Tailoring the microstructural characteristic and improving the corrosion resistance of extruded dilute Mg-0.5Bi-0.5Sn alloy by microalloying with Mn

Yng Liu,Wei-li Cheng,b,*,Xiong-jie Gu,Yn-hui Liu,Ze-qin Cui,b,Li-fei Wng,b,Hong-xi Wng,b

a School of Materials Science and Engineering,Taiyuan University of Technology,Taiyuan 030024,China

b Shanxi key Laboratory of Advanced Magnesium-Based Materials,Taiyuan University of Technology,Taiyuan 030024,China

Abstract Mg-0.5Bi-0.5Sn alloys with and without microalloying with 0.5wt% Mn were subjected to extrusion,and the effect of Mn microalloying on the microstructural characteristic and corrosion behavior of the extruded alloys was investigated.The results indicated that the average grain size and the density of dislocations decreased,and a new Mg26.67Mn65.47Fe7.86 second phase as well as grain boundary segregation of Sn atoms could be observed in certain micro-regions of the extruded dilute Mg-0.5Bi-0.5Sn-0.5Mn alloy.The tailoring of microstructure resulted in the significan enhancement in corrosion resistance(Rp increased from 1095.91Ωcm2 to 5008.79Ωcm2).In addition,grain boundary segregation resulted in intergranular corrosion and led to the dissolution of Sn atoms.Hence,the dissolution rate of the matrix in Mg-0.5Bi-0.5Sn-0.5Mn alloy could be inhibited by the corrosion product fil containing an intermediate product(SnO2).? 2020 Chongqing University.Publishing services provided by Elsevier B.V.on behalf of KeAi Communications Co.Ltd.This is an open access article under the CC BY-NC-ND license(http://creativecommons.org/licenses/by-nc-nd/4.0/)Peer review under responsibility of Chongqing University

Keywords:Mg alloy;Extrusion;Microalloying;Microstructure;Corrosion behavior.

1.Introduction

Mg alloys are an attractive option as next-generation biodegradable implant materials due to the excellent biocompatibility,similar density and elastic modulus to human bone,and outstanding biodegradability[1-3].However,a long-standing obstacle to their wide application is the unsatisfie degradation rate,which leads to the loss of mechanical integrity of Mg alloys prior to implanting treatment being accomplished[4,5].Hence,it is critical to decrease the degradation rate of Mg alloys and explore the corrosion mechanism in simulated body flui(SBF)solution.

Alloying is one of the favorable methods that can decrease the corrosion rate effectively.A number of research works have been dedicated to the influenc of various elements alloying on the corrosion resistance of Mg alloys.The results show that the corrosion properties could be enhanced through grain refinemen[6,7],modifying the distribution and fraction of second phases[5,8,9],removing impurities like Fe,Ni and Cu[10-12],and altering the component of corrosion product fil[1,13,14].However,high alloying leads to a great amount of second-phase particles,which promotes the formation of micro-galvanic couples and thus accelerates corrosion processing.Similarly,?zarslan et al.[15]found that Mg-4Sn-1Ge alloy possessed the best corrosion resistance among the investigated Mg-4Sn-xGe(x=1,2,4wt%)alloys.Liu et al.[13]also claimed that the corrosion rate of Mg-1Zn-1Bi-0.6Ca alloy was much lower than that of high-alloyed Mg-Zn based alloys.

Recently,Mg-Bi alloy system has attracted great attention and presents great potential to be developed as biomaterials,which is attributed to the non-toxic and not bio-accumulative as well as the formation of thermal-stable Mg3Bi2phase[16-18].Furthermore,Tok et al.[19]demonstrated that if the Bi element being in solute state inside the matrix,the corrosion rate of Mg-Bi based alloys could be suppressed.Hence,taking both the mechanical performance and the corrosion resistance into consideration,dilution seems to be the best way to balance them and to decrease possibly harmful effects related to alloying elements.

