Materials Science and Engineering C 32 (2012) 2570–2577

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Materials Science and Engineering C

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Effects of Zn on microstructure, mechanical properties and corrosionbehavior of Mg–Zn alloys

Shuhua Cai a, Ting Lei a,⁎, Nianfeng Li a,b, Fangfang Feng a

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

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

0928-4931/$ – see front matter © 2012 Elsevier B.V. Allhttp://dx.doi.org/10.1016/j.msec.2012.07.042

a b s t r a c t

a r t i c l e i n f o

Article history:Received 3 November 2011Received in revised form 3 July 2012Accepted 27 July 2012Available online 3 August 2012

Keywords:MagnesiumBiomaterialsElectrochemical impedance spectraCorrosion propertiesMechanical property

In this study, binary Mg–Zn alloys were fabricated with high-purity rawmaterials and by a clean melting pro-cess. The effects of Zn on the microstructure, mechanical property and corrosion behavior of the as-cast Mg–Zn alloys were studied using direct observations, tensile testing, immersion tests and electrochemical evalu-ations. Results indicate that the microstructure of Mg–Zn alloys typically consists of primary α-Mg matrixand MgZn intermetallic phase mainly distributed along grain boundary. The improvement in mechanical per-formances for Mg–Zn alloys with Zn content until 5% of weight is corresponding to fine grain strengthening,solid solution strengthening and second phase strengthening. Polarization test has shown the beneficial ef-fect of Zn element on the formation of a protective film on the surface of alloys. Mg–5Zn alloy exhibits thebest anti-corrosion property. However, further increase of Zn content until 7% of weight deteriorates the cor-rosion rate which is driven by galvanic couple effect.

© 2012 Elsevier B.V. All rights reserved.

1. Introduction

Magnesium alloys have attracted great attention as an orthopedicbiodegradable implant material due to their perfect biocompatibilityand close mechanical properties to natural bone [1]. However, mag-nesium alloys are extremely susceptible to corrosion, leading to lossesof strength and toughness, and thus limiting their practical applica-tion as implant biomaterials [2]. Element alloying is one of the mosteffective methods to improve the corrosion resistance and mechani-cal properties of magnesium.

To date, most of the reported biomedical magnesium alloys con-tain aluminum and/or rare earth (RE) elements. Unfortunately, it isfound that the administration of Al and RE may induce latent toxicand harmful effects on the human body [3–5]. Consequently, alloyingelements must be chosen with careful consideration of the possibletoxic effects. Clearly, from a biological point of view, Al and RE are un-suitable alloying elements, and thus it is necessary to develop a novelbiodegradable magnesium alloy without Al, RE or other harmful ele-ments for biomedical application. Song [6] explored in vitro corrosionrates of several magnesium alloys, pointing out that Ca, Mn and Zncould be appropriate candidates. MTT (3-(4,5-Dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide) results also clearly indicatedthat Mg, Zn and Ca did not have cytotoxicity [7]. It is also found thatzinc is one of the most abundant nutritionally essential elements inthe human body, and has basic safety for biomedical applications[8]. Additionally, the addition of Zn can help to reduce the deleterious

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effects of metallic impurities, i.e. Fe and Ni [9]. Therefore, Zn-containingMg alloys have been paid more attention and developed as promisingcandidates for biomedical applications.

Some recent studies discussed the mechanical property and corro-sion behavior of binary Mg–Zn alloys. Zheng et al. [10] studied thebinary Mg–1Zn alloy and reported its enhanced mechanical proper-ties and corrosion resistance by addition of Zn as an alloying element.A binary Mg–6Zn alloy revealed suitable tensile strength and elonga-tion for implant application, as well as a reduced in vitro degradationrate and good in vivo biocompatibility [11,12]. Also, the findings ofMg–3Zn alloy by different heat treatments showed that solutiontreatment enhanced corrosion resistance while aging treatment de-creased the corrosion resistance [13]. A study by Boehlert [14] onMg–Zn alloys containing 0–4.4 wt.% Zn suggests that Zn was a potentgrain refiner and strengthener for Mg, where the optimal Zn contentis 4 wt.%. All these aforementioned studies showed the enhanced cor-rosion resistance and mechanical property of magnesium by the incor-poration of zinc in magnesium and great potential of binary Mg–Znalloys in biomedical applications. However, there is a lack of details onthe influence of volume fraction and existence format of secondaryphases. Besides therewere disagreements over the optimal Zn addition,which was either low (1 wt.%) or high (6 wt.%).

