Improvement of the Corrosion Resistance of NiTi Stents by Surface Treatments
- PDF / 1,124,990 Bytes
- 6 Pages / 414.72 x 648 pts Page_size
- 64 Downloads / 179 Views
treatments 6,89. The oxide layer is very protective and may be very promising to improve both corrosion resistance and biocompatibility of NiTi9-'°. Also, it is able to sustain large deformations induced by the shape memory effect 9 . It has been shown that laser surface melting 8 treatment promotes the oxidation of NiTi and improve the corrosion resistance of the alloy8. However, this technique is very expensive and not very appropriate for implants of complex geometry such as stents. Modification of the oxide layer can also be achieved by more conventional methods such as chemical polishing and heat treatments in salt bath and in air' 1. Also, a study on Ti based implant materials demonstrated a reduction in the dissolution of ions by aging the surface oxide or by thermal oxidation 12. Electropolishing and nitric acid passivation 3 are two other techniques recommended for surface treatments of medical devices (ASTM-F86)' . These methods are simple, non expensive and effective to treat implants of different shapes. The present study investigate the effect of electropolishing, heat treatment (in air and in a salt bath) and nitric acid passivation to modify oxide layer properties on NiTi stents. Techniques such as potentiodynamic polarization tests, Scanning Electron Microscopy (SEM) and Auger Electron Spectroscopy (AES) have been used to develop relationships between corrosion behavior, surface characteristics and surface treatment. EXPERIMENT Material NiTi stents (50.8 at% Ni) have been manufactured by Nitinol Devices and Components (California, USA) by laser cutting diamond shaped apertures in NiTi tubings. The stents are 14 mm long and have a diameter of 4 mm in the expanded state. Five different groups of samples were prepared: 1. Non treated (NT) 2. Electropolished (EP) 3. Air Aged (AA) 4. Heat Treated (HT) 5. Passivated (PA) The first group of stents are supplied directly after the machining and expansion of the stents with an heavy oxide layer. Electropolished stents (EP) have been first, micro-abraded to remove mechanically the primary oxide layer, then, chemically polished at room temperature and finally, electropolished. After electropolishing, a mirror-like surface finish is obtained. Air aged samples (AA) are electropolished stents that have been air aged at 450TC to produce a light yellow oxide layer. The heat treated stents (HT) are electropolished stents that were heat treated in salt at 500TC to produce a dark blue oxide layer. The passivated stents (PA) are electropolished stents that were passivated in a nitric acid solution at room temperature. They optically exhibit the same surface finish as the electropolished stents. All surface treatments were performed by Nitinol Devices and Components. Method The corrosion resistance of 15 stents (3 for each surface treatment) was evaluated by anodic polarization tests. The potentiodynamic experiments were carried out using a computer controlled potentiostat (EG&G Princeton Applied Research, model 273). The tests were conducted in 37°C Hank's physiologica
Data Loading...
