Corrosion Science 54 (2012) 270–277

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Corrosion Science

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On the corrosion behaviour of newly developed biodegradable Mg-basedmetal matrix composites produced by in situ reaction

Ting Lei a,⇑, Wei Tang a, Shu-Hua Cai a, Fang-Fang Feng a, Nian-Feng Li a,b

a State Key Laboratory of Powder Metallurgy, Central South University, Changsha 410083, Chinab Xiangya Hospital, Central South University, Changsha 410008, China

a r t i c l e i n f o a b s t r a c t

Article history:Received 16 July 2011Accepted 20 September 2011Available online 28 September 2011

Keywords:A. Metal matrix compositesA. MagnesiumB. EISB. Weight lossC. Anodic dissolution

0010-938X/$ - see front matter � 2011 Elsevier Ltd.doi:10.1016/j.corsci.2011.09.027

⇑ Corresponding author. Fax: +86 731 88710855.E-mail address: tlei@mail.csu.edu.cn (T. Lei).

Biodegradable Mg-based metal matrix composite (Mg-MMC) reinforced by MgO ceramics and Mg–Znintermetallics were prepared by in situ reaction using a powder mixture of pure magnesium and20 wt% ZnO as raw materials. The corrosion behaviour of Mg-MMC was evaluated by electrochemicalmeasurements and immersion tests in Hanks’ solution. Results show that the newly developed Mg-MMC is composed of a-Mg matrix and uniformly distributed MgO ceramic and Mg–Zn intermetallicsin matrix as reinforcements. The Mg-MMC possesses a corrosion behaviour comparable to pure magne-sium and exhibits enhanced improvement in mechanical properties and corrosion resistance.

� 2011 Elsevier Ltd. All rights reserved.

1. Introduction

Magnesium alloys have attracted great attention as an orthope-dic biodegradable implant material due to their close mechanicalproperties to natural bone and perfect biocompatibility [1,2]. How-ever, Magnesium is active (�2.37 V(SHE)) [3], and the currentlydeveloped magnesium alloys are extremely susceptible to galvaniccorrosion, leading to losses of strength and toughness, and thusretarding its practical application as implant biomaterials [4].Therefore, there is a high demand to design magnesium alloys withcontrollable corrosion rates and suitable mechanical properties [5].A number of recent studies have well documented the corrosionbehaviour of Mg alloys for biomedical applications [6–9].

Recently, Mg-based metal matrix composites (MMCs) gained anincreasing interest as promising biomaterials based on their higherspecific stiffness, strength and minimised sensibility to galvaniccorrosion [10]. Witte et al. [11] firstly reported a MMC made ofAZ91D as a matrix and hydroxyapatite (HA) particles as reinforce-ments and found the AZ91D/HA composites were cytocompatiblebiomaterials with adjustable mechanical and corrosive properties.Similar findings were also reported on magnesium–fluorapatitenanocomposites and ZM61-HA composites [12,13]. However, frombiodegradable point of view, these Mg/HA composites are not fulldegradable materials due to the very low solubility of HA in vivo

All rights reserved.

[14], which has detrimental effect especially when it will be usedas tissue engineering scaffolds.

MgO is one of the main constituents of bioglass and has antibac-terial properties [15,16]. Secondly, MgO is degradable because itmay transform to Mg(OH)2 by hydration and further decomposeto Mg2+ [17], the same decomposed product as pure Mg in aqueousmedium. Gupta and co-workers [18] reported an improvement inmacrohardness, yield and tensile strengths of MgO nanoparticlesreinforced Mg/MgO composites. These increased mechanical char-acteristics are requisite and suitable for a biomaterial for load bear-ing applications. Accordingly, the combination of metallic and MgOwould be a promising approach to fabricate a metal matrixcomposite as biomaterials capable of full degradation, wherein,addition of MgO to Mg matrix may concurrently enhance itsmechanical properties as compared to pure Mg matrix.

