Growth and field crystallization of anodic films on Ta–Nb alloys

The growth behavior of amorphous anodic films on Ta-Nb solid solution alloys has been investigated over a wide composition range at a constant current density of 50 Am -2 in 0.1 mol dm -3 ammonium pentaborate electrolyte. The anodic films consist of two layers, comprising a thin outer Nb 2O 5 layer and an inner layer consisting of units of Ta 2O 5 and Nb 2O 5. The outer Nb 2O 5 layer is formed as a consequence of the faster outward migration of Nb 5+ ions, compared with Ta 5+ ions, during film growth under the high electric field. Their relative migration rates are independent of the alloy composition. The formation ratio, density, and capacitance of the films show a linear relation to the alloy composition. The susceptibility of the anodic films to field crystallization during anodizing at constant voltage increases with increasing niobium content of the alloy. © 2011 Springer-Verlag.