Distinctive transpassivation behavior and characteristics of passive films of laser powder bed fusion produced and annealed Ti–6Al–4V in Hank's solution

Passivated Ti alloys typically demonstrate enhanced corrosion resistance due to the formation of passive films. Intriguingly, in comparison to the annealed Ti–6Al–4V, laser powder bed fusion (LPBF) produced Ti–6Al–4V exhibits significantly attenuated transpassivation behavior in Hank's solution. This work investigated the electrochemical responses of both samples before and after the transpassivation region to illustrate the underlying mechanism for the phenomenon. In the passivation region, both samples displayed analogous passivation behavior, with the annealed and LPBF-produced Ti–6Al–4V having the current densities of 6.64×10 -6 and 8.14×10 -6 A cm -2 at 1 V, respectively. Between 1.5 V and 2.6 V, the oxygen evolution reaction occurred, resulting in the transpassivation phenomenon of the annealed Ti–6Al–4V. The current density increased substantially because of the consumption of the passive film. Following the transpassivation region, both samples underwent repassivation. In the repassivation region, the LPBF-fabricated Ti–6Al–4V had a current density (4.42×10 -4 A cm -2 ) that was merely one-tenth of that of the annealed counterpart (5.54×10 -5 A cm -2 ). Through various electrochemical investigations, attenuated transpassivation behavior is attributed to the reduced oxygen vacancies in the passive film of LPBF-produced Ti–6Al–4V. Under 1 V, the films formed on ATC4 and LTC4 had oxygen vacancy densities of 3.01×10 19 and 1.69×10 19 cm -3 , respectively. Therefore, fewer hydroxide radicals were trapped on the film surface. As such, both the oxygen evolution reaction and the consumption of passive film are mitigated for the LPBF-produced Ti–6Al–4V. The attenuated transpassivation behavior confers upon the LPBF-produced Ti–6Al–4V a low corrosion current density and a high biocompatibility in the repassivation region.