Jian Sun,Wenbo Du,Junjian Fu,Ke Liu,Shubo Li,Zhaohui Wang,Hongxing Liang

Faculty of Materials and Manufacturing,Beijing University of Technology,Beijing 100124,China

Abstract The vulnerable corrosion resistance of Mg alloys is regarded as one of the main disadvantages restricting their application,while it can be used as an extraordinary specialty in some particular fields,such as petroleum exploitation and medicine.In recent years,many Mg alloys with high corrosion rate and high strength have been developed for fracturing temporary plugging tools in the oil exploitation.This review briefly introduces the performance requirements of the degradable fracturing tools classified into mechanical and corrosion properties.Recent progress on corrosion behavior of degradable Mg-Al,Mg-Zn,Mg-RE alloys and Mg matrix composites is then summarized and discussed.Finally,the factors influencing the degradation rate of Mg alloys are analyzed and divided into secondary phase,texture,dislocation,grain size and surface film.From the summary,it can be found that addition of Ni or Cu to the degradable Mg alloys is a common and effective method to enhance the degradation rate due to increasing the amount of secondary phases and deteriorating the corrosion product layers.For the as-extruded degradable Mg alloys,grain size,texture and dislocation are the key factors affecting the corrosion rate under different processing conditions.We expect this review is helpful for those who are working on developing Mg-based functional materials with superior degradation rate.

Keywords:Review;Mg alloys;Degradable fracturing tools;Corrosion rate;Secondary phase.?Corresponding author.

1.Introduction

Magnesium(Mg)alloys have received extensive attention because of their excellent characteristics,including low density,high specific strength,and good machinability,so they have considerable application prospects in the aeronautical,automotive and 3C industries[1–4].However,one of the main obstacles impeding the application of Mg alloys is their weak corrosion resistance,due to the extremely negative standard potential(–2.37 V vs.SHE)and low Pilling-Bedworth ratio(0.81,the oxide films are porous and unprotected)of Mg[5].Therefore,the majority of studies have concentrated on enhancing the corrosion resistance of Mg alloys,such as through alloying,heat-treatment,deformation and surface treatment[6–10].From another point of view,the poor corrosion resistance of Mg alloys can be used as an extraordinary merit among metallic structural materials in some particular fields,such as biodegradable and anodic materials[11–15].Nonetheless,the corrosion rate of them does not need too fast.Therefore,the rapidly degradable specialty of Mg alloys is still not fully utilized.

Oil and natural gas have been the most important energy sources for decades.Due to technological advancements in petroleum exploitation,the productions of unconventional oil and gas fields with low porosity and low permeability are gradually increasing,especially the hydraulic fracturing[16,17].In the process of fracturing operations,balls or plugs are utilized to seal the pipes and withstand the high liquid pressure to create downhole cracks.After fracturing,the balls or plugs should be flowed back to the ground or drilled out,which is difficult to operate,time-consuming and labori-ous.Since 2009,Xu et al.[18–20]developed a new type of controlled electrolytic metallics(CEM)ball,which could be quickly dissolved in the fracturing fluid after operation without drilling,saving the production costs and improving the construction efficiency.Therefore,various degradable metal materials have been investigated and become a good choice for fracturing balls.For example,the dissolvable aluminum alloys are developed based on electrolytic system or Al-water reactivity[21–23].

Fig.1.Schematic of the horizontal multistage fracturing system[25,26].

Because of the unique advantages,Mg alloys are considerable ideal materials for fracturing tools[18].In comparison to the traditional mild steel and aluminum fracturing balls,Mg alloys balls have lower density and higher corrosion rate,which can be more easily flowed back or rapidly degraded after the fracturing process,reducing the risk of blockage of the oil outlet channel[24,25].Compared with the traditional polymer materials,Mg alloys have relatively high strength and high temperature resistance to bear high liquid pressure,ensuring that the fracturing process is accomplished successfully[24].Therefore,many researchers have focused on improving both corrosion rates and mechanical properties of Mg alloys over the past decade by various methods,including alloying,heat treatment and hot deformation.In the present review,the current studies on Mg alloys for the application of degradable fracturing tools are summarized.

2.Performance requirements of degradable fracturing tools

Currently,multistage fracturing is a widely used hydraulic fracturing technology,and its schematic diagram is given in Fig.1,which mainly consists of the packer,ball,ball seat and sliding sleeve[25,26].According to the geological and technical requirements,the open hole section of a wellbore is divided into several stages by using the packer,and the sliding sleeve is set at the specific location that needs to be stimulated[27].During the fracturing operation,the ball is pumped down the hole to break the pins which lock the ball seat under pressure and drive the sliding sleeve to the open position,and then the fracturing fluid containing proppants is injected into the formation through ports on the sliding sleeve.When the pressure of fracturing fluid exceeds the strength of the rock,micro-cracks are formed and lead to the release of reservoirs,increasing the permeability and production of unconventional fields.After fracturing,the ball will be flowed back to the surface in the conventional process.However,the fracturing ball is stuck inside the pipe and sometimes cannot be flowed back to the ground due to insufficient reservoir pressure or severe deformation of the ball.In this case,the ball needs to be drilled and milled,otherwise the production of the well will be critically affected.The degradable fracturing balls are easily decomposed after fracturing,avoiding the potential risk of flowback failure and pipe blockage.

Because most of unconventional reservoirs are more than 3 km deep,the fracturing tools are located in a high temperature environment,bearing the high pressure of fracturing fluid at the same time[28].The working temperature and pressure of the dissolvable bridge plugs generally range from 50 °C to 150 °C and 50 MPa to 105 MPa,respectively,and the corresponding solubility is also an important performance indicator of the tools which also classified into three levels(as given in Table 1)[29].Therefore,the dissolvable fracturing tools require high temperature stability,high mechanical property and high corrosion rate.Furthermore,temperature will simultaneously change the latter two capabilities for most materials.At present,the performance requirements of the degradable fracturing tool mainly contain strength and corrosion rate,which need to provide enough strength and dimension integrity before service,and then be corroded rapidly after fracturing operation,as shown in Fig.2[19].

