The Influence of Chemical Pre-treatment and Magnesium Surface Enrichment on Bonding of Succinic Acid Molecules to Aluminium Alloy

In the present study a combined Fourier Transform Infrared Reflection–Absorption Spectroscopy (FT-IR–RAS) and Auger electron spectroscopy (AES) analysis was performed to investigate the bonding of organic coatings to oxides formed on Al–Mg based alloys. The interfacial bonding of a model molecule, succinic acid, mimicking the functional groups used in practical coatings was studied in detail on various differently pre-treated aluminium alloy (AA5182, 4.8 wt% magnesium) surfaces. Besides these differently pre-treated alloy surfaces, also pure magnesium and aluminium were studied for reference. The surface and near-surface composition of the alloys was studied by Auger electron spectroscopy, ion-milling away the surface oxides to obtain a depth profile. The results show that after heat treatment, the oxides are as much enriched in magnesium as aluminium although the bulk concentration of magnesium is only 4.8 wt%. FT-IR–RAS shows that bonding of the succinic acid molecules takes place in a range of configurations, varying between bridging bidentates and unidentates. The relative portion of unidentates increases with the magnesium surface concentration. This is attributed to the difference in bond lengths of Mg–O and Al–O, forcing the carboxylic acid groups to stretch or to attach to only one oxide group (unidentate) instead of two (bidentate). Also the mixed OH/CO3 surface after Mg enrichment at the surface has less room for bonding with two groups, thus affecting the bonding configuration in a similar way.