Behaviour of copper and generation of oxygen during anodizing of Nb-Cu alloys

The anodic oxidation behaviour of two compositions of Nb-Cu alloy is examin ed with particular reference to the oxidation of copper at the alloy/film i nterface and the generation of oxygen within the anodic film. Anodizing of an Nb-3 at. % Cu alloy results in a uniform, niobia-based anodic film, with copper species, which migrate faster than Nb5+ ions, distributed throughou t the film. The oxidation of copper proceeds without significant enrichment of copper in the alloy. Further, the film is free of bubbles of oxygen, wh ich is not generated in significant amounts during film growth to relativel y high voltages. In contrast, copious amounts of oxygen are generated durin g anodizing of an Nb-30 at,% Cu alloy, with the consequential development o f a highly flawed anodic film of non-uniform thickness. The oxygen is prese nt mainly in numerous bubbles located within the niobia-based film material . Bursting of the bubbles, with release of trapped oxygen, leads to a highl y damaged film. The continuous generation of oxygen consumes much of the cu rrent and limits the anodizing voltage to a low value. The large-scale gene ration of oxygen for the more concentrated alloy is suggested to be associa ted with the distribution of copper in the alloy, with copper having a tend ency to form copper-rich clusters, which are enhanced in size by an initial brief period of uniform film growth. Oxygen is then generated, probably fo llowing oxidation of such clusters. Copyright (C) 2000 John Wiley & Sons, L td.