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Electrochimica Acta 87 (2013) 283– 293

Contents lists available at SciVerse ScienceDirect

Electrochimica Acta

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otentiometric scanning electrochemical microscopy for the localharacterization of the electrochemical behaviour of magnesium-based materials

avier Izquierdoa,b, Lívia Nagyc, István Bitterd, Ricardo M. Soutoa,b,∗, Géza Nagyc,∗∗

Department of Physical Chemistry, University of La Laguna, E-38200 La Laguna, Tenerife, Canary Islands, SpainInstituto Universitario de Materiales y Nanotecnologías, 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, Hungary

r t i c l e i n f o

rticle history:eceived 4 June 2012eceived in revised form0 September 2012ccepted 11 September 2012vailable online 17 September 2012

a b s t r a c t

The applicability of scanning electrochemical microscopy for the local detection and quantificationof relevant species participating in the corrosion of magnesium-based materials is presented. Theiron–magnesium galvanic pair exposed to aqueous NaCl solution was adopted as model system for thispurpose. Mg2+ ion concentration and pH profiles were investigated using ion selective electrodes, con-taining a liquid membrane and Sb/Sb2O3 as sensor elements, respectively. Oxygen consumption at thesubstrate related to the cathodic reaction was also monitored with the antimony-based electrode though

eywords:otentiometric SECMagnesium

Hon selective micropipettes

operated amperometrically. Data show a major production of hydroxyl anions at the cathodic sites asresult of the oxygen reduction half-reaction, whereas in the vicinity of the magnesium surface pH isgreatly affected by the anodic dissolution process instead. The later produces the release of metal cationsaccompanied by hydrogen evolution.

© 2012 Elsevier Ltd. All rights reserved.

ntimony electrode

. Introduction

Electrochemical corrosion is one major cause of economicosses in the industrialized countries [1]. Despite their attrac-ive mechanical properties, most metals and alloys spontaneouslyndergo corrosion if certain conditions occur, especially in aque-us environments. Particularly, due to its high strength-to-weightatio, magnesium and its alloys are attractive materials for sev-ral applications in the automotive, aerospace, electronics andnergy-production industries [2–5]. And they are often found innavoidable contact with other metals such as aluminum or steel5]. However, these materials lack stability against corrosion andsually require additional protection. In fact, thanks to its tendencyo degrade in aqueous environments, it has been considered as aotential biodegradable and biocompatible material for non per-

anent implants [6–9]. Moreover, its position in the galvanic series

as allowed magnesium to be evaluated for the protection of otheretals by acting as sacrificial anode, either as a component of a

∗ 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: rsouto@ull.es (R.M. Souto), G-nagy@gamma.ttk.pte.huG. Nagy).

013-4686/$ – see front matter © 2012 Elsevier Ltd. All rights reserved.ttp://dx.doi.org/10.1016/j.electacta.2012.09.029

protective coating, or via direct galvanic connection [10–14].Besides, several Mg-containing alloys like those aluminum-basedones, tend to suffer from preferential dissolution of this component,usually the most active in the material [5,15,16].

Local microcells are originated in a metallic material sub-jected to corrosion, with the subsequent spatial distributions ofanodes and cathodes on its surface when left at its open cir-cuit potential (OCP). In this way, dissolution occurs as result ofmetal oxidation at the anodes, whereas this process is main-tained by the reduction of some species from the environment atthe cathodes, usually oxygen in neutral and moderately alkalinemedia, or protons in acidic solutions. Since these local cells areoriginally formed within the range of micrometers and nanome-ters, a comprehensive understanding of the mechanistic aspectsof corrosion requires the synthesis of data in those scales. Unfor-tunately, conventional electrochemical techniques lack spatialresolution and provide little information on behavior at sites ofcorrosion initiation or at defects. The recent advent of a variety oflocal probe techniques is greatly contributing to overcome theselimitations when applied to the investigation of corrosion pro-cesses in situ. Following this trend, researchers are exploring awide variety of methods to characterize local electrochemical pro-

cesses, which range from the design of microelectrochemical cellsand setups, to the use of scanning probe techniques [17]. Amongthe later scanning electrochemical microscopy (SECM) has becomeone of the most powerful local techniques for corrosion research

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84 J. Izquierdo et al. / Electroc

ue to the wide variety of operation modes which contribute toive a great versatility to the technique [18–20].

The corrosion of metals produces local changes in the concen-rations of ionic and molecular species present in the adjacentiquid phase. Thus, the Mz+ ions produced at the anodic sitesre transferred under diffusion control to the bulk, and the sys-em can be studied by amperometric SECM [21]. This feature haseen exploited to image the generation of specific metal cations

n several studies that include visualization of metastable pits onustenitic steel in situ by SECM at the open-circuit potential [22], tohe detection of metal dissolution either from inclusions in alloys23–25] or from defects in polymer-coated metals [10,26,27]. Onhe other hand, in a neutral aqueous medium, the reaction ofissolved oxygen from the electrolyte at the cathodic areas maye analyzed through monitoring of the subsequent depletion ofxygen content in the solution volume adjacent to the cathodicites [28–32]. However, for the detection of concentration distri-utions in certain corroding samples, especially for metals withufficiently negative redox potentials in aqueous environments,he use of Pt microelectrodes is limited by the onset of oxygeneduction and hydrogen evolution reactions [10,27,31]. Thus, forffective amperometric operation of the SECM, it becomes neces-ary to either purge out oxygen and to use modified electrodes tovoid early evolution of hydrogen gas [33,34]. Yet, more robust andelective procedures are to be developed for the analysis of theseetal cations in order to gain a better understanding of the mech-

nisms involved in corrosion and passivation. As it is well known,orrosion processes are dramatically affected by pH too. Further-ore, both corrosion half-cell reactions may lead to local changes

n pH of the surrounding environment. On one hand, alkalinizationlways occurs in the cathodic sites due to either H+ consumptionr OH− release, depending on nature of sample and electrolyte,nd it may facilitate the formation of passive films. On the otherand, some corrosion-produced metal ions may undergo hydrol-sis at the anodic sites, and consequently reduce the local pH, aeature that usually accelerates the corrosive attack. Though thextent of pH variation associated with the hydrolysis of metal ionsainly depends on the equilibrium constant for their hydrolysis

rocess [35], thus greatly varying from one metal to another, itay become a major feature affecting the onset of local corrosionicrocells. Therefore, the measurement of the local alterations of

H in the vicinity of a corroding system and their time evolution is matter of major interest.

