The corrosion behavior of Ce-implanted magnesium alloys
Xuemin Wang a,?,Xiaoqin Zeng a ,Shoushan Yao a ,Guosong Wu a ,Yijian Lai b
a National Engineering Research Center of Light Alloys Net Forming,Shanghai Jiaotong University,Shanghai 200030,PR China b
Instrumental Analysis Center,Shanghai Jiaotong University,Shanghai 200030,PR China
A R T I C L E D A T A
A B S T R A C T
Article history:
Received 27February 2007Received in revised form 11May 2007
Accepted 11May 2007The aim of this study was to investigate the effect of cerium ion implantation on the corrosion behavior of AZ31magnesium alloys.Samples were implanted with doses of 5×1016,1×1017and 5×1017ion/cm 2,respectively,using a metal vapor vacuum arc (MEVVA)source at an extraction voltage of 45kV.Auger electron spectrometry (AES)and X-ray photoelectron spectroscopy (XPS)were used to analyze the depth distribution and the valence states of elements in the implanted layer,respectively.The potentiodynamic polarization technique was applied to evaluate the corrosion resistance of the implanted samples in a 3.5wt.%NaCl solution saturated with Mg(OH)2.The results showed that under an optimal dose the corrosion resistance of the implanted sample was improved compared with that of the bare sample.Finally,the corrosion mechanism of the Ce-implanted samples was discussed.
?2007Elsevier Inc.All rights reserved.
Keywords:Ion implantation Magnesium alloys Corrosion resistance
X-ray photoelectron spectroscopy (XPS)Auger electron spectroscopy (AES)
1.Introduction
As the least noble material (E °=?2.34V vs normal hydrogen electrode),magnesium has a high tendency to atmospheric and galvanic corrosion [1,2].The galvanic corrosion can cause severe pitting in the metal,which leads to the decreased mechanical stability and an unattractive appearance.Conse-quently,this disadvantage has greatly hindered the wide-spread use of magnesium alloys as light materials in electronic,aerospace and automobile industries [3,4].Addi-tions of rare earth elements into magnesium alloys have been proved to increase the corrosion resistance,but alloying Mg with RE elements is relatively difficult in practice.Therefore,surface modification seems to be an effective method to overcome this disadvantage.Up to now,many surface techniques such as electrochemical plating [5,6],microarc oxidation [7],physical vapor deposition (PVD)[8,9],laser cladding [10]and ion implantation [11,12],etc.have been utilized to improve the corrosion resistance of magnesium alloys.
Within these surface techniques,ion implantation offers the unique possibility to induce a controlled concentration of an element into a thin surface layer.Nearly all kinds of dopant can be implanted into the substrate.The temperature of the process is relatively low,which prevents from the unwanted modification of bulk properties [13,14].Thus,ion implantation has been applied and proved to be feasible to increase the corrosion resistance of materials such as stainless steels and zircaloy alloys [15,16],etc.Although several mechanisms have been proposed to understand the effect of ion implantation on the corrosion properties,the corresponding mechanisms are still not very clear.Moreover,in the case of magnesium alloys,only a few works have been reported on the corrosion behavior of the alloys after ion implantation treatment [17,18].
In the present study,an attempt has been made to improve the corrosion resistance of AZ31magnesium alloys by cerium ion implantation.The characterization of the implanted layer was analyzed by X-ray photoelectron spectroscopy (XPS)and Auger electron spectroscopy (AES).The corrosion resistance of
M A T E R I A L S C H A R A C T E R I Z A T I O N 59(2008)618–623
?Corresponding author.Tel./fax:+862162933487.
E-mail addresses:wangxuemin75@https://www.doczj.com/doc/8011207875.html, ,wangxuemin75@https://www.doczj.com/doc/8011207875.html, (X.
Wang).1044-5803/$–see front matter ?2007Elsevier Inc.All rights reserved.doi:
10.1016/j.matchar.2007.05.006
the implanted samples was investigated by potentiodynamic polarization tests.
