Spatially resolved measurement of electrochemical activity and pH distributions in corrosion processes by scanning electrochemical microscopy using antimony microelectrode tips

1 Spa ially- esol ed measu emen o elec ochemical ac i i y and pH dis ibu ions in co osion p ocesses by scanning elec ochemical mic oscopy using an imony mic oelec ode ips Ja ie Izquie doa, Lí ia Nagyc, Ágnes Va gac, Juan J. San anaa,b, Géza Nagyc, Rica do M. Sou oa,d a Depa men o Physical Chemis y, Uni e si y o La Laguna, E-38200 La Laguna, Tene i e, Cana y Islands, Spain b Depa men o P ocess Enginee ing, Uni e si y o Las Palmas de G an Cana ia, E-35017 Las Palmas de G an Cana ia, Cana y Islands, Spain c Depa men o Gene al and Physical Chemis y, Facul y o Sciences, Uni e si y o Pécs, 7624 Pécs, I júság ú ja 6, Hunga y d Ins i u o Uni e si a io de Ma e iales y Nano ecnologías, Uni e si y o La Laguna, E-38200 La Laguna, Tene i e, Cana y Islands, Spain Abs ac A new me hod o he spa ially- esol ed measu emen o pH du ing co osion p ocesses wi h he scanning elec ochemical mic oscope (SECM) is p esen ed. An imony ips a e employed because he dual- unc ion cha ac e is ics o his ma e ial allow he combined ampe ome ic/po en iome ic ope a ion o he SECM. The applicabili y o his echnique is illus a ed by conside ing he gal anic co osion o a model zinc-i on pai imme sed in 0.1 M NaCl aqueous solu ion. Spa ially esol ed images o pH and oxygen concen a ion abo e he me al specimens could be ob ained in he same expe imen . Keywo ds: Scanning elec ochemical mic oscopy; gal anic co osion; pH dis ibu ion; an imony mic oelec ode. 2 1. In oduc ion Local mic ocells a e o med du ing he spon aneous co osion o me als exposed o aqueous en i onmen s. Tha is, spa ial dis ibu ions o anodes and ca hodes on he me allic su ace a e de eloped when he ma e ial is le a i s open ci cui po en ial. The me al is oxidized a he anodes, whe eas he co osion p ocess is main ained by he educ ion o some species om he en i onmen a he ca hodes ( o ins ance, oxygen in neu al and mode a ely alkaline media, o p o ons in acidic solu ions). Because he co oding me al usually p oduces Mz+ ca ions ans e ed unde di usion con ol a he anodic si es, he sys em can be s udied by SECM [1-3]. This ac has been exploi ed o image me as able pi s on aus eni ic s eel in si u by SECM a he open-ci cui po en ial [4], and o de ec me al dissolu ion ei he om inclusions in alloys [5-7] o om de ec s in polyme -coa ed me als [8-10]. On he o he hand, in a neu al aqueous en i onmen , oxygen om he elec oly e is consumed a he ca hodic a eas, and he eac ion may be de ec ed by he scanning ip as local deple ion o he oxygen concen a ion in he elec oly e adjacen o ca hodic si es [11-15]. These s udies ha e ga he ed new spa ially- esol ed in o ma ion on he mechanisms o localized co osion eac ions in o e whelming cases employing me al mic odisks as he ip and he ampe ome ic ope a ion o he SECM. Tha is, he elec ochemical in o ma ion is ob ained om he measu emen o he a adaic cu en lowing in he ul amic oelec ode ip as a unc ion o ei he ime o i s loca ion abo e he sample. Co osion p ocesses a e e y sensi i e o pH, and he p og ess o he co osion eac ions may p oduce changes in he pH o he en i onmen oo. Ca hodic hal -cell eac ions o en in ol e a p og essi e alkalinisa ion o he elec oly e, ei he by consuming p o ons i he elec oly e is o iginally acidic, o by eleasing hyd oxyl ions in he case o neu al o alkaline solu ions. Addi ionally, he co osion o he me al a he anodic si es esul s in he gene a ion o me al ions, and some o hem unde go hyd olysis eac ions wi h he aqueous en i onmen , p oducing local acidi ica ion o he medium. Thus, he measu emen o pH in he icini y o a co oding su ace and i s isualiza ion as a unc ion o he loca ion a he su ace, and i s e olu ion wi h ime a e o majo