Table 1The standard performance requirements of dissolvable bridge plugs[29].

2.1.Mechanical properties

During fracturing,the working pressure generally ranges from 50 MPa to 105 MPa,and most metal materials can satisfy this condition.But in fact,the maximum stress of the ball is generated in the contact area with the ball seat and much greater than the working pressure of the operation[26,30].Zheng et al.[26]investigated the deformation characteristics of aluminum alloy fracturing balls by finite element analysis(FEA),and the result showed that the maximum von Mises stress of the ball was 359 MPa under 60 MPa fluidpressure.For Mg-based degradable fracturing balls,the maximum stresses are not exceeding 300 MPa under 70 MPa in related studies[31–34].Moreover,the strength of metallic materials generally decreases with increasing temperature[35].In other word,an appropriate room temperature strength may be unsuitable under an elevated temperature environment.But up to now,few investigations have been concentrated on the stress analysis of Mg alloys under fracturing processes at high temperature and high stress.The mechanical property standards of Mg alloys for degradable fracturing tools are not clear and need to be supplemented.

Fig.2.Variation tendency of strength and corrosion rate with time[19].

At present,the mechanical properties of degradable Mg alloys are primarily measured by compression test at room temperature,and the recent experimental results summarized below are also based on this method.The size of cylinder specimens and strain rate of compression test are generally ranged fromΦ3 mm×6 mm toΦ10 mm×20 mm and 0.5 mm/min to 3 mm/min,respectively.

2.2.Corrosion properties

During fracturing,the fracturing ball must withstand high pressure of fracturing fluid,which is the corrosion medium for degradable tools as well.The components of fracturing fluid are different based on the geological characteristics of reservoirs,and the main additives include thickener,crosslinker,breaker,fungicide,pH buffer,and clay stabilizer[36].In the early stages of fracturing,the fracturing fluid obtains high viscosity through reactions of the thickener and crosslinker,transporting proppants into microcracks to maintain a conductive channel[37].After fracturing,due to the chemical action of the breaker,the viscosity of the fracturing fluid is decreased to accelerate the flowback.According to the investigation of Pei et al.[27],the decomposition rate of a particular Mg alloy is low in the guar gum fracturing fluid,which can degrade rapidly in KCl solution.Generally,the corrosion process of Mg alloys in aqueous solution is an electrochemical process.Because of the high viscosity of the fracturing fluid at the beginning of operation,the ion diffusion rate is slow,leading to a low corrosion rate[38]and avoiding the risk of mechanical failure at the same time.After fracturing,the corrosion rate of Mg alloys may increase dramatically with the decreased viscosity of the fracturing fluid.In addition,NaCl and KCl are commonly used as clay stabilizers[39,40].Under the chloride ion and high temperature conditions,Mg alloys are able to dissolve promptly,fulfilling the high corrosion rate of degradable fracturing tools.However,due to the different environments and the fracturing parameters of various reservoirs,the standard of corrosion rate for the degradable fracturing tools is not clear,which also needs to be supplemented.

Currently,the corrosion rate of Mg alloys is mainly evaluated by weight loss(PW),evolved hydrogen(PH),Tafel extrapolation of polarization curves(Pi)and electrochemical impedance spectroscopy(Pi/EIS).Among the four methods,PHis typically in good agreement withPW,but the electrochemical measurements(PiandPi/EIS)are typically lower than the other two methods according to the results of Atrens et al.[41].Therefore,the corrosion rates summarized below are selected from the results ofPWandPHunder a similar corrosion medium(3.0–3.5 wt.% NaCl or KCl solution)and temperatures(approximately 25 °C and 93 °C)for convenient comparison.Moreover,the corrosion rate data(mm·y–1)are provided in the literatures or calculated by formula(1)and(2)as following[42]:

where,ΔW(mg)is the weight loss of specimen after immersion,A(cm2)is the exposed surface area of specimen,andt(day)is the immersion duration.

where,VH(mL·cm–2·day–1)is the hydrogen evolution rate.

3.Mg-based degradable fracturing tools

Due to the relatively low mechanical properties and corrosion rate of pure Mg,it cannot be used as a degradablefracturing tool.Adding alloy elements and/or reinforcements is an effective way to improve the performance of degradable Mg alloys.Up to now,many relevant studies have focused on Mg-Al series alloys,Mg-Zn series alloys,Mg-RE series alloys and Mg matrix composites.In the following subsections,we summarize the mechanical properties and corrosion rates of these materials based on recent references.

Fig.3.Microstructures of Mg-Al-Zn alloys:(a)Mg-15Al,(b)Mg-20Al,(c)Mg-25Al,(d)Mg-20Al-1.5Zn,(e)Mg-20Al-5Zn and(f)Mg-20Al-10Zn[43].

3.1.Mg-Al series alloys

Aluminum(Al)is one of the most common alloying elements in Mg alloys.Its maximum solid solubility inα-Mg matrix is 12.7 wt.% at 437 °C,and the primary secondary phase isβ-Mg17Al12.The addition of moderate Al markedly enhances the mechanical properties of Mg alloys because of the solid-solution strengthening of Al solute atoms and the age hardening of theβ-Mg17Al12phase[1].However,an excessive Al concentration will significantly reduce both strength and corrosion resistance of Mg-Al alloys due to the formation of network-likeβ-Mg17Al12phase,so the content of Al generally does not exceed 9.0 wt.%for current commercial applications.From another perspective,theβ-Mg17Al12phase is beneficial to accelerate the corrosion rate of the degradable Mg-Al series alloys,so the Mg alloys containing high Al content are developed.The mechanical properties and corrosion rates of the degradable Mg-Al series alloys under appropriate processing conditions are listed in Table 2.

