Effect of Laser Shock Peening on the Passivation Behavior of Subtractively and Additively Manufactured Ti–6Al–4V Alloys in pH 2 Buffer Solution

The effects of laser shock peening (LSP) on subtractively manufactured (SM) and additively manufactured (AM) Ti–6Al–4V alloys in pH 2 buffer solution were investigated. LSP increased the surface roughness from 0.25 ± 0.05 μm to 0.6 ± 0.1 μm, raised Vickers hardness by 12–16%, and introduced compressive residual stresses of 400–950 MPa. Microstructural analysis indicated that LSP promoted β-phase formation and grain refinement in SM alloys, while reducing the α′-phase fraction in AM alloys. Electrochemical testing revealed that all LSP-treated specimens exhibited active–passive transitions, unlike the stable passive response of unpeened samples. The corrosion rate (icorr) decreased from approximately 5 × 10−6 to 1 × 10−6 A·cm−2 after LSP. During 24 h potentiostatic polarization at 1.3 VSCE, the passive current density stabilized at 10−8–10−7 A·cm−2, with LSP AM specimens exhibiting values approximately twice those of their unpeened counterparts. Mott–Schottky analysis confirmed that the donor density (ND) in the SM alloy changed negligibly after LSP, indicating a stable passive alloy. In contrast, the ND for the AM alloy increased from 1 × 1019 to 3 × 1019 cm−3, suggesting an oxygen-vacancy-rich, less stable passive film. Overall, LSP reduces the corrosion rate primarily through the introduction of compressive residual stress but may impair the long-term passive-film stability of AM Ti–6Al–4V owing to defect generation. In contrast, the SM alloy maintains passive-film stability under identical treatment conditions.