In spite of the improvement in mechanical properties of Mg-based metal matrix composite, some recent studies on corrosionbehaviour indicate that reinforcements like carbon nanotubes andSiC particles deteriorated the corrosion resistance of magnesiumcomposite due to galvanic corrosion [19–22]. Neubert and co-work-ers [23] reported a reduction in the corrosion resistance of Al2O3 fi-bres strengthened magnesium composite in NaCl solution, and yetno evidence of galvanic corrosion between the fibres and matrixwas observed. To date, there are few studies on corrosion behaviourof magnesium composite materials reinforced by MgO ceramic. Inthis work, we attempt to fabricate Mg-based metal matrix compos-ite (Mg-MMC) by in situ reaction sintering technique, wherein Mgpowder is allowed to react with ZnO powder at a temperature be-low the melting point of Mg to form MgO and Mg–Zn intermetallic

20 30 40 50 60 70 80

38 39 40 41 42

BInte

nsity

(a.u

.)

Angle (2θ )

Mg MgO MgZn Mg2Zn3Mg7Zn3

A Pure Mg

A

B Mg-MMC

Fig. 1. X-ray Diffraction patterns of pure Mg and Mg-MMC.

T. Lei et al. / Corrosion Science 54 (2012) 270–277 271

phases as reinforcements to toughen the Mg-based matrix. Thus, inrelation to the preparation of Mg-MMC, the mechanical propertiesand corrosion behaviour of composites are evaluated.

2. Experimental

Magnesium powders of 99.5% purity and 100 lm in diameterwere used as the matrix material, and nano-size ZnO particles of99.5% purity with a mean diameter of 36 nm were used as the rein-forcement. The synthesis of Mg-based metal matrix composite in-volved ball-milling a powder mixture of Mg and 20 wt.% ZnO andsubsequent hot-press sintering from room temperature to 550 �Cat a rate of 10 �C per minute under 30 MPa for 4 h in a graphite cru-cible to form 30-mm-diameter discs with a thickness of 65 mm.Thus, in this paper the as-prepared magnesium composite isnamed as Mg-MMC for easy description. The whole processesincluding raw materials weighing, mixing, die charging and sinter-ing were conducted under highly pure argon atmosphere. Cylindri-cal specimens with a diameter of 13.5 mm and a height of 5 mmwere machined by linear cutting and ground with SiC emery pa-pers up to 2000 grit, and successively polished with 1 lm diamondpaste, then ultrasonically cleaned in pure ethanol and dried underan air pressure stream prior to experiments. Pure Mg specimenswere made in the same way as counterpart. Note that the prepara-tion of composites from a powder mixture of Mg and 5 wt.% ZnO or10 wt.% ZnO was also performed, respectively. However, the as-prepared composite with 20 wt.% ZnO as a raw material exhibitsoptimisation performances, and thus it is discussed hereafter.

Electrochemical measurements and immersion tests were car-ried out in simulated body fluid [24] (containing NaCl 8.0 g/L, KCl0.4 g/L, CaCl2 0.14 g/L, NaHCO3 0.35 g/L, D-C6H6O6 0.35 g/L,MgSO4�7H2O 0.2 g/L, KH2PO4 0.1 g/L, Na2HPO4�12H2O 0.06 g/L).The pH value of the solution was adjusted to 7.4 at 37 ± 0.5 �C with1.0 mol/L HCl and tris (hydroxymethyl) aminomethane (CH2OH)3

CNH2 solution. Immersion tests were carried out in accordance withASTMG31-72 [25] (the ratio of surface area to solution volume was1 cm2:20 ml). The pH value of the solution was recorded during theimmersion tests (PHS-3C pH meter, Lei-ci, Shanghai). The weightlosses of the specimens were obtained by immersion test. Afterthe immersion of 5 days, the samples were removed from the solu-tion and cleaned with chromate acid (200 g/L CrO3 + 10 g/L AgNO3)for 5 min to remove surface corrosion products without removingany amount of metallic Mg [26]. Then the samples were rinsed withdistilled water, cleaned ultrasonically in acetone, and dried in air.The dried specimens were weighed and the corrosion rate (CR) canbe calculated by Eq. (1) [25]:

CR ¼DmAt

ð1Þ

where CR is the corrosion rate in mg cm�2 h�1, Dm is the weight lossin mg, A is the original surface area exposed to the corrosive mediain cm2, and t is the immersion time in hours.

