An electrochemical impedance study of a natural aged Al-Cu-Mg alloy in NaCl

Electrochemical Impedance measurements in the range of 50 kHz to 50 mHz were applied to the study of an Al-Cu-Mg aluminium alloy (2024) in T4 condition and naturally aged for 6 y. Since the first hours of testing, the alloy was in an active state with high corrosion rates. At the beginning, the shape of the impedance plots suggested that the system is under charge transfer resistance control. However, when the experimental time increased, the system changed towards a mixed control. The impedance of this system is characterised by a high frequency capacitive are which gives information on the performance of the unattacked surface. At lower frequencies, a diffusion control is apparent suggesting the specific contribution of the surface in which the localised attack takes place. A good agreement was found between the experimental results and a model circuit. (C) 1997 Elsevier Science Ltd.