In contrast, though rather scarcely applied, these physicochemi-al parameters may be studied using ion-selective electrodes (ISEs)s tips. In this case, the SECM is operated in the potentiometricode [36–38], which gives greater chemical selectivity. The appli-

ability of Zn2+ ion-selective microelectrodes (ISMEs) for the studyf model corrosion systems with SECM [39,40] has been recentlyemonstrated, similarly to what has already been achieved withhe Scanning Ion Electrode Technique (SIET) [41]. In studies apply-ng SIET, a variety of ion-selective systems have been investigated42–50], including Mg2+ species released during corrosion of theuried metal from defects operated on coated magnesium alloys42,45,48]. Yet investigations of magnesium substrates with SECMave only been carried out in amperometric mode [51,52] or foreasurements of the electrolyte resistance between the tip and

he substrate [53], using always Pt microelectrodes as the probe.n those studies, visualization of active regions in the surface wasccomplished, however selective potentiometric detection of theocal concentration of the corrosion produced Mg2+ ions was notttempted.

Most often liquid type microelectrodes have been employed fortudying pH distributions above corroding metal surfaces [42–50].evertheless, for corrosion purposes using SECM, it would bereferable to use microelectrodes based on the characteristics of

Acta 87 (2013) 283– 293

certain oxides of transition metals such as antimony [54–56] andiridum [57]. These solid state microelectrodes are much morerobust than ion selective micropipettes. Furthermore they have theability to behave as double function electrodes. This means thatwhile in potentiometric mode they detect local pH, they can alsofunction as amperometric SECM tip to detect electroactive species.

In this work, we report preliminary results on the characteriza-tion of a model corrosion system formed by an iron–magnesiumgalvanic couple which were obtained from SECM measurementsusing antimony and Mg2+-ion selective electrodes as the tips. Theconcentration distributions of dissolved oxygen and Mg2+ ions, aswell as the pH distributions that result of the galvanic corrosionprocess, were measured and analyzed in terms of the reactionsoccurring in the system.

2. Experimental

2.1. Chemicals and solutions

High purity antimony in powder presentation (Aldrich) wasemployed for the fabrication of the antimony electrode. Solutionswere prepared from Millipore deionized water and analytical gradechemicals. The inner reference solution employed in the lumenof the Mg2+-ISE was 10 mM MgCl2 + 0.25 M KCl. Different concen-trations of the same magnesium salt, dissolved in 10 mM NaClsolution, were employed for calibration of the ISE.

The ionophore employed in the fabrication of the Mg2+ ISEwas bis-N,N-dychlohexyl-malonamide, which was synthesizedfollowing the method described in reference [58]. The compo-sition of the ion selective cocktail was the following: 2.1 wt.%ionophore, 34.0 wt.% high molecular weight poly(vinyl-chloride),and 63.9 wt.% o-nitrophenyl-octyl-ether. 70 mol.% potassiumtetrakis(p-chloro-phenyl) borate was subsequently added. Allthe components in the ionophore membrane were supplied bySigma–Aldrich. Ion-selective microelectrodes were prepared usingmicropipettes pulled from borosilicate glass capillaries B200-116-10 (Sutter, Novato, CA, USA). The inner wall of the pipette tips werehydophobized exposing them to a solution of dimethyldichlorosi-lane in carbon tetrachloride.

2.2. Samples and instruments

The galvanic Fe–Mg couple employed for this study consistedon one sheet of each metal immobilized in Epofix resin (Struers,Ballerup, Denmark). Iron was cut from 1 mm thick sheet of 99.99%purity purchased from Goodfellow Materials Ltd. (Cambridge, UK),in order to obtain a 1 mm × 2 mm strip. For the magnesium surface,metal ribbon of 99% purity supplied by Panreac (Barcelona, Spain),with a cross section of 200 �m × 3 mm, was employed. The metalwas not analyzed, but typical impurity levels (in ppm) were cited bythe supplier as: Cu 5, Fe 50, Ni 5, Pb 5, Zn 20. Both metal strips werefixed vertically in the resin, facing up their rectangle cross sectionsas the active surfaces to be analyzed. The separation between thetwo metals was ca. 5 mm, which was assumed to be enough dis-tance to avoid alterations in either of the active metal areas due tothe reactions taking place on the other one for the duration of thetests performed in this study. The resulting mount with the embed-ded metals had a circular top surface of 3 cm diameter. The surfaceof the specimen was flat ground with abrasive SiC paper up to 4000grit, rinsed with ethanol, dried in air, and finally wrapped aroundlaterally by sellotape that reached over the mount surface for about

6 mm. Thus a small container of approximately 5 ml of electrolytewas created for the microelectrochemical measurements.

