Anodic oxidation of Al-Ce alloys and inhibitive behaviour of cerium species

The anodic oxidation of Al-Ce alloys, containing up to 27 at.% Ce, has been investigated in ammonium pentaborate and sodium hydroxide electrolytes of pH 8.3 and 12.0 respectively. Cerium ions and Al3+ ions are incorporated into the mainly amorphous anodic oxide films at the alloy/film interface in proportion to the concentration of the respective elements in the bulk alloys. The cerium ions migrate outward in the films more rapidly than Al3+ ions, by factors from about 1.4 to 2.5, increasing with reduction of the cerium content of the alloy. The more rapid migration of cerium ions leads to the formation of a layer of relatively pure cerium oxide, or for dilute alloys a cerium-enriched layer of alumina, adjacent to the alloy/film interface, which thickens as the films grows. The films can be grown at relatively high efficiency under alkaline conditions, at least to pH 12, which would not normally support usual anodic film growth on high purity aluminium. The sustained film growth to high pH is due to the anodic inhibition provided by the outer cerium-rich layer of film, which prevents the direct ejection of Al3+ ions from the film to the electrolyte. (C) 1998 Elsevier Science Ltd. All rights reserved.