文档库

最新最全的文档下载
当前位置:文档库 > A chrome-free conversion coating for magnesium–lithium alloy by a phosphate–permanganate solution

A chrome-free conversion coating for magnesium–lithium alloy by a phosphate–permanganate solution

A chrome-free conversion coating for magnesium –lithium alloy by a

phosphate –permanganate solution

Hua Zhang a,?,Guangchun Yao a ,Shulan Wang b ,Yihan Liu a ,Hongjie Luo a

a

School of Materials and Metallurgy,Northeastern University,Shenyang 110004,China

b

School of Science,Northeastern University,Shenyang 110004,China

Received 28March 2007;accepted in revised form 30July 2007

Available online 17August 2007

Abstract

A chrome-free conversion coating on magnesium –lithium alloy was obtained from a phosphate –permanganate solution.The morphology,the composition and the corrosion resistance of this coating were examined.The thin and non-penetrating cracked morphology with some deposits existed on the phosphate –permanganate conversion coating.The main elements of the conversion coating were Mg,O,K,P and Mn.The results of the electrochemical measurements and the immersion tests demonstrated that the corrosion resistance of the magnesium –lithium alloy has been improved by the phosphate –permanganate conversion treatment.?2007Elsevier B.V .All rights reserved.

Keywords:Magnesium –lithium alloys;Chrome-free conversion coating;Corrosion resistance;Phosphate –permanganate

1.Introduction

Magnesium –lithium alloys are the lightest structural metallic alloys currently available [1,2].The addition of lithium to magnesium results in a considerable weight saving and makes them lighter than conventional magnesium alloys.Therefore,they are undoubtedly most promising in light-weight engineering applications,aerospace field and automobile industry [3–5].Besides,magnesium –lithium alloys have advantages of high electrical and thermal conductivity,good ratio of strength to weight,high ability to dampen shock waves,easy forming at room temperature and so on [6].However,the inclusion of lithium makes magnesium –lithium alloys possess poor corrosion resis-tance,which greatly restricts their use in practice [7].Therefore,it is urgently necessary to find a suitable protection method to improve the corrosion resistance of magnesium –lithium alloys.There are a number of available methods to protect magnesium alloys.These include conversion coatings [8],anodizing [9],vapor-phase processes [10],electroless plating [11]and laser surface modification technique [12].Among

them,the conversion coating treatments are comparatively cheap,easily to operate and hence are widely used.It is well known that conversion coatings for magnesium alloys have been traditionally based on chromate ions.However,the use of chromate solution is being progressively restricted due to the high toxicity of the hexavalent chromium compounds [13].Recently,lots of chromate free conversion coatings are developed,such as phosphate/phosphate –permanganate con-version coatings,rare earth conversion coatings,stannate conversion coatings [14–16].However,there has been little published information on conversion coating treatments for magnesium –lithium alloys,except that A.K.Sharma has obtained highly stable chromate conversion coatings on magnesium –lithium alloys [6].As mentioned above,chromate is harmful to the environment;therefore,it is urgently needed to develop new environment-friendly conversion treatments for magnesium –lithium alloys.

The aim of the present study is to develop a chrome-free conversion coating on magnesium –lithium alloy.The selected solution was phosphate and permanganate and encouragingly resulted in the formation of a continuous and adherent layer on the substrate.The morphology,the composition and the corrosion resistance of this coating were examined.

Available online at http://www.wendangku.net/doc/f0800614af1ffc4fff47ac32.html

Surface &Coatings Technology 202(2008)1825–

A chrome-free conversion coating for magnesium–lithium alloy by a phosphate–permanganate solution

A chrome-free conversion coating for magnesium–lithium alloy by a phosphate–permanganate solution

1830

http://www.wendangku.net/doc/f0800614af1ffc4fff47ac32.html/locate/surfcoat

?Corresponding author.Tel.:+862483686462;fax:+862483682912.E-mail address:zh2506@http://www.wendangku.net/doc/f0800614af1ffc4fff47ac32.html (H.Zhang).

