AFM-Assisted Investigation of the Corrosion Behaviour of Magnesium and AZ91 Alloys in an Ionic Liquid with Varying Water Content

Abstract

Atomic force microscopy (AFM)-assisted corrosion measurements of poly- and microcrystalline magnesium, of AZ91D magnesium alloys, and of AZ91D alloys with defined amounts of metallic impurities (Cu, Ni, and Si) in the ionic liquid 1-butyl-3-methylimidazolium trifluoromethylsulfonate with variable water contents is reported. Whereas both magnesium and the AZ91 alloys show a tremendous corrosion in aqueous solutions, they are practically inert in the water-free ionic liquid. The apparent electrochemical window of the water-free ionic liquid on magnesium and its alloys can reach values of 10 V and more. The low corrosion rate of AZ91 alloys with metallic impurities, in any case, follows the trend in aqueous solutions: in comparison to the base alloy, the addition of nickel and silicon leads to higher corrosion rates. Upon addition of water to the ionic liquid two effects are observed. The higher the water content in the ionic liquid, the lower the corrosion potential and the higher the corrosion rate. Poly- and microcrystalline magnesium shows a surprisingly complicated corrosion behaviour, which gives rise to different processes during electrochemical polarization. As a proof-of-principle, in-situ AFM measurements were performed on the corrosion of AZ91D alloy in the ionic liquid. No corrosion products are observed at the surface in the water-free ionic liquid by in-situ AFM, even after several hours at +3 V versus a saturated calomel electrode (SCE), which indicates surface passivation in the ionic liquid.