Xuan Liu ,Zijian Zhu,Jilai Xue

School of Metallurgical and Ecological Engineering,University of Science and Technology Beijing,Beijing 100083,China

Abstracts The electrochemical behaviors and corrosion resistance of the wrought Mg-Y-Zn based alloys with high Y/Zn mole ratio have been investigated in details.The results show that the corrosion resistance of the investigated Mg-Y-Zn based alloys are dependent on the modifie arrangement of LPSO phase by adjusting Y/Zn mole ratios.Increasing the Y/Zn mole ratio not only greatly decreases the size of LPSO phase plates,but also leads to the precipitation of Mg24Y5 phase.The corrosion rate of Mg-Y-Zn based alloys greatly increases from 7.4 mg·cm-2·day-1 to 11.3 mg·cm-2·day-1 with increasing the Y/Zn mole ratio up to 3.It should be attributed to the decreasing size of LPSO phase plates as cathodes,further increasing the hydrogen evolution kinetics.The related corrosion mechanism is discussed in details.? 2020 Chongqing University.Publishing services provided by Elsevier B.V.on behalf of KeAi Communications Co.Ltd.

Keywords: Mg-Y-Zn alloys;Y/Zn mole ratio;Corrosion resistance;Electrochemical behaviors;Microstructures.

1.Introduction

Magnesium alloys are promising candidates for lightweighting structural materials in automotive applications,due to their low density and high specifi strength,etc.[1-3].However,there is still a great gap between the high-volume applications and current capability of Mg based alloys (low strength,poor deformability,etc.[4-6]).

The past decades proceed an on-going research upsurge on the ultra-high strength Mg-Y-Zn based alloys via a broad branch of technical routes (e.g.Ingot casting/powder metallurgy with subsequent severe plastic deformation and aging process) [7-13].Such high performances of Mg-Y-Zn based alloys have been widely attributed to the significan reinforcement by the heat-resistant ternary Mg-Y-Zn phase(long period stacking ordered phase (LPSO,Mg12YZn),the icosahedral quasicrystal I-Mg3Zn6Y phase,and the cubic WMg3Y2Zn3phase) [14,15].In turn,these reinforcing phases give rise to the weak aging hardening response of the Mg-YZn based alloys [16].The phase constitutions of Mg-Y-Zn based alloys are mainly dependent on the Y/Zn mole ratios[9,17].The LPSO phase becomes the primary ternary phase as the Y/Zn mole ratio is larger than 1.

Meantime,the corrosion behaviors of Mg-Y-Zn based alloys have gradually received sustained attention so as to develop high performance Mg-Y-Zn based alloys with both high strength and good corrosion resistance.The corrosion behaviors of the Mg-Y-Zn based alloys bearing I-Mg3Zn6Y and W-Mg3Y2Zn3phase have been early reported [18-20].Recently,those alloys bearing LPSO phase received much more attention.It has been widely confirme that the volume fraction and arrangement of LPSO phase played crucial role in the corrosion rate of Mg-Y-Zn based alloys [21-23].The related corrosion rate-controlling mechanism for LPSO and W-Mg3Y2Zn3phase has also been proposed by Bao et al.[24,25].Furthermore,minor Gd and Al has also been considered to improve the corrosion resistance of the as-cast Mg-Y-Zn alloys [26,27].The research stream mainly covered the corrosion behaviors of Mg-Y-Zn based alloys with Y/Zn mole ration around 1-2.However,those Mg-Y-Zn alloys with high Y/Zn mole ratio (over than 2) have been scantly investigated.

Thus,this work investigated the corrosion resistance of a wrought Mg-Y-Zn-Zr alloy with high Y/Zn mole ratio(Y/Zn=3),by comparing the corrosion performance of a referenced Mg-Y-Zn-Zr alloy with similar total amount of alloying addition but common Y/Zn mole ratio (Y/Zn is around 1-2).The electrochemical behaviors have been investigated in detail to help understand the corrosion behaviors of Mg-YZn based alloys with high Y/Zn mole ratios.Meanwhile,the microstructures before and after corrosion were also investigated aimed to providing a connection between corrosion performance and microstructures characteristics.

