Journal of The Electrochemical Society, 162 (9) C487-C494 (2015) C487

Behavior of Alloying Elements during Anodizing of Mg-Cuand Mg-W Alloys in a Fluoride/Glycerol Electrolyte

M. S. Palagonia,a A. Nemcova,b I. Kubena,c M. Smıd,d S. Gao,b H. Liu,b X. L. Zhong,bS. J. Haigh,b M. Santamaria,a,∗ F. Di Quarto,a,∗ H. Habazaki,e,∗ P. Skeldon,b,∗,z

and G. E. Thompsona,∗∗

aElectrochemical Materials Science Laboratory, DICAM, University of Palermo, 90128 Palermo, ItalybSchool of Materials, The University of Manchester, Manchester M13 9PL, United KingdomcCEITEC IPM, Brno 616 62, Czech RepublicdInstitute of Physics of Materials, AS CR, Brno, 616 62, Czech RepubliceDivision of Materials Chemistry, Faculty of Engineering, Hokkaido University, Sapporo, Hokkaido 060-8628, Japan

Anodizing of sputtering-deposited magnesium and Mg-0.75at.%Cu and Mg-1.23at.%W alloys has been carried out in a fluo-ride/glycerol electrolyte. The aims of the study were to investigate the enrichment of alloying elements in the alloy immediatelybeneath the anodic film and the migration of alloying element species in the film. The specimens were examined by electronmicroscopy and ion beam analysis. An enrichment of copper is revealed in the Mg-Cu alloy that increases with the anodizing timeup to ∼6×1015 Cu atoms cm−2. Copper species are then incorporated into the anodic film and migrate outwards. In contrast, noenrichment of tungsten occurs in the Mg-W alloy, and tungsten species are immobile in the film.© The Author(s) 2015. Published by ECS. This is an open access article distributed under the terms of the Creative CommonsAttribution 4.0 License (CC BY, http://creativecommons.org/licenses/by/4.0/), which permits unrestricted reuse of the work in anymedium, provided the original work is properly cited. [DOI: 10.1149/2.0761509jes] All rights reserved.

Manuscript submitted May 29, 2015; revised manuscript received June 19, 2015. Published June 30, 2015.

Anodizing treatments have long been used for the protection ofmagnesium alloys against corrosion and wear and interest still remainsin improving and extending the range of available technologies.1–6

Nevertheless, fundamental knowledge of anodizing of magnesium,7

for instance relating to the migration rates and transport numbers offilm species, is comparatively limited, in part due to the difficultyof forming films of uniform composition and thickness, and also totheir reactivity to water. The formation of barrier-type anodic filmshas been extensively investigated on the valve metals, especially foraluminum, niobium, tantalum, titanium and zirconium.8 These studieshave shown that oxide films of uniform thickness are formed under ahigh electric field that depends upon the film composition and the rateof film growth.8–10 The films on aluminum, tantalum and niobium areusually amorphous,8 and their formation involves migration of metalions and oxygen ions, with significant contributions of both types,e.g. the transport numbers of Al3+, Nb5+ and Ta5+ are ∼0.40, 0.24and 0.24 respectively.8 Further, an outer region of the oxide filmsoften contains a low concentration of species derived from the anionsof the electrolyte.11 In contrast, the films on zirconium are usuallynanocrystalline and form mainly by migration of O2− ions,12,13 whilefilms on titanium undergo a transition from amorphous oxide to amixture of amorphous and crystalline oxide.14 Barrier-type films canalso be formed on magnesium, although such films have receivedmuch less attention and, consequently, less is known of the detailsof their composition, structure and growth mechanism. Films formedin aqueous electrolytes are often reported to consist of MgF2, MgOand/or Mg(OH)2.5,7,15–17 Barrier-type films can also be formed usingnon-aqueous electrolytes and the formation of uniform films in suchelectrolytes provides the opportunity for systematic studies of theanodizing behavior.18,19

