A comparison study of Ce/La and Ca microalloying on the bio-corrosion behaviors of extruded Mg-Zn alloys

J.H.Chu,L.B.Tong,Z.H.Jing,D.N.Zou,Q.J.Wng,S.F.Liu,H.J.Zhng

a School of Metallurgical Engineering,Xi’an University of Architecture and Technology,Xi’an 710055,China

b State Key Laboratory of Rare Earth Resources Utilization,Changchun Institute of Applied Chemistry,Chinese Academy of Sciences,Changchun 130022,China

c Key Lab of Automobile Materials,College of Materials Science and Engineering,Jilin University,Nanling Campus,Changchun 130025,China

Received 28 June 2019;received in revised form 9 September 2019;accepted 19 September 2019 Available online 25 June 2020

Abstract The influences of Ca and Ce/La microalloying on the microstructure evolution and bio-corrosion resistances of extruded Mg-Zn alloys have been systematically investigated in the current study.Compared with single Ca or Ce/La addition,the Ca-Ce/La cooperative microalloying results in an outstanding grain refinement,because the fine secondary phase particles effectively hinder the recrystallized grain growth.The coarse Ca2Mg6Zn3 phases promote the formation of Ca3(PO4)2 or hydroxyapatite particles during the immersion process and accelerate the dissolution of the corrosion product film,which destroys its integrity and results in the deterioration of anti-corrosive performance.The Ce/La elements can be dispersed within the conventional Mg7Zn3 phases,which reduce the internal galvanic corrosion between Mg matrix and the secondary phases,leading to an obvious improvement of corrosion resistance.Therefore,the Ca-Ce/La cooperative microalloying achieves a homogenous fine-grained microstructure and improves the protective ability of surface film,which will pave a new avenue for the design of biomedical Mg alloys in the coming future.

Keywords:Extruded Mg-Zn alloy;Ca and Ce/La microalloying;Microstructure evolution;Bio-corrosion behaviors.

1.Introduction

Compared with the conventional metallic implant materials,such as titanium alloys,stainless steel or cobalt alloys,magnesium(Mg)and its alloys have attracted great concern,due to their low density,high specific strength,excellent biocompatibility and the ability for promoting the formation of new bones[1–3].More importantly,Mg alloys can be gradually biodegraded in the human body after the tissues have been healed,which can effectively reduce the long-term complications and avoid a second surgical procedure[4–6].The Young’s modulus of Mg alloys(40–45GPa)is close to the human bone(5–23GPa),and the stress shielding effect from the mismatch between the implants and bones can be dramatically decreased[7–10].Additionally,Mg is one of necessary elements in human body(the daily intake is about 300–400mg for a normal adult),and the redundant Mg2+ions from degradation process are non-toxic and can be excreted through excrement or urine[11,12].Therefore,as new class of degradable and resorbable biomaterials,Mg alloys are promising candidates for biomedical materials.

Unfortunately,the poor mechanical properties and corrosion resistances of pure Mg implant destroy its structural integrity,leading to the premature failure before the damaged bone tissue is healed[13–15].Alloying treatment(such as Al,Zn,Mn,rare earth,RE)is an effective approach to improving the mechanical properties and anti-corrosive performances of pure Mg[16–18].However,it has been proved that Al is harmful to neurons,osteoblasts and promotes the risk of Alzheimers disease,and thus the commercial Mg-Al alloys are not applicable as implant materials[19,20].Zn is one of the abundant nutritionally essential elements in the human body and proved to enhance the strength and anti-corrosive performance of pure Mg[21,22],due to its ability of reducing the impurity(Ni or Fe).Therefore,Mg-Zn alloy has been considered as a potential candidate for biomedical materials,but its coarse-grained structure and poor age-hardening response hinder the improvement of yield strength.Ca element is essential for the formation of hydroxyapatite,which promotes the bone growth and prevents the bone fractures induced by osteoporosis.Ca has been proved as an effective grain refiner for Mg-Zn series alloys,which is benefit to the grain boundary strengthening effect[23,24].Compared with the expensive rare earth(Nd,Pr or Gd)elements,as the byproducts of Nd/Pr separation process,low-cost Ce/La mischmetals are safe in human body,when their releases are below the critical limit[25].In addition,Ce/La microalloying can remarkably improve the mechanical properties of Mg alloys[26,27].Recently,many studies have concentrated on improving the mechanical and anti-corrosive properties,through Ca or Ce/La microalloying and thermomechanical processing(extrusion,rolling and forging),and developed new wrought Mg-Zn-Ca or Mg-Zn-Ce/La series alloys for biodegradable materials[28–31].However,the detailed biological corrosion behaviors of wrought Mg-Zn alloys alloyed with Ca or Ce/La elements are controversial,especially the Ca-Ce/La cooperative addition[32,33],which still remain unclear and need further understanding.

