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 (Ecorr) for HA-Cu-Col at +0.54 V vs. Ag/AgCl reference electrode, while the lowest corrosion current density (Icorr) was achieved with HA at 0.027 µA/cm2, followed by HA-Cu-Col at 0.040 µA/cm2, 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 (Rcoat) of 2885, 13268, and 12851 Ω.cm2 for HA, HA-Cu, and HA-Cu-Col, respectively, whereas the highest charge transfer resistance (Rct) of 1078 kΩ.cm2 was observed for HA-Cu-Col, indicating superior suppression of interfacial electrochemical reactions despite the moderate Rcoat 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.