From previous work, anodizing of magnesium alloys can result inthe enrichment of alloying elements beneath the anodic film. Suchenrichments have been reported for copper, tungsten and zinc be-neath oxide/hydroxide films formed on model alloys in an aqueouselectrolyte20,21 and of zinc beneath fluoride-based films formed on acast commercial alloy in an organic electrolyte of the compositionused in the present work.19 The enrichments are located in alloy lay-ers of a few nanometres thickness that lie just beneath the anodic

∗Electrochemical Society Active Member.∗∗Electrochemical Society Fellow.

zE-mail: p.skeldon@manchester.ac.uk

films. Enrichments of an apparently similar nature occur followinganodizing of aluminum alloys when the alloying element oxides havea Gibbs free energy of formation per equivalent (�Go/n) that is lessnegative than that of Al2O3.22 A critical level of enrichment is neces-sary before the alloying element can be oxidized and incorporated intothe anodic film, which depends approximately linearly on �Go/n fordilute binary alloys. For magnesium systems, the factors that controlthe enrichment of alloying elements during anodizing are less under-stood. Recent studies have also identified enrichments of iron,23 andneodymium and zinc,24 associated with the formation of corrosionfilms on magnesium and magnesium alloys.

In the following, the behaviors of two alloying elements, namelycopper and tungsten, are compared in binary alloys produced by mag-netron sputtering. The alloys were anodized in a fluoride/glycerolelectrolyte with 5 vol.% of added water. This composition of elec-trolyte has previously been used to form porous or nanotubular an-odic films on titanium25 and barrier films on a magnesium alloy.19 Theanodizing behavior of the alloys is compared with that of sputtering-deposited magnesium. The alloys selected for the present study, Mg-0.75at.%Cu and Mg-1.23at.%W, represent alloys that have revealedenrichments following anodizing in aqueous electrolytes.20,21 How-ever, their anodizing behaviors in an organic-based electrolyte thatgenerates a fluoride-based film have not been considered. Sputtering-deposition was selected in order to produce metastable solid-solutionalloys, since the alloying elements have low equilibrium solubili-ties in magnesium. The Mg-1.23at.%W alloy is a model systemfor understanding alloying element behavior, with the mass contrastbetween magnesium and tungsten assisting the analysis of speci-mens by transmission electron microscopy (TEM) and Rutherfordbackscattering spectroscopy (RBS). In contrast, copper is an alloy-ing element in the high-strength ZC series of commercial magnesiumalloys.

Experimental

Magnesium, Mg-0.75at.%Cu and Mg-1.23at.%W alloys of respec-tive thicknesses of ∼220, 1950 and 450 nm were deposited onto elec-tropolished aluminum substrates of size 5×2 cm using an Atom Techfacility with separate targets of 99.99% magnesium, 99.997% copperand 99.99% tungsten. The substrates had been first electropolishedfor 3 min at 20 V in a perchloric acid/ethanol mixture (20/80 by vol.)at 278 K in order to create flat surfaces for deposition of the mag-nesium and alloy layers. The sputtering chamber was evacuated to

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C488 Journal of The Electrochemical Society, 162 (9) C487-C494 (2015)

3×10−5 Pa; sputtering was then carried out in 99.999% argon at 0.5Pa. The specimens were subsequently masked with lacquer, leavinga working area of ∼1 cm2, then anodized at a constant current den-sity of 5 mA cm−2 in 0.35 mol dm−3 NH4F/glycerol with 5 vol.% ofadded water at room temperature (∼20◦C). The anodizing was termi-nated at selected voltage increments (i.e. the difference between thefinal voltage and the voltage surge due to the resistance of the elec-trolyte). A three-electrode cell was employed with a titanium disk asthe counter electrode and a platinum wire as a pseudo-reference elec-trode. The electrolyte was stirred during anodizing. The current wassupplied by a DC constant current source (Metronix Model 6912).The voltage-time responses were recorded during anodizing usingin-house software based on Labview. After anodizing, the speci-mens were rinsed in de-ionized water and dried in a flow of coolair.