The antimony electrode employed in SECM for combinedamperometric/potentiometric operation was fabricated following

J. Izquierdo et al. / Electrochimica Acta 87 (2013) 283– 293 285

F easure1

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awsSF[gftmcfsaLp

ig. 1. Sketches of the two types of potentiometric tips employed in the SECM m0−2 M MgCl2 + 0.25 M KCl as the internal solution).

he procedure described elsewhere [55]. In brief, the fabricationas initiated by heating the antimony powder in a melting potith a gas flame. When antimony melted, it was introduced in

thick glass Pyrex capillary using syringe suction. The resulting,ntimony filled capillary was then pulled manually with standardlass blowers technique using metallic tweezers. This resulted in

small diameter glass fiber filled with antimony. A pipette with00 �m tip diameter was prepared from borosilicate glass capil-

ary and a section of the previously prepared antimony fiber wasnserted in its lumen with the tip reaching out for about 15 mm,

hereas ca. 20 mm long staying in the lumen. Mercury metal and aopper wire were then inserted into the lumen of the thicker cap-llary to provide electrical contact resulting in the assembly shownn Fig. 1A. Loctite® adhesive was used to seal both ends. The final

icroelectrode had an antimony disk of 175 �m with an RG value of.9. The electrochemical response of the antimony microelectrodeor amperometric operation was tested in two electrolytes, namelyerated 10 mM NaCl, and deaerated 5 mM [Ru(NH3)6]Cl3 + 0.1 MaCl solutions. Potentiometric operation was directed to monitor

he effect of pH on the potential of the electrode. Calibration waserformed using 9 buffer solutions covering the range 4 ≤ pH ≤ 12.

linear relationship between the potential of the antimony tip andhe solution pH was observed between 4 and 11, and the slope ofhe calibration plot amounted 42.3 mV/pH unit.

Potentiometric ion-selective operation was performed using conventional micropipette type Mg2+-ISE, that was preparedith liquid internal filling solution and a silver chloride coated

ilver wire of 250 �m diameter, as internal reference electrode.chematic drawing of the micropipette electrode is shown inig. 1B. It was fabricated according to well established procedures59,60]. In brief, two micropipettes were first pulled from onelass capillary, and subsequently silanized by introducing inside aew microliters of 5 vol.% dimethyldichlorosilane solution in carbonetrachloride. The hydrophobic layer was obtained by keeping the

icropipettes at 80 ◦C in an oven for about 30 min. The ionophoreocktail was then filled into the micropipette by applying vacuumrom the back side with an attached syringe. Finally, the internal

olution was backfilled with the assistance of a very thin capillaryttached to a syringe, and the Ag/AgCl was placed and fixed withoctite® adhesive. Due to the relatively big size of the metal sam-les employed in this study, micropipettes with about 180 �m tip

ments: (A) antimony electrode, and (B) micropipette-based Mg2+-ISE (containing

diameter were prepared and used. As it is well known micropipetteISE’s can be easily prepared with submicron diameter, however itwas not justified to use so small tips in this work, because theypresent bigger resistance and high noise picking up character.

2.3. Instruments

Measurements were performed in an SECM equipment builtby Sensolytics (Bochum, Germany), controlled with a personalcomputer. The electrochemical interface employed in the amper-ometric operation was an Autolab bipotentiostat provided witha frequency response analyzer (Metrohm, Herisau, Switzerland).In this case, the electrochemical cell was completed with anAg/AgCl/KCl (3 M) electrode as reference, and a Pt wire as counterelectrode. Alternating current (AC) mode was also performed usingthese instrument and cell configurations. AC voltage signals of10 mVpp amplitude around the open circuit potential of the tip inthe electrolyte were applied. A total of 26 frequency values in the351–55,493 Hz frequency range were used, and the values spacedlogarithmically. The same SECM instrument was employed forthe potentiometric measurements using ion selective electrodes,though in this case a home made voltage follower based on a1012 � input impedance operational amplifier was interconnectedbetween the cell and the potentiometric input of the system.

3. Results

3.1. Mg2+ distribution over the magnesium surface

In order to study the distribution of Mg2+ concentration in theelectrolyte from metal corrosion, SECM in potentiometric opera-tion was employed with a Mg2+-ISE as probe. The metal sample wasplaced at the bottom of the small electrochemical cell, and the MgISE tip was fastened onto the tip holder. Before the test electrolytewas introduced in the cell, the tip was placed over the center of Mgmetal surface using the X, Y motors of the instrument. A video cam-era was employed to assist the process. Next, the tip was carefully

moved towards the metal surface by means of the Z motor until ca.75 �m vertical tip–substrate distance was reached. At this stage,the test solution was added to the measuring cell, the referenceelectrode was introduced, and the two metals in the sample were

286 J. Izquierdo et al. / Electrochimica Acta 87 (2013) 283– 293

0 1 2 3 4 51

2

3

4

pM

g

d/a

Retreat, galvanic connection

Approach, galvanic connection

Retreat, electrically disconnected

Fig. 2. Distribution of Mg2+ ion concentration above the magnesium strip measuredwith the Mg2+-ISE as the height of the tip is changed. The iron–magnesium samplewas immersed in 10 mM NaCl solution, whereas the two metals were either con-nected electrically (i.e. galvanic coupling) or disconnected. Both the direction of thett

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Galvanic connection

Disconnected

Fig. 3. Potentiometric SECM scan lines measured above the magnesium strip in

Fig. 3. That is, scanned X lines correspond to several of the cross

ip movement and the electrical condition of the metal are given in the graph. d/a ishe dimensionless distance between the sample and the tip. Scan rate: 5 �m s−1.

lectrically connected at the rear of the sample by means of a copperire. The evolution of the e.m.f. of the cell was monitored with the

lapse of time. After a few minutes, a steady voltage was attained.hen the tip was slowly removed from the metal surface, at a speedf 5 �m/s moved upward, while recording the electrode poten-ial at the ISE. Once the chosen maximum tip–substrate distanceas reached, the tip–substrate distance was kept constant for twoinutes, and subsequently the tip was moved down at the same

ate, 5 �m/s, to approach the Mg metal surface again. The elec-rode potential vs. tip–substrate distance curves could be convertednto pMg2+–tip–substrate distance plots using the calibration curvef the electrode. Normalized tip–substrate distances were used byetermining the ratio between the tip radius, a, and the verticalip–substrate distance, d. The pMg2+–d/a curves measured duringhe initial withdraw (black curve) and the subsequent approachred curve) steps are shown in Fig. 2. Next, the electric connectionetween the Fe and Mg metals was removed, and a new retreatMg2+–d/a curve was registered for the electrically-disconnectedagnesium metal (cf. the blue curve in Fig. 2).As it was expected, the Mg2+ ion concentration was higher

hen the tip was placed closer to the substrate. Nevertheless,he approaching (black) and the withdrawing (red) experimentalurves for the galvanically-coupled metals are quite different. Thisifference can be attributed to unavoidable convection in the elec-rolyte produced by the motion of the tip. That is, as the tip is moved,t displaces some electrolyte from the Mg2+-rich surroundings ofhe metal to the bulk solution, resulting in the measurement of big-er values for ion concentration during the retreat movement. Andhe opposite behavior is observed when recording the approachurve.

