The Special Role of Anodic Second Phases in the Micro-galvanic Corrosion of EW75 Mg Alloy

The Special Role of Anodic Second Phases in the Micro-galvanic Corrosion of EW75 Mg Alloy

The Special Role of Anodic Second Phases in the Micro-galvanic Corrosion of EW75Mg Alloy

Jinhui Liu a ,b ,Yingwei Song b ,*,Jiachen Chen c ,Peng Chen b ,Dayong Shan b ,En-Hou Han b


Northeastern University,Shenyang 110004,China


National Engineering Center for Corrosion Control,Institute of Metal Research,Chinese Academy of Sciences,Shenyang 110016,China c

School of Chemical Engineering and Technology,Xi'an Jiaotong University,Xi ’an 710049,China


Article history:

Received 10November 2015

Received in revised form 10December 2015Accepted 10December 2015

Available online 12December 2015


Second phases

Micro-galvanic corrosion Mg-RE alloys Micro-anodes SKPFM


It is well known that second phases act as micro-cathodes in the corrosion of traditional Mg alloys.However,the effect of second phases on the corrosion behavior of Mg-rare earth (RE)alloys is ambiguous in view of the second phases consisting of Mg and more active RE elements.The role of second phases in the corrosion of cast EW75(Mg –5Y –7Gd –1Nd –0.5Zr)was studied by scanning electron microscopy (SEM)observations,Scanning Kelvin Probe Force Microscopy (SKPFM)analysis,immersion and electrochemical tests.It is found that the second phases in EW75are more active than Mg matrix and preferentially dissolved at the initial corrosion stage.It indicates that the second phases act as micro-anodes,which are greatly different from the role of second phases in traditional Mg alloys.

?2015Elsevier Ltd.All rights reserved.


As the lightest engineering metal,Mg and its alloys have been more and more attractive in automotive and aerospace industries [1].However,the poor corrosion resistance severely restricts their widespread applications [2,3].

It is well known that second phases play a key role in the corrosion resistance of Mg alloys.As for the traditional Mg alloys such as Mg –Al and Mg –Zn systems,the discrete second phases consisting of Mg and more inert alloying elements are nobler than Mg matrix and act as micro-cathodes to accelerate the corrosion of Mg matrix [4–6].Moreover,Song et al.[7]found that the continuous second phases may act as a barrier to inhibit the corrosion propagation in Mg matrix.

Presently,rare earth (RE)Mg alloys are popular because of their higher creep resistance and strength at high temperature [8–11].Especially,it is found that the corrosion resistance of traditional Mg alloys such as AZ91and AM60is improved with addition of REs.It is attributed to the reasons as follows:(1)decrease potential difference between second phases and Mg matrix [12–14]by formation of more active second phases containing REs;(2)purify the melt by formation of intermetallic compounds with impuri-ties;(3)form a more compact REs oxide ?lm [15].Besides the

addition of REs into the traditional Mg alloys,there exist another type of Mg-RE alloys which only contain pure magnesium and REs.In this type Mg-RE alloys the second phases consist of Mg and more active REs [3–17].The electrochemical stability of these second phases in comparison with Mg matrix is ambiguous.Zhang et al.reported the cathodic role of second phases in Mg-5Y-7Gd-1Nd-0.5Zr just according to the corrosion morphology of 2h immersion [18].Birbilis et al.[16]reported the inert second phases in Mg-RE alloys (where RE ?Ce,La,Nd)based on the comparison of the polarization curves of Mg-RE intermetallic compounds and pure Mg.These previous works are strongly in ?uenced by the traditional opinion of the nobler second phases.There are no enough proofs to support the cathodic role of second phases in Mg-RE alloys.In our previous work,it was found that the second phases in Mg-RE alloys preferentially corrode,which is greatly different from the typical corrosion status.Thus,this work aims to clarify the role of second phases in Mg-RE alloys and further to disclose the in ?uence of REs on the corrosion resistance.

2.Experimental details

Cast EW75(Mg –5Y –7Gd –1Nd –0.5Zr)and pure Mg were used for this investigation.EW75magnesium alloy,produced by General Research Institute for Nonferrous Metals,PR China,is composed of (wt.%)7.04Gd,4.53Y,1.29Nd,0.49Zr,and Mg balance.The chemical composition (wt.%)of pure Mg,produced by Shanxi Yinguang Huasheng Magnesium Co.,Ltd.,PR China,is

*Corresponding author.Tel.:+862423915897;fax:+862423894149.E-mail address:ywsong@ (Y.Song). Ltd.All rights reserved.

