Unveiling the Contribution of Lactic Acid to the Passivation Behavior of Ti–6Al–4V Fabricated by Laser Powder Bed Fusion in Hank’s Solution

In actual physiological environments, bacteria can activate the immune system and release lactic acid. However, the detailed contribution of lactic acid to the passivation behavior of titanium (Ti) alloys is still unclear. The current work investigated the in vitro passivation behavior of Ti–6Al–4V (TC4) alloys fabricated by laser powder bed fusion in Hank's solution with and without adding lactic acid. Electrochemical methods, inductively coupled plasma atomic emission spectrometer, and X-ray photoelectron spectroscopy were jointly used. Adding lactic acid decreases the corrosion resistance of samples by degrading the formed passive film. The film formed in the (lactic acid)-containing solution exhibits a higher level of oxygen vacancies and a lower thickness, attributed to the suppressed formation of Ti⁴⁺ transformed from Ti³⁺ and Ti²⁺. Moreover, the presence of lactic acid would increase the open circuit potential, relieve the ions release, and hinder the deposition of calcium phosphates within 24 h immersion.