studies of electron transfer at aluminum alloy surfaces by scanning electrochemical microscopy

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Studies of Electron Transfer at Aluminum Alloy Surfacesby Scanning Electrochemical MicroscopyMark B. Jensen,a,* Audrey Guerard,a Dennis E. Tallman,b,*,z andGordon P. Bierwagenb,*aDepartment of Chemistry, Concordia College, Moorhead, Minnesota 56562, USAbDepartment of Coatings and Polymeric Materials, North Dakota State University, Fargo,North Dakota 58105, USA

Scanning electrochemical microscopy �SECM� has been used to study electron transfer at Al and Al alloy surfaces, employing themediators hydroquinone, hydroquinone sulfonate, anthraquinone-2,6-disulfonate, and anthraquinone-2-sulfonate. Both oxidationand reduction processes are examined. SECM, in combination with scanning electron microscopy/energy-dispersive X-ray analy-sis, reveals that Cu-containing secondary phase particles exhibit the highest electron transfer activity for both oxidation andreduction reactions on the AA 2024-T3 surface. Although electron transfer is fastest at the intermetallic particles, it also occurs onthe alloy matrix under acidic conditions. The charge carried by the mediator appears to have little influence on the electron transferprocess despite the fact that the matrix oxide surface is positively charged below pH 7.© 2008 The Electrochemical Society. �DOI: 10.1149/1.2916734� All rights reserved.

Manuscript submitted January 15, 2008; revised manuscript received March 17, 2008. Available electronically May 9, 2008.

0013-4651/2008/155�7�/C324/9/$23.00 © The Electrochemical Society

The electrochemical activity of aluminum alloys is of fundamen-tal interest in corrosion studies. The low density and high tensilestrength of these materials have led to their common use in aircraftconstruction. However, the addition of alloying elements, most no-tably copper, makes these materials susceptible to microgalvanic �orlocalized� corrosion. Copper-rich particles in aluminum alloys areknown to undergo a dealloying process that results in a surfaceenriched in spongelike copper clusters that behave as localcathodes.1 Additionally, the number and the relative activity �kinet-ics� of electron transfer sites at Al alloys are important for develop-ing active coatings for corrosion control and for understanding themechanism of their action. Such coatings function by electroniccommunication �i.e., electron transfer� between the active coatingand the underlying substrate.2-5 The method used to prepare thealloy surface prior to application of the active coating will almostcertainly influence this electronic communication. Pretreatment pro-cedures typically involve mechanical polishing, deoxidation, and/orchemical or electrochemical anodization. Such surface pretreatmentsundoubtedly influence the formation of native surface oxides and,thus, the number of defects and/or electron transfer sites on thesurface �including intermetallic inclusions�. Understanding the influ-ence of surface preparation is a longer term goal of this work.

This work is also motivated by the need to better understand thedetails of direct electrodeposition of conducting polymers at Al alloysurfaces employing electron transfer mediators.6-9 Nucleation andgrowth of the polymer occur at electron transfer sites on the alloysurface where the mediator is oxidized in an initial step. Our previ-ous work suggests that the structure and charge of the mediatorinfluences the electrodeposition process.9 Thus, it is important toobtain further insight into the initial electron transfer steps of thisoxidative polymerization process.

Our laboratory is studying electron transfer at Al and at AA2024-T3 substrates using conductivity atomic force microscopy�C-AFM� for measurements in air �i.e., nonimmersed� and scanningelectrochemical microscopy �SECM� for measurements under im-mersed conditions. The C-AFM results will be the subject of a forth-coming paper.10 In this paper, we describe the use of SECM to probeelectron transfer reactions �ETRs, both cathodic and anodic� at thealuminum alloy AA 2024-T3 and, for comparison, also at pure Al.

Previous reports on the use of SECM for studying electron trans-fer at Al and its alloys include studies of localized corrosion11 andETR12 at pure Al, ETR at Al alloy13 and Al composites,14 localdissolution of Al alloy employing an integrated AFM/SECM,15-17

* Electrochemical Society Active Member.z E-mail: [email protected]

address. Redistribution subject to ECS terms128.252.67.66aded on 2014-05-26 to IP

and H2 evolution over a scratch on the surface of AA 2024-T3coated with either poly�methyl methacrylate� �PMMA� or apoly�aniline�/PMMA blend.18 Most relevant among these is a reportby Serebrennikova and White showing that, on pure aluminum, re-duction reactions occur at structural and/or electronic flaws in thenative oxide �measurements made in nonaqueous solvent�12 and areport by Seegmiller and Buttry showing that reduction reactionsoccur predominately at Cu-rich intermetallic inclusions on AA2024-T3.13 Other than the local dissolution studies noted above,there appear to have been few if any SECM studies of oxidativeETR at Al and Al alloys.

