Structure of the oxide film on Tia6Ta alloy after immersion test in 8 mol/L boiling nitric acid medium

By using X-ray photoelectron spectroscopy (XPS), X-ray diffractometer (XRD) and scanning electron microscopy (SEM), we investigate the corrosion behavior and the structure of the oxide film of Tia6Ta alloy that is subjected to the immersion corrosion test in 8 mol/L boiling nitric acid for 432 h. Based on the phase constitution indentified by depth profile XPS, the oxide film could be divided into three sub-layers along its thickness direction: the chemical stable TiO2 and Ta2O5 are present in layer I; the sub-oxide Ti2O3 and TaO are present in the layer II and layer III, and the high valence oxide evolutes from their sub-oxide gradually. Owing to the shielding effect of Ta2O5, the corrosion rate of the Tia6Ta alloy decreases from 0.051 mm/y to 0.014 mm/y with increasing immersion time, showing an excellent corrosion resistance in 8 mol/L boiling nitric acid.