Xiao et al.systematically studied a variety of degradable Mg-Al series alloys,which were alloyed with Zn,Cu,RE and Si to satisfy the demands of application,and found that the corrosion rate was significantly changed by secondary phases.First,theβ-Mg17Al12phase gradually increased with increasing Al content,as shown in Fig.3(a–c)[43],and a similar result was found in Ref.[44].After adding Zn to Mg-20Al alloy,the partialβphase transformed intoτ-Mg32(Al,Zn)49phase and the total volume fraction of secondary phases increased,as shown in Fig.3(d–f)[43].Meanwhile,the corrosion rate of the alloys first increased and then decreased with Al or Zn addition,as indicated in Table 2,which was caused by the galvanic effect and barrier effect of secondary phases on the corrosion behavior of Mg alloys.Due to tiny solid solubility inα-Mg matrix and high standard electrode potential of Cu,Cu-containing phases are formed and markedly accelerated the corrosion rate of the degradable Mg-Al series alloys with Cu addition[43,45–48],as indicated in Table 2.However,the volume fraction of the T-MgAlCuZn phase increased and its morphology changed from fine particle along the grain boundary to hollow rod with further increasing the Cu concentration in the Mg-17Al-3Zn-xCu alloys,as shown in Fig.4,resulting in a better barrier effect and leading to a lower degradation rate[48].Among the alloys of this research[48],the Mg-17Al-3Zn-5Cu alloy held both the highest strength(438 MPa)and highest corrosion rate(468.7 mm·y–1).

In contrast,the effects of Al addition on the corrosion behavior of as-cast Mg-Cu alloys have been investigated[49,50],and the results show that the corrosion rate dramatically reduced(as indicated in Table 2),which was associated with the secondary phases and corrosion product of them.First,the secondary phase of Mg-Cu-xAl alloys transformed from Mg2Cu to MgAlCu andβ-Mg17Al12.As shown in Fig.5(a–f,i),the Volta potential difference(ΔVPD)between theβ-Mg17Al12phase andα-Mg(164 mV)is much lower than that of Cu-containing phases(800–1000 mV),and the volume fraction of Cu-containing phases decreased with Al addition,resulting in a lower galvanic effect and leading to a lower corrosion rate.On the other hand,the Al 2p spectrum of the corrosion product was detected after adding Al into as-cast Mg-Cu alloys,as shown in Fig.5(g),which implied the presence of denser Al(OH)3or Al2O3films and inhibition of corrosion process.Furthermore,the volume fraction of second phases varied with heat treatment,which was also contributed to the variation of corrosion rate of the alloys.As shown in Fig.5(h),the Al content inα-Mg of Mg-Cu-Al alloys increased after solution treatment(Alloy 2–5),while the Cu content inα-Mg of the heat-treated Mg-Cu-xAl alloys displayed no significant change.However,the volume fraction of Cu-containing phases and the corrosion rate of Mg-Cu-xAl alloys decreased after solution treatment(Alloy S1,S2 and S3)and increased after aging treatment(Alloy A1,A2 and A3),as shown in Fig.5(i).It also can be found that as-aged Al-containing alloys exhibit higher corrosion rates compared with as-cast alloys.Besides,Wang et al.[51]studied the effect of In on the corrosion behavior of Mg-9Al-3Cu alloy,and the result showed that the In addition obviously accelerated the corrosion rates of the alloys,as indicated in Table 2.After adding 2 wt.% In into Mg-9Al-3Cu alloy,the Al con-tent inα-Mg decreased from 5.9 wt.% to 5.1 wt.%,and the average widths of theβ-Mg17Al12phase decreased from 9.4 to 6.1 μm,while its volume fraction increased from 6.1%to 6.8%,weakening the barrier effect and strengthening the galvanic effect simultaneously.Meanwhile,the resistance of surface film(Rf)of equivalent circuit fitted by EIS of Mg-9Al-3Cu-2In alloy disappeared and its open circuit potential(OCP)dropped rapidly after immersion compared with Mg-9Al-3Cu alloy,as shown in Fig.6,which implied that the activation effect of In on the deterioration of the oxide film of the alloys.

Table 2(continued)

Table 2Corrosion rates,tested in 3.0–3.5 wt.% NaCl or KCl solution,and mechanical properties of the degradable Mg-Al series alloys.

Fig.4.Microstructures of Mg-17Al-3Zn-xCu alloys:(a)Mg-17Al-3Zn-1.5Cu,(b)Mg-17Al-3Zn-3Cu and(c)Mg-17Al-3Zn-7Cu;(d)Mg-17Al-3Zn-10Cu[48].

Fig.5.SEM images,surface potential maps,and potential profiles of as-cast Mg-3Cu-xAl alloys:(a–c)Mg-3Cu and(d–f)Mg-3Cu-8Al;(g)XPS spectra of Al 2p of corrosion products of as-cast Mg-3Cu-4Al alloy;(h)Cu and Al contents in α-Mg and(i)volume fraction of the second phases and corrosion rate of Mg-3Cu-xAl alloys with different heat treatment[50].

In order to improve the mechanical properties and corrosion resistance of the alloys used as degradable fracturing balls,RE and Si elements are added into the Mg alloys with high Al and Zn contents.It can be found that RE-containing phases formed and attached around theβ-Mg17Al12to become continuous network structures after adding trace RE elements(Gd,Y and La),weakening the galvanic effect and strengthening the barrier effect simultaneously[52–54].In addition,the passive surface film of the alloys was further enhanced by RE addition,improving the corrosion resistance of the alloys.Besides,the compressive strength of the alloys increased as indicated in Table 2,which was mainly caused by the grain refinement and dispersion strengthening of RE.Furthermore,a similar effect of Si addition on properties of degradable Mg-Al series alloys has been found[55].With increasing the Si content,the volume fraction of Mg2Si phase of Mg-17Al-5ZnxSi alloys increased,and its morphology gradually changed from a fine spherical shape to a coarse polygonal type and eventually to a dendrite crystal,and the distribution and volume fraction of theβ-Mg17Al12phase were also significantly affected,as shown in Fig.7.As a result,the compressive strength of the alloys first increased and then decreased,and the corrosion rate had an inverse variation tendency,as indicated in Table 2.