The Archimedes’ principle was used to measure the density ofMg/MgO composite and an average of three readings was takenfor each reported density. A dog-bone specimen with a gaugelength of 20 mm, a thickness of 2 mm and a width of 6 mm wasmachined for tensile test [27]. Tensile strength was tested on an In-stron3369 materials testing machine at a displacement rate of2 mm min�1. Three parallel specimens were taken for each groupin the tensile test. An extension meter with a gauge length of10 mm was used to measure the elongation. Brinell hardness mea-surements were performed using a Brinell hardness tester (HB-RNu-1875) with a steel ball indenter of 2.5 mm in diameter underthe load of 612.5 N and maintaining for 30 s.

Electrochemical measurements were performed on CHI-660Celectrochemical workstation with a three-electrode system

comprising the as-cleaned magnesium alloy slice as working elec-trode by sealing in a Teflon jacket with an exposed geometric areaof 1 cm2, a platinum wire as auxiliary electrode, and a saturatedcalomel electrode (SCE) as reference electrode.

Before the measurements, open circuit potential Eocp was testeduntil it was stabilized. Potentiodynamic polarisation curves wereobtained in the potential range of Eocp ± 200 mV at a scan rate of2 mV s�1. Electrochemical impedance spectroscopy (EIS) measure-ments were performed at Eocp with the scan frequency ranged from100 kHz to 0.01 Hz, and with the perturbation amplitude of 5 mV.

For the characterisation of the sample morphology and compo-sition a field-emission scanning electron microscope HitachiFE-SEM S4800 equipped with an Energy dispersive X-ray (EDX)analyser was used (the sample was sputtered with Au before thetest). The phase composition of the coatings was analysed byX-ray diffractometry (XRD: D/MAX-255) with the Cu-Ka1 radia-tion (wavelength k = 1.5406 Å). The tube voltage and the tube elec-tric current of XRD were 40 kW and 250 mA, respectively.

3. Results and discussion

3.1. In situ reaction process and microstructure of Mg-MMC

When the well-mixed powder mixture of Mg and ZnO by ballmilling was subjected to sintering in an argon atmosphere at thetemperature of 550 �C, two possible reactions would take placeas described as follows [28]:

MgðsÞ þ ZnOðsÞ ¼MgOðsÞ þ ZnðlÞ ð3Þ

MgðsÞ þ ZnðlÞ !Mg—ZnðlÞ ð4Þ

where the subscripts s and l indicate a solid and liquid state, respec-tively. As a result, ZnO in the powder mixture may react with thenearby magnesium granules, leading to the oxidation of Mg intoMgO accompanied by in situ reduction of ZnO to Zn(l). Simulta-neously, as long as Zn(l) is formed, the newly formed Zn(l) may reactwith Mg to produce Mg–Zn(l) intermetallic. Afterwards, during thecooling process from 550 �C to room temperature, the single Mg–Zn(l) phase will conduct the reaction as described by Eq. (5), givingrise to multi-intermetallics with different atomic ratio of Mg to Zn[28].

Mg� ZnðlÞ !MgðsÞ þMgxZnyðsÞ ð5Þ

272 T. Lei et al. / Corrosion Science 54 (2012) 270–277

The X-ray diffraction (XRD) patterns of pure Mg and Mg-MMCare shown in Fig. 1. As could be seen in Fig. 1, only peaks corre-sponding to a-Mg matrix phases were found in the XRD patternof pure Mg sample. For the sample of Mg-MMC, peaks related toa-Mg, MgO and Mg–Zn multi-intermetallics including Mg7Zn3,MgZn and Mg2Zn3 were identified. In addition, no diffraction peakscorresponding to that of ZnO or Zn are found. This suggested thatin situ chemical reaction had taken place during sintering processat 550 �C, leading to the oxidation of Mg to MgO and completedreduction of ZnO to Zn. Subsequently, the in situ formed Zn mayreact with Mg to produce Mg–Zn intermetallic compounds as con-firmed by the multi-intermetallic peaks as shown by the enlargedinset in Fig. 1. Such a result is consistent with the findings reportedin Ref. [28].

