Neutral and n-Doped Conjugated Polymers for corrosion control of Aluminum alloys

Corrosion Control Coatings for Aluminum Alloys Basedon Neutral and n-Doped Conjugated PolymersM.C.Yan,a D.E.Tallman,a,*,z S.C.Rasmussen,b and G.P.Bierwagen a,*a Department of Coatings and Polymeric Materials andb Department of Chemistry and Molecular Biology,North Dakota State University,Fargo,North Dakota58108-6050,USAConjugated polymers͑CPs͒continue to be of interest for use as components of corrosion control coatings.For aluminum alloys, however,there is controversy and uncertainty regarding the protection mechanism͑s͒,or indeed even whether corrosion protection by a CP is possible.Recent results from our laboratory indicate that,for an aluminum alloy coated with the oxidized form of polypyrrole͑PPy͒,a passivation mechanism is not likely and that the dopant release mechanism may not be effective,especially in the presence of oxygen.An alternative approach is to utilize the neutral or reduced͑n-doped͒form of a conjugated polymer to cathodically protect the alloy and/or to serve as an oxygen scavenging coating.The CP coatings and alloy studied were neutral PPy and neutral or n-doped poly͑2,3-dihexylthieno͓3,4-b͔pyrazine͒on AA2024-T3.Results from the scanning vibrating electrode technique and galvanic coupling measurements are presented to demonstrate the efficacy of this approach.©2009The Electrochemical Society.͓DOI:10.1149/1.3190162͔All rights reserved.Manuscript submitted April14,2009;revised manuscript received June22,2009.Published August20,2009.This was Paper 1691presented at the Honolulu,Hawaii Meeting of the Society,October12–17,2008.Conjugated polymers͑CPs͒,such as polyaniline,polypyrrole ͑PPy͒,polythiophene,and their derivatives,continue to be of inter-est for use as components of corrosion control coatings1-3and are often viewed as promising environmentally benign replacements for conventional chromate-based coatings.Chromate-containing coat-ings are under increasing scrutiny due to the toxicity of Cr͑VI͒and issues related to its safe handling and disposal.4CPs can exist in several redox͑or doped͒states:Oxidized͑or p-doped͒,where elec-trons are removed from the polymer backbone;neutral͑or undoped͒, which are typically insulating or semiconducting;and reduced͑or n-doped͒,where electrons are added to the polymer backbone. Clearly the interaction of a CP with an active metal should depend on its oxidation state.For an oxidized͑or p-doped,thus,electronically conducting͒CP, several types of interactions with an active metal͑such as Al,Fe,or their alloys͒are possible,including electronic interactions͑nonre-dox͒,electrochemical interactions͑redox͒,and chemical interactions ͑involving polymer and/or dopant anions͒.2The corrosion behavior of the metal may be influenced by any or all of these interactions, depending on the nature of the metal substrate,the characteristics of the CP,the manner of metal surface preparation,and the method of CP coating deposition.2The most important mechanisms for steels seem to be anodic protection͑or passivation,especially for stainless steels͒5-7and corrosion inhibition by released dopant ions͑espe-cially for mild steels͒.8,9However,there is controversy and uncer-tainty regarding the protection mechanism͑s͒for the more active aluminum alloys,or indeed even whether corrosion protection of Al alloys by a CP is possible.Suggested mechanisms for aluminum alloys include ennobling of the alloy,thereby minimizing O2and H+ reduction at the metal surface,10anodic protection/passivation,11,12 galvanic and ion-exchange release of oxygen reduction inhibitors,13 and barrier protection by the reduced form of the polymer following an initial galvanic coupling.14Recent results from our laboratory indicate that,for p-doped PPy-coated aluminum alloys,a passivation mechanism is unlikely and that the dopant release mechanism may not be effective,espe-cially in the presence of oxygen.15This conclusion may extend to other p-doped CPs as well.An alternative approach is to utilize either the neutral or the reduced͑n-doped͒form