Exploring the electrochemistry of 6xxx series aluminium alloys as a function of Si to Mg ratio, Cu content, ageing conditions and microstructure

The electrochemical response of 6xxx series Al-alloys, which are based on the Al-Mg-Si-(Cu) system, was systematically studied. Nine custom Al-Mg-Si-(Cu) alloys were produced on the basis of varying Si to Mg ratio and Cu content. The alloys were further aged at four different ageing conditions, and analysed by potentiodynamic polarisation and careful quantification of metastable pitting events from potentiostatic transient testing. The relevant alloy microstructures were characterised using transmission electron microscopy, revealing differences in nanoscale precipitates that evolve as a function of alloy composition and ageing conditions. It was revealed that the pitting events depend primarily on the thickness of the precipitates, rather than their length or number density. In the case of Cu-containing 6xxx alloys, precipitates were found to be surrounded by an apparently single atomic layer of segregated Cu atoms, which appeared to inhibit the precipitate growth, thus inhibiting the number of pits occurring. This study provides significant insights into both the understanding of, and the control of, the genesis of pitting corrosion in 6xxx series Al alloys.