The mediators used in this study are hydroquinone �HQ� andhydroquinone sulfonate �HQS� for probing reduction at the alloysurface and anthraquinone-2-sulfonate �AQS� and anthraquinone-2,6-disulfonate �AQDS� for probing oxidation. Our choice of media-tors is motivated by the observation that certain dihydroxy benzenecompounds are effective mediators for the mediated electrodeposi-tion of polypyrrole on AA 2024-T3,9 and also by the fact that thesemediators yield steady-state currents in constant potential amperom-etry at the Pt microelectrode probe of the SECM.

Experimental

The SECM experiment.— The SECM is a scanning probe instru-ment that can be used to identify local variations in electrochemicalactivity on an electrode surface. In a typical imaging experiment amicroelectrode probe is used to either oxidize or reduce an electro-chemical mediator compound in solution. The probe is held at aconstant height over a substrate electrode and scanned back andforth in the x–y plane. A bipotentiostat allows for independent con-trol of both probe and substrate potentials, and the probe current isplotted vs the probe position to create a two-dimensional map of thesubstrate activity.19 In most experiments, the tip generation/substratecollection �TG/SC� mode of operation was used, whereby the me-diator was oxidized �reduced� at the probe tip and the product sub-sequently reduced �oxidized� at active sites on the alloy surface. Afew experiments employed the substrate generation/tip collection�SG/TC� mode, in which the mediator was oxidized �reduced� at thesubstrate and the product subsequently reduced �oxidized� at theprobe tip.

Materials and sample preparation.— HQ from MCB, hydro-quinone sulfonic acid potassium salt �HQS, from Alfa Aesar�,anthraquinone-2,6-disulfonic acid disodium salt �AQDS, fromFluka�, and anthraquinone-2-sulfonic acid sodium salt �AQS, fromFluka� were reagent grade and used as received. Solutions of eachmediator were made using 0.1 or 1.0 M Na2SO4 in either water,0.001 M Na2SO4, or 0.005 M H2SO4. All solutions contained 0.01M mediator. AA 2024 panels were obtained from Q-Panel, and Al

) unless CC License in place (see abstract). ecsdl.org/site/terms_use of use (see

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�99.99%� panels were purchased from Alpha Aesar. Samples werecut into sizes appropriate for the SECM cell, sanded with 600-gritsilicon carbide, rinsed with hexane, and air dried. A commercialengraver �Gravograph� was used to cut a 2 � 2 mm square positionmarker into samples to be used for both SECM and SEM/EDX�scanning electron microscope/energy-dispersive X-ray� analysis.

Instrumentation.— SECM images and �at selected sites� ap-proach curves were obtained with a CHI900B scanning electro-chemical microscope �CH Instruments� utilizing a 10 �m Pt micro-electrode probe, Pt counter electrode, and a saturated Ag/AgClreference electrode. To explore the behavior of different mediatorsover the same substrate area, a cell was designed to allow efficientsolution replacement with no disturbance of the probe position. TheSECM probe height was ca. 5 �m for most images. Potentials arereported with respect to the saturated Ag/AgCl electrode. The open-circuit potential �OCP� of the alloy in the various electrolytes wasca. −0.8 � 0.1 V.

Samples for SEM/EDX analysis were removed from the SECMcell, rinsed with water, placed on aluminum mounts, and coated withgold using a Technics Hummer II sputter coater. Images were ob-tained using a JEOL JSM-6300 SEM. X-ray information was ob-tained via a Thermo energy-dispersive spectroscopy detector using aVANTAGE digital acquisition engine at 15 keV.

Rotating disk voltammetry was carried out using a Pine Instru-ment system consisting of an AFE6R1PT Pt ring assembly withreplaceable 19.6 mm2 disks �AA-2024-T3, Al, and Cu were used inthis work� controlled by an AFCBP1 bipotentiostat, AFMSRX rota-tor, and PineChem software. The Al, AA-2024-T3, and Cu diskswere sanded with 600-grit silicon carbide and rinsed with hexaneprior to use.

