Yujin Nie ,Jianwei Dai ,Xuan Li ,Xiaobo Zhang,*

a School of Materials Science and Engineering,Nanjing Institute of Technology,Nanjing 211167,China

b Jiangsu Key Laboratory of Advanced Structural Materials and Application Technology,Nanjing 211167,China

Abstract Corrosion is one of the most drawbacks which restricts the wide applications of Mg alloys.In the last decade,the corrosion behaviors of Mg alloys with stacking fault (SF) and/or long period stacking ordered (LPSO) structures have obtained increasing attention.However,the corrosion mechanism of the SF-or LPSO-containing Mg alloys has not been well illustrated and even reverse results have been reported.In this paper,we have reviewed recent reports on corrosion behaviors of SF-or LPSO-containing Mg alloys to better clarify and understand the significanc and mechanism.Moreover,some deficiencie are presented and advises are proposed for the development of corrosion resistant Mg alloys with SF or LPSO structures.

Keywords: Magnesium alloys;Corrosion behavior;Stacking fault;Long period stacking ordered.

1.Introduction

Mg is one of the lightest metal materials with a density of 1.738g/cm3which is～2/3 of aluminum and～1/4 of steel[1-3].Mg alloys have high specifi strength and specifi stiffness,in addition,they have the characteristics of good damping,castability,machinability,thermal conductivity,and strong electromagnetic shielding ability,and hence show wide prospects in automobile,transportation,electronics,and other field [4-8].Moreover,Mg ions are the fourth most abundant cations in human body and can accelerate the growth of bone tissue by promoting biological activity[9-11].Compared with traditional biomedical titanium alloys and stainless steels,Mg alloys exhibit favorable biocompatibility and biodegradability[9,10,12].They can be completely degraded in the physiological environment after the disease was cured,thus eliminate the second surgery to remove the implants and make patients avoid the burden of second surgery [13-15].Furthermore,the elastic modulus of Mg alloys is close to that of the human bones,leading to the avoidance of stress shielding [10,14,16,17].As a result,biodegradable Mg alloys have been considered as the next generation biomedical materials.

Although Mg alloys have such unique characteristics,their application still has many limitations.Firstly,they have low absolute strength and thus cannot be used as the main force components.Secondly,the mechanical properties of Mg alloys at elevated temperature decrease rapidly,so most of Mg alloys cannot be applied at high temperature [18-22].Lastly and importantly,the corrosion resistance of Mg alloys is relatively poor due to low corrosion potential [23-25].Therefore,it is necessary to develop Mg alloys with satisfactory corrosion resistance and mechanical strength.In order to achieve this goal,rare earth (RE) elements have been added into Mg alloys,and the performance of them was greatly improved[7,26-30].

In 2001,Kawamura et al.[31] developed a series of Mg-Zn-Y alloys by rapid solidificatio powder metallurgy process and found that Mg97Zn1Y2(at%) alloy showed excellent room temperature mechanical performances:The yield strength was over 600MPa and the elongation was up to 5%.It was believed that such excellent mechanical properties of the alloy had a significan relationship with a unique lamellar microstructure,which was subsequently define as long period stacking ordered (LPSO) structure [32].From then on,more and more attention had been paid on the LPSO structure in Mg alloys [33-36].The Mg-12Gd-2Y-1Zn-Mn alloy was developed by extruded and subsequent aged by Su et al.[34],with an ultimate tensile strength of 509MPa and an elongation of 5% at room temperature.Homma et al.[35] reported a Mg-1.8Gd-1.8Y-0.7Zn-0.2Zr (at%) alloy with LPSO structure prepared by hot extrusion and ageing,which exhibited an ultimate tensile strength of 542MPa,yield strength of 473MPa and elongation of 8.0%.And the mechanical properties of Mg-9Gd-3Y-1Zn-0.8Mn-1.5Ag alloy with LPSO structure were investigated by Wang et al.[36],which was prepared by solution treated at 520°C for 10h,then extruded and aged at 200°C.They found that the ultimate tensile strength was 533MPa,yield strength was 399MPa and elongation was 9.0%.

Moreover,stacking fault (SF) can also effectively enhance the comprehensive mechanical properties of Mg alloys,which can be transformed into LPSO [22,26,37].Zhang et al.[26] found that the tensile yield strength,ultimate tensile strength,and elongation of the Mg-6Ho-1Zn alloy with profuse SF were 224MPa,325MPa,and 17%,respectively,which were much higher than those of the Mg-6Ho alloy without SF.The tensile yield strength and ultimate tensile strength of Mg-4Er-4Gd-1Zn alloy with SF were 253MPa and 358MPa,respectively,whereas those of the Mg-8Er alloy without SF were only 153MPa and 260MPa [27].And Zhang et al.[37] found the ultimate tensile strength and elongation of Mg-6.5Gd-2.5Dy-1.8Zn (wt%) alloy with SF were 392MPa and 6.1%.Recently,Jian et al.[38] reported that the mechanical properties of Mg-8.5Gd-2.3Y-1.8Ag-0.4Zr (wt%)alloy by conventional hot rolling were investigated,introduced high density basal SF,ultra-high yield and ultimate strength were 575MPa and 600MPa,respectively.

Generally,the LPSO and SF structures can restrict the growth of grains and thus play a significan role in grain refinemen [39-41].Under the effect of applied stress,LPSO and SF structures easily interact with dislocations and block dislocations,resulting in strength improvement [40-43.Moreover,they can also effectively accumulate dislocations,and thus increase the strain-hardening rate and help maintain ductility[41,42].In addition,LPSO-or SF-containing Mg alloys have lower value of basal slip Schmid factor than those without LPSO or SF structures,which is beneficia to strength enhancement [40,44].

In the past two decades,most of the reports on Mg alloys with LPSO or SF have focused on their microstructures and mechanical properties.However,the effects of LPSO or SF on corrosion behaviors have been lack of research.Several studies showed that LPSO or SF structures in Mg alloys could result in better corrosion resistance [26,30,45-48].Zhang et al.[45] reported that the corrosion rates of Mg-11.3Gd-2.5Zn-0.7Zr with LPSO structure was only 0.17mm/y in simulated body flui (SBF),which is much better than that of Mg-10.2Gd-3.3Y-0.6Zr alloys (0.55mm/y) without LPSO structure.Zhang et al.[30] found that most of the corrosion reactions occurred at the junctions of LPSO structure and Mg matrix,and the morphology and volume fraction of the LPSO structure were important for improving the corrosion resistance of the Mg-Zn-Y alloys.Zhang et al.[47] reported that a novel Mg-8Er-1Zn alloy with profuse SF exhibited low corrosion rate(0.34mm/y)in SBF.Nevertheless,some reports pointed out that the presence of LPSO structure reduced the corrosion resistance of Mg-Gd-Zn(-Zr) alloys [49,50].As for SF,there is a consistent consequence that SF is beneficia to corrosion resistance of Mg alloys according to available literature.But the order of preferential corrosion ofα-Mg or SF is controversial [26,47,51]:Some researchers reported that SF was firstl corroded,and the others considered theα-Mg was dissolved preferentially.Therefore,effects of SF and LPSO structures on the corrosion behaviors of Mg alloys are essential to be clarified

In conclusion,the corrosion resistance of Mg alloys is of great importance for their development and applications,and LPSO and SF structures have shown significan impact on corrosion behavior except for mechanical properties.In this paper,the SF and LPSO structures of Mg alloys were briefl introduced,and recent developments on corrosion behaviors and mechanism of SF-containing and LPSO-containing Mg alloys were highlighted and summarized.