In the current study,the microstructure evolution and biocorrosion behaviors of extruded Mg-Zn alloys after single Ca,Ce/La and their corporate additions,are systematically investigated.The anti-corrosive performances are evaluated using immersion and electrochemical tests in simulated body fluid(SBF),and the corresponding corrosion mechanisms will be clarified through analyzing the microstructure evolution of corroded surfaces and corrosion product films,which will open a new window for the design of Mg alloy with superior anti-corrosion performances for the biomedical implant materials.

2.Material and methods

2.1.Preparation of extruded Mg-Zn-(Ca-Ce/La)alloys

Pure Mg(99.96%),Zn(99.90%),Ce/La mischmetal(mass ratio of 4:1)and Mg-30%Ca master alloy were used to prepare the Mg-6.0Zn-1.0Ca(ZX61),Mg-6.0Zn-1.0Ce/La(ZE61)and Mg-6.0Zn-0.5Ca-0.5 Ce/La(ZXE600)(wt.%)alloys,via gravity casting and hot extrusion,the details of preparation process are described elsewhere[31].The actually chemical composition was measured by inductively coupled plasma atomic emission spectrometry(ICP-AES)(Table 1).

Table 1Actually chemical compositions of the Mg-Zn-Ca-Ce/La alloys(wt.%)measure by ICP.

2.2.Microstructure characterization

The Mg alloy discs(11.3×2.0mm3)were cut from extruded bars perpendicular to extrusion direction(ED).All of specimens were ground from 1200 to 4000 grit paper and finely polished using diamond paste.The microstructures of extruded Mg alloys were observed using a JEOL JSM-7000F scanning electron microscopy(SEM)equipped with an energy dispersive X-ray spectroscopy(EDX),and the average grain size was calculated by Image-Pro 5.0 software.

2.3.Immersion tests

The immersion tests was carried out in simulated body fluid(SBF)at 37 °C for 0–30h,the nominal ion concentrations of the current SBF solution,and the pH value of the solution was adjusted to～7.4 with 1.0M HCl(0–5ml)[34].During the immersion test,the hydrogen bubbles were gradually generated from the soaked Mg alloy discs were collected into a measuring burette.The samples after immersion tests were put in the chromic acid solution with AgNO3,to remove the corrosion products before the weight measurement,and then rinsed in alcohol solution and dried by high-purity N2.The corrosion rate(CR)can be calculated according to the Eq.(1)(ASTM G31-72 standard)[35]:

whereg is the weight loss of Mg alloy,A is the exposed area(cm2),t is the immersion time in hours(h),andρis the measured density of each Mg-Zn-Ca-Ce/La alloy(g cm?3).Furthermore,the surface morphologies of Mg alloys with different immersed hours were analyzed by SEM,and the compositions and constitutions of corrosion products were investigated using EDX and XRD analysis.

2.4.Electrochemical tests

Electrochemical tests were carried out in the SBF solution at 37 °C,using an electrochemical system(PARSTAT 4000,Princeton Applied Research,USA).A classical threeelectrode cell consisted of a platinum plate as counter electrode,a saturated calomel electrode(SCE)as reference electrode and the Mg specimen with exposed area of 1.0 cm2as working electrode.The electrochemical impedance spectroscopy(EIS)was measured under the open circuit potential after an initial delay of 300s,with the scan frequency from 105to 10?1Hz,and the EIS data were fitted by ZSimpWin 3.10 software.The potentiodynamic polarization experiments were carried out from?2.0 to?1.0V using a scan rate of 10 mV s?1.Tafel extrapolation method was used to calculate the corrosion potential(Ecorr)and corrosion current density(icorr),and evaluate the corrosion resistance.

Fig.1.Typical SEM microstructures of extruded Mg alloys:(a)ZX61,(b)ZE61,(c)ZXE600.

Table 2EDX analysis of extruded Mg-Zn-(Ca-Ce/La)alloys(at.%).

