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Electrochimica Acta 59 (2012) 398– 403

Contents lists available at SciVerse ScienceDirect

Electrochimica Acta

j ourna l ho me pag e: www.elsev ier .com/ locate /e lec tac ta

canning electrochemical microscopy for the investigation of corrosionrocesses: Measurement of Zn2+ spatial distribution with ion selectiveicroelectrodes

avier Izquierdoa, Lívia Nagyb, Ágnes Vargab, István Bitterc, Géza Nagyb,∗∗, Ricardo M. Soutoa,d,∗

Department of Physical Chemistry, University of La Laguna, E-38200 La Laguna, Tenerife, Canary Islands, SpainDepartment of General and Physical Chemistry, Faculty of Sciences, University of Pécs, 7624 Pécs, Ifjúság útja 6, HungaryBudapest University of Technology and Economics, Budafoki u. 8, 1111 Budapest, HungaryInstituto Universitario de Materiales y Nanotecnologías, University of La Laguna, E-38200 La Laguna, Tenerife, Canary Islands, Spain

r t i c l e i n f o

rticle history:eceived 15 June 2011eceived in revised form 26 October 2011ccepted 26 October 2011vailable online 6 November 2011

eywords:

a b s t r a c t

Ion-selective microelectrodes can be employed as tips in scanning electrochemical microscopy (SECM)for chemical imaging of corrosion processes. They present higher chemical selectivity than conventionalamperometric microdisks, and may be the only effective option to visualize the dissolution of metalswith negative redox potentials in aqueous environments when the use of Pt microelectrodes is limitedby the onset of oxygen reduction and hydrogen evolution reactions. A robust micro-sized ion selectiveelectrode has been developed which allows the spatial distribution of Zn2+ during galvanic corrosion

canning electrochemical microscopyotentiometric operationon selective microelectrodeincalvanic corrosion

of a model Fe/Zn couple to be investigated using SECM. Owing to the low internal contact potentialachieved with the novel design, the resistance of the micropipette electrodes is only fractions of theresistance of conventional micropipette electrodes of the same size. As a result, no special shielding ofthe microelectrodes is required and higher scanning rate can be used for scanning in the potentiometricmodes using these micropipette tips. Concentration profiles over corroding surfaces measured with thistechnique will be presented.

. Introduction

Scanning electrochemical microscopy (SECM) has become aowerful technique for studying the complex processes involved

n Corrosion Science due to its ability to image the topography ando probe chemical reactivity in micrometer- and submicrometer-ized dimensions [1–4]. The use of an ultramicroelectrode (UME)hat is scanned across the surface of a sample, allows interfaces toe characterized with spatial resolution whereas almost any kindf electrochemical measurement can be performed at the SECM tipnd at the substrate surface. To date, SECM has found applicationn the detection of precursor sites [5–7] and the visualization of

etastable regime [8] for pitting corrosion, the characterization of

he chemical stability of thin surface films [9–12] and organic coat-ngs applied on metals [13–26], the detection of metal and inclusionissolution [25–27], the recognition of anodic and cathodic areas

∗ Corresponding author at: Department of Physical Chemistry, University of Laaguna, E-38200 La Laguna, Tenerife, Canary Islands, Spain. Tel.: +34 922318067;ax: +34 922318002.∗∗ Corresponding author.

E-mail addresses: G-nagy@ttk.pte.hu (G. Nagy), rsouto@ull.es (R.M. Souto).

013-4686/$ – see front matter © 2011 Elsevier Ltd. All rights reserved.oi:10.1016/j.electacta.2011.10.076

© 2011 Elsevier Ltd. All rights reserved.

[28,29], hydrogen permeation [30], etc. These studies have sup-plied valuable information using metal microdisks as the tip, andamperometric operation of the SECM. That electrochemical infor-mation is derived from the measurement of the faradaic currentflowing through the ultramicroelectrode tip as a function of eithertime or its position above the sample. Yet, limited application of thistechnique occurs when more than one chemical species can simul-taneously react at the microdisk tip, thus compromising chemicalselectivity [31], and when the species sensed at the tip reacts withit, giving rise to changes in the surface condition and compositionof the material. Thus, there is need for new techniques and pro-cedures in SECM that may provide greater selectivity for chemicalimaging of corrosion processes.

