Mechanical properties and corrosion behavior of artificially aged Al-Mg-Si alloy

Abstract

This article reports the influence of the state of artificial ageing on the hardness, tensile properties and corrosion behavior of an Al-Mg-Si alloy. Pitting corrosion in 0.1M NaOH solution and susceptibility to intergranular corrosion (IGC) have been systematically investigated, covering highly under-aged to highly over-aged states. In alkaline solution, corrosion rate increases with the progress of ageing; while for a given state of ageing, it initially increases with immersion time followed by rapid reduction since corrosion is controlled by the competition of formation and dissolution of Al(OH)3 plus Al2O3 film as determined by FESEM, EDS, and XRD characterizations. The IGC susceptibility of the selected alloy is found to be governed by the microstructure as determined by the state of ageing since it is controlled by the anodic dissolution of the precipitate free zones while closely neighboured grain boundary precipitates act as cathode. It is identified as slight to moderate pitting in highly under-aged, moderate to heavy pitting in under-aged, pitting with localized ICG in peak-aged, localized to uniform IGC in over-aged, and uniform IGC to etching in highly over-aged state. It is adjudged that apart from the inclusions like AlCrMnFeSi, the changes in the precipitation such as β” (Mg5Si6), β' (Mg9Si6) and β (Mg2Si) within the grains and at the grain boundaries with progress of artificial ageing play a pivotal role on mechanical properties and corrosion characteristics of the Al-Mg-Si alloy.