2.SF and LPSO structures in Mg alloys

Generally,SF and LPSO structures are generated in Mg-RE-Zn/Ni/Cu/Co systems [19,26,47,52-55],where RE corresponds to the following rare earth elements:Dysprosium(Dy),gadolinium (Gd),yttrium (Y),erbium (Er),thulium (Tm),terbium (Tb),and holmium (Ho) [27,45,47,56-58].In these series alloys,the addition of RE elements would decrease the stacking fault energy,resulting in abundant generation of planar faults with lamellar structure [59].With the change of stacking fault energy,both stacking orders and composition are satisfied and the LPSO structure is created [60,61].Both SF and LPSO structures are easily observed by optimal microscopy (OM) and scanning electron microscopy (SEM),however,they cannot be clearly identifie by OM or SEM.Thereby,transmission electron microscopy (TEM) and selective area electron diffraction (SAED) analyses are required to identify if the such structures are SF or LPSO.SF is not as regular or periodic as LPSO,thus,if there are some equal spaced diffraction spots adjacent to the Mg matrix along caxis,the stacking structure is confirme to be LPSO;otherwise,it is SF [47].

2.1.SF in Mg alloys

Fig.1.SEM image (a),TEM image (b),and SAED pattern (c) of SF in Mg-8Er-1Zn alloy [47].

SF in Mg alloys has received lots of attentions due to their special solute-atom arrangement and good mechanical properties.Compared with LPSO-containing Mg alloys,the reports on corrosion behavior of Mg alloys containing SF were relatively fewer,but most of SF-containing Mg alloy reports are focused on firs principle calculation,and microstructure evolution or mechanical properties of LPSO and SF [62-66].

SF usually distributes inα-Mg grains,and there is a specifi direction in eachα-Mg grain.SF is closely related to partial or incomplete dislocations of the crystal.For the close-packed crystal structure,SF is formed because partial dislocations are nucleated and slide on the close-packed crystal planes [67].The space between mutually parallel laminae ranges from a few nanometers to～100 nanometers[26,27,47,51].Sometimes,the width of these basal SF can reach as large as 1.0-1.5μm [67].While the periodic supercells for four basal SF,I1,I2,E and T2,are 1×1 unit cells with 20 layers (21 for E stacking fault),as showed in Table 1 [42,68].Fig.1 shows SEM image,TEM image,and SAED pattern of SF in the as-extruded Mg-8Er-1Zn alloy[47].The lamellar microstructures can be observed directly in Fig.1(a) with various orientations in different grains.TEM image in Fig.1(b) shows numerous parallel SF in aα-Mg grain.No periodic extra spots but streaks between the diffraction spots along c-axis in Fig.1(c) confir that these lamellar microstructures are SF.SF have been found not only in Mg alloys containing RE elements,such as Mg-Gd-Zn,Mg-Ho-Zn,Mg-Er-Zn,and Mg-Y-Zn alloys,but also in Mg alloys without RE elements such as Mg-Al-Zn,and Mg-Sn-Ca systems [26,27,47,66,69-71].

Table 1 Stacking sequences of SF in hcp magnesium [68].

2.2.LPSO in Mg alloys

LPSO structure is a unique lamellar microstructure and distributed at the second phase and/or the matrix grains[18,53,61,72-75].Compared with SF,periodic extra diffraction spots along c-axis can be found from SAED pattern in LPSO structure [30,48,76-78].Fig.2 shows the lamellar structure of Mg-2.2Dy-1Cu (at%) under the observation of SEM and TEM with corresponding SAED pattern [43].According to the SAED pattern,there are some equal spaced diffraction spots at the interval of 1/14 of distance between(0002)Mgand reflectio direct spot,so it is proved the 14HLPSO structure,and the width between mutually parallel laminae is～6nm.

In the past,LPSO structures can be divided into six types according to their different stacking sequences,called as 6H,10H,12R,14H,18R,and 24R [57,79-83],as listed in Table 2.The SAED patterns and STEM images of the LPSO structures along the c-axis are showed in Fig.3 to demonstrate 10H-,12R-,14H-,18R-,and 24R-LPSO structures [81,83].Recently,29H,51R,60H,72R,102R and 192R have been discovered in Mg-Co-Y alloy,and each LPSO structure contains either AB′C′A or AB′C building block and has its own stacking sequences,as showed in Table 2 [84].As for 51R (AB′CBCBC′A′BABAB′C′ACA C…),the positions of atoms in the 1st,18th,35th and 52th layers are A,C,B and A,respectively.As for 72R(AB′CBCBC′A′BABAB′C′ACACAC′BCBC B…),if we assume the atoms in the 1st layer are at positions of A,the atoms in the 25th,49th,and 73th layers are at positions of B,C and again A,respectively.As for 102R(AB′CBCBC′A′BABAB′C′ACACAC′BCBCB′ABABA′CACA C…),the atoms in the 1st,35th,69th layer are at positions of A,C and B,respectively,while those in the 103th layer are at positions of A which is the same as those in the firs layer.As for 192R (AB′CBCBC′A′BA BAB′C′ACACAC′BCBCB′ABABAB′CBCBC′A′BABAB′C′ACACAC′BCBCBC′ACACA′B′CBC B…),the positions of atoms in the 1st,65th,129th and 193th layers are A,B,C and A,respectively.LPSO structures feature AB′C building blocks in 15R,12R,21R,AB′C′A building blocks in 10H,18R,14H,and 24R,and AB′C and AB′C′A building blocks in 29H,51R,60H,72R,and 102R [53,85].

Table 2 The stacking sequences of various structures of LPSO found in Mg alloys.

Fig.2.TEM micrograph (a),Fourier-filtere HTEM image of position A (b),and SAED pattern from 14H-LPSO structure along the [0001] direction (c) of the Mg-2.2Dy-1Cu alloy [43].

