Influence of preparation methods on the properties of self-assembled films of octadecylphosphonate on Nitinol: XPS and EIS studies

Abstract

The NiTi alloy (Nitinol), with its favorable micro-structured properties and self-passivity (resembling that of pure Ti) is used as an implant material for arterial stents and orthodontic wires. During the long term contact of the alloy with aggressive environment of human body, corrosion by releasing Ni2+ ions can occur. Thus, the usefulness of such material can be dramatically enhanced if its interface structure and surface chemistry are controlled. The octadecylphosphonate interface (ODP) synthesis, which involves a self-assembled covalently (monodentate type) bonded film of octadecylphosphonic acid (ODPA) on the oxide covered NiTi surface, produces stable and corrosion resistant interfaces. This paper introduces integrated approach to the characterization of the NiTi/ODP interfacial architecture as well as the structure of the electrified ODP/solution interface using high-resolution XPS and in situ EIS measurements. The main focus of this work was to determine the influence of the ODPA deposition method (spray and immersion) on the depth-dependent structural characteristics and orientation of ODPA molecules in the surface film by means of angle resolved XPS. Mechanically strong and chemically stable NiTi/phosphonate interfaces have the potential for their successful implementation in stent technologies.