3.Results

3.1.Initial microstructure

Fig.1 shows the sectional microstructures(perpendicular to ED)of the extruded alloys,and the compositions(EDX)of secondary phases are shown in Table 2.All of the alloys represent the typical equiaxed grain structure,which implies that the fully dynamic recrystallizations occur during the hot extrusion process.The average grain size is calculated as 4.9μm,7.2μm and 2.7μm in ZX61,ZE61 and ZXE600 alloys,respectively,indicating that the combination of Ca and Ce/La is beneficial to the grain refinement.

For the ZX61 alloy(shown Fig.1a),most of necklacelike secondary phase particles with bright contrast are mainly distributed along the grain boundaries,the atomic ratio of Mg,Ca and Zn is accordance with Ca2Mg6Zn3phase(Point A),which has been widely reported in the previous studies[36,37].While the secondary phase particles in ZE61 alloy mainly consist of Mg and Zn,and the Ce/La content is no more than 4.0wt.%(Point B).The average diameter of these particles is remarkably increased compared with that of ZX61 alloy,and therefore the pinning effect for grain growth may be weakened,leading to the grain coarsening behavior.Two types of secondary phase particles are observed in the ZXE600 alloy.A small amount of Ce/La elements can be detected in the coarse spherical phases(Point C)and hardly observed within fine ruptured particles(Point D).Furthermore,the Ca content is much lower than ideal Ca2Mg6Zn3phase,which is related to the eutectic structures of Mg-Zn and Ca2Mg6Zn3phases.The T1’phase in Mg-Zn-Ca-Ce alloy mainly contained the Ca2Mg6Zn3and Mg59Zn33Ce6.5Ca1.5phases[38],which contributes to the ultra-low content of Ca and Ce/La.Fine precipitates are homogenously distributed along the grain boundaries(shown by red arrows),which can effectively hinder the grain growth.

3.2.Bio-corrosion behaviors

As shown in Fig.2,the hydrogen evolutions and corrosion rates of ZX61,ZE61 and ZXE600 alloys immersed in SBF solution for different times are investigated.In the initial stage of immersion(0–5h),the corrosion rates of ZX61 and ZXE600 alloys are much higher than that of ZE61 alloy,implying that single Ce/La element addition results in a superior corrosion resistance,compared with that of Ca or Ca-Ce/La synergistic microalloying.With the increase of immersion time,the corrosion rates of ZE61 and ZXE600 alloys are dramatically decreased,while the ZX61 exhibits a high corrosion rate.Therefore,it can be concluded that Ce/La microalloying of extruded Mg-Zn alloy results in an excellent anti-corrosive property than that of Ca element.

Fig.2.Immersion tests of Mg alloys in SBF solution:(a)hydrogen evolution curves,(b)corrosion rates.

Fig.3.Nyquist plots of Mg alloys immersed in SBF solution:(a)ZX61,(b)ZXE600,(c)ZE61.

As shown in Fig.3 of Nyquist plots,the corrosion characteristics of the extruded Mg alloys immersed in SBF solution are measured through EIS tests.Two well-defined capacitance loops can be observed,which describe the impedance behaviors of double electric layers and surface films,respectively.In the initial corrosion stage of the ZX61 alloy(Fig.5a),the dimension of high-frequency capacitance loop is remarkably increased with increasing immersion time,and the maximum value appears in the sample immersed after 9h,representing the best anti-corrosive performance.With the prolongation of immersion times(9–30h),the dimensions of capacitance loops are obviously reduced,indicating that the corrosion resistances are weakened.Furthermore,the diameter of the secondary capacitance loop is firstly increased and then reduced with increasing immersion time,and the evolution of corrosion resistance may be related to the corrosion product film.In contrast,for the ZXE600 or ZE61 alloys,the dimensions of plots are continuously increased with immersion times,and the secondary capacitance loops represent the similar tendency(Fig.5b and c),which implies that the compact corrosion product films may be formed during the immersion,exhibiting the superior protective abilities.

Fig.4.Bode plots of Mg alloys in SBF solution:(a)ZX61,(b)ZXE600,(c)ZE61.

Fig.4 shows the Bode plots of frequency vs.impedance,the low-frequency impedance value of ZX61 alloy is dramatically increased with immersion times during the initial stage,and reaches the maximum after 9h immersion.With further corrosion from 9 to 30h,the impedance value is obviously reduced,manifesting that the corrosion resistance is gradually worsened.The characteristic of Bode plots for ZXE600 or ZE61 alloy is quite different from that of ZX61 alloy,the impedance values are linearly increased with immersion times,and the corrosion resistance is continuously improved.Especially for the ZXE600 alloy,the maximum impedance value appears in the sample with immersion of 30h,and the corresponding results are well in accordance with that of Nyquist plot.

