Surface & Coatings Technology 258 (2014) 1152–1158

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Corrosion resistance of Mg–Al-LDH coating on magnesium alloy AZ31

Fen Zhang a, Zhen-Guo Liu a, Rong-Chang Zeng a,⁎, Shuo-Qi Li a, Hong-Zhi Cui a, Liang Song b,⁎⁎, En-Hou Han c

a College of Materials Science and Engineering, Shandong University of Science and Technology, Qingdao 266510, Chinab Qingdao Institute of Bioenergy and Bioprocess Technology, Chinese Academy of Sciences, Qingdao 266101, Chinac Institute of Metals Research, Chinese Academy of Science, Shenyang 110016, China

⁎ Corresponding author. Tel.: +86 532 80681226.⁎⁎ Corresponding author. Tel.: +86 532 80662759.

E-mail addresses: rczeng@foxmail.com (R.-C. Zeng), so

http://dx.doi.org/10.1016/j.surfcoat.2014.07.0170257-8972/© 2014 Elsevier B.V. All rights reserved.

a b s t r a c t

a r t i c l e i n f o

Article history:Received 30 January 2014Accepted in revised form 4 July 2014Available online 18 July 2014

Keywords:Magnesium alloyLayered double hydroxideCoatingCorrosion

Mg–Al-layered double hydroxide (LDH) coatings were fabricated by a combined co-precipitation method andhydrothermal process on an AZ31 alloy substrate. The characteristics of the coatings were investigated usingSEM, XRD, FT-IR and EDS. The corrosion resistance of the LDH coatings was studied using potentiodynamicpolarization and electrochemical impedance spectrum. The results demonstrated that the LDH coatings,characterized by nanoplates stacked vertically to the substrate surface and ion-exchange ability, possessexcellent corrosion resistance.

ngliang@qibebt.ac.cn (L. Song).

© 2014 Elsevier B.V. All rights reserved.

1. Introduction

Recently, magnesium alloys have attracted considerable attentiondue to their applications in the automotive, aerospace, power tools,recreational equipment, mobile phone and computer industries.Unfortunately, magnesium alloys possess a lower corrosion resistance,hence restricting their use on a larger scale. Therefore, an improvementin corrosion resistance is of critical importance for magnesium alloys.Numerous surface treatments have been adopted to enhance thecorrosion resistance of magnesium alloys, including chemical conver-sion coatings [1–3], polymer coatings [4,5], microarc oxidation (MAO)or plasma electrolyte oxidation (PEO) coatings [6–8], and silanecoatings [9,10]. Among these surface treatments, chemical conversioncoatings have advantages, including lower cost and simplicity of opera-tion [11]. Many types of chemical conversion coatings have beenapplied to magnesium alloys, including coatings containing chromate[12], phytic acid [13,14], phosphate [15,16], stannate [17,18], vanadium[19], stearic acid [20], dawsonite [3], or rare-earth elements such ascerium [21], lanthanum [22] or hydrotalcite [23–26]. Nevertheless,some of these conversion coatings, such as chromate- and vanadate-based coatings, present environmental hazards, and stannate is notapplicable for practical use at this stage.

Layered double hydroxides (LDHs) are a promising class of protec-tion coatings for magnesium alloys. LDHs can be expressed by thegeneral formula [M2+

1 − x,M3+x(OH)2]x+(A)n−x/2·mH2O, where thecations M2+ and M3+ occupy the octahedral holes in a brucite-likelayer, and the anion An− is located in the hydrated interlayer galleries

[27]. Recently, extensive studies have focused on the potential applica-tions of LDHs as coatings to protect metals such as magnesium alloys[26], aluminum alloys [28], and steel [29,30]. To date, two mainmethods have been developed for preparing LDH coatings. One is thein situ method, and the other is the two-step method, in which theLDH power precursor is first synthesized by the coprecipitationmethodand then the coating is obtained using a certain process.

