Journal of The Electrochemical Society, 156 共4兲 C135-C146 共2009兲
`0013-4651/2009/156共4兲/C135/12/$23.00 © The Electrochemical Society
`
`C135
`
`Electrochemical Evaluation of Constituent Intermetallics
`in Aluminum Alloy 2024-T3 Exposed to Aqueous
`Vanadate Inhibitors
`K. D. Ralston,a,*,z T. L. Young,b and R. G. Buchheita,**
`
`aFontana Corrosion Center, Department of Materials Science and Engineering and bDepartment
`of Chemistry, The Ohio State University, Columbus, Ohio 43210, USA
`
`Experiments were conducted to determine how inhibiting forms of vanadate interact with complex Al alloy microconstituent
`intermetallics to impart corrosion protection. Cathodic polarization experiments on Al 2024-T3 indicate a strong correlation
`between inhibition and the presence of tetrahedrally coordinated vanadate. Anodic and cathodic polarization curves were measured
`on bulk synthesized Al2Cu, Al2CuMg, Al7Cu2Fe, and Al20Cu2Mn3 in alkaline 0.5 M NaCl solutions with and without 10 mM
`NaVO3. Vanadate additions generally decreased Ecorr, increased Epit, and decreased the cathodic kinetics of all tested materials.
`Because of decreased cathodic kinetics, open-circuit potentials 共OCPs兲 were shifted in the active direction in aerated solutions
`when vanadate was present. This shift pins the OCP just below the observed pitting potential for Al2CuMg in vanadate solution,
`effectively preventing breakdown and subsequent support of rapid oxygen reduction by Cu-enriched clusters. Ecorr, Epit, Erp, icorr,
`ipass, and i at −1.3 VSCE data from polarization experiments were summarized in cumulative distribution plots, and averages are
`presented in tabulated format. Scanning electron microscopy images of Al 2024-T3 used for 4 h of OCP measurement show that
`vanadate greatly decreased circumferential trenching around intermetallic particles in both aerated and deaerated solutions. Po-
`tentiostatic hold experiments were used to show suppression of Al2CuMg dissolution in vanadate solutions.
`关DOI: 10.1149/1.3076147兴 All rights reserved.
`© 2009 The Electrochemical Society.
`
`Manuscript submitted October 6, 2008; revised manuscript received January 5, 2009. Published February 9, 2009.
`
`Aluminum 2024-T3 is a high-strength age-hardened aluminum
`alloy commonly used in the aerospace industry. Al 2024-T3 con-
`tains, by weight percent, 3.8–4.9 Cu, 1.2–1.8 Mg, 0.3–0.9 Mn, and
`small quantities of Si, Fe, Zn, Cr, and Ti.1 Alloy additions result in
`both superior mechanical properties and a heterogeneous micro-
`structure which renders the alloy susceptible to localized corrosion.1
`Appreciable quantities of copper, magnesium, and manganese,
`added as strengtheners, remain in solid solution. However, through
`heat-treatments and natural aging, a dispersion of fine Cu and Mg
`particles and insoluble intermetallic precipitates form within the ma-
`trix phase. The main constituent particles in Al 2024-T3 include
`Al2CuMg, Al7Cu2Fe, Al2Cu, and Al20Cu2Mn3. The effect of inter-
`metallic particles on corrosion of aluminum alloys has been widely
`studied.2-11 For Al 2024-T3, it has been found that intermetallics
`containing Cu are typically noble or become noble to the surround-
`ing aluminum matrix during exposure to many electrolytes, and
`these
`particles
`are
`capable
`of
`supporting
`rapid
`cathodic
`kinetics.4,5,8,9,12 Such cathodic particles drive corrosion in the sur-
`rounding matrix,
`leading
`to
`pitting
`and
`trenching
`attack
`morphologies.10,11,13 Al2CuMg 共S phase兲 intermetallic particles are
`of particular interest, because Al2CuMg is one of the most abundant
`intermetallic particles found in Al 2024-T3 and in large part has
`been found to be responsible for susceptibility of Al 2024-T3 to
`localized corrosion.4,5 The corrosion of Al2CuMg is complex; under
`free-corrosion conditions, the intermetallic is initially anodically po-
`larized by the matrix, leading to selective dissolution of Mg from the
`intermetallic and nonfaradaic liberation of Cu, which can then be
`oxidized to form ions that can be reduced on the surrounding
`matrix.4,5 Often what remains of the particle is an enriched Cu rem-
`nant, which acts as a local cathode, supporting rapid oxygen reduc-
`tion and corrosion in the surrounding matrix.4,5 Prevention of Mg
`dissolution from Al2CuMg and, as a result, the subsequent forma-
`tion of local Cu cathodes capable of supporting rapid oxygen reduc-
`tion could be an effective way to increase the resistance of Al
`2024-T3 to localized corrosion.14
`Historically, chromate-based pigments and coatings have been
`used successfully to prevent corrosion of aluminum alloys.15 How-
`ever, due to environmental and carcinogenic risks associated with
`chromate use, more “green” alternative inhibitors and coatings have
`
`* Electrochemical Society Student Member.
