Stress corrosion cracking of AISI 321 stainless steel in acidic chloride solution

Transcription

1 Bull. Mter. Sci. Vol. 25 No. 1 Februry 2002 pp Indin Acdemy of Sciences. Stress corrosion crcking of AISI 321 stinless steel in cidic chloride solution YANLIANG HUANG Mrine Corrosion nd Protection Lbortory Institute of Ocenology Chinese Acdemy of Sciences 7 Nnhi Rod Qingdo Chin MS received 21 August 2001; revised 21 November 2001 Abstrct. The stress corrosion crcking (SCC) of AISI 321 stinless steel in cidic chloride solution ws studied by slow strin rte (SSR) technique nd frcture mechnics method. The frctured surfce ws chrcterized by clevge frcture. In order to clrify the SCC mechnism the effects of inhibitor KI on SCC behviour were lso included in this pper. A study showed tht the inhibition effects of KI on SCC were minly ttributed to the nodic rection of the corrosion process. The results of strin distribution in front of the crck tip of the ftigue pre-crcked plte specimens in ir in the blnk solution (cidic chloride solution without inhibitor KI) nd in the solution dded with KI mesured by speckle interferometry (SPI) support the unified mechnism of SCC nd corrosion ftigue crcking (CFC). Keywords. Stress corrosion crcking; chloride; stinless steel; inhibitor. 1. Introduction Stress corrosion crcking (SCC) is one of the most importnt cuses of filure of stinless steel equipment nd fcilities (Collins nd Monck 1973). Studies on the seshore fcilities showed tht n cidic chloride solution film formed on the surfce of steel nd the stinless steel in this medi ws in ctive dissolution stte (Gnnmoorthy 1990; Kin 1990). It is generlly thought tht SCC cnnot occur when the mteril is in ctive nodic dissolution stte but it occurred nd some csulties (Oldfield nd Todd 1990) were cused nd this roused the ttention to the study of SCC. A number of mechnisms concerning SCC of mterils hve been proposed (Hung 1988; Newmn nd Procter 1990) but they cnnot be used dequtely to explin this kind of SCC. Some ppers were devoted to the inhibition of SCC in stinless steels in cidic chloride-contining solutions (Chen nd Wu 1985; Co et l 1991). Chen nd Wu (1985) studied the inhibition of series of inorgnic nd orgnic dditives on the SCC nd pitting corrosion of AISI 321 stinless steel in n cidic chloride solution of ph 2 6 t 90 C using U-bend specimens nd it ws concluded tht the inhibition effects were ttributed to the synergistic effect between orgnic mines nd hlide ions. Co et l (1991) studied some orgnic nd inorgnic compounds in the inhibition of SCC of AISI 321 stinless steel in cidic chloride solution nd found tht mong the effective inhibitors iodides were the most effective ones. In this pper the study on SCC mechnism nd SCC inhibition were orgniclly integrted. The study of inhibitor on nodic dissolution nd deformtion t crck tip provided some evidence of possible SCC mechnism. This lso fcilitted the selection of SCC inhibitors on this kind of SCC. The ppliction of speckle interferometry (SPI) (Suezou nd Hiroyosi 1985) in the in situ mesurement of specimen in corrosion medi ws developed recently nd it ws found tht this technique ws suitble for the study of environment sensitive frcture of mterils. A more detiled investigtion into the role of I in the inhibition of the SCC of AISI 321 stinless steel ws then crried out by using SPI s well s conventionl techniques nd the results re presented here. 2. Experimentl 2.1 Mterils nd specimens The mterils used for plte nd rod specimens were both AISI 321 ustenitic stinless steels. The specimen for the mesurements of polriztion curves ws rod 10 mm in dimeter. The working surfce ws the cross-section. The rod specimen ws used in slow strin rte (SSR) tests to determine SCC sensitivity nd the effects of inhibitor KI on SCC. The single-edge pre-crcked plte specimens were used for the mesurement of SCC crck propgtion nd strin distribution in front of the crck tip. The plte specimens were cut from 3 mm thick plte prllel to the rolling direction. All the specimens were het treted t 1050 C for 30 min in slt bth nd then quenched in wter. When the specimens were treted s such the microstructure of the specimens ws single phse ustenite. After het tretment the specimens were pickled for 47