Scanning electron microscopy (SEM) was used for investigationof the surfaces of the specimens, employing Zeiss EVO 50 and ZeissUltra 55 instruments operated at 15 and 1.5 kV respectively. Cross-sections of anodized specimens were prepared TEM using either aQuanta 3D dual beam focused ion beam (FIB) instrument, with theOmniProbe in-situ lift-out technique, or a TESCAN Lyra 3 XMU in-strument equipped with a FIB milling facility. Specimens were coatedwith platinum to prevent damage during ion beam bombardment. TheFIB milling was carried out using a 30 kV ion beam, with currents inthe range 1–7 nA for rough cutting and pre-thinning and 100 – 200pA for final cleaning. High angle annular dark field (HAADF) STEMimaging was performed using a probe aberration corrected FEI TitanG2 80–200 ChemiSTEM (S)TEM instrument, operated at 80 kV, witha convergence angle of 18 mrad and a HAADF inner angle of 54mrad. Energy dispersive X-ray spectroscopy (EDXS) was performedusing the Titan’s four windowless silicon drift detector system with atotal collection solid angle of 0.7 srad. TEM investigations were alsoperformed using a JEOL 2100F instrument operated at 200 kV andequipped with an Oxford instruments X-max EDXS facility. Elemen-tal distributions in sections were measured in the STEM mode withAztec software.

The compositions of the anodic films and the enrichments of al-loying elements were investigated by RBS employing 2 MeV 4He+

ions supplied by the van de Graff generator at the University of Na-mur, Belgium. The incident ion beam was normal to the specimensurfaces, with scattered ions detected at165◦ to the direction of the in-cident beam. The data were interpreted using SIMNRA software. Thecarbon, oxygen and fluorine contents of specimens were assessed bynuclear reaction analysis (NRA) using the 12C(d,p0)13C, 16O(d,p1)17Oand 19F(d,p11,12)20F reactions, employing 0.87 MeV 2H+ ions, withdetection of emitted protons at 150◦ to the direction of the incidentbeam. The oxygen contents were quantified using a reference speci-men of anodized tantalum. The total oxygen contents of the films weredetermined using the natural abundance of 16O of 99.792%. Detailsof the analysis of 16O by NRA can be found elsewhere.26 The carboncontent was estimated from the ratio of the yields of the 12C(d,p0)13Cand 16O(d,p1)17O using literature values of the cross-sections for therespective reactions.27 Fluorine could not be quantified by NRA dueto the lack of data on the reaction cross-sections. The accuracy ofthe measurements of the fluorine and oxygen contents of films, deter-mined by RBS and NRA respectively, was ∼5%. The areas of analysisfor RBS and NRA were ∼ 1 mm2.

X-ray-diffraction (XRD) analysis employed a Bruker D8 Discoverinstrument, with a low incident angle (3◦), a scanning range in 2θfrom 25◦ to 85◦ and a step size 0.02◦. Phases were identified using theICDD PDF4+ database.

Elemental depth profiles through the coating thickness were deter-mined using glow discharge optical emission spectroscopy (GDOES)employing a GD-Profiler 2 instrument (Horiba JobinYvon), with acopper anode of 4 mm diameter, an argon pressure of 635 Pa, a powerof 35 W, a flush time 30 s, rfof 13.56 MHz and a sampling intervalof 0.01 s. The emission lines used were 396.157 nm for Al, 324.759nm for Cu, 383.834 nm for Mg, 130.223 nm for O and 429.467 nmfor W.

Figure 1. (color online) Voltage-time responses for anodizing sputtering-deposited magnesium and Mg-0.75at.%Cu and Mg-1.23at.%W alloys at 5mA cm−2 in 0.35 mol dm−3 NH4F/glycerol with 5 vol.% of added water atroom temperature (∼20◦C).

Results and Discussion

Cell voltage-time behavior.— Figure 1 shows the typical cell volt-ages during anodizing of the sputtering-deposited magnesium and theMg-Cu and Mg-W alloys. The voltages display a surge of ∼15 to 25V due to the resistance of the electrolyte, a subsequent slow increasefor ∼ 3 to 10 s, and a final, approximately linear, rise. The responsesfor the Mg-Cu and Mg-W alloys were generally similar to that ofthe magnesium. Anodizing of the magnesium and Mg-W alloy waslimited to ∼300 V, since the deposited layers were then almost com-pletely oxidized. Anodizing of the thicker Mg-Cu alloy was limitedto ∼400 V by dielectric breakdown.