of a CP to cathodi-cally protect the alloy and to serve as an oxygen scavenging coating. For example,the neutral form of PPy reacts with oxygen and under-goes a process sometimes referred to as oxygen doping.15Such a coating applied between the metal alloy and a top coat may protect the alloy by consuming the oxygen that would otherwise reach the alloy surface.Indeed,Ocon et al.found that an undoped PPyfilm was better for the corrosion protection of mild steel than the p-doped form,but the improvement was attributed to the compactness and barrier property of the undopedfilm.16An n-doped CP could also function as an oxygen scavenging coating,but might also protect metal exposed in a coating defect by a cathodic protection mechanism similar to that exhibited by certain metal-filled coatings.17The use of n-doped CPs in electroactive polymer-based supercapacitors is well known,although it was ques-tioned whether the reduction in certain CPs is true n doping or charge trapping.18For the purpose of our discussion,we do not distinguish between these two possibilities,and we refer to a re-duced CP as being n-doped.The n-doped CPs are notoriously reac-tive with oxygen and water,18a prerequisite if these materials are to function as oxygen scavenger and/or cathodic protection coatings for active metals.In this paper,we describe the application of the scanning vibrat-ing electrode technique͑SVET͒and galvanic coupling measure-ments to the study of neutral and n-doped CP-aluminum alloy͑AA 2024-T3͒interactions.Neutral PPy and neutral and n-doped poly͑2,3-dihexylthieno͓3,4-b͔pyrazine͒͑PC6TP͒are used to demon-strate the efficacy of the approach.ExperimentalMaterials and substrate preparation.—The AA2024-T3͑from Q-Panel͒was pretreated by polishing with600grit SiC paper fol-lowed by degreasing with hexanes.The PPyfilms were galvanostati-cally electrodeposited onto AA2024-T3as described previously19 using sodium4,5-dihydroxy-1,3-benzenedisulfonate͑Tiron͒as the dopant anion and electron-transfer mediator.The as-deposited PPy film was in the oxidized state.The neutral state of PPy was obtained by electrochemically reducing the PPyfilm atϪ1.0V͑vs Ag/AgCl͒for1h.The PC6TP was synthesized by Grignard metathesis poly-merization as described previously.20The PC6TP powder was dis-solved in chloroform solvent to saturation,the solution wasfiltered, and the polymer was applied to the AA2024-T3substrate by either airbrush or solvent casting.The as-deposited PC6TPfilm was in the neutral state.To obtain the n-doped form of the polymer,the PC6TP film was reduced atϪ1.0V͓vs Ag wire quasi-reference electrode ͑QRE͔͒in acetonitrile containing0.1M Bu4NPF6.The approximatefilm thickness was3␮m for PPy and2␮m for PC6TP.Structures of the neutral forms of PPy and PC6TP are shown in Fig.1.SVET measurement and data analysis.—The current distribu-tion on CP-coated substrates was measured using a SVET system*Electrochemical Society Active Member. z E-mail:dennis.tallman@ Journal of The Electrochemical Society,156͑10͒C360-C366͑2009͒0013-4651/2009/156͑10͒/C360/7/$25.00©The Electrochemical SocietyC360from Applicable Electronics employing a Pt–Ir microelectrode ͑with a tip diameter of 10␮m,from Microprobe,Inc.͒onto which was deposited a sphere of Pt black ͑ca.20␮m diameter with a capaci-tance of ca.40nF ͒.The microprobe was scanned 200␮m above the sample surface with a vibration amplitude of 20␮m in the X and Y directions at frequencies in the range of 200–300Hz.The sample ͑1cm square ͒was masked by polyester tape so as to form a scan area of 3ϫ3mm.An artificial defect ͑ca.0.1–0.3mm 2͒was intro-duced in the center of the scan area by scribing through the coating with an awl,a common procedure which introduces some variability in defect area from sample to sample.The probe made measure-ments on a 20ϫ20grid ͑requiring ca.10min ͒,generating a 400-point mesh across the surface.The SVET measurements were per-formed at ambient temperature ͑ca.20°C ͒under open-circuit ͑free-corrosion ͒conditions in a cell containing ca.5mL dilute Harrison’s solution ͑DHS,0.35wt %͑NH 