2.Experimental
The samples were die cast AZ31magnesium alloys (Al:3wt.%,Zn:1wt.%,Mg:balance),and the corresponding sizes were 10mm×10mm×3mm.Before implanting,one side of each sample was mechanically polished with silicon carbide paper up to 1500grid,and then ultrasonically rinsed in acetone.Then,cerium ion implantations,at nominal doses of 5×1016,1×1017and 5×1017ion/cm 2,were undertaken using an MEVVA source with an extraction voltage of 45kV.The background pressure of the metal vapor vacuum arc implanter target chamber was 1.4×10?3Pa.Although the implantation system had no magnet analytic capability,the extracted cerium ions were expected to consist of 3%Ce +,83%Ce 2+,14%Ce 3+.During implantation process,the samples were not cooled.Therefore,the implantation temperature depended on the beam current density.To reduce the heating effects,the ion current density
was controlled to be below 26.6μA/cm 2.Thus,the maximum implantation temperature was expected to be no more than 523K.
After argon ion erosion of a 50nm depth outermost layer,the valence states of elements in the implanted layer were analyzed by X-ray photoelectron spectroscopy (THERMO ESCALAB 250,Al K α1486.6eV).To compensate for the systematic error in XPS measurement,all the binding energies were first adjusted to the C 1s signal at 285eV.Auger electron spectrometer (PHI-550ESCA/SAM)was applied to obtain the composition and depth profiles of elements in the implanted layer.A 2keV argon ion beam with a current density of 100μA/cm 2was used to measure the depth profiles,and the sputter rate was estimated to be approximately 10nm/min.
To evaluate the corrosion behavior,potentiodynamic polarization tests were performed using a Parstat 2273potentiostat system.The tests were carried out in a 3.5wt.%NaCl solution saturated with Mg(OH)2,and the scan rate was 1mV/s.In addition,the micro-morphologies of the samples after immersion test were observed by Scanning electron microscopy (FEI,
SIRION-200).
Fig.1–Ce 3d spectra for cerium ion implantation:(a)5×1016ions/cm 2;(b)1×1017ions/cm 2;(c)5×1017ions/cm 2
.
Fig.2–Mg 2p spectra for cerium ion implantation:(a)5×1016ions/cm 2;(b)1×1017ions/cm 2;(c)5×1017ions/cm 2
.Fig.3–Al 2p spectra for cerium ion implantation:(a)5×1016ions/cm 2;(b)1×1017ions/cm 2;(c)5×1017ions/cm 2
.
Fig.4–O 1s spectra for cerium ion implantation:(a)5×1016ions/cm 2;(b)1×1017ions/cm 2;(c)5×1017ions/cm 2.
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3.
Results and Discussion
3.1.
Valence States of Elements in the Implanted Layer
Figs.1–4show the adjusted high resolution XPS spectra of Ce 3d,Mg 2p,Al 2p and O 1s,respectively.As shown in Fig.1,in all cases,the Ce 3d region shows a position (BE Ce 3d 5/2=885.6eV [19])and a band-shape typical for Ce 3+species [20].Besides Ce 3+species,the presence of Ce 4+is confirmed by the presence of
the satellite structure at 916.2eV (Fig.1),contributed to a “shake-down ”charge transfer process [21].Although,the concentration of Ce 3+and Ce 4+can be determined from the following equation [22]:?Ce 3t
?Ce 3t
Ce 3t
tCe 4t
e1T
where the notations Ce 3+and Ce 4+represent the corresponding sums of the integrated peak areas,related to the Ce 3+and Ce 4+XPS signals,respectively.Due to the effect of ultra high vacuum or the X-ray beam induced reduction of Ce 4+to Ce 3+,it is difficult to exactly determine the concentration of Ce 3+in ceria by Eq.(1).So,it can only be confirmed that both Ce 2O 3and CeO 2exist in the implanted layer.The formation of Ce 2O 3and CeO 2may be explained by the normal free enthalpy of them (?1818and ?1087kJ/mol for Ce 2O 3and CeO 2,respec-tively [23]),and cerium has a great chemical affinity to oxygen.
