Impact of impurity elements on the corrosion performance of high strength 6xxx aluminium alloys

High strength and high crush resistant AA6xxx AlMgSiCu aluminium alloys are presently the preferred choice for applications in the automotive industry for lightweight vehicle structures, including the body-in-white and for the light-weighting and the electrification of both on- and off-road vehicles. However, the increasing global demand for aluminium alloys will only be met in the near term by the circulation of secondary metals, which inevitably introduces impurities to alloys. Increased additions of impurities like iron or deliberate additions like copper and nickel can be detrimental to corrosion resistance particularly in the accelerated tests used for qualification with automotive OEMs. This means that there is a strong research incentive to improve the corrosion performance of wrought AA6xxx automotive alloys in response to this demand and to formulate these alloys from end-of-life recycled aluminium to reduce embedded carbon level to below 2 tonnes CO2e/tonne of aluminium as a supplied component and eventually to Net Zero carbon. Understanding of the role of surface microstructure is critical for the understanding and control of the corrosion performance of extruded AA6xxx alloys in automotive applications particularly for alloys formulated from end of life scrap with higher levels of impurities compared to those formulated from prime aluminium.

The objective of the PhD project is to study the surface microstructure of the HSA6 family of high- performance automotive alloys formulated both with deliberate impurity additions and from end-of-life scrap sources like vehicle hulks and from the demolition of buildings. The work will be focused on the influence of alloy microstructure, including die lines and other surface features, constituent intermetallic particles and grain boundary precipitates, on the initiation and propagation of corrosion using correlative accelerated laboratory testing, electrochemical analysis, in situ optical microscopy supported by analytical electron microscopy. The newly developed X-ray and electron-based nanotomography will also be employed to spatially determine the microstructural features that are responsible for the initiation and propagation of corrosion.