in e es o a be e unde s anding o he co osion p ocesses in mic ome ic and submic ome ic scales. Though pH imaging is possible by using SECM in po en iome ic ope a ion since an an imony ip was de eloped o ion-selec i e po en iome ic mic oscopy in 1993 [16], his me hod has no been applied in Co osion Science ye . The elec ic po en ial o his ma e ial changes in esponse o a ia ions in he acidi y o he en i onmen due o i s endency o be coa ed by a Sb2O3 ilm. Thus, an imony ips ha e been employed as pH senso in SECM o he cha ac e iza ion o a a ie y o sys ems [17-19], as well as mic osenso s o o he echniques [20-28]. Due o he me allic 3 p ope ies o an imony, ips om his ma e ial can also be employed in ampe ome ic SECM o image he opog aphy o he subs a es unde in es iga ion [16,18], hus allowing mo e in o ma ion o be gained om hese sys ems. Tha is, an imony ips a e dual- unc ion mic oelec odes applicable o bo h ampe ome ic and po en iome ic SECM. This is a majo ad an age compa ed o o he ISMEs employed o pH moni o ing in SECM which can only be employed o he po en iome ic ope a ion [29-34] o wi h he scanning ion-selec i e elec ode echnique (SIET) [35,36]. This dual- unc ion p ope y has applica ion no only o SECM, because i has been employed o he de ec ion o hea y me als wi h squa e-wa e ol amme y when ca bon-based mic oelec odes we e coa ed wi h an imony hin ilms [37-39]. In his wo k we a e epo ing p elimina y esul s ob ained om SECM measu emen s wi h an an imony ip o cha ac e ize a model co osion eac ion. The expe imen al sys em selec ed o his wo k was he gal anic co osion o a zinc-i on pai , an expe imen al sys em p e iously cha ac e ized by ampe ome ic SECM [12], scanning ib a ing elec ode echnique (SVET) [12,40], and SIET [36]. The dual unc ion o he ip ma e ial was explo ed o he combined po en iome ic moni o ing o pH dis ibu ions and he ampe ome ic de ec ion o dissol ed oxygen employed as edox media o [13]. 2. Expe imen al An imony mic oelec odes we e made o wo pa s as shown in Figu e 1. The ou e glass capilla y p o ides he elec ic connec ion, whe eas he ac ual an imony ip is inse ed in o he lumen o he i s . High pu i y an imony in powde p esen a ion (266329, Ald ich) and bo osilica e glass capilla ies (B200-116-10, Su e Ins umen s, No a o, CA, USA) we e used. The ab ica ion o he mic oelec ode is ini ia ed by hea ing he an imony powde in a mel ing po wi h he help o a gas lame. When an imony mel s, one glass capilla y is illed wi h i using sy inge suc ion. The illed capilla y is hen pulled using me allic weeze s o p oduce a smalle size glass capilla y illed wi h an imony. Fu he pulling wi h a mic opipe e pulle (mod. P-30/P, Su e ) allowed one sec ion o his conic ip, wi h an app oxima e diame e o 15 – 20 μm, o be ab ica ed. A second glass capilla y was also pulled o p oduce a i s end a capilla y o dia. 100 μm, and he an imony ip was nex inse ed in i s lumen wi h he ip eaching ou o abou 15 mm, and abou 20 mm long s aying in he lumen. Me cu y me al and he coppe wi e we e hen inse ed in o he lumen o he hicke capilla y o p o ide elec ical con ac . Loc i e adhesi e was used o seal bo h ends. In his wo k, we in es iga ed he co osion beha iou o a zinc-i on gal anic pai . The samples we e p epa ed om high pu i y s ips o each me al supplied as shee s by Good ellow Ma e ials L d. (Camb idge, UK), wi h nominal pu i ies o 99.5% o i on and 99% o zinc. The me als we e cu in o 1x1 mm2 s ips, and moun ed in an Epo ix epoxy slee e (S ue s, Balle up, Denma k) 4 wi h a sepa a ion o ca. 1 mm be ween hem. The moun s wi h he samples we e polished wi h silicon ca bide pape down o 800 g i , and subsequen ly polished wi h alumina mic opolish o 1 and o 0.3 µm pa icle size. The esul ing su aces we e ho oughly insed wi h Millipo e deionised wa e , d ied wi h ace one and inally su ounded la