Comparative Electrochemical and Time-Dependent Passivation Behavior of Titanium Materials with Different Phase Constitutions in 3.5 wt.% NaCl at 37 °C

Ti alloys are widely deployed in chloride-containing marine and biomedical environments, where the stability of the passive film governs long-term corrosion resistance; however, systematic comparisons of different titanium grades under identical exposure protocols remain limited. This work comparatively investigates the phase constitution, microstructure, and time-dependent interfacial electrochemical responses of commercially pure titanium (CP-Ti), the metastable β alloy Ti-10Mo-6Zr-4Sn-3Nb (Ti-B12), and Ti-6Al-4V (TC4) in 3.5 wt.% NaCl solution at 37 °C. X-ray diffraction, optical microscopy, scanning electron microscopy, electron backscatter diffraction, open-circuit potential (OCP) monitoring, electrochemical impedance spectroscopy (EIS), and potentiodynamic and potentiostatic polarization were combined to examine whether the interfacial-resistance ordering persisted across different exposure histories and how it related to potential- and current-based electrochemical measures. CP-Ti exhibited a coarse single-α microstructure, Ti-B12 a nearly single-β equiaxed microstructure, and TC4 a fine lamellar α + β microstructure, respectively. TC4 displayed the most positive OCP and Ecorr but the lowest fitted Rct and the highest apparent icorr. Ti-B12 yielded the highest fitted Rct and the lowest apparent icorr. Initial fitted Rct values followed the order Ti-B12 (1.127 MΩ·cm2) > CP-Ti (0.760 MΩ·cm2) > TC4 (0.401 MΩ·cm2), a ranking that remained unchanged during 12–36 h natural immersion and after potentiostatic polarization at 1.0 VSCE. These results indicate that electrochemical nobility, reflected by OCP/Ecorr, and the kinetic interfacial response, represented by Rct/icorr, capture distinct aspects of the electrochemical behavior. The persistent material ranking is suggestively associated with the collective differences in phase constitution, microstructure, and alloy chemistry among the three material states. However, owing to the concurrent variations in these factors, their individual contributions as well as the specific roles of passive-film chemical homogeneity and defect density remain to be further clarified through targeted characterization.