It is known that Sn is one of the trace elements in the human body and beneficia to the wound healing process.Previous work validated that the hydrogen evolution rate of Mg alloys could be inhibited by the dissolution of Sn due to the higher hydrogen evolution overpotential of Sn[20,21].In addition,it is also reported that Sn alloying could decrease the grain size as well as promote the formation of passive fil in Mg-Al based alloys[22].However,excessive Sn alloying would result in the precipitation of the Mg2Sn phase,which will provide cathodic sites to induce micro-galvanic corrosion[23,24].Thence,the addition of Sn should be controlled at a certain low level.

Mn is also one of the non-toxic elements and plays a critical role in the enzyme systems.Some studies reported that the Mn addition could improve the corrosion resistance of high alloyed Mg alloys through removing the harmful impurities and/or resulting in the generation of passive fil[5,12,25].While,to our knowledge,limited work is committed to exploring the effect of Mn microalloying on the corrosion behavior of dilute Mg-based alloys.

In addition,extrusion is an effective method of refinin the microstructure and modifying the distribution of solutes,which could improve the corrosion resistance of Mg alloys[26].In this work,Mg-0.5Bi-0.5Sn and Mg-0.5Bi-0.5Sn-0.5Mn alloys were developed and subjected to extrusion.The effects of microalloying Mn on the grain size,dislocation density,second-phase particles and the distribution of solutes of the extruded dilute Mg-0.5Bi-0.5Sn alloy were investigated.Furthermore,how the tailoring in microstructure influence the corrosion behavior and underlying mechanism was also explored in details.

2.Experimental work

The casting ingots with the nominal composition of Mg-0.5wt% Bi-0.5wt% Sn and Mg-0.5wt% Bi-0.5wt% Sn-0.5wt% Mn alloys were produced through melting down pure Mg(99.99wt%),Bi(99.99wt%),Sn(99.99wt%),and Mg-10wt% Mn in an electrical resistance furnace.The melting process should be carried out under the protection of CO2and SF6gas mixture to avoid oxidation.Molten metals(at 983K)were cast into a preheated copper molds(approximately 473K)to produce a cylindrical ingot(length:80mm;diameter:60mm).The ingots were homogenized at 693K for 1h and 773K for 3h followed by water-quenching.And then,preheated samples(at 573K for 30min)were extruded at 573K as well as an extrusion ratio of 25.

The microstructure characterization of studied samples was examined using Scanning electron microscope(SEM;Mira 3XMU),and Electron back scatter diffraction(EBSD;JEOL JSM-7000F).The average grain sizes(AGS)were measured by the software(Nano Measurer 1.2),which were calculated from three optical micrographs and the averages were taken as the fina results.Transmission electron microscopy(TEM;JEM-2100F)equipped with an energy dispersive X-ray spectroscope(EDS)was used to analyze phase constitution and detailed microstructural characteristics.

All electrochemical tests and immersion tests were conducted at 37 °C in SBF solution.The solute composition in a litre of SBF solution was listed in Table 1.The electrochemical workstation(CorrTest,Wuhan,China)was used to measure polarization curves and electrochemical impedance spectroscopy(EIS)of studied samples.The standard of immersion tests was ASTM G31-72.Details of the experiments mentioned above had been described in our previous paper[27].The morphologies of the corroded samples with and without corrosion products were observed using SEM and 3D corrosion morphologies were examined using Digital microscope(DM,Olympus DSX1000).X-ray photoelectron spectroscopy(XPS,K-Alpha)was applied for the chemical composition of the corrosion products of the studied alloys.

Table 1The solute composition of SBF in a litre of solution.

Table 2Fitted electrochemical parameters of the extruded alloys obtained from electrochemical polarization curves.

All testing samples were mechanically ground with 1000-4000 grit SiC papers and polished,then washed with alcohol and air-dried completely.The dimensions of testing samples were 10mm×10mm×4mm and the testing plane was parallel to the extrusion direction(ED)-transverse direction(TD)plane of the extruded bar.