Lin [25,26,31] adopted the in situ method to fabricate Mg,Al-hydrotalcite conversion films on AZ91 alloys in aqueous HCO3

−/CO3

2− medium and demonstrated that the resulting films exhibit goodcorrosion resistance. Chen [23,24,32] also applied the in situ method,preparing Mg–Al hydrotalcite conversion films on AZ31 alloys. Thecontent of aluminum in the magnesium alloy substrate used wasdifferent in the studies conducted by Lin and Chen. In Chen's study, anAl panel was placed into the solution to synthesize hydrotalcite. The insitu preparation process of the Mg–Al-LDH coatings is significantlyaffected by the chemical compositions of the substrates, such as theconcentration of aluminum. Moreover, it is difficult to adjust thechemical compositions of the LDH layers and the species of the anionsin the interlayers of the LDH coatings. Furthermore, the LDH coatingpreparation processes described above are complicated and have notyet achieved the desired level of corrosion resistance.

Fuente [33] synthesized Al–Zn-vanadate hydrotalcite on analuminum alloy using the co-precipitation and air-spraying method.Zhang investigated the corrosion protection provided by molybdatepillared hydrotalcites [34,35] and tungstate hydrotalcites [36] asthe corrosion inhibiting pigments in organic coatings on AZ31 alloys.Those coatings were fabricated using the two-step method andgreatly improved the corrosion resistance of their substrates;however, the adhesion of the coating to the substrate was muchpoorer. Thus, the preparation of LDH coatings on magnesium alloys

Fig. 1. Themicrographs of the top coating of the LDH/magnesium alloy with different magnification (a, b, c) and the cross-section microstructure (d) (the crystallization time was 48 h).

1153F. Zhang et al. / Surface & Coatings Technology 258 (2014) 1152–1158

with high corrosion resistance and adhesion to the substrate using asimple technological process remains a considerable challenge.

The combined co-precipitationmethod and hydrothermal process isexpected to synthesize LDH coatings with multiple chemical composi-tions, including MgAl-LDH, ZnAl-LDH, and LiAl-LDH, regardless of thesubstrate. In addition, various species of anions in the interlayer canalso be adjusted.

This study aims to prepare nano-sized Mg–Al-LDH coatings onAZ31 substrates using the combined method of co-precipitation andhydrothermal crystallization and investigates the effect of the hydro-thermal crystallization time on the corrosion resistance. In addition, acorrosion mechanism model is proposed.

Fig. 2.XRD patterns of the LDH powder (a), AZ31magnesium alloy substrate (b) andMg–Al-LDH coatings (c, d). Peaks marked with ◆ are contributed by the Mg–Al-LDH coatingsand ● are due to the magnesium alloy substrate.

2. Experimental

2.1. Synthesis of Mg–Al-LDH

AZ31 magnesium alloy was used for this study. Before the prepara-tion of the LDH coating, the substrate surface wasmechanically grindedwith SiC papers up to 2000 grit to ensure the same surface roughness.The substrate surfaces were then ultrasonically cleaned in ethyl alcoholand dried under a steam of air.

The Mg–Al-LDH coatings were prepared using a combinedco-precipitation and hydrothermal processing technique on themagne-sium alloy substrates. Mg(NO3)2·6H2O and Al(NO3)3·9H2O with a

Fig. 3. FT-IR spectra of the as-prepared powder and the powder scraped from the Mg–Al-LDH coating/magnesium alloy sample (the crystallization time was 48 h).

Fig. 4. Tafel polarization curves in 3.5 wt.% NaCl solution of the baremagnesium alloy, andthe LDH coatings on magnesium alloy with different crystallization time.

1154 F. Zhang et al. / Surface & Coatings Technology 258 (2014) 1152–1158

Mg2+/Al3+ molar ratio of 3:1 were dissolved in de-ionized water toobtain solution A, which was then kept in a three-neck flask. Na2CO3

was dissolved in deionizedwater and thenmixedwith a certain volumeof NaOH to form solution B. Solution B was then added dropwise intothe three-neck flask to form a slurry, which was stirred vigorously in awater bath at a temperature of 338 K for 48 h and then aged for 12 h.Afterwards, the resulting slurry was transferred to a Teflon-lined auto-clave to immerse the pretreated magnesium alloy. The Teflon-linedautoclave was then heated at 398 K for 24 h and 48 h. Theas-prepared samples were rinsed with deionized water and driedunder ambient conditions.

To obtain the LDH powder, the slurry was filtered and washedseveral times with deionized water, followed by drying at 353 K for10 h.

Fig. 5. Bode diagrams (a, b) and Nyquist diagrams (c) for LDH coating/magnesium alloyand the bare magnesium alloy electrodes immersed in 3.5 wt.% NaCl solution.

