Characteristics of oxide films on Ti-(10–75)Ta alloys and their corrosion performance in an aerated Hank's balanced salt solution

This study presents detailed quantitative characterisation of the native oxide films on Ti-(10–75)Ta alloys (in wt.%) using X-ray photoelectron spectroscopy (XPS) and transmission electron microscopy (TEM) and their corresponding corrosion performance in an aerated Hank's balanced salt solution. The oxide film was identified to be amorphous irrespective of the bulk Ta content, and its thickness decreases with increasing bulk Ta content. Noticeable enrichment of Ta2O5 in the oxide film occurred when the bulk Ta content reached 30 wt.%Ta and beyond, corroborated by the fraction of the Ta5+ valence state in the oxide film. Angle resolved-XPS (AR-XPS) analysis revealed that the surface oxide films are homogeneously distributed with no segregation in the top-most oxide film. A distinct inflection point in corrosion performance and capacitive response was observed at 30 wt.%Ta, beyond which corrosion resistance displayed a marked improvement. To ensure high consistency in the surface oxide structure and therefore excellent corrosion performance, the minimum bulk Ta content is recommended to be 40 wt% for Ti-Ta alloys. This threshold Ta content gives rise to about 20 vol.%Ta2O5 in the oxide film, likely forming a percolating network to offer the required corrosion resistance.