Effect of Zn Content on Orthopedic Mg Alloy as Smart Implant

In recent years, smart implants take the most attention in the field of bone manufacturing. Our study seeks to develop the biodegradability of Mg alloys to use orthopedic implants for the biomedical applications to avoid post removal of the implant. Mg and Zn are very important to human body and have no toxicity. Mg 6% wt Zn biodegradability is studied in simulated body fluid for two and four weeks. Four electro-deposition bathes are used to deposit a coat on the substrate to improve the corrosion resistance of this alloy in the media of simulated body fluid. The following analyses were studied to emphasize the research aim. Scanning electron microscope (SEM), Energy dispersive XRay (EDX) analysis shows the surface morphology and the elements of the coat phases components. The results also confirmed by X-Ray diffraction Pattern (XRD) that show the phases that confirmed the formation of hydroxyapatite HA phase, Fourier-Transform Infrared Spectroscopy (FTIR) to investigate the functional groups of the phases coats that confirm the formation of hydroxyapatite and the electrochemical measurements that investigate the improvement of corrosion resistance. The results indicated that the fourth bath gives the best coat and four weeks immersion gives more corrosion resistance than two weeks.


1.Introduction
Smart implants or biodegradable implants development in simulated body fluid is the powerful aim to attract the interest of scientists in the few recent years. [1,2]. The main advantage is to dispense with the second surgery to extract the slice from the body after the stage of full recovery [1,[3][4][5]. Element alloying with magnesium is a good bath to enhance its corrosion resistance and the mechanical proprieties. [2,4] Magnesium implants have been enhanced to simulate the formation of new bone when used as bone fixtures [6]. Zinc is essential element in the human body and used to rise the corrosion potential and decrease corrosion rate of Mg. [7,8] Zinc can passivate anodic kinetic of Mg in corrosive medium due to the formation of ZnO-based coating [9,10] and also decrease the deleterious effects of metallic impurities as Fe and Ni [11]. Some recent studies discussed the improvement of the Mg-Zn binary alloys. [2] Zinc can be added less than 1 wt% or up to 6 wt% for biomedical application [12], 6.2 wt % [13] at the eutectic temperature 341 o C [2,14]. Also shuhua Cai [2] study the Mg-Zn binary alloy (Zn= 1, 5 and 7%) and enhanced the corrosion resistance of the alloy. Mg-1%Zn studied and enhanced it's mechanical prosperities and corrosion resistance by the addition of Zn as alloying element. [15,16] Mg-3%Zn alloy was studied by different heat treatment to enhance corrosion resistance. [17] Mg -6% Zn alloy has suitable tensile, strength and elongation for implant application [18,19]. Calcium phosphate bioceramics, hydroxyapatite have similar chemical composition to the bone tissue. [20][21][22][23]

-Materials
Mg-6% Zn alloy sheets Specimens of Mg-6% wt Zn is cutting with dimensions 2x2.5 cm 2 and 5mm thickness. The alloy with a chemical composition of Mg-6% Zn is prepared from pure Mg and Zn using a laboratory resistance furnace. The melt is transferred to a semi-continuous casting machine at 900 o C.

2.2.Preparation of Electrolyte Solution
Electroplating bathes include Ca(NO3) 2 .4H2O as the calcium ion source, ammonium di-hydrogen phosphate NH4H4PO4 as the phosphate ion source, sodium nitrate NaNO3 to increase conductivity, hydrogen peroxide (H2O2) to erase in Table (1). .90 g/l 9.90 g/l 11.00 g/l 11.00 g/l NH4H2PO4 2.87 g/l 4.00 g/l 2.87 g/l 4.00 g/l NaNO3 1.27 g/l 1.27 g/l 1.27 g/l 1.27 g/l Na3PO4 2.00 g/l 2.00 g/l 2.00 g/l 2.00 g/l H2O2 20.00 ml/l 20.00 ml/l 20.00 ml/l 20.00 ml/l The optimum conditions are pH 7.4 and temperature 37 o C. Fixed potential was set to 1.4V and time 30min. The circuit is connected so that, the magnesium alloy substrate became negatively charged (as cathode). The specimens are mechanically polished abraded to 2000 grit finish with SiC paper, the specimen are carefully rinsed in distilled water, dried with hot dry air and stored in a desiccators if not immediately examined. The samples are weighed before and after plating and the difference in weight is calculated.

Coating process
The cleaned samples are coated electrochemically for a definite time 30 min at room temperature. Agitation of the solution took place at 250-500 r.p.m. we use four baths for the deposition of coat, I, II, III and VI.
The optimum condition for coating is done for the bath I then, uses the same conditions for the other three bathes in order to compare between them.

