Behavior of Passive Film on New Ti-Zr-Ta-Ag Alloy Surface in Simulated Biofluid
Authors: Vasilescu Cora , Drob Silviu Iulian , Osiceanu Petre , Calderon Moreno Jose Maria , Prodana Mariana , Ionita Daniela , Vasilescu Ecaterina , Popa Monica 
Published: February 2020

We elaborated a new Ti-20Zr-5Ta-2Ag alloy with innovative composition and properties to fulfil the necessities of corrosion resistance, biocompatibility and antibacterial protection; this alloy was complete and complex characterized: its microstructure by scanning electron microscopy (SEM); its native passive film by X-ray photoelectron spectroscopy (XPS); the surface wettability by contact angle measurements; the electrochemical behaviour in simulated human biofluid (Ringer solution of different pH values) by cyclic and linear polarization, electrochemical impedance spectroscopy (EIS) and monitoring of the open circuit potentials for 1000 exposure hours; the quantity of ions released in time by inductively coupled plasma-mass spectroscopy (ICP-MS).

         The new alloy has bi-phase, α + β, acicular microstructure, homogeneous and fine; also, lamellas of α and β phases, separated by distances of few microns can be observed (SEM images).

         Both survey and high resolution XPS spectra showed that the native passive film on Ti-20Zr-5Ta-2Ag alloy surface consists from the protective TiO2, ZrO2, Ta2O5 oxides, Ti and Ta suboxides and metallic Ag, which compact, reinforce and thicken the alloy passive film in comparison with that of Ti. The new alloy native passive film has a thickness of about 12 nm that is well above of that for Ti (1.3 – 3.7 nm).

         The contact angle for the new Ti-20Zr-5Ta-2Ag alloy has value of 64.54 0; it is a surface with high hydrophilic properties; its surface energy is 29.4 mJ/m2, representing a good value that promotes the interactions with biological fluids, cells and tissue and leads to a better biocompatibility.

          The corrosion and passivation potentials of the new alloy have more electropositive values than those of Ti, as result of the beneficial effect of Zr and Ta elements (which take part to the formation of the new alloy passive film, as XPS data showed) and of Ag that has a higher mixed potential. The passive current densities for Ti-20Zr-5Ta-2Ag alloy have lower values (of about 6 times) than those of Ti; corrosion current densities and rates are about 25-28 times lower and the polarisation resistances are about 21-28 times higher than those of Ti; this behaviour is due to the more protective passive layer existing on the new alloy surface.

          Nyquist spectra displayed capacitive loops associated with passive film like an insulator. Bode impedance spectra presented higher impedance values for the new alloy suggesting a more protective passive film than that on Ti surface. The new Ti-20Zr-5Ta-2Ag alloy exhibited higher phase angles than those for Ti, fact that demonstrates a nobler behaviour, a more capacitive film, a more resistant layer as result of its more compact, thicker, more complex composition.

          The impedance results were modelled with an electric equivalent circuit with two time constants corresponding to those two phase angles; the high phase angle represents a very capacitive, very resistant, barrier layer; the lower phase angle reflects the defective, less resistant, porous layer. The electrical parameters evinced slightly more unfavourable values for Ti, indicating that the passive film on its surface has lower insulating properties than the passive film on Ti-20Zr-5Ta-2Ag alloy surface; this fact is normal because the passive film on the new alloy surface is more compact, thicker, and, in addition, contains very protective ZrO2, Ta2O5 oxides, Ta suboxide and metallic Ag (as results from XPS data).

          The new Ti-20Zr-5Ta-2Ag alloy releases into Ringer solution low quantities of metallic ions, order of parts per million (ppm) beginning after 120 immersion hours; the quantity of the released ions increased over time. Silver ions were released in very low amount that enhanced in time, assuring long term antibacterial activity of the new alloy (ICP-MS data).

          The values of the open circuit potentials for the new Ti-20Zr-5Ta-2Ag alloy have more electropositive values than those of Ti, due to the alloy thicker, more compact (XPS results), more resistant (electrochemical and corrosion parameters) passive film. After about 500 immersion hours, all open circuit potential values tended to constant levels and maintained very constantly till 1000 hours, proving the alloy good stability.

            Open circuit potential differences had very low values situated under limits of 500 mV ÷ 600 mV necessary to form galvanic cells. Therefore, both Ti and new Ti-20Zr-5Ta-2Ag alloy are not susceptible to galvanic corrosion. The coupling of Ti with noble metals (Pt, Au, Ag) conducts to a higher mixed potential, because the noble metals are catalyst for the reducing both of the oxygen and water reactions. Thus, a quantity of 2% Ag is capable to increase the corrosion potentials of the new alloy.