Scanning electron microscopy.— Figure 2 displays scanning elec-tron micrographs of the magnesium before and after anodizing. Themagnesium initially reveals approximately hexagonal crystals, withsizes of up to ∼200 nm (Fig. 2(a)). After anodizing to 22 V, thehexagonal features are less evident due to the growth of the anodicfilm (Fig. 2(b)). With further film growth, to 66 V, the surface devel-ops a nodular appearance (Fig. 2(c)), with nodules of a size similarto the crystals of the as-deposited magnesium. After anodizing to

Figure 2. (color online) Scanning electron micrographs (secondary electrons)of sputtering-deposited magnesium, before anodizing (a) and following an-odizing to (b) 22, (c) 66 (c) and (d) 267 V in 0.35 mol dm−3 fluoride/glycerolelectrolyte containing 5 vol.% of added water at room temperature (∼20◦C).

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Journal of The Electrochemical Society, 162 (9) C487-C494 (2015) C489

Figure 3. (color online) Transmission electron micro-graph (dark field) and EDXS elemental maps of a cross-section of sputtering-deposited magnesium following an-odizing to 267 V in 0.35 mol dm−3 fluoride/glycerolelectrolyte containing 5 vol.% of added water at roomtemperature (∼20◦C).

267 V, the surface was relatively flat, with shallow dimples and fur-rows (Figs. 2(d)). The Mg-Cu and Mg-W alloys before and afteranodizing presented features similar to those of the magnesium.

Transmission electron microscopy.— EDX spectrum imagingwithin the scanning transmission electron microscope was used to re-veal elemental distributions within a cross-section of the magnesiumfollowing anodizing to 267 V (Fig. 3). Together with the accompany-ing HAADF images, the section reveals a film of uniform thicknessattached to a ∼20 nm-thick layer of magnesium. The film is 300 ± 20nm thick, which indicates a formation ratio of 1.1 ± 0.1 nm V−1. Theformation ratio compares with values of ∼1.24 nm V−1 for an anodicfilm formed on magnesium in a fluoride/ethylene glycol electrolyte,18

and ∼1.2 to 1.4 nm V−1 for an anodic film formed at the matrixregion of a commercial magnesium alloy in a similar electrolyte tothe present one.19 The elemental maps extracted from the EDXS datashow the presence of magnesium, fluorine and oxygen throughout thefilm.

Figure 4 shows EDXS elemental linescans perpendicular to thefilm surface. The composition of the film appears to be relatively uni-form. The small decrease in the fluorine signal and a small increase inthe oxygen signal, which occurs in a region at a depth of ∼0.54 of thefilm thickness (i.e. the ratio of the depth of the region to the total filmthickness), is due to the incorporation of the oxide/hydroxide film that

Figure 4. (color online) Transmission electron micrograph (dark field) andEDXS elemental linescan of a cross-section of sputtering-deposited magne-sium following anodizing to 267 V in 0.35 mol dm−3 fluoride/glycerol elec-trolyte containing 5 vol.% of added water at room temperature (∼20◦C).

was initially present at the magnesium surface, as indicated in earlierstudies.18,19 This layer acts as a marker in the film. The observationsindicate that the film forms by migration of cations outward, leadingto growth of the film at the film surface, and migration of anions in-ward, leading to growth of the film at the metal/film interface. Theenhanced concentration of oxygen at the base of the magnesium layeris generated at the start of the deposition of the alloys due to the pres-ence of oxygen-containing impurities in the sputtering chamber or onthe surface of the sputtering targets. The XRD pattern for the speci-men, shown in Fig. 5, reveals peaks due to the aluminum substrate,the residual magnesium, MgF2 and possibly MgO. The small peak at∼58.2◦ could not be identified. MgF2 has been previously detected infilms formed in the present electrolyte19 and also in a fluoride/ethyleneglycol electrolyte.18

