Magnesium alloys would get considerable attention as biodegradable temporary implants if their corrosion resistance could be enhanced to the required level. In this study, the calcium phosphate (Ca-P) coating was deposited on a biodegradable magnesium alloy using the electrochemical deposition method with a view to controlling the degradation rate as well as enhancing the bioactivity. The coating morphology and chemistry were characterized using scanning electron microscopy (SEM), energy-dispersive x-ray spectroscopy (EDXS), and x-ray diffraction (XRD). The resistance to electrochemical degradation, as a result of the Ca-P coating on the magnesium alloy, in modified simulated body fluid (m-SBF) was evaluated using potentiodynamic polarization and electrochemical impedance spectroscopy (EIS). The results of this study show that the Ca-P coating significantly decreases the degradation rate of the magnesium alloy, indicating the potential of the Ca-P coating to be used as a corrosion barrier for temporary implant materials.
Magnesium and its alloys have gained an increasing interest in research for their use as biodegradable, temporary implant materials. This interest is because of their low density (ρ= 1.74 g cm−3 to 2.0 g cm−3) and elastic modulus (E = 41 GPa to 45 GPa), both of which are similar or closer to those of human bones (ρ= 1.8 g cm−3 to 2.1 g cm−3; E = 3 GPa to 20 GPa).1 Magnesium ions are not only non-toxic but also the fourth major cations in the human body. These ions are also essential to the human metabolism as a cofactor for many enzymes.2 The use of magnesium alloys as biodegradable, temporary implants may offer many advantages over metallic implants of titanium alloys, stainless steels, and Co-Cr alloys, including:
—possibility of avoiding the second surgical operation owing to their biodegradability
—avoiding the stress shielding problem
—being considerably cheaper as compared to the traditional implant materials
In spite of remarkably attractive properties of magnesium alloys, they have found very limited application as body implants. The major limitation arises because of the unacceptably poor corrosion resistance of the magnesium alloys in the chloride-containing solutions, including physiological environments (with a pH of 7.4 to 7.6).1,3–7 As a result of corrosion-assisted degradation, magnesium alloys may lose their mechanical integrity in the physiological environment before the required service duration is over.8–9 Rapid generation of hydrogen gas during the corrosion process of magnesium is another problem that may hinder growth of host tissues.10–12 Nevertheless, the use of magnesium alloys as temporary biodegradable implant material such as plates, pins, wires, screws, and stents is still promising if their corrosion resistance could be enhanced to a desired level.
While the choices of the alloying elements for developing the biodegradable magnesium alloys with inherent degradation resistance remain limited, surface modification of these alloys with biocompatible coatings may be a possible solution for achieving the desired level of corrosion resistance as well as for addressing the issue of hydrogen evolution. Surface modification of the magnesium alloys by the calcium phosphate (Ca-P) coating could be an attractive choice. Calcium and phosphorous that are naturally found in the human body are also the major components in human bones.13 The Ca-P coating has been used extensively for improving the biocompatibility of the implants of titanium or titanium alloys.14 The application of the Ca-P coating on magnesium alloys will not only improve the biocompatibility of bone-implant interface but also reduce the degradation rate.15 Recently, Song, et al.,16 reported that the calcium phosphate coating on magnesium alloy AZ31, developed at room temperature, could improve significantly the corrosion resistance during the initial stages of immersion, which might allow implants to maintain their mechanical integrity in the bone-healing phase. However, the AZ31 alloy is unlikely to be used in practice since the major alloying element, aluminum, in the AZ series alloys, is known to cause various neurological disorders such as dementia and Alzheimer's disease.17 Accordingly, for the magnesium alloy used in the present study, not-toxic elements Ca and Zn have been chosen as main alloying elements. Both Ca and Zn are present in the human body.18 Moreover, Zn addition helps to improve the mechanical properties of the alloy through a solid solution hardening mechanism.19–20 The addition of Ca is also reported to refine the microstructure as well as improve the corrosion resistance of the magnesium alloys.6,21
A few techniques can be used to deposit the Ca-P coating on magnesium alloys, which includes plasma spraying, electrophoretic deposition, sol-gel, laser melting, physical vapor deposition, and electrochemically assisted deposition.13 In the present study, the electrochemically assisted deposition (ECAD) method was selected specifically because of its distinct advantages, such as:
—ease of operation;
—ability to coat the complex structures such as screws, stents, and pins.
