Time-Resolved Corrosion Behavior of Transition-Metal-Based High-Entropy Alloy in Saline and Phosphate Buffer Solutions

This study focuses on time-resolved surface modifications of a single-phase Ti25Zr25Nb15V15Ta20 high-entropy alloy (HEA) when immersed in 0.9 wt% NaCl and phosphate-buffer solutions (PBS) at 37 °C. A remarkable transition from high ionic diffusion to electron conduction was observed in PBS, whereas the existing conductivity in NaCl solution was further enhanced after 3 h of exposure. During in-situ testing, NaCl improved passivation conceived by the decrease in passivation-current density and increase in Tafel slope. Heterogeneously dispersed oxide particles with NaCl could have accounted for the moderate increase in conductivity while not affecting the capacitive behavior. The Tafel slope decreased after 2 h of immersion in PBS linked to K+ and P−3 accumulation on the surface. The pronounced change in the post-PBS treated sample was also revealed by a four-fold increase in HEA-electrolyte resistance. A visible decrease in the constant-phase-element parameter of the HEA-electrolyte interface after long-term PBS immersion indicated a rise in electrode conductivity and ionic build-up on the surface. The findings suggest that compared to PBS, the selected HEA has a faster passive-layer formation in NaCl with smaller changes in interface resistivity upon long-term immersion, which is promising for enhanced protein-adsorption rates and loading amount.