Exfoliation corrosion of aluminium alloys examined by electrode impedance

The aluminium alloys exhibit, when submitted to certain aggressive conditions, a particular corrosion feature named exfoliation, leading to the formation of thin laminar corroding foils. Three different alloys having distinct corrosion features; 2024 T351, 2219 T87 and 6013 T6 were immersed in the EXCO solution, and their electrode impedance was measured repetitively during 24 h. In a short immersion period, up to two hours, the impedance measurements showed a marked decrease of the charge transfer resistance, thus indicating the acceleration of the corrosion rate, whereas the electrode potential shifted towards a more anodic value. This phenomenon is interpreted by the acceleration of the cathodic process. Then the corrosion rate decreased gradually corresponding to the exhausting of aggressive ions, namely Hf, verified by the measurements of solution pH. For 2024 and 6013 alloys the electrode impedance depicted clearly the existence of potential and current distribution inside the intergranular corroded areas or between the metallic foils delaminated by exfoliation. The progress of the corrosion along with increase of the alloy-electrolyte interface area was clearly indicated by a marked increase of the interfacial capacitance. Electrochemical impedance spectroscopy therefore gives valuable information for understanding this kind of corrosion and may be used as a future fast and reliable laboratory test. Copyright (C) 1996 Elsevier Science Ltd