Surface characterization of the coatings
The properties of electro-plating coating as plated and after immersion in simulated body fluid (SBF) ( Table 2), for 2 and 4 weeks at 37⁰C, are investigated. Scanning electron microscope (SEM), FTIR of coatings, X-ray diffraction (XRD), EDAX and corrosion resistance are used for investigation.

2.4.1-Scanning electron microscope (SEM)
SEM (Quanta 250 FEG, Taiwan) is used to demonstrate the surface morphology. The examination is carried out on different samples employing an accelerating voltage of 30kv. Samples are mounted using carbon paste to ground them. The results are obtained as computer printout. https://doi.org /10.37358/Rev. Chim.1949

2.4.2-Fourir-Transform Infrared Spectroscopy (FTIR)
FTIR spectra are obtained using pearl bruker IFS 125 HR and are recorded in the 4500-400 cm -1 region with 0.0024 cm -1 resolution by using KBr pellet technique.

2.4.3-X-ray Diffraction (XRD)
An X-ray diffractometer (D8 ADVANCE X-RDIFFRACTOMETER, Germany) with copper target and nickel filter is used for identification of the phases, amorphous and crystalline structure of the deposits.

2.4.4-Corrosion Resistance
Electrochemical study tests are carried out using a classical three electrode cell with Pt as counter, Ag/AgCl electrode as reference electrode, and the samples as working electrode. The potentiodynamic curves are obtained using a model CHInstrument.