Previously, a few researchers have electrodeposited the Ca-P coating on various magnesium alloys and have reported a distinct decrease in corrosion rate.16,22 However, there are no reports on time-dependent degradation mechanism of the coated alloys in the physiological environment. Therefore, the goal of this study was to develop the Ca-P coating on the as-cast samples of a Mg-Zn-Ca alloy using the ECAD method, and then to investigate the degradation kinetics of the coated specimen in the physiological environment over the time period using electrochemical impedance spectroscopy (EIS).
Test Alloy and Electrochemical Degradation Test Environment
Magnesium alloy, with a nominal composition of Mg-3 wt% Zn-1 wt% Ca (Mg3Zn1Ca), was produced by induction melting of high-purity Mg, Zn, and Ca in a steel mould crucible under an argon atmosphere and casting. The specimen coupons were sectioned from the as-cast billet and ground progressively on silicon carbide (SiC) papers up to 2,500 grit followed by rinsing with acetone (CH3COCH3) and deionized water before electrochemical testing. For comparison of corrosion resistance of the bare and coated alloys, electrochemical tests were carried out in modified simulated body fluid (m-SBF) at a temperature of (36.5 ± 0.5)°C. The m-SBF solution was buffered with 2-(4-[2-hydroxyethyl]-1-piperazinyl) ethanesulfonic acid (HEPES) at a physiological pH of 7.4.23 The composition of the m-SBF is given in Table 1. During the electrochemical testing, a submersible pump and water bath were used to simulate the in vitro flow rate of body fluid and the common body temperature of (36.5 ± 0.5)°C, respectively.
All electrochemical experiments were carried out in a conventional three-electrode cell that had a saturated calomel electrode (SCE) as the reference electrode and a platinum mesh as the counter electrode.
ECAD of the Ca-P coating was performed at room temperature, using a potentiostatic method, in the electrolyte containing 0.05 M calcium nitrate (Ca[NO3]2) and 0.025 M ammonium dihydrogen phosphate (NH4H2PO4). The pH of the electrolyte solution in the coating bath was 4.2. During the ECAD process, a constant cathodic potential of −3 VSCE was applied on the flat rectangular specimens (25 by 25 by 5 mm) for 2 h. The ratio of the coating solution to the electrode surface area was kept at 30 mL/cm2. After completion of the coating process, the coated specimens were rinsed with deionized water and dried in an oven at 80°C for 1 h.
The potentiodynamic polarization scans were carried out starting at 250 mV more negative to the open-circuit potential in the m-SBF solution at a scan rate of 0.5 mV/s. Electrochemical impedance spectra experiments were carried out by applying a sinusoidal potential wave at open-circuit potential with an amplitude of 10 mV. Impedance response was measured over frequencies between 1 MHz and 10 mHz, recording 10 points per decade of frequency. All EIS and potentiodynamic polarization tests were performed after 2 h of immersion, the time required for the stabilization of the open-circuit potential. Impedance analysis was carried out based on appropriate equivalent electrical circuit (EEC). All the electrochemical tests were duplicated to examine the reproducibility of the results.
Composition of the deposited coating was analyzed with x-ray diffraction (XRD) using a Cu Kα line generated at 40 kV and 25 mA. The morphology and elemental composition of the coating were observed using scanning electron microscopy (SEM) and energy-dispersive x-ray spectroscopy (EDXS).
RESULTS AND DISCUSSION
Chemical Composition and Morphology of the Electrodeposited Ca-P Coating
XRD patterns of the bare and Ca-P-coated samples are shown in Figure 1. There were a few additional peaks in the spectrum for the Ca-P-coated specimen as compared to the bare alloy. The spectrum for the Ca-P-coated specimen possessed distinct peaks for the crystalline brushite (CaHPO4·2H2O) at 2θ values of 11.68, 20.93, and 29.26 degrees. Magnesium peaks were also detected in the case of the coated specimens, indicating that the Ca-P coating was thin enough, at least at some locations, to allow detection of the magnesium alloy substrate.