`** Electrochemical Society Active Member.
`z E-mail: ralston.34@osu.edu
`
`soluble vanadates,
`In particular,
`recently received attention.
`vanadate-based coatings, and inhibitor pigments have been observed
`to inhibit the corrosion of aluminum alloys and have shown promise
`as chromate replacements.16-23 However, unlike chromates, aqueous
`vanadates have a relatively complex aqueous chemistry, dependent
`on pH, concentration, and ionic strength.24-26 This convolutes a
`straight forward understanding of inhibition. In a simplified descrip-
`tion of aqueous vanadate speciation, tetrahedrally coordinated spe-
`cies, metavanadates, and pyrovanadates, predominate in alkaline so-
`lutions,
`octahedrally
`coordinated
`species,
`decavanadates,
`predominate in acid solutions, and single tetrahedral species exist
`over a wide pH range at low concentrations. Previous work has
`shown that the extent of inhibition depends strongly on vanadate
`speciation, with the greatest inhibition from tetrahedrally coordi-
`nated species, which are predominant in alkaline solutions.18,20-22,27
`Tetrahedrally coordinated vanadates have been shown to act prima-
`rily through decreased oxygen reduction; however, vanadates have
`also been observed to be modest anodic inhibitors independent of
`aeration.18,22 Decavanadate ions, which are combinations of 10 oc-
`tahedrally coordinated vanadate units, are predominate in acidic so-
`lutions of appropriate vanadium concentration and have been shown
`to be poor inhibitors of oxygen reduction.18,20-22,24,25,27 There is evi-
`dence that decavanadate increases the cathodic kinetics in acidic
`NaCl solutions.18 However, small increases in pH, as found near
`sites supporting oxygen reduction, can trigger the decomposition of
`noninhibiting octahedrally coordinated species into inhibiting tetra-
`hedral species, which helps explain corrosion protection observed
`from pigments containing decavanadate.16,18,28,29
`Evidence exists that inhibiting tetrahedrally coordinated vana-
`dates suppress the dissolution of Al2CuMg intermetallics.18,21,27
`Ralston et al. noted suppressed Mg dissolution from Cu–Mg par-
`ticles exposed to alkaline and mildly acidic 50 mM NaCl solutions
`with NaVO3 compared to particles exposed to NaVO3-free
`solutions.18 Iannuzzi and Frankel used in situ atomic force micros-
`copy scratching to observe that additions as small as 0.1 mM of
`metavanadate to 0.5 M NaCl suppressed the attack of Al2CuMg
`particles, while corrosion in the surrounding matrix was still
`observed.27 Iannuzzi further found that 5 mM of metavanadate pre-
`vented transient Al2CuMg dissolution, resulting in increased corro-
`sion resistance at open-circuit potential 共OCP兲.20,21 The mechanism
`of suppression of transient dissolution was not clear, but it was
`speculated that monovanadates on the matrix prevent or displace Cl−
`adsorption on the surface, which hinders subsequent oxide film
`breakdown.20 Generally, it is not certain whether vanadates slow
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`

`Journal of The Electrochemical Society, 156 共4兲 C135-C146 共2009兲
`0013-4651/2009/156共4兲/C135/12/$23.00 © The Electrochemical Society
`
`C135
`
`Electrochemical Evaluation of Constituent Intermetallics
`in Aluminum Alloy 2024-T3 Exposed to Aqueous
`Vanadate Inhibitors
`K. D. Ralston,a,*,z T. L. Young,b and R. G. Buchheita,**
`
`aFontana Corrosion Center, Department of Materials Science and Engineering and bDepartment
`of Chemistry, The Ohio State University, Columbus, Ohio 43210, USA
`
`Experiments were conducted to determine how inhibiting forms of vanadate interact with complex Al alloy microconstituent
`intermetallics to impart corrosion protection. Cathodic polarization experiments on Al 2024-T3 indicate a strong correlation
`between inhibition and the presence of tetrahedrally coordinated vanadate. Anodic and cathodic polarization curves were measured
`on bulk synthesized Al2Cu, Al2CuMg, Al7Cu2Fe, and Al20Cu2Mn3 in alkaline 0.5 M NaCl solutions with and without 10 mM
`NaVO3. Vanadate additions generally decreased Ecorr, increased Epit, and decreased the cathodic kinetics of all tested materials.
