collection impedance spectra which show a variation with increasing exposure time iind so is appropriate for recording these stages. The Nyquist diagram is initially described by a well defined capacitive semicircle, which suggests a corrosion process under activation control. This response is linked to the existence of small pits on the surface of the working electrode. However, when the immersion time increases, the attack causes the appearance of large pits on the electrode, leading to an impedance response comprised by two overlapping semicircles; the high-frequency semicircle related with the flat part of the working electrode and the low frequency are associated with the part covered by pitting.