Deconvolution analysis of the XPS spectra for Mg 2p (Fig.2)indicates that the binding energy at 50.6eV corresponds to MgO,while that at 49.8eV to Mg [24].Fig.3shows the high resolution XPS spectra for Al 2p,which display a single peak centered at 73.4eV.The peaks at 73.4eV may be related with the presence of Al 2O 3[25].As regards O 1s (Fig.4),the peaks present two components.One at 530.3eV can be attributed to both Ce 2O 3[26]and MgO.While the other one at the shoulder at 532.5eV can be due to the H 2O absorbed on the surface [27].The oxygen may come from the residual gas in the vacuum chamber because of the not very high vacuum level of the target chamber.Therefore,magnesium and cerium can be concluded to exist in the implanted layer in the form of MgO and Ce 2O 3,CeO 2,respectively.
3.2.Depth Profiles of Elements in the Implanted Layer
AES measurements were carried out to determine the concentration of elements in the implanted layer as a function of depth.Fig.5(a)–(c)shows the AES spectra of the samples implanted with cerium ions using doses of 5×1016,1×1017and 5×1017ion/cm 2,respectively.From the figures,it is found that the greater the dose is,the thicker the oxide layer has (～110,～130and ～210nm for 5×1016,1×1017and 5×1017ion/cm 2
,
Fig.5–(a)–(c)AES spectra of the samples implanted with doses of:(a)5×1016ions/cm 2;(b)1×1017ions/cm 2;(c)5×1017ions/cm 2
.
Fig.6–Mg AES peaks across the implantation layer for 5×1017Ce ions/cm 2.
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respectively).This can be due to the promotion of the surface oxidation of magnesium alloys because of cerium ion implan-tation.In all cases,the shapes of cerium distributions fit Gauss shape,and the peak concentrations are 16at.%,21at.%,26at.%corresponding to 5×1016,1×1017and 5×1017ion/cm 2,respectively.For magnesium distributions,the profiles follow a V-like distribution,due to the overlap of the decrease of Mg 2+and the increase of metallic magnesium across the implanted layer [28].This result is further confirmed by the shift of magnesium Auger peak position from air/implanted layer interface to substrate (see Fig.6),which denotes the change of MgO to Mg.In the outermost layer,the ratio of Mg/O is greater than 1.0,which indicates this layer is composed of MgO with excess Mg 2+[29].
3.3.Corrosion Property
The potentiodynamic polarization curves of the implanted and bare samples are shown in Fig.7.Fig.8presents the passive current density (i corr )as a function of dose (values are summarized in Table 1).As shown in Figs.7and 8,when the dose is 1×1017ions/cm 2,the corrosion current density of the sample is the lowest in all implantation doses and one order
lower than that of the bare sample.The corresponding corrosion potential (E corr )as a function of dose is plotted in Fig.9(values are summarized in Table 1).In agreement with the results in Fig.8,the sample implanted with the dose of 1×1017ions/cm 2possesses the noblest corrosion potential.Therefore,these findings demonstrate that with an optimal implantation dose the corrosion resistance of magnesium alloys can be improved.
Fig.10shows the micro-morphologies of the bare sample and the sample implanted with dose of 1×1017ions/cm 2after immersion in 3.5wt.%NaCl solution for 1.5h.In Fig.10,it can be seen that the morphology of the bare sample exhibits a porous structure,which is composed of tiny erect laminas.While that of the implanted sample changes into a relatively dense structure.Such a relatively dense structure can be beneficial to decreasing the corrosion rate of the substrate.Moreover,for the bare sample,the damage is found on most of the surface and pitting corrosion initially takes place at the edges of the sample.While for the implanted sample,no obvious corrosion damage is observed.This demonstrates that after cerium ion implantation with dose of 1×1017ions/cm 2the corrosion resistance of AZ31sample can be improved,in agreement with the conclusion of the above potentiodynamic polarization tests.
