Electrochemical Behavior of Laser Powder Bed Fusion (L-PBF) Ti-6Al-4V Alloy: Influence of Phase and Grain Boundaries on Surface Passive Film Formation

Electrochemical dissolution behavior of laser powder bed fusion (L-PBF) Ti-6Al-4V alloy is investigated in a 3.5 wt% NaCl solution through electrochemical impedance spectroscopy (EIS), potentiostatic polarization, and Mott-Schottky analysis. To investigate the effect of microstructure on the corrosion passivation film and corrosion resistance, the microstructure (martensitic alpha '/alpha + beta phases and alpha grain boundaries (alpha GB)) of L-PBF Ti-6Al-4V alloys was modified via heat treatment. The microstructural results indicate that the heat treatment at 950 degrees C (HT-950) lead to a higher volume fraction of beta phase and finer alpha-laths, compared with those under other heat treatment temperatures. Intriguingly, the corrosion resistance of the HT-950 sample is significantly higher than that of the others because the refinement of grains contributes to preventing corrosion. The stable passivation film is attributed to the decrease in donor density (oxygen vacancies) and vacancy diffusion coefficients with increasing the film formation potential. This work illustrates the mechanism of passivation film during the corrosion process of L-PBF Ti-6Al-4V alloy, and would be helpful to develop additively manufactured Ti-6Al-4V alloys with a better corrosion resistance.