`Because of decreased cathodic kinetics, open-circuit potentials 共OCPs兲 were shifted in the active direction in aerated solutions
`when vanadate was present. This shift pins the OCP just below the observed pitting potential for Al2CuMg in vanadate solution,
`effectively preventing breakdown and subsequent support of rapid oxygen reduction by Cu-enriched clusters. Ecorr, Epit, Erp, icorr,
`ipass, and i at −1.3 VSCE data from polarization experiments were summarized in cumulative distribution plots, and averages are
`presented in tabulated format. Scanning electron microscopy images of Al 2024-T3 used for 4 h of OCP measurement show that
`vanadate greatly decreased circumferential trenching around intermetallic particles in both aerated and deaerated solutions. Po-
`tentiostatic hold experiments were used to show suppression of Al2CuMg dissolution in vanadate solutions.
`关DOI: 10.1149/1.3076147兴 All rights reserved.
`© 2009 The Electrochemical Society.
`
`Manuscript submitted October 6, 2008; revised manuscript received January 5, 2009. Published February 9, 2009.
`
`Aluminum 2024-T3 is a high-strength age-hardened aluminum
`alloy commonly used in the aerospace industry. Al 2024-T3 con-
`tains, by weight percent, 3.8–4.9 Cu, 1.2–1.8 Mg, 0.3–0.9 Mn, and
`small quantities of Si, Fe, Zn, Cr, and Ti.1 Alloy additions result in
`both superior mechanical properties and a heterogeneous micro-
`structure which renders the alloy susceptible to localized corrosion.1
`Appreciable quantities of copper, magnesium, and manganese,
`added as strengtheners, remain in solid solution. However, through
`heat-treatments and natural aging, a dispersion of fine Cu and Mg
`particles and insoluble intermetallic precipitates form within the ma-
`trix phase. The main constituent particles in Al 2024-T3 include
`Al2CuMg, Al7Cu2Fe, Al2Cu, and Al20Cu2Mn3. The effect of inter-
`metallic particles on corrosion of aluminum alloys has been widely
`studied.2-11 For Al 2024-T3, it has been found that intermetallics
`containing Cu are typically noble or become noble to the surround-
`ing aluminum matrix during exposure to many electrolytes, and
`these
`particles
`are
`capable
`of
`supporting
`rapid
`cathodic
`kinetics.4,5,8,9,12 Such cathodic particles drive corrosion in the sur-
`rounding matrix,
`leading
`to
`pitting
`and
`trenching
`attack
`morphologies.10,11,13 Al2CuMg 共S phase兲 intermetallic particles are
`of particular interest, because Al2CuMg is one of the most abundant
`intermetallic particles found in Al 2024-T3 and in large part has
`been found to be responsible for susceptibility of Al 2024-T3 to
`localized corrosion.4,5 The corrosion of Al2CuMg is complex; under
`free-corrosion conditions, the intermetallic is initially anodically po-
`larized by the matrix, leading to selective dissolution of Mg from the
`intermetallic and nonfaradaic liberation of Cu, which can then be
`oxidized to form ions that can be reduced on the surrounding
`matrix.4,5 Often what remains of the particle is an enriched Cu rem-
`nant, which acts as a local cathode, supporting rapid oxygen reduc-
`tion and corrosion in the surrounding matrix.4,5 Prevention of Mg
`dissolution from Al2CuMg and, as a result, the subsequent forma-
`tion of local Cu cathodes capable of supporting rapid oxygen reduc-
`tion could be an effective way to increase the resistance of Al
`2024-T3 to localized corrosion.14
`Historically, chromate-based pigments and coatings have been
`used successfully to prevent corrosion of aluminum alloys.15 How-
`ever, due to environmental and carcinogenic risks associated with
`chromate use, more “green” alternative inhibitors and coatings have
`
`* Electrochemical Society Student Member.
