Development of iron-rich layers during anodic oxidation of sputter-deposited Al-4 atom % Fe alloy

The role of iron during anodizing of a non-equilibrium Al-4 atom % Fe alloy at high faradaic efficiency has been examined using high resolution transm ission electron microscopy, Rutherford backscattering spectroscopy, and X-r ay photoelectron spectroscopy. Anodic oxidation proceeds with initial forma tion of an anodic alumina film, free from iron species, and subsequent form ation of an iron-contaminated alumina film, which is eventually contaminate d by iron species throughout its thickness. During the initial oxidation of aluminum, iron atoms are accumulated in a thin layer of alloy, about 2-3 n m thick, immediately beneath the anodic film. When the average composition of the enriched alloy layer reaches approximately Al-25 atom % Fe, both alu minum and iron are oxidized and are incorporated into the film in their app roximate alloy proportions in the presence of the enriched alloy layer. The incorporated iron species are in the trivalent state, and migrate outward in the growing anodic film about 1.8 times faster than Al3+ ions. The iron species eventually reach the film/electrolyte interface, forming a thin, ir on-rich, hydrated layer probably composed of gamma-FeOOH. (C) 1999 The Elec trochemical Society. S0013-4651(98)11-076-5. All rights reserved.