CORROSION MECHANISM OF AL-ZN-IN ALLOYS IN CHLORIDE SOLUTIONS

Transcription

1 CORROSION MECHANISM OF AL-ZN-IN ALLOYS IN CHLORIDE SOLUTIONS A. G. MUÑOZ, S. B. SAIDMAN nd J. B. BESSONE Instituto de Ingenierí Electróquimic y Corrosión (INIEC) Dpto. de Ing. Quimic Universidd Ncionl del Sur Av. Alem Bhí Blnc Argentin jessone@cri.edu.r Astrct- The electrochemicl ehvior of Al-Zn- In lloys ws investigted in chloride medi y mens of potentiodynmic techniques, complemented y Scnning Electron Microscopy (SEM), Energy Dispersive X-Ry Anlysis (EDX) nd Trnsmission Electron Microscopy (TEM). The influence of locl lkliztion produced y previous cthodiztion ws nlyzed t potentils more negtive thn the rest potentil. The electrochemicl response of lloys is given y the sum of those corresponding to the different present phses. Thus, the electrochemicl ehvior of ech metl nd imetllic lloy (Al-In, In-Al nd Zn-In) ws nlyzed in the sme solution conditions t potentil region ner the corrosion potentil of the ternry lloy. The low soluility of In in Al fvors its segregtion t grin oundries, where the formtion of Zn-In lloys ws detected. The initition of the ttck in the Al-Zn-In lloy is minly loclized in these zones. This is consequence of the enhnced dsorption nd surfce ccumultion of Cl - promoted y In t potentils more positive thn V SCE. Close similrities of the nodic ehviors of the Al-In nd ternry lloys led us to propose mechnism where the In distriution plys the mjor role. The In dissolution from homogeneous solid solution phses llows n enrichment of this element t the Al nd Zn rich phses y displcement rections. (Al,Zn)In + Cl - InCl e - 2 InCl Zn 2 In Zn Cl - This, in turn, ssures n ccumultion of dsored Cl -, which mintins n ctive stte of the Al mtrix. Thus, the rpid Zn dissolution rection ensures etter redistriution of In, so ringing the system towrds potentils ner those for Cl - dsorption on In. The ttck morphology ws lso nlyzed in order to explin the smooth wide cvities emrcing mny grins. Keywords- Aluminium, Al-Zn-In lloy, corrosion of Aluminium, Chloride dsorption.. I. INTRODUCTION Regrding commercilly produced Al scrificil nodes, there is little dout tody tht Al-Zn-In se nodes present the est performnce in sewter (Wolfson, 1994). The verge operting potentil of these nodes in this medi is V vs. SCE. The ctivting elements (Zn, In) ensure its uniform dissolution nd void polriztion. Then, it seems resonle tht one wy to otin fundmentl understnding of this ehvior lys in etter knowledge of the electrochemistry of the different phses constituting the ternry Al-Zn-In lloy. Previous studies hve suggested tht the initil step in the dissolution mechnism of the Al-In lloy cn e interpreted in terms of chloride dsorption process (Sidmn et l., 1995). Thus, the more negtive potentil of zero chrge (pzc) of In, compred with tht of Al, llows the dsorption of Cl - nd the destiliztion of the luminium oxide film to occur t more negtive vlues. The present contriution gives etter knowledge of the electrochemicl response of the Al-Zn-In lloy. A commercil Al-Zn-In-Si node nd ternry lloy, prepred in our lortory, were used s working electrodes. The nodic ehvior of pure metls (Zn e In) nd the inry lloys Al-5%Zn, Zn-5%In, nd Al-5%In re compred with tht of the ternry Al-5%Zn-0.02%In in 0.5 M NCl solution of ph 5. In order to otin more informtion out mechnistic spects of surfce ctivtion, the nlysis of the synergistic effect produced y the deposition of Zn nd In on Al, when their ctions re present in solution, ws lso considered. II. EXPERIMENTAL The ternry lloy Al-5%Zn-0.02%In ws otined using pure Al, Zn nd In (Aldrich Chemicl Co., with impurities ccording to Tle I). They were first etched in 2% HNO 3 ethnol solution nd then introduced in qurtz furnce with Ar tmosphere. It ws heted t pproximtely 20 C higher thn the highest element melting point, shcked nd then quenched in cold wter. Due to the very low soluility of In nd Zn in Al t room temperture (Bker nd Okmoto, 1992) nd the nominl composition used, the presence of rich In nd Zn phses is expected. Higher soluilities, however, re possile s consequence of quenching. A commercil Al-Zn-In-Si lloy, whose composition nd impurities re lso shown in Tle I, ws used for the ske of 275