`** Electrochemical Society Active Member.
`z E-mail: ralston.34@osu.edu
`
`soluble vanadates,
`In particular,
`recently received attention.
`vanadate-based coatings, and inhibitor pigments have been observed
`to inhibit the corrosion of aluminum alloys and have shown promise
`as chromate replacements.16-23 However, unlike chromates, aqueous
`vanadates have a relatively complex aqueous chemistry, dependent
`on pH, concentration, and ionic strength.24-26 This convolutes a
`straight forward understanding of inhibition. In a simplified descrip-
`tion of aqueous vanadate speciation, tetrahedrally coordinated spe-
`cies, metavanadates, and pyrovanadates, predominate in alkaline so-
`lutions,
`octahedrally
`coordinated
`species,
`decavanadates,
`predominate in acid solutions, and single tetrahedral species exist
`over a wide pH range at low concentrations. Previous work has
`shown that the extent of inhibition depends strongly on vanadate
`speciation, with the greatest inhibition from tetrahedrally coordi-
`nated species, which are predominant in alkaline solutions.18,20-22,27
`Tetrahedrally coordinated vanadates have been shown to act prima-
`rily through decreased oxygen reduction; however, vanadates have
`also been observed to be modest anodic inhibitors independent of
`aeration.18,22 Decavanadate ions, which are combinations of 10 oc-
`tahedrally coordinated vanadate units, are predominate in acidic so-
`lutions of appropriate vanadium concentration and have been shown
`to be poor inhibitors of oxygen reduction.18,20-22,24,25,27 There is evi-
`dence that decavanadate increases the cathodic kinetics in acidic
`NaCl solutions.18 However, small increases in pH, as found near
`sites supporting oxygen reduction, can trigger the decomposition of
`noninhibiting octahedrally coordinated species into inhibiting tetra-
`hedral species, which helps explain corrosion protection observed
`from pigments containing decavanadate.16,18,28,29
`Evidence exists that inhibiting tetrahedrally coordinated vana-
`dates suppress the dissolution of Al2CuMg intermetallics.18,21,27
`Ralston et al. noted suppressed Mg dissolution from Cu–Mg par-
`ticles exposed to alkaline and mildly acidic 50 mM NaCl solutions
`with NaVO3 compared to particles exposed to NaVO3-free
`solutions.18 Iannuzzi and Frankel used in situ atomic force micros-
`copy scratching to observe that additions as small as 0.1 mM of
`metavanadate to 0.5 M NaCl suppressed the attack of Al2CuMg
`particles, while corrosion in the surrounding matrix was still
`observed.27 Iannuzzi further found that 5 mM of metavanadate pre-
`vented transient Al2CuMg dissolution, resulting in increased corro-
`sion resistance at open-circuit potential 共OCP兲.20,21 The mechanism
`of suppression of transient dissolution was not clear, but it was
`speculated that monovanadates on the matrix prevent or displace Cl−
`adsorption on the surface, which hinders subsequent oxide film
`breakdown.20 Generally, it is not certain whether vanadates slow
`
`Downloaded on 2017-05-08 to IP
`
`66.192.64.35
`
`
`
` address. Redistribution subject to ECS terms of use (see ) unless CC License in place (see abstract). ecsdl.org/site/terms_use
`
`
`
`

`

`
`
`C136C136
`
`Journal of The Electrochemical Society, 156 共4兲 C135-C146 共2009兲
`
`corrosion of Al 2024-T3 through acting exclusively on Al2CuMg or
`if suppressed Al2CuMg dissolution is a consequence of overall cor-
`rosion inhibition. Although previous work has established that solu-
`tions containing predominately tetrahedrally coordinated vanadates
`prevent or slow Mg dissolution from Al2CuMg, and in turn the
`formation of Cu-rich cathodes capable of supporting rapid oxygen
`reduction, the precise relationship between tetrahedral vanadates,
`Al2CuMg, and the matrix is not currently understood.