2.2. Characterization

The surface microstructure, chemical composition and thickness ofthe coatings were observed using a field-emission scanning electronicmicroscope (FE-SEM, Hitachi S-4800) equipped with an energy disper-sive X-ray spectrometer (EDS). All of the samples for the SEM observa-tion were sputtered with gold. The structures of the LDH coatings aswell as the LDH powder were examined using an X-ray diffractometer(XRD) (D/Max 2500PC) with a Cu target (λ = 0.154 nm). Fourier-transform infrared (FT-IR) spectra were obtained on a TENSOR-27spectrophotometer using the KBr pellet technique. Potentiodynamicpolarization curves and electrochemical impedance spectra (EIS) wereobtained in a cell with 3.5 wt.% NaCl solution using (Princetonpotentiostat model 2273). A classical three-electrode system was usedwith the sample as the working electrode (1 cm2), a saturated calomelelectrode (SCE) as the reference electrode, and a platinum plate as thecounter electrode. The samples were immersed in the medium for

Table 1Corrosion potential (Ecorr), corrosion current density (Icorr) and breakdown potential (Eb) of th

Treatment process Test solution

AZ31 substrate 3.5 wt.% NaClLDH coating-24 h 3.5 wt.% NaClLDH coating-48 h 3.5 wt.% NaClIn situ LDH film 0.1 M NaCl

(0.58 wt.% NaCl)In situ LDH film modified by phytic acid 0.1 M NaCl

(0.58 wt.% NaCl)

20 min before the electrochemical tests. The polarization curves wererecordedwith a sweep rate of 2 mV/s. EISmeasurementswere acquiredfrom 100 kHz down to 10 mHz using a 5-mV amplitude perturbation.

e samples.

Ecorr(V/SCE)

Icorr(μA/cm2)

Eb(V/SCE)

Reference

−1.56 30.4 −1.37 –

−1.38 3.16 −1.25 –

−1.18 0.0652 −1.10 –

−1.47 4.53 −1.28 [24]

−1.54 0.760 −1.44 [40]

Fig. 6. Fit results of EIS data and the equivalent circuit for LDH coating-48 h sample.

Fig. 7. SEM surfacemorphologies of the original LDH-coated sample (a) and the immersedsample (b). The insets are the corresponding EDS spectra.

Fig. 8.XRD spectra of the LDH coating-48 h sample before and after immersion in 3.5 wt. %NaCl solution for 72 h.

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3. Results and discussion

3.1. Characterization of Mg–Al-LDH

Fig. 1a demonstrates that the Mg–Al-LDH coating hydrothermallytreated for 48 h is compact over the entire magnesium alloy substrate.Fig. 1b and c present images at higher magnifications, demonstratingthat the coating consists of uniform nanoplates that grew roughlyvertically on the substrate, with a length of 300–700 nm and a widthof 20–40 nm. Fig. 1d shows the cross-sectional microstructure of theLDH coating, demonstrating the presence of a dense, uniform coatingon the substrate. The coating contains two structural layers: the denseinner thick-layer and the porous outer thin-layer. The thickness of thecoating is approximately 7.0 μm.

Fig. 2 presents the XRD spectra of the Mg–Al-LDH coatings formedon AZ31 magnesium alloy, the as-prepared Mg–Al-LDH powders andthe magnesium alloy substrate. The XRD pattern of the Mg–Al-LDHpowder (Fig. 2a) exhibits a typical layered structure characteristic ofthe LDH (Mg6Al2(OH)16CO3·4H2O) with identical peaks correspondingto the (003)/(006) reflections of the LDH phase, indicating that thenanocrystallized LDH was obtained using the co-precipitation method.The powder displays a typical well-ordered layer structure with thebasal spacing distances d003 and d006 corresponding to 0.78 nm0.38 nm, respectively. These results are consistent with the publishedd-spacing values of Mg–Al-LDH with carbonate anions in the interlayer[37]. Fig. 2b presents theXRD pattern of theAZ31 substrate. As observedin Fig. 2c and d, the XRD patterns of theMg–Al-LDH coatings onmagne-sium alloy display obvious peaks corresponding to the LDH phases. Theresults illustrate that the LDH coatings are successfully formed on thesubstrate using the combined co-precipitation and hydrothermalprocess. In addition, lengthening the hydrothermal treatment periodincreases the crystallinity of the films on the alloys.

