Electrochimica Acta 203 (2016) 337–349

The use of the electrochemical micro-cell for the investigation ofcorrosion phenomena

F. Andreatta*, L. FedrizziUniversity of Udine, Department of Chemistry, Physics and Environment, Via del Cotonificio 108, 33100 Udine, Italy

A R T I C L E I N F O

Article history:Received 29 August 2015Received in revised form 12 January 2016Accepted 14 January 2016Available online 16 January 2016

Keywords:local electrochemical measurementselectrochemical micro-cellmicro-capillary celllocalized corrosioninhibition

A B S T R A C T

Electrochemical micro-cell systems based on the use of glass micro-capillaries enable to performstandard electrochemical measurements on areas with dimension in the micro- or submicrometer range.The base principle is the miniaturization of the working electrode, which is obtained using a micro-capillary to position the electrolyte on the sample surface. This technique has been extensively employedfor the investigation of localized corrosion in passive surfaces like steels and aluminium alloys.This paper presents examples of the study of localized corrosion and its inhibition in aluminium alloys

by means of the electrochemical micro-cell. These are discussed trying to highlight the major advantagesand the critical aspects related to the use of the technique. Moreover, the combined use of theelectrochemical micro-cell with other electrochemical and surface analysis techniques highlights thatthe micro-cell technique is a very powerful method for the investigation of corrosion phenomena.

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1. Introduction

Many electrochemical methods to study localized corrosion arebased on large-scale techniques with exposed areas in the mm2–

cm2 range. These enable the characterization of the overallelectrochemical behaviour of a system, but they do not giveaccess to local electrochemical information, which is important inorder to understand corrosion mechanisms. The recent develop-ment of local electrochemical techniques like scanning probemicroscopy (SPM) [1–5], scanning vibrating electrode technique(SVET) [6–8] and electrochemical micro-cell systems based on theuse of glass micro-capillaries [9–13] has introduced the possibilityto study the local electrochemical behaviour of different materials(aluminium and magnesium alloys, steels, etc.) in the micro- orsubmicrometer range. The local electrochemical techniques can bedivided into two major groups: scanning techniques based on amicro-reference electrode and small area techniques [14].

In the scanning techniques based on a micro-referenceelectrode, immersed samples (with area in the mm2–cm2 range)are investigated using micro- or ultramicroelectrodes. Thesetechniques measure the potential and/or current gradients byscanning a reference micro-electrode over the sample surface. Themost important are: scanning reference electrode technique(SRET), SVET [6–8], local electrochemical impedance spectroscopy

* Corresponding author. Tel.: +39 0432 558838 Fax: +39 0432 558803.E-mail address: francesco.andreatta@uniud.it (F. Andreatta).

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(LEIS) [15], scanning Kelvin probe (SKP) and scanning Kelvin probeforce microscopy (SKPFM) [1–5]. The experiments might beperformed under an open circuit condition or under potential orcurrent control. Depending on the technique used, a lateralresolution down to a few nanometers is possible. However, thescanning methods do not allow measuring local corrosion currentssince the current flow of the whole immersed surface area isusually recorded. Therefore, kinetic information of a localelectrochemical process cannot be gained directly [1–8].

Small area techniques are based on the miniaturization of theelectrochemical cell in order to enable selective characterisation ofdifferent areas of the sample surface. By decreasing the size of theexposed area, it is possible to localize the electrochemicalprocesses. This can be achieved by thin embedded wires,photoresist techniques, a droplet cell or small glass capillaries[14]. In contrast to scanning techniq-ues discussed above, thesetechniques enable to carry out electrochemical measurements(potentiostatic, potentiodynamic and galvanostatic measurementsand electrochemical impedance spectroscopy) on very smallregions of the sample surface [14]. Therefore, local corrosioncurrents can be evaluated directly with a high resolutionpotentiostat. However, small area techniques usually do not showsuch a high spatial resolution as the scanning methods [16–18].

Among techniques based on a small area, the electrochemicalmicro-cell is extremely interesting because it enables to performstandard electrochemical measurements on areas with dimensionin the micro- or submicrometer range. This technique was

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introduced by T. Suter et al. at ETH (Swiss Federal Institute ofTechnology, Zurich, Switzerland) in the nineties [9–13].

In these works of T. Suter [9–13], the terms “microelectro-chemical system” and “micro-cell” were employed referring to themethod. Other works, used the name “scanning droplet cell”[19,20] or “micro-capillary cell” [14]. In this review paper, the term“electrochemical micro-cell” will be used for this local technique.

The electrochemical micro-cell employs a glass micro-capillaryto position the electrolyte on the sample surface (Fig. 1A and B).The micro-cell with connectors for reference (RE) and counter (CE)electrodes is usually mounted on the revolving nosepiece of anoptical microscope in order to enable precise positioning of theglass capillary on the working electrode (WE).

This paper reviews the application of the electrochemicalmicro-cell for corrosion studies and presents selected examples ofthe study of localized corrosion and its inhibition in aluminiumalloys. The paper highlights the major advantages and criticalaspects related to the use of this technique. Moreover, thecomplementary use of the electrochemical micro-cell with otherelectrochemical and surface analysis methods is considered in thiswork.

2. Electrochemical micro-cell setup

In typical setups of the electrochemical micro-cell, theminiaturization of the area of the working electrode can beobtained either by means of a glass micro-capillary with freedroplet or by a micro-capillary with silicone gasket (Fig. 2A and B).The capillary size can be in the micrometer range or even in thesubmicrometer range [2,11,12,21]. This makes it possible forelectrochemical analysis of single heterogeneities on a surface.The exposed area corresponds with the size of the electrolyte incontact with the sample for the setup with free droplet (Fig. 2A) orwith the size of the glass capillary mouth for the setup with thesilicone gasket (Fig. 2B). Therefore, the wetted area on the samplesurface defines the working electrode for both setups.

