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 mH z were applied to the study of an Al-Cu-Mg aluminium alloy (2024) in T 4 condition and naturally aged for 6 y. Since the first hours of testi ng, 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 experi mental time increased, the system changed towards a mixed control. The impedance of this system is characterised by a high frequency capacit ive are which gives information on the performance of the unattacked s urface. At lower frequencies, a diffusion control is apparent suggesti ng the specific contribution of the surface in which the localised att ack takes place. A good agreement was found between the experimental r esults and a model circuit. (C) 1997 Elsevier Science Ltd.