3.Results and discussion

3.1.Microstructural characteristics

Fig.1 shows the inverse pole figur(IPF)maps,grain size distribution maps,and(0001)pole figure of the studied alloys.The alloys exhibited a fully recrystallized microstructure and the AGS decreased from 11.04μm to 9.72μm after Mn alloying.Similarly,Liao et al.[28]found that the AGS of Mg-1Sn alloy decreased from 13.1μm to 2.8μm by the addition of 2wt% Mn,which illustrated that Mn was one of effective alloying elements suitable for grain refinemen of Mg alloys.In addition,the scatter texture of Mn-free alloy contained two texture component,which were the basal poles from the normal direction(ND)rotated around 60° towards the ED as well as the splitting of basal poles titled around 60°-90° from the ND rotated towards the ED,while the texture of Mn-containing alloy was ED-split texture and its texture peak tilted from the ND to the ED by around 30°.Lee et al.[29]validated that the texture intensity could be modifie via lattice rotation of dynamic recrystallized(DRXed)grains by dislocation slip during deformation.It should be noted that the average Schmidt factor of Mn-free and Mncontaining alloys were 0.185 and 0.152,respectively.Thence,the dislocation of Mn-free alloy was more likely to slip due to its relatively high Schmidt factor and the texture was more scatter and weaker than that of Mn-containing alloy.

Fig.1.EBSD inverse pole figur maps,grain size distribution map and(0001)pole figure from the ED-TD plane of the extruded alloys,(a),(b),and(c)Mg-0.5Bi-0.5Sn,(d),(e),and(f)Mg-0.5Bi-0.5Sn-0.5Mn.

Fig.2(a)and(b)shows the SEM micrographs of the extruded alloys.It could be observed that only few nano-scaled particles distributed along the grain boundaries and in the grain interior in the extruded alloys.TEM measurement together with EDS analysis were carried out further to figur out the nano-scaled particles(Fig.2(c)-(f)).The globular particles(points 1 and 3)could be observed in both Mn-free and Mn-containing alloys,which could be confirme as Mg3Bi2phase.Note that the fraction and size of Mg3Bi2phase particles were extremely minor,so it could be deduced that the particles exhibited weak effect on corrosion of the studied alloys.Nevertheless,certain irregular block particles(point 2)containing Mg,Mn and Fe elements could be formed in the grain interior of the Mn-containing alloy,which indicated that Mn element could combine with impurity elements to form Mg26.67Mn65.47Fe7.86phase.The standard potentials of Mg,Mn,and Fe were?2.37V,?1.029V and?0.037V,respectively.The potential of Fe was nobler than that of Mn and Mg,therefore the Mg-Mn-Fe phase replaced the more detrimental impurity(Fe)could weaken the potential between impurity andα-Mg,which reduced the harmful effect of impurity Fe on the corrosion process of Mg alloy[30].

Fig.3(a)and(b)shows the bright fiel TEM images of the extruded dilute alloys.Mn-free alloy exhibited relative more dislocation and the dislocation pile-up in the grain interior than that of Mn-containing alloy.Previous studies reported that the crystallographic defects such as dislocations could lead to materials in these regions corroded preferentially due to the local high energy and atomic arrangement[31,32].Therefore,the lower density of dislocation could decrease the matrix dissolution rate of Mg alloy.As indicated in Fig.3(c),some atoms were segregated in the area neighboring to the grain boundary could be observed in Mn-containing alloy.To further investigate the composition of these atoms,EDX line-scan along the red dashed line in Fig.3(c)was performed and the result is shown in Fig.3(d).It was evident that these segregated atoms were Sn atoms.

3.2.Corrosion tests

Fig.4 shows the hydrogen evolution data and the corresponding corrosion rates of the extruded specimens in SBF at 37 °C.The corrosion rates were calculated by the following equation[33]:

Fig.2.SEM micrographs and bright fiel TEM images of extruded specimens:(a),and(d)Mg-0.5Bi-0.5Sn and(b),(e),and(f)Mg-0.5Bi-0.5Sn-0.5Mn alloys,as well as(c)energy dispersive X-ray spectroscope of Point 1,2,and 3.

Fig.3.Bright fiel TEM images of extruded specimens:(a)Mg-0.5Bi-0.5Sn and(b),and(c)Mg-0.5Bi-0.5Sn-0.5Mn,as well as(d)EDX line-scan along the red dashed line in(c).