0257-8972/$-see front matter ?2007Elsevier B.V .All rights reserved.doi:10.1016/j.surfcoat.2007.07.094

2.Experiments 2.1.Materials

Highly pure Mg (N 99.9%),Li (N 99.9%)as well as Zn (99.9%)were melted and cast in a low carbon steel crucible under the protection of 75%Li +25%LiF and Ar gas at 993K and then the alloys were solidified in a steel mould.Subsequently,the ingots were homogenized for 12h at 523K.Finally,the plates with the thickness of 15mm were cold rolled to 4mm.The chemical composition of the alloy is 10wt.%Li,1wt.%Zn and 89wt.%Mg.2.2.Specimen preparation and surface treatment

Specimens were prepared from as-prepared magnesium –lithium alloys with the size of 15×10×4mm.Prior to formation of conversion coating,specimens were ground up to a 1000-grit finish on silicon carbide paper to obtain an even surface and subsequently washed in acetone and alkaline detergent,respectively.Specimens were rinsed in flow deionized water between each step to remove all the contaminations.Then specimens were immersed in a bath consisting of permanganate

and phosphate for conversion treatment.After the coatings were formed,the specimens were rinsed in flow deionized water and died with hot air.The detail alkaline detergent solution composition,the conversion solution composition and the corresponding operation conditions are shown in Fig.1.The traditional chromate conversion solution was also used for comparison.A digital temperature controller was used to make the temperature of conversion treatment solution constant.2.3.Surface analysis

The surface and cross-sectional morphologies were observed by a SSX-550scanning electron microscopy (SEM),equipped with the energy-dispersive X-ray (EDX)analysis facilities.A MKII X-ray photoelectron spectroscopy (XPS)analysis was used to examine the chemical composition of conversion coating.The X-ray source was the K αpeak of aluminum.All binding energy values were charge-corrected to the adventitious C 1s signal,which was set at 284.60eV.2.4.Corrosion tests

2.4.1.Electrochemical measurements

Chronopotentiometric (E ~t)and potentiodynamic tests were performed using a Princeton Applied Research (PAR)EG&G model 273A.A three-electrode electrochemical cell was used with a saturated calomel electrode (SCE)as the reference electrode and a graphite rod as the auxiliary electrode.Specimens as the working electrode were embedded in the mixture of paraffin and rosin with the volume fraction of 1:1to isolate the non-studying surfaces.A 3.5wt.%NaCl solution was used as the test electrolyte.All experiments were conducted at room temperature.The polarization range was from ?0.25V ~+0.5V versus the open circuit potential.The scanning rate was 1mV/s in anodic and cathodic directions.Before potentiodynamic polarization tests the working electrode was immersed in the solution for 10min.2.4.2.Immersion tests

Salt immersion method was also introduced to test the corrosion resistance of coatings.The time interval between

A chrome-free conversion coating for magnesium–lithium alloy by a phosphate–permanganate solution

the

Fig.1.Flow chart of surface treatment process for magnesium –lithium

A chrome-free conversion coating for magnesium–lithium alloy by a phosphate–permanganate solution

alloys.

Fig.2.Microstructure of the bare magnesium –lithium alloy after rolling.

1826H.Zhang et al./Surface &Coatings Technology 202(2008)1825–1830

beginning of the test and the first corrosive spot appearing on the coating surface could donate the corrosion resistance of the coatings.Because magnesium alloy is sensitive to Cl?for pitting corrosion,the conventional corrosive electrolyte-artifi-cial seawater(3.5wt.%NaCl solution)was used for corrosion test.For comparison,the bare magnesium–lithium alloy and the traditional chromate conversion coating were also tested.All the specimens were immersed in3.5wt.%NaCl solution,then the

surface condition of each specimen was observed carefully.And the corrosion degree of each specimen was used to evaluate the corrosion resistance.

3.Results and discussion

3.1.Microstructure of the bare magnesium–lithium alloy

According to the magnesium–lithium phase diagram[17], magnesium–lithium alloys exhibit two phase structures(α+β) with lithium content between5and11wt.%.Forαphase,it is a solution of lithium in magnesium with hexagonal close-packed (hcp)structure,whileβphase is a solution of magnesium in lithium with body-centered cubic(bcc)structure.Therefore,the alloy with10wt.%lithium content used in this study has the dual phase structure,just as shown in Fig.2.It can be clearly seen that magnesium–lithium alloy after rolling is constituted withαphase andβphase,and the grey irregularαphase is surrounded by the darkβphase.The volume fraction of each phase is calculated based on the phase diagram and is about 28%αand72%β.

3.2.Morphology and composition of the conversion coating

Fig.3shows the surface morphology of the conversion coating on magnesium–lithium alloy treated with the phos-phate–permanganate solution.It can be seen that some white particle-like deposits with different diameters and irregular cracks were formed on the surface of the coating.These cracks are distributed all over the surface of the conversion coating.

