Morphology, composition and structure of anodic films on Al–Cr alloys

Abstract

Anodic films developed on aluminium alloys, Al–Cr for example, are frequently characterised morphologically by transmission electron microscopy, with precise composition, and valence states of relevant species determined by comparatively broad beam techniques, e.g., Rutherford backscattering spectroscopy, X-ray absorption spectroscopy using synchrotron radiation, etc. Here, the combination of transmission electron microscopy with electron energy loss spectroscopy has been utilized for morphological, compositional and structural probing at the necessary resolution in a single instrument. For the Al–Cr alloy, anodizing results in interfacial enrichment of chromium in a thin alloy layer immediately beneath the anodic film; upon achieving a steady-state enrichment, chromium is oxidized and incorporated into the alumina film as Cr 3+O units. The Cr 3+ species have reduced outward mobility relative to Al 3+ ions under the high electric field across the anodic film. Further, oxygen gas development is associated with oxidation and incorporation of chromium; such gas, detected readily by EELS, is present in gas-filled voids within the film section. The presence of oxygen-filled voids evidently influences the outward mobility of chromium species detected from fine volumes of film material.