Results and Discussion

During previous studies of the electrodeposition of polypyrroleon AA 2024-T3 employing various dihydroxybenzene compoundsas the electron transfer mediator, it was observed that the charge ofthe mediator influenced the nucleation and growth stages of polymerformation.9 Based on those observations, we conjectured that nega-tively charged mediators might more effectively interact at the posi-tively charged oxide surface �the isoelectric pH of the oxide is above7�.20 Unfortunately, some of the dihydroxybenzene mediators suchas Tiron �4,5-dihydroxy-1,3-benzenedisulfonate� or catechol �1,2-dihydroxybenzene�, while effective mediators for polypyrrole for-mation, undergo self-oligomerization in the absence of pyrrole. Suchmediators are not useful in the present SECM studies becausesteady-state currents at the SECM microelectrode probe are not ob-tainable. The mediators chosen for this study are shown in Table Ialong with their reduction potentials. HQ and HQS were selected forstudies of reduction at the alloy surface, having similar redox poten-tials but differing in charge �both mediate polypyrroleelectrodeposition9�. Probing oxidative ETR at the alloy surface ismore problematic, because many redox mediators require a potentialsufficiently positive of the alloy OCP that surface modifications re-sult �e.g., oxide growth�. AQS and AQDS were selected to probeoxidation at the alloy surface because they have sufficiently lowreduction potentials.

Reductive ETR at AA 2024-T3.— Figure 1a shows an SECMmap obtained using HQ as the mediator. HQ is oxidized to quinoneat the microelectrode probe and an enhanced current is observedwhen the probe is above an active region where quinone is reducedback to HQ, leading to positive feedback �or recycling�.19 The “hotspot” observed in Fig. 1a is associated with a copper-rich site, asevidenced by the voltammograms in Fig. 1b and further confirmedby SEM/EDX �vide infra�. Figure 1b shows the probe current �uppercurves� at various y-positions �denoted in Fig. 1a� as the substratepotential is scanned. A single substrate voltammogram �lower curveof Fig. 1b� is shown, although each of the probe current curves wasobtained from a separate substrate potential scan. An oxidation peakis observed in the substrate voltammogram centered at ca. 0 V. Re-


duction currents are observed at the probe when the probe is posi-tioned above the hot spot �y = 50 �m�, but not when the probe ispositioned further away �y = 30, 70 �m�. The reduction current atthe probe is attributed to copper deposition, as it is stripped from acopper-rich site �the hot spot� of the substrate. A stripping voltam-mogram obtained at the microelectrode probe �not shown� agreedexactly with that obtained when the experiment was repeated with apure Al substrate containing a pure Cu dot �embedded Cu wire�. Thesubstrate stripping current and the probe deposition current bothincrease by approximately an order of magnitude when the alloy isallowed to stand for 2 h under the acidic conditions of Fig. 1. Thereis very little increase in these peak currents when the experiment isconducted in an unacidified solution. The SECM appears to be apowerful tool for studying the redistribution of Cu on these alloysurfaces. Figure 1b also shows a significant anodic current at theprobe for the y = 50 �m position for substrate potentials less than−600 mV, consistent with the oxidation of H2 generated at the Cu-rich site under the acidic conditions.

Figure 2 compares SECM maps obtained for the same area of theAA 2024-T3 surface using HQ �a neutral mediator� and HQS �anegatively charged mediator� under acidic conditions where the al-loy surface is positively charged �images virtually identical to thesewere obtained for substrate potentials ranging from −0.8 V to ca. 0V�. Nearly identical maps were obtained for each mediator, showinga nearly one-to-one correspondence of the most active and the leastactive sites �note that, although the absolute currents differ in thesetwo images due to the difference in diffusion coefficients of the twomediators, the current range is the same�. This suggests that themediator charge has little influence on the electron transfer activity.As discussed below, the most active reduction sites are the copper-rich secondary phase particles, and mediator charge may have littleinfluence on electron transfer at such sites. The charge of the me-diator may play a more important role for electron transfer at siteson the alloy matrix, as suggested by our studies of mediated elec-trodeposition of polypyrrole.9