Fig.5.Equivalent circuits of the EIS spectra of Mg alloy samples.

The EIS spectra are fitted according to the equivalent circuit(Fig.5),and the corresponding results are listed in Table 3,whereRsis the solution resistance,CPEfandCPEdl(constant phase element)represent the capacity of the surface film and electric double layer,respectively,which are determined byY(f,dl)andn(f,dl)values.Whennis equal to 0 and 1,theCPEis identical to a resistor and a capacitor,respectively.Rtrepresents the charge transfer resistance,andRfis the film resistance.TheRtvalues reflect the dissolution rate of Mg alloy substrate,the higher value implies the better corrosion resistance.For the ZX61 alloy,theRtvalue manifests an obvious tendency of first increase and then decrease with increasing immersion times.The maximumRtvalue of 156.2cm2appears in 9h sample,after which the corrosion resistance is remarkably weakened.For the ZXE600 or ZE61 alloy,theRtvalue is continuously increased with immersion times,implying the surface films represent the excellent protection capabilities.The maximum values are 205.5cm2and 202.2cm2in the 30h samples,respectively.Therefore,it can be concluded that the Ca or Ce/La microalloying has a strong impact on the bio-corrosive properties,through modifying the microstructures of corrosion product films during the corrosion process.

Table 3Fitting results of EIS spectra.

In order to further investigate the evolutions of anticorrosive performances during the immersion,the potentiodynamic polarization curves of Mg alloys with different immersion times are shown in Fig.6.Inspection of anodic side,the suspicious passivation behaviors occur in the alloys without immersion treatments,which may be attributed to the naturally formed MgO films(the breakdown points are shown by black arrows).After immersion with different times,the corrosion potentials are greatly increased in all of the three alloys.The anodic reactions are remarkably hindered,implying that the corrosion resistances are improved,which may be related to the formation of corrosion product films on the surfaces of Mg alloys during the immersion process.In addition,the overpotential for hydrogen evolution reaction is remarkably decreased,which will accelerate the cathodic reaction to a certain degree.It is noteworthy that an obvious passivated film breakdown point is only observed in the ZX61 alloy after immersed for 9h(shown by red arrow in Fig.6a),representing a good protective capability,while the corrosion current densities are almost linearly increased with increasing potential in the other specimens.On the contrary,for the ZXE600 and ZE61 alloys,the passivation trends are more remarkable with prolonged immersion,indicating that the Ca-Ce/La or Ce/La addition will effectively improve the corrosion resistances(Fig.6b and c).TheEcorrandicorrvalues of 30h-immersed ZXE600 alloy are increased to?1.42V and 1.86×10?6A/cm2.

3.4.Morphology evolutions of corrosion product films

In order to inspect the microstructure evolution of corrosion product film,Fig.7 shows the surface morphologies of ZX61,ZXE600 and ZE61 alloys immersed in SBF with different hours.The corrosion products are formed on the surface of ZX61 alloy after 5h immersion,while the film with a crackled appearance is not completed,due to the dehydration of the layer after drying in warm air.With increasing immersion time,the corrosion product film gradually covers the surface,but the tiny corrosion pits can be observed in local regions(shown by red arrows),which indicates that this film cannot effectively inhibit the corrosion of ZX61 alloy.Finally,the large break-down areas after long-time immersion appears on the corrosion product film,and leads to the deterioration of protect capability.

For the ZXE600 alloy,the homogenous and chapped film with a dry river bed’s morphology feature is observed in 5 himmersed sample,which can only provide an inferior protection for corrosion behavior.With further corrosion(9–20h),the average diameter of the corrosion product blocks within the film is remarkably increased,and the volume fraction of cracked region is dramatically decreased,even if the local dissolution behaviors occur.After 30h immersion,the corrosion product blocks are gradually coarsened,while this film retains its completeness,which owns an excellent anti-corrosive performance.

For the ZE61 alloy,the surface morphology evolution in the initial stage of immersion process is highly similar with that of ZX61 alloy.With increasing immersion times,the corrosion product film becomes more homogenous and compacted.The volume fraction of tiny corrosion pits is remarkably decreased,and the large-area local corrosion behaviors cannot be observed.