`Vanadate is a known buffer, and observed inhibition must be
`rationalized in the context of the effects that buffers have on corro-
`sion. The presence of a buffer can have pronounced effects on the
`corrosion of Al–Cu alloys.30,31 In unbuffered systems, oxygen re-
`duction results in an increase in alkalinity at cathodic sites, which
`dissolves the surrounding Al matrix, leading to shallow grooving
`and trenching around intermetallics and, occasionally, widespread
`cathodic corrosion across the matrix.30,31 When a buffer is present,
`local alkalinization and associated cathodic corrosion damage
`modes are suppressed. However, when these modes are suppressed,
`the cathodic reaction is then available to support penetrating local-
`ized corrosion sites such as acid pits and crevices.30,31 This leads to
`deep and discrete pits but comparatively less mass loss than is ob-
`served in unbuffered systems.30
`Due to the small size of constituent intermetallic particles present
`in Al 2024-T3, direct electrochemical testing of different intermetal-
`lics in the matrix is not feasible. However, previous work on alumi-
`num alloys using a microcapillary electrode and synthesized “bulk”
`intermetallics demonstrates that intermetallic-specific electrochemi-
`cal data can be obtained.2,3,32 The microcell is a modified standard
`three-electrode setup that uses a thin, glass, silicon-coated capillary
`connected to an electrolyte reservoir, containing both a reference
`electrode and counter electrode to contact and allow electrochemical
`experiments on micrometer-scale-diameter working electrodes.33,34
`General details of the microcell setup and a specific description of
`the microcell used for this work can be found in the literature.3,33,34
`By choosing bulk intermetallics that are representative of constitu-
`ents in Al 2024-T3, pure Al and Cu, and an Al 4% Cu solid solution
`used as a matrix analog, electrochemical characteristics of specific
`intermetallic phases can be catalogued and used to rationalize ob-
`served behavior of the bulk alloy.
`The objective of this work is to determine how inhibiting vana-
`dates interact with the matrix and constituent particles of Al 2024-
`T3. In addition, this work aims to develop a deeper understanding of
`the suppression of Mg dissolution from Al2CuMg intermetallics in
`inhibiting vanadate solutions.
`
`Experimental
`
`Solution preparation.— Solutions for all experiments were pre-
`pared using reagent-grade chemicals. The NaVO3 for solution
`preparation was purchased from Fluka Chemika with an assay of
`艌98%. Cathodic polarization experiments were conducted in 0.5 M
`NaCl solutions adjusted to pH 5.1 using HCl with 0.25 and
`0.0025 M NaVO3 to show the effect that tetrahedral vanadates have
`on inhibition compared to octahedral vanadates. However, most ex-
`periments were conducted in alkaline 0.5 M NaCl solutions with
`and without 10 mM NaVO3 to characterize the inhibitive effect tet-
`rahedrally coordinated vanadates have on different constituent inter-
`metallics in Al 2024-T3. The initial as-dissolved 0.5 M NaCl
`+ 10 mM NaVO3 solution was yellow and had a pH of 6.37. As
`previously mentioned, vanadates have been shown to provide the
`strongest inhibition when coordinated tetrahedrally, which occurs in
`alkaline solutions. As a result, the pH of the master test solution was
`adjusted with dropwise additions of 10 N NaOH until
`the pH
`reached 9.18. Before experimentation, the test solution was allowed
`to equilibrate for more than 2 weeks, during which a few additional
`drops of NaOH were used to maintain the pH above 9. Once the pH
`was stable, nuclear magnetic resonance 共NMR兲 was used to charac-
`terize the solution. To help ensure that the vanadate species in solu-
`tion were not evolving with time, the solution pH was monitored
`
`daily over the course of microcell experimentation, and NMR spec-
`tra were collected prior to the first experiment and after completion
`of the last experiment. The same 0.5 M NaCl + 10 mM NaVO3
`solution was used for subsequent experiments after completion of
`the microcell polarization work. Because the solution appeared
`stable over the duration of microcell experiments, pH was used as a
`sufficient measure of solution and species stability. Although the
`NaVO3 solution remained stable, the pH of NaCl solutions adjusted
`to approximately pH 9.2 decreased with time. This is likely the
`result of H2CO3 formation from dissolved atmospheric CO2 and the
`− and
`subsequent proton formation from equilibria involving HCO3
`2−, which both have increased solubility in alkaline solutions.35
`CO3
`As a result, care was taken to monitor and measure the pH imme-
`diately prior to and during each testing session. Unless specifically
`stated, the pH of NaCl-only solutions was between 9.05 and 9.20.
