D0I:10.13374/j.issn1001-053x.1989.06.033 北京科技大学学报 第11卷第6期 Vol.11 No.6 1989年11月 Journal of U'niversity of Science and Technology Beijing Nov,1989 Physico-Chemical Behaviour of Rare Earth Elements and Ability for Corrosion Resistant' Li Wenchao(李文超)，Ling Yanping(凌燕平)， Li Ying(李英)，Ye Zhiyuan(叶志远) ABSTRACT:The physico-chemical behavior of rare earth elements in low sulphur 16Mn steel and their ability to enhance the corrosion resistance of the steel in sea water studied in this paper.The results indicate that Ce and La in 16Mn steel all have the abilities such as deoxidation,desulphidation,refining grains and cleaning grain boundaries and matrix etc.The anodic and cathodic polarization curves show that the rare earth elements added in 16Mn steel enhance the corrosion resistance of the steel in sea water. KEY WORDS:rare earth element,corrosion resistance,physico-chemical behavior 16Mn steel is widely used a material in industry for brige,shipping,auto- mobile and boiler container etc.Its ability for corrosion resistant at -40-450C is 20~38%higher than normal carbon steel.But the ability for corrosion resi- stant by sea water in 16Mn steel with rare earth element addition has been reported in a little paper. In paperst1 has been studied modifying of non-metallic inclusions in 16Mn stcel with rare earth elements.The non-metallic inclusions in the steel without rare earth elements were mainly MnS,aluminate and silicate in which the amount of the sulphide inclusions was up to 90%.Luyckx et al.t1 shows that MnS inclusion has completely disappeared when adding rare earth elements in alloys is CREJ/CS)3,whcre RE stands for rare earth elements.In present paper the ability for corrosion resistant by sea water in the 16Mn steel with addition of rare earth element Ce or La has been studied.The deoxidation ability by the elements and an appearance order of rare earth inclusions have been considered by thermodynamics. Manuscript Reccived May 8,1989 ··Dept,of Physical Chemistry 590
第 卷 第 期 北 京 科 技 大 学 学 报 年 月 下 。 一 。 。 一 ’ 床 李文 超 ， 犷 凌 燕 平 ’ 李英 ， ’ 。 叶 志 远 二 尹 一 几 一 ， ， 一 王 、 ， 一 ， ， 一 〕 丁 ， ， · 一 一 一 · 一 一 。 一 ， 。 一 ‘ 、 尸 〔 〕 〔 〕梦 ， 、 ， · · ， 二 丫 ， 户声 DOI ：10．13374／j ．issn1001－053x．1989．06．033
1 Experimental In a laboratory,under an argon atmosphere 16Mn steel with low sulphur melted with pure Fe,C,Mn and Si in MgO crucible was carried out at 1653+5C in a molybdenum wire furnace.After melting the rare earth element Ce or La was added in the melten.Then we took some specimens for analysis of compo- sition.The 5kg 16Mn steel ingots were melted in a induction furnace and added Ce or La at 1580C under an argon atmosphere. Standard impact specimens were made by a ingot which was worked by turning,forging,rolling,planing,milling,and grinding.The impact values were measured at -60~+60C.Metallographic specimens were taken from top of every ingot to study the rare earth inclusions.They were observed with JAX- 50A scanning electron microscope (SEM)and Cambridge Instrument Quantimet 900 (CIQ-900).Specimens for measurement of alloying amount were made as a cylinder shape from the ingot which had been worked by stripping skin, turning and grinding.Then they were electrolysed to study alloying amount of the rare earth element.Polarization specimens were made as a regular geometric crnament.After polishing they were mounted in epoxy resin as a electrode.The anodic and cathodic polarization curves of the 16Mn steel with rare earth addition were tested with CMS-351.After that the ratio of corrosion and electro-che- mical properties have been calculated. 2 Result and Discussion 2.1 Deoxidation and Desulfuration of Rare Earth Element in the 16Mn Steel Rare earth elements have a great affinity to oxygen and sulphur.The expe- rimental results are given in Table 1. Table 1 Results of deoxidation and desulfuration from 16Mn steel with Ce Ce ppm 0.0 50.0 60.0 65.0 0 ppm 20.0 12.0 10.0 17.0 s ppm 25.0 22.0 21.0 21.0 From the Table 1 it has found that deoxidation has a minimun in the range of 50~60 ppm amount of Ce.A reaction of deoxidation with rare earth ele- ments is x〔RE)+y〔O)=RExO, 591