Fig.6.Polarization curves of extruded Mg alloys immersed in SBF solution:(a)ZX61,(b)ZXE600,(c)ZE61.

Fig.7.The surface morphologies of Mg alloys immersed for different hours:(a)～(d)ZX61,(e)～(h)ZXE600,(i)～(l)ZE61.

Fig.8 shows the crystal structures of the various corrosion products on the surface of Mg samples immersed after 20h.Except for Mg(OH)2,the obvious diffraction peaks of Ca3(PO4)2and hydroxyapatite(Ca10(PO4)6(OH)2)are inspected in the ZX61 alloy,which may be related to the original coarse Ca2Mg6Zn3phase particles.For the ZXE600 and ZE61 alloys,some diffraction peaks of(Ce,La)PO4peaks are observed.

Fig.8.XRD patterns of corrosion products on the surfaces of Mg alloys after immersion.

In order to prove the effect of corrosion product film on the anti-corrosive performance of Mg alloys,the SEMEDX results are shown in Fig.9(marked by red regions).The content of Ca element at the broken regions of corrosion product film in ZX61 alloy is much higher than that in ZXE600 and ZE61 alloys,indicating that the Ca-containing corrosion products deteriorate the corrosion resistance.The coarse Ca2Mg6Zn3phases of ZX61 alloy may accelerate the formation of Ca3(PO4)2and hydroxyapatite particles through releasing a large number of Ca2+during the immersion,representing an important role on its corrosive protection,and the detailed mechanisms need further discussed.

Fig.9.EDX results of corrosion products of Mg alloys:(a)ZX61,(b)ZXE600,(c)ZE61.

4.Discussion

The effects of the secondary phase particles on the dynamic recrystallization process can be summarized as follows:(1)the large particles may act as nucleation sites to promote the recrystallization,(2)the closely-packed fine particles can restrict the recrystallization through the significant pinning effect on the grain boundaries.In the current study,two typical secondary phases are proved as Ca2Mg6Zn3and Mg7Zn3in the ZX61 and ZE61 alloys,respectively.Inspecting their morphologies,the diameter of the Ca2Mg6Zn3phase is much smaller than that of Mg7Zn3phase.Therefore,the large Mg7Zn3particles contribute to the nucleation of ZE61 alloy,and cannot hinder the DRX grain growth,leading to a grain coarsening effect.In contrast,the fine Ca2M6Zn3particles in ZX61 alloy effectively hinder the grain growth during the hot extrusion process,exhibiting an obvious grain refinement.For the Ca-Ce/La cooperative microalloying,the types of secondary phases become more complicated.The addition of Ce/La element in the Mg-Zn-Ca alloy promotes the formation of T1’phase particles,the intensive segmentation effect derived from Mg59Zn33Ce6.5Ca1.5phase leads to the refinement of initial Ca2Mg6Zn3particles,which promotes the pinning effect and severely hinders the DRX grain growth,exhibiting the best grain refinement.In addition,the Ce/La elements in the Mg-Zn-Ce/La phases can be partially replaced by the Ca element,leading to the formation of T1’phases.However,the influence of secondary phase particles on the recrystallization is dependent on their volume fraction,size,shape and interparticle spacing.Up to now,the determination and representation of the particle parameters are quite complicated,and the detailed influence mechanisms need further understanding.

Both the immersion tests and electrochemical results have proved that the Ca and Ce/La microalloyings represent the significant influences on the corrosion resistance of extruded Mg-Zn alloys,which is attributed to the evolutions of microstructure(grain size and secondary phase),electrode potentials and the formation of corrosion product film during the immersion process.The fundamental corrosion process of Mg alloy in SBF solution mainly consists of the following reactions:

Generally speaking,the naturally thin and porous magnesium oxide film can be gradually formed on the surface of Mg alloy in the air,or partially transformed into Mg(OH)2or MgCO3within the aqueous and carbon dioxide environments,according to Eqs.(5)and(6).Unfortunately,the Mg(OH)2film will be quickly dissolved via the chemical reaction shown by Eq.(7),due to the existence of aqueous chloride solutions,and thus cannot effectively protect the degradation of Mg substrate.