Fig.3.SAED patterns and Z-contrast STEM images of (a) hcp-Mg,(b)10H-LSPO,(c)14H-LPSO,(d) 18R-LPSO,(e) 24R-LPSO,and (f) 12R-LPSO[81,83].

Fig.4.SEM images of the (a) as-cast Mg96.5Gd2.5Zn1,(b) as-cast Mg96.82Gd2Zn1Zr0.18 alloy,and (c) as-cast Mg96Gd3Zn1 alloy [61,74,91].

In fact,these structures are not always isolated but interrelated.Matsuda et al.[57] found that 10H-,18R-,14H-,and 24R-LPSO could be formed in rapidly solidifie Mg97Zn1Y2alloy after being annealed at 573K for 1h,and it has been confirme that 18R-and 14H-LPSO structures could exist not only in the same specimen but also in the same grain in the annealed Mg97Zn1Y2alloy [57].Moreover,it was also found that various types of LPSO structures can be transformed into each other by certain heat treatment [75,82,86].Itoi et al.[75] reported that 18R-LPSO structure was transformed into 14H-LPSO structure after annealing at 773K for 5h.Furthermore,the present investigations indicate that LPSO structural transformation occurs mainly between LPSO structures with the same type of stacking faults when their lower stacking fault energies are closer.Therefore,intertransformation occurs between LPSO phases with the same SF type when their lower stacking fault energies are close[68].

Usually,the formation of LPSO is available under certain processes,including conventional casting,rapidly solidifie powder metallurgy,heat treatment,and severe plastic deformation [87-90].According to several literatures [28,58,61,69],these alloys can be classifie in two types,typeⅠLPSO forms during the solidificatio at grain boundaries in Mg-RE-Zn (RE including Y,Dy,Ho,Er,and Tm) systems.Type ⅡLPSO does not form in as-cast alloys,but forms after annealing or aging at high temperatures (typically>250 °C) in Mg-RE-Zn (RE including Gd and Tb)systems.However,Wu et al.[61] found that the 14H-LPSO structure can be formed in as-cast Mg96.5Gd2.5Zn1alloy,as shown in Fig.4(a),and the lamellar 14H-LPSO can be converted from eutectic (βphase) after being solution-treated at 773K.Then,the lamellar 14H-LPSO was also detected in the as-cast Mg96.82Gd2Zn1Zr0.18alloy [91],as shown in Fig.4(b).Moreover,it was firstl found that the 18R-LPSO could be formed at grain boundaries in as-cast Mg96Gd3Zn1alloy [74],as shown in Fig.4(c).Table 3 lists transformation of LPSO in some Mg alloys,which shows that LPSO can be formed during cooling for the as-cast alloy or from eutectic phase after soaking or aging.

Table 3 The transformation of LPSO in Mg alloys.

Table 4 The corrosion properties of Mg alloys with and without SF in SBF.

2.3.Relationship between SF and LPSO structures

Fig.5.TTT diagram for precipitation of β′,β1,β and 14H-LPSO structure and for the formation of SF in the Mg97Zn1Gd2 alloy [28].

It has been reported that SF and LPSO structures can be formed in Mg alloys singly or together.It can be assumed that the LPSO structure is generated by introducing the SF into the original compact stacked structure,resulting in a longer period along the stack direction [92].In addition,LPSO structures are multi-layered fault system composed of I1or I2plane faults,and only one type of SF exists in each LPSO structure[68,93].Zhang et al.[37] performed a series of experiments on Mg-Gd-Dy-Zn alloy and the results showed that 14HLPSO was disappeared after solution treatment at 510°C for 10h,and the SF was detected after the next peak-aging at 215°C for 109h.Yamasaki et al.[28] found that SF was transformed into 14H-LPSO structure at 673K in Mg-Gd-Zn alloys.The time-temperature-transformation (TTT) diagram ofβ,β1,β′and 14H-LPSO structure and the formation of SF in the Mg97Zn1Gd2alloy was shown in Fig.5 [28].The gray and black shaded areas indicated the formation of the SF and LPSO structures,respectively.From the diagram,the following formulas can be obtained.

Fig.6.Bright-fiel TEM micrograph (a) and corresponding diffraction pattern (b) of the LPSO structure in the un-recrystallized grains in the extruded-aged Mg-15Gd-1Zn-0.4Mn alloy [73].

Therefore,the SF and LPSO structures formed at medium and high temperatures in Mg97Zn1Gd2alloy,respectively.SF could be transformed into 14H-LPSO at 673K.

Rong et al.found both SF and LPSO in extruded-aged Mg-15Gd-1Zn-0.4Mn alloy [73].In Fig.6,the crude streaks are observed and there are periodic extra diffraction spots corresponding to the{0002}plane of theα-Mg matrix,which determines that the structure is 14H-LPSO.Meanwhile,some thin streaks are also found and have no periodic extra spots beside 14H-LPSO,confirmin that the structure is SF.The similar phenomenon was obtained in as-cast Mg-6Gd-4YxZn-0.5Zr (x=0.3,0.5 and 0.7wt%) alloys [95].

To sum up,both SF and LPSO structures present a lamellar microstructure,and there is an intimate relationship between them:the SF can be transformed into LPSO structure and they can be existed in Mg alloys singly or together under certain conditions.The LPSO structure can be considered to generating by introducing the SF into the original compact stacked structure,resulting in a longer period along the stack direction.

3.Corrosion behaviors of SF-or LPSO-containing Mg alloys

The corrosion resistance of Mg alloys is relatively poor which severely limits their applications on engineering and other field [96-100].The rapid corrosion rate can be mainly ascribed to the following factors.Firstly,Mg is very active,and its standard potential is about -2.37V,which is nearly the lowest among engineering metals [51,98-100].Secondly,many micro-galvanic couples would be established among the second phases,impurities and Mg matrix,which forms electrochemical corrosion [101,102].Finally,the oxide fil formed during corrosion process is loose and porous,and could not provide a good protection to the matrix[103].When Mg alloys are immersed in aqueous environment,the following reactions will be occurred [98,104-106].

Corrosion of Mg alloys are impacted by external and internal factors.The former contains temperature,corrosion]]medium,stress,etc.[11,98,100,104,107-109].The latter includes alloying elements,impurities,microstructures,corrosion product films etc.[110-115].Usually,there are three normal methods for evaluating corrosion of Mg alloys,including mass loss,hydrogen evolution and electrochemical experiments [101,106,116].The mass loss is the simplest,which can provide accurate and clear corrosion data.Usually,after the immersion,the corrosion products will be cleaned with a mixed solution of Cr3O and AgNO3and it will not cause damage to the alloy itself during the cleaning process.The hydrogen evolution can monitor the corrosion process of Mg alloys in real time;however,this method has some errors because it is not guaranteed that all hydrogen is collected,and the hydrogen also has a certain solubility in the solution.Electrochemical experiments provide an instantaneous corrosion information and they also provide information about the kinetic and thermodynamic differences between alloys and solutions,nevertheless,the corrosion rate obtained by electrochemical method often shows apparent difference from that obtained by mass loss and hydrogen evolution methods.Therefore,corrosion behavior of Mg alloys can be better investigated combining these three methods,and the corrosion rates obtained by them are named as PW,PHand Pi,respectively.Here we summarize the effects of SF and LPSO structures on corrosion behaviors and mechanism of Mg alloys.In order to conveniently compare the corrosion rate of the alloys,all the corrosion rates are converted into the same unit mm/y in this review.