There is a high demand to design magnesium alloys with control-lable corrosion rates and suitable mechanical properties. Accordingly,it is necessary to study systematically the effects of zinc content onthe microstructures, mechanical properties and degradation behaviorof binary Mg–Zn alloys for biomedical application. According to theMg–Zn binary phase diagram [15], the maximum solid solubility ofZn in Mg is 6.2 wt.% (i.e. 2.5 at.%) at the eutectic temperature

Fig. 1. The photo of a cylindrical specimen for electrochemical measurements.

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341 °C. For this reason, the alloys with different volume fractions andexistence formats of secondary phases are designed in this study bythe incorporation of Zn content lower than and close to its maximumsolid solubility. And thus three binary Mg–x wt.% Zn alloys (x=1, 5, 7)were prepared and investigated to evaluate their microstructure, me-chanical property and corrosion behavior.

2. Experimental

Mg–Zn alloys, with Zn content of 1, 5 and 7 wt.%, were preparedusing high purity magnesium (≥99.99 wt.%) ingots and zinc(≥99.99 wt.%) granules as raw materials. The melting is conductedin an electronic resistance furnace at 750–800 °C protected by 99%CO2 and 1% SF6 (volume fraction) mixed gas from oxidation. Afterabout 30 min holding and stirring, the melt is cast into a permanentsteel mould preheated to 200 °C to form an ingot. The chemical com-positions of the Mg–Zn alloys were analyzed by an inductivelycoupled plasma atomic-emission spectrometry (ICP-AES) method,as listed in Table 1. According to the nominal zinc contents, theas-cast Mg–Zn alloy ingots were denoted as Mg–1Zn, Mg–5Zn andMg–7Zn, respectively. Pure Mg is used as control group.

Cylindrical specimens with a diameter of 13.5 mm and a height of5 mm (Fig. 1) were machined by linear cutting and ground with SiCemery papers up to 2000 grit, and successively polished with 1 μmdiamond paste, then ultrasonically cleaned in pure ethanol anddried under an air pressure stream prior to experiments.

The Archimedes' principle was used to measure the density ofMg–Zn alloys and an average of three readings was taken for eachreported density. A dog-bone specimen with a gauge length of35 mm, a thickness of 2 mm and a width of 10 mm was machinedfor tensile test [16]. Tensile strength was tested on an Instron 3369materials testing machine at a displacement rate of 2 mm min−1.Three parallel specimens were taken for each group in the tensiletest. An extension meter with a gauge length of 10 mm was used tomeasure the elongation. Cylindrical samples with a diameter of10 mm and a height of 20 mmwere used for compression test. Brinellhardness measurements were performed using a Brinell hardnesstester (HB-RNu-1875) with a steel ball indenter of 2.5 mm in diame-ter under the load of 612.5 N and maintained for 30 s.

Electrochemical measurements and immersion tests were carriedout in simulated body fluid (SBF) [6]. The pH value of the solution wasadjusted to 7.4 at 37±0.5 °C with 1.0 mol/L HCl and tris(hydroxymethyl) aminomethane (CH2OH)3CNH2 solution. Immer-sion tests were carried out in accordance with ASTMG31-72 [17](the ratio of surface area to solution volume was 1 cm2:20 mL). ThepH value of the solution was recorded during the immersion tests(PHS-3C pH meter, Lei-ci, Shanghai). The weight losses of the speci-mens were obtained by immersion testing. After the immersion of5 days, the samples were removed from the solution and cleanedwith chromate acid (200 g/L CrO3+10 g/L AgNO3) to remove surfacecorrosion products without removing any amount of metallic Mg[18]. Then the samples were rinsed with distilled water, cleanedultrasonically in acetone, and dried in air. The dried specimens wereweighed and the corrosion rate (CR) can be calculated by Eq. (1) [17]:

CR ¼ Δm=At ð1Þ

Table 1Chemical composition, wt.%, of pure Mg and the as-cast three Mg–Zn alloys.