The free droplet setup (Fig. 2A) uses the surface tension of theelectrolyte to keep a hanging droplet in position using a capillaryfilled with electrolyte positioned at 10 mm above the samplesurface (Fig. 2A). This was employed by many research groups[19,20]. This review focuses on the micro-cell systems employingglass capillaries with silicone gasket. However, it should beconsidered that the free droplet setup is very interesting since itenables the use of the technique as a scanning method [19,20].

Fig. 1. Electrochemical micro-cell setup (A) and a glass

In the setup employing a glass micro-capillary with a siliconegasket, the electrolyte is confined to the area of analysis using asilicone rubber gasket at the capillary mouth to enclose theelectrolyte (Fig. 2B). The introduction of the silicone rubber gasketby Suter et al [11–13] have strongly improved the versatility, lateralresolution, and reliability of the technique leading to a wide spreaduse over the past two decades. The deformability and thehydrophobic behaviour of the silicone seal enable precisepositioning of the micro-capillary without leakage of electrolyte[12].

In both free droplet and silicone gasket setups, the electrolyte incontact with the sample surface is connected through the capillaryto a reference and a counter electrode enabling to use a standardthree-electrode arrangement for electrochemical measurements(Fig. 3). The glass micro-capillary is hold in position by a cell whichcontains connections for counter and reference electrodes.Moreover, the cell provides the possibility to refresh the electrolytein the glass capillary through an electrolyte inlet.

3. Applications of the electrochemical micro-cell for corrosionstudies

The micro-cell technique was mainly developed to performelectrochemical studies on single microstructural heterogeneitiesin different metals and alloys [1,2,6–8]. The corrosion resistance ofpassive materials like stainless steels or aluminium alloys islimited by their microstructural heterogeneities. Inclusions,precipitates and impurities play a key role in being potentialinitiation sites for localized corrosion. Suter et al. [11–13] have usedthe silicone rubber based method for various investigations. Theinvestigation of pit-initiation at MnS inclusions in stainless steels[11] has clearly shown that the electrochemical micro-cell enablesthe distinction between active and non-active inclusions. Mostimportantly, this work demonstrated for the first time that theelectrochemical micro-cell is a powerful tool for the investigationof local phenomena. This work from Suter et al. [11] was followedby other publications from the same research group on characteri-zation of inclusions in stainless steels [12,22–29]. These indicatedthat the electrochemical micro-cell could provide informationabout the corrosion resistance of single phases in the range of10 mm diameter, while this information is not accessible withmacro-electrochemical polarization experiments. This approachwas followed by other research groups [30–35] for the study ofpitting resistance of stainless steel, which is strongly impaired by

micro-capillary engaged on the sample surface (B).

Fig. 2. Glass capillary with free droplet (A) and with silicone gasket (B).

Fig. 3. Three-electrode setup of the electrochemical micro-cell.

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sulfur species due to dissolution of the inclusions. Initiation ofattack at MnS inclusions was also studied combining differentelectrochemical and surface analysis techniques at the local scale[35]. In particular, micro-cell characterization was combined withSVET [35]. In the case of SVET measurements, a glass micro-capillary was employed to inject an aggressive electrolyte nearMnS inclusions in order to promote pitting corrosion. Inclusions instainless steels were also investigated by other authors whoconstructed a setup similar to that developed by Suter et al. [36,37].

Micro-electrochemical studies were reported for differentphases (ferrite and austenite) in duplex stainless steels [38–41].V. Vignal et al. [42] and H. Krawiec et al. [43] investigated the effectof friction on local electrochemical behaviour of duplex stainlesssteel carrying out measurements on ferrite and austenite phaseswith the electrochemical micro-cell. These works proved thatthere is a drastic change of the electrochemical behaviour of the

phases induced by straining. Data acquired with the electrochemi-cal micro-cell and with a scanning droplet cell system on 316Lstainless steel were discussed by H. Krawiec et al. [44] in order tocompare the electrochemical behaviour under static (micro-cell)and under flow conditions (scanning droplet cell system). It wasshown that there is a discrepancy in the cathodic currentsmeasured with the two techniques. This indicated that experi-mental parameters (setup, crevice geometry, specimen surface-capillary gap distance) might affect the results. This aspect was alsotargeted by numerical simulation [44,45]. Electrochemical char-acterization by means of micro-cell technique was also reportedfor active materials such as austempered ductile iron [46,47] andduplex stainless steel [48,49].

The electrochemical micro-cell was employed by A.W. Hasselet al. [20] for the investigation of structured oxide films onaluminium with a free droplet set up. A very important applicationof the electrochemical micro-cell in the field of aluminium alloys isrepresented by the study of pit initiation at single inclusions suchas AlCuMg and AlCuFeMn intermetallics in aluminium alloyAA2024-T3 [50]. In particular, this paper discussed the influence ofthe size of the exposed area on pit initiation in potentiodynamicpolarization curves acquired on Al 99.999% and AA2024-T3 in0.01 M NaCl solution. The polarization curves were recorded withthe electrochemical micro-cell using a 100 mm glass micro-capillary and with a conventional cell with exposed area of 1 cmdiameter. The reduction of the exposed area leads to a shift of thebreakdown potential (pitting potential) in the positive direction forboth Al 99.999% and AA2024-T3. This shift was more marked in thecase of AA2024-T3 than for Al 99.999%. This evidenced for the firsttime that there is a statistical effect related to the number of weakspots in the investigated area (intermetallics, inclusions, grainboundaries, oxide defects) related to the reduction of the exposedarea (working electrode) in comparison to conventional electro-chemical measurements [50]. Moreover, this paper showed thatthe local electrochemical behaviour of intermetallic particles couldsignificantly change as a function of their composition. Succes-sively, breakdown at intermetallic compounds in AA7075 wasinvestigated by electrochemical micro-cell and by SKPFM[14,51,52]. These works showed that there was good correlationbetween local breakdown potentials measured with the electro-chemical micro-cell and Volta potential measurements by SKPFMtechnique. An extensive survey of electrochemical characteristicsof different types of intermetallic particles in 7xxx aluminium