The surface morphology of conversion coating can be closely related to the microstructure of the substrate[18].In this paper,the magnesium–lithium alloys exhibit two phase structures,consisting of magnesium-richαand lithium-richβphases.The electrochemical potential of binary alloys is related to the thermodynamic equilibrium potentials of the two components of the alloy[19].The standard electrochemical potential of lithium(?3.045V versus normal hydrogen electrode(NHE))is lower than that of magnesium(?2.375V versus NHE)[20],therefore,the potential of the lithium-richβphase should be also lower than that of the magnesium-richαphase.This creates a local cell effect betweenαphase andβphase and the micro-current must be formed in the substrate due to the potential difference of the dual phase structure when the magnesium–lithium alloy was immersed in the conversion solution.Consequently,βphase(the anodic area)dissolved and the hydrogen released fromαphase(the cathodic area).This release of hydrogen caused the cracks to be formed on the surface of the coating during the conversion treatment,as shown in Fig.3.

Table1shows the EDX results of different areas in Fig.3. The compositions of the conversion coating are mainly compounds of Mg,O,P,K and Mn.The P,K and Mn in the coating were from the phosphate–permanganate solution,while the Mg was believed to be from the substrate alloy.The ions of P,Mn and K in treatment solution have been involved in the conversion reactions resulting from their presence in EDX. At the same time,it can be seen that these elements'contents (wt.%)at different points were different.The white deposit (spot A)had relatively high content of O,P,K and Mn.There was also certain amount of O,P,K and Mn in the top surface of the conversion coating(spot B),while in the crack of the coating(spot C),Mg is highly enriched.

Fig.4a shows the cross-sectional microstructure of the conversion coating on magnesium–lithium alloy treated with the phosphate–permanganate solution.It is apparent that a thin layer has indeed adhered compactly to the surface of the magnesium–lithium alloy.Fig.4b is the EDX of the phosphate–permanganate conversion coating on magnesium–lithium alloy. According to the element distribution from the coating surface to the substrate along the line labeled in Fig.4a,it can be seen that the coating produced by the phosphate–permanganate process does not have obvious boundary with the substrate. Therefore,the coating was connected tightly to the substrate.

XPS was used in order to better understand the chemical composition of the conversion coating.The XPS analyses of conversion coating on magnesium–lithium alloy treated with the phosphate–permanganate solution are shown in Fig.5.The XPS analysis on the surface by large area scanning coating are Table1

EDX results of the phosphate–permanganate conversion coating

Different zone Mg,wt.%O,wt.%P,wt.%Mn,wt.%K,wt.% Spot A26.3640.2416.2011.13 6.07 Spot B52.2124.8712.147.46 3.32 Spot C74.4712.458.68 2.27

A chrome-free conversion coating for magnesium–lithium alloy by a phosphate–permanganate solution

2.13 Fig.

3.Surface morphology of the conversion coating on magnesium–lithium

alloy treated with the phosphate–permanganate solution for20min at328K.

1827

H.Zhang et al./Surface&Coatings Technology202(2008)1825–1830

composed of Mg,Mn,O,P and K (Fig.5a),which was consistent with the EDS result.From the Mg2p and Mn2p analyses (Fig.5b and c),it can be found that magnesium takes the form of Mg(OH)2and MgO,and manganese in the coating takes the form of Mn 2O 3and MnO 2.Phosphate and potassium have not been identified due to the lack of reference information,but they should also be included in the conversion coating,according to XPS results.

3.3.Chronopotentiometric (E ~t)measurements

E ~t measurement was used to record the change of open circuit potential (OCP).The change of OCP as a function of immersion time can be used to monitor the chemical stability and corrosion process of specimens during immersion.In this study,the corrosion processes of the bare alloy and the phosphate –permanganate conversion coating in 3.5wt.%NaCl solution for approximately 2000s are represented in Fig.6.

In the case of the bare magnesium –lithium alloy,the corrosion potential (E corr )increased from an initial value of ?1.639V SCE to a stable value of E =?1.624V SCE .The open potential increased to 0.015V SCE within the determination time.It has the relatively low equilibrium open potential value (?1.624V SCE ).This

indicated that the bare magnesium –lithium was not steady in 3.5wt%NaCl solution compared with the phosphate –perman-ganate conversion coating (the equilibrium OCP is ?1.603V SCE ).For the phosphate –permanganate conversion-coated mag-nesium –lithium alloy,the corrosion process can be divided into two stages.In the first stage,an obviously jagged potential fluctuation was observed in the range I in the Fig.6.The OCP

A chrome-free conversion coating for magnesium–lithium alloy by a phosphate–permanganate solution

of

Fig.4.Cross-sectional microstructure of as-prepared conversion coating on magnesium –lithium alloy (a)and qualitative chemical analyses (b)scanning from the coating surface to the substrate along the line labeled in the

A chrome-free conversion coating for magnesium–lithium alloy by a phosphate–permanganate solution

figure.