2 comprison. Cylindricl rods were mounted in PTFE holder configuring disk electrode with dimeter of 7 mm. Electrodes were polished with 1000 emery pper, 0.3 µm grit lumin suspension nd finlly rinsed with undnt triple distilled wter. Tle 1 Composition nlysis of the Al-Zn-In nd the commercil Al-Zn-In-Si lloys. Elements Al-Zn-In Al-Zn-In-Si Zn In Si Fe Cu Cd P Al Reminder Reminder Chloride solutions were prepred from nlyticl grde chemicls nd thrice distilled wter. The ph ws djusted with HCl. A lrge Pt sheet ws used s counter electrode. As reference, SCE connected through Luggin-Her cpillry tip ws used nd potentil in the text re referred to this. The experiments were crried out in purified N 2 sturted tmosphere t 25 C. Runs were mde using potentiostt-glvnostt PAR Model 173, fter 20 min of stiliztion t the rest potentil. Potentiodynmic polriztions were performed t 0.001V s -1, initilly towrds cthodic potentils nd then in the nodic sense. Results were corrected for ohmic drop using the solution resistnce otined from impednce mesurements. From results of t lest three runs, firly good reproduciility with vritions elow ± 5% ws ttined. A rte of 2600 rpm ws used with rotting disk experiments. A dul stge ISI DS 130 SEM nd n EDAX 9600 quntittive energy dispersive nlyzer were used to exmine the electrode ttck. A. Pitting of Al III. RESULTS In order to otin etter insight into the fundmentl spects of pitting of Al, it ws nlyzed in non-queous chloride solutions. Thus, the possiility of locl cidifiction, s the min cuse for pit initition, my e ignored. Pitting initites t potentil ner 0.76 V, in similr mnner s it occurs in queous chloride solutions (Fig.1). Only slight influence of electrode rottion on the hysteresis cycle is detected, indicting surfce controlled process. I / µ A Fig. 1: Cyclic voltmmetries of Al with nd without electrode rottion in 0.5 M LiCl (Dimethylformmide). () ω = 0 rpm; () 1800 rpm. V = V s -1. E s, = V, E s,c = V. B. Electrochemicl studies The polriztions of the ternry Al-Zn-In nd commercil lloys, otined in dereted 0.5 M NCl solution, re shown in Fig.2. A corrosion potentil (E corr ) of V is found for the ternry lloy. This potentil differs from V otined in the cse of the commercil lloy. This is consequence of n enhnced hydrogen evolution given y the presence of impurities. The shpe of the nodic curve is, however, similr to tht for the ternry lloy, ut displced out 100 mv towrds more positive potentils. Dissolution ws preferentilly strted t the grin oundry regions, where segregted Zn nd Si were detected. Indium could not e detected y X-ry nlysis due to its low level in the commercil lloy. For the ske of comprison, polriztions on Zn, In (Fig.2) nd the inry lloys Al-5%In nd Zn-5%In (Fig.2) were crried out under the sme conditions. It is interesting to note, tht the presence of In shifts the rest potentil of oth Zn nd Al lloys to vlue of pproximtely V. This fct my e relted to necessry threshold potentil for Cl - dsorption. Indium on the other hnd, remins cthodic t this potentil region. To ssess the effects of chloride dsorption cused y the presence of In, the polriztion of the In-5%Al lloy ws lso studied t different chloride concentrtions (Fig.3). The curves exhiit n nodic pek followed y current plteu. At more positive potentils rekdown process tkes plce. The presence of n nodic pek, more evident t the most concentrted solutions, cn e oserved t potentil very close to the corrosion potentils for Al-In lloys. Additionl nd interesting conclusions out the ehvior of the ternry lloy could e found with the polriztion initited t V with nd without electrode rottion (Fig.4). 276