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alloys was reported by N. Birbilis et al. [53,54]. MgSi particles in Al-Mg-Si alloys were investigated by F. Eckermann et al. [55]. Thesame author used the electrochemical micro-cell for the investi-gation of exfoliation corrosion in AA6016 to prove that grainboundaries are preferentially attacked with respect to the graininterior [56]. Moreover, the electrochemical micro-cell wasemployed for the study of intermetallic particles in AA2024-T3 exposed to aqueous vanadate solutions [57]. The same approachhas been recently employed for the investigation of inhibition bycerium species at the location of intermetallic compounds in bareAA2024-T3 and in alclad AA2024 [58,59]. M. Jariyaboon et al. [60–64] employed a different equipment than that developed by T.Suter et al. [12] for the investigation of friction stir welded AA2024-T351 and AA7010-T7651. In this case, a plastic pipette substitutedthe glass capillary in order to make contact with a localized regionof the working electrode. This equipment was also employed forthe study of atmospheric corona treatment on AA1050 aluminium[65].

Another important research topic regarding the application ofthe electrochemical micro-cell is the investigation of the deformednear-surface region of rolled aluminium alloys. The existence ofdisturbed oxide-rich subsurface layers causes different filiformcorrosion behaviour than the underlying bulk material in rolledaluminium alloys [66–68]. In particular, the electrochemicalmicro-cell was employed to carry out electrochemical measure-ments on areas in the sub-surface region with and without rolled-in layers [68]. In this work, glow discharge optical emissionspectroscopy (GDOES) was employed in order to expose craters atdifferent depths for characterization by micro-electrochemicalcell. This approach enabled to highlight the electrochemicalbehaviour of rolled-in layers from that of the bulk material. Asimilar experimental approach was followed for the study of theinfluence of surface microstructure on AC electrograining ofaluminium [69]. Recently, the combination of GDOES andelectrochemical micro-cell was used for the in-depth study ofcladded brazing sheet [70] and aluminized steel [71].

Micro-electrochemical studies were conducted on AlCu4Mgand AlMg2 as cast aluminium alloys in order to study the effect ofmetallurgical and mechanical heterogeneities (precipitates, cracksand strain concentration) on corrosion behaviour [72,73]. Inparticular, regions containing micro-cracks and strain concen-trations are more susceptible to localized corrosion than areaswithout mechanical defects. The influence of the crystallographicorientation of grains on corrosion behaviour of pure aluminiumhas been recently reported combining the electrochemical micro-cell with X-ray diffraction [74]. This work showed that crystallo-graphic orientation might change the current density in thecathodic branch and the breakdown potential in potentiodynamicpolarization curves. The effect of surface stresses on local corrosionbehaviour was investigated also in AA2050-T8 [75].

Modeling of anodic dissolution of pure aluminium was targetedby O. Guseva et al. [76]. This work presented a mathematical modelfor simulating a passive aluminium surface with a pit in whichactive electrochemical metal dissolution occurred. The modelincluded hydrolysis products of Al and species obtained as a resultof homogeneous reactions between chloride and Al3+ ions and Alhydrolysis products. The model was applied to a real glass micro-capillary geometry used for experimental measurements with theelectrochemical micro-cell. Numerical modelling of resultsobtained with local probe techniques (electrochemical micro-celland SVET) was considered in the case of aluminium alloys [77].This paper evidenced that a critical step for the application ofnumerical mass transport models was the definition of the localdissolution rate of aluminium matrix as a function of the chemistry(pH, chloride concentration), which was not possible to reach bylocal probes.

Application of the electrochemical micro-cell technique in thefield of magnesium alloys is limited. Measurements with amicroprobe (0.1 mm testing diameter) were performed by H.Hoche et al. [78] on PVD-coated magnesium alloys by comparingintact coating areas with specific coating defects produced byVickers indentations. H. Krawiec et al. [79] reported an investiga-tion of corrosion behaviour of second phase particles inAZ91 magnesium alloy. Recently, I. Kot et al. [80] reported anotherstudy on as cast AZ91 alloy. Sudholz et al. [81] investigated Mg–Ybinary alloys to reveal the individual effect of Y additions on theelectrochemical behaviour of the alloys.

Other applications of the electrochemical micro-cell includeinvestigation of single microparticles [82] or grains [83–85]. nano-electrochemical depositions [86–88], electrochemically machinedmaterials [89,90], WC-Co composites [91], welded materials[92,93], multimetallic materials [94], switch electrodes forelectronic applications [95,96], plastically deformed materials[97,98], Ti alloys [99] and copper patina [100]. Another importantapplication of the electrochemical micro-cell deals with localizedimpedance measurements [101–107].

4. Advantages of the electrochemical micro-cell

Since the electrochemical micro-cell has progressively becomeover the past years a well-established technique for localizedcorrosion measurements, several papers are available in literaturediscussing different setups, applications, advantages and criticalaspects of this technique. Among these papers, the ones from T.Suter et al [22,26]. M.M. Lohrengel et al [101,103,108,109]. N.Birbilis et al [102]. V. Vignal et al [35] and F. Arjmand et al. [110]should be mentioned. The major advantages of the electrochemicalmicro-cell in comparison to other local techniques and conven-tional large scale electrochemical methods can be listed as follows:

� The spatial resolution of the technique is high. As discussedabove, this is in the order of few tens of micrometers in manyapplications or even in the submicrometer range in a limitednumber of applications [2,11,12,21,88].

