Improvement of corrosion resistance of magnesium metal by rare earth elements

Electrochimica Acta53(2007)117–121Improvement of corrosion resistance of magnesiummetal by rare earth elementsToshihide Takenaka∗,1,Takami Ono,Yuji Narazaki,Yusuke Naka,Masahiro KawakamiDepartment of Production Systems Engineering,Toyohashi University of Technology,Toyohashi441-8580,JapanReceived29November2005;received in revised form6March2007;accepted9March2007Available online19March2007AbstractMg metal containing rare earth metals(REs)can be electrowon directly by molten salt electrolysis.The clarification of the optimum RE content in Mg is necessary tofix the electrolytic conditions in the direct electrowinning of Mg with RE.From this point of view,effect of RE addition in Mg metal on its corrosion property was studied in detail in this study.The specimen was prepared by adding La,Nd,or Ce in melted Mg metal, and its corrosion resistance was examined by an immersion test in3mass%-NaCl solution at room temperature.The corrosion resistance of Mg was improved greatly by adding a small amount of RE,whereas the excess addition of RE deteriorated the corrosion resistance.The optimum RE content was about0.5mass%.In this study,the corrosion property of Mg with an artificial surface oxide layer was also studied to clarify the effect of surface oxide.The corrosion resistance of Mg was particularly strengthened by conversion coating in a solution including La(NO3)3,Nd(NO3)3, or Ce(NO3)3,with Mg(NO3)2.This result suggests that the surface oxidefilm consisting of both Mg and RE gives ideal corrosion resistance to Mg metal.Mg metal with conversion coating including RE should also be of use as a corrosion-resistant material.©2007Elsevier Ltd.All rights reserved.Keywords:Magnesium;Rare earth;Corrosion resistance;Conversion coating;Surface oxide1.IntroductionMg metal is widely used because of its superior characteris-tics,and its demand for many purposes,such as in automobile manufacturing,should rise from now on.Mg is usually used as Mg alloys to improve its mechanical properties and corro-sion resistance.A rare earth element(RE)is a major alloying element,and some Mg–RE alloys have been commercialized. Since molten salt electrolysis is one of the typical produc-tion processes of both Mg and RE,the direct production of a Mg–RE alloy should be promising.In actuality,it was reported that Mg metal containing RE was obtained by electrolysis in molten salts with a RE chloride orfluoride[1,2].Our group also reported that Mg metal containing La could be produced by molten salt electrolysis,and that the La content in Mg could be controlled by the electrolytic conditions[3].The clarifica-∗Corresponding author.Tel.:+81532446695;fax:+81532446690.E-mail address:takenaka@pse.tut.ac.jp(T.Takenaka).1ISE member.tion of the optimum RE content in Mg is very important tofix the electrolytic condition in the direct electrowinning of Mg with RE.One of the important effects of RE on the corrosion resistance of Mg is the so-called“scavenger effect”,i.e.,some impurity elements in Mg,such as Fe,severely deteriorates its corro-sion resistance[4],and RE is said to cancel their influence by the formation of intermetallic compounds with the impurities. Meanwhile,the corrosion resistance of Mg mainly depends on the surface oxidefilm,i.e.,the corrosion property of Mg should be affected by RE addition since the surface oxidefilm changes with the RE addition.In this study,the corrosion resistance of Mg containing some REs against salt water was measured.The influence of RE addi-tion on the corrosion resistance of Mg was investigated,and the possibility of the direct electrodeposition of Mg metal with the optimum RE content was discussed.The corrosion resistance of Mg with surface oxide coating was also measured,and the results were compared with those of Mg containing RE.Based on these experiments,the mechanism of the improvement of the corrosion resistance of Mg by RE was discussed.0013-4686/$–see front matter©2007Elsevier Ltd.All rights reserved. doi:10.1016/j.electacta.2007.03.027118T.Takenaka et al./Electrochimica Acta53(2007)117–121Table1Composition of Mg metalElement ContentMg(%)>99.93Impurity(ppm)Al110Si40Mn170Fe280Zn100Cu20Ni52.Experimental2.1.SpecimenCommercial