3.1.Corrosion behavior of SF-containing Mg alloys

Only limited reports can be available on corrosion of Mg alloys with SF [26,47,51,117,118].The limited results are shown in Table 4,which demonstrate that SF are beneficia for corrosion resistance of Mg alloys,but the corrosion mechanisms are different.

The corrosion behavior of Mg-6Ho without SF and SFcontaining Mg-6Ho-1Zn alloy was evaluated in SBF by Zhang et al.[26] It was found that the corrosion rate of the latter was lower compared with that of the former.Moreover,the corrosion morphology of the latter exhibited more uniform corrosion mode.They also investigated the corrosion properties of Mg-6Ho-0.5Zn and Mg-6Ho-1.5Zn alloys compared with Mg-6Ho alloy and found that the corrosion rate of Mg-6Ho-0.5Zn alloys containing SF was lower than those of Mg-6Ho and Mg-6Ho-1.5Zn alloys without SF [51].Furthermore,it was observed that the corrosion was proceeded along the SF in the interior ofα-Mg grains [51].Similarly,the corrosion rate of Mg-8Er-1Zn alloy containing lamellar SF throughout the whole grains was much lower in comparison with that of the Mg-8Er alloy without SF,and the SF was corroded in preference toα-Mg matrix [47].Schematic diagrams of corrosion processes of both alloys were proposed in Fig.7 to illustrate corrosion behavior.In the Mg-8Er alloy,the second phase particles were distributed randomly in the grains and established the micro-current couplings with the Mg matrix,which cannot hinder the expansion of corrosion and form serious pitting corrosion.While in the Mg-8Er-1Zn alloy,SF was formed and located throughout the wholeα-Mg matrix and it was preferentially corroded during the corrosion process.Thus,the corrosion expansion was difficul to spread across the grain boundaries because the orientation of the SF was different in different grains.

Fig.7.Schematic diagrams of corrosion processes for the Mg-8Er alloy without SF (a1-a5) and the new Mg-8Er-1Zn alloy with profuse SF (b1-b5);the local amplificatio of (b1-b5) were represented in (c1-c5) [47].

Fig.8.HAADF-STEM images of as-cast GZ31K alloy with SF before and after immersed in 0.1M NaCl for 1min;(A) A lower magnificatio image of solute-rich SF before corrosion;higher magnificatio images of alloy before (a1) and after (b1-b6) corrosion in quiescent 0.1M NaCl for 1min immersion;(a2-a6) and (b2-b6) are element distribution maps corresponding to (a1) and (b1) [66].

Zhang et al.[66] studied corrosion behavior of Mg-3Gd-1Zn-0.4Zr (GZ31K) alloy by quasi in-situ scanning transmission electron microscopy (STEM).And the high angle annular dark fiel (HAADF-STEM) images of soluterich SF in theα-Mg matrix were shown in Fig.8 before and after immersion in 0.1M NaCl for 1min.It was precisely and directly proved thatα-Mg phase around SF was dissolved preferentially,which is different from the finding in Mg-Ho-Zn [26,51] and Mg-Er-Zn [47] alloys.It was considered that the corrosion potential of SF was higher than that ofα-Mg due to the segregation of Gd and Zn solute atoms in the SF,which was confirme by electrochemical tests.Therefore,the galvanic couples were generated between the SF andα-Mg,resulting in the preferential corrosion of theα-Mg.Then the microstructure and corrosion properties of as-cast,T4-treated,and as-extruded GZ31K alloys were studied in SBF [117].It was proved that the as-cast GZ31K alloys containing SF at the edges of Mg matrix showed the lowest corrosion rate,and the T4-treated GZ31K alloy had the highest corrosion rate because of the galvanic corrosion between matrix and precipitates.Moreover,the as-extruded one exhibited the most uniform corrosion mode.The corrosion schematic diagrams of the GZ31K alloys under different condition were shown in Fig.9 to demonstrate different corrosion rates and modes.Lately they investigated the microstructures and corrosion properties of the as-extruded Mg-6Gd-0.5Zn-0.4Zr (GZ60K) and Mg-6Gd-1Zn-0.4Zr (GZ61K) alloys[118].SF was discovered withinα-Mg matrix grains in both alloys,which acted as a barrier to slow corrosion rate of the alloys.At the meantime,the half-time of yield strength for mechanical degradation of GZ60K and GZ61K was determined to be 125 days and 85 days,respectively,revealing that uniform corrosion mode plays positive role in mechanical integrity of biodegradable Mg alloys in corrosive medium.

Fig.9.Schematic diagrams of the corrosion processes of GZ31K alloy under different conditions (a-d) as-cast GZ31K;(e-h) T4-treated GZ31K,and (i-l)as-extruded GZ31K [117].

According to the above reports,SF is beneficia to corrosion resistance of Mg alloys,but contrary corrosion sequence of phases has been observed in different alloys:In Mg-Ho-Zn and Mg-Er-Zn alloys,the SF was firstl corroded,but theα-Mg dissolved preferentially in Mg-Gd-Zn-Zr alloys.The different results may be ascribed to different chemical compositions of the alloys.The SF rich in different solute atoms exhibit various corrosion potential as compared to theα-Mg matrix,resulting in different dissolution orders.Moreover,the Mg-Ho-Zn and Mg-Er-Zn alloys were immersed for a long time and the corrosion products were removed by acid solution before observing the corrosion morphology,while the corrosion morphology of the Mg-Gd-Zn-Zr alloys was observed directly after immersed in NaCl for only 1min,so the different characterized methods may also cause contrary corrosion sequences.

3.2.Corrosion behavior of LPSO-containing Mg alloys

The LPSO is currently popular due to its special lamellar microstructure and has an important role on corrosion resistance.A series of experiments of Mg alloys containing LPSO have been investigated in order to elucidate the relationship between corrosion resistance and LPSO.It can be known that the effect of LPSO on corrosion behaviors of Mg alloys is significant and there are several factors that contribute to it,such as volume fraction,distribution,morphologies,and types of LPSO.In terms of the reports on corrosion resistance of LPSO-containing Mg alloys,some of the results are listed in Tables 5 and 6.The metallurgical condition and microstructure of Mg alloy and the characteristics of LPSO are concluded in Table 5.Table 6 describes the corrosion behavior of Mg alloy,including the corrosion rates (Pw,PH,Pi),corrosion medium and time.