The most intense sites of reduction �highest probe currents� onthe Al alloy surface are Cu-rich secondary phase particles. Figure 3shows the almost exact correspondence between the most activereduction spots in the SECM and the Cu-rich sites on the alloysurface. This is in agreement with the SECM results of Seegmillerand Buttry, who employed the protonated form of �dimethylami-no�methylferrocene, a positively charged mediator, in a boratebuffer of pH 8.5.13 Comparison of the Cu EDX image in Fig. 3 withone collected from an AA-2024 panel in the as-received state �datanot shown� indicates that Fig. 3 shows a significant enrichment of

Table I. Structures and potentials of mediators used in SECMimaging.



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copper on the surface. This phenomenon is known to be a result ofsanding during surface preparation.21 Additional Cu enrichment onthe surface occurs upon immersion in acidic solution as discussedabove in conjunction with Fig. 1b.

A more detailed analysis of a smaller 75 � 75 �m area fromFig. 3 is shown in Fig. 4, in which an SECM image is shown alongwith EDX maps for a variety of elements in addition to copper. Sixareas of high cathodic activity are labeled a–e and each of thesecorresponds to a Cu-rich area. Areas a, c, e, and f show only Cu,while areas b and d show both Mn and Fe as well. These results arein agreement with those of Seegmiller and Buttry, who used SECM


and EDX to correlate areas of high cathodic activity on AA 2024with elemental maps of Cu, Fe, Mg, and Mn.13 They identified spotsrich in Cu, but not Fe, Mg, or Mn �regions a, c, e, and f in Fig. 4�and attributed them to either Al2Cu ��-phase� or Al2CuMg �S-phase� intermetallic particles. Large active areas rich in Cu, Fe, andMn �such as region b in Fig. 4� were attributed to AlCuFeMn par-ticles. They also identified activity from particles rich in both Cuand Mg, but we see no such areas. In fact, each active area identifiedin our work appears to be depleted of both Mg and Al, similar toEDX results reported by Buchheit et al., showing S-phase particledissolution in salt water,22 though we note that our immersion con-

Figure 1. �a� SECM map showing an ac-tive region for reduction on AA 2024-T3using HQ as a mediator. Solution con-tained 0.01 M HQ, 1.0 M Na2SO4, 0.005M H2SO4. E�probe� = 1 V, E�substrate�= −0.8 V �TG/SC mode�. �b� Probe/substrate voltammograms showing a sub-strate metal stripping peak �lower curve�and copper deposition at the microelec-trode probe �upper curves� for various po-sitions of the probe along the y-scan direc-tion; E�probe� = −0.3 V.

Figure 2. SECM maps of AA 2024-T3�200 � 200 �m� obtained using �a� HQas mediator and �b� HQS as mediator. So-lutions contained 0.01 M mediator,1.0 M Na2SO4, 0.005 M H2SO4.E�probe� = 1.0 V, E�substrate� = −0.75V �TG/SC mode�.

Figure 3. �a� SECM map of AA 2024-T3�200 � 200 �m� obtained using HQS asmediator �solution conditions as in Fig. 2�;E�probe� = 1.2 V, E�substrate� = OCP�TG/SC mode� and �b� elemental EDXmap for Cu. Lines are drawn to emphasizethe patterns of correspondence.




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ditions differ from theirs. S-phase intermetallic particles, initiallyanodic due to the dealloying of Mg and Al, quickly switch to Cucathodes for oxygen reduction;22,23 thus, the behavior of S-phaseparticles is time dependent. Our SEM/EDX results suggest that deal-loying was mostly complete by the time we were able to obtain anSECM image. The fact that Seegmiller and Buttry saw activity fromparticles rich in both Cu and Mg may have been due to their use ofan anodic scan in borate buffer prior to SECM imaging, therebyforming a protective oxide film on these intermetallic particles.13