Fig. 3 presents the FT-IR spectra of the as-prepared Mg–Al-LDHpowders, which were scraped from the surfaces of the samples afterhydrothermal treatment for 48 h. As observed, bands correspondingto the H\O\H stretching vibration and the O\H symmetric contrac-tion of the water molecules between layers of the LDH coating and the

Table 2The corresponding fitted parameters of the Nyquist plot for the LDH-coating 48 h sample.

RS(Ω·cm2)

Cfp(μF·cm−2)

Rfp(Ω·cm2)

Y0(μΩ−1·cm−2·s−1)

n

8.054 0.021 84 1.515 0.5877

water molecules absorbed on the LDH surface are observed at approxi-mately 3696, 3409 and 1633 cm−1. The absorption band at 1394 cm−1

is attributed to the asymmetric stretching mode of C\O in carbonatemolecules. The shoulder band at approximately 2931 cm−1

Rfd(Ω·cm2)

Cct(μF·cm−2)

Rct(Ω·cm2)

Zw(μΩ−0.5·cm−2·s−1)

60,910 18.61 48,130 14.32

Fig. 9. FT-IR spectra of the LDH coating-48 h sample before and after immersion in3.5 wt. % NaCl solution for 72 h.

1156 F. Zhang et al. / Surface & Coatings Technology 258 (2014) 1152–1158

corresponds to a CO32−\H2O stretching vibration, suggesting the

presence of water molecules hydrogen bonded to the carbonate ionspresent in the interlayer. The peaks at 665 cm−1 and 467 cm−1 areassigned to the Al\O and Mg\O vibration modes. Therefore, based onthe above analysis of the FT-IR spectrum, the hydrothermal crystalliza-tion treatment could successfully yield a Mg–Al-LDH coating on theAZ31 substrate. A 48-h crystallization period was required to developa well-crystallized Mg–Al-LDH conversion coating on the surface ofthe magnesium alloy. The spectrum of the as-prepared LDH power isalmost the same as that of the LDH powder scraped from the magne-sium alloy sample, with the exception of a lower transmittance. TheLDH powders scraped from the magnesium alloy sample may bemixed with the magnesium powders, leading to a low purity, whichresulted in a low transmittance in the FT-IR spectrum.

3.2. Corrosion resistance of the LDH coatings

Potentiodynamic polarization [38] is a commonly used techniquethat was employed to investigate the corrosion resistance of the LDHcoatings. Fig. 4 presents the potentiodynamic polarization curves ofthe LDH-coated samples as well as the magnesium alloy substrate.Table 1 lists the corrosion potential (Ecorr), corrosion current density(Icorr) and breakdown potential (Eb) of Mg–Al-LDH coatings preparedusing different treatment processes.

Ecorr of themagnesium alloy sample is−1.56 V vs. SCE, while that ofthe LDH coating-24 h sample and that of the LDH coating-48 h sampleare −1.38 V vs. SCE and −1.18 V vs. SCE, respectively, which aremore positive than that of the AZ31 substrate. Icorr of the magnesiumalloy sample is 3.04 × 10−5 A cm−2, while the values of theLDH-coated samples are 3.16 × 10−6 A cm−2 and 6.52 × 10−8

A cm−2 for hydrothermal treatment periods of 24 h and 48 h, respec-tively. Icorr for the LDH-coated samples (48 h) decreased by threeorders of magnitude compared with that of the magnesium alloysubstrate. Eb of the AZ31 magnesium alloy is −1.37 V vs. SCE, whilethe values of the LDH coating-48 h sample and LDH coating-24 hsample are −1.10 V vs. SCE and −1.25 V vs. SCE, respectively. Areasonable explanation for this result is that the LDH coatings havea self-healing ability. The corrosion resistance of the LDH coatingsprepared by the method here far exceeded that of the LDH coatingsprepared by the in situ synthesis method [24,39] (Table 1).