� Only the area under investigation is wetted by the electrolyte.This is very important since it enables to carry out multiplemeasurements on small samples or to investigate the electro-chemical behaviour of single microstructural heterogeneities indifferent metals and alloys [1,2,6–8].

� The three electrode setup enables to carry out DC and ACelectrochemical measurements [22,101–103,110].

� Different complementary surface analysis techniques likescanning electrode microscopy—energy dispersive X-ray spec-troscopy (SEM-EDXS), time of flight-secondary ion massspectroscopy (TOF-SIMS), X-ray photoelectron spectroscopy(XPS), atomic force microscopy (AFM) and glow dischargeoptical emission spectroscopy (GDOES) can be combined withthe electrochemical micro-cell [35,50,58,59]. Moreover, thislocal technique can be used in combination with X-raydiffraction (XRD) and electron backscatter diffraction (EBSD)[74,111]. X-ray microtomography was employed for microstruc-tural analysis of samples characterized with the electrochemicalmicro-cell [112,113]. The complementary use of the electro-chemical micro-cell with surface analysis methods is veryimportant in order to ensure a correct interpretation of data andto establish a direct link between microstructure of the systemunder investigation and its local electrochemical behaviour.Moreover, spectrophotometric techniques can be complemen-tary used with the micro-cell. As an example, M. Lohrengel et al.[90,114] used a ultraviolet-visible spectrophotometer (UV-VIS) toquantitatively analyse corrosion products during electrochemi-cal machining of iron in neutral NaNO3 solutions. In this work, a

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flow-through micro-cell was employed to carry out electro-chemical measurements. This system enables the flow of theelectrolyte in the micro-cell. Therefore, it was possible to analysethe composition of corrosion products dissolved in the electro-lyte by connecting the electrolyte outlet of the cell to the flow-through cuvette of an UV–vis spectrophotometer [90,110]. Asimilar experimental approach was followed for the develop-ment of flow-cell system and downstream UV–vis analytics toobtain synchronized electrochemical and spectroscopic data in afully automated mode [115]. N. Homazava et al. [116,117]developed a micro-cell setup combined with inductively coupledplasma mass spectroscopy (ICP-MS) for in situ investigation ofcorrosion processes. This setup was employed for the study ofdifferent materials [118–122]. This concept was further devel-oped in a flow-type scanning droplet cell microscope (FT-SDCM)connected to an ICP-MS for locally studying the electrochemicaldissolution of Hf–Ta alloys [123,124]. M. Voith et al. [125]combined a flow-through micro-cell with atomic absorptionspectroscopy.The electrochemical micro-cell setup can bemodified in order to measure additional parameters like pHduring the corrosion experiment [26]. The local pH can bemeasured with a pH sensitive microelectrode inserted inside theglass micro-capillary. T. Suter et al. [26] employed a 25 mmtungsten wire insulated with 5 mm Polyimid for pH measure-ments with sensitivity better than 0.5 pH unit. The electrochem-ical micro-cell can be also modified in order to control thetemperature, to apply a stress to the sample during the corrosionexperiment or to perform corrosion measurements with friction[26]. Electrolyte flow inside the glass micro-capillary is possibleby inserting an additional micro-capillary inside the maincapillary of the micro-cell [26]. with a special type of capillary(theta capillary) [89,109] or with a specific design of the cell[123,124]. Lohrengel et al [89] employed capillaries made of glasstubes with an internal partition (theta capillaries) to obtain twoseparated channels. One channel is used as electrolyte inlet, theother as the outlet. The electrolyte flow was provided through agear pump. A limitation of the flow-through concept is the largersize of the glass capillary compared to conventional capillaries.This results in a decreased spatial resolution of the techniquewhen flow-through capillaries are employed [89].

� The sample does not need to be flat and no special surfacepreparation is needed for the characterization [50]. However,surface polishing is often necessary in order to enable the

Fig. 4. Crevice formation (A) or leakage and O2 diffusion

identification of small microstructural features (heterogeneitiessuch as inclusions or intermetallic compounds) with opticalmicroscope. Moreover, this favours precise positioning of theglass micro-capillary on the area of interest [14].

Aggressive or concentrated solutions can be used in the glasscapillary. This is an important advantage of the method incomparison to other local techniques like SVET, which requiresthe use of electrolytes with low concentration [101].

5. Critical aspects related to the use of the electrochemicalmicro-cell

The use of glass micro-capillaries has introduced a number ofcritical aspects related to the application of the electrochemicalmicro-cell. These are mainly related to the necessity of measuringvery small currents (in the range of pA or fA) and to the manualpreparation of the capillaries. The main critical aspects related tothis local technique can be listed as follows.

� A potentiostat with high current resolution is required (usually10 fA for most applications). M. Lohrengel et al. [108] proved thatthe spatial resolution of the method is actually limited by thecurrent resolution of the potentiostat rather than by the size ofthe glass micro-capillary.

