The Study of AA2024 De-Alloying Using Luminol Electrogenerated Chemiluminescence Imaging

Abstract

This paper describes the use of luminol electrogenerated chemiluminescence (ECL) to image the re-distribution of electrochemically active copper (copper in electrical contact with the substrate) at the surface of AA2024-T351 corroding in NaCl electrolyte. AA2024 samples are thermo-mechanically treated to produce a macroscopically inhomogeneous distribution of intermetallic particles. The AA2024-T351 samples are allowed to corrode by immersion in 0.51 M aqueous NaCl and (ex-situ) ECL measurements are performed to follow copper re-distribution. Luminol ECL light emission is orders of magnitude more intense on copper than aluminum and macro and microscopic images are obtained to elucidate the re-distribution of electrochemically active copper. S phase de-alloying produces a radical re-distribution of copper on a microstructural scale (1-10 μm) but macroscopic copper distributions (1-10 mm) remain largely unchanged. In-situ experiments using the SVET show that macroscopic copper distribution directs localization of corrosive attack such that net anodic activity (pitting/intergranular attack) is concentrated in regions with lowest surface coverage of copper and net cathodic activity is concentrated in regions of highest coverage. Results are explained in terms of the role of solution chemistry and pH in influencing the transition from transient to stable pitting.