Alloys name Zn Fe Cu Mn Mg

Pure Mg 0.001 ≥99.9Mg–1Zn 1.12 b0.0016 b0.002 b0.001 BalanceMg–5Zn 5.10 BalanceMg–7Zn 7.05 Balance

where CR is the corrosion rate in mg cm−2 d−1, Δm is the weight lossin mg, A is the original surface area exposed to the corrosive media incm2, and t is the immersion time in days. The weight loss wasconverted to the average corrosion rate, Rw (mm/year) using [19]:

Rw ¼ 2:1CR: ð2Þ

Electrochemical measurements were performed on CHI‐660Celectrochemical workstation with a three-electrode system compris-ing the as-cleaned Mg–Zn alloy slice as working electrode by sealingin a Teflon jacket with an exposed geometric area of 1 cm2, a plati-num wire as auxiliary electrode, and a saturated calomel electrode(SCE) as reference electrode.

Before the measurements, open circuit potential Eocp was testeduntil it was stabilized. Potentiodynamic polarization curves wereobtained in the potential range of Eocp±200 mV at a scan rate of2 mV s−1. The corrosion potential (Ecorr) and corrosion current density(Icorr) were derived directly from the polarization curves by Tafel regionextrapolation. The corrosion current density, Icorr (mA/cm2), is relatedto the corrosion rate (mm/year) using [19]:

Ri ¼ 22:85Icorr: ð3Þ

Electrochemical impedance spectroscopy (EIS) measurementswere performed at Eocp with the scan frequency ranged from100 kHz to 0.01 Hz, and with the perturbation amplitude of 5 mV.

Specimens for optical microscopy were etched with a solutionconsisting of 10 g picric acid, 175 mL ethanol, 25 mL acetic acid,and 25 mL distilled water. For the characterization of the samplemorphology and composition, a field-emission scanning electronmicroscope Nova NanoSEM 230 equipped with an Energy disper-sive X-ray (EDX) analyzer was used. The phase composition of thethree Mg–Zn alloys was analyzed by X-ray diffractometry (XRD:D/MAX-255) with the Cu–Kα1 radiation (wavelength λ=1.5406 Å). The tube voltage and the tube electric current of XRDwere 40 kW and 250 mA, respectively.

3. Results and discussion

3.1. Microstructures of the as-cast Mg–Zn alloys

Fig. 2 displays the optical metallographic images of pureMg and theas-cast Mg–Zn alloys with different Zn contents. It can be seen that themicrostructure of the as-cast Mg–Zn alloys typically consists of primaryα-Mg matrix and second phase mainly distributed along grainboundary. It clearly shows that the grain size decreased with increasingZn content. The pure Mg material exhibits a grain size of 350 μm asshown in Fig. 2a, while the average grain sizes in Mg–1Zn, Mg–5Zn

Fig. 2. Optical micrographs showing the microstructures of (a) pure Mg, (b) Mg–1Zn, (c) Mg–5Zn and (d) Mg–7Zn.

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and Mg–7Zn are about 100 μm, 55 μm and 56 μm as observed inFig. 2(b)–(d), respectively, indicating the addition of Zn element until5% of weight toMg can significantly refine the grain size, but the refine-ment efficiency is not significant with further addition of Zn over7 wt.%. When the Zn content increased from 5 to 7 wt.%, the secondphase formed a network structure of dentrite along the grain boundary,as shown in Fig. 2d. The microstructure of pure Mg and Mg–Zn alloyswas also imaged using backscattered electron (BSE)mode in SEM to re-veal compositional contrast, as shown in Fig. 3. Because of the large dif-ference in atomic number between Mg and Zn, such images clearlyreveal that pure Mg contains only α-Mg phase as shown in Fig. 3a,while Zn‐rich second phase can be detected in the microstructure ofMg–5Zn and Mg–7Zn alloys. As could be seen in Fig. 3c and d, thewhite island-like regions are corresponding to the second phase distrib-uting at the grain boundary. The chemical composition of the grain areawas identified by EDS to be attributed to α-Mg matrix as shown inFig. 3e, while the second phase was identified to be composed of Mgand Zn elements as shown in Fig. 3f, and the atomic ratio of Mg to Znwas about 7:3, likely attributing to Mg7Zn3 phase. Moreover, thevolume fraction of the second phase also increased with the increaseof Zn content from 5 to 7 wt.%.