2.Experimental

2.1.Alloy preparation

The investigated MgY2.8Zn1.9Zr0.16and MgY3.6Zn1.2Zr0.16alloys (at.%) with similar total amount of alloying addition were prepared by melting pure Mg,pure Zn (99.9wt%,the same below),Mg-30%Zr and Mg-50%Y master alloys in the mild steel crucible with the protective atmosphere of 0.5%SF6+CO2at 710 °C,and cast into a preheated steel mold with a diameter of 60mm.The cast ingots were extruded intoΦ12 mm rods after the homogenization at 510 °C for 16 h.The extrusion ratio,temperature and ram speed were 18:1,400 °C and 0.2 m·min-1,respectively.

2.2.Microstructural characterization

Microstructures of the investigated alloys were observed by an optical microscope (OM),and scanning electron microscope (SEM) coupled with an energy dispersive X-ray analyzer (EDX),after mechanical polish and chemical etching.An X-ray diffractometer (XRD) was taken to detect the phase composition.The scanning range was 15-80° with a speed of 8°/min.The XRD patterns were indexed using PDF standard card (2004).

2.3.Electrochemical tests

Potentiodynamic polarization curves were obtained by an CHI660E electrochemical workstation in 3.5wt% NaCl solution.Three electrodes system was employed with Pt foil as a counter electrode,saturated calomel electrode (SCE) as reference and samples as working electrode.Samples with the surface area of 1.13 cm2were ground up to 3000 grit with SiC abrasive papers before test.The polarization curves were recorded at a scan rate of 1 mV·s-1after different immersion periods.Following different immersion periods,the electrochemical impedance spectra (EIS) were on the independent sample over the frequency range from 100kHz to 10 mHz.The measured EIS were then fitte with the ZSimpWin software.All electrochemical measurements were performed in triplicate to ensure good reproducibility.

Fig.1.XRD patterns of the investigated as-extruded alloys.

2.4.Hydrogen evolution and mass loss tests

The samples were sealed with epoxy to ensure a fi ed exposure area,and ground with 2000 grit SiC paper.The hydrogen gas evolved was collected for samples immersed in 3.5wt% M NaCl (25 °C) for 30h,using an inverted funnel and burette above the immersed specimens.In parallel to the hydrogen collection,the mass loss tests were also performed by removing the surface product with the boiled chromic solution (200 g·L-1).

2.5.Immersion tests

The immersion corrosion tests were conducted in a 3.5wt%NaCl aqueous solution (25 °C) for specifi periods.The asextruded alloys (Φ12×5mm) were sealed by epoxy with an exposing area of 1.13 cm2(The exposed surface is perpendicular to the extruding direction).The specimens were abraded on successively fine SiC paper up to 2000 grade.The immersion tests were performed in triplicate to check the reproducibility.The corroded surfaces and cross-sections are also observed using both OM and SEM after removing the surface products.

3.Results and discussion

3.1.Phase compositions and microstructures

Fig.1 shows the XRD patterns of the investigated asextruded alloys.The MgY2.8Zn1.9Zr0.16alloy consists ofα-Mg phase and the LPSO phase(Mg12YZn).It has been widely reported that the phase composition of Mg-Y-Zn based alloys was dependent on the Y/Zn mole ratios [9,17].The LPSO phase becomes the primary ternary Mg-Y-Zn phase as the Y/Zn mole ratio is larger than 1.Additional Mg24Y5phase is indexed from the XRD pattern of MgY3.6Zn1.2Zr0.16alloy(The Y/Zn mole ratio is 3).Such high Y/Zn mole ratio makes additional Y solute forms the binary Mg24Y5phase with Mg via the eutectic reaction [28].

Fig.2.Micrographs of the investigated alloys perpendicular to the extruding direction.(a),(b)and(c)MgY2.8Zn1.9Zr0.16 alloy;(d),(e)and(f)MgY3.6Zn1.2Zr0.16 alloy.

Fig.2 shows the microstructures of the as-extruded Mg-Y-Zn alloys.The MgY2.8Zn1.9Zr0.16alloy is comprised of the DRXed (dynamically recrystallized)α-Mg grains embedded with a quantity of grey bulk LPSO phase plates,as shown in Fig.2a and b.Furthermore,several dissociative cubic particles distribute around the bulk LPSO phase,as shown in Fig.2c.A further EDS analysis confirm them as the Zr-rich cuboids(Mg-52.5 at.%Zr-31.4 at.%Zn).The MgY3.6Zn1.2Zr0.16alloy has a similar microstructure,but the embedded LPSO phase plates have much smaller size than those in MgY2.8Zn1.9Zr0.16alloy,as shown in Fig.2d.Similar Zr-rich cuboids can also be observed,as shown in Fig.2e.The measured area fraction of the LPSO phase for MgY2.8Zn1.9Zr0.16and MgY3.6Zn1.2Zr0.16alloy are 84%and 69.8%,respectively.Such difference should have a great impact on the corrosion resistance of the investigated alloys.

