ANODIC-OXIDATION OF AL-CE ALLOYS AND INHIBITIVE BEHAVIOR 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 electr olytes 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 e lements in the bulk alloys. The cerium ions migrate outward in the fil ms more rapidly than Al3+ ions, by factors from about 1.4 to 2.5, incr easing with reduction of the cerium content of the alloy. The more rap id migration of cerium ions leads to the formation of a layer of relat ively pure cerium oxide, or for dilute alloys a cerium-enriched layer of alumina, adjacent to the alloy/film interface, which thickens as th e films grows. The films can be grown at relatively high efficiency un der alkaline conditions, at least to pH 12, which would not normally s upport usual anodic film growth on high purity aluminium. The sustaine d film growth to high pH is due to the anodic inhibition provided by t he outer cerium-rich layer of film, which prevents the direct ejection of Al3+ ions from the film to the electrolyte. (C) 1998 Elsevier Scie nce Ltd. All rights reserved.