The X-ray diffraction (XRD) patterns of pure Mg and Mg–Zn alloysare shown in Fig. 4. As could be seen in Fig. 4, only peaks correspond-ing to α-Mg matrix phase were found in the XRD pattern of pure Mgsample, while MgZn peaks can be clearly identified in the as-cast Mg–5Zn and Mg–7Zn samples. Furthermore, the diffraction intensity ofMgZn increased with increasing Zn content. According to the binaryalloy phase diagrams [15], Mg7Zn3 may conduct eutectic reaction at325 °C and decompose into α-Mg and MgZn intermetallic duringcooling process. The high concentration of Mg atom in the secondphase with a high atomic ratio of Mg to Zn (7:3) derived from EDSanalysis is likely attributed to the existence of α-Mg in eutectic(MgZn+α-Mg). Accordingly, with respect to the results of XRD andEDS, it is reasonable to conclude that the second phase is MgZn

intermetallic, which explained the presence of MgZn phase in XRDanalysis for Mg–5Zn and Mg–7Zn samples. Moreover, Zn has a rela-tively high solubility, and up to 1.6 wt.% Zn can fully dissolve inα-Mg matrix at room temperature [15]. Therefore, the alloying ele-ment Zn tends to be incorporated into the α-Mg matrix when its ad-dition is below the solubility limit. Accordingly, Mg–1Zn wasessentially a single-phase alloy with Zn dissolved as a solute in theα-Mg matrix as observed in Fig. 3b, and thus no MgZn phase wasdetected at such a low Zn content.

In this study, the microstructure of the as-cast Mg–Zn alloys clearlyshows that the addition of Zn can significantly refine the grain size ofMg matrix, which is consistent with the previous findings [20–23]. Asimilar result was also reported in binary Mg–Y alloys, where thegrain size significantly decreased with increasing Y content [24]. Asfor the reason for the grain refinement, it was widely accepted thatthe segregation of zinc at the front of grain growth forms an intensiveconstitutional undercooling in a diffusion layer ahead of the advancingsolid/liquid interface and then restricts the grain growth and promotesthe nucleation of the primary Mg, and thus refines the grain size [25].The refinement efficiency of a solution element can be determined bythe calculation of a growth restriction factor (GRE) [26]. It has beendocumented [26] that Zn had a higher GRE value (5.31) than Al (4.32)and Y (1.70), meaning that Zn has more powerful growth restrictionand better refinement efficiency.

3.2. Mechanical properties

Table 2 summarizes themechanical properties of the as-castMg–Znalloys in comparison with the reported properties of natural bone [27].Compared to pure magnesium, the hardness and ultimate strength inyield, tension and compression of Mg–Zn alloys increased with theincrease of Zn content until 5% of weight. On the contrary, Mg–7Znalloy displays deteriorated mechanical properties. The elongation ofMg–1Zn alloy exhibits the maximum value as high as 13.77%. When

Fig. 3. BSE-mode SEMmicrograph of the surfacemorphology of alloys (a) pureMg, (b) Mg–1Zn, (c) Mg–5Zn and (d)Mg–7Zn; (e) EDS analysis corresponding to assigned A area; (f) EDSanalysis corresponding to assigned B area.

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the Zn contentwas increased to 5 wt.%, the elongationwas significantlyreduced to 8.5%. Further addition of Zn content until 7% of weight led toa serious drop in elongation to 6.0%, even less than the value of pureMg.It is noted that values in elongation of 8.4 and 18.8% were reported forMg–4.4Zn and extruded Mg–6Zn alloy, respectively [11,14].

The increase in tensile strength and yield strength for Mg–Znalloys with Zn content until 5% of weight can be explained, on theone hand, by Hall–Patch relationship [28], which is supported by anumber of findings [20–23]. On the other hand, according to theMg–Zn binary phase diagram [15], the maximum solubility of Zn inmagnesium is 1.6 wt.% at room temperature in the equilibriumstate, and thus Zn element mainly dissolves into primary Mg tosome extent, generating solid-solution strengthening [14,27,29]. Inaddition, when the addition of Zn content is 5 wt.%, a number of

MgZn phases will precipitate from Mg matrix along grain boundaries,which promote the strength of Mg–Zn alloys by dispersion strength-ening [14,30]. Thereby, fine grain strengthening, solid solutionstrengthening and second phase strengthening contribute to theimprovement in mechanical performances of Mg–Zn alloys with Zncontent until 5% of weight.