4.Discussion
It is well known that for magnesium alloys,pitting corrosion is the main corrosion form in solution containing Cl ?.But if a compact MgO layer was formed,the transfer of Cl ?would be reduced and become more difficult than that of
water
Fig.7–Effect of cerium ion implantation on the
electrochemical behavior of AZ31:(a)5×1016ions/cm 2;(b)1×1017ions/cm 2;(c)5×1017ions/cm 2;(d)
bare.
Fig.8–Corrosion current density vs the implanted doses.Table 1–The relationship between the passive current density,corrosion potential and the implantation doses Dose
(ions/cm 2)
Passive current density (A/cm 2)
Corrosion potential
(V vs SCE)
None 2.91×10?5?1.455×1016 2.88×10?5?1.481×1017 2.81×10?6?1.375×1017
3.26×10?5
?
1.47
Fig.9–Corrosion potential vs the implanted doses.
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molecules.As a result,the initial reaction could be described as the following Eqs.(2)and (3)[30]:MgO tH 2O →Mg eOH T2e2TMg t2H 2O →Mg eOH T2tH 2
e3T
According to the above AES results,cerium ion implanta-tion is found to promote the surface oxidization of magnesium alloys (110nm →210nm).Moreover,the oxide layer can be compacted by the ion bombardment effect,and the higher the implantation dose is,the greater the ion bombardment effect is.Therefore,the induced MgO layer can improve the corrosion resistance of magnesium alloys,in agreement with other report [31].
On the other hand,reactive elements such as yttrium,cerium and other rare earths play an important role in improving the corrosion resistance [32–34].They can be presented as alloy or oxide dispersoid addition.The oxide dispersoid addition can act as a barrier to decrease the corrosion current density.In our study,after ion implantation,cerium exists in the forms of Ce 2O 3and CeO 2as an oxide dispersoid addition.Therefore,the improvement of the corrosion resistance may be ascribed to the formed oxide dispersoid addition after cerium ion implantation.
Under the effects of both MgO layer and the oxide dispersoid addition,it would expect that the higher the implantation dose was,the better the corrosion resistance of the sample became.However,the best corrosion resistance
sample is it that implanted with the dose of 1×1017ions/cm 2(see Section 3.3).This fact may be due to the implantation damage effect.It is known that the implantation damage would reduce the corrosion resistance of metals [35,36].Moreover,the damage level is linearly increased with the implantation dose.In this way,if only considering the effect of implantation damage,the corrosion resistance of the samples would decrease with the increasing of the implantation dose.Therefore,under the hybrid effects of MgO layer protection,oxide dispersoid addition and implantation damage,there is an optimal implantation dose which can best improve the corrosion resistance of magnesium alloys.In our study,the implantation dose with 1×1017ions/cm 2may be optimal.
5.Conclusions
Surface analyses were carried out on the cerium ions implanted AZ31magnesium alloys.The XPS and AES data indicated that cerium existed in the form of Ce 2O 3and CeO 2as an oxide dispersoid addition in the implanted layer.Moreover,the ion implantation promoted the oxidization of the magne-sium surface,and the higher the implantation dose was,the thicker the oxide layer had.After cerium ion implantation,the improved corrosion resistance of magnesium alloys has been achieved.Under the hybrid effects of oxidization protection,oxide dispersoid barrier and implantation damage,the sample implanted with dose of 1×1017ions/cm 2possessed the best corrosion resistance.
Acknowledgements
The authors would like to acknowledge the support of Huiliang Zhou from the Shanghai Testing Center of China for the Auger electric spectra analysis,and the XPS analysis from the physical and chemical analysis research center of the University of Science and Technology of China.
