Chemical surface ageing in ambient conditions of an Al–Fe–Cr approximant phase

The γ-Al65Cr27Fe8 phase is a complex metallic alloy with interesting electrochemical properties. Here we present a detailed study of the surface ageing of this alloy when exposed to ambient conditions for a long time. A combination of x-ray reflectivity, photoemission spectroscopy and secondary neutral mass spectroscopy measurements is used to provide a model of the modification of the surface structure and its composition as functions of ageing time. The near surface structure is described by the stacking of three layers. The first layer on top of the substrate corresponds to a mixed metal oxide and is amorphous. The intermediate layer consists of pure aluminum oxy-hydroxide while the outermost layer corresponds to carbonated contaminations. The total thickness of this near-surface region evolves with ageing time, reaching a stable state only after several days.