Atomic Layer-Deposited Al₂O₃ Coatings on NiTi Alloy

Atomic layer deposition is introduced as a method suitable for preparation of Al₂O₃ layers on the surface of NiTi medical devices such as stents because of the excellent thickness control and conformal protective coating on complex structures. The corrosion properties of NiTi plates with Al₂O₃ coatings of various thicknesses in an environment similar to that occurring in the human body were studied using open circuit potential, potentiostatic electrochemical impedance spectroscopy, and cyclic polarization tests. It shows that the layer thickness plays a key role in the inhibition of corrosion. The thinner layers are more diffuse and make it easier for anodic reaction of passive NiTi with protective TiO₂ underneath of Al₂O₃, while the thicker layers have the barrier effect with local pores initiating pitting corrosion. The results of our electrochemical experiments consistently show that corrosion properties of thick Al₂O₃ coatings on NiTi plate are inferior compared to the thin layers.