Enhanced oxidation current measured at the probe �enhancedrelative to that measured when the probe is far removed from thesubstrate surface� may result not only from positive feedback of themediator, but also from oxidation of H2 generated at active reduc-tion sites on the alloy surface under the acidic conditions of thisstudy �e.g., see Fig. 1b and the accompanying discussion�. Thesetwo sources of current enhancement can be distinguished by theproper choice of probe potential. Figure 5a shows an SECM mapobtained with HQS as the mediator with the probe potential set at1.2 V at which oxidation of both HQS and H2 can occur. Threeparticularly active sites of reduction are observed in this map. Figure5b shows an SECM map of the same region with the probe potentialset at 0 V where only H2 oxidation can occur, indicating that two ofthese sites are also producing H2 �verified by cyclic voltammetry�CV� at the probe while positioned above these spots�. Clearly, notall sites that reduce the mediator produce H2. Note also that thecurrents in Fig. 5b are more than an order of magnitude lower thanthose in Fig. 5a, suggesting that H2 production does not significantlyinterfere with ETR maps obtained using the mediator. The SEMbackscatter image �Fig. 5c� and the EDX map for Cu �Fig. 5d�reveal that both processes �mediator reduction and H2 generation�occur at Cu-rich sites.

Figure 6 shows a more detailed elemental composition of thesurface displayed in Fig. 5, revealing that the composition of thesecondary phase particles influences the type of ETR that occurs.For example, the hydrogen evolution reaction occurs preferentiallyon Cu sites devoid of Mg, Mn, and Fe �most likely �-phase and/ordealloyed S-phase particles�, while mediator reduction occurs at allthese sites, though the highest current appears at the AlCuFeMnparticles. More quantitative analysis of the secondary phase particlesand of their surface oxides will be necessary for a complete under-standing of ETR at these complex surfaces.


Oxidative ETR at AA 2024-T3.— The mechanism for the medi-ated electrodeposition of polypyrrole on AA 2024-T3 begins withthe oxidation of the mediator, which in turn oxidizes the pyrrolemonomer. Therefore, we are interested in using the SECM to iden-tify areas of anodic activity on the alloy surface where mediatoroxidation can occur. One approach is to use the SECM in SG/TCmode, in which a high substrate potential is used to oxidize themediator and a low probe potential is used to reduce it back to itsoriginal form. We have attempted this with both HQ and HQS me-diators, but have found that the high substrate potential required formediator oxidation results in a passive �oxide or perhaps oligomeric�layer that prohibits us from obtaining consistent images. Interest-ingly, in the presence of pyrrole, no such passive layer forms, andvirtually all charge flow leads to polymer deposition.7

As an alternate approach we have chosen to use the TG/SC modewith the oxidized form of a mediator having a low E°. The mediatoris reduced at the probe and then reoxidized at the substrate at apotential that ideally minimizes passive oxide layer formation. AQSand AQDS �Table I� are functionally similar to the most effectiveelectrodeposition mediators we have tested.9 The solid line labeledsubstrate in Fig. 7 shows the redox behavior of 0.01 M AQDS in 0.1M Na2SO4 on AA 2024 �the decreased salt concentration was nec-essary due to the lower solubility of AQDS in comparison to ourother mediators�. This behavior is reproducible for many successivescans, although the activity slowly decreases over time, presumablydue to the adsorption of impurities or minor reaction products. Thesurface can be easily reactivated by dropping the lower potential ofthe voltammetric scan to −1.1 V.

Figure 8 displays SECM images identifying areas of both highcathodic �Fig. 8a� and anodic �Fig. 8b� activity in the same 80� 80 �m area of the alloy. The image in Fig. 8a was collectedusing conditions similar to Fig. 2a above, and the areas of highcathodic activity are presumably rich in Cu �as verified previously�.After collection of this image, the tip was raised slightly and the cellsolution was replaced with 0.01 M AQDS in 0.1 M Na2SO4. The tipwas lowered back to its original position and the image in Fig. 8bwas collected with the substrate at −0.2 V and the probe at −0.6 V.Two areas of high anodic activity are apparent; these correspond totwo of the cathodically active areas. The anodic activity of thesespots is confirmed by the probe current �dotted� curves in Fig. 7.

Figure 4. EDX maps for Mg, Al, Mn �top,left to right� and Fe, Cu �bottom, left toright� with corresponding SECM image�bottom right panel�. These images corre-spond to an enlarged �75 � 75 �m� viewof the upper-left quadrant of the images inFig. 3.




