Effect of thermal oxidation on the surface characteristics and corrosion behavior of a Ta-implanted Ti-50.6Ni shape memory alloy

A NiTi shape memory alloy (SMA) modified by Ta ion implantation was subjected to oxidation treatment in air at 723 and 873 K. Atomic force microscopy (AFM), Auger electron spectroscopy (AES), and grazing incidence X-ray diffraction (GIXRD) measurements were conducted to investigate the surface characteristics, including surface topography, elemental depth profiles, and surface phase structures. The surface roughness of the Ta-implanted NiTi increases after oxidation, and the higher the oxidation temperature is, the larger the value is. The surface of the Ta-implanted NiTi oxidized at 723 K is a nanolayer mainly composed of TiO₂/Ta₂O₅ and TiO with depressed Ni content. The Ta-implanted NiTi oxidized at 873 K is mainly covered by rutile TiO₂ in several micrometers of thickness. Potentiodynamic polarization tests indicated that the corrosion resistance of the Ta-implanted NiTi was improved after thermal oxidation at 723 K, but a negative impact was found for the Ta-implanted NiTi oxidized at 873 K.