Formation Features of Anodic Oxide Films in the KNO3 Melt

            The present communication  is considered as an extension of earlier investigations of niobium anodized in nitrate melts of salts at oxide recrystallization temperatures. Studied patterns of growth and crystallization of oxide layers are currently subject to be completed with investigations of AOF phase and elemental composition considering the available general data on applicability of the process of metal substrate oxygen saturation and inclusion of electrolyte components to oxide at anodic oxidation [1].

Examination of phase composition applying Auger spectroscopy at layer-by-layer etching with Argon ions has shown  that generated films instead of being entire niobium pentoxide layer, as it was seemed to be, has occurred to be a «sandwich» of Nb₂O₅ , NbO₂  and NbO phases (in the direction from oxide surface to niobium). This is similar to what is taking place at thermal oxidation [2]; at the same time within anodic oxidation oxygen is intensively dissolved in niobium substrate, penetrating into it on the depth which substantially exceeds the oxide thickness.

Studying of the possibility of electrolyte components being included into oxide has lead to a result which from the first sight may be considered as contradictory: at growth anodic oxide is saturated not only with anions (which could have been expected similar to anodic oxidation in hydrous electrolytes), but also with cations.

Considering the fact that the issue of cations presence in AOF is of great interest, potassium quantitative content in oxide has additionally been inspected by the method of fluorescent X-ray test as to intensity of K-fluorescent X-rays excluding background. As for potassium model KCl crystal has been applied. Assuming 50% potassium content in the model and linear dependence of radiation intensivity from concentration of the element it has been calculated potassium concentration value constituting the rate close to 9%, which together with the abovementioned data of X-ray diffractometry survey allows to point on formation of potassium solid compounds in niobium oxide at AOF crystallization.