� Ohmic resistance between working electrode and counterelectrode is another critical issue with the micro-cell methods[13,102,110]. This can be quantitatively evaluated by means of thecurrent interrupted method or impedance measurements[13,102]. T. Suter [13] showed that the ohmic resistance for aglass micro-capillary with 150 mm internal diameter is about2 � 104 ohm. For measured currents in the range of pA and fA,this leads to an ohmic drop between 0.2 and 20 mV depending onthe current level. This ohmic drop can be considered relativelylow. Moreover, N. Birbilis et al. [102] demonstrated for a stainlesssteel electrode in 0.1 M NaCl solution that ohmic drop dependson the capillary diameter, strongly increasing for diametersbelow 100 mm. Based on the results reported by T. Suter [13] andN. Biribilis et al. [102]. it can be expected that the omhicresistance between working and reference electrodes will lead toa significant ohmic drop in the case of measurements withcapillary diameters below 100 mm or with not highly conductiveelectrolytes. In particular, a large ohmic drop could be possible

(B) in the case of glass capillary with silicone gasket.

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when large currents are passing between the working andcounter electrodes. This is the case of potentiodynamicpolarization of active metals like magnesium alloys or for largecathodic currents. Passive materials like aluminium alloys andstainless steel usually exhibit currents in the pA or even fA rangein potentiodynamic polarization curves performed using glasscapillaries with size below 100 mm [102]. For such currentslevels, the ohmic drop is expected to be not significant since thecurrents flowing in the cell are very small. For passive materials,the current becomes relatively high only for potentials above thebreakdown potential or for large rates of cathodic reactionleading to the risk of significant ohmic drop only for polarizationat very positive or negative potentials. Moreover, it should beconsidered that that potentiodynamic polarization measure-ments for passive materials are often interrupted just above thebreakdown potential limiting the risk of a significant ohmic droponly in the case of polarization at large cathodic currents. Theelectrochemical micro-cell setup can be modified in order toreduce cell resistance between working electrode and counterelectrode [19,20,101]. The most effective approach is thepositioning of the counter electrode inside the glass micro-capillary near the sample surface, as done for example in thesetup developed by M. Loherengel et al [101,103,108,109].

If the reference electrode is not placed exactly in the proximityof the working electrode surface, some fraction of the ohmicresistance, related to the uncompensated resistance betweenworking and reference electrode, will be included in the measuredpotential by the potentiostat. The uncompensated resistancebetween working and reference electrodes might be high inelectrochemical micro-cell setups due to limitations related to theconstruction of the cell. In fact, the reference electrode is oftenpositioned far from the working electrode or even outside the cell[13]. This is done in order to reduce the size of the micro-cell formounting the cell on the revolving nosepiece of the opticalmicroscope, as can be seen in Fig. 1. Although, this solutionfacilitates precise positioning of the glass micro-capillary on thesample surface, it should be considered that the ohmic drop relatedto the uncompensated resistance should be corrected when largecurrents are passing in the cell. Another drawback related to the

Fig. 5. Large scale potentiodynamic polarization curves acquired in a 0.1 M NaClsolution after 24 h immersion of clad AA2024 aluminium alloy in differentinhibitive solutions [59]. With permission from Elsevier.

position of the reference electrode outside the micro-cell could bethe formation of gas bubbles in the tube connecting the referenceelectrode to the cell. This might lead to the disconnection of thereference electrode. In this case, the open circuit potential will notbe detected by the potentiostat and it will be impossible to carryout any measurement with the cell.

� Preparation of glass micro-capillaries involve manual operations(pulling the glass pipette, grinding and polishing to obtain therequired capillary size and application of the silicone gasket). Inparticular, the flatness of the capillary tip and the shape of thesilicone gasket might affect the quality of the contact betweenmicro-capillary and sample surface. This leads to the risk ofcrevice corrosion, electrolyte leakage or oxygen diffusion(Fig. 4A and B). A crevice can be created in the contact regionbetween micro-capillary and sample if the capillary tip is not flator the thickness of the silicone gasket is not uniform, asschematically represented in Fig. 4A. The formation of a crevicemight favour the establishment of conditions leading to crevicecorrosion (rapid consumption of oxygen in the crevice,suppression of cathodic reactions and migration of aggressivespecies like OH� and Cl� ions from the bulk solution to thecrevice). In particular, the risk of crevice corrosion might be highin the case of passive materials. The existence of a crevice mightaffect also mass transportation and distribution of species insidethe glass micro-capillary, as shown by H. Krawiec et al. [44].Incorrect sealing of the silicone gasket (Fig. 4B) might causeelectrolyte leakage impairing the acquired data. Moreover, thismight favour O2 diffusion in the micro-cell affecting theelectrochemical processes inside the glass micro-capillary. Thismight be a critical issue because it can significantly affect theshape of the cathodic branch of potentiodynamic polarizationcurves. Using Ar shielding at the tip of the glass micro-capillary,R. Oltra et al. [126] demonstrated for a Pt electrode in 0.5 M NaClsolution that the oxygen reduction reaction could be affected byoxygen diffusion even if there is perfect sealing of the glassmicro-capillary.

� The glass micro-capillary defines a confined volume of electro-lyte inside the micro-cell. Usually, the electrolyte can berefreshed inside the cell only at the end of each measurement.Therefore, the electrolyte inside the cell might be subjected tochemistry changes, especially in the case of relatively longmeasurements. In particular, this might affect the transportationof species involved in the electrochemical reactions like oxygenand protons. It has been demonstrated that the concentration ofprotons in the capillary depends on flow conditions and micro-cell geometry [44]. Moreover, the cathodic reaction might leadto a localized pH change inside the glass micro-capillary forinstance during cathodic polarization [44].

