Use of acoustic emission technique for the early detection of aluminum alloys exfoliation corrosion

Exfoliation corrosion of aluminum alloys is a form of localized corrosion which affects many industries, specially aeronautics. The study of this corrosion mode using only electrochemical techniques is not fully efficient for the detection and control on line of this phenomenon. Therefore, we developed a non-destructive testing technique based on the acoustic emission recordings in order to follow-up this form of corrosion on aluminum alloys. Indeed, recent works have shown the interest of the acoustic emission for the detection, the monitoring and the localization of pitting corrosion on aluminum alloys. This pitting corrosion phenomenon is currently well understood and the experimental methodology acquired during that study is transposed to the study of exfoliation corrosion of aluminum alloys. The present study is conducted on two aluminum alloys: (Al 2024 T3, and Al 7449 T6 and T7). Samples are immersed 4 days in the modified ASTM STP 1134 saline solution. Observations of the structures after tests show that the exfoliation corrosion sensitivity of alloy 7449 T6 is more important than for alloy 7449 T7 which exhibits only the presence of small and non-occluded pits. Very severe exfoliation corrosion was also observed on Al 2024 T3, but after a longer immersion time or in a more acid solution. The recording of the acoustic emission activity shows evident links between this activity and the exfoliation corrosion rate. The analysis of the signal's characteristics reveals a population corresponding to the release of hydrogen bubbles. A few more energetic signals have also been observed, Their source can be either, the cracking resulting from the separation of sheets of metal, or the development and evolution of hydrogen bubbles formed inside blisters during exfoliation corrosion. (C) 2002 Elsevier Science Ltd. All rights reserved.