3 A. G. MUÑOZ, S. B. SAIDMAN, J. B. BESSONE 10 5 Commercil lloy Al-5%Zn-0.02%In 10 4 Al-5%Zn-0.02%In Zn i / µ A cm In i / µ A cm Al-5%In Zn-5%In Fig. 2: Comprison of the potentiodynmic polriztions of Al-Zn-In with other lloys in reted 0.5 M NCl solution, ph = 5. () Al-Zn-In lloy, commercil Al-Zn-In-Si lloy, Zn nd In; () inry lloys Zn-5%In nd Al- 5%In. v = V s -1.E i = E corr. i / ma c m time t -1.8 V, previous to the potentiodynmic run in the positive sense, would ensure qusi-re Al surfce. The profile recorded without rottion shows tht the current density chnged its sign t V nd tends to constnt vlue c. 30 µa cm -2 (Fig.4). This ehviour my e minly ttriuted to Al dissolution through the qusi-dissolved oxide generted y locl lkliztion (Yn et l., 1996). Under rottion, the possiility of locl ph rise ws minimized nd similr curve to tht shown in Fig.2 is otined. The electrochemicl ehvior otined t potentils more positive thn V is very similr to tht otined without previous cthodiztion (Fig.2) Fig. 3: Influence of Cl - concentrtion on the polriztion of the In-5%Al lloy. v = V s -1. E s,c = V. NCl concentrtion: (.) 0.01 M; ( _.. _ ) 0.1 M; ( _. _ ) 0.5 M; ( _ ) 1.0 M. It is known tht the hydrtion nd/or dissolution of Al oxide tke plce y locl lkliztion, when Al nd Al lloys re polrized t reltively high cthodic potentils. Thus, polriztion for determined period of C. Attck morphology studies A preferentil ttck t the grin oundry extending to the grin ody ws found fter the ternry lloy ws polrized from the rest potentil towrds nodic vlues nd removed t V (Fig.5). At higher mgnifiction, corrosion products t the grin oundry cn e oserved (Fig.5). X-ry mps indicte n enrichment of In nd Zn. Initition nd propgtion of the ttck is ssocited with the microstructure nd composition of the lloy (Fig.5c). Dissolution ws preferentilly strted t interdendritic zones present t the grin oundry regions. The presence of corrosion products, indicting the initition of ttck, cn e lso oserved distriuted long the cell 277

4 ω/rpm i / µ A cm Fig. 4: Potentiodynmic polriztion of the Al-Zn-In lloy in dereted 0.5 M NCl solution, ph = 5 with nd without electrode rottion. v = V s -1. E i = -1.8 V. ω= 2600 rpm. In Zn oundries formed during the retrogrde solidifiction process (k < 1, where k: prtition coefficient). The sme type of ttck ws oserved when the polriztion ws egun from potentils more negtive thn the rest one. Thus, this fct suggests n independence of the properties of the ctive surfce from previous lkliztion. The Al-Zn-In se nodes show uniform dissolution under operting conditions, giving rise to the wellknown higher efficiency. Fig.6 shows section perpendiculr to the corroded surfce of n Al-Zn-In lloy fter operting 206 dys in sewter. As cn e oserved, smooth hemisphericl cvities were developed t the node surfce. Etching with Keller s regent indictes tht these cvities re not ssocited with preferentil ttck long prticulr regions, i.e., grin oundry zones (Fig.6). IV. DISCUSSION The ctivtion of Al my e interpreted in terms of penetrtion of Cl - through the defective sites of the oxide nd its dsorption on the Al surfce. This process is only possile when the pzc of Al is nodiclly exceeded (Muñoz nd Bessone, 1999). The polriztions performed in non-queous solutions demonstrte tht the formtion nd stility of pits will e given y the presence of dsored Cl - t the Al/oxide interfce (Fig.1). Then, the ppernce of wide hysteresis cycle gives evidence for the formtion of slt film, which mkes difficult the repssivtion process. At potentils more negtive thn the dsorption one, the pit growth stops nd further dissolution is ohmic controlled. Thus, due to the sence of wter, the stility of pits in terms of locl cidifiction my e rejected. The effects of Fig. 5: () SEM microgrph of the Al-Zn-In lloy fter eing nodiclly polrized from E corr nd removed t 0.95 V in dereted 0.5 M NCl solution, ph = 5. v = V s -1. () Higher mgnifiction of the corrosion products t the grin oundry. (c) X-ry mp of the corrosion products corresponding to () (Zn-Lα nd In- Lα signls). incorporting lloying elements on the ctivtion of Al will e, thus, nlyzed in terms of the dsorility of Cl - on the different lloy phses. The opertion potentil of sed Al-Zn-In lloys hs een widely reported to e round V in different chloride medi. Then, due to