Fig.5.XPS analyses of the phosphate –permanganate conversion coating.

1828H.Zhang et al./Surface &Coatings Technology 202(2008)1825–1830

the phosphate–permanganate conversion coating in NaCl

solution decreased rapidly from the initial potential of ?1.628V SCE to the lowest value(?1.681V SCE)and then increased quickly to the highest value(?1.570V SCE).Such

violent fluctuation of the OCP indicated the heterogeneous

reactions,which may arise from the non-uniform composition

and structure of the conversion coating.In the second stage

(range II in Fig.6),the OCP fluctuation was diminished to

rather low amplitude and the potential was decreased slightly

and steadied around?1.603V SCE.This relative stable stage of

OCP illustrated that there were some slow and regular reactions

in the conversion coating in this period.The OCP increased to

0.025V SCE within the test time.Its equilibrium open potential

value(?1.603V SCE)was higher than that of the bare

magnesium–lithium alloy(?1.624V SCE).This indicated that

the stability of the specimen with phosphate–permanganate

conversion coating was better than that of the specimen with

nature oxide film on magnesium–lithium,that is,the corrosion

resistance of magnesium–lithium alloy can be improved through the conversion treatment in a solution containing phosphate and permanganate in appropriate conditions.

3.4.Potentiodynamic polarization studies

Corrosion current density(i corr)and corrosion potential (E corr)are frequently used to evaluate corrosion resistance of

A chrome-free conversion coating for magnesium–lithium alloy by a phosphate–permanganate solution

the Fig.7.Potentiodynamic polarization curves of the magnesium–lithium alloy

without and with phosphate–permanganate conversion coating in3.5wt.%

NaCl

A chrome-free conversion coating for magnesium–lithium alloy by a phosphate–permanganate solution

solution.

Fig.6.The curves of open circuit potential-time for the magnesium–lithium

alloy without and with the phosphate–permanganate conversion coating in

3.5wt.%NaCl

A chrome-free conversion coating for magnesium–lithium alloy by a phosphate–permanganate solution

solution.

Fig.8.Surface morphologies of the magnesium–lithium alloy without(a)and with

traditional chromate conversion coating(b)after1h immersion and with

phosphate–permanganate conversion coating after10h immersion(c)in3.5wt.%

NaCl solution.

1829

H.Zhang et al./Surface&Coatings Technology202(2008)1825–1830

conversion coating.For the two polarization curves,the anodic curve was the important feature related to the corrosion resistance,while the cathode reaction corresponded to the evolution of the hydrogen[21].Fig.7shows the polarization curves of the bare alloy and the phosphate–permanganate conversion coating.

The corrosion potential of the coating treated by phosphate–permanganate process was?1.570V SCE,which was obviously higher than that of the bare alloy(?1.625V SCE).The corrosion current density of the former was somewhat lower than that of the latter.At the same time,at a given potential,the corrosion current density of the phosphate–permanganate conversion coating was lower than that of the bare alloy.According to Lu's work on the electrochemical corrosion behavior of chromate conversion coatings of Ni-based alloys in0.5M H2SO4solution [22],conversion coating exhibited a decrease in the corrosion current at a given potential,demonstrating that the corrosion resistance of the substrate alloys has been improved.Therefore, the phosphate–permanganate conversion coating played a protection for the magnesium–lithium alloy.

3.5.Immersion tests

In order to further evaluate the corrosion resistance of conversion coatings,immersion tests were performed.

As soon as the bare alloy and the specimen with the chromate conversion treatment were immersed in3.5wt.%NaCl solution, some continuous streams of gases quickly evolved from their surfaces.And the quantity of gases from the former was larger than that of the latter.For the bare alloy,some homogeneous corrosion pits emerged on its whole surface only for5min immersion,and the corrosion areas on the alloy gradually expanded with the increasing immersion time.After immersion for1h,grey–white corrosion products were observed on the overall surface of the alloy.The specimen with chromate conversion coating revealed a different corrosion process. Localized corrosion was randomly distributed on the surface of chromate coating after the immersion of5min.Increasing immersion time also resulted in extensive surface corrosion,but the corrosion degree was lower than that of the bare alloy in the same test time.Fig.8a and b shows the surface morphologies of the bare alloy and the chromate conversion coating after immersion in salt solution for1h,respectively.It is clearly seen that the bare alloy was uniformly covered with corrosion products,while the specimen with chromate conversion coating was pitting corrosion.However,for phosphate–permanganate conversion coating,a dense structure was formed and acted as a barrier to corrosive medium as shown in Fig.4a.In the period of the immersion,almost no gas bubbled on the surface of the specimen and no obvious corrosion trace could be observed even up to10h immersion.From Fig.8c,it is apparent that the phosphate–permanganate conversion coating was still intact and almost no obvious corrosion products were formed on it.