2 48 Ynling Hung 5 min in 5% HF + 20% HNO 3 solution t room temperture rinsed in distilled wter thoroughly followed by rinsing in cetone nd then dried. For the rod specimens they were coted with polytetrfluoroethylene except for the 20 mm working section tht ws totlly immersed in the test solution. The plte specimens were ftigue pre-crcked in ir ~ 1 5 mm beyond the notch tip. The plte ws thin enough so tht plin stress conditions could be ssumed. The stress intensity fctor K t the crck tip cn be clculted from the following formul (Chinese Reserch Institute of Aeronuticl Technology 1981) K = Fσ F = π π tg π π sin b + π cos (1) where is the crck length b the plte width σ the stress. 2.2 Test solution It ws known from previous work (Co et l 1991) tht AISI 321 stinless steel is sensitive to SCC in solution of 0 5 mol/l NCl mol/l HCl (blnk solution) t 55 C so ll tests were performed under this condition. The blnk solution ws prepred with pure regent grde chemicls nd distilled wter. KI with pure regent grde ws used s the dditive. The concentrtion of KI ws mol/l. 2.3 Apprtus nd procedures 3 observed with 30 microscope. The strin distribution in front of the crck tip of the pre-crcked plte specimens in ir in the blnk solution nd in the solution with KI ws mesured by SPI (Suezou nd Hiroyoshi 1985). For the electrochemicl mesurements sturted clomel electrode (SCE) ws used s the reference electrode nd ll the vlues of the electrode potentil re reported vs SCE. The polriztion curves were mesured on PAR 332 electrochemicl mesuring system. All the tests were repeted three times to confirm the relibility of the results. The frctured surfces of the filed specimens were observed by scnning electron microscope (SEM). 3. Results 3.1 Stress strin curves Figure 1 shows plots of the stress vs strin of the steel in blnk solution nd in the solution contining KI. The results show tht KI exerts significnt inhibitive effects on the SCC of the system studied. The frcture strin in solutions with KI is significntly higher thn tht in blnk solution. 3.2 Effects of cross-hed speeds on crck propgtion rte Figure 2 shows crck rte stress intensity fctor K. It cn be seen tht the crck propgtion rte decresed with the decrese of cross-hed speeds but when the cross-hed speed ws lower thn mm/s the plteu rte remined the sme. The SCC tests were crried out on smll SSR tensile test mchine. The crck propgtion of plte specimens ws Figure 1. Stress strin curves of 321 stinless steel in blnk solution nd the solution contining mol/l KI ( blnk solution; solution s in curve mol/l KI). Figure 2. Crck propgtion rte d/dt-k curves for 321 stinless steel in 0 5 mol/l HCl + 0 5mol/l NCl solution t different cross-hed speeds ( mm/s; mm/s; mm/s; mm/s).

3 Stress corrosion crcking of AISI 321 stinless steel Effects of inhibitor KI on crck propgtion As the crck rte remined the sme t cross-hed speeds of m/s nd lower this ws chosen to evlute the effect of KI on crck propgtion. Figure 3 shows the crck propgtion rte stress intensity fctor K. When KI ws dded to the test solution the crck propgtion rte decresed. 3.4 Polriztion curves Polriztion curves for AISI 321 stinless steel in blnk solution nd in the solution with KI re shown in figure 4. It cn be seen tht when KI ws dded into the test solution the nodic dissolution of steel ws gretly reduced. 3.5 Strin distribution in front of crck tip Figure 5 shows the strin (ε xx ) distribution in front of the crck tip of plte specimens loded with 24 MP to 36 MP t loding rte of 0 02 MP/s in ir in the blnk solution nd in the solution with mol/l KI. The mximum strin nd the re encircled by the contour with the sme strin level in front of the crck tip in the blnk solution re lrger thn those in ir. The deformtion in front of the crck tip in the solution with KI is reduced compred with tht in the blnk solution the mximum strin nd the re encircled by the contour with the sme strin level in front of the crck tip in the solution with KI re smller thn those in the blnk solution. 3.6 Crck morphology The frctured surfces of filed specimens were observed by scnning electron microscopy (SEM). A lot of crcks cn be seen on the side surfce of the rod specimen frctured in blnk solution nd the morphology of the frctured surfces is typicl of brittle clevge. In the solution dded with KI few crcks cn be found on the side surfce of the rod specimen nd the frctured surfce exhibits lot of dimples tht re typicl of ductile frcture. Figure 6 shows the crcked surfce morphology of the pre-crcked specimens in blnk solution. The frctured surfce is of brittle type t low K stge. At high K stge the frctured surfce is chrcterized s brittle clevge with dimples in between. It mens tht the propgtion of SCC crcks is discontinuous. 4. Discussion Co et l (1991) nd Hung et l (1993) proposed quntittive method for describing the effects of inhibitors on nodic nd cthodic rections nd deduced equtions s follows: I = f I = f I corr E E exp β c corr E Ecorr Ic = fc Ic = fc Icorr exp (2) β where I corr nd E corr re respectively the corrosion current density nd corrosion potentil nd β nd β c re the nodic nd cthodic Tfel slopes in nturl logrithmic scle in the uninhibited solution; I nd I c re the respective nodic nd cthodic current densities in the solution with n dditive nd I nd I c re those in the uninhibited solution under the sme electrode potentil E; f nd f c re the corresponding ction coefficients of the c Figure 3. Crck velocity d/dt-k curves for single-edge precrcked specimens in 0 5 mol/l HCl mol/l NCl solution nd in the solution with mol/l KI t cross-hed speed of mm/s (O 0 5 mol/l HCl mol/l NCl; s in O mol/l KI). Figure 4. Polriztion curves for AISI 321 stinless steel (A 0 5 mol/l HCl mol/l NCl; B s in A mol/l KI).