Scanning electrochemical microscopy can also be operated ina potentiometric mode by employing an ion-selective microelec-trode (ISME) as the tip [32–35]. They present higher chemicalselectivity than conventional amperometric microdisks, and maybe the only effective option to visualize the dissolution of metalswith negative redox potentials in aqueous environments when the

use of Pt microelectrodes is limited by the onset of oxygen reductionand hydrogen evolution reactions [36,37]. However a quick surveyof the scientific literature concerning the use of SECM, serves toobserve that the potentiometric method has been applied rather

himica Acta 59 (2012) 398– 403 399

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METEX, Seoul, Korea) and home-built control software. A video

J. Izquierdo et al. / Electroc

carcely compared to conventional amperometric modes. The rea-on for this situation arises from the fragility, very high resistancend limited stability exhibited by the ISMEs, which require carefulhielding of the electrochemical cell to improve the signal to noiseatio, and very slow scanning rates to account for their rather longesponse time. The resistance value in an ISME arises from the rel-tively thick organic cocktail layer and liquid contact between theonophore containing cocktail and the filling solution of the innereference. This is the reason that the potentiometric SECM has beenlmost completely limited to pH monitoring by using solid stateicroelectrodes based on the characteristics of certain oxides of

ransition metals, or solid state chloride electrode [32,38–40]The situation has greatly changed due to the development

f robust micro-sized ion selective electrode containing a solidontact between the ionophore and the inner reference solution41,42]. Owing to the low internal contact potential and smaller

embrane thickness achieved with the novel design, the resistancef the micropipette electrodes are only fractions of the resistance ofonventional micropipette electrodes of the same size. As a result,o special shielding of the microelectrodes is required and highercanning rate can be employed in the potentiometric mode usingicropipette tips. Very recently, the fabrication of a solid contacticropipette ion selective microelectrode for Zn2+ ions has been

eported [43]. Since zinc is a metal widely employed in our techno-ogical society, the corrosion of this metal in aqueous environmentsas been extensively investigated up to now. In particular, the rolef corrosion products to inhibit the cathodic reaction and the possi-ility of additional self-healing effects for corrosion protection dueo additives in the composition of the metal alloy or arising fromurface treatments, are a hot research topic in the case of galva-ized steels [44–47]. To understand the role of corrosion productsf galvanized steels, it is necessary to measure the concentrationistributions of Zn2+ and OH− ions in the solution above the sur-ace. Whereas pH monitoring is already available using the SECM38–40], the measurement of the distribution of the metal ions wasot possible with this technique until now.

The present study investigated the applicability of a solid con-act Zn2+-ISME to measure the spatial concentration distribution ofissolved zinc over corroding surfaces measured with SECM oper-ted in potentiometric mode

. Experimental

.1. Samples

The substrate consisted of 99.5% purity iron and 99.95% purityinc surfaces mounted into an epoxy resin sleeve, such that onlyn 1 mm × 1 mm square end surface formed the testing metal sub-trates. The separation between the two metals was 5 mm. Theseetals were supplied as sheets of thickness 1 mm by Goodfel-

ow Materials Ltd. (Cambridge, UK). The front side of the mountsas ground with silicon carbide paper down to 4000 grit, and fol-

owed by a polishing step with 0.3 �m alumina slurries. Samplesere then washed thoroughly with ultra-pure deionized water and

llowed to dry in air. The corrosive medium was 10 mM NaCl solu-ion in contact with air, quiescent and at ambient temperature.

.2. Zn2+-ion selective microelectrodes

The ion selective cocktail was prepared using was N-henyliminodiacetic acid bis-N′,N′-dicyclohexylamide as zinc

onophore, potassium tetrakis(4-chlorophenyl)borate (KTCPB), 2-itrophenyl octhyl ether (NPOE) and PVC in tetrahydrofuraneTHF). The composition of the mixture was: 57.7 wt.% NPOE,.6 wt.% Zn ionophore, 3.4 wt.% PVC, 1.3 wt.% KTCPB, and 34.0 wt.%

ments: (A) solid contact, and (B) conventional liquid contact. The diameter of thepipette tips used in the studies ranged between 10 and 50 �m.

THF. The synthesis of N-phenyliminodiacetic acid bis-N′,N′-dicyclohexylamide zinc ionophore can be found in Ref. [48].