In addition,there are several particles with different chemical composition (Mg-21.7 at.% Y-1.6 at.%Zn) from the LPSO phase (Mg-6.4 at.%Zn-8.4 at.%Y) and the Zr-rich cuboids.Such additional particles should be the indexed Mg24Y5phase by the XRD pattern.It can be also found that the over high Y/Zn mole ratio decreases the size of LPSO phase plates and also results in the formation of additional Mg24Y5phase particles.

3.2.Electrochemical behaviors

Fig.3 shows the electrochemical properties of the investigated Mg-Y-Zn alloys.Both of the two alloys ex-hibits stable open circuit potential,as shown in Fig.3a.Furthermore,the average OCP value (-1.614V vs.SCE) of MgY3.6Zn1.2Zr0.16alloy is about 25mV more negative than that of MgY2.8Zn1.9Zr0.16alloy.It probably suggests that the MgY3.6Zn1.2Zr0.16alloy exhibits higher electrochemical activity than the MgY2.8Zn1.9Zr0.16alloy.The polarization curves of the two alloys are shown in Fig.3b.Meanwhile,the Tafel fittin results from the polarization curves are lists in Table 1.The corrosion potentials of MgY2.8Zn1.9Zr0.16and MgY3.6Zn1.2Zr0.16alloy are -1.472 and -1.506V (vs.SCE),respectively.They have great difference from the measured OCP values,which should be attributed to the dynamic potential during measurement.The corrosion current density is obtained by extrapolating the cathodic branch back to the corrosion potential.The MgY3.6Zn1.2Zr0.16alloy exhibits a bit higher current density (38.1 μA·cm-2) over the MgY2.8Zn1.9Zr0.16alloy (20.3 μA·cm-2).The fitte corrosion current densities are consistent with the similar as-cast Mg-Y-Zn alloys [22,25],but a bit higher than those of similar as-extruded Mg-Y-Zn based alloys (11-13 μA·cm-2) [8].However,it should be noted that the fitte corrosion current density merely reflect the instantaneous corrosion rate of the measured alloys [29].

Table 1 Tafel fittin results derived from the polarization curves in 3.5wt% NaCl solution.

Table 2 The fitte EIS data on the basis of the equivalent circuits presented in Fig.6.

Fig.3.Electrochemical properties of the investigated alloys in 3.5wt% NaCl solution.(a) OCP curves;(b) Polarization curves.

Moreover,it should be noted that the MgY3.6Zn1.2Zr0.16alloy exhibits larger current response at the cathodic branch,as shown in Fig.3b.This may be due to the enhanced microgalvanic corrosion.The cathodic branch of the polarization curves reveals the hydrogen evolution reaction(HER)kinetics[30,31].The higher current response probably indicates faster hydrogen gas evolution rate.

3.3.Hydrogen evolution

Fig.4 shows the hydrogen evolution of the investigated Mg-Y-Zn alloys immersed in 3.5wt%NaCl solution for 24h.At the initial immersion stage (2h),the MgY3.6Zn1.2Zr0.16alloy evolves a bit less volume of hydrogen gas than the MgY2.8Zn1.9Zr0.16alloy.However,the hydrogen evolution rate of the MgY3.6Zn1.2Zr0.16alloy accelerates greatly as the immersion time increases up to 6h.The fina hydrogen volume per area of MgY3.6Zn1.2Zr0.16alloy is 10.5 ml·cm-2after immersion for 24h.The hydrogen evolution rate is～0.438 ml·cm-2·h-1,which is 1.5 times larger than that of MgY2.8Zn1.9Zr0.16alloy (0.288 ml·cm-2·h-1).The hydrogen evolution test is consistent with the results of high current response of MgY3.6Zn1.2Zr0.16alloy at the cathodic branch of polarization curves (Fig.3b).

Fig.4.Hydrogen evolution of the investigated alloys immersed in 3.5wt%NaCl solution.

