Surface evolution and corrosion characteristics of an air-heat-treated biocompatible alloy based on TiNi

Abstract

For nickel–titanium products intended for medical applications, surface stability in physiological environments is one of the key factors determining the long-term performance of the material. Despite the presence of a naturally formed passive layer, TiNi alloys may undergo corrosion degradation, accompanied by changes in the surface state and the risk of nickel ion release. Therefore, selecting surface-treatment conditions that reduce the corrosion activity of the material without the use of complex or multistep technologies remains an important task. This study investigates the effect of heat treatment in air at 300–500 ℃ for 60 min on the morphology, elemental composition, roughness, and corrosion behavior of a cast Ti49.8Ni50.2 alloy. The sample surfaces were examined using scanning electron microscopy, energy-dispersive X-ray spectroscopy, and atomic force microscopy. Corrosion behavior was evaluated by linear sweep voltammetry in a 0.9% NaCl solution. Heat treatment at 300–400 ℃ was shown to reduce the corrosion rate to 0.08–0.12×10−3 mm/year. The most effective treatment temperature was 350 ℃, at which the corrosion rate decreased by approximately a factor of 31 compared with the untreated sample. Increasing the temperature to 450–500 ℃ produced the opposite effect because of the pronounced development of the surface topography. The correlations obtained provide a basis for understanding thermally induced surface transformations in TiNi and for selecting controlled air-oxidation conditions aimed at forming a corrosion-resistant surface state of relevance to the medical application of nickel–titanium-based alloys.