`Additionally, the reservoir, capillary, and tubing of the microcell
`were frequently flushed with fresh solution throughout experimen-
`tation.
`
`NMR.— Vanadates have a complex aqueous speciation depend-
`ing on both concentration and pH, and as a result, small changes in
`pH can have dramatic effects on the type and concentration of spe-
`cific species in solution. NMR was used to characterize solutions
`used for cathodic polarization experiments on an Al 2024-T3 sheet
`in pH 5.1 NaVO3 solutions and microcell polarization experiments
`in alkaline 10 mM NaVO3 solution, which were expected to take a
`number of weeks to complete and for which the possibility of solu-
`tion evolution with time was a concern. After allowing approxi-
`mately 2 weeks for solution stabilization, NMR spectra were col-
`lected immediately prior to microcell work in vanadate solutions
`and 10 days later after completion of experimentation. NMR spectra
`for cathodic polarization experiments in pH 5.1 solutions were col-
`lected immediately after the pH of the vanadate solutions was ad-
`justed. A Bruker DPX 400 MHz superconducting magnet was used
`to collect high-resolution 51V 共105.2 MHz兲 NMR spectra. An indi-
`rect detection probe was used with a 90° pulse duration of 10.38 ␮s.
`Spectra were collected using 8192 transients, a spectral window of
`73,529 Hz, a 0.051 s acquisition time, and a 0.20 s relaxation delay.
`Each spectrum had the subsequent process parameters applied:
`10.0 Hz line broadening, zero filling 共25 K points兲, and baseline
`correction. A solution consisting of 20% v/v VOCl3 in C6D6
`共␦51V = 0 ppm兲 was used as an external standard to reference the
`51V chemical shifts. Peaks were identified by comparison to
`literature.26,29,36
`
`the microcapillary
`using
`polarization
`Potentiodynamic
`electrode.— To characterize the inhibitive effects of tetrahedrally
`coordinated vanadates, anodic and cathodic polarization curves were
`collected on representative bulk versions of intermetallics found in
`Al 2024-T3 in alkaline 10 mM NaVO3 + 0.5 M NaCl and
`NaVO3-free 0.5 M NaCl solutions. Samples used for potentiody-
`namic polarization experiments using the microcell were sourced
`from previous work and commercial suppliers. The 99.999% Al and
`99.9% Cu samples were obtained from Alfa Aesar. The intermetallic
`samples used in this study were prepared and studied previously:
`Al7Cu2Fe and Al20Cu2Mn,32 Al 4%Cu,3 Al2CuMg,4 and Al2Cu.37
`Samples were ground in 200 proof ethyl alcohol to 1 ␮m using SiC
`grinding papers, followed by polishing using 6 and 1 ␮m diamond
`pastes. All electrochemical experiments presented in this paper were
`made using an Autolab PGSTAT 100 potentiostat in conjunction
`with General Purpose Electrochemical Systems data-acquisition
`software. Both anodic and cathodic polarization experiments were
`preceded by 30 s of OCP measurement and were carried out in
`aerated solutions using a 0.01 V /s scan rate. Anodic polarization
`curves were initiated at −0.03 V vs OCP and reversed at either
`0.0 V vs OCP or manually at approximately 0.05 V above any ob-
`served breakdown. Cathodic polarization experiments were initiated
`at 0.03 V vs OCP and terminated at −2.0 V vs saturated calomel
`electrode 共SCE兲, although capillary tip leaking often resulted in
`
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`
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`
`
`
` address. Redistribution subject to ECS terms of use (see ) unless CC License in place (see abstract). ecsdl.org/site/terms_use
`
`
`
`

`

`Journal of The Electrochemical Society, 156 共4兲 C135-C146 共2009兲
`
`
`
`C137C137
`
`early termination of the experiment. The contact area used for area
`normalization of data was estimated from digital images taken after
`individual experiments.
