Corrosion of NiTi Wires with Cracked Oxide Layer

Monitoring Tensile Fatigue of Superelastic NiTi Wire in Liquids by Electrochemical Potential

“…A thermodynamic analysis suggested that the most stable interface structure is that with Ti vacancies in the NiTi surface. This is in perfect agreement with frequent literature evidence [14,18] for Nirich layer in the subsurface NiTi matrix.…”

“…As the oxide cracks are extremely thin, they can be observed by SEM only under applied stress since they close as the shear band moves back upon unloading furnace at 510°C for 4 min and cooled in water for 3 min. As a result of this treatment, all wire samples used in this study have the same microstructure and same surface oxide as the NiTi spring samples studied in our earlier closely related work [13][14][15] (*70 nm TiO 2 layer with excess Ni on the top and *40-nm-thick Ni-enriched layer underneath [14]). Capillaries were clamped on both ends of the wire sample, electrically isolated with Teflon and epoxy and gripped into the testing rig.…”

“…This work deals with an experimental investigation of structural fatigue of superelastic NiTi wire transforming cyclically in simulated biofluids at constant body temperature under cyclic tension. A motivation comes from our earlier work on fatigue of superelastic NiTi springs cycled in simulated biofluids [13][14][15] targeting particularly the problem of unexpected failure of gastrointestinal esophageal, and/or tracheal NiTi stents [16] braided from NiTi wires or orthodontic wires which are exposed to mechanical loads in corrosive biofluids-i.e., NiTi implants which transform martensitically in the body. Clinical reports of unexpected random failures of implanted braided NiTi stents show that the NiTi wires of implanted stents unexpectedly fractured soon after implantation and the explanted stents exhibited serious embrittlement [17].…”

“…A question whether the unexpected embrittlement and fracture of implanted NiTi stents shall be ascribed to mechanical fatigue stemming from the bulk or to corrosion attack stemming from the surface was addressed in our recent work [13][14][15]. It was concluded that both are probably involved in the gradual fatigue degradation and that cracking of the thin TiO 2 oxide layer (grown by thermal oxidation on the wire surface during the shape setting heat treatment) probably plays a key role in the degradation process.…”

“…Since the NiTi stent is typically heavily deformed during the microinvasive implantation, the oxide fractures and the wire of the implanted NiTi stent is protected against corrosion by the ''cracked TiO 2 oxide.'' Figure 1 shows a SEM image of the cracked oxide layer on a superelastic NiTi wire transforming via shear band propagation in tension [14]. It can be seen that the oxide layer behind the shear band front is broken by high density of regularly spaced thin oxide cracks oriented perpendicularly to the wire axis.…”