Higher pMg2+ data were determined when the two metals wereisconnected. This indicates that magnesium dissolves less vigor-usly in the environment when it is not acting as sacrificial anodeor iron. From the plots given in Fig. 2, it is also obvious that when

agnesium was galvanically-coupled to iron, the curves wereougher than the one determined over the unconnected metal. Therigin of this fact could be explained with the assistance of the video

amera, because gas evolution occurred on the surface of magne-ium when the two metals were electrically connected. That is,as evolution was not exclusively restricted to the iron strip, but

the iron–magnesium sample during immersion in 10 mM NaCl. The metals were:(– � –) with, and (– © –) without galvanic connection. Vertical tip–substrate dis-tance: 75 �m; scan rate: 5 �m/s.

it was even more vigorous over magnesium. It is known from basicchemistry that the gas evolving at the magnesium–water interfacemust be hydrogen resulting from the reaction between this metaland water. Interestingly the magnesium acts as an anode in theMg–Fe couple, but hydrogen is reduced during its corrosion pro-tecting function, a distinctive feature for this metal and its alloys[5].

The electric connection of the two metals originates the devel-opment of a high negative potential on the iron metal. It wasobserved to be more negative than −1.60 V vs. Ag/AgCl/KCl (3 M)at all times. These potential values lay outside the stability rangeof water. And hydrogen gas evolution is also occurring at the ironsurface.

Concentration profiles of Mg2+ ions were also determined byrastering the ISE parallel to the surface. Line scans were measuredalong the 200 �m cross section of the Mg area, both with (red curve)and without (black curve) galvanic connection between the twometals. Results are given in Fig. 3 for single scan lines determinedwhen the tip was maintained at ca. 75 �m vertical distance fromthe surface. As it was observed before, the signal registered nearthe corroding metal indicated higher Mg2+ concentration whenthe metal was acting as sacrificial anode in the iron–magnesiumgalvanic pair, than those measured when the two metals were dis-connected. The maximum dissolution of magnesium was detectedwhen the tip passed over the central position of the metal strip,which supports that the whole area was behaving anodically. Con-versely, this maximum over the center of the Mg strip surface wasnot observed when the metals were not in electrical contact. Inthis case, the shape of the concentration profile was not symmetricover the metal, and a separation between the portion acting as theanode (at the left side of the line scan), and the area functioning ashydrogen-reducing cathode (right side), could be noticed.

Additionally, several concentration profiles were measured atdifferent locations over the Mg strip while the metal was under gal-vanic connection. In this way, heterogeneities in the location of thecorrosion process over the metal could be detected. Fig. 4 depicts sixconcentration profiles measured when the tip was moved along theX axis at different Y positions, covering a total length of 100 �m inthe Y axis, whereas keeping the same scan direction used to record

sections taken in the 200 �m width direction of the magnesiumarea, which were measured over a central region of the larger 3 mmside. Despite the fact that the tip is actually too large to spatially

J. Izquierdo et al. / Electrochimica Acta 87 (2013) 283– 293 287

0

500

100 0

0.5

1.0

1.5

2.0

2.5

3.0

0

20

40

60

80100

Distance along X Axis,m

Y = 0 m

Y = 20 m

Y = 40 m

Y = 60 m

Y = 80 m

Y = 100 m

pM

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Distance

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Y Axis

,m

Fig. 4. Potentiometric SECM scan lines measured when the tip crossed (X direc-tion) over the magnesium strip in the iron–magnesium galvanic pair immersed in1wd

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above resin, disconnected

above Fe, disconnected

above Fe, galvanic connection

Fig. 5. Normalized Z-approach curves measured with the antimony electrode inamperometric SECM operation using oxygen reduction at the tip as redox mediatorreaction. The plots were measured above the resin sleeve or the iron strip as indi-cated in the graph. The iron and the magnesium strips were either disconnected,or connected electrically during the experiments, as indicated in the legend. Testelectrolyte: 10 mM NaCl. Tip potential: −0.70 V vs. Ag/AgCl/KCl (3 M); scan rate:10 �m/s; tip diameter: 175 �m; RG = 2.9. i/ilim is the dimensionless tip current, and

0 mM NaCl, taken in a plane parallel to the surface of the metal. The different curvesere obtained after shifting the tip in the Y direction in 20 �m steps. Tip–substrateistance: 75 �m; scan rate: 5 �m/s.

esolve the system in these dimensions, the concentration profileeasured for Y = 20 �m shows the higher values, indicating a region

n the anodic surface of the metal displaying greater electrochem-cal activity. Further studies would be necessary to determine thectual heterogeneity of the whole system and its possible relation-hip to any microstructural features as the nucleation sites for theventual generation of corrosion pits as result of it.