Two different kinds of zinc-ion selective microelectrodes wereprepared using micropipettes pulled from acid washed, and driedborosilicate glass capillaries (B100-50-10 and B200-116-10; Sut-ter Instruments, Novato, CA, USA), namely solid contact andconventional microelectrodes. The conventional micropipette ionselective electrodes were fabricated according to the establishedmethodology [49,50], whereas the solid contact ion-selectivemicroelectrodes were manufactured with the new proceduredescribed in Refs. [41,43]. In brief, these novel micropipetteelectrodes were made of two parts as shown in Fig. 1A. Twomicropipettes were pulled from glass capillaries using a pipettepuller, and subsequently silanized by introducing inside their tipa few microliters of 5% dimethyldichlorosilane solution in carbontetrachloride. The hydrophobic layer was completed after heatingthe pipettes at 80 ◦C in an oven for about 30 min. The cocktail wasbackfilled into the tip through a very thin glass capillary attachedto the syringe needle used. The internal solid contact is providedby a PEDOT-coated carbon fibre of 33 �m diameter (obtained as agenerous gift from Specialty Materials, Lowell, Massachusetts, USA)dipped in the cocktail. To ensure electric contact between the innerfibre and the copper wire for the connection of the ISME with themeasuring instrument, mercury was introduced in the inner capil-lary as shown in Fig. 1A. Conventional micropipette electrodes usedan internal filling solution (0.25 M potassium chloride containing10 mM ZnCl2) in contact with the cocktail and a chloride coatedsilver wire introduced as internal reference electrode as depictedin Fig. 1B.

2.3. Instrumentation

The SECM instrument has been described previously [51]. Maincomponents are a 3D positioning stage driven by stepper motors(MFN 25PP; Newport, Irvine, CA, USA), a battery-powered voltagefollower based on an operational amplifier (TL082; National Semi-conductor, Santa Clara, CA; USA), a digital multimeter (M-3630D;

camera was used to further assist positioning of the tip close tothe surface. The electrochemical cell was composed by the ISME astip, and an Ag/AgCl/3 M KCl as reference electrode. The completeset-up is schematically shown in Fig. 2.

400 J. Izquierdo et al. / Electrochimica Acta 59 (2012) 398– 403

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Fig. 2. Diagram showing the main components of the SECM instrument.

. Results and discussion

.1. Characterization of the Zn2+-ion selective electrodes

We were primarily concerned with the fabrication of zinc ionelective microelectrodes for SECM operation. The calibration pro-edure consisted in the introduction of the microelectrode in

sequence of ZnSO4 solutions with 10−x molar concentration,ith 6 ≤ x ≤ 1. The procedure was initiated with the most diluted

olution and subsequently exposed to solutions of increased con-entration. After a brief period overpotential values occurring uponlectrolyte exchange, the electrode always attained a steady poten-ial value in each solution. With the potential values taken fromach solution, the calibration plot shown in Fig. 3 was drawn. It cane observed that there is a linear relationship between the potentialf the ISME and the minus logarithm of concentration of zinc ions,nd the slope of the plot is 29.8 mV decade−1. The activity coeffi-ients were calculated using the Debye–Hückel approximation.

.2. Potentiometric SECM

The performance of the solid contact Zn2+-ISME to image metalon concentrations originating from a corroding surface using SECM

as explored on a zinc–iron galvanic pair exposed to a 10 mM NaClqueous solution, open to air and at ambient temperature. Fig. 4Aepicts SECM scan lines obtained above the zinc specimen for dif-erent exposure times ranging from 1 to 177 min. That is, the ISMEip is moved in the X direction at a constant vertical distance of5 �m from the substrate, starting from a position on the resinleeve close to the zinc squared strip. Indeed, the scanning probeasters the zinc surface for 220 < X < 1460 �m, and subsequentlyontinues its movement on the epoxy sleeve in direction towardshe location of the iron sample. The tip movement was stoppedefore reaching the iron surface in this case, though a sufficiently

ong excursion above the insulating resin was made as to allow therobe to travel in a portion of the electrolyte with a concentra-ion distribution characteristic of the bulk electrolyte. All the linecans were taken on the same line passing through the center ofoth metal samples, in order to unambiguously monitor changes

elated to the advancement of the corrosion process. The onset ofalvanic corrosion leads to the oxidation of zinc with the releasef Zn2+ ions, whereas the cathodic process is located on the ironurface (not shown in the figure).