Mg metal,whose composition is shown in Table1,was melted with a certain amount of La,Ce,or Nd (Hirano-Seizaemon,3N)at about973K in an electric furnace filled with high-purity Ar(5N).The melt was stirred with Mo impellers for a few hours,and cooled in the furnace.The ingot was cut,and the segregation was checked by SEM(JEOL,JSM-6300).The center part of the ingot was cut rectangularly,and embedded in resin form except for one face(about10mm2).The face without resin was polished with emery paper(#1500),and degreased with ethanol and acetone.To form an artificial surface oxide layer on Mg,conversion coating was done by using a solution containing Mg(NO3)2 (Kanto-chemical,hexahydrate,special-grade)of a concentra-tion of10−3mol/dm3,and/or RE nitrate of10−3mol/dm3. La(NO3)3,Ce(NO3)3,and Nd(NO3)3(Kanto-chemical,hexahy-drate,special-grade)were used as the RE nitrate.A piece of Mg metal mentioned above was embedded in resin form except for one face(about10mm2).The face without resin was pol-ished with emery paper(#1500),and degreased with ethanol and acetone.The specimen was immersed in the nitrate solution at room temperature for1day,and dried under airflow for1day at363K in order to denitrate.The surface of the specimen was wiped softly with a tissue paper to remove powdery deposits on the surface.2.2.Corrosion resistivity measurementThe specimen was immersed in a NaCl solution of30g/dm3 at room temperature.The specimen was taken out periodically, and the corrosion product on the surface was removed with a hard plastic brush.The specimen was weighed quickly,and immersed again in the solution.This procedure should have accelerated the corrosion in comparison with simple immersion.The surface of the specimen before and after the corrosion test was observed by SEM and analyzed by EDX(Shimadzu,ED AX DX-4)and XRD(Rigaku,RINT-2500).The polarization curve of the specimen was measured at room temperature.The specimen was immersed in a NaCl solution of30g/dm3,and the potential was shifted from its immersion potential with10mV/min by a potentio-stat(Hokuto Denko, HA-305)and a function generator(Hokuto Denko,HB-104). The changes in the potential and current were recorded by a personal computer connected with the potentio-stat.3.Results and discussion3.1.Corrosion resistance of Mg containing RETypical appearances of the Mg containing La after the corro-sion test of50h are shown in Fig.1with those of Mg without La and commercial Mg alloy,AZ31.Mg without La was seri-ously corroded as shown in Fig.1(a).The corrosion resistance of Mg containing0.3mass%-La was significantly improved as shown in Fig.1(c).Mg metal containing0.6mass%-La shown in Fig.1(d)had almost equal corrosion resistivity with AZ31 shown in Fig.1(b).Only Mg(OH)2was detected by XRD in the corrosion product of each specimen.Fig.2shows the weight changes in the Mg containing La during the corrosion test.When the La content was less than 1mass%,the larger the La content was,the greater the corro-sion resistance.The weight change in the specimen containing 0.6mass%was smaller than that of AZ31.However,the cor-rosion resistance worsened with the increase in the La content when the La content was more than1mass%.The Mg containing 3mass%-La still had ideal corrosion resistivity,but the Mg con-taining9mass%-La was corroded considerably.Fig.3shows the weight changes in the Mg containing Nd.The corrosion resis-tance of Mg was significantly improved by the Nd addition.Even the addition of0.1mass%-Nd was very effective.The specimen had better corrosion resistance than AZ31when the Ndcontent Fig.1.Appearances of(a)Mg,(b)AZ31,(c)Mg with0.3mass%-La and(d) Mg with0.6mass%-La after50h immersion in salt water.T.Takenaka et al./Electrochimica Acta 53(2007)117–121119Fig.2.Weight change of Mg containing La in saltwater.Fig.3.Weight change of Mg containing Nd in salt water.was about 0.1–1.0mass%.Fig.4shows the weight changes in the Mg containing Ce.Although the corrosion resistance was considerably increased by the Ce addition,the effect of the Ce addition seemed inferior to those of La and Nd.The optimum Ce content for the corrosion protection was 0.3mass%,but the corrosion resistance was still worse than that of AZ31.The