4͒2SO 4and 0.05wt %NaCl ͒.Scans were initiated ca.5min after immersion and were repeated every 30min.All solutions were prepared with analytical grade reagents and Milli-Q water.Display of the SVET data and statistical analysis of the data were performed with Origin software.The current densities are displayed in three-dimensional ͑3D ͒maps,showing the spatial distribution of the vertical component of the current density as a function of the ͑X ,Y ͒position in the scan region.In these SVET maps,anodic cur-rents are positive and cathodic currents are negative.A contour map of the current density is projected onto the bottom of the 3D map.In some cases,the data are also presented as current vectors superim-posed onto an optical micrograph of the sample,showing both hori-zontal and vertical components of the current.A minimum of four replicate SVET measurements was made for each sample type.Re-sults presented are representative of the set.Galvanic coupling measurements .—Galvanic coupling experi-ments were carried out in a two-compartment enclosed glass cell described in our previous work.15The cover of each compartment had four openings for inserting the working electrode,a Luggin capillary connected to a saturated calomel electrode,and gas in and gas out tubes.The working electrode in one compartment was bare AA 2024-T3͑subsequently referred to as the AA compartment ͒and the working electrode in the other compartment was CP-coated sub-strate ͑subsequently referred to as the CP compartment ͒,the sub-strate being either indium tin oxide ͑ITO ͒or AA 2024-T3.The ex-posed surface of the AA 2024-T3was a pinhole of 0.04cm 2͑simulating a coating defect ͒and the area of the CP was 1.0cm 2,yielding an area ratio ͑CP to AA ͒of ca.25.The working electrodes were electrically connected through a zero resistance ammeter ͑ZRA ͒and the mixed potential and the coupling current between the CP and the AA were measured simultaneously as functions of time using the Gamry PC4/300potentiostat in ZRA mode.To simulate the condition for a top-coated CP ͑where a top-coat would protect the CP from direct oxygen access ͒,the solution in the CP compart-ment was sparged with N 2,while the solution in the AA compart-ment was sparged with air.At least four replicate galvanic coupling measurements were performed for each sample type and results pre-sented are representative.Electrochemical impedance spectroscopy .—All electrochemical impedance spectroscopy ͑EIS ͒and Mott–Schottky measurements were performed at ambient temperature ͑ϳ20°C ͒in DHS sparged with N 2employing a saturated Ag/AgCl electrode as the reference electrode.EIS measurements on PC 6TP-coated AA 2024-T3͑exposed area of 1cm 2͒were carried out in an enclosed three-electrode cell using a Gamry PC4/300potentiostat/galvanostat ͑EIS300software ͒con-trolled by Gamry Framework Software ͑version 4.35͒.A spectrum was obtained at each of several dc base potentials ranging from the open-circuit potential ͑OCP,Ϫ0.78V ͒to Ϫ1.5V ,corresponding to PC 6TP films ranging from the ͑as cast ͒neutral state to the n-doped state.The sample was polarized at each base potential for 5min before the acquisition of the spectrum.The spectra were acquired over the frequency range of 105−0.1Hz at 10points per decade using an ac signal amplitude of 5mV root mean square ͑rms ͒.For Mott–Schottky analysis,the neutral PC 6TP-coated AA 2024-T3͑exposed area of 1cm 2͒was immersed in DHS for 1h to permit the OCP to stabilize,after which the potential was stepped to more negative values in 50mV steps,each of 7s duration ͑an effective scan rate of ca.7mV/s ͒.The capacitance was calculated at each step from the imaginary part of the impedance,21obtained us-ing a 1000Hz ac perturbation having an amplitude of 5mV rms.Scanning electron microscopy .—Scanning electron microscopy ͑SEM ͒and energy-dispersive X-ray ͑EDX ͒analysis images were obtained using a JEOL JSM-6300scanning electron microscope.X-ray information was obtained via a Thermo EDS detector using a V ANTAGE Digital Acquisition Engine at 15keV .Results and DiscussionNeutral PPy .