An in vitro study of Ti and Ti-alloys coated with sol–gel derived hydroxyapatite coatings

Electrochemical–corrosion properties of Ti, Ti6Al6Nb and Ti6Al4V implant materials coated with calcium phosphate (CaP) coatings were investigated in vitro at the open circuit potential using non-destructive electrochemical impedance spectroscopy (EIS) technique in simulated physiological-Hankʹs Balanced Salt Solution (HBSS). Characterization of the structure and composition of CaP coatings obtained by the sol–gel method was performed using X-ray diffraction analysis. The interpretation of the EIS results was based upon a two-layer model of the surface film consisting of an inner barrier TiO2 (CaTiO3) layer and the outer porous hydroxyapatite (HAP)+β-tricalcium phosphate (β-TCP) layer, described by using the high frequency time constant (τ1HF) and the low frequency time constant (τ2LF), respectively. Spontaneously passivated surfaces of naturally grown surface films on all three substrates in HBSS were used for reference. The chemical composition of the substrate as well as the composition and crystallinity of the CaP coating have a direct influence on electrochemical–corrosion properties of the implant materials under in vitro conditions. Well-crystallized HAP and β-TCP exhibited a beneficial corrosion protection effect on the substrate during prolonged exposure to HBSS. In the case of poorly crystallized coatings containing a greater amount of carbonate incorporated in the structure of HAP, the total impedance of the system markedly changed with prolonged exposure to physiological solution due to degradation of the coating.