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During CV of the substrate, the probe was held at −0.6 V andplaced at the imaging height ��5 �m� above either point a or pointb in Fig. 8. The probe current at point a shows no activity in the


SECM image, and the probe shows a minimal response to thechanging substrate potential in Fig. 7. When the probe was placed atthe active point b, however, a dramatic change in the probe current

Figure 5. SECM maps showing �a� reduc-tion sites, E�probe� = 1.2 V, and �b� H2

generation sites, E�probe� = 0 V, on an80 � 80 �m area of AA 2024-T3 �HQSmediator; solution conditions as in Fig. 2�,E�substrate� = −0.8 V. �c� SEM back-scatter image and �d� EDX map for Cu ofthe same area. Lines are drawn to empha-size the patterns of correspondence.

Figure 6. EDX maps for Mg, Mn, Fe�top, left to right� and Cu, Al �bottom, leftto right� with corresponding SECM imagefrom Fig. 5b �bottom right panel�. Thecircled regions in the EDX maps corre-spond to the sites of H2 production in theSECM image.




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was observed. At substrate potentials below −0.45 V the probe cur-rent is lower in magnitude than at point a due to competition be-tween the probe and substrate for reduction of AQDS. At substratepotentials above −0.45 V the probe current becomes greater inmagnitude than at point a because the probe is now re-reducing theAQDS that had been regenerated at the substrate.

The question arises as to why some areas are highly active forboth anodic and cathodic behavior and others only for cathodic. Wespeculate that the active areas may have different elemental compo-sitions, as shown in Fig. 4. We are currently attempting to generatedata correlating anodic and cathodic activity data with EDX imagingto determine if this is the case. After the image in Fig. 8b wascollected, the solution was replaced with the original acidic HQSsolution and another SECM image was collected using the originalconditions. One would expect to see the original image �Fig. 8a�once again, but instead only the two spots that were visible with theAQDS solution were observed. It is possible that the missing spotwas due to passivation of that site, a result of the more positivesubstrate potential required to obtain the image in Fig. 8b. Clearlythis behavior warrants further investigation.

Dynamic nature of the AA 2024-T3 surface at low pH.— Figure9 shows probe approach curves �PACs� collected in TG/SC mode ata single location on an AA 2024-T3 substrate as a function of im-mersion time in acidic hydroquinone solution. The shape of a PACcan be used to determine the absolute probe position and heteroge-neous rate constants, as well as to classify the substrate as insulatingor conducting under particular conditions.19 In general, as the probe


approaches an insulating surface, diffusion to the probe microelec-trode is blocked, resulting in a decrease in the probe current. Alter-natively, if the surface is conducting, regeneration of the mediatorwill result in an increase in the mediator concentration and the probecurrent will rise. Figure 9 shows that the behavior of AA 2024-T3appears to change from insulating �slow ETR rate� to conducting�fast ETR rate� with increasing immersion time, illustrating the dy-namic nature of this material in acidic solution. Furthermore, theSECM images tend to get fuzzier with time, as though the probewere retracted further from the substrate surface, consistent with agradual etching of the substrate surface �the PAC also indicated anincreasing distance between the probe and substrate surface�. Whena similar set of experiments is conducted under more neutral condi-tions using the same solution but with no added H2SO4, the shape ofthe PAC remains constant with immersion time. This dynamic be-havior makes determination of the absolute probe position difficult,as reported previously.13 We estimate the probe position in all of ourSECM images to be approximately 5 �m above the substrate sur-face, but it may vary between 2 and 10 �m.

While this dynamic behavior observed in Fig. 9 does vary withprobe position, the general trend of increasing activity is consistentfor the entire substrate surface. An SECM image was collected aftereach of the PACs obtained in Fig. 9. The image obtained after 9 minof immersion is shown in Fig. 10, along with a plot of the intensityof three spots as a function of immersion time. Two of these spots,A and B, are in regions of higher cathodic activity, while spot C isconsidered an “average” background spot. The intensities of each of

Figure 7. CV of 0.01 M AQDS in 0.1 MNa2SO4 on AA 2024-T3 with probe placedat two different positions above the sur-face. Probe positions correspond to spotslabeled in Fig. 8. Scan rate: 100 mV/s,initial E�substrate� = −0.2 V, E�probe�= −0.6 V.

Figure 8. SECM images from thesame 80 � 80 �m area of AA 2024-T3.�a� 0.01 M HQS in 1.0 M Na2SO4,0.005 M H2SO4; E�probe� = +1.2 V,E�substrate� = OCP. �b� 0.01 M AQDS in0.1 M Na2SO4; E�probe� = −0.6 V,E�substrate� = −0.2 V. Arrows point toprobe positions in Fig. 7.