The SEM morphology is shown in Fig. 2a. As could be seen, theMg-MMC was composed of the dark background and the evenlydistributed gray interfacial boundaries surrounding it. The EDXanalyses of the assigned area as shown in Fig. 2b and c indicate thatthe dark background is mainly Mg and these boundaries containmost of the Mg, O and Zn elements. In terms of the EDX and XRDanalysis in Fig. 1, it is reasonable to conclude that the dark back-ground is a-Mg matrix and those along and/or adjacent to the a-Mg dendrite boundaries in a continuous manner are MgO andMg–Zn intermetallics. It is worthy to note that this homogenousand continuous distribution of the second phases is in contrast toagglomerated apatite in Mg–HA composites [11–13].

Fig. 3 shows a line scan EDX analysis from boundary region toMg matrix, and atomic percentage of elements along the scan

Fig. 2. (a) SEM micrograph of the surface morphology of Mg-MMC; (b) EDS analysis co

direction were depicted in Fig. 3b. Five sites are chosen and markedfrom left to right as A, B, C, D and E, respectively. It could be seenthat from site A to site E, the atomic percentage of Zn decreasesprogressively, while that of Mg was in an opposite trend, and max-imum O is observed at site C. The atomic ratio of Zn to Mg in site Aapproached 7:2, likely attributing to Mg2Zn3 phase, while theatomic ratio of O to Mg was almost 1:1 in site C, indicative of thepredominant presence of MgO. Based on the same calculation,there probably existed Mg2Zn3 and MgZn phases from site B to siteC, Mg7Zn3 and a-Mg phases from site C to site D, and MgZn and a-Mg phases from site D to site E, respectively. In brief, the constitu-ents in the interfacial boundary region are mainly composed ofMgO and MgxZny phases including Mg7Zn3, MgZn and Mg2Zn3,which are consistent with the previous XRD analysis. It is worthyto note there is trace of O atoms from site A to site B and from siteD to site F. This was probably due to the oxidation of the sampleson the surface in pretreatment process.

The microstructure of the polished surface of pure Mg and thecomposite was imaged using backscattered electron mode inSEM to reveal compositional contrast, as seen in Fig. 4a and b.Fig. 4c presents the SEM surface appearance of the corroded com-posite by chromic acid. Obviously, there was only single a-Mgphase present for pure Mg specimen as illustrated in Fig. 4a. It isclearly depicted from Fig. 4b that the composite was mainly com-posed of Mg matrix and the evenly distributed MgO and Mg–Znintermetallics in the matrix. The enlarged inset morphology re-vealed that this second boundary phase contained compact andfine heterogeneous particles. After corroded for a short time by

rresponding to assigned A area; (c) EDS analysis corresponding to assigned B area.

Fig. 3. (a) SEM morphology of Mg-MMC, on which a line scan EDS analysis from boundary region to Mg matrix was performed; (b) The average atomic percentages of Mg, Oand Zn in the locations labelled A–E in the micrograph of (a).

Fig. 4. SEM micrograph of the surface morphology (a) BSE-mode SEM image of pure Mg; (b) BSE-mode SEM image of Mg-MMC and (inset) the second boundary phase, thelocation of which is denoted by the arrow (c) corroded Mg-MMC surface by chromic acid and (inset) the second boundary phase, the location of which is denoted by thearrow.

T. Lei et al. / Corrosion Science 54 (2012) 270–277 273

10.0

10.5

274 T. Lei et al. / Corrosion Science 54 (2012) 270–277

chromic acid, some cracks and voids are observed in the boundaryphases as shown by the inset in Fig. 4c. These defects are likely dueto the detachment of MgO that are located in the grain boundaryregions.

0 2 4 6 8 107.0

7.5

8.0

8.5

9.0

9.5

Immersion time (h)

pH v

alue

pure Mg Mg-MMC

Fig. 5. The pH value of SBF as a function of immersion time for pure Mg and Mg-MMC.