Figure 6 displays bright field transmission electron micrographs ofanodic films of uniform thickness formed to 400 V on the Al-Cu alloyand to 300 V on the Mg-W alloy. The films contain nanocrystals inall regions of the film thicknesses. A lighter band is evident near themiddle of the film on the Mg-Cu alloy, which is due to incorporationof the oxide/hydroxide film of the original alloy surface. Figures 7and 8 present dark field images and EDX elemental maps of the re-spective specimens. The presence of the nanocrystals in the films wasconfirmed by the accompanying electron diffraction patterns. Fromexamination of several bright and dark field micrographs, thicknessesof 520 ± 40 and 396 ± 30 nm were determined for the films on theMg-Cu and Mg-W alloys respectively. The ratio of the film thicknessto the anodizing voltage is ∼1.3 ± 0.1 nm V−1 for both specimens.Magnesium, fluorine and oxygen species are distributed throughoutthe film thicknesses. Copper is enriched at the alloy/film interface, butwas not detected in the film. In contrast, no enrichment of tungsten

Figure 5. (color online) X-ray diffraction pattern for sputtering-depositedmagnesium anodized to 267 V in 0.35 mol dm−3 fluoride/glycerol electrolytecontaining 5 vol.% of added water at room temperature (∼20◦C).

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C490 Journal of The Electrochemical Society, 162 (9) C487-C494 (2015)

Figure 6. (color online) Transmission electron micrographs (bright field) ofthe (a) Mg-0.75at.%Cu and (b) Mg-1.23at.%W alloys following anodizing to200 V at 5 mA cm−2 in 0.35 mol dm−3 NH4F/glycerol with 5 vol.% of addedwater at room temperature (∼20◦C).

was resolved at the alloy/film interface, while tungsten was presentin the inner ∼50% of the film thickness. Oxygen was enriched in athin region at the base of the alloys, as observed for the magnesiumlayer. Local enhancements of oxygen and fluorine in the Mg-W al-loy are probably due to the presence of cracks or interstices in theas-deposited alloy that allow ingress of the electrolyte at the start ofanodizing. The surface area accessible to the electrolyte is thereforeinitially greater than the nominal surface area of the specimen, result-ing in a low initial rate of voltage rise that lasts until the voids are

filled. The striations that appear in the anodic film and residual alloylayer in the dark field image of Mg-W alloy, which are associatedwith a bubble-like network in the bright-field image, are created bythe ion beam during the thinning of the section. An EDX line scanacross the striations revealed no significant variations in the film oralloy composition.

Ion beam analysis and glow discharge optical emissionspectroscopy.— Figure 9a shows the experimental RBS spectra forthe sputtering-deposited magnesium before anodizing and after an-odizing to 66 and 267 V; Fig. 9 (b) shows an example of a fittedspectrum. The locations of elements at the specimen surfaces areindicated by arrows. Following anodizing, magnesium, fluorine andoxygen are detected in the anodic film. Hydrogen may also be present,which is not detected by RBS. The fitting of the spectra required in-clusion of ∼6 at.% oxygen within the magnesium layer. Figure 10shows peaks due to the 19F(d,p11,12)20F and 16O(d,p1)17O reactions inNRA spectra for the specimens anodized to 22, 66,172 and 267 V.The NRA measures the total of the oxygen in the anodic film, on elec-tropolished aluminum substrate and within the magnesium layer. Thespectra illustrate an increase of fluorine and decrease of oxygen withthe progress of anodizing. The peak due to the 12C(d,p0)13C reaction(not shown) revealed no dependence on the voltage, which indicatedthat the carbon is mainly a contaminant derived from either residuesof glycerol from the electrolyte and/or organic species adsorbed afteranodizing. The estimated upper limit on the carbon for the film formedto 267 V was ∼1×1017 atoms cm−2.