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thus the contant of deoxidation is k=aREa=(fREC%RE])*(fo%]) then take the logarithm of it logk=xlogfRE+xlog〔%RE)+ylogfo+ylog〔%O〕 (1) because logfRE=eREC%REJ+eREC%O] logfo=e8c%O]+eC%RE] thus from formula (1)get logk=〔%RE)(xe+ye8)+〔%O〕(xeRe+ye8) +x/2.303ln〔%RE)+y/2.303ln〔%O) (2) by differentiation with respect to fcrmula (2)get d〔%O〕_ 0%RE)=-xe5+ye8)+￥/2.303C6RE) (xeRe+ye8)+y/2.303〔%O〕 If content〔%O〕with〔%RE)has minimum,it should be 0=0and0>0 IC dc%0)=0, dC"RE] We can get x/2.303C%RE]=-(xeRE+yeE) (4) Calculated with known data,we obtain that the content of oxygen has a minimum,while C%Ce]=51ppm in the 16Mn steel.It is almost in conformity with the data in Table 1. The products of deoxidation and 400, desulfuration are as rare earth inclu- 0. sions floating on the melten surface. The results from the observations in 200 SEM and CIQ-900 shown in Fig.1 Total area of inclusions 。Sr每n diamer>um 01 exhibit that a suitable amoumt of rare earth elements in the 16Mn stecl plays 20 40 a part in refinement of the inclusions RE,ppm,一 and cleaning the matrix.The total quantum of inclusions and the amount Fig.I Relation of an amount of of grain are minimum,while the amo- inclusions with a content of unt of rare earth elements in the rare earth olements 592
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16Mn steel is [RE]<60ppm.But C%RE]>60ppm,the total quantum and the size of inclusions are increased.This coincides with Luyckx4. Rare earth elements are surface activity.They can decrease a surface ene- rgy,that is,the energy of formation of nucleus with critical size is also decrea- sed.So the number of nucleus is increased. 2.2 Thermodynamic Analysis of Formation of Rare Earth Inclusions From the observations in CIQ-900 it found that the sulphide inclusion MaS in the 16Mn steel without any addition of rare earth elements is a strip shape along a rolling direction.When added a rare earth element achived [REJ/CS) 2.0,MnS inclusion has changed to spherical rare earth inclusion.And in this case Al2Os inclusion is completely disappeared.How explain these by thermo- dynamics.First all,It got the data of standard free energy of formation of rare earth compounds from paperst2,31 and calculated the activity products (taken Iwt as a standard state).They are shown in Table 2.Then using equation a=f and Igf,=e,where a:is activity of ith composition;f is activity coefficient of composition:is the precent content ofj compo- sition;e is interaction coefficient betweenih andj composition in the liquid 16Mn steel at 1873 KI6).The activitis and activity coefficients of every ele- ment in the steel before and after addition of rare earth elements are calculated. The results are listed in Table 3. Table 2 The standard free energy of formation of rare earth inclusions and activity products at 1873Kt2,3] Reaction △G°J/mole Activity product 〔Ce〕+2Co]=Ce02) -852720+249.96T Π.1=0.19×10-10 〔CeJ+3/2O)=1/2Ce20,() -714380+179.47T Π.2=0.29×10-10 〔Ce]+〔0]+1/2cS]=1/2Ce202S(.)-675700+165.50T Ⅱ.3=0.63×10-10 CCe)+〔S)=CeS(a) -422100+120.38TΠ.4=0.33×10-5 〔Ce)+3/2rS〕=112Ce2S3(a) -536420+163.86TⅡ.5=0.4×10-6 [Ce]+4/3CS]=1/3Ce,S( -497670+146.30TΠ.6=0.58×10-8 2CA1)+3〔0]=A12Oa() -1224740+390.00TΠ.7=0.16×10-19 [Ce]+[A]+3C]=CeAlOs( -1366460+364,30TⅡ.a=0.83×10-19 CCe)+〔N)=CeN() -173050+81.17TΠ.9=0.26 It also has calculated the free energy of formation RE inclusions in the 16Mn steel,which composition is 0.17%C,0.52%Si,1.54%Mn,0.014%P, 0.002%S,0.001%0,0.004%N,0.15%Al,0.004%Ce,as follows for reaction t CRE〕+y/xCO]-1/xCRE]xOyt)using the thermal equation△G=△G°+RTln 593
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(II/),where II.is activity product of reaction products:I.is activity product of reactants.The results have been listed in the Table 4. Table 3 The activities of elements In molten 16Mn steel 41×10-3 fo MffA1∫e。 Not added RE0.1730.9711.0391.0281,043 0.173149,502.084,11156.5 Added RB 0.1730.9711.0211,0281.0390,3470,173149.502.044.11155.911.39 Table 4 The free energy of formation of rare earth Inclusions in the 16Mn steel Reaction △GJ/mo1 △G1873,kJ/mol 〔Ce]+〔N)=CeN() -172890+181.53T +167.12 〔Ce]+2c0)=Ce0,t) -852720+448.67T +123.61 CCc)+3/2c0)=1/2Ce20() -714380+342.44T -72.98 〔Ce〕+〔0)+1/2〔S)=1/2Ce202S()-675700+317.95T -80.18 CCe)+CAl]+3]=CeAlO,( -1366460+650.58T -147,93 〔Ce)+[S)=CeS) -422100+226.57T +2.26 [Ce]+32CS]=1/2Ce2S3() -536420+295.80T +17.61 [Ce]+4/3CS]=1/3Ce,S( -497670+263.65T +7.39 〔A1)+3/2C0)=1/2Al2O,() -612370+318.47T +15.88 From the Table 4 it is concluded that Ce2O3()Ce2O2S()and CeAlO3( can be formed.Since Ce,O3()and Ce2O2S()can exchange each other,we must consider what is the final product.It has compared Ce23()with Cea O2S)as following,for the reaction Ce,0s)+〔S)=Ce202S()+〔O〕 △G°=RT1n(n,/Π？2) and △G=△G°+RTln(a(o)/a()=RTln(n3a(o,/Ⅱi2a() if AG<0,i.e.(I3())/(H32a(<1 or a(o)/a(<22/23=0.212.But at 1873K a(o)/a()=0.085,so inclusion Ce2O2S()should be formed in this experi- ments.And the same to CeA103(),the Ce202S()and CeAlO3(have been already observed in the experiments (see Fig.2). 2.3 Improving the Mechanical Properties of the 16Mn Steel by Addition of Rare Earth Elements In Fig.3 it is indicated that the fractography of impact samples without rare earth addition is a intercrystallic fracture,and that of the sample with opti- 594
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