Effect of Nb and Ta alloying on corrosion behavior and passive film chemistry of biomedical Ti–Zr-based medium-entropy alloys in PBS solution

Abstract

Ti–Zr-based medium-entropy alloys are promising candidates for biomedical implants owing to their favorable biocompatibility and ability to form protective passive films in physiological environments. However, the relationship between Nb/Ta-induced changes in α′/α″/β phase constitution and the corresponding corrosion and passivation behavior of Ti–Zr-rich alloys remains insufficiently understood. In this work, Ti–Zr–Nb and Ti–Zr–Ta medium-entropy alloy series with different Nb/Ta contents were fabricated by vacuum arc melting, with commercially pure Ti and Ti–6Al–4V used as reference materials. Their microstructure and surface electrochemical heterogeneity were characterized, while corrosion behavior and passive film chemistry were evaluated in PBS solution; Gibbs free energy analysis and early cell adhesion assessment were also performed. XRD and EBSD results reveal that increasing Nb/Ta content promotes the transition from α′/α″/β multiphase structures to β-dominated matrices, with Nb showing a stronger β-stabilizing effect than Ta. Electrochemical results demonstrate that corrosion resistance does not improve monotonically with increasing Nb/Ta content. Ta5 exhibits the lowest corrosion current density of 1.13 × 10⁻⁶ A cm⁻², whereas Nb4 shows the lowest passive current density among the designed alloys, 2.60 × 10⁻⁵ A cm⁻². Although Nb10 and Ta10 display higher interfacial impedance near OCP, they also exhibit higher corrosion/passive current densities and earlier current rise behavior during anodic polarization. XPS analysis identified TiO₂/ZrO₂-based passive films containing Nb- or Ta-containing oxide species, while the Gibbs free energy data indicate that the formation of these high-valence oxides is thermodynamically favorable. These results indicate that high impedance near OCP does not necessarily guarantee stable anodic passivation. Instead, relatively low Nb/Ta additions provide a better balance between corrosion resistance, passive current response, high-potential film stability, and early cell adhesion.