CHEM.RES.CHINESE UNIVERSITIES 20 1 0,26(5),822—828 Numericalumerical Simulation for imulation Ior reviCreviqce orrosion of Corrosion of 304 Stainl,ess Steel Steel —in Sodium Chloride Solution WANG Wei , ,SUN Hu.yuan , ,SUN Li-juan ,SONG Zu.wei ' and ZANG Bei.ni , ,。 1.Institute ofOceanology,Chinese Academy ofSciences Qingdao 266071 P.R China; 2.Graduate University ofChinese Academy ofSciences,Beo'ing 100049,P R.China; 3.ShandongProvincialKeyIaloratory ofCorrosionScience,Qingdao266071,P R.China Abstract The authors presented a mechanistic model describing the chemical reactions within a corroded thin.nar. row crevice.In the mathematical model,a two-dimensional steady-state was used to predict the crevice pH profile by taking into account dissolved oxygen and hydrogen ions within the crevice.It consists of six parallel electrochemical reactions:multi naodic reactions(Fe,Cr,Ni dissolution reactions)and htree cathodic reactions(hte oxygen reduction, the hydrogen reaction and water dissociation).Current density distribution and oxygen concentration distribution were determined to be corresponding to the evolution of potential distribution wimin the crevice.The contribution of each metal reaction to the overall corrosion process was in proportion to the mole fraction.and the simulation pro. vided fl good agreement with published experimental results for the crevice corrosion of stainless steel in sodium chloride solution. Keywords Crevice corrosion;Finite element method(FEM);Current densi ;pH;Multi naodic reaction Article ID 1005-9040(20101.05.822.07 1 IntrOduCtiOn was applied to crevice nad pitting corrosion in carbon Crevice corrosion is one of the most destructive stee1.And the model was used to simulate various ex. periments reported in the literature[9' ̄ ̄】Wang Pt [1l】 .forms of the localized corrosion of stainless stee1. which occurs as a result of the occluded eell that presented a probabilistic model to investigate the ini. ofrms in a crevice on the steel surface.It Icads to the tial pit size,corrosion piting current,and material existence of a locally occluded region where anode properties via the experimental data for aircraft mate. dissolution can Occur.It iS well known that crevice rials.And the numerical model was developed based corrosion is initiated when the crevice solution rea- on an oxygen reduction control process.for crevice corrosion of Lap ioints[12】Chang已t 【 】focused .ches the chemical reaction condition under which the crevice can be locally activated.Corrosion of iron is their attention on evaluating transport processes and chemical reactions during the initiation stage of cre. costly and,hence,has been the subject of many stu. dies.Owing to the high cost nad long duration of cor- vice corrosion in 3 1 6L stainless steels via their ad. vanced mathematical mode1.Vuillemin et a1.【 pre. rosion experiments,development of improved predic— tion models by numerical simulation is required. sented a simple approach for heterogeneous reactions There have been a number of mathematical mo— in a post--processing step via determining the condi-- dels of evolution of hte chemical conditions within a tions of potentia1.external chemistry or geometry un. passive crevice reported in tl1e literature[ 一】der which gas or solid phases could appear in the ere- .In some cases.the predicted values of many models in the li— vice.Kennell et a1.【” presented a numerical modeJ of terature have been compared with experimenta1 data. crevice corrosion without considering the hydrogen Sources of possible inaccuracy in both the models’ evolution reaction,it predicted the pH profile and formulation and the input data have been identified in demonstrated t}le net deposition of metal at the crevice some of htese studies.Sharland/ developed a pre. tip caused by an overall potential drop.Agarwal et a1.【 J developed na analytical mode1 for hte current dictive and self-consistent mathematical model which distribution on an elec仃ode film in crevice corrosion. Corresponding author.E—mail:sun@ms.qdio.ac.cn Received October 20,2009;accepted March 22,20 1 0. Supported by the National Natural Science Found ̄ion of China(No.40776044)and hte Knowledge Innovation Program of Chinese Acadenry of Sciences(No.KZCX2-YW-2 10). NO.5 Ⅵ NGWei 口Z 823 It identifled the conditions for the saturation of the cathodic current as well as the trends in the variation of the cathode capacity as a function of elec仃ode length.However,the overall theoretical approach served as all inspiration for the present work. The authors presented a new model for the evo— lution of the crevice corrosion reactions within a pas— sive crevice.It includes a detailed description Of bOth the chemical reactions and the ionic trnasport within the crevice.In order to limit the complexiyt of the model we have used a steady state solution of the mass transport equations in a