Kinetics of passivity of NiTi in an acidic solution and the spectroscopic characterization of passive films

Abstract

Anodic polarization of nitinol in acetic acid under galvanostatic conditions produces oxide films composed mainly of TiO 2 . An exponential current-field relation is valid during ionic conduction through the growing oxide, in which the field coefficient is related to the jump distance. Transport processes in anodic films have been discussed in terms of a cooperative mechanism developed for amorphous oxide films on valve metals, in which both metal and oxygen ions were involved in ionic conduction. For more crystalline oxide structure of passive films on nitinol, formed during a prolonged potentiostatic conditions, the charge transfer takes place only through the oxygen vacancies as mobile species via a high-field-assisted mechanism. Based on the results of the Mott-Schottky analysis, these films behave as n-type semiconductors indicating that oxygen vacancies formed during the film formation and growth act as electron donors. The barrier/protecting and electronic/ semiconducting properties of the passive films as well as their chemical composition were studied using electrochemical impedance spectroscopy and X-ray photoelectron spectroscopy.

Semiconductor-to-Metal Transition in Rutile TiO2 Induced by Tensile Strain

“…Due to the oxophilic nature of titanium, calcination at elevated temperatures results in the formation of a capping TiO 2 film. Our initial efforts are focused on 50 and 100 nm rutile TiO 2 films (Figure S8) that were grown by aerobic oxidation at 500 °C for 30 and 120 min, respectively. − …”

“…The foils were then oxidized at 500 °C under aerobic conditions for 30 or 120 min. Because of the oxophilic nature of titanium, calcination at elevated temperatures results in a rutile TiO 2 capping later, wherein the thickness can be controlled by annealing time and temperature. , From UV–vis interferograms, the thickness of the TiO 2 film is estimated to be ∼50 and 100 nm for samples oxidized 30 and 120 min, respectively . All TiO 2 films showed some nickel on the surface by XPS, and its concentration was Nitinol batch dependent.…”

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

“…) in terms of biocompatibility, the nickel content should be kept as low as possible to minimize the possibility of its release. The following Ti/Ni ratios were achieved by other methods: 33.1 [3] and 7.1 [31] by water boiling, 23.4 by water autoclaving [3], 11.5 by anodization in acetic acid [33], 3.6 by oxidation at 600°C [31], and 2.5 by chemical etching [3].…”

“…The spectra recorded on NiTi electrode covered with ODP layers prepared by immersion or by spraying were evaluated using parameters of standard peaks (Table 1) [31][32][33]. During the deconvolution of the spectra, all peak parameters, except height, were kept constant.…”

“…The Ti 2p 3/2 peak for metal state is centered at 454.4 eV, and that for Ni metal shows a main 2p 3/2 peak at 852.9 eV and a satellite (sat. )~6.5 eV above it [31][32][33]. As a standard for TiO 2 , XPS spectrum for Ti thermally oxidized for 1 h in an oxygen atmosphere at 450°C was recorded [32].…”