Evolution of the passive film on mechanically damaged nitinol

Abstract

The corrosion behavior of Nitinol-based medical implants is critical to their success in vivo. Contemporary Nitinol-based medical implants are typically chemically passivated or electrochemically polished to form a protective passive film. However, mechanically formed surfaces caused by handling damage, fretting, or fatigue fracture may also be present on a device in vivo. In this study, mechanically polished surfaces are used to simulate mechanically damaged surfaces such that analytical techniques, including electrochemical impedance spectroscopy, open circuit potential monitoring, X-ray photoelectron spectroscopy (XPS), and Mott-Schottky analysis may be used to monitor the evolution of the passive film on mechanically damaged Nitinol. These mechanically polished Nitinol surfaces are compared with chemically passivated and electrochemically polished Nitinol surfaces and mechanically polished titanium surfaces in phosphate buffered saline solution. The mechanically polished Nitinol exhibits lower impedance at low frequencies, empirically modeled to a thinner film with lower film resistance than chemically passivated and electrochemically polished Nitinol and mechanically polished titanium. Moreover, the passive film on mechanically polished Nitinol continues to develop over time, increasing in its thickness and film resistance. This characterization demonstrates that mechanically formed surfaces may be initially less protective than chemically passivated and electrochemically polished Nitinol surfaces, but continue to become thicker and more resistant to electrochemical reactions with exposure to saline solution.

The intrinsically high pitting corrosion resistance of mechanically polished nitinol in simulated physiological solutions

“…Mechanically polished nitinol is covered by a thin oxide film of roughly 3 nm in saline solution under open circuit potential as measured by EIS, which agrees well with the previous results by EIS, 21,26 XPS 22,26 and AES. 41 The film polarization resistances of the investigated alloys are indicators of their general corrosion properties (Table III).…”

“…Our chemical composition analysis by energy dispersive spectroscopy (data not shown) suggests that these particles are of Ti 2 NiO x type intermetallic compounds and this agrees well with previous reports. 7,26 To quantitatively compare these surfaces of nitinol samples, we calculated the surface height parameters using the AFM image height data, based on the ISO standard 25178. The results are shown in Table I.…”

“…Mechanically 26 In addition, the surface roughness of nitinol has no significant effects on these electronic parameters.…”

“…Ti samples show the highest resistances ($750 kX/cm 2 ) which indicates that Ti has the lowest corrosion rate; all nitinol samples have a lower oxide film resistances (400-500 kX/ cm 2 ) in agreement with the literature. 26 However, the oxide film resistances on nitinol samples are still 10 times higher than the ones on Ni. This indicates that Ti oxides play a dominant role for the electron transfer between the solution and the surface film on nitinol.…”

“…The mechanically polished samples also more accurately represent the working conditions for stent applications where an oxide film develops on an abraded surface since these surfaces are being subjected to fretting or mechanical scratching during handling, surgery or service. 25,26 The aim of this study is to test if nitinol is intrinsically vulnerable to pitting corrosion in a simulated physiological solution. Furthermore, we evaluate various factors that affect the pitting corrosion property of nitinol, particularly the effects of the alloying chemistry, inclusions, surface roughness and the semiconducting properties of the oxide films formed on nitinol.…”