Fig. 3. Calibration of the potentiometric response of the solid contact Zn2+-ion selec-tive microelectrode to solutions of varying Zn2+ concentration. (a) Dynamic responsetransients of the Zn ISME to metal concentration; and (b) calibration plot.

The first scan line recorded just after the electrolyte was addedto the cell container and the chosen probe-substrate distance wasattained, closely describes the initial conditions of the system. Thepotential recorded at the ISME shows an almost constant read-ing below the lower limit of detection of the microelectrode asfrom the calibration curve in Fig. 3, regardless the probe travellingabove either the zinc strip or the insulating resin. This conditionis described in the graph by pZn values close to 7. Yet, a moredetailed inspection of the plot corresponding to this initial con-dition does exhibit some trends consistent with the dynamics ofthe corroding system. The pZn values show a tendency to decreasefrom left to right while the probe is passing above the zinc strip.Though zinc concentrations are below the range for their quantita-tive determination, the progress of the corrosion reaction and thesubsequent release of Zn2+ ions, leads to the probe travelling intopositions of greater metal concentrations during the time periodof the scan. The situation changes after the other side of the zincsquare is reached for X = 1460 �m, and the concentration valuesderived from the potential recording at the ISME returns to thosefound at the beginning of the scan line. And the situation remainsalmost constant until the end of the probe travel.

Quantitative determinations of the concentration of Zn2+ ionsin the system are attained in all the subsequent line scans plot-ted in Fig. 4A. They all show a clear accumulation of metal ions

above the zinc strip, which quickly fades away as the probe moveson the resin sleeve to greater distances from the metal. This isalready the case for the line scan measured after 13 min, whichshows the progressive enrichment of the electrolyte in metal ions

J. Izquierdo et al. / Electrochimica Acta 59 (2012) 398– 403 401

Fig. 4. Potentiometric SECM scan lines measured above the zinc specimen in azinc–iron galvanic pair immersed in 10 mM NaCl after different times as indicatedin the plots. Two different Zn2+-ISMEs were employed as SECM tips, namely: (A)the novel solid-contact micropipette system described in this work, and (B) a con-v2

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to the values characteristic of the bulk electrolyte for the furthestdistance covered in this experiment. All these observations are con-sistent with previous studies based on the measurement of ionic

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bove the zinc strip during the duration of the probe travel asell. This feature is not observed in the scan lines measured for

onger exposures, because the higher metal concentrations exist-ng at those times are not significantly modified by the metal fluxeseaving the corroding surface below. The concentration of metalons continue to increase with the elapse of time until about 30 minfter immersion, and a stationary is observed for the rest of thexperiment duration. This behaviour is more clearly observed bynspection of Fig. 5, which gives the maximum metal ion concen-ration obtained from each line scan, thus effectively plotting theariation of Zn2+ concentration with exposure time. It is interestingo notice the development of a maximum concentration of metalons after ca. 30 min, and the concentration remains practicallynchanged for the remaining of the experiment. Since it is unlikelyhat the corrosion process might have stopped after this short expo-ure when the metal is galvanically coupled to iron in 10 mM NaClqueous solution, the attainment of a maximum concentration for

n2+ ions in the electrolyte must be due to the combined effectf the corrosion reaction (that should lead to further increase ofinc ion concentration), and the consumption of the metal ions

Fig. 5. Time evolution of the Zn2+ concentration measured above the zinc specimenfrom the potentiometric scan lines depicted in Fig. 4A.

during the precipitation of corrosion products through reaction[52]:

Zn2+ + nH2O → [Zn(OH)n](2−n) + nH+, pK1 = 7.96

Accordingly, the local pH of the electrolyte in contact with thesurface of zinc should be ca. 3.9, a value that agrees well with thepH values observed for the same system when a pH-sensitive anti-mony electrode was employed as the SECM tip instead [40]. Thoughfrom the inspection of the Pourbaix diagram of zinc [53], precipi-tation of corrosion products would not be expected for pH valuesbelow 7, the calculations given in Ref. [52] support our observa-tions. Furthermore, similar results were recently reported usingSIET as observed during the self-repair processes occurring in adefective coating applied on galvanized steel [47].

Another feature is that the greater change in metal ion concen-trations when departing from the corroding sample occurs within ashort distance from the metal limit (in excess of two decades in thevalues of pZn), which is followed by a rather slow asymptotic decay

Fig. 6. Distribution of Zn ion concentration above the zinc specimen measuredwith the solid contact Zn2+-ISME as the height of the tip is changed. The zinc–irongalvanic pair has been immersed in 10 mM NaCl for the immersion times given inthe graph. The approach and retreat lines closely match each other.