3.4.Electrochemical impedance spectroscopy (EIS)

Fig.5 shows the electrochemical impedance spectra of the investigated alloys with and without polarization in both Nyquist and Bode plots.The EIS in Nyquist plots of the two Mg-Y-Zn alloys without polarization consist of a high frequency capacitive loop and an inductive loop at low frequency,as shown in Fig.5a.Furthermore,the MgY2.8Zn1.9Zr0.16alloy exhibits a much larger capacitive loop than the MgY3.6Zn1.2Zr0.16alloy,which indicates larger corrosion resistance [32,33].The EIS in Bode plots further reveal the impedance and phase angle vs.frequency.Each peak of phase angle stands for one time constant [34].The investigated Mg-Y-Zn alloys generally consist of two peaks of phase angle,as shown in Fig.5b.However,the peak at high frequency (100-10,000Hz) shows a broad peak width,which is commonly attributed to the overlap of two time constants[35,36].

Fig.5c shows the EIS in Nyquist plots of the two alloys with polarization.A significan shrinkage of the capacitive loop can be generally identified compared to those without polarization (Fig.5a).Furthermore,they consist of a high frequency capacitive loop,middle range frequency capacitive loop and an inductive loop,which can be confirme by the three peaks of phase angle in Fig.5d.There are two peaks of phase angle around the frequency range from 10 to 10,000Hz,instead of an overlapped peak with broad width (Fig.5b).

Fig.5.Electrochemical impedance spectra of the investigated alloys with and without polarization.(a),(c) Nyquist plots;(b),(d) Bode plots.

Fig.6 shows the equivalent circuits for the presented EIS of the investigated Mg-Y-Zn alloys.Table 2 lists the fitte data based on the equivalent circuits.The EIS of the two alloys without polarization can be equivalent to the circuit in Fig.6a.In the selected circuit,Rsand Rctare the solution resistance and charge transfer resistance,respectively.The electric double layer (interface between the electrode and electrolyte) is represented by the constant phase element CPEdl,which is define by two values of Ydland ndl(dispersion coefficient) If ndlis 1,CPEdlis identical to a capacitor;if ndlis 0,CPEdlrepresents a resistance.Rfand CPEfrepresent the fil resistance and capacity,respectively.RLand L stand for the resistance and inductance to describe the low frequency inductive loop.It implies the initiation of localized corrosion [37].The charge transfer resistance (Rt) of MgY3.6Zn1.2Zr0.16alloy is much smaller than that of MgY2.8Zn1.9Zr0.16alloy,as listed in Table 2.Furthermore,the comparison of fil resistance(Rf) is also similar to that of charge transfer resistance.It probably suggests that the MgY3.6Zn1.2Zr0.16alloy may have an inferior corrosion resistance over the MgY2.8Zn1.9Zr0.16alloy.

Another circuit has been selected to stand for the EIS of the two Mg-Y-Zn alloys with polarization,as shown in Fig.6b.It can be seen that additional constant phase element(CPEp) and resistance (Rp) have been supplied to the equivalent circuit.The additional time constant may be attributed to the formation of micro corrosion pores after severe polarization,since corrosion pores gradually form under the localized corrosion,inferred by the inductive loop in Fig.5a.In turn,these micro corrosion pores greatly increase the surface area of immersed alloys,making the aggressive electrolyte penetrating into the inner layer of alloy surface.This should be responsible for the great decrease in the charge transfer resistance,as listed in Table 2.

Fig.6.Equivalent circuits for the EIS of investigated alloys.(a) without polarization;(b) with polarization.

3.5.Corrosion morphologies

Fig.7 shows the surface morphology evolution of the investigated alloys immersed in 3.5wt% NaCl solution.A significan deposition of the surface products can be observed in the early stage of immersion (2h).After immersion for 4h,the surface products cover most of the surface area of the two investigated Mg-Y-Zn alloys.Finally,they gradually shield the fresh metal with an immersion period of 12h.Even so,a small portion of fresh metal is still uncovered after the MgY2.8Zn1.9Zr0.16alloy is immersed for 12h.The degree of coverage by corrosion products on MgY3.6Zn1.2Zr0.16alloy is close to that on MgY2.8Zn1.9Zr0.16alloy immersed for 4h.However,the surface of MgY3.6Zn1.2Zr0.16alloy has been totally corroded.Huge corrosion holes can be observed due to the propagation of severe corrosion attack,as shown in Fig.7.The surface evolution during immersion is quite consistent with the collected hydrogen evolution (Fig.4).Thus,the MgY2.8Zn1.9Zr0.16alloy exhibits better corrosion resistance over the MgY3.6Zn1.2Zr0.16alloy.The mass loss measuring method further confirm that the corrosion rate of the MgY2.8Zn1.9Zr0.16and MgY3.6Zn1.2Zr0.16alloy are 7.4 and 11.3 mg·cm-2·day-1,respectively.