The hydrogen evolution curves could be divided into four stages.Firstly,the hydrogen volume of Mn-containing alloy was slightly less than that of Mn-free alloy during the stage I.It should be noticed that the hydrogen volume of Mn-free alloy increased obviously after 72h immersion,while,the time of increased rates of hydrogen volume was delayed with the Mn microalloying,which illustrated that the addition of Mn could delay the time of the corrosion product fil rupture and improved the stability of corrosion product fil[34].During stage II,the fittin slope of the hydrogen volume evolution curve of Mn-containing alloy was steeper than that of Mnfree alloy,demonstrating that the corresponding corrosion rate of Mn-free alloy was lower than that of Mn-containing alloy.After immersion for 168h,the corrosion rate of Mn-free alloy increased with the increase in immersion time.However,the corrosion rate of Mn-containing alloy decreased significantl and it was roughly stable at 0.5mm/y,which may be related to the dynamic balance between the formation and breakdown of corrosion product film

Fig.4.The hydrogen volume evolution(a)and the corresponding corrosion rates(b)of the extruded specimens after immersion for 384h in SBF at 37 °C.

Fig.5.(a)anodic branches of the polarization curves and(b)anodic current density values measured at?1.55V,?1.50V and?1.45V from the polarization curves and(c)cathodic branches of the polarization curves.

Fig.5(a)shows the anodic polarization curves of the extruded alloys.It can be found that theEcoordecreased from?1.57V in Mn-free alloy to?1.63V in Mn-containing alloy,indicating that the kinetics of the anodic reaction was sensitive to the Mn addition.On the one hand,the Mn-containing alloy had lower dislocation density than that of Mn-free alloy,which led to a decrease in the dissolution rate of anode.On the other hand,the occurrence of grain boundary segregation in Mn-containing alloy might lead to the grain boundaries corroded preferentially and increased the anodic dissolution rate,which would be meticulously discussed in Fig.8.In this work,the density of dislocations and dislocation pile-up were relatively lower and slighter,it could be inferred that the decrease in the density of dislocations had less influenc on the kinetics of the anodic reaction than the intergranular corrosion of the extruded Mn-containing alloy with fin DRXed grains.Therefore,the Mn-containing alloy possessed a larger anodic current density.

The anodic current density values measured at?1.55V,?1.50V and?1.45V,respectively,and the corresponding results are shown in Fig.5(b).The difference in current density between Mn-free alloy and Mn-containing alloy initially decreased slightly and then increased sharply with increasing potential,which was attributed to the passivation tendency of the anodic branch of Mn-containing alloy.When the potential was between?1.45V and?1.50V,the passive fil formed on the Mn-containing alloy surface could protect the matrix of Mn-containing alloy[35].However,the anode current density of Mn-containing alloy enhanced sharply when the potential exceeded the critical value,which was mainly related to the breakdown of passive film After breakdown,the bare metal surface was directly exposed to the SBF solution and dissolved quickly,therefore,the anode current density increased significantl.

The cathodic branches of the polarization curves are shown in Fig.5(c).As shown,the cathodic branches of the polarization curves exhibited liner Tafel region,so the intersection of the cathode tangent line with theEcorrvalues could be used to estimate the corrosion current density(icorr)[36].The fittin parameters of the polarization curves are listed in Table 2.The values oficorrwere decreased significantl after the addition of Mn,which was mainly affected by the generation of passive fil and the Mg26.67Mn65.47Fe7.86phases.The passive fil on the sample surface could protect the matrix and inhibit the cathodic kinetics in the corrosion process.Furthermore,the effect of the Mg26.67Mn65.47Fe7.86phases with lower potential on micro-galvanic corrosion was weaker than that of impurity.The corrosion rate(Pi)was calculated by the following equation[13]:

ThePidecreased from 0.59mm/a to 0.22mm/a after the addition of Mn,indicating that the cathodic activity of Mncontaining alloy was suppressed.

