The biocompatibility of commercially pure zirconium and its alloys is closely related to their surface properties with both the composition of the protecting oxide film and the surface topography playing animportant role. This articleis a study of corrosion behavior of new zirconium alloys for orthopedic implants, which are supposed to be used instead of some implant materials that have a higher citotoxicity. For this reason, zirconium and its alloys will be employed widely in biomedical applications. The higher stability and corrosion resistance exhibited by zirconium are due to the spontaneous formation of a passive zirconium oxide film, which protects the metal from further oxidation. Potentiodynamic polarization and electrochemical impedance spectroscopy (EIS) are used as electrochemical techniques. Measurements were carried out to investigate the corrosion behavior of zirconium and Zr2.5Nb, Zr3Ta, and Zr2.5Nb3Ta alloys in aerated Hank solution at 37 ± 0.2 ◦ C. The results of EIS were compared with those obtained by potentiodynamic polarization techniques. Impedance spectra were represented both in complex impedance diagram (Nyquist plot) and Bode plots. The EIS measurements have confirmed this stability range and pointed out the formation of oxide layers on the electrode surfaces.
Keywords: metallic biomaterials; corrosion; passive range; polarization curves
Metallic materials are being increasingly used in medical applications as implants to restore lost functions or release organ functions below acceptable levels. Zirconium alloys are among the most used metallic biomaterials, particularly for orthopedic applications. They possess a set of suitable properties for these applications such as low specific weight, high corrosion resistance, and biocompatibility. The nature of the oxide layer of zirconium-based alloys formed during corrosion was determined by different researchers[1 – 8] through scanning electron microscopy (SEM), study of the oxide–metal interface region as well as X-ray diffraction (XRD) and electron diffraction(ED)analyses. Themetallographic observations and corrosion kinetics studies show that variations in the oxidation rate among the zirconium-based alloys result from the differences in structure of the interphase layer at the oxide–metal interface. It was shown that the corrosion and associated hydrogen ingress behavior of zirconium-based alloys are often considered in terms of the effects of an interphase layer at the oxide–metal interface. This interphase layer can significantly reduce hydrogen ingress and oxidation rates provided that the interphase layers remains intact and protective. There is thus a great interest in developing and understanding the nature of the oxide–metal interface in zirconium-based alloys. However, there are relatively few reports of studies of the oxide close to the oxide–metal interface in zirconium-based alloys.[8 – 15] This lack of extensive study is attributed to the difficulties in preparing SEM and transmission electron microscopy (TEM) specimens that contain the oxide–metal interface. In this article, we studied the corrosion behavior of zirconium and some zirconium-based alloys in aerated Hank solution at 37 ± 0.2 ◦ C.
Using an induction furnace through a melting and remelting process, we have obtained three new zirconium-based alloys namely Zr2,5Nb, Zr3Ta, and Zr2,5Nb3Ta with chemical composition presented in Table 1. A VoltaLab 40 model electrochemical combined with dynamic electrochemical impedance spectroscopy (EIS) was used for the electrochemical polarization. The polarization behavior of zirconium-based alloys was studied in a classical electrolytic cell with three electrodes. A platinum plate electrode and a saturated calomel electrode (SCE) were used as counter and reference electrodes, respectively. All three electrodes (including working electrode) were placed in a cell, which was connected to a UltraThermostat type U10 with external recirculation of heating water to maintain the temperature inside the cell close to 37 ◦ C. The working electrode was made of zirconium and zirconiumbased alloys (Zr2,5Nb, Zr3Ta, and Zr2,5Nb3Ta). Samples with 1 cm2 geometric surface area were used. Prior to experiments, the electrodes were polished with SiC emery paper down to #4000. After polishing, the electrodes were degreased in acetone, washed with Millipore water, and then introduced into the measurement cell. The working electrode potential was scanned in the potential range of −1000 to +1200 mV/SCE with scan rate of 0.5 mV/sec.