.2. Oxygen consumption

When a sacrifice anode is used for corrosion protection, oxygenonsumption occurs at the cathodic sites of the protected metal sur-ace if these metals are placed in non-acidic environments. In ordero study the oxygen concentration profile over the protected ironurface, an antimony microelectrode was employed as the SECMeasuring tip while the technique operated in the amperometricode. Since the SECM was operated in the amperometric mode, the

ip–substrate distance could be adjusted using the negative feed-ack effect when approaching a zone of the resin sleeve locatedetween the two metals. Furthermore, variations in the local oxy-en concentration could also be visualized. For this purpose, thelectrode was placed as close to the surface of the iron strip asossible with the assistance of a video camera, and subsequentlyetracted 1000 �m in the Z direction. Next, the tip was shifted hor-zontally 1.5 mm to be located above the resin sleeve. After the testlectrolyte was added to the system, and while the two metals wereept electrically disconnected, the Z-approach curve was recorded.

constant potential of −0.70 V vs. Ag/AgCl/KCl (3 M) was appliedo the tip, a potential value at which oxygen reduction occurs. Thelosest tip–substrate distance in Z-approach curve shown in Fig. 5ccurs for a negative feedback of ca. 60% (black curve). This reduc-ion in the faradic current measured at the tip corresponds to aeight of 85 �m over the surface if we consider the electrode to be

microelectrode of 175 �m diameter with RG = 2.9 [61]. After thisurve was recorded, the tip was lifted up, and moved parallel tohe surface to a position located just above the center line of theron strip to record the corresponding Z-approach curve. The samecan parameters were employed as before, still without galvanic

onnection between the two metals. The corresponding approachlot is also given in Fig. 5 (blue curve). Though it could be expectedhat the corrosion of the disconnected iron should result in somexygen consumption, the negative feedback effect actually seemed

d/a is the dimensionless distance between the sample and the tip.

to be smaller on iron than above the resin. This apparent discrep-ancy could be explained by considering differential polishing on theresin and the metal piece. That is, due to the difference in hardnessbetween the materials, the metal sample can be expected to pro-trude above the surrounding resin holder. Indeed, the Z-approachcurve measured above the iron strip could be superimposed overthat obtained on the resin by slightly shifting the curve along the Xaxis. In the forecoming, this observed distance difference was takeninto account in order to establish the actual tip–substrate distance.On the other hand, the fact that the Z-approach curves measuredboth on the resin and on the iron strip could be superimposed, indi-cates that the extent of iron corrosion in this environment does notlead to a significant depletion of the oxygen concentration at earlyexposures.

On the other hand, the red curve in Fig. 5 shows the Z-approachcurve measured above the same location on iron strip that wasmeasured after the two metals were electrically connected. In thiscase, local depletion of oxygen is readily observed in the plot. Thus,the corrosion of Mg drives a very intensive electrochemical oxygenreduction at the iron surface. And oxygen diffusion from the bulkelectrolyte could not compensate its consumption at the metal.

Two dimensional images of oxygen consumption were obtainedwhen the antimony tip scanned the surface in an X–Y plane set at175 �m vertical tip–substrate distance, and they are shown in Fig. 6.Image A was recorded when the two metals were disconnected,while image B was achieved when they were galvanically-coupled.In Fig. 6A, the image of the iron substrate shows up among the sur-rounding resin areas. Above the iron surface smaller current valueswere registered, showing smaller oxygen concentrations could beobtained. We can conclude that the SECM image shows the cor-rosion of iron in this environment. On the other hand, the imageof the iron strip shown in Fig. 6B is much sharper. In this case, tipcurrents close to 0 nA were measured over the metal, thus showingthat oxygen was absent from the thin electrolyte volume com-prised between the tip and the iron strip. The dimensions of thearea showing the lowest current values in the image closely match

the dimensions of the iron surface (1 mm × 2 mm). This proves thatthe whole metal is behaving as a cathode. Therefore, the iron strip

288 J. Izquierdo et al. / Electrochimica Acta 87 (2013) 283– 293

F the ani nnectt f 2500

it

3

btgtitttSeriawafimt

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ig. 6. Amperometric SECM images depicting the absolute current measured at

mmersed in 10 mM NaCl. The metals were: (A) without, and (B) with galvanic coip–substrate distance: 175 �m; scan rate: 30 �m/s. The figures represent an area o

s effectively protected from corrosion through its connection withhe sacrificial anode of magnesium.

.3. pH distributions

The antimony electrode was also employed to image pH distri-utions over the substrate during the corrosion reaction. Thoughhe electrode was usually placed as close as possible to the investi-ated surface with the assistance of a video camera, the possibilityo more precisely establish the tip-to-substrate distance by apply-ng an alternating potential to the tip was investigated. In this way,he tip could remain unbiased, provided a sufficiently small exci-ation AC signal was employed. The rationale for not polarizinghe antimony tip was to avoid the chemical modification of theb/Sb2O3 surface of the electrode. AC operation of the SECM wasmployed, and the magnitude of the current and its phase shift withespect to the applied AC potential modulation were recorded asmpedance values. The near-field interaction between the SECM tipnd the surface could be employed to obtain AC Z-approach curvesithin the frequency range. A location above the insulating resin,

pproximately equidistant from both metal specimens, was chosen

or the determination of the tip–substrate distance, and the results given in Fig. 7. The plots depict the variation of the normalized

odulus (Fig. 7A), and the phase shift (Fig. 7B) of the impedance ashe tip approached the substrate for a selection of frequencies of the

0 1 2 3 4 5 6 70.8

1.2

1.6

2.0

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55.0 kHz

23.1 kHz

9.68 kHz

4.06 kHz

1.70 kHz

715 Hz

300 Hz

A

ig. 7. Normalized AC Z-approach curves towards the resin sleeve at a position located0 mM NaCl with the 175 �m antimony electrode. The tip was left at its spontaneous opxcitation signal amplitude was 10 mVpp, and frequencies are indicated in the figure. (A)

he dimensionless distance between the sample and the tip.

timony tip travelling over the iron strip when the iron–magnesium sample wasion. Tip potential: −0.70 V vs. Ag/AgCl/KCl (3 M); tip diameter: 175 �m; RG = 2.9;

�m × 3000 �m in X and Y directions.

AC perturbation. Modulus values have been normalized by divid-ing the value recorded at each point by that measured in the bulksolution (e.g., sufficiently far from the substrate to give a stationaryvalue). The higher impedance observed at the end of the curve forevery frequency and the decrease in phase angle, are indicationsof the greater resistance offered to ionic current flow when the tipapproached the insulating surface due to the smaller electrolytevolume, though the extent of this blocking effect greatly dependson the frequency of the AC excitation signal. Greater contrast wasobserved in the low-frequency range, whereas the system becamevery noisy at 55 kHz.