Table 5 The Characteristics of LPSO in Mg alloys.

Table 5 (continued)

Table 6 The corrosion properties of Mg alloys with LPSO corresponding to Table 5.

Table 6 (continued)

3.2.1.Mg-Y-Zn alloys

Mg-Y-Zn alloys have been paid much attention since they exhibit good mechanical properties mainly caused by LPSO structure.And due to the requirement of the actual applications,the corrosion behaviors of the alloys have been studied in the last decade [30,39,57,82,119,123-126].Izumi et al.[125] firstl reported the corrosion properties of the LPSO-containing Mg97.25Zn0.25Y2alloy prepared by gravity casting under different cooling rates.They found that the higher cooling rate contributed to the better corrosion resistance because of refine grains,supersaturatedα-Mg solid solution and LPSO structure.In addition,the increased cooling rate could hinder the occurrence of filifor corrosion.Nevertheless,the effect of LPSO on corrosion resistance had not been referred in that work.

Actually,LPSO plays a complex role in corrosion resistance of Mg alloys:It can form micro-galvanic corrosion withα-Mg matrix and thus accelerate the corrosion during the corrosion process or act as a corrosion barrier for decreasing the corrosion rate[8,46,48,119,127,128].Zhang et al.[30] prepared a series of Mg-Zn-Y alloys containing LPSO and found that the LPSO accelerated the corrosion ofα-Mg matrix,and pitting corrosion became more severe with more LPSO.The Mg94Zn2Y4alloy presented fastest corrosion rate,it was 106mm/y.In order to illustrate the effect of LPSO structure on corrosion resistance of Mg-Y-Zn alloys,the relative voltage potential difference between the LPSO andα-Mg matrix was evaluated by scanning Kelvin probe force microscopy (SKPFM),as presented in Fig.10 [18].It was found the corrosion preferentially emerged at the interface of the LPSO andα-Mg matrix since the LPSO which distributed at grain boundaries with higher corrosion potential,acted as micro-cathode and promoted the corrosion of the surrounding matrix.Then the LPSO was corroded progressively,following by other nobler particles.

However,on the other hand,the LPSO could enhance corrosion resistance of some Mg-Y-Zn alloys owing to the corrosion barrier effect.It was found that the corrosion rate of Mg-Zn-Y-Gd alloys was decreased with the increase of LPSO,the corrosion rate of Mg96.5Zn1Y2Gd0.5alloy with the largest volume fraction of LPSO was 11.55mm/y [48].It was also found that LPSO had a positive effect on corrosion resistance of Mg-Zn-Y-Li alloy,which weaken the micro-galvanic corrosion and prevented theα-Mg matrix from corrosion to some extent [46].Furthermore,the corrosion behavior of MgY1.72Zn2.81Zr0.17,MgY3.83Zn3.03Zr0.17and MgY2.58Zn1.27Zr0.15were investigated [119],their corrosion rates were 23.39,19.20 and 6.87mm/y,respectively.And it was found that the second phase in the Mg-Y-Zn-Zr alloys transformed from W-phase to LPSO with increasing Y/Zn mole ratio.And the Y/Zn ratio changed the volume and morphologies of the LPSO structure,specificall .The corrosion mechanism was schematically illustrated in Fig.11.For the MgY1.72Zn2.81Zr0.17alloy,the microgalvanic couplers betweenα-Mg and W-phases formed,and theα-Mg matrix located at grain boundaries corroded preferentially,then the corrosion front was extended to the W-phases and next grains,accelerating the corrosion process.For the MgY3.83Zn3.03Zr0.17and MgY2.58Zn1.27Zr0.15alloys,theα-Mg matrix was separated by LPSO and the extension of corrosion front was tough.And the reduction ofα-Mg matrix resulted in low amount of microgalvanic couplers.These two reasons were responsible for the improved corrosion resistance.But it was also indicated that the compact LPSO structure could effectively prevent the penetration of corrosive medium,so MgY2.58Zn1.27Zr0.15alloy showed better improvement of corrosion resistance.While the wide plate-like LPSO structure with high volume fraction acted as large cathode and led to higher corrosion rate.

Fig.10.SKPFM results of Mg-3.1Zn-7.6Y alloy (a) surface voltage potential map and(b)line-profil analysis of relative voltage potential in Fig.10(a)[18].

Fig.11.Schematic illustrations of the corrosion mechanisms (a) MgY1.72Zn2.81Zr0.17,(b) MgY3.83Zn3.03Zr0.17,and (c) MgY2.58Zn1.27Zr0.15 alloy [119].

In addition,it was found that the morphology of the LPSO and heat treatment also had an important impact on the corrosion behavior of Mg-Y-Zn alloys.Cheng et al.[120]prepared Mg-4.86Zn-8.78Y-0.6Ti alloy under different heat treatment processes(Alloy A:As-cast,Alloy B:400?C×10h,Alloy C:520?C×6h,and Alloy D:520?C×6 h+400?C×10h),the LPSO structure with different morphologies were observed according to the microstructures.The corrosion rates of these four alloys presented the following order:Alloy B with lamellar LPSO and block-like LPSO(15.16mm/y)>Alloy D with lamellar LPSO and rod-like LPSO (8.82mm/y)>Alloy A with block-like LPSO (5.65mm/y)>Alloy C with rod-like LPSO (2.97mm/y).The cross-section morphologies and corresponding high-magnificatio images of the alloys at different conditions are showed in Fig.12.Herein,the continuous LPSO structure could play an effective corrosion barrier role,and the more uniform corrosion mode and better corrosion resistance were obtained in Alloy C,which was attributed to uniform distributed rod-like LPSO structure and compact surface fil formed during the corrosion process.The corrosion properties of the Mg95.33Zn2Y2.67alloys under different conditions,including as-cast,heat treatment and heat treatment after as-extruded,were investigated by Wang et al.[129].They found that the morphologies of the alloys were varied from block-like to rod shape with a lot of lamellar LPSO inα-Mg matrix to homogeneous rod-like LPSO.The alloy,which was extruded and then heat treated at 540°C for 5h,exhibited best corrosion resistance (1.35mm/y) due to the existence of rod-like LPSO that acted as corrosion barrier.However,heat treatment may accelerate corrosion rate of LPSO-containing Mg-Y-Zn alloys.MgY3Zn2Al alloys after heat treatment at 400,430 and 460 °C showed decreased corrosion resistance because of the reduced and discontinuous LPSO [128].