—We previously described both SVET measure-ments 22and galvanic coupling experiments 15for the p-doped form of PPy and its interaction with AA 2024-T3.The SVET measure-ments revealed a well-defined oxidation current centered at the de-fect due to anodic dissolution of the alloy.Reduction current was rather uniformly distributed over the polymer surface,a combination of polymer reduction and oxygen reduction.15,22Hydrogen evolution typically accompanied the anodic current at the defect.The oxida-tion current at the defect gradually diminished after several hours as corrosion product built up in the defect area.This behavior distinctly differs from that observed for a common barrier coating,such as an epoxy coating,where both anodic and cathodic processes are con-fined to the defect and initiate almost immediately upon immersion.22-24The galvanic coupling measurements confirmed the SVET observations and demonstrated the role of oxygen in the gal-vanic interaction,including the ability of neutral PPy to react with oxygen.15In contrast to the behavior described above for the oxidized PPy,the alloy coated with neutral PPy initially exhibited strong reduction across most of the sample surface with oxidation current being ob-served only at the corners of the sample ͑Fig.2͒.Importantly,no oxidation was detected at the defect during the first 2h of the ex-periment.However,the total charge during this initial 2h period exhibited an imbalance,with cathodic charge exceeding anodic charge as apparent from Fig.2a .The reason for this imbalance is not entirely clear.There was no visual evidence of electrolyte seepage under the coating or under the tape mask ͑see micrograph in Fig.2a ͒.We speculate that during the first 2h,oxygen reacts directly with the polymer,oxidizing it to the p-doped form,resulting in the oxidation current observed at the corners.Normally a direct oxida-tion occurring uniformly across the polymer would be transparent to the SVET.This process does not occur uniformly as evidenced by Fig.2.Furthermore,if anion incorporation into the polymer during this initial oxygen doping process was sluggish,the SVETwouldFigure 1.Structure of the neutral forms of PPy ͑left ͒and PC 6TP ͑right ͒.C361Journal of The Electrochemical Society ,156͑10͒C360-C366͑2009͒detect a net cathodic current.Clearly this is an issue that needs to be examined further.Galvanic coupling experiments performed during the initial 2h did not reveal any significant coupling current.Thus,cathodic protection of the defect by the polymer does not appear to play a role.After 2h,the PPy was so sufficiently oxidized that it transitioned to the behavior described above for p-doped PPy and the total charge balanced for the remainder of the experiment ͑40h ͒.Figure 2c shows the SVET map after a 4h immersion,illustrating oxidation current centered at the defect and reduction current distributed over much of the polymer surface.Such a transition from the behavior in Fig.2a to that in Fig.2c suggests that the polymer ͑and not the metal ͒was being oxidized in Fig.2a .In another experiment,alloy coated with neutral PPy was exposed to air for 48h.SVET current maps for this sample obtained immediately upon immersion were similar to those obtained from freshly prepared p-doped PPy-coated alloy,further confirming the reactivity of PPy with oxygen.The PPy films used here were quite thin and were exposed directly to aerated electrolyte,so the initial protection was rather short lived.With a top coat,such oxygen scavenging films may afford a substantial degree of corrosion protection to Al alloys.Indeed,neutral ͑undoped ͒coatings of poly ͑3-octylpyrrole ͒25and of poly ͑2,5-bis ͑N -methyl-N -hexylamino ͒phenylenevinylene ͒26with top coats showed good corrosion protection of Al alloys,suggested to result from their oxygen scavenging ability.2N-doped poly(2,3-dihexylthieno[3,4-b]pyrazine).