e ally by sello ape, hus c ea ing a small con aine o he es elec oly e solu ion. The wo me als we e elec ically connec ed a he back o he epoxy moun o o m a gal anic couple. The elec ochemical cell was comple ed wi h an Ag/AgCl, 3M KCl e e ence elec ode, and a pla inum wi e as he coun e elec ode. A home-buil SECM sys em was employed [17], using a 3D posi ioning de ice d i en by p ecision s ep mo o s wi h 75 nm minimal s ep size. A ideo came a was used o u he assis posi ioning o he ip close o he su ace. The dis ance be ween he ip and he subs a e was es ablished by allowing he p obe o gen ly es on he sample, and subsequen ly he p obe was e ea ed o he chosen ope a ion dis ance wi h he aid o he Z-posi ioning mo o . Tip-su ace dis ance was also es ima ed using he nega i e eedback unc ion measu ed o he oxygen educ ion cu en when he ip was loca ed o e he epoxy pa . Selec ed expe imen s we e also pe o med using a pla inum mic oelec ode o 25 μm diame e o check he e ec o he me al used o build he ip in ampe ome ic SECM. Reagen s o analy ical g ade and bidis illed wa e we e employed o p epa e all he solu ions. The pH calib a ion o he an imony mic oelec odes was made in he bu e solu ions lis ed in Table 1. They we e p epa ed om 0.1 M solu ions o he basic species, and he pH was adjus ed by adding HCl. Co osion es s we e ca ied ou in aqueous 0.1 M NaCl solu ion. 3. Resul s and discussion 3.1. Calib a ion o he an imony mic oelec ode An imony mic oelec odes we e calib a ed by measu ing hei open ci cui po en ials in he bu e solu ions gi en in Table 1. The ypical calib a ion p ocedu e was pe o med by in oducing he mic oelec ode in a sequence o bu e solu ions ini ia ed wi h he mos alkaline solu ion. In his way, he ip was exposed o solu ions o inc eased acidi y as shown in Figu e 2. The o e po en ial alues obse ed in he plo occu ed upon elec oly e exchange, and nex he elec ode a ained a s eady po en ial alue in each solu ion. No s eady alue o he elec ode po en ial could be obse ed when i was in oduced in he bu e solu ion o pH 3.07, because he an imony elec ode was no s able in his medium due o acid a ack. In his medium, he ini ial po en ial alue in he ansien was aken o calib a ion pu poses. Finally, he elec ode was ein oduced in he mos alkaline bu e solu ion o check he ep oducibili y o i s po en ial esponse. The po en ial alue measu ed a e a qui e sho s abiliza ion pe iod was he same as ha eco ded a he beginning o he expe imen wi hin 5 expe imen al e o o 1.5 mV. Wi h he po en ial alues aken om each bu e solu ion, he calib a ion plo shown in he inse o he igu e was d awn. I can be obse ed ha he e is a linea ela ionship be ween he po en ial o he an imony ip and he solu ion pH in he in e al be ween 3 and 11.5. The slope o he plo amoun s 46.1 mV/pH uni . 3.2. SECM isualiza ion o pH and oxygen concen a ion dis ibu ions du ing he gal anic co osion o a zinc-i on pai The dual unc ion o he an imony mic oelec ode used as ip in scanning elec ochemical mic oscopy was explo ed on a zinc-i on gal anic pai imme sed in na u ally ae a ed 0.1 M NaCl solu ion a ambien empe a u e. The ip-subs a e dis ance was es ablished by allowing he an imony ip o gen ly es on he su ace o he epoxy slee e hal way be ween he wo me als. A e he ip was e ea ed o a heigh o 25 µm, SECM scan lines o a ays we e egis e ed abo e he zinc specimen by scanning he ip pa allel o he su ace. Figu e 3 displays he pH alues measu ed wi h he an imony ip ope a ed in po en iome ic mode while scanning a line passing abo e he cen e o he wo me als a e hey ha e been imme sed in he es solu ion o abou 120 min. Alkalinisa ion o he elec oly e occu ed in he p oximi y o he i on sample, which is he noble me al in he gal anic pai . Thus, he ca hodic p ocess akes place on his me al. In an acidic elec oly e, p o ons a e emo ed om he elec oly e as hey