Electrochimica Acta 189(2016)190–195

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Electrochimica Acta

j o u rn a l h o m e p a g e :w w w.e l s e v i e r.c o m /l o c a t e /e l e c t a c t

The Special Role of Anodic Second Phases in the Micro-galvanic Corrosion of EW75 Mg Alloy

The Special Role of Anodic Second Phases in the Micro-galvanic Corrosion of EW75 Mg Alloy


0.01Si,0.01Al,0.0032Fe,0.0012Cu,0.01Mn,and Mg balance.For metallographic characterization,specimens were wet ground with water through successive grades of SiC abrasive papers from P320to P5000,followed by polishing with1m m diamond paste. Specimens for corrosion morphology observation were immersed in the3.5wt.%NaCl solution for different time intervals.Corrosion products were removed by chromic acid solution consisting of 180g là1CrO3.Surface and cross-section morphologies were observed by a scanning electron microscopy(SEM;Phillips XL30FEG)with the acceleration voltage of14kV.Sputtering was not carried out because the corrosion products were removed before SEM tests.Chemical composition was analyzed by energy dispersive X-ray spectroscopy(EDX)elemental mapping.Volta potential distributions of EW75were probed using a Scanning Kelvin Probe Force Microscope(SKPFM;Multimode3D,Bruker Corporation)in work function mode.The probe used in SKPFM measurements was magnetic etched silicon probe(MESP) (k=2.8N/m,Bruker Corporation,CA,USA)with piezoelectric tube size of125?125?5m m.Prior to each experiment,the tip was checked by performing a potential measurement on a gold reference sample.A dual scan mode was applied with tapping mode to obtain the surface topography signal and the cantilever to record the potential signal.The results were analyzed by Nano-Scope Analysis software.The specimens for Volta potential test were polished to1m m diamond paste and cleaned in an ultrasonic bath using ethanol[19].Electrochemical measurements were conducted in 3.5wt.%NaCl solution at room temperature (25?1 C)using a PARSTAT4000electrochemistry test system (Princeton Applied Research,USA).A classical three-electrode cell was used with platinum as counter electrode,saturated calomel electrode SCE(+0.242V vs SHE)as reference electrode and the samples with an exposed area of1cm2as working electrode. Polarization measurements started fromà250mV vs.OCP at a constant scan rate of0.5mV sà1and terminated until a?nal current density of approximately10mA cmà2.The scan frequency of Electrochemical Impedance Spectroscopy(EIS)ranged from 100kHz to10mHz with a perturbation amplitude of20mV.The EIS spectra were?tted using the ZSimpWin3.20software.Initial

The Special Role of Anodic Second Phases in the Micro-galvanic Corrosion of EW75 Mg Alloy

delay Fig.1.BSE-SEM morphologies and EDX elemental mapping of cast EW75:(a,b)Surface morphologies;EDX elemental mapping of:(c)Gd,(d)Y,(e)Nd and(f)Mg.

J.Liu et al./Electrochimica Acta189(2016)190–195191

of300s was set for polarization and EIS measurements to ensure a stable testing system.The tests were repeated at least three times to ensure the accuracy.

3.Results and discussion

3.1.Microstructural characterization

The microstructure of EW75is shown in Fig.1.It can be found from the low magni?cation morphology(Fig.1a)that many white second phases are discretely distributed at the grain boundaries. The Mg matrix near grain boundaries presents a light color(see arrow in Fig.1a),meaning the enrichment of REs.Two different second phases can be observed from the magni?ed image in Fig.1b, the block-shaped second phases(marked by I)rich in Y and Gd (Fig.1c and d)and the dendritic-shaped second phases(marked by II)rich in Y,Gd and Nd(Fig.1c–e).Both second phases consist of Mg and the more active REs.The volume fraction of the block-shaped second phases is much lower than that of dendritic-shaped second phases.The nobility of second phase in comparison with Mg matrix is investigated below.As for the microstructure of pure Mg[20], there is no precipitation phase observed.

