Corrosion of NiTi Wires with Cracked Oxide Layer
Corrosion behavior of superelastic NiTi shape memory alloy wires with cracked TiO2 surface oxide layers was investigated by electrochemical corrosion tests (Electrochemical Impedance Spectroscopy, Open Circuit Potential, and Potentiodynamic Polarization) on wires bent into U-shapes of various bending radii. Cracks within the oxide on the surface of the bent wires were observed by FIB–SEM and TEM methods. The density and width of the surface oxide cracks dramatically increase with decreasing bending radius. The results of electrochemical experiments consistently show that corrosion properties of NiTi wires with cracked oxide layers (static load keeps the cracks opened) are inferior compared to the corrosion properties of the straight NiTi wires covered by virgin uncracked oxides. Out of the three methods employed, the Electrochemical Impedance Spectroscopy seems to be the most appropriate test for the electrochemical characterization of the cracked oxide layers, since the impedance curves (Nyquist plot) of differently bent NiTi wires can be associated with increasing state of the surface cracking and since the NiTi wires are exposed to similar conditions as the surfaces of NiTi implants in human body. On the other hand, the potentiodynamic polarization test accelerates the corrosion processes and provides clear evidence that the corrosion resistance of bent superelastic NiTi wires degrades with oxide cracking.
Keywordsbending electrochemical corrosion tests martensitic transformation shape memory alloy superelastic NiTi wires
This research has been supported by the 7RPEU project SMARTNETS (FP7-NMP2010-SMALL-262806) and by the projects P107/12/0800 and P108-12-P111 of the Grant Agency of the Czech Republic. The authors would like to express special thanks to prof. Jan Van Humbeeck from MTM Department KU Leuven for consultancy and providing access to the FEI SEM facility as well as to Neil Othen from GATAN for the same regarding the tensile stage Microtest 2000EWN.
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