Characterization of passive oxide film on a Ti–5%Ta–1.8%Nb alloy on exposure to severe oxidizing conditions

This paper presents the results of a study on the characteristics of the passive oxide film that forms on the surface of an α + β Ti–5%Ta–1.8%Nb alloy, which possesses good corrosion resistance in severe oxidizing environment of boiling 11.5 M nitric acid. Through systematic structure–property studies, the microstructure with low corrosion rate (< 1 mpy) in liquid, vapor and condensate phases of nitric acid was identified. The characteristics of the passive film, which imparts corrosion resistance to the alloy, are influenced by its microstructure, temperature and concentration of the acid. The microstructure, thickness and composition of the oxide film were characterized using different techniques. TiO2, Nb2O5 and Ta2O5 formed on exposure to vapor and condensate phases, while TiO2 was observed on exposure to the liquid phase. Detailed microstructural studies showed that the passive film consists of nano-crystalline phases of titanium and tantalum oxides, predominantly anatase in an amorphous matrix. Based on these studies, the mechanism of corrosion of the alloy is derived.

Research Highlights►Liquid phase corrosion results in a thicker and protective oxide film. ►Oxide film is a mixture of amorphous and nano-crystalline anatase. ►Higher amounts of Nb2O5 and Ta2O5 form on vapor and condensate phase corrosion. ►High corrosion rate in condensate phase is due to selective dissolution of Ti in α phase.