ON THE REPASSIVATION BEHAVIOR OF HIGH-PURITY TITANIUM AND SELECTED ALPHA, BETA, AND BETA-ALLOYS IN AQUEOUS CHLORIDE SOLUTIONS(ALPHA TITANIUM)

The repassivation characteristics of a titanium thin film evaporated f rom a high-purity Ti source as well as selected alpha (commercially pu re 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 w ere used in aqueous chloride solutions (0.6 M NaCl, 5 M HCl, 5 M LiCl, 5 M HCl + 1 M TiCl3,). Bare surface open-circuit potentials followed the relationship E(V-SCE) = - 1.20 (pH = 0) - 0.043 pH based on the mi xed potential established between the anodic Ti/Ti+3, Ti/TiO2, and wat er or H+ reduction reactions. Oxide formation after depassivation was of low overall current efficiency on all titanium materials; a large p ercentage of the anodic charge following depassivation contributed to dissolution. Consequently, an empirical expression was used to describ e the anodic current density decay during repassivation; i = i(0)(t/t( 0))(-n). Potentiostatic current transients on rapidly fractured thin f ilm Ti produced plateau bare-metal i(0) values greater than 100 A/cm(2 ) which were below the theoretical ohmic limit, m = 1.0 to 1.4 dependi ng on solution and potential and t(0) values from 20 to 30 mu s. Two a nodic Tafel regions and a single cathodic region best described IR-cor rected E - log i relationships for bare Ti in all electrolytes. LiCl a nd TiCl3 inhibited bare surface dissolution but slightly delayed curre nt density decay. Minimal differences between any of the repassivation parameters utilized were observed for selected alpha, beta, and beta + alpha titanium alloys. The similarity was attributed to dominance of Ti+3 production in the total anodic charge during repassivation and p redominantly TiO2 formation in the passivating oxides of all alloys.