Characterization of Intergranular Corrosion Defects in a 2024 T351 Aluminium Alloy

In the 2xxx series alloys, intergranular corrosion is generally related to the strong reactivity of copper-rich intergranular precipitates leading to a copper enrichment of these particles. While the nature of the oxides formed inside the intergranular corrosion defects was assumed to strongly influence the intergranular corrosion propagation rate, it was not clearly identified due to the thickness of the oxide layer formed which required to use high resolution analytical techniques. The present work aims to characterize the intergranular corrosion defects formed for a 2024-T351 aluminum alloy after a 24 hours continuous immersion in a 1 M NaCl solution and compares the results to literature data concerning the oxide layers formed on copper-rich model alloys. A combination of focus ion beam (FIB) technique, transmission electron microscopy (TEM) observations and energy dispersive X-Ray spectroscopy (EDX) analyses was used to accurately characterize both the morphology and chemical composition of the intergranular corrosion defects. Results evidenced the dissolution of intergranular copper-rich particles, the formation of a 10-200 nm-thin metallic copper-rich layer at the oxide/metal interface and the incorporation of copper inside the amorphous alumina oxide film leading to the formation of structural defects of the oxide film.