The morphology of the coating developed with the ECAD process is shown in Figures 2(a) and (b). The coating developed on this alloy consists of loose, porous, and irregular crystalline flakes.
These coating characteristics are evidenced in the SEM micrograph of the cross section of the coated specimen (Figure 3). Figure 3 also suggests the coating thickness to be ~10 μm to 15 μm. The EDXS spectrum of the Ca-P-coated specimen (Figure 2[c]) shows that the coating is rich in Ca and P, with a Ca/P atomic ratio of ~1, which is consistent with the Ca/P ratio of the brushite. EDXS analysis also reveals the Mg from the substrate, which is also consistent with the XRD analysis (Figure 1).
Brushite is deposited on the magnesium alloy substrate according to the following reactions:
The available Ca2+ ions readily react with HPO42− in the aqueous solution to form the brushite:
Potentiodynamic Polarization Scans
Potentiodynamic polarization scans for the coated and bare alloy in m-SBF at 36.5°C are shown in Figure 4. The current densities in the corresponding anodic and cathodic parts of the polarization scan of the coated specimen were significantly lower than those of the bare alloy. Also, the corrosion potential (Ecorr) of the coated alloy was approximately 50 mV more noble as compared to the bare alloy, implying less susceptibility of the Ca-P-coated alloy to corrode. In addition, a considerable decrease in the cathodic current density observed in the case of the coated specimen suggests a substantial decrease in cathodic hydrogen evolution, which is one of the major concerns in using the magnesium alloys as implant materials.11
The electrodeposited Ca-P coating on the Mg3Zn1Ca magnesium alloy appears to provide a considerable physical barrier and the corrosion resistance during the initial immersion period. However, the main component of coating, i.e., brushite, is known to be soluble in the physiological environment.24 Hence, it may be important to characterize the time-dependent electrochemical degradation of the coated alloy.
Electrochemical Impedance Spectroscopy of the Bare and Ca-P-Coated Samples
The kinetics of time-dependent electrochemical degradation of the Ca-P-coated and bare Mg3Zn1Ca alloy was studied using EIS. Figure 5 presents the evolution of impedance spectra (Nyquist plots) of the bare and Ca-P-coated Mg3Zn1Ca alloy after immersion for different durations in m-SBF. The Nyquist plots for the Ca-P-coated, as well as bare, alloys consist of two capacitive loops. The capacitive loop in the higher frequency range relates to the charge-transfer processes, whereas the loop in the intermediate/lower frequency range corresponds to the mass-transfer processes.25 The polarization resistance is determined by the combined diameter of both the capacitive loops. On this basis, it is evident from Figure 5 that the Ca-P coating on the Mg3Zn1Ca alloy considerably improves the polarization resistance for the initial phase of immersion (2 h). The combined as well as the individual diameter of the coated specimen decreased gradually with an increase in immersion time (Figure 5), indicating the gradual vanishing of the corrosion resistance provided by the Ca-P coating. This possibly can be attributed to the dissolution of coating with time, as the main component of coating, i.e., brushite, is known to be soluble in the physiological environment.24
The impedance data of the bare and Ca-P-coated Mg3Zn1Ca alloy after different durations of prior immersion are also shown in Bode impedance plots (Figure 6). In the Bode impedance plot, the magnitude of impedance at the lowest frequency represents the polarization resistance. It is noted from Bode impedance plots (Figure 6) that the impedance of the CaP-coated specimen at the lowest frequency decreases gradually with an increase in immersion time. The phase angle plots (Figure 7) indicate two time constants for all the cases, one appearing at intermediate/higher frequency (~100 Hz) and other at lower frequency (~1 Hz). The two time constants arising in the case of the Ca-P-coated specimen may be attributed to the presence of two interfaces, i.e., the Ca-P coating/solution interface and the metal/solution interface. This is consistent with several other coated metal systems.26–30 The presence of the two time constants in case of the bare alloy indicates the development of a film and, therefore, two interfaces, i.e., the film consisting of corrosion products/solution interface and the metal/solution interface.