—A series of high purity dialkyl-functionalized poly ͑thieno ͓3,4-b ͔pyrazine ͒s was produced via electropolymerization.27The cyclic voltammetry of these polymers ͑including PC 6TP,the dihexyl polymer used in this work ͒was described previously,illustrating the ability to reduce ͑n dope ͒this class of polymer.We present here complementary data from EIS that supports our contention that the reduced polymer is an n-type semiconductor.Figure 3shows a series on Nyquist plots for PC 6TP-coated AA 2024-T3immersed in DHS obtained at various potentials,starting from the OCP and stepping to progressively morenegative potentials.The sample was polarized at each potential for 10min before the acquisition of the spectrum.Clearly the imped-ance of the coated alloy collapses as the polymer is reduced,indi-cating more facile charge transfer across the PC 6TP film at the more negative potentials.Following the experiment at the most negative potential,the sample was allowed to remain in the electrolyte at an open circuit ͑with continuous N 2flow above the electrolyte ͒.After 12h immersion at an open circuit,the impedance spectrum reverted to the OCP spectrum shown in Fig.3,indicating that the polymer was slowly oxidized by adventitious oxygen and/or by hydrogen ions from the slightly acidicDHS.Figure 2.͑Color online ͒SVET current density maps for the neutral ͑undoped ͒PPy deposited on AA 2024-T3with a de-fect near the center of the sample.͑a ͒Cur-rent map ͑left ͒and optical image with cur-rent vectors superimposed ͑right ͒obtained immediately after reduction of the PPy,0h.͑b ͒Current map after 1h immersion.͑c ͒Current map after 4himmersion.Figure 3.͑Color online ͒EIS ͑Nyquist plots ͒of PC 6TP on AA 2024-T3as a function of potential.Electrolyte:N 2-sparged DHS.Exposed area of 1cm 2,amplitude of 5mV ,and frequency range of 0.1–105Hz.Sample was polar-ized at each potential for 5min before the acquisition of the spectrum.C362Journal of The Electrochemical Society ,156͑10͒C360-C366͑2009͒An expanded view of the spectrum at Ϫ1300mV is shown inFig.4͑in Bode format ͒along with a theoretical fit obtained from the equivalent circuit shown in the figure ͑each Q in the circuit repre-sents a constant phase element ͒.This circuit is very similar to model circuits used in the previous work to model the semiconducting properties of CPs exhibiting a space charge capacitance.21,28Fur-thermore,the magnitude of the impedance and the frequency depen-dence observed in Fig.3and 4for n-doped PC 6TP are quite similar to that observed for n-doped poly ͓4,4Ј-bis ͑methylsulfanyl ͒-2,2Ј-bithiophene ͔.29Table I summarizes the values for the model elements used to generate the fit shown in Fig.4.The electrolyte resistance is modeled by R s .The CP impedance is modeled by Q cp ͑sc ͒͑representing the CP film capacitance having a space charge component ͒,a resistance R cp ͑representing the CP film resistance ͒,and Q d ͑representing a diffusion process,usually associated with ion diffusion within the polymer ͒.The contribution of Q d is apparent at the lowest frequencies of Fig.4.The CP-Al alloy interface is mod-eled by Q cp /Al ͑sc ͒and a charge transfer resistance ͑R ct ͒.The capaci-tance at this interface might also have a space charge contribution.28Evidence for the space charge capacitance is shown in the Mott–Schottky plots of Fig.5obtained at a frequency of 1kHz.The bare alloy shows little contribution to the space charge capacitance at thisfrequency,whereas the PC 6TP-coated alloy exhibits a pronounced linear region that follows the Mott–Schottky equation 30C −2=2␧␧0eN D A 2ͩE −E FB−kTeͪwhere C is the capacitance,E is the potential,N D is the charge carrier density,E FB is the flatband potential,and the other symbols have their usual meanings.The positive slope of the plot indicates that the polymer behaves as an n-type semiconductor.Assuming that the space charge capacitance is the smallest ͑thus,most dominant ͒contribution to the measured capacitance,as is usually the case,31and assuming a dielectric constant ␧of 10for the polymer,a rea-sonable value for CPs 28and other semiconducting materials,32a flatband potential of Ϫ1.78V and a charge carrier density of 1ϫ1019cm −3are estimated from the plot ͑the kT /e term is negli-gible at