Corrosion behavior of under-, peak-, and over-aged 6063 alloy: A comparative study

The mechanical property of age-hardenable Al-alloys is governed by the state of ageing, which determines the microstructure and consequently, their corrosion behavior which is a vital aspect for a number of applications. This article presents a comparative assessment of corrosion behavior of under-, peak- and over-aged Al-Mg-Si alloy. Corrosion characteristics have been determined via immersion tests in 0.1 M ortho-phosphoric acid solution and intergranular corrosion (IGC) tests. Corroded surfaces are examined by field emission scanning electron micrographs-energy dispersive spectroscopy and 3D optical profilometer. The obtained results reveal that the corrosion rate at a specific immersion time as well as the depth of IGC increases in the order for under-, peak-, and over-aged states. Irrespective of the state of ageing, corrosion loss increases linearly but the rate of corrosion decreases rapidly with increasing immersion time. The dominant mode of corrosion in under-aged alloy is identified as localized pitting, while peak-aged is highly susceptible to IGC in contrast extensive pitting corrosion is observed for over-aged alloy. The observed differences in corrosion behavior are explained considering characteristics of precipitates. Formation of beta (Mg2Si) in case of over-aged alloy and presence of inclusions like AlFeMnSi particles are found to accelerate pitting corrosion.