Characterization of intergranular corrosion defects in a 2024 T351 aluminium alloy

Abstract

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 aluminium 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. An intergranular defect obtain after cyclic immersion (8 hours continuous immersion, 16 hours emersion) was also observed and characterised. In order to obtain a thin sample in a localized region, i.e. in an intergranular corrosion defect, a Focused Ion Beam (FIB) / Scanning Electron Microscope (SEM) FEI HELIOS 600i equipped with a field emission gun (FEG) was used. The thin sample preparation was done using conventional lift out procedure; it is summarized in Fig. 1. Location of interest (intergranular corrosion defect) was chosen (Fig. 1a) and a platinum coating was deposited using electron beam prior to using ion beam to protect the area beneath from being contaminated by the Gallium (Ga ions) (Fig. 1b). Using a large beam current for fast ion milling, two tranches were milled on either side of the Pt coating. The sample of size (10x10x7 µm 3 ), so prepared, was then mounted on a TEM sample holder. A cross‐section view of the intergranular corrosion defect was therefore obtained (Fig. 1c). It was then polished using successive lower beam current. Finally, the sample was thinned to 100 nm or less using 1 keV ion beam to minimize the artefacts from sample preparation (Fig. 1d). A transparent section was obtained. Some intermetallic precipitates were visible inside and all around the intergranular corrosion defect. Then, a combination of transmission electron microscopy (TEM) observations (Fig. 2) and electron energy loss spectroscopy (EELS) 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.