a e elec o educed o hyd ogen a he ca hodic si es: 2H+ + 2e- → H2(g) (1) whe eas in neu al and alkaline en i onmen s, he elec o educ ion o dissol ed oxygen eleases hyd oxide ions in o he solu ion acco ding o: O2 + 4H2O + 4e- → 4OH- (2) Since he ini ial elec oly e is sligh ly acidic, equa ion (1) could be conside ed o occu i s on he su ace o i on, ollowed by he elec o educ ion o oxygen a a la e s age. Ye , he simul aneous occu ence o oxygen elec o educ ion can be checked by using he an imony mic oelec ode as con en ional ip in ampe ome ic SECM. In his case, he po en ial o he ip was se a -0.65 V s. (Ag/AgCl, 3M KCl) o de ec he concen a ion dis ibu ion o dissol ed oxygen abo e he i on sample. The same ea u e is clea ly obse ed when he cu en alue no malized o he bulk cu en (I/Ilim) du ing a scan line aken o e he cen e o he image, was de e mined wi h he an imony mic oelec ode (see Figu e 4A). In he la e , he Y axis has been in e sed o show smalle ca hodic 6 a adaic cu en s o all lowe in he g aph, hus e ec i ely co esponding o lowe oxygen con en s. In o de o check he alidi y o his me hod, a pla inum ip was also employed. The scan a ay gi en in Figu e 4B displays a egion o deple ed oxygen concen a ion ex ending o e he dimensions o he i on sample as eco ded wi h he pla inum mic oelec ode. In summa y, oxygen is consumed a he ca hodic si es h ough equa ion (2) a a highe a e han di usion can supply i om he bulk o he elec oly e. Thus, he consump ion o oxygen can be moni o ed by ampe ome ic SECM wi h an imony ips as hey p o ide he same esul s han con en ional pla inum mic oelec odes. The alkalinisa ion p ocess con inues wi h he elapse o ime o he du a ion o he expe imen , and e en highe pH alues a e measu ed a e 26 hou s imme sion o he gal anic pai in he elec oly e. The dis ibu ion o pH om he su ace owa ds he bulk o he elec oly e can be ob ained simply by s epwise changing he ip-sample dis ance du ing he measu emen s. This p ocedu e is illus a ed in Figu e 5A which co esponds o he ip posi ioned di ec ly abo e he cen e o he i on specimen. The ip was i s emo ed om he icini y o he su ace o a dis ance o 900 µm, and hen he di ec ion o he ip mo emen was e e sed o he p obe o app oach he su ace again. The pH d ops by almos 4 uni s du ing he e ea ing mo emen om he i on su ace, whe eas a s a iona y alue is obse ed in he app oach scan o dis ances in excess o 400 µm. This pH alue co esponds o he bulk elec oly e a e he gal anic co osion p ocess has p oceeded o 26 hou s. A sho e dis ances, a as inc ease o pH alues is obse ed as he ip con inues app oaching he i on su ace un il he ini ial alue on he p oximi y o he me al is measu ed again. The wo pH-dis ance cu es ob ained du ing e ea and subsequen app oach o he ip o he su ace hus desc ibe a hys e esis loop. The o igin o his ea u e can be a ibu ed o he mo emen o he elec oly e ha p oduces con ec i e e ec s in he elec oly e. The slowe decay o pH om he su ace o he bulk elec oly e du ing he e ea ing mo emen happens because he ip lea es a hyd oxide- ich egion ha cons i u es he con ined elec oly e olume o med be ween he ip and he i on su ace o he p e ious 26 hou s, and hus i s mo emen is accompanied by a po ion o his elec oly e ha dilu es slowe han he ip mo es om he su ace. This e ec is less p onounced du ing he app oach mo emen as he ip mo es om a olume wi h e y small concen a ion o hyd oxide ions o much iche concen a ions. The esponse ime o mos senso s is smalle in di ec ion o concen a ion inc ease han in he opposi e di ec ion. The dilu ion e ec s due o ip mo emen a e below he de ec ion limi o he expe imen al echnique in his case. The spa ial dis ibu ion o pH can be imaged when scan a ays a e measu ed ins ead. They can be eco ded ei he in a plane pa allel o he su ace, he a ays e ec i ely being a composi ion o scan lines as ha cons i u ing