402 J. Izquierdo et al. / Electrochimica Acta 59 (2012) 398– 403

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ig. 7. Distribution of Zn2+ ion concentration in a plane perpendicular to the surfaaCl for 5 h. Y axis indicates the height position of the ISME in relation to the substra

uxes in the electrolyte adjacent to the same zinc–iron galvanicystem made by SVET [29].

The behaviour of the novel solid contact Zn2+ ISME was com-ared with that exhibited by conventional micropipette electrodesy conducting a similar set of experiments on the same experi-ental system. Fig. 4B shows a selection of the line scans recorded

fter different exposures. In this case, the ion-selective electrodeoes not provide data with enough stability and reproducibility toecord quantitative data in potentiometric SECM. The plots exhibitig oscillations, and they are greatly affected by the scan directionor this scan rate, whereas the solid contact ISME did not show theseimitations. In this way, the superiority of a solid contact ISME forhe potentiometric operation of the SECM could be established.

The distribution of Zn2+ species in the electrolyte phase over aorroding sample could also be monitored as a function of the dis-ance from the surface. In this case, the ISME probe moved over thepecimen until it was placed approximately above the center of then sample, and subsequently retracted from the surface by meansf the Z positioner. Fig. 6 depicts the corresponding plot measuredhereas the ISME was moved from the distance of closest approach

n our work (i.e. 25 �m) until a distance of 1200 �m. The concen-ration of Zn2+ ions diminished steadily during this operation. Thetability of the microsensor is further evidenced by switching thecan direction in order to measure the corresponding approachurve. Both plots are well superimposed, a further evidence of aobust and stable potentiometric probe. Thus, the distribution ofetal ions can be imaged by scanning in the XZ plane as it is

hown in Fig. 7. This image is effectively a composition of scan linesecorded in the X direction in a movement parallel to the surface ofhe sample. In this way, each scan line extends for 2500 �m. Oncehe scan line has been completed, the probe is retracted on the Zxis to a new height, and the next scan line was recorded. In order toeduce the time required to acquire a complete map up to a distancef 1000 �m, well inside the bulk of the electrolyte scan lines haveeen recorded only at a selection of probe-surface distances, andhe composed map given in Fig. 7 shows some blank lines accord-ngly. Yet, the concentration profiles in both the X and Z directionsan be clearly deduced from the observation of this image.

. Conclusions

This study reveals that solid contact Zn2+ ion sensor microelec-rodes can be used to monitor concentration distribution of zincations in a corroding system and the influence of corrosion prod-cts precipitation with the scanning electrochemical microscope

perated using rastering conditions similar to those employed withhe amperometric modes.

The formation of zinc soluble species is identified in the anodicone covering the zinc strip coupled to iron when exposed to an

he zinc specimen registered after the iron–zinc pair has been immersed in 10 mMface at which each line was recorded. Colors in the graph correspond to pZn values.

aqueous 10 mM NaCl solution. The time evolution of the corrodingsystem could thus be monitored.

SECM images depicting the concentration distribution of Zn2+

species over the reactive system can be obtained by recording scanlines in the XY plane parallel to the investigated surface, or as depthprofiles when the XZ plane (i.e. perpendicular to the surface) istaken instead.

Summarizing, this work demonstrates the complementary useof the potentiometric operation of the scanning electrochemicalmicroscope to detect local changes of the ion activity as result ofcorrosion reactions.

Acknowledgements

The authors are grateful to the National Office for Research andTechnology (NKTH, Budapest, research grant ES-25/2008 TeT) andto the Spanish Ministry of Science and Innovation (MICINN, Madrid,Acción Integrada No. HH2008-0011) for the grant of a CollaborativeResearch Programme between Hungary and Spain. J.I. and R.M.S.are grateful for financial support by the MICINN and the EuropeanRegional Development Fund (Brussels, Belgium) under Project No.CTQ2009-12459. A Research Training Grant awarded to J.I. by theMICINN (Programa de Formación de Personal Investigador) is grate-fully acknowledged. L.N., G.N. and A.V. acknowledge support from“Developing Competitiveness of Universities in the South Trans-danubian Region (SROP-4.2.1.B-10/2/KONV-2010-0002)”.

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