Abstract
The anodic oxidation of niobium has been studied in a variety of electrolytes at temperatures of 20° and 90°C under conditions of constant current density. Although thin films exhibiting interference colours are produced in all electrolytes studied, it has been established that with weak electrolytes (e. g. acetic acid) the film thickens with continued passage of charge resulting in the formation of a thick grey absorbing coating. The ionic efficiency, [i+]/i+ +i- has been studied by gravimetric methods and also determinations have been made of the V/t (i constant) and -1/C /t (i constant) relationships. As acetic acid gave a higher efficiency than other electrolytes, studies have been largely confined to this electrolyte. A characteristic of V/t relationship of niobium in acetic acid is the change in slope of the curve at 20-30V and the formation of a pronounced plateau at 70-90V. It has been established by x-ray and electron diffraction and electron microscopy that although the oxide surface is amorphous below the plateau it crystallises at the plateau. The phenomenon is considered to be an example of field crystallisation where an amorphous oxide can crystallise when subjected to a high electric field; this phenomenon was first observed for tantalum. The change in the slope at 20-30V may be significant and may indicate a change in the chemical or physical nature of the amorphous oxide. The plateau has been discussed and is considered to be due to field crystallisation which commences at the metal/oxide interface, the amorphous Nb[2]O[5] being transformed to the crystalline state with consequent rupture of the overlying amorphous oxide. A comparison has been made between the results obtained in the present work and the results obtained by other workers on the high temperature oxidation of niobium.