R E F E R E N C E S
[1]Mathieu S,Rapin C,Hazan J,Steinmetz P.Corrosion behaviour
of high pressure die-cast and semi-solid cast AZ91D alloys.Corros Sci 2002;44:2737.
[2]Subba Rao RV,Wolff U,Baunack S,Eckert J,Gebert A.
Corrosion behaviour of the amorphous Mg65Y10Cu15Ag10alloy.Corros Sci 2003;45:817.
[3]Gray JE,Luan B.Protective coatings on magnesium and its
alloys —a critical review.J Alloys Compd 2002;336:88.[4]Clow BB.Magnesium industry review.Adv Mater Process
1996;150:33.
[5]Gu C,Lian J,Li G,Niu L,Jiang Z.Electroless Ni –P plating on
AZ91D magnesium alloy from a sulfate solution.J Alloys Compd 2005;391:104.
[6]Sharma AK,Suresh MR,Bhojraj H,Narayamurthy H,Sahu RP.
Electroless nickel plating on magnesium alloy.Metal Finish 1998;96:6.
[7]Liang J,Guo B,Tian J,Liu H,Zhou J,Xu T.Effect of potassium
fluoride in electrolytic solution on the structure and
properties of microarc oxidation coatings on magnesium alloy.Appl Surf Sci
2005;252:345.
Fig.10–(a)–(b)Micro-morphologies of the samples after 1.5h immersion in 3.5%NaCl solution:(a)the bare sample;(b)the sample implanted with dose of 1×1017Ce ions/cm 2.
622
M A T E R I A L S C H A R A C T E R I Z A T I O N 59(2008)618–623
[8]Wu GS,Zeng XQ,Li GY,Yao SS,Wang XM.Preparation and
characterization of ceramic/metal duplex coatings deposited on AZ31magnesium alloy by multi-magnetron sputtering.
Mater Lett2006;60:674.
[9]Hoche H,Blawert C,Broszeit E,Berger C.General corrosion
and galvanic corrosion properties of differently PVD treated magnesium die cast alloy AZ91.Adv Eng Mater2003;5:896.
[10]Jun Y,Sun GP,Wang HY,Jia SQ,Jia https://www.doczj.com/doc/8011207875.html,ser(Nd:YAG)
cladding of AZ91D magnesium alloys with Al+Si+Al2O3.
J Alloys Compd2006;407:201.
[11]Tian XB,Wei CB,Yang SQ,Fu RKY,Chu PK.Corrosion
resistance improvement of magnesium alloy using nitrogen plasma ion implantation.Surf Coat Technol2005;198:454. [12]Zeng XQ,Wu GS,Yao SS.Formation by reactive magnetron
sputtering of TiN coating on Ti-implanted magnesium alloy.
Mater Lett2006;60:2252.
[13]Ittermann B,Fullgrabe M,Stockmann HJ,Kroll F,Peters D,
Ackermen H.Vacancy distribution around isolated
implantation sites.Appl Phys Lett2005;87:131907.
[14]Nordlund K,Keinonen J,Ghaly M,Averback RS.Coherent
displacement of atoms during ion irradiation.(London)
1999;49:398.
[15]Pham MT,Rao KRM,Manna I,Richter E.Enhanced corrosion
resistance of austenitic stainless steel by plasma immersion ion implantation of nitrogen.J Mater Sci Lett2003;22:1099. [16]Peng DQ,Bai XD,Chen XW,Zhou QG,Liu XY,Yu RH.
Comparison of electrochemical behavior of zirconium and
zircaloy-4implanted with Y and Ce ions.Appl Surf Sci
2004;221:259.
[17]Bakkar A,Neubert V.Improving corrosion resistance of
magnesium-based alloys by surface modification with
hydrogen by electrochemical ion reduction(EIR)and by
plasma immersion ion implantation(PIII).Corros Sci
2005;47:1211.