Furthermore,the corrosion rates of Mg97Y2Zn1alloys in longitudinal sections and transversal sections were discrepant:The corrosion rate of the longitudinal sections was 10 mm/y,which slightly slower than that of the transversal sections(40mm/y) at the initial stage of immersion about 5h but the contrary result was found for the long-term immersion (the former was 30mm/y,the latter was 20mm/y).They explained such difference was attributed to various orientation of LPSO in longitudinal and transversal sections [130].

As for the LPSO-containing Mg-Y-Zn series alloys,in general,the characteristics of the LPSO structure,including the volume fraction,morphology,and distribution,which can be changed by alloying,heat treatment,and other processes,have an important influenc on corrosion behavior.There are two distinct findings One is that the corrosion is repressed due to the increasing of LPSO,this can be contributed to the compact and continuous distribution of LPSO.The other is that the corrosion rate is accelerated because the galvanic couples were formed between LPSO andα-Mg matrix.Furthermore,the morphologies of LPSO also have a great influ ence on corrosion resistance,when they distribute uniformly and form compact surface film the corrosion resistance of Mg alloys is improved.

3.2.2.Mg-Gd-Zn alloys

Appropriate addition of Gd can enhance the corrosion resistance of Mg-Gd-Zn alloys due to the formation of LPSO,and the formation,volume fraction,and distribution of LPSO can be modifie by heat treatment and other processes,which impact corrosion behavior of the Mg-Gd-Zn alloys apparently[45,90,121,131-135].

The effect of LPSO structure on corrosion resistance of Mg-Gd-Zn series alloys was primitively investigated by Zhang et al.[45].They found the corrosion rate of Mg-11.3Gd-2.5Zn-0.7Zr alloy with 14H-LPSO structure in Hanks’ solution was 0.17mm/y which was lower than that of Mg-10.2Gd-3.3Y-0.6Zr alloy (0.55mm/y) without LPSO.Moreover,the corrosion morphology of Mg-11.3Gd-2.5Zn-0.7Zr alloy was much more uniform.Subsequently,the corrosion performances of Mg-5Gd-1Zn-0.6Zr (GZ51K) under different conditions including as-cast,T4,T6,and asextruded conditions were investigated [12,90,121,132].The corrosion behavior of the as-cast GZ51K alloy with 14HLPSO located at the outer edge of theα-Mg matrix grains exhibited more uniform corrosion and better corrosion resistance as compared to those of the T6-treated GZ51K alloy(solution treated at 535°C for 12h and aged at 200°C for 12h)without LPSO[121],as shown in Fig.13.The schematic diagrams were plotted to explain the effect of LPSO on corrosion behavior,as shown in Fig.14.

Generally,the potential of the Mg matrix was more negative than that of eutectic phases,and the potential of the LPSO structure was between that of the matrix and eutectic phase.The LPSO distributed at the out edge of the matrix grains acted as a barrier between the Mg matrix and the eutectic phase,which would decrease the effect of galvanic corrosion and protected the matrix from corrosion.Hence,the corrosion process of the as-cast GZ51K alloy with LPSO structure was slow and uniform.While for the T6-treated alloy without the LPSO structure,much precipitates distributed discontinuously in the grains,which formed micro-galvanic and accelerated the corrosion.Therefore,the T6-treated alloy underwent rapid and localized corrosion.Moreover,they also found that the volume fraction of the LPSO distributed at the outer edge of matrix grains was impacted by solution temperature,and higher LPSO volume fraction caused by proper T4 treatment parameters (350-450°C for 5h) led to better corrosion resistance as well as more uniform corrosion mode.Nevertheless,solution treated at 500°C for 5h resulted in fast and localized corrosion due to the disappearance of the LPSO,the formation of Zr-rich precipitates,and grain growth [132].In addition,the LPSO structure,which was disappeared after T4-treatment,was again formed throughout whole matrix grains in the GZ51K after hot extrusion [90],then it was retained after aging at 180-220°C [12].The as-extruded GZ51K alloy with LPSO structure exhibited more uniform corrosion mode and lower corrosion rate as compared to the T4-treated one without LPSO [90],and subsequent aging treatment on the as-extruded alloy further improved corrosion resistance for the reduced internal stress and crystal defects [12].

Fig.12.Cross-section corrosion morphologies (a) and corresponding high-magnificatio images (b-e) of the Mg-4.86Zn-8.78Y-0.6Ti alloys after immersion in 3.5% NaCl solution for 24h [120].

Fig.13.Macro (a,b) and micro (c,d) corrosion morphologies of the as-cast (a,c) and T6-treated (b,d) GZ51K alloys after removing corrosion products formed during immersed in SBF for 120h [121].

Fig.14.Schematic diagrams of corrosion process for the as-cast GZ51K alloy with LPSO structure to show uniform corrosion (a-d) and T6-treated GZ51K without LPSO structure to show localized corrosion (e-h) with prolonging immersion time in SBF [121].

Fig.15.Surface corrosion morphologies of (a) as-cast,(b) T4-treated,and cross-section corrosion morphologies of (c) as-cast and (d) T4 Mg-Gd-Zn-Zr alloys [134].

It has been confirme that the distribution of LPSO structure affects corrosion behavior of the as-cast Mg-6Gd-xZn-0.4Zr alloys (x=0.5,1,and 2 wt%,denoted as GZ60K,GZ61K,and GZ62K) [133].The as-cast GZ60K and GZ61K alloys with LPSO distributed at the outer edge of the matrix grains exhibited more superior corrosion resistance and uniform corrosion than the as-cast GZ62K alloy with LPSO distributed throughout whole matrix grains.When LPSO structures distributed at the outer edge of grains,they played as bridges between the eutectic phases and the matrix,thereby weakened the micro-galvanic reaction.However,the function of bridges was impaired when the LPSO located throughout grains.

Recently,Liu et al.investigated the role of LPSO structure on corrosion properties of Mg-15Gd-2Zn-0.39Zr alloys [134].In the corrosion process,the LPSO promoted the micro-galvanic corrosion of the matrix due to higher potential than the matrix,and it was also noted that the corrosion product film played a positive role as a barrier,which hindered the expansion of corrosion front,as shown in Fig.15.Otherwise,the relative potential and volume fraction of LPSO was lower thanβ-(Mg,Zn)3Gd,which reduced the micro-galvanic corrosion rate in some degree.Therefore,the corrosion resistance of T4-treated Mg-Gd-Zn-Zr alloy(5.68mm/y) improved significantl compared with the as-cast Mg-Gd-Zn-Zr alloy (48.25mm/y).Shuai et al.[135] studied the corrosion behavior of Mg-3Zn-0.5Zr-xGd alloys and found the LPSO played a positive role in corrosion resistance because the LPSO was an unstable phases and corroded firstl,then formed a compact corrosion product films which protected the Mg matrix from the corrosion medium.Furthermore,the lamellar LPSO can hinder the corrosion extending to the next grains.

