Pitting corrosion mechanisms of Ti-Cu-(Pd-) based metallic glasses in simulated physiological solution

Abstract

Ti-based bulk metallic glasses with very high strength-to-stiffness ratio are promising candidates for biomedical applications. However, their substantial Cu contents cause biocompatibility issues regarding cytocompatibility and corrosion resistance. This work focuses on the analysis of chloride ion-induced pitting corrosion processes of two prominent glassy alloys under free corrosion and anodic polarisation conditions in phosphate-buffered saline (PBS) solution (pH 7.4) at 37 °C. The Ti₄₇Zr_7.5Cu₃₈Fe_2.5Sn₂Si₁Ag₂ glass demonstrates high susceptibility to passive film degradation and breakdown during long-term testing under OCP condition and upon anodic polarisation (LVA). Electrochemical Impedance Spectroscopy (EIS) indicates the formation of a porous, non-continuous surface film with low resistance. The shape of the anodic polarisation by charging curves (APCC) indicates a limited ability for pit repassivation. The Ti₄₀Zr₁₀Cu₃₄Pd₁₄Sn₂ glass exhibits higher resistance to pitting corrosion, which alters the nature of surface degradation. Surface state analysis by means of SEM, EDX, XRD and XPS reveal that the corrosion products within the pits and their surroundings are enriched in Cu, Zr and Pd. Mechanisms for the development of pitting corrosion for the two alloys, including nanoligament formation and advanced degradation, are proposed and especially the role of Cu and Pd therein is discussed. These findings provide valuable insights into the corrosion mechanisms of Ti-Cu based glassy alloys and serve as a base for developing corrosion resistance strategies.