Although almost studies of degradable Mg alloys are concentrated on composition adjustment,there are some attempts through different processing methods for the low-alloyedMg-Al alloys.Wang et al.[56]proposed a novel Mg-4.5Al-1.5Sn-0.5Ca alloy via sub-rapid solidification(SRS)followed by homogenization treatment,which led to the spheroidized and refined CaMgSn phase,in comparison to its conventional solidification(CS)counterpart.As a result,the ultimate tensile strength and elongation increased from 220 MPa and 21%to 236 MPa and 24%,respectively.Moreover,the homogenized SRS alloy exhibited homogeneous corrosion and an increased corrosion rate due to the large contact area and short distance between the refined CaMgSn particles andα-Mg matrix.Generally,coatings on Mg alloy substrates can provide a hindrance layer to isolate the matrix from corrosive electrolytes,leading to a significant improvement of corrosion resistance without adjusting the microstructure[9,10].However,Liu[97]developed an iron-bearing phosphate chemical conversion(PCC)coating method on the as-extruded AZ31 alloy with a self-catalytic degradation function,and the schematic diagram of accelerating degradation mechanism is given in Fig.8.After dipping in the 3.5 wt.% NaCl solution,the coating was permeated by H2O,Cl–,H+and OH–,leading to the initial degradation of the coating and Mg substrate.Then,Fe2+ions were released and reduced from the coating,making galvanic couples with the Mg substrate and further accelerating the degradation.In addition,a stronger galvanic corrosion effect and more porous corrosion product layer were obtained with a higher Fe concentration,resulting in a higher degradation rate.

Fig.6.Electrochemical test results of Mg-9Al-3Cu(AC9)and Mg-9Al-3Cu-2In(ACI92)alloys in 3.5 wt% NaCl solution at room temperature:(a)Nyquist plot,(b)Bode plot,(c)equivalent circuit of ACI92,(d)equivalent circuit of AC9,(e)OCP[51].

3.2.Mg-Zn series alloys

Zinc(Zn)is another momentous alloying element in Mg alloys with a maximum solid solubility of 6.2 wt.% inα-Mg matrix.As a result,Zn has effects of solid-solution strengthening and age hardening.Furthermore,as one of the most essential nutrients in the human body,Zn is widely used in biodegradable Mg alloys.However,adding Zn to Mg alloys forms Mg-Zn phases,which dramatically weaken the corrosion resistance of the alloys,and the tolerance limit for Zn in biodegradable Mg alloys is 2.5 wt.%[1,58].Conversely,Zn,with respect to degradable application,is favorable element to increase both strength and corrosion rate of Mg alloys.The mechanical properties and corrosion rates of degradable Mg-Zn series alloys under appropriate processing conditions are listed in Table 3.It can be found that the corrosion rate of degradable Mg-Zn series alloys significantly increases by Cu or Ni addition,which is mainly caused by the forma-tion of Cu-or Ni-containing phases with high electrochemical potential,such as MgZnCu,MgZnNi,Mg2Cu and Mg2Ni phases[59–63].In particular,the highest corrosion rate in 3.0 wt.% KCl solution at room temperature is obtained in the Mg-4Zn-4Ni alloy(more than 3000 mm·y–1)among all reported degradable Mg alloys so far,while its compressive strength is relatively low(265 MPa).Subsequently,the effects of heat treatment and hot extrusion on the corrosion behavior and mechanical properties of Mg-4Zn-2Ni alloy are investigated[64].Compared with the as-cast Mg-4Zn-2Ni alloy,thecompressive strength of the as-homogenized alloy markedly increased,while its corrosion rate dramatically decreased,as indicated in Table 3.After hot extrusion,the grain size of the Mg-4Zn-2Ni alloy was obviously refined,and the secondary phase distribution became much more uniform,leading to a significant improvement of mechanical properties.Meanwhile,the corrosion rate of the as-extruded alloys notably increased due to precipitated phase and unDRXed region containing high density dislocations,where the galvanic corrosion preferentially occurred,as shown in Fig.9.In addition,the volume fraction of unDRXed region and precipitated phase decreased at a higher extrusion temperature(250 °C),resulting in a lower degradation rate.Moreover,the properties of different compositions of as-extruded Mg-4Zn-xNi alloys were measured[65].As a result,the as-extruded Mg-4Zn-4Ni alloy had both high strength(585 MPa)and high corrosion rate(912.9 mm·y–1).

Table 3Corrosion rates,tested in 3.0–3.5 wt.% KCl solution,and mechanical properties of the degradable Mg-Zn series alloys.

Fig.7.OM images of Mg-17Al-5Zn-xSi alloys:(a)Mg-17Al-5Zn,(b)Mg-17Al-5Zn-0.5Si,(c)Mg-17Al-5Zn-1Si and(d)Mg-17Al-5Zn-3Si[55].

Fig.8.Schematic diagram of an iron-bearing phosphate chemical conversion coating,which accelerates AZ31 alloy degradation[57].