3.2.Scanning Kelvin Probe Force Microscopy analysis

Volta potential distribution of EW75is shown in Fig.2.The dark regions in the optical micrograph of Fig.2a are the second phases and the marked areas are selected for SKPFM mapping.It is obvious that the second phases exhibit a darker color than the surrounding Mg matrix in Fig.2b.Because the block-shaped second phases with small size are mixed in the dendritic-shaped second phases,it is dif?cult to distinguish.Also,this result indicates the similar Volta potential of both second phases.Under the work function mode of SKPFM,the bright areas correspond to the more positive potential, while the dark areas correspond to the more negative potential.It indicates that the Mg matrix is nobler than second phases.Base on this result,it can be speculated that the second phases will act as micro-anodes to corrode preferentially,which are greatly different from the role of second phases in traditional Mg alloys.However, the Volta potential between the Mg matrix and second phases are only aboutà35mV according to the line-pro?le analysis result in Fig.2c,while second phases are100mV more positive than Mg matrix in other Mg alloys such as Mg–Zn and Mg–Al systems.The low Volta potential difference will imply a weak micro-galvanic corrosion of EW75.Because the volume fraction and size of the dendritic-shaped second phases are much higher than that of block-shaped second phase,the dendritic-shaped second phases will play a key role in the corrosion of EW75.Thus,more attention is paid to the dendritic-shaped second phase in the following research.

3.3.Corrosion behavior of EW75alloy in3.5wt.%NaCl solution

The corrosion morphologies of the specimens immersed in NaCl solution for different time intervals are shown in Fig.3.Most of the second phases on the surface have been dissolved while the matrix keeps intact after10min immersion(Fig.3a).In this stage,the second phases act as micro-anodes to corrode preferentially, whereas Mg matrix is left intact as micro-cathode.This result is in good agreement with SKPFM.After immersion time of30min, some shallow pits are visible on the Mg matrix(Fig.3b).With increasing immersion time to2h(Fig.3c),second phases can

hardly be seen and the Mg matrix becomes rough because of more severe corrosion.In addition,from the cross-section corrosion morphologies of30min and2h immersion(Fig.3e and f),the corrosion depth of the second phases is approximately4and 20m m,respectively.Though the second phases are corroded gradually,the Mg matrix is still remained.This case provides a convictive support for the anodic role of the second phases in EW75alloy.Moreover,some small corrosion pits are visible in

The Special Role of Anodic Second Phases in the Micro-galvanic Corrosion of EW75 Mg Alloy

the Fig. 2.SKPFM results of cast EW75:(a)optical micrograph,(b)surface Volt potential map,and(c)line-pro?le analysis of relative Volta potential through second phase.

192J.Liu et al./Electrochimica Acta189(2016)190–195

Mg matrix at 2h immersion from the observation of cross-section image in Fig.3f,which is rougher than that in Fig.3e.In this stage,the second phases are gradually dissolved out accompanying the corrosion of Mg matrix.At a longer immersion time,the corrosion morphology changes a little,just like the specimen after 72h immersion in Fig.3d.The second phases are slightly sunken in comparison with the surrounding Mg matrix.This corrosion status can be attributed to the removal of whole surface layer due to general corrosion.

3.4.Electrochemical tests

The surface of cast EW75after 2h immersion is in absence of second phases.It is speculated that its surface is homogeneous like pure Mg,but the matrix is solid solution with REs like EW75.To further clarify the surface status of EW75after 2h immersion,the specimens of pure Mg,as-received EW75and EW75after 2h immersion are compared by electrochemical tests.

From the comparison of polarization curves in Fig.4a,the cathodic side of EW75after 2h immersion is greatly different from EW75but greatly similar to pure Mg.In the case of anodic sides,EW75after 2h immersion shows a lower anodic current density

than pure Mg,which can be associated with the effect of surface oxide ?lm due to the solid solution of REs in the remained Mg matrix.The polarization curve results indicate that the surface status of EW75in the absence of second phases is similar to pure Mg.

The EIS results are shown in Fig.4b.All three plots consist of a high frequency capacitance loop followed by a medium frequency capacitance loop and a low frequency inductance loop.The high frequency capacitance loop is owing to the electric double layer at the interface of Mg substrate and solution [20].The dimension of EW75after 2h immersion is the largest one,indicating the lowest metal dissolution rate of EW75after 2h immersion.This result can be attributed to the weak micro-galvanic effect in the absence of second phases and little impurities in the remained Mg matrix.The medium frequency capacitance loops relate to the surface ?lm.The EW75after 2h immersion and as-received EW75are covered with more compact ?lms due to the effect of REs oxide,resulting in the larger radius of medium frequency loops than pure Mg.This result is in good accordance with the polarization curves of the lower anodic current of EW75after 2h immersion than pure Mg.