This scanned potential range was chosen taking into account that the potential pH diagram for physiological conditions generally showed that the potential value of a metallic biomaterial may vary from −1.0 to +1.2 V/SCE in the human body. Excellent reproducibility was achieved when a potentiostatic reduction was applied for 10 min. The corrosion current density (icorr) was determined by extrapolation of the anodic and cathodic curves in the Tafel potential range. Impedance measurements were performed using VoltaLab 40 dynamic EIS with VoltaMaster 4 software on the frequency range between 100 kHz and 1 MHz with an a.c. wave of ± 5 mV (peak-topeak) overlaid on a d.c. bias potential and the impedance data were obtained at a rate of 10 points per decade change in frequency.
Using ZView software we obtained the Nyquist and Bode diagrams for all the studied experimental cases and fit results, and we also proposed equivalent circuits for the electrode/electrolyte interface. All tests have been performed in Hank solution (Table 2) at 37 ± 0.2 ◦ C under atmospheric oxygen conditions without agitation.
The XRD measurements were performed using a Bruker-AXS X Ray Diffractometer type D8 ADVANCE device with the following characteristics: X-ray tube with Cu anode (λ = 1.54184 Å); 40 kV/40 mA, Ni filter kβ ; Step 0.04◦ , measuring time on point 2 s.
Results and Discussions
The potentiodynamic polarization curves given in Fig. 1 showed that the working electrode was directly translated to a stable passive behavior from the Tafel region without exhibiting an active–passive transition. The electrode passivity was observed on a large potential range (approximately 700 mV for Zr2,5Nb alloy). At more positive potentials, the current densities increased due to anodic oxidation with formation of new oxides of alloying elements and also due to both transpasivation and oxygen evolution reactions. The corrosion current densities calculated from the potentiodynamic polarization curves from Fig. 1 are given in Table 3. Analyzing Fig. 1 and Table 1, it can be observed that in Hank solution at 37 ◦ C the lowest value of the corrosion current density is for Zr3Ta alloy for which we obtained the highest value of polarization resistance (RP) and the lowest value for the diffusion limit current density. We can conclude that in Hank solution, in the same conditions, the corrosion resistance of studied metallic biomaterials increases in the following direction: RZr < RZr2,5Nb < RZr2,5Nb3Ta < RZr3Ta. Hence in all the cases, the addition of the alloying elements led to the increase of corrosion resistance and consequently to the increase of the stability of these metallic biomaterials in Hank solution.
Further, we studied the EIS behavior of these metallic biomaterials in the same conditions.
The EIS measurements are carried out at open circuit potential (OCP) after 10 min of immersion. By analyzing the Nyquist diagram for zirconium (Fig. 2(a)), it can be observed that at high frequencies there appears a capacitive loop very well defined, which isfollowed by diffusive branch at medium and low frequencies. This behavior is pointed out by the Bode diagram (Fig. 3). As we can see from the Bode diagram on the phase angle versus logfrequency curve, there appears a maximum very well-defined curve that corresponds to phase angle of 75◦ , which indicates a capacitive behavior of zirconium electrode in this environment. At low frequencies on the Bode diagrams, there appears a second time constant with phase angle of approximately 27◦ , which indicates an inductive behavior
In Fig. 2(a), the Nyquist diagrams for all studied samples are presented. By analyzing this figure, it can be observed that, for Zr2,5Nb alloy, the Nyquist diagram presents a very high and wide capacitive loop at high and medium frequencies followed by a diffusive region.
It is very clear that the value of polarization resistance of Zr2,5Nb alloy is very high in comparison with the value of the polarization resistance of the other alloys. For the other alloys, the Nyquist diagrams are explicitly presented in Fig. 2(b). By analyzing this figure, we can see that for Zr3Ta alloy the Nyquist diagram presents three capacitive loops at different frequencies. The first capacitive loop appears in the high-range frequencies and it is followed by a second capacitive loop at the medium frequencies, while at the low and very low frequencies there appears a large capacitive loop. These results are in accordance with Bode diagram where for the Zr3Ta alloy appear three time constants on the curve phase angle versus log frequency, which corresponds to the three capacitive loops on the Nyquist diagram. We presume that these results obtained for Zr3Ta alloy are due to the formation of different oxidation compounds, when Ta and Zr appear in different oxidation states.