3.2. Mechanical properties of composites

Hardness measurements were performed on composite sam-ples and each hardness value reported was an average of five mea-surements taken randomly from different positions of the sample.Table 1 summarises the density and mechanical properties of pureMg and Mg-MMC. It should be noted that the densities of pure Mgand magnesium composite were rather close to that of naturalbones [29]. The overall average hardness of composite is 66.2 HB,increased by 30% as compared to pure Mg. The tensile strengthof magnesium composite increased to 131.8 from 60.1 MPa forpure Mg. Apparently, the evenly distributed MgO and Mg–Zn inter-metallics in Mg matrix as reinforcements greatly enhanced thetensile strength of composite. Likely, the decreased grain size ofMg matrix and strengthening effect of uniformly distributed MgOand Mg–Zn intermetallics are the most important reason for the in-crease of the tensile strength of Mg-MMC. However, the tensilestrength of Mg-MMC is lower than that of reported Mg–HA com-posites, wherein, 264.3 and 137 MPa for AZ91D/HA and Mg/15HAcomposites were reported [11,12], respectively. On the other hand,compared to pure Mg, the elongation of Mg-MMC was significantlyreduced to 6.0%. Thus, it is reasonable to deduce that MgO and Mg–Zn intermetallics have deleterious effect on the ductility of Mgcomposites.

3.3. Immersion tests

Fig. 5 shows the pH variation of SBF as a function of immersiontime. It can be seen that the pH values of the solution correspond-ing to pure Mg specimens increased rapidly from 7.2 to 10.1 in theinitial 4 h of immersion. Afterwards, the pH value of SBF increasedslowly with immersion time and became stabilized. In contrast, thepH values of the solution corresponding to Mg-MMC specimens in-creased much slowly and reached 9.6 in the initial 4 h of immer-sion. This implies that the composite exhibited a relativelyslower corrosion behaviour as compared to pure Mg material,probably due to the embedding of boundary phases of MgO andMg–Zn intermetallics in Mg matrix, leading to structural changes.

The corrosion behaviours of pure Mg and Mg-MMC specimenswere also evaluated by immersion test in SBF. Fewer hydrogenbubbles appeared on the surface of the composite during the firstseveral minutes. On the other hand, at the beginning of immersion,large numbers of hydrogen bubbles are evidently observed arisingfrom the surface of the pure Mg substrate due to the reaction of thesubstrate with the corrosive electrolyte. Secondly, during thewhole immersion time, rapid generation of bubbles from pureMg substrate is observed, indicative of a fast hydrogen evolutionrate. After 5 days of immersion, amount of detached corrosionproducts were observed on the bottom of the beaker for both pureMg and Mg-MMC specimens.

The average weight changes of pure Mg and Mg-MMC after5 days of immersion in SBF were depicted in Fig. 6. The weight loss

Table 1Density and mechanical properties of pure Mg and Mg-MMC composite.

Material Density (g/cm3) Tensile strengt

Pure Mg 1.718 60.1Mg-MMC 1.966 131.8Natural bone [29] 1.75 50–172

is an indication that there was corrosion attack on all specimens.The maximum weight loss of 0.95 mg cm�2 h�1 was observed forpure Mg specimens, which is nearly four times that of Mg-MMC(0.25 mg cm�2 h�1). Thereby, it led to the conclusion that Mg-MMC with MgO and Mg–Zn intermetallics as reinforcementsexhibited advantage of corrosion resistance over pure Mg.

Generally, corrosion behaviours of Mg and its alloy are corre-lated to their microstructures and corrosion starts on the Mg ma-trix phase. Mg is rather active in aqueous medium and dissolvesaccording to the following reaction [30]:

MgþHþ þH2O!Mg2þ þ OH� þH2 ð6Þ

Consequently, in the early stage of immersion, the dissolution ofmagnesium consumes H+, but releases OH�, leading to the increaseof pH value of SBF [27], and thus the change of pH value withimmersion time could be used to evaluate the corrosion behaviourof magnesium. With the accumulation of Mg2+ in SBF, Mg2+ wouldreact with OH� to form Mg(OH)2 which may precipitate on samplesurface when reaching to its saturation. With increasing immer-sion time, the newly formed Mg2+, OH� and Mg(OH)2 precipitateswould reach a dynamic equilibrium, leading to a relative stablepH in SBF. Meanwhile, for Mg-MMC, there might be accompaniedby the hydration reaction of MgO, leading to additional formationof Mg(OH)2 precipitations. The Mg(OH)2 has a hexagonal crystalstructure with twice the increase in volume compared to cubicMgO, thus causing cracks easily. Fig. 7a reveals the corrodedappearance of Mg-MMC sample after 5 days of immersion in SBF.Cracks and thick porous deposit layer resulting from corrosionare observed on the sample surface. EDX analysis correspondingto the porous flower-like layer as illustrated in Fig. 7b quantifiesthe atomic ratio of Mg and O is close to 1:2, indicative of the pres-ence of Mg(OH)2. As a result, the deposition of a substantialamount of Mg(OH)2 precipitates on the composite surface mayact as a protective film for the composite and prevent it from indirect contact with corrosion medium. Therefore, the effectively

h (MPa) Elongation (%) Surface hardness HB

7.43 49.66.0 66.22.10 –

0.0

0.2

0.4

0.6

0.8

1.0

1.2

pure Mg Mg-MMC

aver

age

wei

ght l

oss

(mg/

cm2 /h

)

