Calcination condition effect on microstructure, electrochemical and hemolytic behavior of amorphous nanotubes on Ti6Al7Nb alloy

• Ti6Al7Nb anodization in a hybride electrolyte leads to specific nanotubes.
• As formed amorphous nanotubes were annealed at different conditions.
• The microstructure of annealed nanotubes on TiAlNb alloy was investigated.
• Nanotubes’ stability was correlated with rutile and anatase ratios.
• Higher temperature annealing leads to improved stability and hemocompatibility.

Controlled-diameter TiO2 nanotubes were obtained by electrochemical anodization of Ti6Al7Nb in a hybrid organic–inorganic electrolyte as glycerol:H2O 60:40 vol.% + 0.5 wt.% NH4F. The anodization condition was 15 V for 2 h. According to microstructural and X-ray diffraction (XRD) analysis, as-formed TiO2 nanotubes are amorphous but by subjecting to thermal treatments the structure becomes crystalline with a specific ratio of the crystalline phases of TiO2, anatase and rutile. The nanotubes' diameter and length were evaluated by scanning electronic microscopy (SEM) analysis, porosity was computed and morphological parameters were also discussed as a function of the applied heat treatment conditions. The electrochemical behavior of annealed and as-formed samples was evaluated with Tafel plots and electrochemical impedance spectroscopy (EIS) and was further correlated with microstructural characteristics. This investigation brings to attention, the desirable characteristics of structurally oriented TiO2 nanotubes for bioapplications. Amorphous nanotubes are converted into a crystalline phase and after calcination a range of stability and performance was proposed taking into account the corrosion rate, the resistance of the barrier layer determined from EIS measurements and hemocompatibility.