Corrosion products and mechanism on NiTi shape memory alloy in physiological environment

Abstract

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 long-term 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.

The Electrochemical Behavior of the NiTi Alloy in Different Simulated Body Fluids

“…NiTi alloy is susceptible to pitting corrosion in chloride containing solutions 30 . Attack by Cl -in NiTi result in Ni being released into the solution and decrease in the local Ni concentration at the pitting sites 31 . The remaining Ti reacts with dissolved oxygen from the solution to form titanium oxides in the corroded area 31 .…”

“…Attack by Cl -in NiTi result in Ni being released into the solution and decrease in the local Ni concentration at the pitting sites 31 . The remaining Ti reacts with dissolved oxygen from the solution to form titanium oxides in the corroded area 31 . The corrosion product layer expands over the entire surface and is composed of TiO 2 , Ti 2 O 3 , and TiO with depleted Ni 31 .…”

“…The remaining Ti reacts with dissolved oxygen from the solution to form titanium oxides in the corroded area 31 . The corrosion product layer expands over the entire surface and is composed of TiO 2 , Ti 2 O 3 , and TiO with depleted Ni 31 . The change in the Ni release rate is related to the corrosion defects such as pitting pores on the NiTi specimens 31 .…”

“…The corrosion product layer expands over the entire surface and is composed of TiO 2 , Ti 2 O 3 , and TiO with depleted Ni 31 . The change in the Ni release rate is related to the corrosion defects such as pitting pores on the NiTi specimens 31 . In the early immersion period, nickel ions area released gradually into the surrounding solution 31 .…”

“…The change in the Ni release rate is related to the corrosion defects such as pitting pores on the NiTi specimens 31 . In the early immersion period, nickel ions area released gradually into the surrounding solution 31 . As the immersion continues, corrosion results in the formation of pitting pores that promote nickel release, leading to a high Ni release rate 31 .…”