It is necessary to compare the obtained corrosion rates in this work with those in previous publications.Li et al.reported three as-cast Mg-Y-Zn-Zr alloys with low corrosion rate around 2-4.5 mg·cm-2·day-1[22].This should be due to the different amount of alloying addition (less than 11 wt%in total).Bao et al.reported the corrosion rate of an as-cast MgY3.83Zn3.03Zr0.17alloy with comparable amount addition of alloying element was 9.6 mg·cm-2·day-1(after unit conversion) [25].Furthermore,the as-extruded Mg97Zn1Y2alloy exhibited a corrosion rate of around 20-30mm/year [23].The converted values in this work are 13.8 and 20.9mm/year,respectively.Thus,it suggests that the obtained corrosion rates in this work are in same order with the reported values.To be frankly,the Mg-Y-Zn based alloys exhibit superior mechanical properties,due to such large amount of alloying addition.In turn,such large amount of strengthening second phase greatly decrease their corrosion resistance,which are much inferior to the most common commercial magnesium alloys.

Fig.8 shows the micrographs of the surface morphology of the two Mg-Y-Zn alloys immersed in 3.5wt% NaCl solution for 12h.The corroded surface of MgY2.8Zn1.9Zr0.16alloy is rough and full of deep corrosion pores,as shown in Fig.8a.A further magnifying view (Fig.8b) reveals that some bulk LPSO phase(Mg-6.1 at.%Y-5.4 at.%Zn,confirme by EDX)distribute around these corrosion pores.It seems that the Mg matrix around the bulk LPSO phase get corroded in priority.A quantity of fin corrosion pores spread over the corroded surface of MgY3.6Zn1.2Zr0.16alloy,as shown in Fig.8c.These corrosion pores are fine than those in the MgY2.8Zn1.9Zr0.16alloy.It should be noted that similar LPSO phase lamellas(Mg-7.8 at.% Y-5.5 at.% Zn) distribute around these pores,as shown in Fig.8d.It further verifie the prior corrosion of Mg matrix around the LPSO phase.

Fig.9 shows the cross-sectional morphologies of the investigated alloys after immersion for 12h.The cross-sectional surface of MgY2.8Zn1.9Zr0.16alloy is quite smooth,as shown in Fig.9a.The MgY3.6Zn1.2Zr0.16alloy exhibits a rough crosssectional surface with the matrix severely corroded,as shown in Fig.9b.Furthermore,it can be clearly observed that the matrix around the LPSO phase plates gets corroded in priority.It suggests that the MgY3.6Zn1.2Zr0.16alloy exhibits a lower resistance to the corrosion attack,which is consistent with the analyzed result from the electrochemical impedance spectra (Fig.5).

4.Discussion

In the case of similar total amount of alloying addition,increasing the Y/Zn mole ratio (up to 3) not only greatly decreases the size of LPSO phase plates,but also leads to the precipitation of Mg24Y5phase.Such high Y/Zn mole ratio gives rise to the insufficien Zn solute for LPSO phase.As a result,the total surface of LPSO phase (area fraction,69.8%) in MgY3.6Zn1.2Zr0.16alloy is smaller than that in MgY2.8Zn1.9Zr0.16alloy (area fraction,84%),as shown in Fig.2.In turn,the high Y/Zn mole ratio also means the excess amount of Y solute,which should be responsible for the formation of Mg24Y5phase.These Mg24Y5phase get decomposed during the high temperature homogenization.They partially dissolve back into matrix,which makes the Mg-YZn alloy heat-treatable [9].The remnant Mg24Y5phase get crushed into small particles after severe extrusion deformation,as shown in Fig.2f.Hence,the corrosion resistance of the investigated Mg-Y-Zn alloys is mainly dependent on the arrangement of LPSO phase.

Fig.7.Evolution of the surface morphology of the investigated alloys immersed in 3.5wt% NaCl solution.

Fig.8.Micrographs of the surface morphology of the investigated alloys immersed in 3.5wt% NaCl for 12h (with the removal of surface products).(a) and(b) MgY2.8Zn1.9Zr0.16 alloy;(c) and (d) MgY3.6Zn1.2Zr0.16 alloy.