To further determine the characteristics of the corrosion inhibitioneffect of the Mg–Al-LDH coatings, EIS was performed to analyze thecorrosion resistance of the coatings. Fig. 5a, b present the typical Bodediagrams, while Fig. 5c presents typical Nyquist plots. It is well known

thatmaterials with a higher Zmodulus at lower frequencies exhibit bet-ter corrosion resistance on metal substrates [40,41]. It can be observedfrom the Bode diagrams that the LDH-coated samples exhibit larger im-pedance at low frequency (Fig. 5a). Note also that the low frequency im-pedance was considerably higher, approximately 105.0 Ω·cm2, for theLDH-coated sample prepared using hydrothermal treatment periodsof 48 h. Concurrently, it can be observed from the Nyquist plot(Fig. 5c) that the largest radius of curvature for the LDH coating-48 hsample indicates that this sample possesses the highest corrosion pro-tection ability. These results demonstrate that the impedance for theLDH-coated samples increasedwith prolongedhydrothermal treatmentperiod, leading to the full crystallization of the LDH coating in accor-dance with the XRD results presented in Fig. 2.

In addition, at the lower frequency of the Bode andNyquist diagram,the trendof the curves indicates a diffusion process,whichwas attributedto the ion-exchange reaction. The coatings with better EIS performancecan effectively prevent the diffusion/penetration of the Cl− ions to themagnesium alloy substrate, thus reducing the corrosion rate of themagnesium alloy substrate.

The EIS spectrum of the LDH coating-48 h sample was analyzedbased on the equivalent circuit, as shown in Fig. 6. Table 2 summarizesthe corresponding fitted parameters. Rs represents the solutionresistance. Rfp, which represents the resistance of the outer layer ofthe LDH coating, is only 84 Ω·cm2, indicating that the outer layer ofthe LDH coating possesses a porous structure, in accordance withFig. 1d. Jun-Yen Uan's research group [25,26] also reported that theMg–Al-LDH produced by the in situ method possessed a porous outerlayer and a dense inner layer as evidenced by TEM micrographs.

Cfp represents the capacitance of the outer layer of the LDH coating atthe interface. Rfd, which represents the resistance of the inner layer ofthe LDH coating, is 60,910 Ω·cm2, indicating that the inner layer hasexcellent corrosion resistance. Constant phase element (CPE) is usedin place of a capacitor to compensate the non-homogeneity in thesystem, which is defined by two values, Y0 and n. If n is equal to 1,CPE is identical to a capacitor. CPEfd represents the capacitance of theinner layer of the LDH coating. Rct represents the charge-transferresistance, and Cdel is the electric double layer capacity at the interface.Generally, larger values of Rct correspond to better anti-corrosionperformance of the coating. Table 2 indicates that the value of Rct is48,130 Ω·cm2, suggesting that the LDH-coated samples exhibit fairlygood corrosion resistance. Zw represents the diffusion resistance of theLDH coating, demonstrating that the LDH coatings possess the abilityfor ion-exchange.

3.3. Protection ability of the LDH coatings

Fig. 7 presents the SEM morphologies and corresponding EDSspectra of the original LDH-coated sample and the immersed sample.After immersion for 72 h (Fig. 7b), most of the area of the sample stillexhibits the nanoplate-like structure, similar to the original sample(Fig. 7a). However, the plate size decreased after immersion, whichmay be due to the dissolution of the plate. It can be observed from theinset of Fig. 7 that the original LDH coating (Fig. 7a) is mainly composedof Mg, Al, O and C elements, with no Cl and Na signals observed.However, Cl andNa signals (Fig. 7b) were observed on the same sampleafter immersion, which indicates that the LDH coating exhibits ion-exchange ability by absorbing Cl− and Na+ from NaCl solutions [42]and that the interlayer of the LDH is able to retain Cl− and Na+ in theLDH structure.

The XRD spectra of the LDH coating-48 h sample before and afterimmersion in 3.5 wt.% NaCl solution for 72 h are presented in Fig. 8.It is apparent that most of the peaks are the same for both samples,suggesting that the as-prepared LDH coatings possess good corrosionresistance. The XRD pattern of the immersed sample contains Mg(OH)2peaks, implying the occurrence of corrosion between the LDH layerand the magnesium alloy substrate.

1157F. Zhang et al. / Surface & Coatings Technology 258 (2014) 1152–1158

Further FT-IR investigations shown in Fig. 9 also demonstrate ananalogous result to that obtained from the XRD and EDS. Thecharacteristic bands assigned to LDH remained the same after the

Fig. 10. The plain views of the as-prepared LDH-coated sample before (a) and after 72 himmersion in 3.5 wt. % NaCl solution (b), and the plain view of the AZ31 substrate after72 h immersion in 3.5 wt. % NaCl solution (c).

immersion tests, which demonstrated that the LDH coating has avery stable structure.