The anodic and cathodic polarization curves of the extruded alloys showed reverse trend.Previous work reported that the polarization curves were obtained by short-term electrochemical tests and the corrosion pits on the alloy surface quickly became corrosion cavities after the passive fil was broken down,therefore,the repassivation was unlikely to occur during the corrosion process[34].It could be deduced that the corrosion rates of the extruded alloys were close to the cathodic reaction rates.Furthermore,the hydrogen evolution rate of Mn-containing alloy increases rapidly after the breakdown of the corrosion product fil(Fig.4).While,as the immersion time exceeded 172h,the hydrogen evolution rate of Mn-containing alloy was much lower than that of Mnfree alloy,which also demonstrated that the corrosion rates of the extruded alloys were mainly controlled by the kinetics of the cathodic reaction.

Fig.6(a)shows the Nyquist plots of the studied alloys,it could be found that the Nyquist plots contained high frequency capacitive loop and intermediate frequency capacitive loop,which corresponded to the charge transfer process between the sample surface and the electrode as well as the formation of corrosion product film respectively[13].

To explore the corrosion characterization of the extruded alloys,the fittin results of the Nyquist plots are listed in Table 3 and the equivalent circuit with the surface model is shown in Fig.6(b).In this circuit,the SBF solution provided the solution resistance(Rs)and the substrate of Mg alloy could be considered as working electrode.The Mg2+generated at the interface whenα-Mg was in contact with the electrolyte,which corresponded to the elements of the charge transfer resistance(Rct)and the electric double-layer capacity(CPE1).As the matrix of Mg alloy dissolved,the corrosion products precipitated on the surface,which protected the Mg matrix and prevented ions diffusion.The corresponding elements were the resistance(Rfil)and the capacitance(CPE2)of corrosion product layer[37].As shown in Table 3,theRctincreased from 2289Ωcm2to 3998Ωcm2after Mn microalloying,indicating that the dissolution rate of the Mncontaining alloy matrix decreased.Furthermore,the higher values ofRfilof Mn-containing alloy illustrating that the corrosion product fil on the surface was more compact and denser.Previous study reported that the constant phase element(CPE)was define byCPE-Tand n,theCPEwas identical to an ideal capacitor when n was equal to 1[27].However,others considered that the parameterCPE-Tcould not completely represent the value of capacitance,the effective capacitance of corrosion product fil could be calculated by the fl wing equation[34]:

Table 3Fitting results of EIS spectra.

Furthermore,the polarization resistance(Rp)was usually calculated by the following equation[27]:

Fig.6(c)shows theCfandRpof the extruded alloys,it could be found that theRpincreases from 1095.91Ωcm2to 5008.79Ωcm2after the addition of Mn.In addition,Mncontaining alloy possessed the lower value ofCf,suggesting that the thinner corrosion product fil formed on the surface.The thinner corrosion fil and higher value ofRpmeant lower corrosion rate,therefore,Mn-containing alloy exhibited better corrosion resistance.

The corroded surface of the Mn-free alloy removing corrosion products after immersed in SBF solution for different times and the EBSD IPF map corresponding to grains with size<10μm as well as 3D corrosion morphology are shown in Fig.7.The corrosion pits and strip-like corrosion morphologies could be observed on the surface of Mn-free alloy after 10min immersion,which may be related to the fin grain size and non-basal crystal orientation.Previous studies reported that grains with basal orientation exhibited better corrosion resistance,which was attributed to that these grains possessed the highest atomic density(1.13×1019atoms/m2)and the lowest surface energy(15.4kJ/mol)among all oriented grains[38-40].Furthermore,Luo et al.[32]reported that the grain size also had an effect on the corrosion rate of grains.The relatively small grains(smaller than average grain size)were preferentially corroded when Mg alloys immersed in the corrosive medium and the basal plane was more stable than the non-basal plane with the same grain size.As shown in Fig.7(b),the small grains with size<10μm(close to average grain size)marked by the blue rectangles exhibited a strip-like distribution,which was similar to the corrosion morphologies on the surface of Mn-free alloy.Moreover,the corresponding(0001)pole figur exhibited that most of the small grains are non-basal oriented grains.After immersion for 30min,in addition to the pitting corrosion and strip-like corrosion morphologies,the filifor corrosion also could be observed on the surface of Mn-free alloy.The presence of filifor corrosion suggested the generation of passive fil in the initial stage of immersion[13],which would be meticulously discussed in Fig.10.It could be found that corrosion cavities formed on the corroded surface after 3h immersion.As the immersion time increased,the huge corrosion cavities could be observed,which implied that the small corrosion pits could coalesce and quickly grow into corrosion cavities causing more Mg matrix to be exposed to the SBF solution.Therefore,the repassivation process was less likely to occur in the growth of corrosion cavity and the corrosion rate of Mn-free alloy increased with the increase of immersion time.