� The shape of the glass micro-capillary might affect thepotentiodynamic polarization curves acquired with the electro-chemical micro-cell. O. Guseva et al. [76] developed a model forthe description of electrochemical processes occurring inside apit on the surface of pure aluminium exposed to the electrolyteinside a glass micro-capillary. According to this model, theconcentration of species involved in the electrochemicalprocesses are strongly affected by the shape of the glassmicro-capillary. H. Kraviec et al. [44] showed that the currentdensity measured for 316L stainless steel in 1.7 M a NaCl solutioncould be affected by the shape of the glass micro-capillary (angleof the capillary at the tip and thickness of the silicone gasket).Abodi et al. [127] developed a model to describe the electrolytepotential and the concentration distribution of species in orderto simulate potentiodynamic polarization curves for aluminiumalloy AA2024 in regions containing different phases (interme-tallic compounds). This evidenced a strong effect of species

Fig. 6. Potentiodynamic polarization curves in a 0.05 M NaCl solution acquired with the electrochemical micro-cell on areas containing an Fe-rich intermetallic (16 scans ondifferent regions of the surface) in clad AA2024 in the degreased condition (A) and SEM micrographs of the areas investigated before and after potentiodynamic polarizationfor the curve indicated in the figure (B and C, respectively) [59]. With permission from Elsevier.

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concentration on the simulated potentiodynamic polarizationcurves. Since the shape of the glass micro-capillary can stronglyaffect the acquired data, it is advisable to carry out a full set ofmeasurements using the same glass micro-capillary.

� Blockage of the glass micro-capillary by corrosion products isanother issue related to the existence of a confined volume ofelectrolyte inside the glass capillary. The accumulation ofcorrosion products at the mouth of the glass capillary mightrestrict the access of the electrolyte to the metal surfaceimpairing the electrochemical measurements. It should benoted that in most applications, the measurements with theelectrochemical micro-cell are interrupted immediately afterthe passivity breakdown in order to avoid blockage of the glasscapillary by corrosion products [14,50]. This might be alsocaused by the formation of gas bubbles inside the capillary,especially in the case of cathodic reactions like hydrogenevolution. Issues related to the confined volume of electrolyte

inside the micro-capillary might be limited using the flow-through concept for the micro-cell [89].

� Sweep rate is another important aspect related to acquisition ofpotentiodynamic polarization curves with the electrochemicalmicro-cell. In order to avoid cell leakage or blockage of thecapillary, it is often necessary to increase the sweep rate for theacquisition of potentiodynamic polarization curves [14,51,52]. Insuch cases, it is very important to properly validate the results bycomplementary techniques since high sweep rates might lead tothe acquisition of misleading electrochemical data [102].

� Local measurements carried out with the electrochemicalmicro-cell often exhibit different behaviour than conventionallarge scale measurements carried out on the same material. Thestatistical effect related to the reduction of the size of theexposed area might affect the reproducibility of results obtainedwith the electrochemical micro-cell [50]. This should beconsidered as an intrinsic characteristic of the method due toits increased resolution as compared to conventional large scale

Fig. 7. Potentiodynamic polarization curves in a 0.05 M NaCl solution acquired with the electrochemical micro-cell on areas containing an Fe-rich intermetallic (10 scans ondifferent regions of the surface) in clad AA2024 in the alkaline etched condition after immersion for 24 h in a 2.5 mM CeCl3 aerated solution (A) and and SEM micrographs ofthe areas investigated before and after potentiodynamic polarization for the curve indicated in the figure (B and C, respectively) [59]. With permission from Elsevier.

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electrochemical techniques. Although a direct correlationbetween measurements with the micro-cell and conventionalmeasurements is often not straightforward, conventional largescale measurements are often useful for the correct interpreta-tion of results obtained with the micro-cell. Moreover, multipleexperiments should be carried out for a correct understanding ofthe behaviour of the system under investigation.

6. Selected examples related to the use of the electrochemicalmicro-cell for corrosion studies

In order to further highlight the main advantages and criticalaspects related to the use of the electrochemical micro-cell forcorrosion studies, some selected examples taken from theexperimental work carried out in our research group are discussedbelow.

6.1. Localized corrosion and inhibition by Ce species in clad AA2024aluminium alloy

Clad AA2024 aluminium alloy was investigated by means of theelectrochemical micro-cell in order to highlight the effect of Cespecies on local electrochemical behaviour of Fe-rich intermetalliccompounds in the AA1050 clad layer [59]. Different inhibitivesolutions were considered including an aerated 2.5 mM CeCl3solution, an oxygen saturated solution and an oxygen saturatedsolution with addition of chlorides. Moreover, the substrate wasinvestigated in the degreased and in the alkaline etched conditions.Details of the experimental procedures are reported elsewhere[59]. In this paper, the focus will be on samples immersed for 24 hin the 2.5 mM CeCl3 solution.

Fig. 5 shows large scale cathodic and anodic potentiodynamicpolarization curves recorded in a 0.1 M NaCl solution for thealkaline etched substrate (alkaline etched) before and afterimmersion for 24 h in the 2.5 mM CeCl3 solution (alkaline

Fig. 8. Anodic (A) and cathodic (B) polarization curves acquired in a 0.05 M NaClsolution with the electrochemical micro-cell on different regions of themicrostructure of AA2024-T3 alloy [58]. With permission from Elsevier.

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etched + 24 h CeCl3). Large scale potentiodynamic polarizationmeasurements were well reproducible. The current density in thecathodic branch decreases after immersion in the solutioncontaining Ce species due to a decrease of the oxygen reductionrate. Moreover, the anodic branch of the potentiodynamicpolarization curve for the alkaline etched + 24 h CeCl3 sampleexhibits a rather marked reduction of the anodic current densityrelative to the alkaline etched substrate. This trend could beassociated to the deposition of a layer containing Ce compounds onthe entire sample surface, as indicated by TOF-SIMS analysis notreported here [59].