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the spots increases with immersion time. Also, the current at eachspot after �5 min of immersion surpasses the diffusion-limitedprobe current at a large tip–sample separation ��37.8 nA�, indicat-ing that the entire surface appears to be exhibiting reductive ETRactivity.

As noted above, this dynamic behavior is not observed in moreneutral solution, and Pourbaix diagrams for Al indicate that alumi-num oxide �Al2O3·H2O� is unstable below pH 4.24 The pH of ouracidified solution was between 2 and 3; therefore, the increase inoverall activity is most likely related to dissolution of the nativeoxide film. The probe currents at spots A and B �Fig. 10a� increasewithin the first 10 min of immersion, then remain rather constant,possibly reflecting a rapid dissolution of an oxide on Cu-rich par-ticles. In contrast, the current at spot C increases more slowly and ina more monotonic fashion, probably reflecting a slow attack on thematrix oxide layer, leading to more rapid ETR. We do not believehydrogen evolution contributes significantly to the observed dy-namic behavior, even though H2 is oxidized at the probe potentialused in these experiments �+1.0 V�. The current from H2 genera-tion is much smaller and is highly localized as discussed earlier �seeFig. 5 and accompanying discussion�.

Oxidative and reductive ETR at pure Al.— For comparison withthe AA 2024-T3 results, SECM images were obtained at pure�99.99%� Al substrates. Figure 11 shows representative results forreductive ETR at two different Al substrates using two differentmediators, HQ �Fig. 11a and c� and HQS �Fig. 11b and d� underacidified �Fig. 11a and b� and nonacidified �Fig. 11c and d� condi-tions. Reductive ETR occurred at localized regions of the Al surface

Figure 9. Probe approach curves obtained at the upper-left corner �0,0 �m�of the SECM image of Fig. 10 as function of solution immersion time.Solution: 0.01 M HQ, 1.0 M Na2SO4, 0.005 M H2SO4; E�probe� =+ 1.0 V, E�substrate� = −0.80 V.


under both pH conditions, with similar images being obtained fromeach of the two mediators. Serebrennikova and White12 also ob-served localized ETR at Al substrates in nonaqueous solvent �aceto-nitrile� using nitrobenzene as the mediator and attributed the reduc-tion of nitrobenzene on aluminum to electroactive defect sites in thenative oxide film on the aluminum. The size �typically �10 �m�and density ��1 � 105 per cm2� of ETR sites obtained in ouraqueous solution studies appear comparable to that observed bythese workers in nonaqueous solution, although it was noted that thedensity of sites could vary by 2 or 3 orders of magnitude from thesame source of Al, suggesting that electronic conduction in the na-tive oxide is very sensitive to surface preparation.25

Oxidative ETR can also be observed at the Al substrate andappears to occur at the same sites as does reductive ETR. Figure 12shows SECM images for reduction �Fig. 12a, TG/SC mode� andoxidation �Fig. 12b, SG/TC� for the same substrate area employingHQS as the mediator under acidic conditions �similar images areobtained in neutral or nonacidified solution�. It is likely that somesurface modification �e.g., oxide growth� occurred at the rather posi-tive potential required to obtain Fig. 12b, yet oxidative ETR stilloccurred at the same location as the reductive ETR in Fig. 12a�although there is a slight shift in the region of highest activity�,consistent with the existence of one or more defects in the oxidelayer at this location.

Two aspects of the results obtained at the pure Al substrateshould be noted. First, on average, the probe currents obtained at themost active areas of pure Al were lower than those for the mostactive areas �i.e., Cu-rich sites� of the Al alloy �for similar experi-mental conditions�, indicating lower rates of ETR at the pure Al.Occasionally, the highest probe currents on pure Al were smallerthan the current at large probe–substrate separation, reflecting therather sluggish rate of ETR. Second, while there was some increasein probe currents with immersion time in acidic solution, the dy-namic behavior of pure Al in acidic solution was much less than thatobserved for the alloy, suggesting that the oxide layer of the alloymatrix was more susceptible to acidic attack.

