Surface morphology and electrochemical corrosion behavior of hydroxyapatite reinforced sintered Ti-Mn alloy in NaCl solutions

This study investigates the surface morphology, microhardness, and electrochemical corrosion behavior of Ti-Mn alloys reinforced with hydroxyapatite (HA) at 5, 10, and 15 wt% concentrations, fabricated via spark plasma sintering (SPS), in 0.9 wt% and 3.5 wt% NaCl solutions. Scanning electron microscopy (SEM) revealed a dense Ti-10Mn microstructure with polygonal α-Ti grains and needle-like titanium-manganese intermetallic phases, while higher HA content increased particle aggregation, porosity, and microvoids. The microhardness of titanium increased from 229 HV0.3 to 352 HV0.3 with manganese addition due to β-Ti phase formation and lattice strain. Further reinforcement with 5, 10, and 15% hydroxyapatite raised the values to 543, 657, and 710 HV0.3, respectively, by limiting dislocation motion through particle strengthening. Electrochemical tests, including potentiodynamic polarization and electrochemical impedance spectroscopy (EIS), were employed to evaluate the corrosion performance. Pure Titanium showed superior corrosion resistance due to the formation of a stable TiO2 passive layer. Mn addition, increased corrosion susceptibility, whereas 10 wt% HA significantly enhanced corrosion resistance, with the lowest corrosion current density (Icorr = 9.261 µA/cm² in 0.9 wt% NaCl; 6.868 µA/cm² in 3.5 wt% NaCl). EIS analysis, using Nyquist and Bode plots with a constant phase element (CPE) model, confirmed the 10 wt% HA composite’s highest polarization resistance (Rp = 5.46E + 05 Ω·cm² in 0.9 wt% NaCl; 5.10E + 05 Ω·cm² in 3.5 wt% NaCl) and near-ideal capacitive behavior (α = 0.954 and 0.955), indicating a strong passive film. The 15 wt% HA composite exhibited reduced polarization resistance (Rp = 1.13E + 03 Ω·cm² in 0.9 wt% NaCl) due to porosity and HA clustering. These results highlight the optimal HA content for Ti-Mn composites in biomedical and structural applications.