0.25

0.95

Mg-MMC

pure Mgmg/cm2/h

Fig. 6. Average corrosion rates determined from mass loss testing for pure Mg andMg-MMC.

-1.7 -1.6 -1.5 -1.4 -1.3 -1.2-7

-6

-5

-4

-3

-2

b Mg-MMC

b

log

Cur

rent

Den

sity

(A/c

m2 )

Potential (V vs.SCE)

a pure Mg

a

Fig. 8. Potentiodynamic polarisation curves of (a) pure Mg and (b) Mg-MMC in SBF.

T. Lei et al. / Corrosion Science 54 (2012) 270–277 275

protective effect of the corrosion film on composite surfaces slo-wed down the hydrogen evolution rate and retarded further degra-dation as compared to pure Mg.

3.4. Electrochemical corrosion measurements

The corrosion resistance of pure Mg and Mg-MMC was furtherdetermined in SBF solution using potentiodynamic polarisationtests as shown in Fig. 8. In general, the cathodic polarisation curveis attributed to hydrogen evolution reaction due to the reduction ofwater, while the anodic polarisation curve is associated with thedissolution of Mg, leading to the formation of Mg2+ [31]. Normally,for Tafel curves, the more positive the corrosion potential alongwith much lower the corrosion current, the more slower wouldbe the corrosion rate, in other words, the corrosion resistance isbetter. It was found that the Tafel curve for Mg-MMC exhibits acorrosion potential at �1.475 V(SCE), which is positively shiftedabout 51 mVSCE relative to pure Mg with corrosion potential at�1.526 V(SCE). Meanwhile, the corrosion current of Mg-MMCwas 95.02 lA cm�2, approximately six times lower than that ofpure Mg (549.2 lA cm�2). The polarisation test results lead directlyto the conclusion that the corrosion resistance of Mg-MMC in SBF

Fig. 7. (a) SEM microstructure of corroded appearance of Mg-MMC after 5 days of im

corrosive medium was better than that of pure Mg, which was ingood agreement with the results of immersion tests.

Electrochemical impedance spectra (EIS) measurements forpure Mg and Mg-MMC were carried out at open circuit potential(Eocp) as illustrated in Fig. 9a. It could be seen that all these imped-ance diagrams are characteristic of one well-defined capacitiveloop at high frequency followed by an inductive loop in the lowerfrequency domain, implying that the composite possesses a corro-sion behaviour comparable to pure Mg. In general, the high fre-quency capacitive loop could be attributed to the relaxationprocess of electrochemical reaction impedance corresponding tothe dissolution of Mg and electric double layer capacitance (Cd1)at the electrode/electrolyte interface [32]. Thereby, the electrodereaction process correlated to high frequency capacitive loop canbe described by a parallel circuit of Cd1 and charge transfer resis-tance Rt [32]. The presence of an inductive loop in low frequencyregion is often observed on the impedance spectra of magnesium[33–35]. It is widely believed that low frequency inductive loopmay arise from the partial protection of the surface oxide film thatis in relation to the surface layer and precipitation formed by an-ode dissolution [35,36]. Moreover, impedance diagrams of com-posite obtained at Eocp for 30 min, 5 and 10 h of immersion inSBF show that high-frequency capacitive loop and low-frequency

mersion in SBF (b) EDX analysis corresponding to the porous flower-like layer.