cor-rosion resistance worsened with the increase in the Ce content when the Ce content was more than 0.3mass%.Fig.4.Weight change of Mg containing Ce in saltwater.Fig.5.Change in polarization curve with La addition in Mg.It was reported that the effect of the La addition on the corro-sion resistance of Mg was related to the change in the structure of Mg with La addition [3].Only one phase was found when the La content was less than 1mass%,whereas two phases,such as a Mg-based solid solution and a eutectic texture of Mg and Mg 9La,were seen when the content was more than 1mass%.The difference between the effects of La,Nd,and Ce may depend on their solubilities in Mg,though the accurate values at room temperature are still uncertain [5].Fig.5shows the change in the polarization curves with the La addition.The average corrosion current densities estimated from the weight change after the corrosion test of 50h are also plotted as an index of the static immersion test.The average corrosion current density of the Mg without La seemed slightly larger than that expected from the polarization curve.This deviation can be explained well by the increase in the surface area during the long-term corrosion test.Conversely,the result of the polarization curve at Mg containing 0.3mass%-La does not agree with that of the static immersion test seemingly.The immersion potential shifted less-noble with the La addition and the current density increased overall,though the corrosion resistance was improved by the La addition in the immersion test.The cathodic curves of the Mg containing 0.3mass%-La changed gradually near its immersion potential,while that of the Mg without La seemed to immediately reach the limiting current.This result suggests that the cathodic reaction on Mg is inhibited by the La addition.The corrosion resistance of Mg was improved by the La,Nd,or Ce addition.The optimum RE content was about 0.5mass%,though their effects of each RE were somewhat different.It was reported that Mg metal containing this optimum La or Nd con-tent could be electrodeposited in a molten NaCl–KCl–MgCl 2mixture with LaCl 3[3]or NdCl 3.These results also suggest that Mg metal with mixed REs has ideal corrosion resistance,and that it can be electrowon directly in molten salt including mixed RE chlorides.3.2.Corrosion resistance of Mg with conversion coating A thin coating layer was formed on Mg by the conversion coating.The surface after the coating was colored pale yellow,120T.Takenaka et al./Electrochimica Acta53(2007)117–121Fig.6.SEM image of Mg surface with conversion coating by using solution containing10−3mol/dm3-Mg(NO3)2and10−3mol/dm3-La(NO3)3.but the Mg substrate was seen through the layer.Fig.6shows the SEM image of the surface after the coating by using the solution containing both Mg(NO3)2and La(NO3)3.A typical island-like structure was seen on the whole surface by the SEM observation, while no crack was found by laser microscopic observation.The crevices seen in Fig.6seemed to befilled with a substance with different composition or different density.The coating layer was not seen clearly in the cross section of the specimen.The layer should not be thick nor strong mechanically.Mg,La and O were mainly detected in the area in Fig.6 by the EDX analysis as shown in Table2.The XRD patterns of the specimens treated in the solution containing Mg(NO3)2 and La(NO3)3are shown in Fig.7(a),and that of Mg(NO3)2 and Ce(NO3)3are shown in Fig.7(b).In Fig.7(a),the peaks of the Mg substrate mainly appear in the pattern.A peak at around2θ=140◦is not due to Mg,but cannot be identified as any Mg nor La compound.The pattern of Fig.7(b)is similar to that of Fig.7(a).The peaks due to Mg are mainly seen in Fig.7(b),but the small peaks of CeO2are also found.No peak of Mg oxide and hydroxide are observed in Fig.7(a)and(b). These results indicate that the coating layer was thin and that the low crystallized compounds might have been formed under the experimental condition.The composition in Table2can also involve the influence from the substrate.Fig.8shows the weight change in the specimen with or without the conversion coating during the corrosion test.The Table2Composition of Mg with conversion coating by using solution