Moreover, many studies [30,31] have pointed out that precipitatedsecond phase can dramatically improve the strength while decreasethe plastic of alloys. On the one hand, the second phase may hinderthe dislocation reduction and increase the dislocation density. Onthe other hand, the second phase dispersed at the grain boundarycould be new crack source, which expands easily and eventually re-sults in brittle failure. As could be seen in Fig. 2d, plenty of secondphases in Mg–7Zn alloy formed a network structure with dendritic

20 30 40 50 60 70 80

∇

Mg-7Zn

Mg-5Zn

∇

∇

Inte

nsity

(a.u

.) ∇

Pure Mg

Mg-1Zn

MgMgZn∇

Angle (2θ)

Fig. 4. X-ray diffraction patterns of pure Mg and as-cast Mg–Zn alloys.

0 10 20 30 40 50 60 707.0

7.5

8.0

8.5

9.0

9.5

10.0

10.5

11.0

pH v

alue

immersion time(h)

Pure MgMg-1ZnMg-5ZnMg-7Zn

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

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segregation along grain boundaries, resulting in residual defects, andthus sharply aggravated the strength and elongation of alloy. Overall,Zn is a promising alloying element and more importantly, the addi-tion of Zn content has to be controlled to promote the mechanicalproperties of Mg alloys to make them more suitable for human im-plant materials.

0.5

0.6

0.7

s (m

g/cm

2 /h)

Pure Mg

3.3. Immersion test

The pH variation of SBF as a function of immersion time is shown inFig. 5. It can be seen that the pH values of the solution corresponding topure Mg specimens increased rapidly from 7.4 to 10.3 in the initial 10 hof immersion. Afterwards, the pH value of SBF increased slowly withimmersion time and became stabilized at 10.2. In contrast, the pHvaluesof the solution corresponding to Mg–1Zn, Mg–5Zn and Mg–7Zn speci-mens increased much slowly and reached 9.16, 9.04 and 9.17 in the ini-tial 10 h of immersion, and finally stabilized at 9.57, 9.46 and 9.67 at theend of the immersion testing, respectively.

It is well known that corrosion behaviors of Mg and its alloy arecorrelated to their microstructures and corrosion starts on the α-Mgmatrix phase. Mg is rather active in aqueous medium and dissolvesaccording to the following reaction [32]:

Mg þ Hþ þ H2O→Mg

2þ þ OH− þ H2: ð4Þ

Consequently, in the early stage of immersion, the dissolution ofmagnesium consumes H+, but releases OH−, leading to the increaseof pH value of SBF [16], and thus the change of pH value with immer-sion time could be used to evaluate the corrosion behavior of magne-sium. It is reasonable to deduce from the pH variation trend that allthree Mg–Zn alloys exhibit a relatively slower degradation rate ascompared to pure Mg material.

The corrosion behavior of the three Mg–Zn alloys was also evalu-ated by immersion test in SBF. In the early stage of immersion, large

Table 2Mechanical properties of the as-cast three Mg–Zn alloys.

Material Modulus(GPa)

Yieldstrength(MPa)

Tensilestrength(MPa)

Elongation(%)

Compressionstrength(MPa)

Hardness(HB)

Naturalbone[28]

5–23 – 35–283 1.07–2.10 164–240 –

Pure Mg 1.86 29.88 100.47 7.43 183.09 37.10Mg–1Zn 24.23 60.62 187.73 13.77 329.60 47.33Mg–5Zn 36.47 75.60 194.59 8.50 334.12 53.80Mg–7Zn 39.60 67.28 135.53 6.00 353.11 56.26

numbers of hydrogen bubbles are evidently observed arising fromthe surface of pure Mg specimens, indicating a fast rate of hydrogenevolution due to the reaction of Mg matrix with the corrosive electro-lyte, while the numbers of hydrogen bubbles arising from the surfacesof the three Mg–Zn alloys are comparatively fewer. After a few hours,the pure Mg samples were covered with white corrosion products. Asthe immersion time increased, corrosion products are observed on allspecimens and are increased with time. It is noted that detached cor-rosion products are found on the bottom of the beaker containingpure Mg specimens at the end of immersion testing.