room temperature ͒.We found no reports of charge carrier densities for n-type CPs,but for p-type CPs,the charge carrier den-sity ranges from ca.1015to ca.1019cm −3,dependent on dopant ion,metal substrate,and pH.21Our estimate is similar to those reported for PPy on stainless steel ͑0.7ϫ1019cm −3͒,33polyaniline on plati-num ͑1.4ϫ1019cm −3͒,34and polybithiophene on Pt ͑2.2ϫ1019cm −3͒.35Figure 6shows the SVET current density map for AA 2024-T3coated with the n-doped PC 6TP after 5min of immersion in DHS.A rather intense reduction current is observed in the center of the sample at the location of the defect,presumably due to oxygen reduction at the exposed alloy surface ͑no bubbles suggestive of the hydrogen evolution reaction were observed in the defect area ͒.Oxi-dation current is distributed more or less evenly over the polymer surface,attributed to polymer oxidation from the n-doped state to the neutral state.This result strongly suggests that the n-dopedpoly-Figure 4.͑Color online ͒EIS ͑Bode plot ͒of PC 6TP on AA 2024-T3at a potential of Ϫ1300mV ͑conditions as in Fig.3͒.Points are experimental;lines are from a theoretical fit using the equivalent circuit shown ͑curve fitting was performed using ZSimpWin from Echem software;the average RSD of the fit was 0.4%͒.Table I.Model circuit parameters from the least-squares fit shown in Fig.4.SymbolR s ͑⍀͒Q cp ͑sc ͒͑␮S s n ͒n cp R cp ͑⍀͒Q d ͑␮S s n ͒n d Q cp /Al ͑sc ͒͑␮S s n ͒n cp /Al R ct ͑⍀͒Value ͑RSD,a %͒44.8͑7.3͒8.90͑16͒0.54͑2.7͒394͑4.3͒2100͑9.9͒0.50͑fixed ͒4.84͑1.8͒0.77͑0.50͒7130͑0.62͒aRelative standarddeviation.Figure 5.͑Color online ͒Mott–Schottky plots for a 2␮m PC 6TP coating on AA 2024-T3͑᭺͒and for bare AA 2024-T3͑ᮀ͒immersed in DHS.Potential scanned from positive to negative in steps of 50mV with step durations of 7s.Exposed area of 1cm 2,amplitude of 5mV rms,and frequency of 1000Hz.C363Journal of The Electrochemical Society ,156͑10͒C360-C366͑2009͒mer cathodically protects the alloy exposed at the defect.To the best of our knowledge,this is the first example of sacrificial protection of a metal by a fully organic coating.Because the coating was rather thin ͑ca.2␮m ͒and was exposed to aerated electrolyte in the SVET cell ͑it is difficult to exclude oxygen in the SVET experiments ͒,the direct oxidation of the polymer by oxygen also occurs,reducing the lifespan of the protection in these experiments to ca.30min,at which time the behavior transitioned to that observed for the neutral PC 6TP,described in the next section.The influences of a top coat and film thickness on delaying this transition are currently being assessed.To further verify that the behavior observed in the SVET experi-ment was due to galvanic coupling between the polymer and the alloy,galvanic coupling measurements were performed in the two-compartment cell described earlier.15This cell permitted careful control of the oxygen levels in each electrode compartment.Figure 7shows the result for such a measurement performed with aerated DHS in the AA 2024-T3compartment and nitrogen-sparged DHS in the PC 6TP compartment.These conditions reduced the exposure of the n-doped PC 6TP to oxygen ͑and thus to direct oxidation ͒while permitting oxygen to interact with the exposed alloy surface in theAA compartment.Figure 7verifies that cathodic protection of the alloy was provided by the polymer and was maintained for several hours,evidenced by both the reduction current occurring in the AA compartment and the negative shift in the alloy potential by ca.200mV when it was coupled with the polymer.After a few hours,the polymer was oxidized to its neutral form,although it continued to provide protection of exposed alloy,as evidenced in Fig.7and discussed in the next section.Neutral poly(2,3-dihexylthieno[3,4-b]pyrazine).