Fig.6.(a)Nyquist plots of the extruded alloys and(b)equivalent circuit used to fi the EIS spectra and(c)the calculated fil capacitance and polarization resistance for the extruded alloys.The inset in figur(a)shows a high magnificatio image of the high frequency zone.

To further clarify the corrosion morphology of Mn-free alloy,the corroded damage was characterized by 3D topographic.The distance from the highest point(red)to the lowest point(purple)indicated by the color legend shown on the right.It could be observed that the localized corrosion was severe and distributed randomly,which was consistent with the SEM micrograph after immersed for 24h.

Fig.8(a)-(d)shows the SEM micrographs of the decorroded products of the Mn-containing alloy after immersed in SBF solution for 10min,30min,3h,respectively,and the EBSD IPF map corresponding to grains with size<10μm.After immersion in SBF solution 10min,intergranular corrosion occurred on the corroded surface of Mn-containing alloy,which was attributed to the presence of grain boundary segregation(Fig.3).The potential of Sn(?0.14V)was more nobler than that of Mg(?2.37V),therefore,the Mg matrix with different content of Sn element possessed different potential,which induced the micro-galvanic corrosion between grain boundaries and Mg matrix[34,41].Besides that,some corrosion pits also could be found on the surface of Mncontaining alloy.It should be mentioned that pitting corrosion preferentially occurred on the small grains.With the increased immersion time,intergranular corrosion and pitting corrosion became more severe.In addition,it could be found that falloff of some small grains occurred on the corroded surface and other small grains were not bound commendably to the Mg matrix,which was related to the difference in density of grain boundaries between coarse-grained and fine-graine regions.Consequently,the effect of intergranular corrosion on the fine-graine region was more obvious,and thus the small grains would preferentially fall off with the aggravation of intergranular corrosion.

As to the corrosion morphology after immersion for 3h,the mixed pitting,intergranular and filifor corrosion could be found in uncoated Mn-containing alloy.It could be found that the path of filifor corrosion was induced by the shedding of a large number of small grains.Furthermore,as shown in Fig.8(d),the distribution trend of small grains in Mncontaining alloy was similar to the path of filifor corrosion,therefore,it is easily inferred that the falling-off of small grains provided the filifor corrosion path.

Fig.7.SEM micrographs of the extruded Mg-0.5Bi-0.5Sn alloy removing corrosion products after immersed for(a)10min,(c)30min,(d)3h,(e)24h respectively.Along with(b)EBSD IPF map corresponding to grains with size<10μm and(f)3D corrosion morphology of the extruded Mg-0.5Bi-0.5Sn alloy after immersed for 24h.(The insets in(b)shows its corresponding(0001)pole figur and triangular IPF legend).

The SEM micrography and the 3D corrosion morphology of Mn-containing alloy after 24h immersion are shown in Fig.8(e)and(f),it could be observed that the corroded surface was smooth and the distribution of corrosion damage was homogeneous.Generally,the relatively uniform corrosion morphology could lead to the occurrence of dynamic balance between the formation and breakdown of corrosion product films which resulted in the decrease in corrosion rate of Mn-containing alloy after long-term immersion in SBF solution.

Fig.8.SEM micrographs of the extruded Mg-0.5Bi-0.5Sn-0.5Mn alloy removing corrosion products after immersed for(a)10min,(b)30min,(c)3h,(e)24h respectively.Along with(d)EBSD IPF map corresponding to grains with size<10μm and(f)3D corrosion morphology of the extruded Mg-0.5Bi-0.5Sn-0.5Mn alloy after immersed for 24h.(The insets in(d)shows its corresponding(0001)pole figur and triangular IPF legend).