Rotating disk voltammetry.— Rotating disk electrode voltamme-try was employed to further understand the electrochemical behaviorof the mediators at the AA 2024 substrate. AQS and AQDS wereused in these studies because they have the lowest redox potentialand thus do not require extensive excursions to positive potentialsthat might lead to oxide growth or other substrate surface modifica-tion. Because these mediators behave similarly, only results forAQDS are presented here. Figure 13a shows consecutive potentialscans at the AA 2024-T3 electrode. The initial scan shows a poorlydefined wave suggesting a very sluggish rate of ETR. Upon furtherpotential cycling, the wave becomes more well-defined, eventuallyreaching a stable reproducible wave that reflects more facile ETR,the limiting current of which follows Levich behavior. This finalwave is also displayed in Fig. 13b, where it is compared with the

Figure 10. �a� SECM image of AA2024-T3 in 0.01 M HQ, 1.0 M Na2SO4,0.005 M H2SO4 after 9 min of immer-sion. E�probe� = +1.0 V, E�substrate�= −0.80 V. �b� Plot of probe current vsimmersion time for points A, B, and C in-dicated in the SECM image. Dotted lineshows probe current when sample–probeseparation is large.




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voltammetric response at pure Al and at Cu. Very little current isobserved at the Al electrode even after repetitive cycling �AQS doesshow a larger current under these conditions, reaching ca. −0.1 mAat −1 V�. A well-defined wave is observed at the Cu electrode witha limiting current of ca. −1.5 mA. From the behavior exhibited inFig. 13a and from the similarity of the final wave at AA 2024 to thatobtained at Cu, we conjecture that the changing electroactivity ob-served in Fig. 13a results from dealloying and Cu redistributionupon repetitive potential cycling. This behavior is being further in-vestigated.

Conclusion

SECM in combination with SEM/EDX reveals that Cu-containing secondary phase particles exhibit a significantly higherelectron transfer activity than does the alloy matrix for both oxida-tion and reduction reactions occurring at the AA 2024-T3 surface.
Although electron transfer is fastest at the intermetallic particles, it

also occurs on the alloy matrix. Under acidic conditions, the surfaceis dynamic, probably the result of oxide attack, leading to increasingrates of electron transfer across the entire surface. This is consistentwith our observation of nucleation of conjugated polymer growth onthe matrix.26 The composition of the secondary phase particles ap-pears to play a role. For example, the hydrogen evolution reactionoccurs preferentially on Cu sites devoid of Mg, Mn, and Fe �mostlikely �-phase and/or dealloyed S-phase particles�, while mediatorreduction occurs on all Cu-rich particles, the highest probe currentsbeing observed at AlCuFeMn particles. The charge carried by themediator appears to have little influence on the electron transferprocess despite the fact that the matrix oxide surface is positivelycharged below pH 7. Localized electron transfer is also observed atpure aluminum, with both oxidation and reduction occurring at thesame sites, presumably defects in the oxide film. In contrast to thealloy, the surface of pure Al exhibits less dynamic behavior under

Figure 11. SECM images showing reduc-tion sites on two different substrates ofpure Al obtained using 0.01 M mediatorin 0.1 M Na2SO4. E�probe� = + 1.2 V,E�substrate� = OCP. �a� HQ and �b� HQSin 0.005 M H2SO4 at substrate 1, �c� HQand �d� HQS in neutral �nonacidified� so-lution at substrate 2.

Figure 12. SECM images showing �a� re-duction and �b� oxidation sites onpure Al using 0.01 M HQS in 0.1 MNa2SO4, 0.005 M H2SO4. �a� cathodic ac-tivity, TG/SC mode, E�probe� = 1.2 V,E�substrate� = −0.7 V. �b� anodic activ-ity, SG/TC mode, E�probe� = 0 V,E�substrate� = 2.0 V. Similar images areobtained in neutral �nonacidified� solution.

acidic conditions.




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Acknowledgments

The support of this research by the Air Force Office of ScientificResearch under grant no. FA9550-06-1-0461 and no. FA9550-07-1-0370 and by the Concordia Chemistry Research Endowment isgratefully acknowledged. We also thank Mr. Scott Payne for assis-tance with the SEM/EDX imaging.

North Dakota State University assisted in meeting the publication costsof this article.

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studies of electron transfer at aluminum alloy surfaces by scanning electrochemical microscopy

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