its high nodic dissolution rte, this potentil prcticlly does not chnge in the rnge emrced from 10 2 to 10 4 µa/cm -2. The more negtive rest potentil found in the cse of the ternry Al-Zn-In lloy (E corr = V) my e ttriuted to the sence of cthodic impurities. They enhnce the hydrogen evolution, which in turn polrize the system nd shift the surfce potentil to more negtive vlues. The ctivtion generted y the presence of In nd Zn on Al is indicted y displcement of the rest potentil towrds more negtive vlues. This fct my e nlyzed in terms of higher interction of Cl - with the metl surfce rought out y the presence of In. In the cse of the electrochemicl ehvior of Zn, dsorption of Cl - ions hs een postulted s step on the Zn oxidtion mechnism (Bnás et l.,1986). Then, vlue of V ws reported s the c 278

5 A. G. MUÑOZ, S. B. SAIDMAN, J. B. BESSONE Fig. 6: SEM microgrph of n Al-5%Zn-0.02%In lloy fter 206 dys in se wter: () 17X; () surfce etched with Keller s regent, 4X. pzc in this medi (Bugh nd Lee, 1973). Also, the very rpid nodic dissolution (Fig.2) is controlled y the diffusion of ZnCl n 2-n species from the surfce to the ulk solution (Askur nd Noe, 1984). According to our dsorption model, the presence of Zn would mke possile the dsorption of Cl - to e sustined t potentils more positive thn the pzc of Zn in Al-Zn lloys. This is just the pitting potentil found for the Al-5%Zn lloy, where Zn is minly present in the form of α phse (Slins et l.,1999). Tking into ccount the electrochemicl ehvior reported for In in chloride (Muñoz et l., 1999) nd lkline medi, strong dsorption of Cl - nd/or OH - my e expected once V is nodiclly exceeded. This promotes the formtion of thin slt film efore mssive metl dissolution egins t more positive potentils. It mens tht In rings out strong increse of Cl - concentrtion t the metl surfce once V is overcome. The influence of In on the nodic dissolution of Zn hs een lso recently nlyzed y kinetic studies nd electrochemicl impednce spectroscopy (EIS) performed on Zn nd the inry lloy Zn-5%In in Cl - solutions (Muñoz et l., 2002). The presence of In, s segregted phse t grin oundries or s rich In phses in the interdendritic zones of the grin, promotes higher Zn dissolution rte s well s higher overtension for the hydrogen evolution. Both fcts shift the corrosion potentil of the lloy nd its ctive potentil towrds more negtive vlues. On the other hnd, when the ctivtion effect of In is considered on Al, the strong Cl - dsorption exerted y rich In phses promotes the ctivtion t more electronegtive potentil (E < - 1.1V) thn tht corresponding to pure Al (Sidmn et l., 1995). The ctive dissolution region nd the corrosion potentil of oth the Zn-In nd Al-In lloys re prcticlly coincident with those showed y the ternry lloy (Fig.2). According to the nodic ehvior showed y the In-5%Al lloy (Fig.3), the dissolution mechnism seems to e ssocited with the Cl - dsorption on the different phses present. Then, the nodic pek oserved round V corresponds to the dissolution of Al present s In or Al rich solid solution phses. At more nodic potentils, the ctivtion of pure Al is evident. Which is the role plyed y Zn? The electrochemicl ehvior given y the ternry nd commercil lloys in Fig.2 cn e explined considering the synergistic effect of In nd Zn on Al. It hs een demonstrted tht Al ctivtes when it is plced in neutrl solutions contining In 3+ nd Zn 2+ (Sidmn et l., 1999). Deposition of Zn nd In on re Al occurs y displcement rections giving rise to similr surfce fetures s those found t the ctive surfce of Al-Zn-In lloys used s node mteril. The presence of Zn 2+ seems to fvor n ccumultion of In y dissolution of previous deposited Zn. The presence of different dsorption sttes of Zn would lso mke esier the steps for In 3+ reduction mechnism. The presence of ccumulted In t the ctive Al surfce fvors chloride dsorption, so voiding repssivtion. Furthermore, the hydrolysis of Al 3+ is strong exothermic rection. Thus, the surfce tempertures generted within the pits re high enough to promote the formtion of In-Al phses nd enhnce