[18]Tian XB,Wei CB,Yang SQ,Fu RKY,Chu PK.Water plasma
implantation/oxidation of magnesium alloys for corrosion
resistance.Nucl Instrum Methods Phys Res B Beam Interact Mater Atoms2006;242:300.
[19]Praline G,Koel BE,Hance RL,Lee HI,White JM.X-ray
photoelectron study of the reaction of oxygen with cerium.
J Electron Spectrosc Related Phenom1980;21:17.
[20]Spanier JE,Robinson RD,Zhang F,Chan SW,Herman IP.
Size-dependent properties of CeO2-y nanoparticles as studied by Raman scattering.Phys Rev B2001;64:245407.
[21]DabalàM,Armelao L,Buchberger A,Calliari I.Cerium-based
conversion layers on aluminum alloys.Appl Surf Sci
2001;172:312.[22]Zhang J,Wang BJ,Ju X,Liu T,Hu TD.Structural characteristics
of cerium oxide nanocrystals prepared by the microemulsion method.Chem Mater2001;13:4192.
[23]Knacke O,Kubaschewski O,Hesselmann K.Thermochemical
properties of inorganic substances.Berlin,Heidelberg:
Springer-Verlag;1991.
[24]Hsiao HY,Tsai WT.Characterization of anodic films formed
on AZ91D magnesium alloy.Surf Coat Technol2005;190:299.
[25]Moulder JF,Stickle WF,School PE,Bomben KD.In:Chastain J,
editor.Handbook of X-Ray Photoelectron Spectroscopy.Eden Praine,MN,USA:Perkin-Elmer;1992.
[26]Mullins DR,Overbury SH,Hunsley DR.Electron spectroscopy
of single crystal and polycrystalline cerium oxide surfaces.
Surf Sci1998;409:307.
[27]Abreu CM,Cristobal MJ,Novoa XR,Pena G,Perez MC,Serra C.
Influence of chromium and cerium implantation in the
electrochemical development of passive layers on AISI304L.
Electrochim Acta2004;49:3057.
[28]Zeng XQ,Wang QD,Lu YZ,Ding WJ,Zhu YP,Zhai CQ,et al.
Behavior of surface oxidation on molten Mg-9Al-0.5Zn-0.3Be alloy.Mater Sci Eng A Struct Mater Prop Microstruct Process 2001;301:154.
[29]Zhang WQ.Handbook of Metal Corrosion,vol.56.Shanghai:
Shanghai Science and Technology Press;1987.(in Chinese).
[30]Chen M,Yu QS,Reddy CM,Yasuda HK.Model study
investigating the role of interfacial factors in electrochemical impedance spectroscopy measurements.Corrosion2000;56
(7):709.
[31]Yamamoto A,Tsubakino H.Surface treatment of magnesium
alloys by artificial corrosion–oxidization method.Mater Trans 2003;44:511–7.
[32]Wan Q,Bai X,Liu X.Impact of yttrium ion implantation on
corrosion behavior of laser beam welded zircaloy-4in sulfuric acid solution.Appl Surf Sci2005;252:1974.
[33]Arenas MA,Garcia I.De Damborenea,X-ray photoelectron
spectroscopy study of the corrosion behaviour of galvanised steel implanted with rare earths.Corros Sci2004;46:1033. [34]Peng DQ,Bai XD,Chen XW,Zhou QG,Liu XY,Deng PY.Effect
of cerium ion implantation on the aqueous corrosion
behavior of zircaloy-4.Appl Surf Sci2003;218:7.
[35]Fukumoto S,Yamamoto A,Terasawa M,Mitamura T,
Tsubakino H.Microstructures and corrosion resistance of
magnesium implanted with nitrogen ions.Mater Trans
2001;42:1232.
[36]Nakatsugawa I,Martin R,Knystautas EJ.Improving corrosion
resistance of AZ91D magnesium alloy by nitrogen ion
implantation.Corros Sci1996;52:921.
623
M A T E R I A L S C H A R A C T E R I Z A T I O N59(2008)618–623