Corrosion behavior and in-vitro bioactivity of Ti-based composites: Synergistic and competitive effects of ZrO2 and nHA ceramic reinforcements en en The present study introduces a comparative and synergistic evaluation of corrosion behavior and in-vitro bioactivity of Ti-based composites reinforced with zirconia (ZrO2) and/or nano-hydroxyapatite (nHA). Pure Ti, Ti–10 vol% nHA (TH), Ti–4 vol% ZrO2 (TZ), and Ti– 4 vol% ZrO2–6 vol% nHA (TZH) were fabricated via spark plasma sintering (SPS) at 1200 °C under vacuum to elucidate the individual and combined effects of ceramic phases on passive film formation and degradation mechanisms. Potentiodynamic polarization and electrochemical impedance spectroscopy (EIS) were performed after short-term and 14-day immersion in simulated body fluid (SBF). The TH composite exhibited the lowest corrosion current density (9.22 × 10-6 mA/cm2) and highest polarization resistance (2910 kΩ.cm2), confirming the formation of a dense, stable Ca–P/TiO2 multilayer that effectively blocked electrolyte penetration. EIS analysis further revealed the formation of a stable, highly capacitive passive layer on the TH sample, characterized by phase angles near -80° and impedance values up to 1770 kΩ.cm2. In contrast, the dual-ceramic TZH composite showed micro-galvanic interactions between ZrO2 and nHA phases, leading to localized pitting and reduced long-term stability. The TZ sample showed delayed but noticeable improvement in corrosion resistance during prolonged immersion, indicating that ZrO2 contributes to long-term passivation through the gradual formation of a stable ZrO2-rich barrier. Long-term immersion tests confirmed apatite formation on all samples, with TH exhibiting the most uniform Ca–P-rich layer, as verified by FE-SEM/EDS. 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