Corrosion Properties of Surface Modified Ti-6Al-7Nb Alloy under Pulsed Plasma Nitriding and Nitrogen Ion Implantation Conditions

Abstract

A Ti-6Al-7Nb alloy was surface modified using pulsed plasma nitriding and nitrogen ion implantation in order to study its microstructural changes and corrosion performance. Pulsed plasma nitriding was performed for 30 h at 913 K in a gas mixture of 3:1 N₂:H₂ under a total pressure of 3 mbar. Nitrogen ion was implanted at 100 keV using a 150 keV accelerator at a dose of 2·5 × 10¹⁷ ions cm². Gracing incidence X-ray diffraction was employed to explore the phases formed after treatments. To characterise the nature of the modified layers produced and to correlate with the corrosion behaviour, SIMS was used. The samples were subjected to electrochemical studies, namely, open circuit potential (OCP)-time measurements and potentiodynamic cyclic polarisation in a simulated body fluid (Ringer's solution). The OCP of the pulsed plasma nitrided and N⁺ implanted samples were found to shift towards the noble direction (-189 mV_SCE and -82 mV_SCE, respectively) in comparison with the untreated sample (-338 mV_SCE). The passivation current density and area of the repassivation loop were found to decrease for both treatments compared with those in the untreated condition. Nitrogen ion implantation enhanced the passivability and reduced the corrosion kinetics of the alloy with increasing tendency for repassivation. It shows very low passivation current density compared with pulsed plasma nitriding (0·086 and 0·493 μAcm², respectively). The nature of the modified surfaces and the reason for the variation and improvement in corrosion resistance are discussed.