The average weight changes of pure Mg and Mg–Zn alloys after5 days of immersion in SBF were depicted in Fig. 6. The weight lossis an indication that there was corrosion attack on all specimens.The maximum weight loss of 0.69 mg cm−2 h−1 was observed forpure Mg specimens, while the order of mass loss rate from high tolow is 0.063, 0.04 and 0.025 mg cm−2 h−1 for Mg–7Zn, Mg–1Znand Mg–5Zn, respectively. The correlated average corrosion rate, Rw(mm/year) is listed in Table 3. The immersion result clearly indicatesthat the corrosion resistance of Mg–Zn alloys increases with the in-crease of Zn addition until 5% of weight. Excessive Zn content inMg–Zn alloy until 7% of weight leads to a serious drop in corrosionresistance.

In order to get a better insight on the corroded surface appearance ofall specimens after 5 days immersion testing, the corrosion productswere removed by chromic acid. Fig. 7 showed the SEM surface

-1 0 1 2 3 4 5 6 7 8 90.0

0.1

0.2

0.3

0.4

aver

age

wei

ght l

os

Mg-1Zn Mg-5ZnMg-7Zn

Zn content(wt.%)

Fig. 6. Average corrosion rates determined from weight loss testing for pure Mg andMg–Zn alloys.

Table 3Values measured from weight loss and the polarization curves for pure Mg and Mg–Znalloys in SBF.

Material Ecorr (VSCE) Icorr (μA/cm2) Ri (mm/y) Eb (VSCE) Rw (mm/y)

Pure Mg −1.581 680.1 15.30 – 34.78Mg–1Zn −1.527 23.4 0.53 −1.09 2.01Mg–5Zn −1.477 11.72 0.26 −0.96 1.26Mg–7Zn −1.543 51.79 1.17 −1.25 3.18

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morphologies of the as-cleaned pure Mg and Mg–Zn alloy specimens.As can be seen in Fig. 7a, the surface of pure Mg collapsed severelyand showed lamellarmicrostructures, indicating a substantial corrosionrate during the immersion testing. Fig. 7(b)–(d) presented the surfaceappearances of three Mg–Zn alloys, which were characterized by anumber of deep pits (as indicated by arrows) superimposed on superfi-cial corrosion over thewhole specimen surfaces, implying localized cor-rosion attacks occurred during the immersion process. It is worthy tonote the presence of pits is detrimental to the overall corrosion resis-tance and the destruction of Mg alloys will proceed by means of pittingcorrosion from one or more of them [33]. Mg–5Zn alloy exhibitedslighter pitting corrosion among all three alloys, whereas severe pittingcorrosions accompanied by collapses over the whole surface of Mg–7Znalloy were observed, indicating the slowest corrosion rate of Mg–5Znalloy and substantial heterogeneous corrosion of Mg–7Zn alloy.

Fig. 7. SEM microstructure of corroded appearance after 5 days immersion and after cor

3.4. Electrochemical corrosion measurements

The corrosion resistance of three Mg–Zn alloys was further deter-mined in SBF using potentiodynamic polarization test as shown inFig. 8. In general, the cathodic polarization curve is attributed to hy-drogen evolution reaction due to the reduction of water, while theanodic polarization curve is associated with the dissolution of Mg,leading to the formation of Mg2+ [32]. The corrosion potential(Ecorr) and corrosion current density (Icorr) were derived directlyfrom the polarization curves by Tafel region extrapolation and werelisted in Table 3. The corrosion potential of the as-cast Mg–1Zn andMg–5Zn alloy is −1.53 and −1.47 V, meanwhile decrease in the cor-rosion current density (Icorr) is observed, whereas Ecorr of the as-castMg–7Zn alloy is −1.54 V. Apparently, the addition of the Zn elementuntil 5% of weight shifts the corrosion potential toward noble direc-tion. It is clearly deduced from Table 3 that all three Mg–Zn alloysexhibit superior corrosion resistance to pure Mg material. The in-crease in degradation rate of the alloys is in the following order:Mg–5Zn, Mg–1Zn and Mg–7Zn. It has been reported that zinc is ableto elevate the corrosion potential of magnesium alloys, and thus re-duce the corrosion rate [34], which was in agreement with this study.