For a developing mechanistic understanding of the time-dependent electrochemical degradation of the Ca-P-coated Mg3Zn1Ca alloy, the experimental impedance data were analyzed using an appropriate EEC. The EEC with two time constants, as shown in the inset in Figure 7, was used to simulate the experimental impedance data. The nested EEC was chosen specifically because of the interconnected nature of the porosity, as often reported in the case of the coated alloy systems.27–28
In this EEC, Rs is the solution resistance and the surface film is represented by a parallel combination of the constant phase element (CPE), Q1, and the film resistance, R1. The electrical double layer is represented by another CPE, Q2, and a charge-transfer resistance, R2. In the simulation of experimental data, the CPE behavior is generally attributed to the distributed surface reactivity, roughness, and electrode porosity.25,27 Figures 8(a) and (b) show the fitting of simulated data with experimentally observed data for the Ca-P-coated specimen after 2 h and 7 h of immersion, respectively. As evident from Figures 8(a) and (b), the experimentally measured plots match well with the simulated data in the frequency region between 100 kHz and 0.1 Hz.
The total error in impedance measurement in the simulation of the experimental data, when using the EEC shown in Figure 7 (inset), was less than 7% for all the experiments. The associated chi square values were also relatively low (10−3 to 10−4). The values of different components calculated using this EEC for the coated and bare alloys are shown in Table 2.
It is evident from Table 2 that the Ca-P coating improves the corrosion resistance of the Mg3Zn1Ca alloy. RP for the bare alloy was 160 Ω · cm2, while RP for the coated alloy after 2 h of immersion was observed to be 1,546 Ω · cm2. The EIS results after 2 h of immersion were consistent with the results obtained by potentiodynamic polarization. The magnitude of Q1 (film capacitance) was observed to be increasing with time, reflecting an increase of the active area, which is consistent with dissolution of the coating with increasing immersion time (as also described earlier). The polarization resistance of the coated specimen decreased with increasing immersion time.
As a result of a gradual degradation of the coating, the impedance of the coated specimen eventually becomes similar to that of the bare alloy at 48 h of immersion. Also, a very low magnitude of film resistance, R1, (Table 2) observed in the case of the coated alloy confirms the porous and loose nature of the coating. The degradation of the coating was also observed by a post-corrosion analysis using SEM. The corrosion morphology of the Ca-P-coated specimens with the corrosion products after 2 h and 48 h of immersion are shown in Figures 9(a) and (b), respectively. It can be seen in Figure 9(a) that the crystalline flakes of brushite were intact after 2 h of immersion without any significant corrosion damage. However, the crystalline flakes of brushite were dissolved and completely disintegrated into loose and porous corrosion products after 48 h of immersion (Figure 9[b]), which suggested the availability of more conductive pathways for electrolyte penetration to the substrate surface. This is consistent with the EIS results obtained for the Ca-P-coated specimens after 2 h and 48 h of immersion.
It is suggested that the electrochemically assisted deposition only produced the physically bonded Ca-P coating on the magnesium alloy, rendering the coating susceptible to disruption/dissolution during exposure to an aggressive electrolyte. However, such coatings may still find use where the short-term corrosion resistance is the desired property (such as in the situation of use of the magnesium alloys for temporary biodegradable implant devices, e.g., screws, stents, pins, wires, etc.).
❖ Calcium phosphate coating was developed on the Mg-3Zn-1Ca (wt%) magnesium alloy by electrochemically assisted deposition. The deposited Ca-P coating mainly consists of a flaky brushite (CaHPO4·2H2O).
❖ As a result of the Ca-P coating, the corrosion resistance of the alloy was considerably improved. The corrosion current density of the coated specimen was ~1.5 orders lower than that of the substrate. The durability of the coating over the time period was also evaluated using electrochemical impedance spectroscopy, and a rapid disruption of the coating was observed in 48 h of immersion in the physiological environment, suggesting the need to tailor the coating for further improvement of the corrosion resistance.
The authors would like to acknowledge the support from the Department of Mechanical and Aerospace Engineering, Monash University, Australia.
*Department of Mechanical and Aerospace Engineering, Monash University, Victoria 3800, Australia.
** Department of Chemical Engineering, Monash University, Victoria 3800, Australia.
*** Department of Materials Engineering, Monash University, Victoria 3800, Australia.