Figure 11a,b, presents examples of RBS spectra for the Mg-Cu andMg-W alloys, using specimens anodized to 400 and 300 V respec-tively. The spectra reveal anodic films containing magnesium, oxygenand fluorine species. The Mg-Cu specimen shows an enrichment ofcopper at the alloy/film interface and the presence of copper in the in-ner ∼0.50 of the film thickness. No enrichment of tungsten is resolvedfor the Mg-W alloy, indicating an upper limit of ∼2×1014 W atomscm−2. Tungsten is confined to the inner ∼0.48 of the film thickness,according to the half-height of the tungsten leading edge. The slope onthe edge is due to the roughness of the original alloy surface and thefilling of interstices by film material. The distribution of tungsten inthe film indicates that the tungsten species are immobile. The atomic

Figure 7. (color online) Transmission electron micro-graph (dark field) and EDX elemental maps of the Mg-0.75at.%Cu alloy following anodizing to 400 V at 5 mAcm−2 in 0.35 mol dm−3 NH4F/glycerol with 5 vol.% ofadded water at room temperature (∼20◦C). The electrondiffraction pattern of the film is shown in the inset.

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Journal of The Electrochemical Society, 162 (9) C487-C494 (2015) C491

Figure 8. (color online) Transmission electron micro-graph (dark field) and EDX elemental maps of the Mg-1.23at.%W alloy following anodizing to 300 V at 5 mAcm−2 in 0.35 mol dm−3 NH4F/glycerol with 5 vol.% ofadded water at room temperature (∼20◦C). The electrondiffraction pattern of the film is shown in the inset.

ratios of Cu:Mg and W:Mg in the inner film regions are ≤0.004 and0.025 respectively, which are factors of ≤0.5 and ∼2.1 times the ratiosfor the alloys. The upper limits on the ratios in the outer regions of thefilms are ∼0.001 and 0.0001 respectively. NRA indicated an upperlimit for carbon in the films of ∼3×1016 carbon atoms cm−2.

Figure 9. (color online) (a) Experimental RBS spectra for the as-depositedmagnesium and following anodizing to 66 and 267 V in 0.35 mol dm−3 fluo-ride/glycerol electrolyte containing 5 vol.% of added water at room temperature(∼20◦C); (b) example of fitted spectrum (solid line) for specimen anodized to267 V.

Figure 12 shows the GDOES elemental depth profiles for the Mg-Cu and Mg-W alloys following anodizing to 400 and 250 V respec-tively. Oxygen and magnesium are distributed throughout the filmthicknesses. Fluorine could not be analyzed due to the use of an argonplasma, which has a low efficiency for excitation of optical emissionfrom fluorine.28 Copper and tungsten are present in the inner ∼0.4of the film thicknesses, assuming that the sputtering rate is constantthrough the film thickness. Copper is also enriched immediately be-neath the film, but no enrichment of tungsten is evident resolved forthe Mg-W alloy. The tails on the magnesium, copper and tungstensignal following sputtering through the residual alloy layers are dueto the non-uniformity of the sputtering across the analysis area, whichis associated with the specimen roughness, the crystal structure of thealloys, and background from material deposited at earlier stages ofsputtering. The findings on the enrichments and on the distributionsof alloying elements are consistent with the results of RBS, althoughthe tungsten-containing region appears thinner by GDOES, suggestingthat the sputtering rate is increased by the incorporation of tungsten.

The relationships between the oxygen and fluorine contents ofthe specimens, determined from NRA and RBS respectively, and the

Figure 10. (color online) NRA spectra for sputtering-deposited magnesiumfollowing anodizing to 22, 66, 172 and 267 V in 0.35 M fluoride/glycerolelectrolyte containing 5 vol.% of added water at room temperature (∼20◦C).

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C492 Journal of The Electrochemical Society, 162 (9) C487-C494 (2015)

Figure 11. (color online) Experimental and simulated (solid line) RBS spectraof (a) Mg-0.75at.%Cu and (b) Mg-1.23at.%W alloys following anodizing to400 and 300 V respectively at 5 mA cm−2 in 0.35 mol dm−3 NH4F/glycerolwith 5 vol.% of added water at room temperature (∼20◦C). The inset in (a)shows the region of copper in the inner part of the anodic film.