3.3.Mg-RE series alloys

Adding rare earth(RE)elements to Mg alloys can obtain solid solution strengthening,fine-grain strengthening,precipitation strengthening and dispersion strengthening[66],so as to achieve ultra-high strength combined with heat treatment and deformation[67,68].For example,the Mg-8Gd-3Y-0.4Zr alloy prepared via rotary swaging and aging had yield strength of 650 MPa and ultimate strength of 710 MPa[69].Jia et al.[70]exploited Mg-12Gd-1Er-1Zn-0.9Zr with ultrahigh strength(～549 MPa UTS)and ductility(～8.2% EL)via hot extrusion combined with pre-deformation and two-stage aging treatment.Furthermore,RE elements are generally regarded as beneficial elements for the corrosion resistance of Mg alloys due to their relatively close standard electrode potentials with Mg,grain refinement and more protective corrosion layers[66].However,Mg-RE alloys with both high strength and high degradation rate have also been successfully developed,which are mainly attributed to the formation of Ni-or Cu-containing phases,especially the long period stacking order structure(LPSO)phases.The mechanical properties and corrosion rates of degradable Mg-RE series alloys under appropriate processing conditions are listed in Table 4.It can be found that the corrosion rates of Mg-RE alloys dramatically increased after adding Cu or Ni addition,and the strength of the alloys was obviously enhanced[71–75].Because of the Cu addition,the Mg2Cu phase formed in Mg-2Gd alloy and the LPSO phases of Mg-Y-Zn alloys transformed from 18R-typed Mg12YZn to 14H-typed Mg80Cu15Y5,which acted as active cathode and generated more galvanic coupling and hence accelerated the galvanic corrosion.Meanwhile,the grain size of the alloys was refined with increasing Cu content,resulting in better mechanical properties than that of the counterpart without Cu.Besides,the texture of Mg-2Gd-xCu alloys changed from RE texture of＜111＞//ED to fiber texture of＜100＞//ED with Cu addition.As for as-extruded Mg-10Gd-3Y-0.3Zr-xNi alloys,a large amount of Zr7Ni10phases were precipitated insideα-Mg matrix with 0.2 wt.% Ni addition,resulting in the highest corrosion rate in this study[74].However,the corrosion rate of the alloys obviously decreased with further increasing the Ni contents due to the disappearance of Zr7Ni10phase,while it was stilllarger than that of the counterpart without Ni(as indicated in Table 4)because of the transformation of the secondary phase from Mg5RE to Ni-containing 18R-LPSO phase.Moreover,the strength of these alloys increased due to the strengthening effect of the Ni-containing 18R-LPSO,and the as-extruded Mg-10Gd-3Y-0.3Zr-0.8Ni alloy showed the highest ultimate compressive strength(596.5 MPa)in this study.Zhong et al.[75,76]investigated and compared the different effects of minor Cu and Ni additions on microstructures and properties of the Mg-2Gd alloy,and the schematic diagrams of corrosion progress of the Mg-2Gd-xCu/Ni alloys are given in Fig.10.With the substitution of Cu by Ni,the secondary phase changed from granular Mg2Cu to Ni-containing 18RLPSO phase,and its volume fraction markedly increased from 1.5% to 11.0%.For as-extruded Mg-2Gd-0.5Cu alloy,the Mg2Cu phase acted as cathodic site in micro-galvanic corrosion,henceα-Mg matrix in the vicinity of granular Mg2Cu phases is corroded preferentially,and then Mg2Cu particles exfoliate from the matrix,leading to pit-like peeling marks(Fig.10(c1)).However,the Ni-containing 18R-LPSO phase exhibited lower potential than that of Mg(Fig.11(b,c))and acted as anodic site when coupled with theα-Mg matrix.Therefore,the Ni-containing 18R-LPSO phase was corroded prior to theα-Mg matrix and became channels for the rapidpenetration of corrosive ions into theα-Mg matrix,leading to a higher corrosion rate,as shown in Fig.10(b4,c4).Besides,it is found that adding Ni was more effective in refining grain size and strengthening basal fiber texture than adding Cu,resulting in higher ultimate tensile strength.Therefore,Ni maybe a better alloying element of degradable Mg-RE alloys than Cu.

Table 4Corrosion rates,tested in 3.0–3.5 wt.% NaCl or KCl solution,and mechanical properties of the degradable Mg-RE series alloys.

Table 5Corrosion rates,tested in 3.5 wt.% NaCl solution,and mechanical properties of the degradable Mg-Al-Zn-Fe composites.

Fig.9.Microstructures of the as-extruded Mg-4Zn-2Ni alloys:(a–f)precipitate phase of as-extruded Mg-4Zn-2Ni alloys:(a,d)200 °C,(b,e)250 °C and(c,f)300 °C;(g–l)Corrosion morphologies of as-extruded Mg-4Zn-2Ni alloys after immersing in 3.0 wt.% KCl solution at room temperature and removing corrosion products:Mg-4Zn-2Ni alloy extruded at(g-i)200 °C and(j-l)250 °C;immersion for(g,j)10 s,(h,k)100 s and(i,l)15 min[64].

Furthermore,there are several studies focused on the effects of volume fraction and morphology of secondary phase on mechanical and corrosion properties of degradable Mg-RE series alloys through composition adjustment and heattreatment.With increasing the Ni and RE addition,the volume fraction of secondary phases increased,especially the Ni-containing LPSO phases,and its morphology graduallytransformed from semi-continuous to continuous,leading to increasing compressive strength,while the degradation rate first increased and then decreased due to the dual role of secondary phases on corrosion[78–80].In contrast,the morphology of the Ni-containing LPSO phases of as-extruded Mg-7.2Y-2.8Ni alloy transformed from continuous network bulk-shaped to disordered rod-shaped after annealing,effectively reducing the obstacle to corrosion expansion and resulting in a higher degradation rate,as shown in Fig.12[81].In addition,the morphology of the Ni-containing 18R-LPSO phase of as-cast Mg-11.5Gd-2.2Ni alloy gradually changed from a continuous lamellar to a rod-shaped structure with increasing the solution treatment temperature,and the corrosion rate of the alloys was affected by the microstructure evolution combined with volume fraction of the Ni-containing 18R-LPSO phase[82].A similar result was found in Mg-4Y-2Cu alloy[73],and the corrosion rate of the alloy heattreated at 430 °C enhanced due to the discontinuous morphology,while the lamellar 14H-LPSO phase precipitated in grains and hindered corrosion expansion after aging treatment.When hot extrusion and subsequent aging treatment were conducted,the broken Mg2Cu and the 14H-LPSO phase were uniformly dispersed in the alloys,resulting in a significant increase in corrosion rate.However,the 14H-type lamellar LPSO phase precipitated in Mg-13.2Gd-4.3Ni alloy and acted as a cathodic phase to slightly accelerate the degradation rate after heat treatment at 400 °C for 0.5 h[83].Besides,the bulk-shaped 18R-LPSO phase was delaminated by heat treatment at 480 °C for 8 h,which can effectively reduce the hindering effect on corrosion expansion and lead to a higher degradation rate.It is worth noting that the bulk-shaped 18RLPSO phase acted as cathode during the immersion process for both as-cast and heat-treated alloys,which showed different result with former investigations[74,76,78].As for asextruded Mg-9.5Gd-2.7Y-0.9Zn-0.8Cu-0.4Ni alloy[84],the nano-scaleβ’phase precipitated after aging treatment,which effectively improved the strength and the highest ultimate compressive strength(620.7 MPa)was obtained among all the reported dissoluble Mg alloys so far.However,theβ’phase was corroded preferentially,impeding the dissolution of the Mg matrix and reducing the corrosion rate to a certain extent.