`
`Electrochemical experiments on bulk Al 2024-T3 sheet.— Experi-
`ments on bulk Al 2024-T3 electrodes in alkaline 0.5 M NaCl solu-
`tions with and without 10 mM NaVO3 were used to obtain electro-
`chemical data from the actual alloy for comparison to microcell
`results. Experiments in pH 5.1 NaVO3 solutions were used to show
`the inhibiting effect that tetrahedrally coordinated vanadates have on
`cathodic kinetics compared to octahedrally coordinated vanadates.
`Further, work on bulk Al 2024-T3 was used to gain insight into
`Al2CuMg dissolution. An Al 2024-T3 sheet was used for four dif-
`ferent sets of experiments, cathodic polarization, OCP measurement,
`anodic polarization, and potentiostatic experiments. All samples
`were polished by hand in 200 proof ethyl alcohol to at least 1 ␮m in
`a similar fashion as discussed for microcell sample preparation ex-
`cept for the samples for OCP measurements, which were polished to
`1 /4 ␮m using diamond paste, the samples for potentiostatic experi-
`ments, which were polished to 1 ␮m using a diamond suspension,
`and an automatic polisher rather than diamond paste by hand, and
`the samples used for cathodic polarization, which were polished to
`1200 grit under ethyl alcohol. Experiments on a bulk Al 2024-T3
`sheet were carried out using a standard three-electrode setup, which
`included a SCE reference, a platinum counter electrode mesh, and
`1 cm2 exposed working electrode. Cathodic polarization experi-
`ments in actively aerated pH 5.1 NaVO3 solutions were preceded by
`a 30 min measurement of OCP. The scan was initiated at 0.03 V vs
`OCP, and a scan rate of 0.5 mV/s was used. OCP was measured for
`4 h in actively aerated and deaerated 0.5 M NaCl solutions at ap-
`proximately pH 9.2 with and without 10 mM NaVO3 to determine
`the effect of tetrahedrally coordinated vanadates on OCP with time.
`For deaerated OCP measurements, solutions were deaerated for 1 h
`before the electrolyte came in contact with the sample. Anodic po-
`larization curves on the Al 2024-T3 sheet in 0.5 M NaCl solution
`with 10 mM NaVO3 solution at approximately pH 9.17 were used to
`make comparisons between microcell data and data collected from
`the Al 2024-T3 sheet. Anodic polarization experiments were pre-
`ceded by a 30 min measurement of OCP. The scan was initiated at
`−0.03 V vs OCP, and a scan rate of 0.5 mV/s was used with scan
`reversal at −0.25 VSCE. Potentiostatic hold experiments were con-
`ducted in 0.5 M NaCl solutions between pH 9.1 and 9.23 with and
`without 10 mM NaVO3. These experiments were used to determine
`if tetrahedral vanadates have an effect on the repassivation of the
`surface once activated and to show the suppression of Mg dissolu-
`tion from Al2CuMg intermetallics. Each 100 mL test solution was
`deaerated for 30 min prior to experimentation and sample exposure.
`The samples were held at a conditioning potential of 1 VSCE for 1 s
`and then held at a specific potential for the next 120 s; potential
`holds at −1.2, −0.9, −0.8, −0.7, −0.6, and −0.5 VSCE were used.
`
`Results
`
`Inhibition from tetrahedral vanadate species vs octahedral spe-
`cies.— The effect that tetrahedrally coordinated vanadates have on
`cathodic kinetics compared to octahedrally coordinated species can
`be observed through experiments in mildly acidic NaVO3 solutions.
