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-4.5A1-3V-2Fe-2Mo) and Ti-6A1-4V 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- 1V 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-6A1-4V 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.