Fig.9 shows the XPS analysis results of the corrosion product of the extruded alloys.The peak located at 1302.7eV in the Mg 1s spectrum of the extruded alloys indicating the existence of Mg(OH)2.The high-resolution Ca 2p peak of Mn-free alloy was centred at 351.1eV,which could be fitte by CaCO3,while the Ca 2p peaks of Mn-containing alloy displayed the doublet line splitted via spin-orbit(L-S)splitting into Ca 2p1/2 at 351.1eV and Ca 2p3/2 at 347.0eV,which also could be fitte by CaCO3.The presence of CaCO3may be related to the CO32?and Ca2+in SBF solution,which was consistent with that reported by Pan et al.[5].The Sn 3d spectrum at 532.0eV suggested the presence of SnO2.It should be mentioned that the presence of SnO2was hardly observed in the corrosion products of Mn-free alloy.Furthermore,the O 1s spectrum of Mn-free alloy could be divided into two peaks(banding energy=530.9eV and banding energy=531.4eV),which corresponded to Mg(OH)2and CaCO3,besides the above mentioned two peaks,the O 1s spectrum of Mn-containing alloy also contained a SnO2peak at 532.0eV.

Fig.9.XPS analysis of the formed corrosion product fil on the extruded alloys:(a)high-resolution Mg 1s spectrum,(b)high-resolution Ca 2p spectrum,(c)high-resolution Sn 3d spectrum,(d)high-resolution O 1s spectrum.

According to the results of XPS analysis,the corrosion products formation mechanism could be revealed by the fl wing chemical reactions.The chemical reactions(5)and(6)proceeded preferentially after the extruded alloys immersed in SBF solution.Firstly,when the fresh surface contacted with the SBF solution,the matrix with a negative electrochemical potential could react with H2O to form Mg(OH)2.Meanwhile,the SBF solution contained the CO32?and Ca2+,which could combine to form CaCO3and deposited on the alloys surface.The Mg(OH)2was much less stable and the dissolved Mg(OH)2could be accelerated by reacting with Cl?(chemical reactions 7 and 8).Furthermore,Liu et al.[42]found that the Sn element dissolved in the matrix of the extruded alloys was also corroded with the dissolution of Mg matrix and the SnH4was formed(chemical reaction 9).However,the SnH4was unstable at room temperature due to the boiling point of SnH4was?52 °C,it was spontaneously transformed into

The back-scattered electron(BSE)images of corrosion morphologies of the extruded alloys are shown in Fig.10(a)and(d).It could be observed that the severe cracking formed on the corrosion product fil and the matrix of Mn-free alloy.However,a high density of flocculen corrosion products was precipitated on Mn-containing alloy surface and the fewer micro-cracks could be found on the corrosion product film which validated that the corrosion product fil of Mn-containing alloy possessed better protective ability.

Fig.10.Back-scattered electron images showing the surface micrographs of corrosion products and surface cross-sectional morphologies along with element mappings after immersion in SBF solution for 24h:(a),(b)and(c)Mg-0.5Bi-0.5Sn and(d),(e)and(f)Mg-0.5Bi-0.5Sn-0.5Mn.(Fig.a with high magnifi ation images of local area appended on the upper right corners).