Although large scale measurements presented in Fig. 5 couldevidence a positive effect of the immersion in the Ce containingsolution on the electrochemical behaviour of the substrate, noinformation about the role of the microstructure was accessiblewith a large scale approach. Therefore, the electrochemical micro-cell using capillaries with 50 mm internal diameter was employedto study the behaviour of regions containing Fe-rich intermetalliccompounds with size in the order of few micrometers and regionswithout second phase particles (matrix). Fig. 6A and Fig. 7A showanodic potentiodynamic polarization curves acquired on regions

containing a single Fe-rich intermetallic compound in thedegreased substrate and in the alkaline etched substrate after24 h immersion in the aerated CeCl3 solution. In addition, Fig. 6 andFig. 7 show the areas investigated before and after potentiody-namic polarization for the curves indicated in the figures.

Potentiodynamic polarization curves shown in Fig. 6A andFig. 7A display passive behaviour and a relatively large scatter forthe breakdown potential. The breakdown potential exhibits astandard deviation of 275 mV for the 16 curves reported in Fig. 6Aand 137 mV for the 10 curves displayed in Fig. 7A. The differentbehaviour as compared to large scale measurements in Fig. 5 andthe large scatter in the breakdown potentials reflects the intrinsiclow reproducibility discussed above for measurements carried outwith the electrochemical micro-cell. Results in Fig. 6A and Fig. 7Aindicate that the electrochemical behaviour of regions containingFe-rich intermetallic compounds might significantly change in cladAA2024. SEM micrographs shown in Fig. 6B and C and Fig. 7B and Cshow that the Fe-rich intermetallic compounds are preferentialsites for the initiation of localized corrosion, which takes place inthe form of pitting mainly at the periphery of the intermetalliccompounds. Thus, the breakdown potentials could be directlyrelated to the initiation of localized corrosion in the areainvestigated. The deposition of Ce species on the substrate afterimmersion in CeCl3 solution leads to a marked shift of thebreakdown potentials in the positive direction for clad AA2024 inthe alkaline etched condition (Fig. 7A) than for the degreasedsubstrate (Fig. 6A). This suggests that the deposition of Cecompounds inhibits the localized attack of the substrate at thelocation of Fe-rich intermetallic compounds. It should be notedthat the use of the electrochemical micro-cell could give access toinformation regarding initiation of localized corrosion at the sitesof the Fe-rich intermetallic compounds, which was not accessiblewith a large scale approach. Results obtained with the electro-chemical micro-cell were also validated using other localtechniques (SKPFM and SVET) [128,129].

6.2. Complementary use of electrochemical micro-cell and surfaceanalysis techniques for the investigation of inhibition by Ce species inAA2024-T3 aluminium alloy

The inhibition effect of Ce species was further investigated inour research group for AA2024-T3 [58,130]. which presents a morecomplex microstructure than the clad alloy discussed above. Thedeposition of Ce species on the substrate was obtained with a verysimilar procedure to that discussed above for clad AA2024. In thecase of AA2024-T3, the inhibitor solution was a 0.05 M NaClsolution containing 5 g/l Ce(NO3)3 6H2O. The immersion time inthe inhibitor solution was 12 h. Further details about theexperimental procedures are reported elsewhere [58]. Theelectrochemical micro-cell was employed in a complementaryway with surface characterization techniques in order to propose amechanism for the inhibition effect of Ce species [58,114].

The electrochemical micro-cell was initially employed for thecharacterization of different regions of the alloy surface containingFe-rich and Mg-rich intermetallic compounds. Moreover, regionswithout large second phase particles (matrix) were also charac-terized. Fig. 8 displays representative anodic (A) and cathodicpotentiodynamic polarization curves (B) acquired in a 0.05 M NaClsolution. Anodic potentiodynamic polarization curves displaysimilar behaviour for regions containing Fe-rich intermetalliccompounds and for the matrix. The breakdown potential of theformer regions is more negative than for the matrix. In contrast,regions containing Mg-rich intermetallic compounds display veryactive behaviour with breakdown potential of about –600 mV vsAg/AgCl, in line with other works reported in literature [50]. Thestrong reactivity of regions containing Mg-rich intermetallic

Fig. 9. Anodic (A) and cathodic (B) polarization curves acquired in a 0.05 M NaClsolution with the electrochemical micro-cell on different regions of themicrostructure of AA2024-T3 alloy after 12 h immersion in cerium nitrate solution[58]. With permission from Elsevier.

Fig. 10. GDOES sputter depth profile of a Si-free aluminized steel sample (A) andindication of the sputter times used to make the craters inside the intended

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compounds is confirmed by cathodic potentiodynamic polariza-tion curves, which exhibit one order of magnitude higher cathodiccurrent density than regions containing Fe-rich intermetallics andthe matrix [58]. Oxygen diffusion in the glass micro-capillary andissues related to oxygen permeability of the silicone gasket cannotbe excluded for the cathodic potentiodynamic polarization curvesin Fig. 8. However, all measurements were carried out using thesame glass micro-capillary in order to limit possible drawbacksrelated to oxygen diffusion at the micro-capillary mouth.

The regions containing Fe-rich intermetallics, Mg-rich inter-metallics and the matrix were also investigated after immersion inthe solution containing Ce species. Fig. 9 reports representativeanodic (A) and cathodic (B) polarization curves after 12 himmersion in cerium nitrate solution. Anodic polarization curvesdisplay a marked shift in the positive direction of the corrosionpotential for the three regions investigated after immersion in thesolution containing Ce species. In particular, this effect is evidentfor the region containing Mg-rich intermetallics. The breakdownpotential of the three different regions is also shifted in the positivedirection, with the region containing Mg-rich intermetallics

exhibiting a large passive range and the most positive breakdownpotential in Fig. 9. The trend displayed in Fig. 9 clearly indicatesthat deposition of Ce species during the immersion step in thecerium nitrate solution strongly improves the electrochemicalbehaviour of the system. In particular, this effect is remarkable forregions containing Mg-rich intermetallic compounds, which showa strong reduction of their electrochemical activity after immer-sion in the cerium nitrate solution. This trend is also confirmed bycathodic potentiodynamic polarization curves in Fig. 9.