It has been early recognized that the LPSO phase exhibited positive potential over the Mg matrix[21,22,38,39].Thus,the LPSO phase plates constitute micro-galvanic couples with the surrounding Mg matrix during the corrosion of Mg-Y-Zn alloys immersed in 3.5wt% NaCl solution.The matrix are severely corroded in priority through micro-galvanic mechanism that the LPSO phase serve as cathodes to stimulate the corrosion of Mg phase.This is clearly verifie from the corroded matrix around the LPSO phase plates in this work(Figs.8 and 9).Compared to the MgY2.8Zn1.9Zr0.16alloy,the MgY3.6Zn1.2Zr0.16alloy has much smaller LPSO phase plates.These dispersed small LPSO plates should greatly increase the number of micro-galvanic couples with Mg during corrosion.Thus,the HER kinetics of the MgY3.6Zn1.2Zr0.16alloy are much larger than those of MgY2.8Zn1.9Zr0.16alloy,according to the cathodic branch of polarization curves and collected hydrogen gas (Figs.3b and 4).It is also why the corrosion pores of the MgY3.6Zn1.2Zr0.16alloy are much smaller than those of MgY2.8Zn1.9Zr0.16alloy (Fig.8).In the case of MgY2.8Zn1.9Zr0.16alloy,the large LPSO plates possess dominant area to theα-Mg matrix (at the transversal section,Fig.2a).One hand,the number of micro-galvanic couples should be less than that in MgY3.6Zn1.2Zr0.16alloy.On the other hand,such large surface phase plates cannot only shield the Mg phase beneath but also block the propagation of corrosion attack significantl .Thus,the MgY2.8Zn1.9Zr0.16alloy exhibits lower hydrogen evolution rate and better corrosion resistance over the MgY3.6Zn1.2Zr0.16alloy.It should be noted that the corrosion of MgY3.6Zn1.2Zr0.16alloy at the initial immersion stage is quite close to that of MgY2.8Zn1.9Zr0.16alloy (Figs.4 and 7).This may because the corrosion evolving fil formation and breakup process has not been steady at the early immersion period.The breakup of surface fil on the MgY3.6Zn1.2Zr0.16alloy may be a bit later than that on the MgY2.8Zn1.9Zr0.16alloy,which is responsible for the less hydrogen gas evolution of MgY3.6Zn1.2Zr0.16alloy at the firs 2h.However,such early unsteady stage has a weak influenc on the fina corrosion rate of the investigated alloys.

Fig.9.The cross-sectional morphologies of the investigated alloys after immersion for 12h.(a) MgY2.8Zn1.9Zr0.16 alloy;(b) MgY3.6Zn1.2Zr0.16 alloy.

The precipitated Mg24Y5phase can also play some role in the corrosion resistance of MgY3.6Zn1.2Zr0.16alloy.It has been widely reported that Mg24Y5phase are strong cathodes forα-Mg matrix[40],which makes the corrosion resistance of Mg-Y alloy deteriorate [41,42].Thus,the existence of Mg24Y5phase particles further increases the number of micro-galvanic couples,resulting in the deterioration of corrosion resistance.Similar Zr-rich particles play similar role in increasing the corrosion rate of Mg alloys [43,44].However,the Mg24Y5and Zr-rich particle phase play a less crucial role over the LPSO phase in the corrosion resistance of MgY3.6Zn1.2Zr0.16alloy,due to their low volume fraction.

5.Conclusion

In this work,the electrochemical behaviors and corrosion resistance of the wrought Mg-Y-Zn based alloys with high Y/Zn mole ratio have been investigated in details.In the case of similar total amount of alloying addition,increasing the Y/Zn mole ratio not only greatly decreases the size of LPSO phase plates,but also leads to the precipitation of Mg24Y5phase.The corrosion resistance of Mg-YZn based alloys greatly decreases from 7.4 mg·cm-2·day-1to 11.3 mg·cm-2·day-1with increasing the Y/Zn mole ratio up to 3.The deteriorating corrosion resistance should be attributed to the enhanced micro-galvanic corrosion mechanism by the dispersed small LPSO phase plates,which is further increasing the hydrogen evolution rate.The corrosion resistance of the investigated Mg-Y-Zn based alloys are dependent on the modifie arrangement of LPSO phase by adjusting Y/Zn mole ratios.