Corrosion products and mechanism on NiTi shape memory alloy in physiological environment
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Chenglin Chua) Department of Physics & Materials Science, City University of Hong Kong, Kowloon, Hong Kong; and School of Materials Science and Engineering, and Jiangsu Key Laboratory for Advanced Metallic Materials, Southeast University, Nanjing 211189, China
Yunchang Xin Department of Physics & Materials Science, City University of Hong Kong, Kowloon, Hong Kong; and School of Materials Science and Engineering, Chongqing University, Chongqing 400044, China
Shuilin Wu Department of Physics & Materials Science, City University of Hong Kong, Kowloon, Hong Kong; and China Ministry of Education, Key Laboratory for the Green Preparation and Application of Functional Materials, School of Materials Science and Engineering, Hubei University, Wuhan 430062, China
Kelvin W.K. Yeung Department of Physics & Materials Science, City University of Hong Kong, Kowloon, Hong Kong; and Department of Orthopaedics & Traumatology, The University of Hong Kong, Hong Kong
Paul K. Chub) Department of Physics & Materials Science, City University of Hong Kong, Kowloon, Hong Kong (Received 31 August 2009; accepted 3 October 2009)
Despite many investigations on the corrosion behavior of NiTi shape memory alloys (SMAs) in various simulated physiological solutions by electrochemical measurements, few have reported detailed information on the corrosion products. In the present study, the structure and composition of the corrosion products on NiTi SMAs immersed in a 0.9% NaCl physiological solution are systematically investigated by scanning electron microscopy (SEM), x-ray energy dispersion spectroscopy (EDS), and x-ray photoelectron spectroscopy (XPS). It is found that attack by Cl results in nickel being released into the solution and decrease in the local nickel concentration at the pitting sites. The remaining Ti reacts with dissolved oxygen from the solution to form titanium oxides. After longterm immersion, the corrosion product layer expands over the entire surface and XPS reveals that the layer is composed of TiO2, Ti2O3, and TiO with relatively depleted Ni. The growth rate of the corrosion product layer decreases with immersion time, and the corrosion product layer is believed to impede further corrosion and improve the biocompatibility of NiTi alloy in a physiological environment. It is found that the release rate of nickel is related to the surface structure of the corrosion product layer and immersion time. A corrosion mechanism is proposed to explain the observed results.
I. INTRODUCTION
NiTi SMAs as biomaterials are used in medical implants and devices such as orthodontic wires, selfAddress all correspondence to these authors. a) e-mail: [email protected] b) e-mail: [email protected] DOI: 10.1557/JMR.2010.0051 350
J. Mater. Res., Vol. 25, No. 2, Feb 2010
expanding cardiovascular and urological stents, spine correction rods, bone fraction fixation plate and staples, and so on.1–5 The reasons for adopting NiTi SMAs in biomedical implants are their unique shape memory effects and superelasticity properties, low Young’s modulu
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