On the contrary,it was reported that the LPSO structure promoted corrosion rates of the Mg95.8Gd3Zn1Zr0.2alloy due to their discontinuous distribution within the grains or at grain boundaries [50].Moreover,Srinivasan et al.[4] also found that the corrosion rate was increased with increasing Zn content in Mg-10Gd-xZn (x=2 and 6wt%) alloys because of the existence of LPSO facilitated the filifor corrosion,which can attribute to the discontinuous distribution of LPSO structure.

In brief,in LPSO-containing Mg-Gd-Zn series alloys,the LPSO structure exhibited lamellar morphologies and distributed at grain boundaries or located throughout whole the grains.On the one hand,the LPSO structure had a positive effect on corrosion resistance when they continuously distributed,the LPSO was regarded as a barrier which separated the matrix from the eutectic phase and protected the substrate from corrosion.On the other hand,they played a negative role when the LPSO structure discontinuously located,the microgalvanic couples were established betweenα-Mg matrix and LPSO and eutectic phases.In addition,the volume fraction and distribution of LPSO can be modifie by alloying element addition,heat treatment,and extrusion,which was critical for corrosion behaviors.

3.2.3.Mg-Dy-Zn alloys

Compared with the Mg-Y-Zn and Mg-Gd-Zn alloys,the researches of Mg-Dy-Zn alloys are quite fewer.Generally,the volume fraction and type of LPSO impact corrosion resistance of Mg-Dy-Zn alloys [56,77,122].The corrosion behavior of as-cast Mg-2Dy-xZn (x=0,0.1,0.5,and 1,at%)alloys were studied by Bi et al.[122],and their corrosion rates were 17.71,14.69,27.63 and 101.33mm/y,respectively.They reported that the Mg-2Dy-0.5Zn alloy formed 18R-LPSO structure.Then Mg-2Dy-0.5Zn alloys were investigated after being solution treated at 530°C for 12h and cooled for 0min,5min,8min,20min,4h,24h,and their corrosion rates were 249.76,244.33,358.35,423.50,1183.64 and 2633.30mm/y,respectively.The results were indicated that the alloy which had the lowest volume fraction of 14H-LPSO structure for cooling 5min showed best corrosion resistance [56].

Furthermore,Peng et al.[77] successfully obtained the 14H-and 18R-LPSO in Mg-2Dy-0.5Zn alloy and the results showed that the Mg-2Dy-0.5Zn after solution treatment at 545 °C for 4h contained 14H-LPSO structure and presented better corrosion resistance than the as-cast Mg-2Dy-0.5Zn alloy contained 18R-LPSO structure in 0.9wt% NaCl (Their corrosion rates measured by electrochemistry were 1.06 and 24.06mm/y,respectively),indicating the type of LPSO impacted the corrosion behaviors of Mg-Dy-Zn alloys.In order to explain the phenomenon,the schematic diagrams of corrosion process of 14H-and 18R-LPSO were plotted and shown in Fig.16.

In the initial stage of corrosion process,the oxide fil of the as-cast alloy was firstl formed on the surface of Mg matrix and then expanded toward the second phase.Therefore,it would result in tensile stress because the thickness of the fil was diverse.As the immersion time increased,the tensile stress also increased,which caused the rupture of oxidation fil between Mg matrix and 18R-LPSO structure.Therefore,cracks formed to provide access for corrosive solution to reach fresh surface and release of hydrogen.Moreover,the remediation ability of oxide fil also acted as a vital role in the corrosion resistance of this alloy.The oxide fil on the surface of the alloy was not dense,the corrosive solution could penetrate the oxide layer to reach the Mg alloy surface.And during the process of corrosion,the oxide fil would be eventually decomposed with the release of hydrogen.For the solution treated Mg-2Dy-0.5Zn alloy,the oxide fil exhibited remediation ability after original oxide fil was destroyed,which effectively prevented the reaction between the corrosive solution and the matrix.While for the as-cast Mg-2Dy-0.5Zn alloy,forming a dense oxide fil needed more time,thereby some corrosive solution reached on Mg alloy surface and generated further corrosion.

3.2.4.Mg-Gd-Ni alloys

It has been reported that adding Ni to Mg-RE alloys can promote the formation of LPSO structure,the Mg98.4Gd1.2Ni0.4,Mg98Gd1.5Ni0.5,Mg97.2Gd2.1Ni0.7,and Mg96Gd3Ni1alloys were designed and their microstructure and corrosion behavior were investigated [136,137].The results showed that their corrosion rates were 919.51,715.87,1331.51 and 1416.13mm/y,respectively.Besides the Mg98Gd1.5Ni0.5alloy,the microstructure of the alloys all consisted ofα-Mg and LPSO,with increasing Gd and Ni elements,the morphologies of LPSO changed from discontinuous distribution at grain boundaries to continuous net-like,and the corrosion rate was increased.The existence of LPSO could increase the corrosion rate.For Mg98Gd1.5Ni0.5alloy,the LPSO was exhibited lamellar morphologies and distributed at grain boundaries,which had the lowest corrosion rate among the studied Mg-Gd-Ni series alloys.In order to explain this phenomenon,the corrosion morphologies and localized potential distribution of the alloys were evaluated,as shown in Fig.17.The potential of LPSO was more negative thanα-Mg matrix,causing preferential corrosion of the LPSO,which was quite different from the other LPSO-containing Mg alloys.

In conclusion,the effect of LPSO structures on corrosion resistance of Mg alloys is complicated,and there are still some doubts.Regarding to LPSO-containing Mg-Y-Zn,Mg-Gd-Zn Mg-Dy-Zn and Mg-Gd-Ni series alloys,in general,the characterization of the LPSO phase,including the type,volume fraction,distribution,morphologies and so on,can be essentially modifie by the Zn/RE ratios and processes.While the corrosion rate of some Mg alloys was extremely high,as for Mg-Dy-Zn and Mg-Gd-Ni alloys,this maybe contributed to the significan difference of electrochemical potential difference between LPSO andα-Mg matrix,which promoted the rapid microgalvanic corrosion.