3.Results and discussions
The surface morphology of the as deposited coating is shown in Figure 1. Figure 1(a) show white flake like structure diversing from center towards prefirphery with different dimensions with grain size 7.91 µm. Flake-like structure mixed with rodlike morphology was observed on the surface of Figure  1(b) and its grain size 5.87 µm. Figure 1(c) shows flake-like structure with some black spots with grain size 5.13 µmand Figure 1(d) shows irregular black flake like structurewith grain size 2.05 µm.  Figure 2(a) shows randomly arranged rod like morphology with grain size 9.36 µm. Figure 2 (b) appears to be irregular flake-like with grain size 7.10 µm. https://doi.org /10.37358/Rev. Chim.1949       The FTIR spectra of electrodeposited Mg-6% Zn alloy in the four bathes are shown in Figures.  6(a-d). The most characteristic chemical groups in the FTIR spectra of the four bathes are PO4 -3 , OH -1 and NO3 -1 . Phosphate group (PO4 -3 ) forms intensive IR absorption at 560, 600 cm -1 and at 1000-1100 cm -1 .Absorbed water band are relatively wide from 3600 to 2600 cm -1 with explicit peak at 870 and 880 cm -1 and more intensive peaks between 1460 and 1530 cm -1 . At 3531, 3541and 3543 cm -1 are due to stretching vibration which indicates the presence of water molecule as in Figure 6. The disappearance of the band 3541 cm -1 could be due to the substitution of OH -1 by NO3 -1 ions (direct substitution 2OH -1 ↔NO3 -1 ) into the four bathes. According to Figures 6 a-d the layer precipitated on the surface contained NO3 -1 and PO4 -3 groups.
FTIR characterization of prepared samples confirms the formation of hydroxyapatite due to the presence of various modes of their functional groups.
The effect of immersion of the coated specimens by bath I in SBF for two and four weeks is shown in Figures 7 (a, b). From the Figures the functional groups NO3 -1 and PO4 -3 appeared i.e. formation of hydroxyapatite. The functional groups are shown in Figures 8 (a, b), 9 (a, b) and 10 (a, b) for immersion in bath II, bath III and bath IV for two and four weeks respectively. https://doi.org/10.37358/RC.20.6.8175     The XRD patterns of as coated Mg-6%Zn alloy with bath I, II, III and IV are shown in Figures  11(a-d). Figure 11(a) shows sharp peaks at 2Ɵ equals 11.76, 34.42, 35.52 and 47.95 represent monoclinic calcium phosphate dihydrate CaHPO4 2(H2O). Other sharp peaks with little heights also represent Brushite calcium phosphate dihydrate. At 2Ɵ equal 63.18 represent hexagonal magnesium and also at 2 Ɵ equals 65, calcium oxide phase is shown. The average grain size of bath I coating which is determined by Scherrer method is about 86 to 63.2 nm. Figures 11(b-d) represent the same phases (monoclinic calcium phosphate dihydrate, hexagonal magnesium and calcium oxide) but accompanied by small positional shifts towards higher or lower angles. The average grain size of the coat in bath II is 72.8 -41.6 nm. N.B. All the coatings are crystalline in nature.  X-ray diffraction patterns of the corrosion product from coated specimens with bath I, bath II, bath III and bath IV for two weeks and four weeks are shown in Figures 13( a, b), Figures 14( a, b), Figures  15(a, b) and Figures 16(a, b). https://doi.org/10.37358/RC.20.6.8175   The EDAX analysis of the corrosion product from bath I in case of immersion for two weeks reveal the presence of Cl, Mg, Ca, P and O elements. In agreement with X-ray diffraction since calcium hydrogen phosphate dihydrate CaHPO4 (H2O)2, hydroxyapatite Ca5(PO4)3(OH) and magnesium oxide MgO phases are present. X-ray diffraction of the corrosion products after four weeks give calcium hydrogen phosphate dihydrate CaHPO4 (H2O)2, hydroxyapatite Ca5(PO4)3(OH), magnesium hydroxide Mg(OH)2 and magnesium oxide MgO phases. With increasing of immersion time to four weeks, the peaks of Ca and P continued to increase and Mg peaks continued to decrease as shown in Figure 20(b). This continuous growing of hydroxyapatite with immersion of four weeks, obviously confirms the bioactivity of the coating.
EDAX analysis for the corrosion products obtained from bath II coating after two weeks immersion Figure 18(a) and for four weeks Figure 18  It is observed from the EDAX spectrum of the coated specimens with bath III, after immersion of two weeks and four weeks in SBF that well resolved peaks corresponding to different elements Ca, P, O, Mg and Cl are present in Figures 19(a, b) and confirm the synthesis of hydroxyapatite Ca5(PO4)3(OH), calcium hydrogen phosphate dihydrate CaHPO4(H2O)2, magnesium oxide MgO and magnesium hydroxide Mg(OH)2 phases. In agreement with X-ray diffraction for two weeks immersion in the SBF. In case of dipping four weeks hydroxyapatite Ca5(PO4)3(OH), magnesium oxide MgOand magnesium hydroxide Mg(OH)2 phases are appeared. Average grain size of the coat in bath III is 53.5 -35.8 nm. A single unwanted peak of Na is observed which might be due to the foreign impurity. Potentiodynamic polarization studies: Figure 21 shows Potentiodynamic polarization curves of the Mg-6% Zn alloy substrate coated with bath I, II, III and IV in SBF. The corrosion potential (Ecorr), corrosion current density (Icorr) and the anodic and cathodic Tafel constants (βa, βc) are extracted directly from the Potentiodynamic polarization curves using Tafel extrapolation and linear polarization methods [13].
The polarization resistance (RP) is calculated by Stern-Geary equation (Eq. (1) [24,25]) All the electrochemical parameters calculated from Tafel plots are listed in Table 2. It is shown that the treated samples with a lower Icorr, positive Ecorr and higher RP have a good corrosion resistance. The corrosion resistance of samples is improved after the surface coating by the four bathes. By comparing the data Ecorr and Icorr of bath IV coated sample are -1.441v and 1.200 x 10 -4 A/cm 2 respectively. Ecorr (-1.710) of coated sample with bath I is slightly less than that of coated by bath IV, while the Icorr (6.16 x 10-4A/cm 2 ) is reduced. In addition, RP (543.65) of bath IV coat is reduced to 142.73 for bath I coat. It is indicated that bath IV treated sample has higher corrosion resistance than bath III, bath II and bath I. The refinement of the microstructure is good for improving the corrosion resistance of the coated Mg-6%Zn. The corrosion protection of coated samples in SBF follows the sequence Bath IV ›Bath III › Bath II › Bath I.  For four weeks show the best corrosion resistance than which immersed for two weeks and finally the coated substrate.

4.Conclusions
We study the electrodeposition of the four bathes on Mg-6%Zn alloy. The surface morphology of the coat have flake-like structure.
FTIR analysis show the present of hydroxyl, nitrate, phosphate, hydrogen phosphate groups.
X-ray diffraction pattern indicated that the structure of the coat incase of I, II, III, and IV is monoclinic calcium phosphate dihydrate, hexagonal magnesium, calcium oxide, magnesium oxide and magnesium hydroxide.
The elements percentage of the constituents in the coat of the implants is shown from EDAX analysis. The elements are Mg, P, Zn, Ca and O.
Then we test the protection of the coated alloys in the SBF immediately and after different periods of immersion two and four weeks.
The Potentiodynamic polarization of the implants uncoated and coated by the different four bathes are studied in simulated body fluid. The results show that IV› III› II › I.
Potentiodynamic polarization measurements show that the coated alloys have more corrosion resistance than the uncoated ones.
Also as the immersion time of the coated alloys increase in SBF, the corrosion protection increase following the sequence IV› III › II › I.