Fig. 10 shows the macro profile of the as-prepared LDH-coatedsample before (a) and after (b) 72 h immersion in 3.5 wt.% NaClsolution. Fig. 10c shows the macro profile of AZ31 substrate after 72 himmersion in 3.5 wt.% NaCl solution. The surface of the LDH coating-48 h sample after immersion (Fig. 10b) is brown in color and almostthe same as that of the as-prepared sample, and no obvious corrosionpits are observed after immersion. However, the substrate had a severelycorroded surface.

3.4. Corrosion mechanism

One possible anti-corrosion mechanism of the LDH coating on theAZ31 alloy is that the coating can act as a barrier layer against chlorideattack because of the high density of the LDH film, protecting themagnesium alloys to a certain extent.

However, the improvement in the corrosion performance of the LDHcoating can also be attributed to the adsorption and retention ofcorrosive Cl− ions and the release of CO3

2− ions. Tedim et al. [43] provedthat nitrate-containing LDHs are effective chloride nanotraps due to theion-exchange process; therefore, the LDHs can delay coating degrada-tion and the initiation of corrosion. The LDH coatings with carbonateintercalation also possess ion-exchange ability. The ion-exchangereaction of the LDH coating on the magnesium alloys in chloride-containing solution can be written as follows [42]:

LDH� CO2−3 þ 2Cl

−→LDH� 2Cl− þ CO

2−3 : ð1Þ

Based on the ion-exchange process, the released CO32− ions concen-

trated on the coating surface, leading to the formation of a diffusionboundary layer containing high concentrations of CO3

2− ions, whichmay react with the dissolved Mg2+ to form a protective MgCO3 film.MgCO3 is much more easily dissolved to form Mg(OH)2 under alkalineconditions. The formation of Mg(OH)2 can inhibit the expansion andspread of the pitting corrosion. In addition, the presence of CO3

2− inthe diffusion boundary layer impairs the adsorption of Cl− on thesurface of the coating due to competitive adsorption. Therefore, thediffusion boundary layer containing CO3

2− can effectively improve thepitting resistance property of themagnesiumalloy surface. The probablereaction can be written as follows:

Mg2þ þ CO

2−3 ¼ ½MgCO3�↓ ð2Þ

MgCO3 þ 2OH− ¼ MgðOHÞ2↓ þ CO

2−3 : ð3Þ

Based on the experimental results, a corrosion protection mecha-nism model of the LDH coating is proposed, and this model is used toillustrate the mechanisms of ion-exchange, deposition and competitiveadsorption. In the model (Fig. 11), the three layers from top to bottomare the diffusion boundary layer, LDH coating and AZ31 substrate.In conclusion, the anti-corrosion mechanism of the LDH coating can bedivided into three parts: ion-exchange, deposition and competitiveadsorption.

4. Conclusions

A uniform and compact Mg–Al-LDH coating has been successfullyfabricated on a magnesium alloy by combining the co-precipitationmethod and the hydrothermal crystallization process. The primaryconclusions were as follows:

(1) The microstructure of the LDH coatings was composed ofcompact LDH (Mg6Al2(OH)16CO3·4H2O) nanoplates orientedalmost perpendicular to the substrate surface. The size of thesenanoplates was 300–700 nm long and 20–40 nm wide.

Fig. 11. The proposed corrosion protection mechanism model of the LDH coating.

1158 F. Zhang et al. / Surface & Coatings Technology 258 (2014) 1152–1158

(2) The corrosion current density of the LDH coatings is decreased bythree orders of magnitude comparedwith that of the AZ31 alloy.The prepared Mg–Al-LDH coatings can be used as potentialcandidates of non-chromium chemical conversion coatings forthe corrosion protection of magnesium-aluminum alloys.

(3) The corrosion protection ability of the LDH coatings may be dueto ion-exchange, competitive adsorption for chloride ions andprotective deposition of Mg(OH)2 on the alloy surface.

Acknowledgments

This work was supported by the Scientific Research Foundation ofShandong for Outstanding Young Scientists (BS2013CL009), theDoctoral Program Foundation of the State Education Ministry(20133718120003), the Applied Basic Research Foundation of Qingdao(13-1-4-217-jch) and SDUST Research Fund.

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