The first time constant corresponds to a phase angle of 45◦ , which means diffusive behavior. The second time constant corresponds to a phase angle of 20◦ , which corresponds to an inductive behavior, and the third time constant to a phase angle of 30◦ , which reveals diffusive behavior with inductive tendencies.
The Nyquist diagram for Zr2,5Nb3Ta alloy is also presented in Fig. 2(b). By analyzing this diagram, it can be observed that in this case there appears a very wide flattened capacitive loop, which is observed at the high and medium frequencies. This capacitive loop is followed by an inductive loop, which represents the adsorption of the reactive species on the electrode surface followed by the relaxation processes. These results were confirmed by Bode diagram where on the phase angle versus log frequency curve there appears two time constants. The first time constant corresponds to a phase angle of 65◦ , which means a capacitive behavior. The second time constant appears at the low-frequency range and corresponds to a phase angle of 25◦ , which means an inductive behavior.
The values of electrochemical parameters obtained from Nyquist and Bode diagrams are presented in Table 4.
Analyzing the XRD diagrams, it can be observed that in the case of Zr2.5Nballoy the sample containsamajor phase of Zr(hexagonal system) with diffraction peaks shifted face compared to diffraction lines position of pure Zr to smaller angles, which means higher cell parameters and this indicates the possibility of an intercalation of Nb atoms in crystalline network of Zr (Fig. 4(a)). The crystallite medium dimension calculated with Debye–Scherrer is 26 nm.
At 2θ = 35.8◦ and 2θ = 26.8◦ , there appears diffraction peaks corresponding to a Nb0,81Zr19 (cubic system) phase. It can be observed that only the line corresponding to the (111) plane and the line corresponding to the (220) plane indicates that this phase is textured (with preferential orientation of crystallites).
The diffraction lines translocation for Zr phase represents the consequence of Ta entering in the structure, thereby expanding the elementary cell dimension. It can be observed that in the case of Zr2.5Nb3Ta sample (Fig. 4(c)), in which the proportion between Ta and Zr is higher than in the case of Zr3Ta sample (Fig. 4(b)), the translocation of diffraction lines is higher.
The crystallite medium dimension for Zr phase calculated for the line corresponding to the plane (102) is 24.1 nm for the Zr3Ta sample and 37.6 nm for the Zr2.5Nb3Ta sample.
The elementary cell parameters for the samples are presented in Table 5.
The potentiodynamic polarization curves showed that, in all the cases, the working electrode was directly translated to a stable passive behavior from the Tafel region without exhibiting an active–passive transition.
Zirconium and its alloys have a passive behavior on a large potential range. At more positive potentials, the current densities increase again due to both transpasivation and oxygen evolution reaction.
In Hank solution, at 37 ◦ C the lowest value of the corrosion current density is for Zr3Ta alloy for which the highest value of polarization resistance and the lowest value of the diffusion limit current density were obtained.
In all the cases, the addition of the alloying elements such as Nb and Ta led to the increase of corrosion resistance of these metallic biomaterials in Hank solution.
EIS measurements were used to characterize the semiconductive properties of the oxide films. Nyquist and Bode diagrams were plotted at the OCP. The obtained Bode diagrams were in good accordance with Nyquist diagrams (Figs 2, 3).
For analysis of the impedance spectra were proposed equivalent circuits that satisfactorily simulate the spectra as it can be observed in Fig. 5(a), (b), (c), (d).
The XRD diagrams showed that the addition of the alloying elements led to the increase of medium size of crystallites in all the cases.