Once the very close tip–substrate distance was established withAC mode of SECM, the antimony tip was removed to a height of80 �m, and subsequently positioned over the center of the mag-nesium sample, or the iron strip, by moving the tip in the X–Yplane. Then, the potentiometric operation mode was set in order tomonitor pH distributions over the metals. Vertical pH profiles weredetermined over the center line of each of the metals, both whenthey were disconnected, and when galvanic contact was estab-lished between them. In these experiments the following protocolwas followed: first, the pH-sensitive ISE was removed from the sur-

face to a distance of 600 �m. After 10 s waiting time at this height,the tip was moved down towards the surface until a tip–substratedistance of 0.5 normalized (d/a) units. After 10 s waiting time, thetip was removed again from the surface of the metal to the bulk.

0 1 2 3 4 5 6 70

10

20

30

40

50

Ph

ase

, d

eg

ree

s

d/a

B

between the iron and the magnesium strips when the sample was immersed inen-circuit potential in the electrolyte (namely + 0.44 V vs. Ag/AgCl/KCl (3 M)). TheNormalized magnitude, and (B) phase of the impedance measured at the tip. d/a is

J. Izquierdo et al. / Electrochimica Acta 87 (2013) 283– 293 289

0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5

5

6

7

8

9

10

11

0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5

5

6

7

8

9

10

11

pH

d/a

Retreat

Approach

B

pH

d/a

Retreat

Approach

A

F tip as its height was changed relative to the surface. The iron–magnesium sample wasi galvanically-coupled. The direction of the tip movement is indicated in the figures; scanr

TdwctTtFssstTcbaniftibtf

esmtbipdnssmcadM

inn

0 500 1000

5.5

6.0

6.5

7.0

7.5

8.0

8.5

pH

Distance along X axis, m

Fig. 9. pH distribution during a scan line recorded with the antimony electrode overthe magnesium strip. The iron–magnesium galvanic pair was immersed in 10 mMNaCl. Tip–substrate distance: 80 �m; scan rate: 5 �m/s.

0 2 4 6 8 10 1210

11

12

13

14

15

pH

d/a

Retreat

Approach

Fig. 10. pH distributions above the iron strip measured with the antimony tip as

ig. 8. pH distributions above the magnesium strip measured with the antimony

mmersed in 10 mM NaCl, whereas the two metals were (A) disconnected, and (B)

ate: 5 �m/s. d/a is the dimensionless distance between the sample and the tip.

he potential of the ISE was continuously recorded during all theuration of the experiment. Tip movement in the vertical directionas always performed at the speed of 5 �m/s. In this way, verti-

al pH profiles could be obtained as a function of the normalizedip–substrate distance for both the approach and the retreat steps.he same protocol was employed while the two metals were elec-rically disconnected, and when they were galvanically coupled.ig. 8 shows the mentioned vertical pH profiles measured above Mgurface. As it can be seen, in the case of the free-corroding magne-ium sample, higher alkalinity is detected in the vicinity of the Mgurface (see Fig. 8A), which indicates that the cathodic half-reactionakes place on some regions of the metal due to its free corrosion.he pH values determined at the largest distances were close to theorresponding value for the bulk solution, which was measured toe 8.76 when registered with a conventional pH-meter. Though thepproach and retreat curves exhibit a similar behavior, they couldot be overlapped, and they showed the biggest gap for normal-

zed distances smaller than 1.5. This fact can be attributed to theorced convection effect produced by the tip during its movemento and from the surface. Higher pH values were measured dur-ng the retracting process in the major part of the displacement,ecause the electrode was departing from a portion of the elec-rolyte more concentrated in hydroxyl anions during its movementrom the surface towards the bulk of the solution.

The opposite trend was observed when galvanic connection wasnsured (cf. Fig. 8B). The pH of electrolyte adjacent to the magne-ium surface became lower, than the pH of the bulk. Close to theetal surface, pH values as low as 5.5 could be detected. This shift

o the acidic direction occurred despite hydrogen evolution coulde observed with the video camera, and thus a source of alkalin-

zation was still somehow active at the metal. However, the higherresence of protons is more difficult to evaluate due to the greateriffusion rate of this species and, in this specific case, also due to theoise in the system provoked by the evolution of hydrogen gas. Theame effect was observed when the tip was moved parallel to theurface to record the scan line given in Fig. 9. This latest measure-ent shows a clear pH decrease in a section of ca. 200 �m, which

orresponds with the actual X portion of the magnesium strip useds target. It must be noticed that the onset of acidification could beetected at larger distances as a result of the diffusion of H+ andg2+ ions.

Finally, Fig. 10 displays pH–vertical d/a plots obtained above

ron surface when the two metals were connected. In both plots,amely approach and retreat curves, the alkalinization observedear the cathode is extensive, even at distances as large as 2 mm. It is

its height was changed relative to the metal surface. The iron–magnesium galvanicpair was immersed in 10 mM NaCl. The direction of the tip movement is indicatedin the figures; scan rate: 5 �m/s.

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90 J. Izquierdo et al. / Electroc

nteresting to compare this result with the amperometric curveecorded for oxygen detection, previously shown in Fig. 6B, whenhe limiting current was attained at d/a values of ca. 5. That is,onstant oxygen concentration in the electrolyte close to the bulkolution value could already be found at a tip–substrate distance of00 �m. Certainly, the corrosion process originates an increase inhe solution pH here through the reduction of oxygen. But in com-ination with the amperometric observations, if that was the onlyource of this effect, OH− concentration should decrease down tohe values in bulk solution at much smaller tip–substrate distanceshan those found in Fig. 10. Then, hydrogen evolution, probablyesulting from the highly negative OCP experienced by the elec-rically connected system, is likely to be the additional factor thateads to this high pH in the proximity of the iron strip. It must beoted that pH values shown in Fig. 10 for the volume of electrolyte

n the vicinity of the iron surface are far beyond the dynamic mea-uring range of the antimony electrode. Therefore no quantitativenformation can be extracted from these curves.

