EIS diagnosis on the corrosion behavior of TiN coated NiTi surgical alloy

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Abstract

In this work titanium nitride (TiN) coatings were deposited on NiTi surgical alloy by arc ion plating (AIP). The open circuit testing and electrochemical impedance spectroscopy (EIS) have been employed to study the corrosion performance of the TiN coated samples in Troyde’s simulated body fluid. It was found that the TiN coating showed a good corrosion resistance at the beginning of long-term immersion; with the increase of immersion time, however, the corrosion resistance of the coating deteriorated after nearly 24 h of immersion; moreover, the large areas of the coating had fallen off the substrate after 30 days of immersion.

Introduction

Nitinol (NiTi) is both superelastic and biocompatible. Moreover, the narrow temperature range within which Nitinol’s superelasticity is exhibited includes body temperature. Thus, Nitinol has become the “material of choice” for designers of the implants. For applications in the human body, surface properties and corrosion resistance are the most important material characteristics determining the biofunctionality of an implant [1].

Many studies of the in vitro or in vivo corrosion resistance of NiTi have been reported [2], [3]. It has been found that NiTi exhibits poor resistance to localized corrosion in chloride-containing environments, with arguably low pitting potential values. Besides, the self-healing of the passive film on NiTi is a slow and difficult process. And moreover, once a NiTi implant is placed in the physiological environment, Ni ion release will inevitably occur, which will be released into the adjacent tissues, with potentially harmful local and systemic effects.

Surface modification used to improve the corrosion resistance of NiTi seems attractive. Titanium nitride coating has been accepted by the Federal Food and Drug Administration for use with titanium alloy parts because of its biocompatibility [3]. Coating of NiTi with TiN is known to improve corrosion behavior of NiTi alloy [4]. Endo [5] pointed out that the TiN coating by arc ion plating improved corrosion resistance of NiTi alloy in 0.9% NaCl solution in the low-potential region, but increased pitting corrosion susceptibility at potentials above 500 mV. However, detailed discussions of this issue were beyond these papers.

In the present work, the corrosion behavior of TiN coated NiTi alloy by arc ion plating in Troyde’s simulated body fluid (TSBF pH = 7.4) at 37 ± 1 °C was studied. The EIS test was used to detect the pinholes in the TiN coating and assess their effect on the corrosion-resistance behavior over longer immersion time. The degradation mechanism of the TiN coating is also discussed.

Section snippets

Sample preparation and characterization

The substrate material used in this study was the NiTi surgical alloy, which was cut into standard samples with the size of ϕ 12 mm × 2 mm. The substrates were ground and polished to obtain a mirror-like surface, and then cleaned using ultrasonic cleaner; firstly in acetone, and then in ethanol. TiN coating were prepared in a Bulat-6 AIP system using two 99.9% pure titanium targets. The distance between samples and cathodic arc targets was 240 mm or so. Prior to deposition, the substrate surface was

Coating characterization

Fig. 1 shows the X-ray diffraction pattern of the coating. Defined peaks of TiN are exhibited clearly, i.e., the (1 1 1), (2 0 0) and (2 2 0) diffraction peaks.

The planar and cross-section SEM images of the TiN coating are shown in Fig. 2. There are many macro-particles (MPs) on the surfaces of the film, as one can see from Fig. 2(a). These MPs are not uniform, with spindle or round shape, which is characteristic of AIP method. The thickness is about 1.08 μm in size.

Open circuit testing

The variation of corrosion

Conclusions

The failure process of the TiN coating for Nitinol surgical alloy in Troyde’s simulated body fluid (pH = 7.4) has been investigated. Electrochemical tests showed that the process that the TiN coating failed was caused by the pinholes and macro-particles in the film. The coating offered good protection for NiTi alloy at the beginning of long-term immersion. With the increase of immersion time, however, the solution penetrated into the interface through the pinholes and macro-particles, and the

Acknowledgments

The project is supported by NAMCC 863 (No. 2002AA326010). The authors express their gratitude to Prof. Liang C.H. for supporting the EIS tests (School of Chemical Engineering, DUT China).

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