two—dimensional crevice. where multi anodic reactions such as Cr,Ni oxidation. and mass transport of ions tender concentration gra. dients were considered in the simulation.Mass ba. 1ance for oxygen concentration and hydrogen ion concentration,and Laplace’s equation for the steel potential in the crevice solution were used to derive the model equations.The results show that corrosion current densiyt rises remarkably near the crevice mouth and drops rapidly with increasing depth into hte crevice.An oxygen concentrtaion gradient is the cause of the oxygen current density.Partial consump— tion of oxygen in the boundary layer leads to a de- creased iron corrosion rate.The calculated results were compared with a series of experimental data from the literature[10】and other modeling work with the aim of interpreting the empirical results and to help validate the mode1.The model may be used for describing crevice corrosion of a wide range of stain— less steels containing chromium and nickel although input parameters are different. 2 Crevice ModeI 2.1 Description of the Problem and Assumption Usually a crevice has a rectangle shape,and the model which has a rectangular crevice of length,and width w with active walls corroding is defined.Mode】 result presented in this paper was validated against the experimental data of Alavi and Cottis【 刚.In their ex- periment,the dimensions of the crevice were 8 cmx 2.5 cmx90 gm.The crevice walls were made from AISI 304 stainless steel coupled to a cathode made rfom the same steel immersed in the aerated bulk electrolyte which consisted of 0.6 mol/L NaCl solu. tion.The system is schematically illustrated in Fig.1. The present model system has the following hy- potheses: Bulk electrolyte Pas Fig.1 Schematic diagram of corroding crevice (1)The precipitates such as Fe(OH)2,Cr(OH)3 and Ni(OH)z do not provide a barrier to the transport of reaction species in the crevice solution. (21 The stee1 surface outside the crevice is as. sumed to be covered wiht a passive film which is an excellent barrier to oxygen,hydrogen ion permeation nad there is sufifcient cathodic charge in the bulk so. 1ution to drive crevice corrosion. (31 No fluid flow effects need be included and the electrolyte is static. (4)Chemical reactions inside the crevice are supposed at equilibrium. (5)Alavi and Cottis[1 ̄】found that the chloride activity within the crevice increased slightly compared tO that of the bulk of 0.6 mol/L NaC1 solution.So.the bulk solution is assumed to be aerated containing Na十 nad C1一supporting ions.The effect of other compo— nents of stainless steel on the crevice corrosion rate is neglected because it is insigniifcant as can be justiifed below 2.2 Description of the Model The corrosion model was developed based on corrosion reactions,oxygen,hydrogen ion difusion and reaction,and overall charge balnace.The six most important reactions for modeling the corrosion beha- vior of steelin a crevice rae three anodic iron dissolu. tions,oxygen reduction,hydrogen ion reduction,and water reduction reaction. 2.2., Governing Equations Under steady state conditions,Laplace equation is used to describe the electricaI potentia1.E in the crevice solution: (1) Mass conservation governs the dissolved oxygen nad hydrogen ion concentration in the crevice solution ofr hte crevice mode1.Hence, 824 CHEM.RES.CHINESE UNIVERSITIES Vl01.26 + 。 ay=。 (2) 耸+耸:0 where Co,is molar concentration of oxygen and CH+is molra concentration of hydrogen ions. 2.2.2 Anodic Reactions The model has the assumption that stainless steel compositions rae 74%Fe,1 8%Cr,8%Ni,while other impurities of the stainless steel are not considered. Passive corrosion in an aqueous solution produces a lfow of metallic ions into the aqueous solution by anodic reactions such as FeM Fe (aq)+2e, 。=ioFe 1 0‘眦 r。f (4) CrM—÷cr3 (aq)+3e,icr=i ̄Cr10‘肼 一 cr(5) NiM—÷Ni (aq)+2e,l‘Ni= i l 0;( N (6) where iM, M, Ml.ref and bM are the current density, exchange current density,equilibrium potential(vs. SCE)and Tafel slope of each metal oxidation,respec— tively.Subscript“M”represents the anodic species of Fe.Cr andNi. All anodic reactions occur simultnaeously in the crevice.The model is assumed that the contribution of each metal reaction to the overall corrosion process is in proportion to the mole fraction.So,the overall naodic reaction is expressed by fc。 0.732iF。+0.192ic +0.076fNi (7) where icorr is corrosion current densiyt. 2.2.3 OxygenReduction Oxygen reduction on active iron was found to be a four electron transfer reaction: o2+2H2O+4e—}4O (8) It follows the relationship: fo 一io2o Cos10‘ 喝・ (9) .一Bulk o2 where CB u is oxygen concentration in the bulk electrolyte. 2 2 4 Water Reduction The reduction of water H2o+e—÷H+0H—was ofundtofollowthe relationship, i :。==一f:,。l O(-e+z 。 ref H2。 (10) 2.2.5 HydrogenReduction The hydrogen discharge reduction is 2H++2e— H2 (11) Sharland[ 】assumed the rate ofthe hydrogen re— duction to depend linearly on hte pH,so hte hydrogen reduction current was assumed to obey the following equation: fH+= cH+exp(一 E/R7) (12) where%+is electrochemical partition coefifcient・ However,the cathodic portion of the more accurate Butler-Volmer equation for hydrogen ion reduction is[ ] 0~_i0仰 ref)/ …: where c Bu is hydrogen ion concentration in the bulk electrolyte, 0+is rtansfer coefifcient of hydrogen ion reduction.In this case the hydrogen ion has 0O・5, H+ nH 1・ For all the reductions,iRe, ,罐 and bRo rae the current density,exchange current densiyt,equi— librium potential(vs.SCE)and Tafel slope of each reduction reaction,respectively ̄Subscript‘‘Re” represents the species of oxygen,hydrogen ion and water reductions. 2.2.6 Boundary Conditions Mass steady difusion follows Fick’s diufsion law: J:一D—Oc (14) where J is the diufsion flux, represents the normal direction at the crevice bottom or walls. In hte crevice active region.the diufsion flux of oxygen equals its reduction rate: ( )=嚣 The hydrogen ion has the same boundary condi— tion: [ =杀 where Doand D:H+rae the diufsion coefifcients of dissolved oxygen and hydrogen ion,respectively. Since the passive film ofmetal surface is an in- sulator to ionic conduction, ]:0 (17) At the mouth of the crevice and the other initial surface,thevalues ofthe steelare asfollows: Co=c BU::lk (18) NO.5 Ⅵ A IGWei P 口, 825 cH+=《: (19) the gradients of concentrations are very large.The Following Ohm’S law,the potential flux at and in the direction perpendicular to steel surface equals the simulation was carried out with input values received from literature[2,5,10j8 刀. total current density multiplied by solution resistiviy,t 3 Results and Discussion Current density and crevice potential in the ere. vice are greatly affected by the chemical change in the crevice solution.It is Valid that dissolved oxygen and 一 一 。 )(20) where,P is the solution resistivity,f10t is the total current density flowing to or out of the steel,defined he hydrogen iton establish steady state quickly after the initiation of crevice corrosion.because oxygen and ft。t=fc。 fO2+iH20+iH .(2 1) the hydrogen ion can penetrate jnto the crevice wi出in a very small distnce farom the mouth and this diflu. sion needs little time to be accomplished as compared t0 mat of other species which need to trnsaport deep jnto the crevice.And a potential gradient jS instntaly established within the crevice before any ion transport. The model is now used to simulate this experi mentL Jand it iS possible to solve systems.which consists of several coupled domains.With the above boundary conditions.we illus仃ate the feasibility of calculating the diferential equations for electroche— mical corrosion behavior of steel in a neutra1 envi— The parameters used in the simulation Of this experi. ment re alisted in Table 1. ronment by finite element method(FEM).The mesh near the crevice mouth iS arranged very dense because Table 1 Parameters used in the calculated model Fig.2 shows the validation of he curtrent model against the experimental data of Alavi and Cottis[ 。】 and two of the leading published crevice corrosion models[2,s]that were also used to simulate the Alavi 826 CHEM.RES.CHINESE UNIVERSITIES Vo1.26 ;≯d{一曩葛 0and Cottis experiment.It is evident from Fig.2 that the oxygen and hydrogen ion difusion.Moving into the 一 6 0 8 O 0 O 2 O current model is an improvement over the other mo- dels.As can be seen in Fig.2,the model result wiht pH variation is closer to hte experimental data. 5 O 4.5 4 O 3・5 3.0 2・5 2.0 1.5 Distance/cm Fig.2 pH哪.distance from crevice mouth for current model。literature[2,sl and experiments[i0】 Sharland offered three diferent iL values(a constant CO/TO- ding rate)in this models. . The pH value is slightly lower than those of pre— vious studies in the crevice solution.and these studies nearly have the same trend curves with pH variation. But the experimental data show the lowest PH (minimum Deak value)near the crevice mouth. Fig.3 shows the comparison of experiments of Alavi and Cottis[1 ̄】and current model for the variation of potentia1 with the distance from the crevice mouth. The experimental potential was studied at 11 6 and 1 64 h,respectively,and the potential varied slowly,per- haps due to the change in the oxygen concentration of the bulk electrolyte.It can be assumed that the expe. riment was at steady state condition iust as the as— sumption of current mode1. Distance/cm Fig.3 Potential .distance from crevice mouth for current model and experimental data[10】 Under this condition.the calcultaed potential drop between the crevice mouth and the crevice bot— tom is 23.5 mV.As a result of the potential drop,there is a current flowing in the crevice solution from the crevice bottom toward the crevice mouth.The crevice potential becomes more negative with