Anodic film growth on Al–Nd alloys

Abstract Anodizing of an Al–10at.%Nd sputtering deposited alloy is examined by a combination of transmission electron microscopy, Rutherford backscattering spectroscopy and X-ray photoelectron spectroscopy, in order to determine the role of neodymium during film growth. An amorphous oxide film forms, containing units of Al 2 O 3 and Nd 2 O 3 , on a mainly amorphous alloy. The film grows by migration of O 2− ions inward and Al 3+ and Nd 3+ ions outward. Nd 3+ ions migrate faster than Al 3+ ions, by a factor of approximately 2.0, and form a thin layer of Nd 2 O 3 , which is readily hydrated, at the surface of the film. The efficiency of film growth is close to 100% under the selected conditions of anodizing. The anodic oxidation of the alloy proceeds without the development of a neodymium-rich alloy layer, which is consistent with the lower Gibbs free energy per equivalent for formation of Nd 2 O 3 relative to Al 2 O 3 . The field strength for film growth is reduced as compared with that for anodic alumina, due to the presence of units of Nd 2 O 3 .