surfce diffusion. The much lower moility of Zn, compred to tht of In, nd its tendency to dissolve from the surfce would lso explin the decresing surfce concentrtion of Zn (Sidmn nd Bessone, 1997). In this wy, the presence of Zn t the ctive surfce cuses In enrichment. This, in turn, ssures the dsorption of Cl - nd the increse of its surfce concentrtion t potentils more positive thn V, considered s the pzc for In. Thus, this surfce condition promotes nd mintins the ctivtion of Al. As expected, the ttck initites t grin oundries, where phses enriched in In nd Zn would e present. Then, it propgtes into the ulk of the grin through interdendritic zones, where the hrdly solule In nd Zn were rejected during the solidifiction process. The corrosion products oserved in Fig.4 nd nlyzed y EDX clerly demonstrte the presence of these rich In nd Zn phses. Thus, the initil lloy dissolution promoted y these enriched In-Zn zones explins the coincidence etween the nodic response of the ternry lloy with the inry Al-In nd Zn-In lloys. 279

6 The high glvnic efficiencies oserved with the ternry lloy in sewter (Bessone et l., 1981) is directly ssocited with the type of ttck. The presence of In ensures the development of smooth wide cvities (Fig.5). This fct cn e explined y continuous redistriution of In t the surfce. The In dissolved from solid solution phses is re-deposited t the nerest zones. This dissolution-deposition process of In, which would e ssisted y Zn dissolution, llows the ctive stte propgting to the region poor in In. Also, the strong exothermic hydrolysis of Al 3+ will fcilitte the surfce diffusion of In. This enhnces the cpcity for Cl - dsorption which extents to mny grins. A slt film is formed nd n ohmic control, leding to smooth wide cvities ttck, is estlished. On compring the ternry with the commercil lloy (Fig.2), prcticlly the sme nodic ehviour is oserved, lthough difference ppers in the E corr vlues. The higher mounts of cthodic impurities present in the commercil lloy (Tle 1), induce higher polriztion of the nodic res, which now re le to rech the chloride dsorption potentil on In-Zn rich zones. Thus, the E corr of the commercil lloy ttins nd mintins V. In the ternry lloy, however, the polriztion produced y the cthodic impurities is not enough to polrize the nodic res to this vlue, nd prcticlly no chloride-ssisted ttck is otined t its E corr. V. CONCLUSIONS According to the electrochemicl nd morphology studies, the following conclusion cn e ddressed: i. The ttck initition of the Al-Zn-In lloy is relted to rich In-Zn zones, segregted to grin or cell oundries. ii. The presence of In in electric contct with Al nd Zn promotes Cl - dsorption t potentils more positive thn V, considered s the pzc of In in such conditions. iii. The presence of Zn fcilittes surfce In enrichment y displcement rection. This, in turn, ssures n ccumultion of dsored Cl -, which mintins the ctive stte of the Al mtrix. Acknowledgements The Secretrí de Cienci y Técnic UNS (UNS-CSU- 170/99) nd The Consejo Ncionl de Cienci y Tecnologi (CONICET) re grtefully cknowledged for the finncil support. REFERENCES Askur S nd K.Noe, Corrosion, 40, (1984). Bker H. nd H. Okmoto (Eds.). ASM Hndook, Vol.3, ASM Interntionl, Mterils Prk, Ohio, (1992). Bnás J, K.G.Schütze nd E. Heitz, J.Electrochem. Soc., 133, (1986). Bugh L.M. nd J.A.Lee, J.Electronl. Chem., 48, (1973). Bessone, J.B., Surez Bldo R.A., De Micheli S.M. de, Corrosion, 37, (1981). Muñoz, A.G., S.B.Sidmn nd J.B.Bessone, J. Electrochem. Soc.,146, (1999). Muñoz A.G. nd J.B.Bessone, Corros. Sci., 41, (1999). Muñoz, A.G., S.B. Sidmn nd J.B.Bessone, Corros.Sci., 44, (2002). Sidmn, S.B., S.G.Grcí nd J.B.Bessone, 25, (1995). Sidmn S.B. nd J.B. Bessone, J. Appl. Electrochem., 27, (1997). Sidmn S.B., A.G.Muñoz nd J.B.Bessone. J.Appl. Electrochem., 29, (1999). Wolfson S.L., Mt. Performnce, 33, (1994). Yn H., J. Downes, P.J. Boden nd S.J. Hrris, J.Electrochem. Soc., 143, (1996). Received: Septemer 16, Accepted for puliction: Ferury 12, Recommended y Guest Editors: J. Cerdá, S. Diz nd A. Bndoni. 280