Generally,SF and LPSO structures impact corrosion resistance of Mg alloys so much that they should be paid more attention.Up to now,the formation and transition of them can be controlled by alloying design and proper processes including heat treatment,deformation,and cooling rate,but systematic and further researches to reveal the influence of SF and LPSO structures on corrosion behavior and mechanism of Mg alloys are still necessary.In terms of multiphase materials,it is important to confir the characteristics of the SF and LPSO structures,such as the type,volume fraction,composition,distribution,and morphology,and the relation between the SF/LPSO and corrosion resistance should be further investigated.The microstructures can be characterized by OM,SEM,TEM,etc.Considering the nano scale of SF and LPSO structures,except for the normal immersion tests (mass loss test,hydrogen evolution test,and pH value test) and electrochemical tests with some products analysis by using X-ray photoelectron spectroscope (XPS),Raman spectrometer,Fourier transform infrared spectroscope (FTIR),etc.,some unusual tests,such as SKPFM and in-situ STEM,are conducive to characterized the influence of SF or LPSO structures on corrosion behavior of Mg alloys,as shown in Fig.18.In summary,only the roles of SF and LPSO structures on corrosion behavior are clearly illustrated can we better improve the corrosion resistance of Mg alloys and enlarge their applications.

Fig.16.The detailed schematic diagrams of corrosion process,a1-a4 graphs correspond to the as-cast Mg-2Dy-0.5Zn (MDZ-C) alloy with 18R-LPSO;b1-b4 graphs correspond to the solution treated Mg-2Dy-0.5Zn (MDZ-545) alloy with 14H-LPSO [77].

Fig.17.SKPFM results of Mg98Gd1.5Ni0.5 alloy:(a) optical micrograph;(b) surface localized potential distribution map and (c) line-profil analysis of relative potential through the LPSO phase in (b) [137].

Fig.18.The methods and analytical tools for microstructure and corrosion behavior characterizations of SF-containing and LPSO-containing Mg alloys.(a)TEM image and corresponding SAED pattern of the as-cast Mg-11Gd-1Zn alloy [78],(b) EBSD of extruded Mg97Zn1Y2 alloys after annealed at 475 °C for 168h [39],(c) OM image of extruded Mg-11Gd-1Zn alloy after solution treated at 500 °C for 8h [78],(d) SEM images along with the EDS mapping results corresponds to as-cast Mg-6.0Gd-1.2Cu-1.2Zr alloy [19],(e) Raman spectra of ZW38 alloy after immersing for 2h in 0.1M NaCl solution [18],(f) H2 evolution curves and corrosion rates of the Mg-5Gd-1Zn-0.6Zr alloy [90],(g) The topography of the Mg-2Zn-0.5Gd-0.5Zr alloy observed by laser confocal scanning after removing the degradation products [10],(h) XPS analysis of Mg96.0Gd3.0Ni1.0 alloy after the immersed in 3.5% NaCl solution for 5min [136],(i) HAADF-STEM image of a solute-rich stacking fault (SF) in the α-Mg matrix of as-cast Mg-3Gd-1Zn-0.4Zr following 1min immersion in quiescent 0.1M NaCl and the corresponding EDXS maps [66],(j) SKPFM Volta potential measurement as-cast GW93 alloy [113],(k) FTIR spectra of the corrosion products of WE43 immersed in Ringer’s solution [2],(l) Corrosion morphologies of Mg-10.2Gd-5.7Y-1.6Zn-0.42Zr alloy (removed thickness from the top surface about 50μm) after immersion for 150h in 3.5% NaCl solution saturated with Mg(OH)2 [127],(m) Electrochemical results of the Mg-Gd-Zn-Zr alloys[118].

4.Summary

In the last two decades,Mg alloys containing SF or LPSO structure have been extensively studied due to their excellent mechanical properties,and the strengthening mechanisms have been well illuminated.However,little attention has been paid to the corrosion behavior of these alloys until last decade.The SF and LPSO structures have been found to play important roles in corrosion resistance of Mg-RE-TM series alloys according to available reports,which is of great importance for their wide applications.This review has presented the recent developments on corrosion behaviors of Mg alloys with SF or LPSO structure.

As for the SF-containing Mg alloys,SF is beneficia to corrosion resistance and uniform corrosion,and the corrosion sequence of theα-Mg and SF is contrary in different kinds of Mg alloys which may be attributed to different alloying compositions or different characterizations.For the limited researches,further work is recommended to better understand the corrosion mechanism of SF-containing Mg alloys,for instance,the effects of volume fraction,orientation,and distribution of SF on corrosion behavior of Mg alloys.

As for the LPSO-containing Mg alloys,LPSO structures,including volume fraction,distribution,morphology,and type,apparently impact corrosion behavior,and the influenc rules and mechanisms are better understood as compared to those of the SF-containing Mg alloys.It has been found that when the LPSO distributes uniformly and continuously,the LPSO may be regarded as a barrier separating the matrix from the eutectic phase,and thus protect the matrix from corrosion.Conversely,LPSO can form galvanic corrosion with theα-Mg matrix and accelerate the corrosion.The volume fraction of LPSO plays a positive or negative role in corrosion for different series alloys.The LPSO structures which distribute at the outer edge of matrix grains in the form of lamellar are good for the improvement of corrosion resistance,whereas,when they locate at the grain boundaries in the form of block and plate or throughout the matrix grains in the form of lamellar,it is harmful to corrosion resistance.Moreover,14H-LPSO structure shows much better corrosion resistance due to the rapider fil remediation as compared to the 18R-LPSO structure for the Mg-Dy-Zn alloy,and the potential of the LPSO is even more negative thanα-Mg matrix in Mg-Gd-Ni alloys.

Based on the available reports,corrosion behaviors and mechanisms of the Mg alloys with SF or LPSO structures have been partially illustrated.However,more attention should be paid to further understand the corrosion mechanism and develop corrosion resistant SF-and LPSO-containing Mg alloys for widely applications.Firstly,when we design Mg alloys,the corrosion potential of the SF and LPSO should be preferentially considered because smaller electrochemical potential difference among each phase is beneficia to the corrosion resistance.Secondly,the influenc factors,such as composition,type,volume fraction,distribution of SF and LPSO,that impact corrosion resistance of the SF-and LPSOcontaining Mg alloys are expected to be fully revealed for better tailoring corrosion behaviors.Thirdly,it is recommended to consider service environments of Mg alloys,for example,stress.It is well known that stress corrosion cracking and corrosion fatigue would occur under stress condition,and different medium may cause various corrosion.Finally,advanced and standard tests are necessary.For the nano-scaled SF and LPSO,it is difficul to directly characterize their corrosion potentials in medium,thus advanced technology are desired for better evaluating them;moreover,it is not convenient to compare the corrosion rates of the alloys because of various testing methods and medium,hence,standard tests are important for the researches to share the data.

Acknowledgments

This project was supported by the Natural Science Foundation of Jiangsu Province for Outstanding Youth(BK20160081),the Natural Science Foundation of Jiangsu Province (BK20181020) the Natural Science Foundation of Higher Education Institutions of Jiangsu Province -Key Project(18KJA430008),the“333 Project”of Jiangsu Province(BRA2018338),the Practical Innovative Project for Postgraduates of Jiangsu Province (SJCX19_0493).