Furthermore, a passivation stage was found in the anodic polariza-tion curve of all three Mg–Zn alloys. The breakdown potential (Eb) isusually indicated by a sudden drop on the polarization curve, which isan indication of the tendency for localized corrosion. A more positive

rosion product removal for (a) pure Mg, (b) Mg–1Zn, (c) Mg–5Zn and (d) Mg–7Zn.

-1.8 -1.6 -1.4 -1.2 -1.0 -0.8 -0.6

-7.5

-7.0

-6.5

-6.0

-5.5

-5.0

-4.5

-4.0

-3.5

-3.0

-2.5

-2.0

-1.5

Eb

Eb

log

Cur

rent

Den

sity

( A

/ cm

2 )

Potential (V vs. SCE)

a

bc

d

(a) pure Mg(b) Mg-1Zn(c) Mg-5Zn(d) Mg-7Zn

Eb

Fig. 8. Potentiodynamic polarization curves of pure Mg and Mg–Zn alloys.

2576 S. Cai et al. / Materials Science and Engineering C 32 (2012) 2570–2577

Eb means a less likely localized corrosion [35]. A breakdown potentialat which pitting corrosion initiated was detected at −1.09, −0.95and −1.25 V for Mg–1Zn, Mg–5Zn and Mg–7Zn alloy, respectively,as shown in Table 3. Therefore, the localized corrosion can more eas-ily occur on the Mg–7Zn alloy. In contrast with the three alloys, puremagnesium was active over all the potential range. The passivationstage suggests that the Zn element could help to form a protectivefilm on the surface of Mg–Zn alloys during corrosion process [36]. Itis obvious that Mg–5Zn alloy shows the lowest anodic current andnoblest corrosion potential followed by a simultaneous increase ofthe breakdown potential, indicative of the best corrosion resistanceamong the three alloys. This conclusion is in good agreement withthe immersion results.

As confirmed in the microstructure characterization, both Mg–5Znand Mg–7Zn alloys are mainly composed of primary Mg matrix phasewith similar grain size and second phase distributed along grainboundary with the same chemical composition of MgZn intermetallic.

0 400 800 1200 1600 2000 2400 2800

0

200

400

600

800

1000

Z''

(ohm

cm

2 )

pure Mg Mg-1Zn Mg-5Zn Mg-7Zn

pure Mg Mg-7Zn Mg-1Zn Mg-5Zn

914 1317 2085 2213

Rt (Ω cm2)

HF

MF

a

b

Z' (ohm cm2)

Fig. 9. (a) Nyquist plots for pure Mg and Mg–Zn alloys after soaking in SBF for 1 h and(b) equivalent circuit used for modeling experimental EIS data of Mg–Zn alloys.

The significant difference in microstructure between them is the vol-ume fraction of the second phase. In general, second phase containingbinary alloy of Mg and Zn has nobler potential than Mg matrix [37].As a result, second phase may act as a cathode and Mg matrix as ananode at the interface between them, resulting in galvanic corrosion.Correspondingly, the discrepancy in corrosion resistance for Mg–5Znand Mg–7Zn is likely attributed to the role of second phases in corro-sion process. The influences of second phases on the corrosion rate ofalloys have been addressed recently [37,38]. In order to confirm theeffect of second phases, the cathodic polarization curve of Mg–7Znwas compared with that of Mg–5Zn. The cathodic current density ofboth curves gradually increased with the enhancement of cathodicpotential. The curve of the cathodic region to some extent representspolarization behavior of the non-corroded surface of a specimen andthe reaction of hydrogen evolution [39]. At the same cathodic poten-tial (marked by the broken line in Fig. 8), the corrosion current den-sity of Mg–7Zn was nearly two orders of magnitude higher thanthat of Mg–5Zn, indicating the fast cathodic hydrogen evolution rateon Mg–7Zn alloy. A similar result was also reported in Mg–Zn–Y–Zralloy with 5.8 wt.% Zn content [37]. The accelerated hydrogen evolu-tion rate can be attributed to the presence of high volume percentsecond phases in Mg–7Zn alloy, which is in the form of a continuousnetwork structure as shown in Fig. 2d. With the increase of Zn con-tent over 5 wt.%, the quantity of MgZn intermetallic phase is in-creased rapidly in the matrix to form a continuous networkstructure, leading to the formation of more anode–cathode sites. Thus,more galvanic corrosions could happen on these sites, resulting in fastcorrosion rate. Accordingly, it can be inferred that corrosion of Mg–7Zn alloy is driven by galvanic couple effect, resulting in the fast disso-lution of α-Mg matrix and thus the decline of corrosion resistance.

