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

Abstract

The anodic oxidation behaviour of two compositions of Nb–Cu alloy is examined with particular reference to the oxidation of copper at the alloy/film interface 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 throughout the film. The oxidation of copper proceeds without significant enrichment of copper in the alloy. Further, the film is free of bubbles of oxygen, which is not generated in significant amounts during film growth to relatively high voltages. In contrast, copious amounts of oxygen are generated during anodizing of an Nb–30 at.% Cu alloy, with the consequential development of a highly flawed anodic film of non‐uniform thickness. The oxygen is present mainly in numerous bubbles located within the niobia‐based film material. Bursting of the bubbles, with release of trapped oxygen, leads to a highly damaged film. The continuous generation of oxygen consumes much of the current and limits the anodizing voltage to a low value. The large‐scale generation of oxygen for the more concentrated alloy is suggested to be associated with the distribution of copper in the alloy, with copper having a tendency to form copper‐rich clusters, which are enhanced in size by an initial brief period of uniform film growth. Oxygen is then generated, probably following oxidation of such clusters. Copyright © 2000 John Wiley & Sons, Ltd.

Growth of barrier-type anodic films on magnesium in ethylene glycol electrolytes containing fluoride and water

“…Thus, the gold is accumulated in the enriched alloy layer without incorporating into the anodic film to high anodizing potentials [35,36]. In addition to aluminum alloys, accumulation of an alloying element beneath the anodic film was found in magnesium alloys [37], although no obvious enrichment of alloying elements occurred for titanium, zirconium and tantalum alloys [38][39][40]. The present study discloses that even for the formation of fluoride-based anodic films, prior oxidation of magnesium occurs with gold atoms that accumulate in a thin alloy layer beneath the anodic film.…”

Effect of Nb addition on anodic behavior of Ti alloy in electrolyte containing calcium and phosphorus

“…Since the outer layer forms under outward migration of metal ion, the Nb ion apparently migrates more slowly than Ti ion. This phenomenon can be accounted for the difference in bond energies of TiO 2 [28] and Nb 2 O 5 [29], which are 319 and 329 kJ/mol, respectively. Good correlation between the migration rate of the outwardly migrating species and their single metal-oxygen bond energies has been reported [30,31].…”