In order to further clarify the corrosion characteristics,the EIS spectra are ?tted by the equivalent circuits as shown in Fig.5.It

The Special Role of Anodic Second Phases in the Micro-galvanic Corrosion of EW75 Mg Alloy


Fig.3.BSE-SEM morphologies of cast EW75immersed in 3.5wt.%NaCl solution for:(a)10min (surface),(b)30min (surface),(c)2h (surface),(d)72h (surface),(e)30min (cross-section)and (f)2h (cross-section)after removing corrosion products.

J.Liu et al./Electrochimica Acta 189(2016)190–195


well known that there are oxide ?lms naturally formed on the surface of Mg alloys.However,it is unavoidable to exit some defects (micro-pores or cracks,etc.)in the oxide ?lms.The 2RQ in series can be used if the defects are uniformly distributed [22].Also,the EIS spectra can well be ?tted by this equivalent circuit.The ?tting results are listed in Table 1.R s is the solution resistance.R ct refers to the charge transfer resistance and Q dl represents the electric double layer capacity at the interface of magnesium alloy substrate and solution (the high frequency capacitance loop).The constant phase element Q is de ?ned by two values,Q dl and n dl .If n is equal to 1,Q is identical to a capacitor (C).Constant phase element is used in place of a capacitor to compensate for the inhomogeneity due to the existence of second phases,scratches and oxide ?lms on the sample surface.The R f and Q f (de ?ned by Q f and n f )represent the ?lm resistance and capacity,respectively (the

medium frequency capacitance loop).R L and L represent the resistance and inductance,respectively,indicating the initiation of corrosion (the low frequency inductance loop)[21,22].

According to the ?tting results in Table 1,the EW75after 2h immersion shows the highest R ct and R f values,implying the lowest metal dissolution rate and highest ?lm protection property.

The corrosion process of EW75can be explained based on the SKPFM,corrosion morphologies and electrochemical tests.The second phases are more chemically active than Mg matrix.In the initial stage,the second phases are preferentially dissolved as micro-anodes and Mg matrix is protected as micro-cathodes.Then slight corrosion of Mg matrix occurs due to the non-uniform microstructure (such as defects and chemical composition)at the interiority of Mg matrix to form corrosion micro-cells.When the second phases are dissolved very deeply (like 20m m)and only the Mg matrix is exposed on the surface,the micro galvanic effect can be ignored.Then the remained Mg matrix is gradually corroded again until the exposure of the internal second phases (see Fig.3d).The corrosion of EW75cyclically develops in the light of this mode.Finally,it presents a uniform corrosion status.


The SKPFM,immersion and electrochemical tests demonstrate that the second phases act as micro-anodes during the corrosion of cast EW75,which is greatly different from the role of second phases in traditional Mg alloys.The Volta potential difference between Mg matrix and second phases is lower,resulting in a relatively uniform corrosion

The Special Role of Anodic Second Phases in the Micro-galvanic Corrosion of EW75 Mg Alloy


Fig.5.Equivalent circuit of samples immersed in 3.5wt.%NaCl solution:(a)EW75,(b)pure Mg and EW75after 2h

The Special Role of Anodic Second Phases in the Micro-galvanic Corrosion of EW75 Mg Alloy

The Special Role of Anodic Second Phases in the Micro-galvanic Corrosion of EW75 Mg Alloy

The Special Role of Anodic Second Phases in the Micro-galvanic Corrosion of EW75 Mg Alloy


Fig. 4.Electrochemical comparison of EW75,pure Mg and EW75after 2h immersion in 3.5wt.%NaCl solution:(a)polarization curves and (b)EIS Nyquist plots.

Table 1

Fitting results of EIS spectra.

R s

(V cm 2)

Q dl

(mV à1cm à2s à1)n dl R ct

(V cm 2)

Q f

(mV à1cm à2s à1)n f R f

(V cm 2)


(H cm à2)


(V cm 2)

Pure Mg 19.0320.40.9256311.760080.6473140.97212



29.1422.20.9072271.240890.8316181.3EW75after 2h immersion










J.Liu et al./Electrochimica Acta 189(2016)190–195


Thanks for the?nancial support by the National Key Basic Research Program of China(No.2013CB632205)and the National Natural Science Foundation of China(No.51471174).


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