—Unlike neu-tral PPy,the neutral PC 6TP did not appear to undergo oxygen dop-ing,even though PC 6TP oxidizes at a rather modest potential in cyclic voltammetry ͑the onset of the oxidation wave in acetonitrile /Bu 4NPF 6is ca.0.5V vs Ag wire QRE ͒.20However,electrochemical interactions between the neutral PC 6TP and the Al alloy did occur.Figure 8shows a series of SVET current density maps obtained above neutral PC 6TP-coated AA2024-T3over a 3day immersion ͑as in previous SVET experiments,a scratch through the coating to the alloy surface was introduced ͒.No significant cur-rent flow was detected within the scan area during the first 3h of immersion.After ca.3h,a small oxidation current appeared above the coating near the defect ͑Fig.8a ͒.With time,a second ͑Fig.8b ͒and a third ͑Fig.8c ͒oxidation peak appeared,accompanied by an increasing reduction current in the defect area.In all replicates of this SVET experiment,the anodic current was localized above coated regions of the substrate,not in the defect area where the substrate was exposed directly to the aggressive electrolyte.The cathodic current was localized at the defect,and the defect remained shiny throughout the experiment ͑Fig.8d ͒.These observations sug-gest that the exposed defect area was somehow cathodically pro-tected by the ͑initially ͒neutral form of PC 6TP.Further evidence that cathodic protection of the exposed alloy occurred was obtained from galvanic coupling experiments.As before,these measurements were performed with aerated DHS in the AA 2024-T3compartment and nitrogen-sparged DHS in the neutral PC 6TP compartment.As ob-served in the SVET experiments,there was an initial period ͑2–3h ͒where little or no current flowed,followed by increasing cathodic current at the bare alloy accompanied by a negative shift in the mixed potential ͑Fig.7͒.Initially the current for the n-doped PC 6TP was greater than that observed with the neutral PC 6TP ͑Fig.7͒as expected,but after ca.25h,the currents were comparable.The n-doped PC 6TP,once it lost its excess negative charge,transitioned to the neutral form,at which point it continued to cathodically pro-tect the defect area.The cathodic protection observed in Fig.7and 8continued for up to 10days,the longest experiments performed to date.The mechanism of protection by the n-doped PC 6TP is easily understood because the n-doped form of the polymer is a strong reducing agent.But how does the neutral form of the polymer provide cathodic protection to the defect area?One possibility is that the PC 6TP undergoes n doping ͑reduction ͒by interaction with the alloy surface,most likely involving dealloying of Mg and Al containing S-phase particles because they are the most active sites on the alloy surface.36Both Al ͑E o =−1.66V ͒and Mg ͑E o =−2.37V ͒are sufficiently strong reducing agents to reduce ͑n dope ͒the PC 6TP.The PC 6TP is capable of complexing metal ions via the nitrogen atoms on the pyrazine rings,27providing an additional thermody-namic driving force for the reaction.The metal ions could also serve as the dopant ions for the n-doping process.Alternatively,electrons released by oxidation of Mg and/or Al may be directly consumed by oxygen reduction in the defect,a pathway that would not involve n doping of the polymer.At the present time we cannot distinguish between these mechanisms.Support for this conjecture comes from galvanic coupling experiments ͑not shown ͒that were conducted as in Fig.7but employing conducting glass ͑ITO ͒in place of AA 2024-T3as the substrate in the PC 6TP compartment.In these ex-periments,the neutral PC 6TP-coated ITO exhibited no cathodiccur-Figure 6.͑Color online ͒SVET current density map showing cathodic pro-tection of AA 2024-T3by an n-doped PC 6TP film containing an artificial defect ͑scratch ͒after 5min immersion in DHS ͑reduction current is nega-tive ͒.Figure 7.͑Color online ͒Coupling current ͑top ͒and mixed potential ͑bot-tom ͒for bare AA 2024-T3͑exposed through a pinhole simulating a coating defect ͒coupled with either neutral or n-doped PC 6TP-coated alloy in DHS.AA compartment:0.04cm 2exposed area in aerated DHS;CP compartment:1cm 2exposed area in nitrogen-sparged DHS.The negative coupling current signifies reduction in the AA compartment.The OCP of the uncoupled alloy in air-saturated DHS was Ϫ0.51V .C364Journal of The Electrochemical Society ,156͑10͒C360-C366͑2009͒。