Therefore,the corrosion resistances of Mg alloys are influenced by various factors,such as grain size,the morphology,composition and distribution of secondary phase particles,as well as the passivation behaviors in SBF solution.Aung et al.[39]reported that the grain coarsening effect resulted in deterioration of corrosion resistance in AZ31B alloy,because the grain boundaries could be acted as the physical corrosion barrier.Alvarez-Lopez et al.[40]also demonstrated that the fine-grained structure remarkably decreased the corrosion rate of AZ31 alloy.However,some conflicting studies reported that the grain refinement through the microalloying or equal channel angular pressing(ECAP)resulted in the decrease of anti-corrosive performance of Mg alloys[41,42].Therefore,the influence of grain size on the corrosion behavior of Mg alloy remains unclear and controversial,which needs further understanding.In the current study,the Ce/La microalloying results in the obvious grain coarsening effect and the outstanding anti-corrosive property,while the ZXE600 alloy with finest grain structure exhibits the moderate corrosion resistance,implying that the grain size effect is not the crucial influence factor,and the corrosion behaviors of extruded Mg alloys are mainly dominated by others.

In order to improve the corrosion resistance of Mg alloy,the alloying element with a close electrochemical potential or that form intermetallic compounds will be preferentially selected,because it can reduce the internal galvanic corrosion behavior.Zn element is nontoxic and represents the excellent grain refinement and strengthening effects,while its standard electrode potential(?0.76V)is much higher than that of Mg(?2.37V),and thus the anti-corrosive performances of Mg-Zn alloys(such as ZK61,ZM21)are very poor[43].In contrast,the wrought Mg-RE based alloys normally exhibit the best corrosion resistance[44],because the electrode potentials of many RE elements(such as Y of?2.37V,Ce of?2.34V,La of?2.38V,Nd of?2.43V)are close to pure Mg[7].In the current study,the trace addition of Ce/La in extruded ZE61 alloy cannot change the crystal structure of conventional Mg7Zn3phase,but most of Ce/La elements are dissolved within the Mg7Zn3particles,leading to the decrease of electrode potential,and weakening the galvanic corrosion betweenα-Mg and Mg7Zn3phase.Similar to that of ZE61 alloy,the Ce/La elements are mainly distributed within the Ca2Mg6Zn3phase particles of ZXE600 alloy,which can also reduce the galvanic corrosion.Moreover,many researchers have reported that the Ca2Mg6Zn3phases represented much higher standard electrode potential than that of Mg matrix[4,5,28,29],and the Ca2Mg6Zn3phase particles as the cathodes would accelerate the galvanic corrosion behavior of ZX61 alloy,resulting in the high corrosion rate.Although no accurate standard electrode potential data of Ca2Mg6Zn3and Mg7Zn3-Ce/La phases are available so far,the experimental results indicate that the extruded Mg-Zn alloy containing the Mg7Zn3phases by modified Ce/La element(ZE61)has more superior corrosion resistance than that of Ca2Mg6Zn3phases(ZX61 or ZXE600),especially in the initial corrosion stage.

Compared with the grain size and electrode potential,the effect of secondary phase particle on the corrosion resistance of extruded Mg-Zn alloy is more remarkable,through modifying the corrosion product film after the prolonged immersion.In the initial stage of corrosion process in SBF solution,the thin corrosion product film gradually covers the surface of Mg alloy.With elongation of immersion time,both the formation of passivation film and its dissolution behaviors occur,exhibiting an obvious competitive effect.Therefore,the various alloying elements and the composition,morphology and distribution of the secondary phase particles intensively influence the evolution of corrosion rate.In addition,the corrosion behavior of Mg alloy in SBF solution is different from that in standard NaCl aqueous solution(3.5wt.%)to some degree,due to the existence of phosphate and carbonate,which might influence the formation of corrosion product film.For the current study,the EDX analysis proves that there exist high content of Ca and P elements within the corrosion product films of all the Mg alloys,implying the formation of calcium phosphate or hydroxyapatite during the immersion and the corresponding chemical reactions in Eqs.(8)and(9)can simultaneously occur.

Fig.10.Schematic illustration of corrosion mechanisms of Mg alloys immersed within SBF solution.