. Discussion

We report here that potentiometric operation of the SECM cane employed to locally measure concentration distributions of keypecies involved in the corrosion of magnesium in aqueous solu-ion. This metal is very reactive in water, thus making difficult toesign accurate experiments to study its corrosion mechanism. Atresent, despite the rather extensive literature on the corrosion ofhis metal and its alloys, the origin of the corrosion process is stillbscure and subjected a big controversy, and a number of experi-ental observations are regarded by some researcher as dubious.

ven experimental techniques are often put to their limits whennvestigating the electrochemical behaviour of magnesium in aque-us solutions due to joint operation of several features, namely:

1) The high dissolution rate of the metal in contact with an aque-ous environment often leads to portions of the material beingphysically detached from the matrix. It also makes difficultthe safe removal of corrosion products from the uncorrodedmaterial. As a result, weight-loss measurements may bearbig experimental errors. Furthermore, they are average mea-surements that give no information on the dynamics of thecorrosion reactions.

2) The corrosion of the magnesium is accompanied by vigor-ous hydrogen evolution [3,4]. A “strange hydrogen evolutionbehaviour” has been described [5]. That is, more hydrogen isevolved when the anodic overvoltage is increased or a higheranodic current density is imposed, conversely to what happenswith the other metals. This phenomenon is called the negativedifference effect (NDE). Hydrogen evolution can greatly affectthe efficiency of electrochemical probes either by blocking theirsurface or even modifying their surface state. Additionally, itcauses convection effects in the electrolyte solution that affectthe current distribution in the electrochemical cell. Conversely,experimental methods have been developed for the quantifica-tion of hydrogen evolving during the corrosion of magnesiumand its alloys [62]. Though they may be regarded highly effi-cient techniques, and procedures have been designed to followthe variation with time of hydrogen evolution, they cannot dis-tinguish whether the gas evolves from anodic or cathodic sites.

3) Magnesium hydrolysis is basically non-acidic. The equilibriumconstant associated with the first deprotonation step of itsaquo-complexes [M(H2O)x]2+, has pK1

a = 11.4 [63]. The acidifi-

cation caused by hydrolysis of Mg2+ can be calculated for givenconcentrations of the metal ion using the equation:

pH = − log√

K1a[Mg2+] (1)

Acta 87 (2013) 283– 293

(4) Using [Mg2+] = 0.0001 and 0.1 M, the expected pH values wouldbe 7.0 and 6.2, respectively [41]. That is, oxidation of mag-nesium is not capable of significantly changing the pH of aneutral corrosion medium. Conversely, either oxygen or protonelectroreduction during the cathodic half-cell reaction producestrong alkaline environments. Strong alkaline environmentsare usually reported around corroding magnesium and mag-nesium alloys [5].

(5) The formation of a passive film of magnesium hydroxide thatexhibits some corrosion protection characteristics is consideredto be formed on magnesium and its alloys, especially at high pH[64]. This passive film is unstable, and the onset of corrosion onmagnesium is regarded to occur at film-free areas on the surfaceof the metal [5,53,65]. Indeed, the open circuit potential of freshpolished samples immersed in aqueous environments usuallyevolves towards more noble potentials [5,53], until a “pittingpotential” is attained at which breakdown of the surface filmoccurs [3,66,67]. Though pitting potential and pit nucleation areterms frequently employed in the literature related to the cor-rosion of this metal, stable pit growth has not been described formagnesium and magnesium alloys [35]. Conversely, retrievedsamples show that the entire exposed surface has been cor-roded [35].

Scanning electrochemical microscopy was selected as theexperimental technique for this work as to gain new spatially-resolved information concerning the chemical reactions respon-sible for the corrosion of CP magnesium. A special concern wastaken in relation to identify if alkalinization occurred at both theanodic and cathodic microcells formed on the metal, or whetherdiffusion of the high concentration of hydroxyl ions formed in thecathodes were responsible for the alkalinization of the electrolyte.Though a small electrolyte volume is typically employed in SECMexperiments, the choice of a small magnesium strip insulated ina much bigger resin mount was considered to allow enough dilu-tion of the formed OH− species as to detect the source of theseions at least for the early stages of metal corrosion. Additionally,the mount contained also an iron strip, thus allowing some exper-iments to be conducted with galvanic coupling of the two metals,which could be simply attained by electrically connecting the twostrips at the back of the mount. In this way, separation of the anodicand cathodic sites could be achieved if the two metal strips wereseparated enough in the mount, though at the cost of both anincrease of the corrosion rate of magnesium and a greater ohmicloss in the electrolyte. Though the latter might be regarded to drivea fraction of the hydrogen evolution reaction from the iron to themagnesium strip in order to minimize the separation between bothhalf-cell reactions, this would result in local alkalinization aroundthose cathodic sites on magnesium. But no evidence of any localalkalinization around magnesium has been observed in the courseof these experiments. Yet, additional experiments are currentlyundertaken in order to completely rule out any eventual effects onthe localization of the hydrogen evolution reaction directly relatedto the significant losses occurring in the electrochemical cell. Varia-tions in both the distance comprised between the two metals in theholder and the ionic strength of the electrolyte are thus considered.

Figs. 2 and 3 show significantly smaller pMg values in a thinelectrolyte layer above the magnesium strip following the galvaniccoupling of this metal to iron. That is, the use of a Mg2+ ISE allowedfor concentration distributions of the dissolved metal to be mea-sured. It must also be noticed that despite the enhancement of

magnesium dissolution due to the galvanic-coupling to iron, thedimensions of the magnesium strip were sufficient for the amountof dissolved magnesium never to saturate the detection limit ofthe ISE (see Supplementary material). That is, the microelectrode

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robe was sensitive to changes in the concentrations of Mg2+ forhe duration of the experiments, contrary to the expectations madey other authors [5]. Another important outcome from our work isbtained from inspection of Fig. 4. Though the magnesium stripas working as the anode in the Fe–Mg galvanic pair, dissolu-

ion of the metal does not occur simultaneously over the entirexposed surface. Indeed, it has been observed from a sequence ofECM measurements passing over the same scan lines (not shown),hat the locations of high Mg2+ concentrations are shifted over thexposed magnesium strip. Our experiments, still of some prelimi-ary nature, agree thus well with the previous observations madesing SVET by Williams and McMurray [35]. We are currently work-

ng on the fabrication of more robust ion-selective electrodes withmaller internal resistances, that will allow higher scan rates to besed with SECM, thus allowing maps instead of selected scan lineso be recorded.