-50 0 50 100 150 200 250 300 350 400 450 500

-50

0

50

100

150

200

Mg-MMC(10h)

150.557

Mg-MMCpure Mg

Z'' (

ohm

cm

2 )

Z' (ohm cm2)

pure Mg (30 min) Mg-MMC (30 min) Mg-MMC (5 h) Mg-MMC (10 h)

322.8Mg-MMC(5h)

Rt (Ω cm2)

478.5

a

b

Fig. 9. (a) Nyquist plots for pure Mg and Mg-MMC after soaking in SBF for differenttime and (b) Equivalent circuit used for modelling experimental EIS data of Mg-MMC.

276 T. Lei et al. / Corrosion Science 54 (2012) 270–277

inductive loop get larger with the increase in immersion time. TheNyquist plots of composite can be simply interpreted using theequivalent circuit shown in Fig. 9b, where Rs is the electrolyteresistance, Cd1 is the double layer capacity, Rt and RL is the chargetransfer resistance of the interfacial reaction and low frequencyloop resistance, respectively, L is the inductance.

The diameter of the semicircle gives the charge-transfer resis-tance (Rt) at the electrode/electrolyte interface. From Rt value, theexchange-current density (j0) could be calculated using the follow-ing expression [36]:

j0 ¼RT

nFRtð2Þ

where n is the number of transferred charges, F is Faraday constant.Apparently, j0 is in inverse proportion to Rt, in other words, thehigher the Rt is, the lower would be the corrosion rate [36]. Conse-quently, charge transfer resistance could be used to evaluate thecorrosion property of the composite. This is because an increasein j0 should correspond to an increase in the corrosion rate. It canbe deduced from Nyquist plots that Rt of Mg-MMC at 30 min ofimmersion was 150.5 X cm2, increased by 200% as compared to thatof pure Mg, and further increased to 322.8 and 478.5 X cm2 after 5and 10 h of immersion, suggesting that the composite is more cor-rosion resistant than pure Mg, which is in good agreement with theresults of immersion tests and polarisation measurements. The in-crease of Rt values observed after a longer immersion time is likelydue to the formation of a stable and protective Mg oxide or hydrox-ide layer, which may become increasingly resistant and insulatingas a function of immersion time. Such a claim is supported by lowfrequency inductive loop observed on Nyquist plot [32,35,37].

From the conductive point of view, MgO is a nonconductor withdielectric constant of 9.65 [38], making the built up of micro-galvanic cell difficult between MgO phases and Mg matrix. Besides,

the dispersion and coexistence of MgO and Mg–Zn intermetallicsmay play a grain refining role, which separated the Mg matrix intoisolated fine grains relative to pure Mg matrix as seen in Fig4. Sucha grain size refinement might be favourable to the improvement incorrosion resistance. Previous study has demonstrated the reduc-tion of corrosion rate of AZ31 magnesium alloy by grain size refine-ment [35]. Furthermore, the uniformly and consecutivelydistributed MgO and Mg–Zn intermetallics in the boundary regionmay also serve as a barrier to electron transfer from one single Mgphase to another, and thus an extra resistance between a-Mgphase and the second phases decreases the effective driving forcefor micro-galvanic corrosion, and therefore restrains the corrosionreaction of Mg matrix and decreases the corrosion rate.

4. Conclusions

Mg-MMC reinforced by MgO ceramics and Mg–Zn intermetal-lics was fabricated by in situ sintering reaction. The in situ reactionprocess between Mg and ZnO powders can be simply described asfollows: Mg powder reacts with ZnO powder at 550 �C, a tempera-ture below the melting point of Mg, to form MgO accompanied byin situ reduction of ZnO to Zn. The newly formed Zn reacts contin-ually with Mg to produce Mg–Zn intermetallic. In the cooling downprocess to room temperature, Mg–Zn intermetallic may transformto multi-phases including Mg7Zn3, MgZn and Mg2Zn3. The Mg-MMC exhibits significant improvement in both mechanical proper-ties and corrosion resistance relative to pure Mg due to grainrefinement strengthening and the second phase strengthening ofthe uniform and consecutive dispersion of MgO and Mg–Zn inter-metallics as reinforcements in Mg matrix. Consequently, theMg-MMC might be a promising candidate as full degradable bio-medical materials.

Acknowledgements

This work was supported by National Natural Science Founda-tion of China (Grant No. 51021063), National Science Fund for Dis-tinguished Young Scholars (Grant No. 50825102) and Open Projectof State Key Laboratory for Powder Metallurgy of CSU.

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