containing 10−3mol/dm3-Mg(NO3)2and10−3mol/dm3-La(NO3)3(atom%by EDX) Element ContentMg75.6La 6.4O16.4Cl 1.0Fe0.6Fig.7.XRD patterns of Mg with conversion coating by using(a)solution containing10−3mol/dm3-Mg(NO3)2and10−3mol/dm3-La(NO3)3,and(b) solution containing10−3mol/dm3-Mg(NO3)2and10−3mol/dm3-Ce(NO3)3. corrosion resistance of Mg was improved by conversion coating by using the solution of Mg(NO3)2to some extent.The spec-imen treated in the solution of La(NO3)3sometimes showed very good corrosion resistivity,but the reproducibility of the results was poor.The corrosion resistance of Mg was remark-ably increased by the coating using a solution containing both Mg(NO3)2and La(NO3)3.The result was reproducible,and did not strongly depend on the concentration of the nitrates.These results suggest that the surface oxidefilm consisting of both Mg and La gives good corrosion resistance to Mg.It should be also emphasized that the specimen with the coating preserved its ideal corrosion resistance even after the periodical polish with a hard plastic brush.Fig.9shows the weight change in the specimen treated in the solution of some RE nitrates with Mg(NO3)2.Conver-sion coating by using the solution of Nd(NO3)3or Ce(NO3)3 Fig.8.Influence of conversion coating on weight change of Mg in salt water.T.Takenaka et al./Electrochimica Acta53(2007)117–121121Fig.9.Influence of rare earth metal on weight change of Mg with conversion coating in salt water.with Mg(NO3)2was also effective in the corrosion protec-tion of Mg,and the difference in the effect of RE was not obvious.The polarization curve of the specimen with the conversion coating was similar to that of the Mg containing La shown in Fig.5.The immersion potential shifted to less-noble with the coating,and the current density increased overall.Since the cathodic curves changes gradually near its immersion poten-tial,it was suggested that the cathodic reaction on Mg was also inhibited by the coating.It was shown that the corrosion resistance of Mg was partic-ularly improved by the surface oxide layer consisting of both Mg and RE.This result suggests that the change in the surface oxidefilm with the RE addition strongly affects the improvement of the corrosion resistance of Mg containing RE.Moreover, the formation of a surface oxide layer consisting of both Mg and RE on Mg should be promising as its corrosion protection process is done without toxic substances,such as chromium or fluorine.4.ConclusionThe corrosion resistance of Mg was significantly improved by the La addition.There was a suitable La content in the cor-rosion protection of Mg,and the excess addition deteriorated the corrosion resistivity conversely.The additions of Nd and Ce in Mg were also effective in the improvement of its corrosion resistance,but their effects were somewhat different.The conversion coating by using the solution containing both Mg(NO3)2and La(NO3)3particularly increased the cor-rosion resistance of Mg,whereas the effect of the coating by using the solution including only Mg(NO3)2or La(NO3)3was limited.The conversion coating using Nd(NO3)3or Ce(NO3)3 with Mg(NO3)2were also effective in the improvement of the corrosion resistance.These results suggest that the corrosion resistance of Mg is bettered by the surface oxide layer consist-ing of both Mg and RE,and that the change in the surface oxide film with the RE addition strongly affects its corrosion resistance of Mg containing RE.Mg with ideal corrosion resistance should be produced easily by molten salt electrolysis because Mg containing the suitable amount of RE can be directly electrodeposited.The formation of a surface oxide layer consisting of both Mg and RE is also promising as the corrosion protection process of Mg without using toxic substances,such as chromium orfluorine.References[1]R.A.Sharma,JOM48(1996)39.[2]D.P.Zhang,D.Q.Fang,J.Wang,D.X.Tang,H.Y.Lu,L.S.Zhao,J.Meng,Proceedings of the1st Asian&9th China–Japan Bilateral Conference on Molten Salt Chemistry&Technology,2005,p.137.[3]T.Takenaka,Y.Naka,N.Narukawa,T.Noichi,M.Kawakami,Electrochem-istry73(2004)706.[4]J.D.Hanawalt,C.E.Nelson,J.A.Peloubet,Trans.AIME147(1942)273.[5]F.A.Shunk,Constitution of Binary Alloys,McGraw-Hill,New York,1969(2nd supplement).。