Ni Release Behavior of Electropolished NiTi Wires After Thermomechanical Pre-treatment

Abstract Risk assessment of NiTi requires careful consideration of Ni release for safe application. Although testing standards have been updated, fundamental aspects affecting Ni release from NiTi remain debated. This study aims to contribute to the fundamental understanding of Ni release by investigating the influence of thermomechanical pre-treatment on short- and long-term Ni release from electropolished NiTi wires. The wires were heat treated and strained + released before immersion in Ringer’s solution. Ni release was determined using inductively coupled plasma mass spectrometry. Microscopic analysis showed a smooth and homogeneous surface before straining, followed by formation of cracks and local removal of oxide after deformation and immersion testing, respectively, correlated with higher Ni release. Higher short-term Ni release was observed for all conditions, decreasing over time regardless of heat treatment duration. Compared to previous release tests on mechanically polished wires, electropolished wires showed lower short-term release of Ni. The results are discussed regarding factors influencing Ni release, including pre-deformation, oxide layer characteristics and the formation of metastable Ni-rich phases beneath the surface oxide. The highest Ni release was observed for conditions correlating with the formation of metastable Ni 4 Ti 3 and NiTi supersaturated in Ni beneath the oxide layer. Graphical Abstract