Electrochemical impedance spectra (EIS) measurements for thethree Mg–Zn alloys were carried out at open circuit potential (Eocp)as illustrated in Fig. 9a. Apparently, the impedance diagram of pureMg is characteristic of one well-defined capacitive loop. In contrast,the EIS spectra of the three Mg–Zn alloys are characterized by twoloops: a capacitive loop in the high frequency region (HF) and a ca-pacitive loop in the medium frequency region (MF), as labeled. Ingeneral, the high frequency capacitive loop is attributed to the relax-ation process of electrochemical reaction impedance correspondingto the dissolution of Mg and electric double layer capacitance (Cdl)at the interface between the metal surface and the corrosive medium[40]. The capacitive loop observed at the middle frequency region isrelated to the presence of surface film influencing the corrosion pro-cess [24]. Thereby, the electrode reaction process correlated to highfrequency capacitive loop can be described by a parallel circuit of Cdland charge transfer resistance Rt [40]. The EIS plots for pure Mg andMg–Zn alloys were different in shape and diameter of the loops, in-dicative of different corrosion mechanisms and corrosion rates. TheNyquist plots of Mg–Zn alloys can be interpreted using the equivalentcircuit shown in Fig. 9b. Since the Nyquist plots of Mg–Zn alloys ex-hibit depressed semicircles, a constant phase element, CPE1 is usedinstead of Cdl in the proposed model. CPE2 and a resistance Rf arealso introduced to account for the capacitance and resistance of thecorrosion product layer formed on the surface of Mg–Zn alloys. TheRf displays correlation with the passivation stage observed on anodicpolarization curve due to protection layer formation. The diameter ofthe high frequency semicircle gives the charge-transfer resistance(Rt) at the electrode/electrolyte interface. From Rt value, theexchange-current density (j0) could be calculated using the followingexpression [41]:

j0 ¼ RT=nFRt ð5Þ

where n is the number of transferred charges, F is Faraday constant.Apparently, j0 is in inverse proportion to Rt, in other words, the higherthe Rt is, the lower would be the corrosion rate [41]. Consequently,

2577S. Cai et al. / Materials Science and Engineering C 32 (2012) 2570–2577

charge transfer resistance could be used to evaluate the corrosionproperty of the alloys. This is because the larger the Rt is, the more dif-ficult is the transfer of charges between solution and the sample sur-face, and thus the corrosion rate is low. It can be deduced from theNyquist plots that Rt ranked from low to high as following: pure Mg(914 Ω cm2), Mg–7Zn (1317 Ω cm2), Mg–1Zn (2085 Ω cm2) andMg–5Zn (2213 Ω cm2). Meanwhile, the order of film resistance Rffrom high to low is 736, 623 and 549 Ω cm2 for Mg–5Zn, Mg–1Zn andMg–7Zn alloy, respectively, which further indicates that Mg–5Zn alloyhas superior corrosion property. As a result, Mg–5Zn alloy exhibits thebest corrosion resistance, and the corrosion tendency displays well cor-relation with the polarization measurement and immersion tests.

4. Conclusions

Microstructure observation showed that the addition of Zn ele-ment significantly refined the grain size of as-cast Mg–Zn alloys andincreased the volume percent intermetallic in the microstructure.The tensile properties and corrosion resistance of the Mg–Zn alloysstrongly depended on the volume fraction and existence format ofsecondary phases. The corrosionmorphologies and immersion testingas well as electrochemical measurements proved that the corrosionresistance increased with increasing Zn content in the range1–5 wt.% despite the presence of the potentially detrimental MgZnintermetallic. Excessive addition of Zn over 7 wt.% resulted in a net-work structure of MgZn intermetallic as a cathode, causing micro-galvanic corrosion acceleration. It was suggested that the addition ofZn content has to be precisely controlled to promote both the me-chanical properties and corrosion resistance of Mg alloys for biomed-ical application.

Acknowledgment

This work was supported by National Natural Science Foundationof China (grant no. 51021063), National Science Fund for Distin-guished Young Scholars (grant no. 50825102) and Open Project ofState Key Laboratory for Powder Metallurgy of CSU.

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