Corrosion response of surface engineered titanium alloys damaged by prior abrasion

During use, bio-implant materials like Ti-alloys need to endure and resist corrosion–wear. Here, a synergy between corrosion and wear takes place; wear is increased by the action of corrosion and corrosion is increased by the action of wear. In this paper, the corrosion response of SP700 (Ti–4.5Al–3V–2Fe–2Mo) and Ti–6Al–4V alloys, with and without surface treatments are described. Both kinds of alloy were surface-treated with: (i) an oxygen diffusion hardening process called “thermal oxidation” (TO) and (ii) a TiN coating procedure known as arc ion plating (AIP). The effect of prior mechanical (abrasive) surface damage on corrosion behaviour was simulated by scratching samples using a diamond indenter. A standard potentiodynamic or cyclic polarisation (CP) procedure, was conducted in de-aerated 0.89 wt.% NaCl (physiological saline) controlled at 37°C at a scan rate of 0.167 mV/s, from −1 V Ag/AgCl up to +4 V Ag/AgCl. Results showed that the TO-treated samples offered the best resistance to the sequential actions of mechanical damage (simulated abrasion) and corrosion. This is attributed to the TO-treatment producing a stable oxide layer, for both Ti-alloys, which displayed a superior repassivation rate and adhesive strength compared to untreated and TiN coated Ti-alloys. The TiN coated Ti-alloys were also prone to pitting and blistering during corrosion testing whilst the TO-treated alloys were not affected by blistering. However, the TO-treated Ti–6Al–4V showed evidence of superficial pitting. On balance, the TO-process appears to offer significant future promise for use in bio-implants and other engineering components subjected to corrosive-wear processes.