Anodic oxidation of Al-Cr alloys and the valence state of chromium

Journal Article · Mon Dec 01 04:00:00 EST 1997 · Journal of the Electrochemical Society

DOI:https://doi.org/10.1149/1.1838169· OSTI ID:603739

Habazaki, H; Skeldon, P; Thompson, G E; Wan, J; Wood, G C; Zhou, X; Laet, J De ^[1]; Shimizu, K ^[2]

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1. Univ. of Manchester Inst. of Science and Technology (United Kingdom). Corrosion and Protection Center
2. Keio Univ., Hiyoshi, Yokohama (Japan). University Chemical Lab.

The anodic oxidation of Al-4 atom percent (a/o) Cr [7.4 weight percent (w/o) Cr] and Al-10 a/o Cr (17.6 w/o Cr) sputter-deposited alloy films was investigated by transmission electron microscopy and by x-ray absorption studies. The oxidation behavior of both alloys is explained by initial formation of a chromium-free anodic alumina film with accumulation of chromium in a thin underlying alloy layer. On sufficient enrichment of the alloy, chromium and aluminum are oxidized at the alloy/film interface in their approximate alloy proportions. The resultant Cr{sup 3+} ions migrate outward through the film more slowly than Al{sup 3+} ions. Oxidation of chromium is followed by production of what is probably oxygen gas within the film, which is likely to generate a high pressure, leading to film rupture and gas release. Consequently, a complex, irregular film morphology, associated with voids, channels and cracks, develops. X-ray absorption studies reveal that Cr(VI) species are detected in significant quantities only after the mechanical breakdown of the film. Such species probably form as a result of the high electrochemical potential of the alloy that is freshly exposed to the environment following film rupture.

• Sponsoring Organization: USDOE
• OSTI ID: 603739
• Journal Information: Journal of the Electrochemical Society, Journal Name: Journal of the Electrochemical Society Journal Issue: 12 Vol. 144; ISSN 0013-4651; ISSN JESOAN
• Country of Publication: United States
• Language: English

Related Subjects

36 MATERIALS SCIENCE
ALUMINIUM BASE ALLOYS
ALUMINIUM OXIDES
CHROMIUM ALLOYS
MORPHOLOGY
OXIDATION
PROTECTIVE COATINGS
RUPTURES
VALENCE