distance into the crevice because the oxygen concentration and hy— drogen ion concentration in the crevice decrease from hte crevice mouth into hte crevice as a result of slow crevice.the crevice potential eventually reaches -0.134 V(vs.SeE).It can be seen htat the trend ofthe model data is in good agreement wiht experimental data.The potential distribution along crevice boun. dary layer is depicted in Fig.4. ■ ■ Potential/mV 14 / 15 C l6 A \ \l7 B l8 l9 20 2l 22 23 Fig.4 Crevice shape(A),meshes of crevice mouth(B) and potential distr.bution(C) along crevice boundary layer Fig.5 shows diferent current densities(/t0t,icorr, 0.732iFe and 0.192icr)from crevice bottom to mouth. While this gives an indication Of the functional form of corrosion current density,it is unfortunately too complex to solve exactly ̄It is clear that i’t0t and orr will rise remarkably near the crevice mouth and drop rapidly with increasing depth into the crevice. 1 2 l・O 0・8 量。一6 0 4 0・2 0 Distance/cm Fig.5 Different current density values(it0t, orr, 0.732iFe and 0.192iCr)of current model . distance from crevice bottom to mouth 0.732iF。and 0.192icr chiefly influencing fc0rr rae the main contributors to anodic reactions.0.732iFe is greater than 0.1 92icr near the crevice mouth.The f 0rr values are close to the i‘t0t values of each point in the crevice.It seems that anodic reactions control the chemical reactions within the crevice,and crevice corrosion occurs violently,especially near the crevice mouth. Fig.6 shows the i and iH20 values within the crevice.0.076iN;is much smaller htan each of 0.732iFe nad 0.192ic so it contributes a litle to the three 一一 眦.《 _【 No.5 一 目《 。_I Ⅵ NGW i口f a, O 1 2 3 4 827 5 anodic reactions.The water reduction current(iHzO)is negative nearly zero which is the smallest current bottom,and then increases near the crevice mouth rapidly. density because of the neutral solution. 7 2 7 6 8 0 8.4 Distance/cin 0 0.4 O.8 1.2 Distance/cm Fig・6 Calculated/Ni(A)and in20(B)of current model vs. distance from crevice bottom to mouth In Fig.7,both in+and/o2 in hte crevice are nega— tive,which means that the corresponding reactions are reduction reactions.The value of io2 is greater than that of打near the crevice mouth,because oxygen reaction is the main reacting factor in the reductions within this active region.The curves of io2 and i+n have the same trend 拿 2 Distance/cm Fig.7 Calculated +( and/o,( of current model vs. distance from crevice bottom to mouth The region of the distance within 2 cm around the crevice mouth is an active area,which could re. ceive enough reactants and energy such as oxygen, water,and e-for the reduction reactions.The hydro gen ion difusing within the crevice is gradually con— sumed by hydrogen reduction.Near the crevice mouth, oxygen depolarizes the steel surface signiifcantly.At hte bottom of hte crevice.there is little oxygen and the steel surface is much】ess depolraized.Hence.the current spreads a longer distance into the crevice.The current densiyt of each curve is nearly zero ta hte bot. tom of hte crevice because no current lfows in and out of hte crevice.The current increases slowly from hte bottom toward the mouth in the crevice,where free corrosion Occurs,while ions difuse slowly at the The distribution of oxygen concentration within the crevice is depicted in Fig.8.Near the crevice mouth,the concentration of oxygen is nearly close to the bulk concentration of oxygen.In the oxygen active region,the concentration of oxygen decreases rapidly with the distance from the crevice mouth.The reason is that oxygen difusion near the crevice mouth is rela— tively higher than tIlat deep into the crevice,while, wiht the increasing of distance,the oxygen difused within the crevice is gradually depleted by corrosion. An oxygen concentration cell forms because of an oxygen concentration gradient in the crevice.This oxygen concentration gradient is a factor of the oxy— gen current density in the crevice.Since oxygen is partially consumed in t(_hc一Ⅲ.e boundar一0ⅢJ/u0IlE扫ua0Iy layer100矗 ,the oxygen 0 concentration at t}le solution iron interface is de. creased,which results in a decreased iron corrosion rate O 16 O l2 0 08 0 04 O 0 2 4 6 8 Distance/c131 Fig.8 Calculated(a)and measured(b)oxygen con— centration distribution data of current model vs.distance rfom crevice bottom to mouth 4 ConcIuSionS Multi anodic reactions were modularly developed for the estimation of crevice corrosion.There is a good agreement between the model results and expe— rimental data in pH nad potential distribution. Current densitv 0f each reaction and oxygen concentration in the crevice were predicted in this study.Fe and Cr oxidation are the mflni reacting fac. tors affecting the anodic reactions.Ni oxidation con— tributs to anodic reactions a litle for the crevice cor. rosion. 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