The integrity of corrosion product film plays a key role in the corrosion resistance of Mg alloy,and the influences of Ca or Ce/La microalloying on the evolutions of corrosion product films ought to be considered.The Ca2+concentration near the Ca2Mg6Zn3phase will be sharply increased during the corrosion process of ZX61 alloy,which can provide the additional sites for Ca3(PO4)2or hydroxyapatite nucleation.Moreover,the existence of hydroxyapatite particle promotes the dissolution behavior of the Mg(OH)2product film and destroys its integrity,leading to the deterioration of corrosion resistance.Fig.12 shows the schematic diagram of the influence mechanisms of corrosion product films formed during the SBF immersion on the corrosion resistances of extruded Mg-Zn alloys.The original coarse Ca2Mg6Zn3phase particles result in a valid segmentation effect of Mg(OH)2film,and the Ca3(PO4)2or hydroxyapatite particles are favorably generated near the Ca2Mg6Zn3phases for ZX61 alloy(Fig.10a).Therefore,the additional interfaces between the Mg(OH)2(deep yellow color)and Ca3(PO4)2/hydroxyapatite(red particles)may be produced in the shadow regions,accelerating the permeation of corrosive medium and enhancing the galvanic corrosion behavior nearby the interfaces,which promotes the dissolution of Mg(OH)2film and worsens the corrosion resistance of Mg alloy.Furthermore,the Ca2+,PO43?and OH?within the SBF solution can be gradually deposited on the surface or within the Mg(OH)2film(purple particles)and form the Ca3(PO4)2or hydroxyapatite,while the influence of these particles on the integrity of corrosion product film is inconspicuous.

The Ca2Mg6Zn3phases are remarkably refined in the ZXE600 alloy,due to the trace addition of Ce/La element,and the integrity of Mg(OH)2film is retained and posses an excellent protection ability(Fig.10b).For the ZE61 alloy,the Ce/La elements are mainly distributed within the conventional Mg7Zn3phases,reducing their electrode potential and improving the corrosion resistance.Different from the Ca2Mg6Zn3phases,the Mg7Zn3phase particles containing Ce/La elements cannot destroy the integrity of Mg(OH)2film(Fig.10c),which keeps its completeness during the immersion and provides a superior corrosion protection.However,the detailed influence mechanisms of Ca2Mg6Zn3and Mg7Zn3-Ce/La particles on the corrosion properties of wrought Mg alloys in SBF solution also need further understanding,even if the experiment data have proved that the anti-corrosive performances of extruded Mg-Zn alloys after Ce/La or Ca-Ce/La microalloyings are superior to that of single Ca addition.

5.Conclusions

The influences of single Ca,Ce/La and Ca-Ce/La cooperative microalloying on the microstructural evolution and biocorrosion properties of extruded Mg-Zn alloys have been systematically investigated in the current study.It can be concluded that the Ca-Ce/La cooperative addition results in a homogenous microstructure with good corrosion resistance,which will open a new avenue for the design of wrought Mg alloy in the future.Furthermore,the volume fraction of the coarse Ca2Mg6Zn3phase particles should be decreased through the chemical composition design(such as decreasing the Zn content,adding the other low-cost alloying element to promote the segmentation effect),in order to achieve its application in industrial application,which is beneficial to the improvement of corrosion resistance,and the key conclusions are drawn as follows:

1.Compared with single Ca or Ce/La addition,the Ca-Ce/La synergetic microalloying results in the remarkable refinement of secondary phase/precipitate of the extruded Mg-Zn alloy,and represents the most effective grain refinement,through hindering the recrystallized grain growth during the hot extrusion.

2.The Ca microalloying leads to the poor anti-corrosive performance,while the Ce/La addition exhibits the most excellent corrosion resistance of Mg-Zn alloy in the initial stage of immersion.With increasing immersion time,the Ca-Ce/La synergetic microalloying results in an obvious improvement of the anti-corrosive property.

3.The Ce/La elements can be uniformly dispersed within the original Mg7Zn3and Ca2Mg6Zn3phase particles,and improves the corrosion resistances of Mg-Zn alloys,through reducing their electrode potentials.Compared with the Mg7Zn3-Ce/La phases,the Ca2Mg6Zn3phases easily destroy the integrity of corrosion product film and result in the deterioration of bio-corrosion resistance,through promoting the formation of Ca3(PO4)2or hydroxyapatite particles in SBF solution.

Acknowledgements

Thanks to the financial aid from the National Natural Science Foundation(Grant nos.51771178,51671152 and 51874225),the Key Research and Development Program of Shanxi Province(Grant nos.2020KWZ-007 and 2018ZDXMGY-149),the Youth Innovation Team of Shanxi Universities and the Natural Science Foundation of Jilin Province(Grant no.20180414016GH).