| Magnesium(Mg)and its alloys are contemplated as possible bioimplant materials for the cardiovascular stents,craniofacial and orthopedic restoration because of its excellent biocompatible features such as closer mechanical properties to the bone,low density,inherent biocompatibility and higher fracture toughness than that of ceramics and polymers.Moreover,due to the characteristics of promoting mineral apposition on implanted bone,they play supportive role for bone remodeling without activating surrounding tissues.On the other hand,gradual dissolution features of Mg and its alloys in physiological environment further encourage their acceptance as biodegradable implant materials and lead to avoid the risk of their existence in the body for a long time,and also skips the necessity of corrective surgery after complete cure of the tissues.However,Mg and its alloys are not accepted as a professional biomedical implant material so far because of their low corrosion resistance in physiological environments as well as releasing the excess amount of hydrogen gas which could reduce the mechanical stability before the complete healing of suffering tissue and limit their clinical applications.In this work,we present an investigation on the corrosion behavior of AZ31 magnesium alloy through in-vitro process in simulated body Fluid(SBF)with the presence of D-fructose and sucrose;and in Hank's solution with the presence of glycine,at 37? and a pH of 7.4,while in-vivo degradation behavior of pure magnesium was investigated in femur bone of rabbit.Various techniques such as potentiodynamic polarization(PDP),potentiostatic polarization,electrochemical impedance spectroscopy(EIS)and hydrogen(H2)gas evolution techniques in combination with surface characterization techniques such as scanning electron microscopy(SEM),energy dispersive spectroscopy(EDS),X-ray diffraction(XRD),X-ray photoelectron spectroscopy analysis(XPS),Fourier transformed infrared(FTIR),Raman spectroscopy,electron probe micro-analyzer(EPMA),optical microscopy(OM)and stereomicroscopy technique were used during the course of the study.The major findings are presented below.In-vivo corrosion behavior of pure Mg was studied after the successful implantation of pure magnesium in femur bone of rabbit through the surface morphology analysis and corrosion products characterization,and in-situ EIS evolution using specially designed three-electrode system.With extending the implantation time,three layered corrosion products were deposited on the surface of pure Mg screw with the elemental composition of C,O,Mg,P and Ca.The basic corrosion products obtained were magnesium hydroxide(Mg(OH)2),dypingite(Mg5(CO3)4(OH)2·5H2O),monohydrocalcite(CaCO3·H2O),kovdorskite(Mg2(PO4)(OH)·3H2O)and hydroxyapatite(Ca5(PO4)3(OH).Magnesium hydroxide was deposited in the inner layer,while dypingite and kovdorskite were deposited in the middle layer as the principal phases,and while accumulation of monohydrocalcite and hydroxylapatite were found in the outer layer as the minor phases.In addition,the content of P and Ca was increased with increasing the implantation time which further verified the increased deposition of Ca-P compounds on the Mg screw.The in-situ EIS measurement reveals that the corrosion resistance of the implanted pure Mg screw was initially decreased,but increased with extending the implantation time,which could be attributed to the deposition of the thick layer of corrosion product on the Mg screw.In SBF solution,the formation of thin surface protective film on the AZ31 Mg alloy was observed in three solutions i.e.blank solution,solution with D-fructose and solution with sucrose,which facilitated the transmission of aggressive ions into the corrosion film and encouraged the localized corrosion of the Mg alloy in the initial stage,in the three different solutions.With increasing the immersion time deposition of both white granular particle and content of O,P and Ca were increased on the sample surface immersed in the three different solutions.However,content of O,P and Ca,and thickness of the corrosion products deposited on the surface of the sample were highest in solution with D-fructose followed by solution with sucrose and lowest in blank solution for the same immersion duration.Furthermore,gradual decrement of the anodic current density for the sample immersed in the three different solutions was observed with increasing the immersion time during potentiodynamic polarization.Nevertheless,lower anodic current density was found for the sample immersed in D-fructose containing solution as compared to rest of solutions for the same immersion period.Meantime,with extending the immersion time,breakdown potential was distinctly observed in solutions containing D-fructose,and with sucrose at the anodic branch which is attributed to the existence of passive state with extending the immersion time,indicating a behavior transfer from pitting corrosion to passive state of AZ31 Mg alloy.However,smaller cathodic current density was observed in solution with sucrose with increasing the immersion time signifying the interreference of sucrose in the cathodic hydrogen reduction mechanism of Mg alloy,which could be possible due to the presence of glucose as a dissociated product of sucrose in SBF.The corresponding hydrogen evolution rate(HER)obtained during the immersion was relatively lower in D-fructose containing solution,and was decreased with increasing the immersion period.Furthermore,the better inhibition efficiency of D-fructose for the anodic dissolution of Mg was found,whereas,suppression of the cathodic hydride formation was observed in solution with sucrose compared to other solutions using the potentiostatic polarization analysis.In addition,with increasing the immersion time,the gradual increament of the corrosion resistance of the sample in the three different solutions was observed.Nonetheless,sample in solution with D-fructose exhibited significantly heigher corrosion reistance followed by lower in solution with sucrose and lowest in blank solutions for the same immersion period,which could be attributed to the deposition of thicker and more compact corrosion product on the surface of AZ31 magnesium alloy with lengthening the immersion time.In Hank's solution,the degradation behavior of AZ31 was significantly influenced by the addition of glycine through the coalesced effects of adsorption and chelation.Initially,the formation of apatite was enhanced on the surface of the AZ31 magnesium alloy due to the adsorption of glycine,where glycine behaved as an inhibitor thereby subduing the dissolution of AZ31 Mg alloy.With extending the immersion time,the content of apatite was decreased due to the coordination of the Ca2+ ions from the apatite with the chelating ligands in glycine forming a soluble compound on decomposition.Moreover,the chelation of glycine with Ca2+ ions induced the formation of cracks on the film surface which is responsible for further promotion of the dissolution of AZ31 Mg alloy thereby forming porous corrosion products layer on the surface of the alloy.As a result,both the continuous dissolution of AZ31 magnesium alloy and the hydrogen evolution rate(HER)were enhanced with increasing the immersion time in Hank's solution.However,Hank's solution without glycine gradually suppressed the corrosion of AZ31 Mg alloy through a continuous deposition of corrosion products on the metal surface as immersion time increases.The deposited corrosion products act as a stable barrier layer and strengthen with increasing the immersion time.Hence,the electrochemical activity and HER of the alloy were gradually reduced with increasing immersion time.Moreover,total current densities,negative deference effect(NDE),Mg dissolution current and Mg hydride formation current densities were relatively lower in solution with glycine compared to without glycine solution at the same applied potentials during potentiostatic polarization.This demonstrates the better inhibition efficiency of glycine thereby inhibiting the anodic Mg dissolution and further suppressed the cathodic hydride formation due to the adsorption of glycine on the surface of AZ31 Mg alloy in the beginning of the immersion time. |
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