It seems that the traditional chromate conversion coating could only offer protection for magnesium–lithium to a certain extent,while the phosphate–permanganate conversion coating could provide a good protection to the substrate and effectively hindered Cl?s to the substrate.

4.Conclusions

(1)A uniform chrome-free conversion coating treated with a

phosphate–permanganate solution was formed on the magnesium–lithium alloy.This conversion coating was obtained in the aqueous solution at the following operating conditions:

KMnO440g/L;KH2PO450g/L;328K;20min. (2)EDX and XPS analyses confirmed that the phosphate–

permanganate conversion coating were mainly composed of elements Mg,O,K,P and Mn.

(3)The results of electrochemical and immersion tests

confirmed that the phosphate–permanganate conversion coating could offer protection for magnesium–lithium alloy.

References

[1]A.K.Sharma,R.U.Rani,A.Malek,K.S.N.Acharya,M.Muddu,S.Kumar,

Met.Finish.94(1996)16.

[2]A.K.Sharma,R.Uma Rani,S.M.Mayanna,Thermochim.Acta376

(2001)67.

[3]A.Sanschagrin,R.Tremblay,R.Angers,D.Dube,Mater.Sci.Eng.,A Struct.

Mater.:Prop.Microstruct.Process.220(1996)69.

[4]J.F.Li,Z.Q.Zheng,S.C.Li,W.D.Ren,Z.Zhang,Mater.Sci.Eng.,A

Struct.Mater.:Prop.Microstruct.Process.433(2006)233.

[5]H.Takuda,T.Enami,K.Kubota,N.Hatta,J.Mater.Process.Technol.101

(2000)281.

[6]A.K.Sharma,Met.Finish.87(1989)73.

[7]S.J.Wang,G.Q.Wu,R.H.Li,G.X.Luo,Z.Huang,Mater.Lett.60(2006)

1863.

[8]K.H.Yang,M.D.Ger,W.H.Hwu,Y.Sung,Y.C.Liu,Mater.Chem.Phys.

101(2007)480.

[9]C.S.Wu,Z.Zhang,F.H.Cao,L.J.Zhang,J.Q.Zhang,C.N.Cao,Appl.

Surf.Sci.253(2007)3893.

[10]R.C.Wolfe,B.A.Shaw,J.Alloys Compd.437(2007)157.

[11]J.Z.Li,Y.W.Tian,Z.Q.Huang,X.Zhang,Appl.Surf.Sci.252(2006)

2839.

[12]A.Kouadri,L.Barrallier,Mater.Sci.Eng.,A Struct.Mater.:Prop.

Microstruct.Process.429(2006)11.

[13]M.Dabalà,K.Brunelli,E.Napolitani,M.Magrini,Surf.Coat.Technol.

172(2003)227.

[14]L.Kouisni,M.Azzi,M.Zertoubi,F.Dalard,S.Maximovitch,Surf.Coat.

Technol.185(2004)58.

[15]K.Brunelli,M.Dabalà,I.Calliari,M.Magrini,Corros.Sci.47(2005)989.

[16]M.A.Gonzalez-Nunez,C.A.Nunez-Lopez,P.Skeldon,G.E.Thompson,

H.Karimzadeh,P.Lyon,T.E.Wilks,Corros.Sci.37(1995)1763.

[17]Z.Shi,M.L.Liu,D.V.Naik,J.L.Gole,J.Power Sources92(2001)70.

[18]O.Lunder,J.C.Walmsley,P.Mack,K.Nisancioglu,Corros.Sci.47(2005)

1604.

[19]K.R.Baldwin,R.I.Bates,R.D.Arnell,C.J.E.Smith,Corros.Sci.38(1996)

155.

[20]Y.Liang,Y.Che,Manual for Thermodynamic Data of Mineral,Northeast

University publishing house,1993.

[21]W.X.Zhang,J.G.He,Z.H.Jiang,Q.Jiang,J.S.Lian,Surf.Coat.Technol.

201(2007)4594.

[22]G.J.Lu,E.T.Adab,G.Zangari,Electrochim.Acta49(2004)1461.

1830H.Zhang et al./Surface&Coatings Technology202(2008)1825–1830