Influence of oxidation temperature on surface morphology and corrosion resistance of a biocompatible cast TiNi-based alloy

Surface properties of TiNi-based alloys strongly influence their corrosion resistance and biocompatibility, which are critical for biomedical applications. In physiological environments, these properties are governed by the stability of a passive oxide film, mainly composed of TiO₂, which limits nickel ion release. This study investigates how air oxidation affects the surface morphology and corrosion behavior of a cast TiNi-based alloy.

Disc-shaped specimens with a diameter of 25 mm and a thickness of 2 mm were cut from the ingot using electrical discharge machining. Oxidation was performed in air at 300, 350, 400, 450, and 500 ℃ for 60 min. Surface morphology and roughness were analyzed by scanning electron microscopy and atomic force microscopy. Corrosion properties were evaluated by linear sweep voltammetry in 0.9 wt.% NaCl solution using a three-electrode cell. Corrosion rates were calculated according to ASTM G102.

Oxidation at 300-350 ℃ reduced the average surface roughness relative to the initial state (S_a = 6.64 nm), indicating the formation of a thin and dense TiO₂ layer. At 400 ℃, roughness increased moderately because of nonuniform oxide growth. At 450-500 ℃, roughness rose sharply to 138.8 nm, reflecting the development of a multilayer heterogeneous oxide structure. Electrochemical measurements showed that these morphological changes strongly affected corrosion behavior. The highest corrosion resistance was obtained after oxidation at 350 ℃, with a corrosion rate of 0.08 × 10^-3 mm/year. Further temperature increase led to a decline in protective properties due to oxide heterogeneity.

The results demonstrate that air oxidation significantly affects the surface structure and corrosion behavior of cast TiNi alloy. Oxidation at 350 ℃ promotes the formation of the most homogeneous oxide layer and provides the highest corrosion resistance, making this treatment the most promising for biomedical applications.

This work was supported by the Strategic Academic Leadership Program «Priority 2030».