Structure and Chemical State of Oxide Films Formed on Crystalline TiNi Alloy and Glassy Ti-Ni-Ta-Si Surface Alloy

The stable oxide layers formed by chemical or electron-beam treatments of TiNi shape memory alloys are able to provide a good corrosion resistance and prevent release of toxic Ni. The mechanisms responsible for corrosion behavior of Ti-bearing alloys are currently not completely clear. In this work, the structure and electrochemical properties of surface oxides on TiNi alloy and Ti-Ni-Ta-Si surface alloy were examined. The glassy structure of the surface alloy was produced by a liquid phase mixing of Ti60Ta30Si10 (at.%) film with a TiNi substrate using a low-energy high-current electron beam. The reference electropolished TiNi alloy and the Ti-Ni-Ta-Si surface alloy exhibit thin (2-4 nm) glassy-crystalline oxide layers varying by chemistry and phase composition as confirmed using high-resolution transmission electron microscopy and x-ray photoelectron spectroscopy. It is shown that during anodic polarization in 0.9% NaCl and Lock-Ringer solutions, the oxide layer on the Ti-Ni-Ta-Si surface alloy possesses an enhanced passivation ability compared to the reference sample. We have revealed that the absence of oxidation species Ni2+ and the lower nickel concentration in the subsurface layer are closely related with corrosion resistance. A scheme describing the formation of corrosion products via transport of nickel ions through the free volumes (interstitial spaces, interfaces between clusters/particles) inside the oxide layer was proposed.