Fig.10.Schematic diagrams of corrosion progress of Mg-2Gd-xCu/Ni alloys.(a1-c1,a2-c2)Mg-2Gd-0.5Cu alloy,(a3-c3,a4-c4)Mg-2Gd-0.25Cu-0.25Ni alloy and Mg-2Gd-0.5Cu alloy;(a1-c1,a3-c3)transverse topographies,and(a2-c2,a4-c4)longitudinal topographies[76].

Fig.11.(a)Surface morphology,(b)surface voltage potential map,(c)line-profile analysis of the relative voltage potential,and(d)enlarged corrosion morphology of the Mg-16.5Gd-2.1Ni alloy after immersion in 3.5 wt.% NaCl solution for 5 s[78].

Fig.12.Microstructures of(a)as-extruded and(b)as-annealed Mg-7.2Y-2.8Ni alloys;Corrosion morphologies of(c)as-extruded and(d)as-annealed Mg-7.2Y-2.8Ni alloys immersion in 3.0 wt.% KCl solution for 50 min[81].

3.4.Mg matrix composites

Although considerable researches have demonstrated that alloying is an effective method to improve the performances of Mg alloys,it also brings a few problems,such as high cost and density,as well as the reduction of thermal conductivity.At present,various reinforcements are widely used in Mg matrix composites to increase the strength,elastic modulus,thermal conductivity and corrosion resistance by combining their superior capacity[85–88].For degradable Mg matrix composites,the reinforcements such as Fe,fly ash cenospheres(FAC),hollow glass microspheres(HGM),graphite and SiC can be used to enhance compressive strength and corrosion rate.The relative data under appropriate processing conditions are listed in Tables 5 and 6.

Zhang et al.[89]investigated the effects of Fe concentration and annealing on the microstructure,mechanical and corrosion properties of Mg-6Al-1Zn-xFe composites,which were prepared by powder metallurgy and hot extrusion.The corrosion rates of the composites increased with increasing in the Fe content,because the majority of Fe existed as insoluble particles in Mg composites(Fig.13)to enhance galvanic corrosion.After annealing,theβ-Mg17Al12phase was partially dissolved inα-Mg matrix,leading to a lower corrosion rate of the composites.In their other studies[90,91],a suitable heat treatment was used to obtain a high volume fraction of secondary phase of the Mg-Al-Zn-Fe composites and the corrosion rate significantly increased.

Yu et al.[92–96]developed numerous degradable Mg composites using stir casting method with FACs or HGMs,which were mainly composed of SiO2as indicated in Table 7.The microstructures of the HGM/Mg composites are shown in Fig.14.With increase in the HGMs addition,the grain of matrix was obviously refined and the Mg2Si phase formed by the interfacial reaction between SiO2and Mg matrix,leading to a higher compressive strength.Besides,a significant percentage of HGMs were added in the HGM/Mg composites(Fig.14(g,h)),resulting in the decrease of density.Meanwhile,as shown in Table 6,the degradation rate of the composites dramatically increased with increasing in FACs and/or HGMs addition,which was due to the formation of more corrosion micro-batteries and the exfoliation of degradation products during the degradation process.Moreover,as for HGM/Mg composites,the corrosion rate increased with increasing in the Ni content,because of the formation of the Al3Ni2phase,which disrupted the local continuity of theβ-Mg17Al12phase and weakened its corrosion barrier effect.

Table 6Weight loss rates,tested in 3.0–3.5 wt.% KCl solution,and ultimate compressive strength of the as-cast degradable Mg matrix composites.

Table 7The chemical composition of FACs and HGMs(wt.%)[92–96].

Liu et al.[97]investigated the influence of graphite content(0–0.5 wt.%)on the microstructure,mechanical and corrosion performance of Mg-Al alloy,and the results showed that theβ-Mg17Al12phase was evenly distributed and gradually transformed into the Al4C3phase with increasing the graphite addition.The grain size of the composites first decreased and then increased,and the strength had an inverse variation tendency.Meanwhile,the weight loss rate and galvanic corrosion rate of the composites gradually increased with increase in the graphite content.Li et al.[98]studied the effect of Zn content on the microstructure,mechanical and corrosion properties of Mg-xZn-Zr-SiC composites.With increasing the Zn addition,the Mg7Zn3phase gradually increased and evolved from short rods to coarsening reticulate shape;at the same time,the surface roughness and wettability of the composites raised(Fig.15),leading to a higher corrosion rate.Moreover,the Mg-xZn-Zr-SiC composites still had relatively high strength after 4 h decomposition test in 3.0 wt.% KCl solution at 90 °C.

3.5.Comparison and discussion

Based on the reported investigations,we summarize the corrosion rate and ultimate compressive strength of the degradable Mg alloys,as shown in Fig.16.It can be found that the as-cast Mg-Al degradable alloys have relatively low corrosion rate and moderate strength.Also,Fe,FACs and HGMs can be used as reinforcements to enhance the corrosion rate of them.The as-cast Mg-Zn and Mg-RE degradable alloys have high corrosion rate but low strength.Although the high corrosion rate is the most important characteristic of degradable fracturing tools,the strength also needs to be considered,which determines the successful fracturing operations.In view of the comprehensive capacities,the asextruded Mg-Zn and Mg-RE series alloys are preferential as degradable fracturing tools,which possess both superior mechanical properties and degradation rate.Furthermore,as disposable tools,the cost of degradable Mg alloys also need to benoticed for large-scale application.Therefore,the as-extruded Mg-Zn alloys may be the best choice for degradable fracturing tools,compared with the as-extruded Mg-RE degradable alloys.