4 50 Ynling Hung () (b) (c) Figure 5. Strin distribution of the specimen in front of the crck tip in ir () in the blnk solution (b) nd in the solution dded with mol/l KI (c). The crck tip is t (00). The minimum strin is nd the step between ech djcent contour is The unit of the coordintes is in mm. Figure 6. The crcked surfce morphology of the pre-crcked specimens in blnk solution. () At low K stge nd (b) t high K stge. dditive on the nodic nd cthodic rections of the corrosion process. The dditive inhibits the corresponding rection if the vlue of the corresponding coefficient is < 1 nd ccelertes the rection if it is > 1. The Tfel slopes for nodic rection in nturl logrithmic scle estimted by technique of curve-fitting in low polriztion region (Co 1985) using the dt shown in figure 4 re respectively 29 4 nd 30 1 in the uninhibited nd KI inhibited solutions. The corresponding cthodic rection slopes re 48 8 nd 52 3 mv in these solutions. The corrosion current densities estimted for the two solutions re 0 51 ma/cm 2 nd ma/cm 2. The corrosion potentils mesured in the two solutions re: 370 mv nd 303 mv. According to these dt nd (2) the clculted vlues of f nd f c re nd 0 34 for KI. The inhibition effect of KI on SCC is minly due to the inhibition effect on the nodic rection of the corrosion process. Jones (1985) stted tht nodic dissolution relieves strin hrdening nd reduces the frcture stress t the crck tip. The relief of strin hrdening occurs by interction of subsurfce disloctions with divcncies generted by the nodic dissolution. The results presented here seem to confirm the hypothesis bove. Since if the mechnism proposed by Jones is correct the deformtion in front of the crck tip of the specimen in the blnk solution would be esier thn tht in ir nd with the inhibitor dded to the blnk solution which hs strong inhibition effect on nodic rection s discussed bove the deformtion in front of the crck tip will be gretly reduced. The inference is verified by the experimentl results shown in figure 5. The strin hrdening relief t the crck tip by nodic rection did not induce ny significnt blunting of the crck tip nd the frctured surfce ws chrcterized by brittle clevge with no obvious evidence of electrochemicl dissolution this cn be explined by the reduction in frcture stress t the crck tip induced by nodic rection. The frctured surfce observtion of pre-crcked specimens lso supports the SCC mechnism presented by Jones (1985). The strin hrdening relief nd the reduction in frcture stress require the vcncies generted by nodic dissolution to diffuse into the crck tip to form divcncies to rect with subsurfce disloctions. At low K vlues the crck propgtion velocity is very slow the vcncies hve sufficient time to diffuse into the crck tip nd thus the frctured surfce is chrcterized by brittle clevge. At high K vlues the vcncies hve little time to diffuse so only comprtively thinner brittle film is formed beyond the crck tip. After the

5 Stress corrosion crcking of AISI 321 stinless steel 51 brittle film frctured in high K vlue the stress t the crck tip ws still high enough for ductile frcture to occur. The crck tip ws blunted by ductile frcture nd the crck velocity ws lowered for short period then the metl t the crck tip rected with the test solution to generte vcncies nd the brittle frcture process repeted s discussed bove. So the chrcteristic of the frctured surfce is brittle clevge with dimples in between. 5. Conclusions (I) The SCC of AISI 321 stinless steel in cidic chloride solution is of ctive dissolution type nd cn be inhibited by the inhibitor KI. The inhibition effect of KI on the SCC is minly due to the effect on the nodic rection of the corrosion process. (II) The results reported support the unified mechnism of SCC nd CFC. Acknowledgement This work ws finncilly supported by the Foundtion of Chinese Eductionl Deprtment for Overses Student. References Co Chu-Nn 1985 Chinese Society for Corrosion nd Protection (in Chinese) Co Chu-Nn Yn Qingong Lu Min nd Lin Hicho 1991 Proc. 7th APCCC (Beijing: Interntionl Acdemic Publishers) pp Chen Xujun nd Wu Shnhe 1985 Proc. 6th Europ. symp. inh. (Ferrr: Elsevier Science Publishers) pp Chinese Reserch Institute of Aeronuticl Technology 1981 Hndbook of stress intensity fctor (Beijing: PRC Science Press) (in Chinese) Collins J A nd Monck M L 1973 Mter. Perf Gnnmoorthy J B 1990 Mter. Perf Hung Shuju 1988 Corrosion nd protection of metls (Chin: Xi n Jiotong University Press) (in Chinese) Hung Y L Co C N Lu M nd Lin H C 1993 Corrosion Jones A 1985 Metll. Trns. A Kin R M 1990 Mter. Perf Newmn R C nd Procter R P M 1990 Br. Corros. J Oldfield J W nd Todd B 1990 Mter. Perf Suezou Nkdte nd Hiroyoshi Sito 1985 Appl. Opt