Figu e 3, o in a plane pe pendicula o he su ace. In he la e case, he dis ibu ion o pH om he su ace in o he elec oly e is imaged ins ead. Figu e 6 shows he pH dis ibu ion ob ained abo e he i on sample. The map is composed by scan lines egis e ed 7 a di e en dis ances om he su ace. Since he dis ance co e ed in each scan line is a he long o allow he pH dis ibu ion o be app ecia ed o e a la ge a ea o he sys em, a he long acquisi ion imes we e equi ed o measu e each scan line, and hey we e egis e ed only a some selec ed heigh s. Ye he spa ial dis ibu ion o pH alues om he su ace in o he bulk elec oly e can be clea ly obse ed om he inspec ion o his image. The pH dis ibu ion o e he zinc sample was also in es iga ed. The pH dis ibu ion o e his me al looks almos homogeneous om he obse a ion o he scan a ays, e en o he longes exposu es as shown in Figu e 7. Ne e heless, some dis ibu ion o pH alues occu s abo e he zinc sample when he da a a e plo ed wi h g ea e esolu ion (c . inse in Figu e 3). Thus, some acidi ica ion happens in his case, hough he magni ude o his e ec is signi ican ly smalle han he alkalinisa ion p ocess occu ing a he ca hode. Fu he mo e, he pH dis ibu ion o e he me al emains qui e homogeneous o e i s whole su ace. I is concluded ha me al dissolu ion occu s a he su ace o he zinc specimen acco ding o eac ion: Zn → Zn2+ + 2e- (3) The dissolu ion p ocess does no p oduce a a ia ion in he pH o he elec oly e as gi en by equa ion (3), unless a ce ain amoun o he eleased me al ions unde go hyd olysis in he elec oly e as gi en by: Zn2+ + nH2O → Zn(OH)n2-n + nH+ (4) In his case, local acidi ica ion o he elec oly e would happen in he p oximi y o he zinc elec ode. Though he bulk elec oly e becomes p og essi ely mo e alkaline wi h he elapse o ime as shown abo e, he zinc su ace emains sligh ly mo e acidic han he bulk e en a e 27 hou s. This ac can be easily obse ed om he e ea and subsequen app oach plo s shown in Figu e 5B, which we e measu ed wi h he ip loca ed abo e he cen e o he zinc specimen. The small concen a ion a ia ions occu ing in his case a e no signi ican ly a ec ed by he mechanical mo emen o he ip, and bo h plo s lie e y close o each o he . 4. Conclusions The use o an an imony mic oelec ode as ip in a scanning elec ochemical mic oscope g ea ly enhances he in o ma ion ga he ed on co osion eac ions by allowing he spa ial dis ibu ion o pH o be imaged (quasi) simul aneously o he con en ional ampe ome ic ope a ion o he ins umen 8 in he gene a ion-collec ion mode. This new me hodology can be used o in es iga e a wide a ie y o co osion sys ems, hough i s mos immedia e applica ion can be ound in he cha ac e iza ion o he sac i icial p o ec ion impa ed by zinc o i on-base alloys in gene al, and o gal anized s eels in pa icula . Acknowledgemen s: The au ho s a e g a e ul o he Na ional O ice o Resea ch and Technology (NKTH, Budapes , esea ch g an ES-25/2008 TeT) and o he Spanish Minis y o Science and Inno a ion (MICINN, Mad id, Acción In eg ada No. HH2008-0011) o he g an o a Collabo a i e Resea ch P og amme be ween Hunga y and Spain. J.I. and R.M.S. a e g a e ul o inancial suppo by he MICINN and he Eu opean Regional De elopmen Fund (B ussels, Belgium) unde P ojec No. CTQ2009-12459. A Resea ch T aining G an awa ded o JI by he MICINN (P og ama de Fo mación de Pe sonal In es igado ) is g a e ully acknowledged. Re e ences: 1. S.E. Pus , W. Maie , G. Wi s ock, In es iga ion o localized ca aly ic and elec oca aly ic p ocesses and co osion eac ions wi h scanning elec ochemical mic oscopy (SECM), Z. Phys. Chem. 222 (2008) 1463–1517. 2. L. Niu, Y. Yin, W. Guo, M. Lu, R. Qin, S. Chen, Applica ion o scanning elec ochemical mic oscope in he s udy o co osion o me als, J. Ma e . Sci. 44 (2009) 4511–4521. 3. R.M. Sou o, S. 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