The influence of recycling on the localized corrosion susceptibility of extruded AA6063 alloys

An approach involving the quantification of microstructure characterized by different techniques such as SEM, EDS, and SKPFM is statistically treated to provide a deeper insight into the influence of recycling AA6063 on localized corrosion susceptibility. Particularly, the intermetallic particles and the two forms of localized corrosion – pitting and intergranular corrosion are systematically documented, measured, and analyzed. Even trace amounts of Cu and Zn introduced into the alloy from recycling had a remarkable effect on the localized corrosion susceptibility. The study found that the initiation and early evolution of the two localized corrosions are in competition, and the predominance of one over the other is closely linked to the composition of the alloy, and microstructure. Recycled variants with higher trace Cu made the alloy more susceptible to pitting attack whereas higher trace Zn is linked with greater IGC susceptibility. The trace amount of higher Zn addition has a particularly beneficial effect on pitting susceptibility as it reduces the likelihood of pitting even in alloys with a higher trace Cu content. The SKPFM results obtained in this study provided a basis for the circumferential pitting susceptibility around intermetallic particles, as a higher volta potential difference (∆V) implied a higher driving force for corrosion. ∆V differences between the different variants were further explained based on trace recycled element distribution in the microstructure.