An important outcome of results presented in Fig. 8 and Fig. 9 isthat it is possible to prove that all regions of the microstructure ofAA2024-T3 are affected by precipitation of Ce species and that theinhibition is very strong in the case of Mg-rich intermetalliccompounds. These results enabled to propose a depositionmechanism taking into account the different reactivity of theintermetallic compounds in the microstructure of AA2024-T3 [58].In order to further support the proposed precipitation mechanismof Ce species, TOF-SIMS and transmission electron microscopy(TEM) analysis was carried out for samples immersed in cerium

sublayers (B) [71]. With permission from Elsevier.

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nitrate solution [114]. TOF-SIMS measurements evidenced ahomogeneous and continuous deposition of cerium compoundson both the matrix and the Fe-rich intermetallic particles, whichare involved in the deposition mechanism via lateral island-growth. TEM analysis indicated that a very thin cerium rich layer ispresent on the matrix while the thickness of this cerium-rich layeris rather high on the Fe-rich intermetallic compounds. Moreover,the existence of very thick cerium layer of Ce species wasconfirmed on Mg-rich intermetallic compounds. The work carriedout in our research group on AA2024 [58,130] strongly highlightsthe importance of complementary use of surface analysistechniques in order to validate electrochemical results obtainedby electrochemical micro-cell.

6.3. In depth study of the electrochemical behaviour of aluminizedDC06 steel

The electrochemical behaviour of hot dip aluminium coatingson DC06 steel was studied by the electrochemical micro-celltechnique [71]. In order to carry out potentiodynamic polarizationcurves inside the different sublayers (free layer, upper and lowerinterdiffusion layers) of the hot dip coating, the GDOES methodwas employed in order to expose the different sublayers forelectrochemical characterization by micro-cell. This experimentalapproach is referred in literature as electrochemical depthprofiling [70]. As an example, the characterization of an aluminizedsteel obtained from an aluminium bath containing 2 wt% Fe isdiscussed in the present paper. The experimental procedures forthe hot dip aluminizing process, for the sample preparation byGDOES and for the characterization by electrochemical micro-cellare reported in detail in another paper [71]. The diameter of thecraters sputtered by GDOES was 4 mm. A glass micro-capillarywith an internal diameter of 800 mm was selected to perform themeasurements. This capillary size was selected because it waslarge enough to obtain data which are less influenced by a changingamount of precipitates and/or defects in the area of investigation,but small enough to acquire 3 potentiodynamic polarization curvesper crater.

Fig.10 illustrates the procedure followed for creating the cratersexposing the different sublayers for the Si-free aluminized sample.Initially, the different sublayers were identified in the depth

Fig. 11. Representative potentiodynamic polarization curves in a 0.1 M NaClsolution of the silicon free coating obtained by the electrochemical micro-cellpositioned in the GDOES sputter craters (for interpretation of the references tocolour in the figure legend, the reader is referred to the web version of the article)[71]. With permission from Elsevier.

profiles of aluminium and iron by their changing slopes as afunction of immersion time, as indicated by the vertical lines inFig. 10A. Based on these data, the sputtering times to reach eachsublayer (free layer, upper interdiffusion layer and lowerinterdiffusion layer) were identified. Successively, three distinctcraters at the depths corresponding to these sputtering times werecreated on the sample for electrochemical characterization, asshown in Fig. 10B.

Fig. 11 displays representative potentiodynamic polarizationcurves acquired by electrochemical micro-cell in the cratersrevealing the different sublayers. Measurements were also carriedout in craters reaching the steel substrate below the metalliccoating. The use of micro-capillaries with 800 mm diameterenabled to obtain clear trends for the different sublayers(Fig. 11). The corrosion potentials shift to more positive valueswhen going deeper into the different coating layers. This enabledto establish galvanic series in a 0.1 M NaCl solution for the differentsublayers showing that each sublayer in the metallic coating cancathodically protect the underlaying sublayers, except for the steelthat can be only be protected by the free layer and not by the upperand lower interdiffusion layers.

These results highlight that the protection scheme in alumi-nized steel is not straightforward as in reference hot dip galvanizedsteel [71]. This further proves that the electrochemical micro-cell,used in combination with surface analysis techniques, is a verypowerful technique.

7. Conclusions

This paper reviews applications of the electrochemical micro-cell for corrosion studies. Moreover, selected examples of the studyof localized corrosion and its inhibition in aluminium alloys arepresented discussing the major advantages and the critical aspectsrelated to the use of this local technique. The micro-cell techniqueenables to perform local electrochemical measurements with highspatial resolution on complex systems like AA2024 aluminiumalloy or thin metallic coatings. This makes it possible to establish alink between the microstructure and electrochemical behaviour. Inparticular, the presented examples highlight that this informationis often difficult to obtain using conventional large scaleelectrochemical techniques. The main critical aspects related toapplication of the electrochemical micro-cell for corrosion studiesinclude demanding requirements for the potentiostat, risk ofcrevice corrosion or oxygen diffusion at the micro-capillary tip,modification of the concentration of species involved in electro-chemical processes, ohmic drop and intrinsic low reproducibilityof electrochemical data. Complementary use of surface analysisand spectrophotometric techniques can support data interpreta-tion making the electrochemical micro-cell a very powerfulmethod for the investigation of corrosion phenomena.

Appendix A. Supplementary data

Supplementary data associated with this article can be found, inthe online version, at http://dx.doi.org/10.1016/j.electacta.2016.01.099.

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