On the repassivation behavior of high-purity titanium and selected α, β, and β + α titanium alloys in aqueous chloride solutions

The repassivation characteristics of a titanium thin film evaporated from a high-purity Ti source and selected alpha (commercially pure Ti, Ti-5Al-2.5Sn), beta , and beta + alpha titanium alloys (Ti-15Mo-3Nb-3Al, Ti-15V-3Cr-3Al-3Sn, and Ti-3Al-8V-6Cr-4Zr-4Mo) were examined. Both the rapid thin film fracture and scratch depassivation methods were used in aqueous chloride solutions (0.6 M NaCl, 5 M HCl, 5 M LiCl, 5 M HCl+1 M TiCl sub 3 ). Bare surface open-circuit potentials followed the relationship E(V sub SCE )=-1.20 (pH=0)-0.043 pH based on the mixed potential established between the anodic Ti/Ti exp +3 , Ti /TiO sub 2 , and water or H exp + reduction reactions. Oxide formation after depassivation was of low overall current efficiency on all Ti materials; a large percentage of the anodic charge following depassivation contributed to dissolution. Consequently, an empirical expression was used to describe the anodic current density decay during repassivation; i=i sub o (t/t sub o ) exp -m . Potentiostatic current transients on rapidly fractured thin film Ti produced plateau bare-metal i sub o values > 100 A/cm exp 2 which were below the theoretical ohmic limit, m=1.0-1.4 depending on solution and potential and t sub o values from 20-30 mu s. Two anodic Tafel regions and a single cathodic region best described IR-corrected E-long i relationships for bare Ti in all electrolytes. LiCl and TiCl sub 3 inhibited bare surface dissolution but slightly delayed current density decay. Minimal differences between any of the repassivation parameters utilized were observed for selected alpha , beta and beta + alpha Ti alloys. The similarity was attributed to dominance of Ti exp +3 production in the total anodic charge during repassivation and predominantly TiO sub 2 formation in the passivating oxides of all alloys.