Fig.13.SEM images and EDS analyses of as-extruded Mg-6Al-1Zn-3Fe alloy[89].

According to the above-mentioned research on degradable Mg alloys,it can be found that the high corrosion rate and high strength of degradable Mg-Al and Mg-Zn alloys are considerably relied on the high content of Cu or Ni elements,while the degradable Mg-RE alloys have comparable properties by adding trace Cu or Ni addition but high RE concentration.But in fact,excellent mechanical properties are huge advantage for Mg-RE alloys in itself.The addition of Cu or Ni is helpful to further improve the strength of Mg-RE alloys combined with a high degradation rate.Therefore,degradable Mg-RE alloys are popular research directions in recent years.It is suggested to develop degradable Mg-RE alloys with high Ni and low RE addition to vastly utilize the improvement of Ni to both mechanical and corrosion properties,reducing the cost of the alloys simultaneously.As for degradable Mg-Al and Mg-Zn alloys,high Cu or Ni content is also indispensable.However,Ni is hardly used as an alloying element in degradable Mg-Al alloys at present.The degradation rate of AZ80 alloy was dramatically accelerated to 6360.9 mm·y–1in dynamic 3.5 wt.% NaCl solution at 90 °C by only 0.1 wt.% Ni addition[99].Besides,the compressive strength and corrosion rate of Mg-15Al-6Zn-2Cu-4HGM were remarkably increased by Ni addition[96].Furthermore,the as-cast degradable Mg-Al alloys have relatively high strength with high Al content.Therefore,the as-cast Mg-Al with high Al and Ni addition may obtain prominent comprehensive capacities for low-cost degradable fracturing tools application.For degradable Mg-Zn alloys,the superior mechanical properties and degradation rate were acquired by high Ni addition and hot extrusion.However,there are less related research and more investigations need to be supplemented.Moreover,the effects of heat-treatment and extrusion process on mechanicaland corrosion properties of degradable Mg alloys need further investigation to achieve better performance.

The daughter had enough to do cracking nuts for him, and at the end of fourteen days she had only one tooth left in her mouth; she had broken all the rest with the nuts

Fig.14.Microstructures of the HGM/Mg composites[95].

4.Factors influencing the corrosion rate of degradable Mg alloys

The corrosion behavior of alloys is complex and determined by many factors.Bahmani et al.[100]proposed a formulation of the corrosion rate of Mg alloys using microstructural parameters including composition,grain size and secondary phase,which can be expressed as:

whereCR0is the corrosion rate of alloys related to the composition.bandcare the contributions of grain size and secondary phase to the corrosion rate,respectively,which are simultaneously changed by alloys and the corrosion solution.fA,mandfA,iare the area fractions of matrix and secondary phase,respectively.Dis the grain size.|ΔE|is the absolute value of the Volta-potential difference of matrix relative to secondary phase.Moreover,CR0can be divided intoCRT,CandCRP,C.CRT,Cis associated with the distribution of different orientations and the matrix surface energy,which is changed by the concentration of solute atoms and density of dislocations.CRP,Cis the passivation effect determined by the surface film of alloys.

Although the abovementioned formulation is developed for alloys with high corrosion resistance,it can also be used to explain the high corrosion rate of recent research on degradable Mg alloys.Adding alloying elements and/or reinforcements is an effective method to affect the degradation rate of Mg alloys,which is caused by the variations including secondary phase,crystallographic orientation,grain size andsurface film.In addition,these factors are also changed by heat-treatment and deformation simultaneously.In the following subsections,these factors influencing the degradation rate of Mg alloys are summarized.

Fig.15.(a–d)Surface roughness and(e–h)droplet static contact angles of as-cast Mg-xZn-Zr-SiC composites[98].

Fig.16.Corrosion rate and ultimate compressive strength of partial degradable Mg alloys and composites(corrosion rates were measured by weight loss or hydrogen evolution in 3.0–3.5 wt.% NaCl or KCl solution at room temperature).(For interpretation of the references to color in this figure legend,the reader is referred to the web version of this article.).

4.1.Secondary phase

Almost all studies on degradable Mg alloys are associated with secondary phase,and it is the primary factor influencing the corrosion rate in terms of its constituent,morphology and volume fraction.Generally,the secondary phase has two inverse effects on the corrosion performance of Mg alloys[101].One is the galvanic effect to accelerate the corrosion development between the secondary phase and the matrix,the other is the barrier effect to stop the corrosion expansion from grain to grain(Fig.17).As a consequence,the degradation rate of alloys increases or decreases with increase in the amount of the secondary phase,as a result of the combined effect without consideration of phase transformation[43,44,50,51,54,65,73,78,80,82,89–91,93,98].

In addition,the different effects of various secondary phases on the corrosion behaviors are determined by theirVolta-potential difference with the Mg matrix.Therefore,Cu,Ni and Fe are generally used as alloying elements in degradable Mg alloys due to their tiny solid solubility and high standard electrode potential,and the corrosion rates are dramatically enhanced even with minor addition[43,45–48,59–63,65,71–76,79,89,91,96].Although some attempts are made to improve the mechanical properties by adding Al and RE elements in Mg-Cu and Mg-Al series alloys,the corrosion rate dramatically decreases partially due to the transformation of lower Volta-potential difference secondary phases[49,50,52–54].

Fig.17.Barrier effect of secondary phases in the corrosion of Mg alloys[51,79].