To further explore the influenc of corrosion products on corrosion behavior,the surface cross-sectional morphologies and the element mappings of the extruded alloys immersed for 24h are shown in Fig.10(b),(c),(e),and(f).The typical local corrosion formed on the surface cross-sectional morphologies of Mn-free alloy and the measurement result of corrosion depth was 73.5μm.However,the local corrosion was alleviated significantl and the thickness of the corrosion products was only 61.3μm in Mn-containing alloy.Furthermore,the corrosion products of Mn-free alloy were mainly composed of CaCO3and Mg(OH)2.Zeng et al.[1]indicated that the enthalpie of Mg(OH)2(?924.66kJ/mol)was much higher than that of CaCO3(?1206.9kJ/mol),and thus CaCO3was more stable than Mg(OH)2.Moreover,Sun et al.[36]found that the Pilling-Bedworth ration(PBR)could reveal the stress state of corrosion product film When the PBR was between 1 and 2,the stress on corrosion product fil was moderate and the fil was more compact and denser.The PBR of CaCO3was 1.43 indicating the corrosion product fil could effectively prevent the penetration of chloride ions at the initial stage of immersion[1],which lead to the formation of filifor corrosion(Fig.7(c)).However,a small amount of CaCO3could not cover the entire Mg substrate and Mg(OH)2on the top of the corrosion products was dissolved into the SBF solution by chemical reactions 7 and 8,therefore,network cracks formed in the local area of the corrosion product film which resulted in the formation of corrosion cavities on the surface of Mn-free alloy.As for Mn-containing alloy,besides Mg(OH)2and CaCO3,an intermediate product layer mainly containing SnO2also was formed.When the fresh surface of Mn-containing alloy contacted with the SBF solution,Mg(OH)2was formed by the dissolution ofα-Mg and CaCO3was generated by the combination of CO32?and Ca2+in SBF solution.With the immersion time prolonged,intergranular corrosion became more severe,which could cause the dissolution of the segregated Sn atoms near the grain boundaries and led to the formation of a corrosion product layer mainly containing SnO2.Previous studies reported that SnO2was more stable than Mg(OH)2and the PBR of SnO2was 1.32,which illustrated that the existence of SnO2was conducive to the generation of compact protective product fil[23,35,42].Finally,the corrosion products of CaCO3and Mg(OH)2were precipitated on the outer layer of corrosion products.Furthermore,a plenty of grain boundaries could act as nucleation sites for corrosion products and facilitated the formation of more dense and homogeneous passive fil[9].Therefore,the corrosion product fil of Mn-containing alloy with fin DRXed grains could protect the matrix effectively.

The schematic diagram is shown in Fig.11 to explore the corrosion process of Mg-0.5Bi-0.5Sn-0.5Mn alloy.After Mn microalloying,certain Sn atoms segregated regions along grain boundaries were formed,which acted as the cathodic sites to induce micro-galvanic corrosion as well as intergranular corrosion.Besides,some corrosion pits could be observed related to the small grains(grain size<10μm)with nonbasal orientation.Thence,the corrosion products of Mg(OH)2and a small amount of CaCO3could be precipitated on the surface of Mn-containing alloy in the SBF solution.As the immersion time prolonged,the dominated intergranular corrosion mode resulted in the dissolution of some Sn atoms being in solute state near the grain boundaries,which led to the formation of a product layer containing mainly SnO2.And then,the corrosion product fil with some micro-cracks formed on the corroded surface and the existence of Sn-containing and Ca-containing products guaranteed the protective ability of corrosion product film Therefore,filifor corrosion could be observed on the surface and the growth path of filifor corrosion propagated through the falling-off of small grains due to higher density of grain boundaries of fine-graine region.Finally,the mixed pitting,intergranular and filifor corrosion occurred on the surface of Mn-containing alloy.

Fig.11.The corrosion mechanisms of Mg-0.5Bi-0.5Sn-0.5Mn alloy immersed in SBF solution.

4.Conclusion

(1)The microstructure of the extruded Mg-0.5Bi-0.5Sn alloy was greatly modifie by microalloying with Mn,leading to the decrease of average grain size,dislocations density and the formation of new Mg26.67Mn65.47Fe7.86second phases together with grain boundary segregation.

(2)Grain boundary segregation of Sn atoms induced micro-galvanic corrosion,which led to the corrosion mode changed from mixed pitting and filifor corrosion in Mn-free alloy to dominated intergranular corrosion in Mn-containing alloy.

(3)The enhanced corrosion resistance of Mg-0.5Bi-0.5Sn-0.5Mn alloy was mainly attributed to the formation of protective product fil consisting of an intermediate product layer(SnO2).

Declaration of Competing Interest

None.

Acknowledgments

This study was jointly supported by the National Natural Science Foundation of China(Grant nos.:51704209,51701060),Natural Science Foundation of Shanxi Province(Grant no.:201801D121088),Shanxi Province Science Foundation for Youths(Grant no.:2016021063),Shanxi Scholarship Council of China(Grant no.:2019032)and the Science and Technology Major Project of Shanxi Province(Grant nos.:20191102008,20191102007).

Supplementary materials

Supplementary material associated with this article can be found,in the online version,at doi:10.1016/j.jma.2020.07.010.