Nanotube formation of Ti-6Al-4V alloy and its corrosion behavior

Abstract

In this study, the nanotube formation and corrosion behavior of Ti-6Al-4V alloy with α- and β-phases were researched with various experimental instruments. For this study, nanotube formation on the sample was performed using the anodic oxidation method by applying a voltage of 30 V using a direct current power supply for 1 h using a 1.0 M H3PO4 + 0.8 wt.% NaF solution at 25 °C. The corrosion properties of specimens were examined by potentiodynamic testing (potential range of -1000–1500 mV) in a 0.9% NaCl solution using a potentiostat. The nanotube morphology of the alloys was observed by field-emission scanning electron microscopy, X-ray diffractometry, and energy dispersive X-ray spectroscopy. The nanotubes in the α-phase were uniformly formed on the surface of the Ti-6Al-4V alloy, but nanotubes in the β-phase were irregularly formed. The nanotube-formed alloy had a wider passivation range and higher corrosion current density than the bulk alloy. The morphologies of the corrosion surfaces show that the edge trace around the β-phase was reduced, and the anatase phase increased on the corroded surface. In cross-sectioned nanotube layer after corrosion test, a deep pit was observed in the part where the nanotubes were removed. XRD analysis showed that the anatase and rutile phases were not formed in bulk alloys, but anatase and rutile phases were detected in the nanotube formed alloy, and the peak of the anatase increased on the corroded surface. From the result of potentiostatic test, the variation of current density from the beginning is almost the same, but the current density of the nanotube formed surface was slightly higher than that of non-nanotube formed surface with time.