Furthermore,heat-treatment and deformation can also change the secondary phase.Generally,the secondary phase dissolved or precipitated after solution or aging treatment,respectively,resulting in a reduction or acceleration of corrosion rate[50,54,65,73,82,89–91].However,the inverse results are found due to the morphology of the secondary phase,which transforms from a continuous network to a semi-continuous structure,resulting in higher corrosion rate after solution treatment with low temperature[73,82],annealing[81]or SRS[56],while precipitates are formed in grains and hinder corrosion expansion,impeding the dissolution of the Mg matrix and reducing the corrosion rate to a certain extent after aging treatment[73,84].In addition,the distribution of secondary phase becomes more uniform after hot extrusion,resulting in a significant increase of the corrosion rate[64,73].

4.2.Texture and dislocation

Degradable Mg alloys with high strength are mainly obtained by extrusion,which usually have a strong texture and many dislocations,somewhat affecting the corrosion rate of the alloys.Generally,the basal plane of Mg alloys has the best corrosion resistance among all the crystallographic planes because of its lower surface energy and more compact corrosion films[102].In addition,the conventional texture in extruded samples consists of basal planes aligned parallel with the extrusion direction(ED),but the texture can be modified by alloying elements and extrusion parameters.For example,the texture with[111]-[201]double fiber orientations was observed in as-extruded Mg-xGd sheets,and the texture intensity was weaken with increasing Gd content[103].In contrast,the RE texture of Mg-2Gd and GW103K alloys was weakened and the basal planes were parallel to the ED by alloying with trace Cu and Ni elements[71,74].Furthermore,adding Ni was more effective in strengthening basal fiber texture than adding Cu of Mg-2Gd alloy[76],as shown in Fig.18.Therefore,more grains tilted their(0001)in a direction parallel to the ED,as well as more(100)and(110)atomic planes were exposed to corrosive medium in Ni-containing alloys(Fig.10(a4)),resulting in a higher corrosion rate.

4.3.Grain size and surface film

Grain size is a significant factor that influences the mechanical properties and corrosion behavior of Mg alloys,and it can be effectively changed by alloying,heat-treatment and deformation.However,the effects of grain size on the corrosion behavior of Mg alloys are controversial,because it is difficult to isolate the influence of grain size from other microstructural parameters such as secondary phase,orientation and dislocation density in many cases.According to the research of Bahmani et al.[100],the grain size may had two different effects on the corrosion rate of Mg alloys.One was accelerating corrosion in very large grain alloys or in nonpassivating solutions,the other was decelerating corrosion in alloys with fine and ultrafine grains by providing coherent and uniform passive films or in passivating solutions.Nevertheless,the effect of grain size on the corrosion rate of degradable Mg alloys is relatively slight,and few studies have concentrated on it.Compared with the as-cast Mg-4Zn-xNi alloys,the corrosion rates of the as-extruded alloys with the same composition were dramatically decreased,which mightbe associated with the decrease in grain size after extrusion to some extent[63–65].

Fig.18.(a–c)IPF with the reference of the ED and(d)the{0002},{110},and{100}PF of Mg-Gd-Cu/Ni alloys[75].

Fig.19.The local misorientation maps of Mg-4Zn-2Ni alloys extruded at different temperatures:(a)200 °C and(b)300 °C[64].

As mentioned above,one of the main reasons for the low corrosion resistance of Mg alloys is their porous oxide films,so the corrosion rate of Mg alloys can be effectively influenced by the chemical composition and protectiveness of the surface film,which can be evaluated by energy disperse spectroscopy(EDS),X-ray photoelectron spectroscopy(XPS)and electrochemical impedance spectroscopy(EIS).After adding Al into Mg-Cu alloys,the Al 2p spectrum of the corrosion product had been detected,which was ascribed to the presence of denser Al(OH)3or Al2O3films[50].Meanwhile,the resistance of the surface film(Rf)of equivalent circuits fitted by EIS increased and its capacitance(Cf)decreased with increase in the Al content,implying the improvement of the protective corrosion product layer and leading to a higher corrosion re-sistance to some extent.Similarly,the Gd 4d spectrums and Y 3d spectrums of the corrosion product of degradable Mg-RE series alloys were observed[78,79,84],perhaps accounting for the decreasing corrosion rate of degradable Mg-Al series alloys with RE addition[52–54].On the other hand,the surface films of Mg alloys were deteriorated by adding Cu,Ni,In and HGM(Fig.20),resulting in an accelerated degradation rate[45,46,51,62,63,65,73–76,78,79,95,96].

Fig.20.Corrosion morphologies of(a)Mg-4Y-2Zn,(b)Mg-4Y-1Zn-1Cu,(c)Mg-4Y-2Cu[73]and(d)cross-section morphologies of Mg-2Gd-0.5Ni alloy[76].

5.Summary and outlook

Degradable Mg alloys are ideal alternative materials for fracturing tools and can be quickly dissolved after fracturing operations due to their poor corrosion resistance.Researching on degradable Ma alloys is becoming an emerging focus of new functional Mg materials over the past decade.In this review,the performance requirements of degradable fracturing tools are briefly introduced,and recent progress of degradable Mg alloys and the factors affecting their degradation rate are systematically summarized.

The following comments are suggested:

(1)Recent progress of degradable Mg alloys has focused on Mg-Al,Mg-Zn,and Mg-RE series alloys and Mg matrix composites.Among them,the as-extruded Mg-Zn and Mg-RE series alloys possess both superior mechanical properties and degradation rates.Considering the comprehensive capacities as well as economics,it is suggested to obtain degradable Mg-Zn alloys with high corrosion rate and high strength by extrusion.

(2)The secondary phase is the primary factor influencing the corrosion rate of degradable Mg alloys.Adding Ni or Cu is a common and effective method to enhance the degradation rate due to their tiny solid solubility in Mg matrix and high standard electrode potential.For the asextruded degradable Mg alloys,grain size,texture and dislocation are the other factors affecting the corrosion rate under different processing conditions.In addition,the effect of composition elements on the surface film of materials also needs to be considered.

(3)The performance requirements of degradable fracturing tools are still not clear,and further development and supplement are necessary.

Declaration of Competing Interest

None.

Acknowledgments

The work is supported by the National Key Research and Development Program of China(2021YFB3701100).