Microstructural evolution and corrosion behavior of electron beam melted Ti55531 alloy: Effects of build height and Mo-rich segregation

Electron beam melting (EBM) of near-β Ti–5Al–5Mo–5V–3Cr–1Zr (Ti55531) alloy produces pronounced microstructural heterogeneity along the build direction, resulting in height-dependent corrosion behavior. In this work, Ti55531 was fabricated with a layer thickness of 50 μm under a powder bed preheating temperature of 780–790 °C, using a beam current of 13.5 mA and a scan speed of 6.3 m s−1 during melting. The microstructure along the build direction and electrochemical corrosion behavior were systematically investigated in naturally aerated 3.5 wt. % NaCl solution. Electrochemical characterization included impedance spectroscopy over 10−2–105 Hz with a 10 mV perturbation and potentiodynamic polarization at 1 mV s−1 for the V1–V3 specimens, while V2 specimens from the same build-height region were potentiostatically polarized at 0.2, 0.4, 0.6, and 0.8 V (versus saturated calomel electrode) for 7200 s to examine the potential-dependent localized corrosion and repassivation behavior. The α/β microstructure became finer and more homogeneous with increasing build height, which was associated with improved overall corrosion resistance. In contrast, spherical Mo-rich segregated regions containing approximately 96 wt. % Mo acted as preferential sites for localized corrosion through local microgalvanic coupling with adjacent Al-enriched regions, promoting localized dissolution and phase spallation. At 0.6 V, the electrochemical response indicated a comparatively protective repassivated state, characterized by enhanced interfacial resistance and suppressed defect transport. Further increasing the potential to 0.8 V resulted in reduced passive-film stability and intensified metastable corrosion, accompanied by repeated local breakdown–repassivation events. These results demonstrate that build-height-dependent microstructural evolution and Mo-rich segregation play distinct roles in governing the overall and localized corrosion behavior of EBM Ti55531 alloy.