Enhanced corrosion resistance and electrochemical stability of hybrid HA-Cu-collagen nanocoatings on Ti-6Al-4V alloy in physiological environments

In this study, the corrosion behavior of hydroxyapatite (HA), hydroxyapatite-copper (HA-Cu), and hydroxyapatite-copper-collagen (HA-Cu-Col) nanocoatings electrochemically deposited on Ti-6Al-4V alloy was systematically investigated. For the hybrid coatings, collagen was incorporated via an immersion process followed by heat treatment at 80°C for 1 hour. Grazing incidence X-ray diffraction (GIXRD) confirmed the retention of the HA phase across all coatings, with Cu addition leading to co-precipitation of metallic Cu phases, evidenced by distinct peaks at 2θ angles of 34.3°, 42°, and 50.4°. Collagen integration, without forming new phases, resulted in a uniform organic-inorganic matrix and enhanced nanoscale porosity. Field-emission scanning electron microscopy (FE-SEM) revealed that the HA coating exhibited a dense needle-like morphology with lengths of 1-5 μm and rod diameters of 200-500 nm, whereas HA-Cu displayed a granular-floret structure interspersed with 20-50 nm copper nanoparticles. The hybrid HA-Cu-Col coating formed a compact fibrous-floret network with nanoscale uniformity. Tafel polarization tests indicated the highest corrosion potential (E _corr ) for HA-Cu-Col at +0.54 V vs. Ag/AgCl reference electrode, while the lowest corrosion current density (I _corr ) was achieved with HA at 0.027 µA/cm ² , followed by HA-Cu-Col at 0.040 µA/cm ² , confirming that the hybrid system maintains low corrosion kinetics while introducing stable passivation behavior. Electrochemical impedance spectroscopy data after 48 hours of immersion showed coating resistances (R _coat ) of 2885, 13268, and 12851 Ω.cm ² for HA, HA-Cu, and HA-Cu-Col, respectively, whereas the highest charge transfer resistance (R _ct ) of 1078 kΩ.cm ² was observed for HA-Cu-Col, indicating superior suppression of interfacial electrochemical reactions despite the moderate R _coat value at extended immersion. Overall, the hybrid HA-Cu-Col nanocoating exhibited enhanced impedance behavior, suggesting improved barrier properties through higher charge transfer resistance, stable passive region formation (0.64-1.58 V), controlled ionic diffusion, and bio-inspired networking. Although initial open-circuit potential (OCP) fluctuations point to interfacial dynamics that warrant further long-term investigation, the coating remains a promising candidate for Ti implant applications in physiological environments.