Another important outcome of this work was the observationf oxygen consumption when magnesium was galvanically con-ected to iron. This dissolved species in the solution was consumed

n the thin electrolyte volume located over the iron strip whencting as the cathode during the corrosion of magnesium. Indeed,he amperometric SECM images in Fig. 6 clearly demonstrate thathe enhanced corrosion rate of magnesium when galvanically-oupled to iron directly corresponds to an enhancement in theonsumption of oxygen, despite the occurrence of hydrogen evo-ution on the metal. Simultaneously, the pH in the thin electrolyteolume located above the magnesium strip becomes slightly acidic,ttaining values between 5.5 and 6. These values are only slightlyore acidic than those predicted by application of Eq. (1) consider-

ng the hydrolysis of Mg2+ ions in the electrolyte. Values as lows 5.3 have already been reported from SIET measurements onorroding AZ31 magnesium alloy, which was ascribed to the co-issolution and hydrolysis of Al along with Mg, since this alloyontains ca. 3% aluminum. In the case of CP magnesium, the possi-le contribution of minute amounts of iron, even in small amounts

n the order of ppm [68], may justify the more acidic values foundn our work. Since the pK1

a of Fe is more than two units smallerhan that of Al (2.2 and 4.5, respectively) [63], a 0.0001 M Fe2+ solu-ion produces almost the same pH decrease than another 0.1 M inl3+ [41]. Not only the pH values can be justified on the basis of

he physicochemical response of CP magnesium, but they are wellnside the linear response of the antimony microelectrode with pH56]. Again the design of the experiment has satisfactorily servedor pH monitoring of magnesium corrosion in an aqueous solution,ithout meeting the warnings raised in ref. [5]. This feature is very

mportant when considering that no alkalinization was observedround the anodic sites in any of the experiments described inur work. And alkalinization is a requirement for the occurrencef the chemical reaction suggested to account for the NDE [65,69].n this model, formation of Mg+ species and their direct reaction

ith water, is proposed:

H+ + 2e− → H2↑ (cathodicpartialreaction) (2)

Mg → 2Mg+ + e− (anodicpartialreaction) (3)

Mg+ + 2H2O → 2Mg2+ + 2OH− + H2↑ (chemicalreaction) (4)

From Eq. (4), both the consumption of protons and the pro-uction of OH− are expected at the anodic sites on corrodingagnesium. Another implication is that hydrogen gas evolution

hould occur at those sites. Yet, evolution of hydrogen gas fromhe corroding magnesium sample can be explained by taking inccount the previous observations by Williams and McMurray [35].

n their work on the corrosion of magnesium in concentrated chlo-ide solution investigated by SVET, they observed that the anodicites occupy only a (small) fraction of the area exposed to thelectrolyte, and that local cathodes can develop in very near

Acta 87 (2013) 283– 293 291

proximity to them on the previously corroded surface. Therefore,hydrogen evolution do not necessarily must occur at the actualanodic microcell sites, but could occur on the surrounding cathodicarea. The results reported in this work could be well described usingthe same interpretation. This explanation may hold even in the caseof magnesium galvanically-coupled to iron. Though the major partof the cathodic reaction occurs on the separated iron strip, somecathodic areas might still develop on the magnesium sample in theclose proximity to the corroding anodic sites. The development ofsuch minute cathodic sites in the otherwise anodic metal in a gal-vanic couple has been recently observed in our laboratory using acombination of scanning microelectrochemical techniques for theFe–Zn galvanic system [70]. This interpretation also accounts forthe electrolyte resistance SECM images given in Ref. [53], in whichthe anodic activity on a free corroding magnesium sample occurredonly on a small portion of the exposed surface.

5. Conclusion

The applicability of the Mg2+-ISE to characterize magnesium-based materials by SECM has been demonstrated. The results showthat anodic dissolution of magnesium anodes takes place pref-erentially in certain most active sites. The increase in Mg2+ ionconcentration occurring in the vicinity of the magnesium strip, wasfound to be two orders of magnitude bigger when the nature of thedegradation process changed from spontaneous self-corrosion togalvanic corrosion. Imaging of oxygen concentration distributionand pH detection were carried out with the antimony electrode, andit was possible to clearly distinguish anodic and cathodic behaviorsoccurring simultaneously on the same surface. Thus, the combina-tion of measurements and electrodes presented here allowed toidentify and to quantify the different reactions taking place duringthe corrosion process. Additionally, the results open a promisingnew route to the elucidation of certain characteristics of the mech-anism involved in the corrosion and passivation of magnesium,which result from greatly localized reactions.

Acknowledgements

The authors are grateful to the Spanish Ministry of Scienceand Innovation (MICINN, Madrid, Acción Integrada No. HH2008-0011) and to the National Office for Research and Technology(NKTH, Budapest, research grant ES-25/2008 TeT) for the grant of aCollaborative Research Programme between Hungary and Spain.J.I. and R.M.S. are grateful for financial support by the MICINNand the European Regional Development Fund (Brussels, Belgium)under Project No. CTQ2009-12459. A Research Training Grantawarded to J.I. by the MICINN (Programa de Formación de PersonalInvestigador) is gratefully acknowledged. L.N. and G.N. acknowl-edge support from “Developing Competitiveness of Universities inthe South Transdanubian Region (SROP-4.2.1.B-10/2/KONV-2010-0002)”. The Authors acknowledge valuable discussions with Dr.Nick Birbilis during the preparation of the manuscript.

Appendix A. Supplementary data

Supplementary data associated with this article can be found, inthe online version, at doi:10.1016/j.electacta.2012.09.029.

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