Surface characteristics and corrosion resistance properties of TiNi shape memory alloy coated with Ta

Ti-50.6 at.% Ni alloy samples were coated with tantalum by the arc ion plating method with the aim to improve their radiopacity. Surface characteristics and corrosion resistance properties were investigated in the present study. The results of XRD show that the film as-deposited was composed of a metastable tetragonal β-phase tantalum which can transform into α-phase tantalum after annealing at 700 °C, 800 °C, 900 °C for 1 h, respectively. TEM observations show that thin nano-structured Ta layers were obtained in the as-deposited samples. Mixture of microcrystallite (approx. 40 nm) and large-size crystal (approx. 150 nm) α-phase tantalum was formed in the film after annealing at 700 °C. When annealed at 900 °C, the film is composed of dominantly uniform large-size α-phase tantalum. The results of XPS survey and high resolution spectra show that a thin oxide film with Ta2O5 in the outmost layer and tantalum suboxides in the inner layer, are formed inside tantalum coating as a result of natural passivation of Ta in the atmosphere. The corrosion resistance was determined by electrochemical methods in 0.9% NaCl solution at 37 °C. Both the as-deposited and annealed samples exhibit excellent corrosion resistance property. Compared to the untreated coating samples, the annealed Ta coating samples exhibit higher open-circuit corrosion potential (Ecorr), breakdown potential (Ebr), and lower passive current density (Ip). The results indicate that the annealing treatment can improve the corrosion resistance property.