`Figure 1 shows the NMR spectra from two different pH 5.1 0.5 M
`NaCl solutions with 共a兲 0.0025 and 共b兲 0.25 M NaVO3. The sub-
`script of the peak labels in the figure describes the number of vana-
`dium atoms in each oligomer. For example, V1 indicates single tet-
`−,rahedrally coordinated vanadium 关VO43−, VO3共OH兲2−, VO2共OH兲2
`
`
`4−, V2O6共OH兲3−兴,
`VO共OH兲3兴, V2 indicates dimeric vanadate 关V2O7
`
`
`
`5−兲,V4 is tetrameric 共V4O124−, V4O136−兲, V5 is pentameric 共V5O167−, V5O15
`
`6−,
`关V10O28
`represents
`decameric
`vanadate
`species
`and V10
`4−, V10O27共OH兲5−兴.25,26,29,36 V1, V2, V4, and V5 are
`V10O26共OH兲2
`tetrahedrally coordinated species, and V10 is octahedrally coordi-
`nated. Also, the vertical scales of the two spectra in Fig. 1 have been
`adjusted so that peaks in both spectra can be observed. As a result,
`
`Figure 1. NMR spectra of pH 5.1 0.5 M NaCl solutions with 共a兲 0.0025 and
`共b兲 0.25 M NaVO3. The dilute NaVO3 solution has a greater proportion of
`tetrahedrally coordinated species 共V1 and V4兲 relative to octahedrally coor-
`dinated species 共V10兲 compared to the more-concentrated NaVO3 solution,
`which contains mostly octahedrally coordinated species.
`
`the two spectra cannot be compared quantitatively. However, the
`data do show that the 0.0025 M NaVO3 solution has a greater pro-
`portion of tetrahedral species relative to octahedral species com-
`pared to the 0.25 M NaVO3 solution, which contains significantly
`more octahedral species than tetrahedral species.
`Figure 2 shows cathodic polarization curves on Al 2024-T3 in
`aerated pH 5.1 0.5 M NaCl with 0.25 and 0.0025 M NaVO3 and
`without NaVO3. These experiments show the inverse relationship
`between NaVO3 concentration and inhibition of cathodic kinetics at
`pH 5.1, where the dilute solutions containing relatively more tetra-
`hedrally coordinated vanadate to octahedral vanadates have a larger
`reduction in cathodic kinetics than more-concentrated solutions with
`relatively more octahedrally coordinated decavanadate.
`
`Tetrahedral vanadate species in alkaline electrolytes.— Small
`changes in solution pH can have a large effect on vanadate specia-
`tion. Concerns that the vanadate test solutions would change over
`the course of microcell experimentation, with consequences for in-
`hibitor behavior, were addressed using NMR. Figure 3 shows the
`spectra from two samples of pH 9.17 0.5 M NaCl + 10 mM NaVO3
`test solution taken immediately before microcell experimentation
`and 10 days later after the conclusion of experimentation in vana-
`date solutions. The solution did remain stable over the course of
`experimentation and was found to contain a number of different
`tetrahedral vanadate species as expected from previous work on
`vanadate inhibition.18,22 The assignments of V4 and V5 are not cer-
`tain, as two different standards available in the literature leave room
`for speculative interpretation.26,36 However, definitive assignment of
`these species is not critical for this work. The solution used for this
`work contained tetrahedrally coordinated species and predominately
`V1; no octahedrally coordinated vanadates were detected.
`
`intermetallics in tetrahedral vanadate solu-
`Polarization of
`tions.— Figures 4a-g show sample anodic polarization curves for
`pure Al, pure Cu, Al 4% Cu, Al2Cu, Al2CuMg, Al7Cu2Fe, and
`Al20Cu2Mn3, respectively. The objective of these experiments was
`to determine the effect of tetrahedral vanadates on the anodic behav-
`
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` address. Redistribution subject to ECS terms of use (see ) unless CC License in place (see abstract). ecsdl.org/site/terms_use
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`C138C138
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`Journal of The Electrochemical Society, 156 共4兲 C135-C146 共2009兲
`
`Figure 3. NMR spectra showing the presence of tetrahedrally coordinated
`vanadates 共V1, V2, V4, and V5兲 in pH 9.17 0.5 M NaCl + 10 mM NaVO3
`solution used for microcapillary electrochemical experiments 共a兲 immedi-
`ately prior to experimentation and 共b兲 10 days later after completion of ex-
`periments.
`
`also appears to slightly increase the corrosion current density. A
`small decrease in cathodic kinetics, as seen by a decrease in current
`density at −1.3 VSCE in vanadate solutions, is largely offset by an
`increase in anodic kinetics, as seen by an increase in ipass observed
`in NaVO3 solutions. NaVO3 increases the breakdown potential of
`pure Al and does not appear to have an effect on the repassivation
`potential on the reverse scan.
`
`Pure Cu.— NaVO3 was observed to have little effect on the cor-
`rosion potential, corrosion current density, or repassivation potential
`on the reverse scan of pur