anodizing voltage are shown in Fig. 13 for the magnesium and theMg-Cu and Mg-W alloys. The gradients for the fluorine contents are∼4.7×1015, 6.0×1015 and 5.2×1015 fluorine atoms cm−2 V−1 for therespective substrates. Those for the oxygen contents are ∼0.8×1015,0.9×1015 and 0.8×1015 oxygen atoms cm−2 V−1. The intercepts indi-cate that the as-deposited Mg-W alloy contained more oxygen than theas-deposited magnesium and Mg-Cu alloy. The presence of oxygen inthe sputtering-deposited layers results in non-faradaic incorporationof MgO into the film. Recent work indicated that oxygen contamina-tion of sputtering-deposited aluminum films originated mainly fromresidual water molecules in the sputtering chamber.29 A base pressureof 10−7 torr was required to reduce the contamination to levels that didnot significantly affect the corrosion behavior of the films. The level issimilar to the base pressure achieved in the present work. However, theobserved presence of oxygen in the magnesium layers suggests thatlower base pressures may be required to reduce the oxygen to negligi-ble concentrations. The oxygen added to the specimens by anodizingis derived mainly from the water molecules of the electrolyte, basedon evidence of anodizing zirconium in a similar electrolyte.30 Smallercontributions may originate from glycerol molecules. The gradientsof the lines indicate that oxygen and fluorine are incorporated intothe films with atomic ratios of ∼0.17, 0.15 and 0.16 for the magne-sium and the Mg-Cu and Mg-W alloys respectively. Using the averageslopes of the voltage responses for the respective substrates, namely5.2, 4.4 and 5.3 V s−1, and assuming the incorporation of O2− andF− ions, the oxygen and fluorine lines are consistent with anodizing

Figure 12. (color online) GDOES elemental depth profiles of (a) Mg-0.75at.%Cu and (b) Mg-1.23at.%W alloys following anodizing to 400 and250 V respectively at 5 mA cm−2 in 0.35 mol dm−3 NH4F/glycerol with 5vol.% of added water at room temperature (∼20◦C).

Figure 13. (color online) Relationship between the fluorine content, deter-mined by RBS, and the oxygen content, determined by NRA, and the an-odizing voltage for the sputtering-deposited magnesium, the Mg-0.75at.%Cualloy and the Mg-1.23at.%W alloy following anodizing in 0.35 mol dm−3

fluoride/glycerol electrolyte containing 5 vol.% of added water at room tem-perature (∼20◦C).

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Figure 14. (color online) Relationship between the enrichment of copper andthe anodizing voltage for the Mg-0.75at.%Cu alloy following anodizing at 5mA cm−2 in 0.35 mol dm−3 NH4F/glycerol with 5 vol.% of added water atroom temperature (∼20◦C).

at current densities of 5.2, 5.5 and 5.3 mA cm−2 respectively. Thevalues are indicative of a high current efficiency; the small differencesin comparison with the experimental current density are due mainlyto the limitations on the accuracy of the estimates.

Figure 14 shows the relationship between the enrichment of copperand the anodizing voltage for the Mg-Cu alloy. The relationship islinear up to ∼250 V with a slope of 2.15×1013 atoms cm−2 s−1.Thereafter, the enrichment either changes at a much reduced rateor is possibly constant considering the accuracy of the data. Theresults indicate that the alloy enriches in copper up to ∼6×1015 Cuatoms cm−2. This amount of copper is contained in a thickness of∼186 nm of the bulk alloy, which is oxidized in 51 s at a currentdensity of 5 mA cm−2, when the voltage had reached ∼250 V. Thisvoltage is close to the transition on Fig. 14 and indicates that little or nocopper is incorporated into the anodic film before then. Subsequently,copper and magnesium are incorporated into the film according tothe concentrations in the bulk alloy, such that no significant increasein the enrichment ensues, similar to the behavior during anodizingof dilute Al-Cu alloys.22 The Cu:Mg ratio in the film relative to thatin the alloy indicates that the copper species migrate outward abouttwice as fast as Mg2+ ions. The TEM images of Figs. 6 and 7 suggestthat the enriched layer is ∼3 nm thick, which contains ∼12.2×1015

Mg atoms cm−2, based on the atomic density of bulk magnesium.Hence, an enrichment of ∼6×1015 Cu atoms cm−2 suggests an averageconcentration of copper of roughly 50 at.%. Whether or not the copperis uniformly distributed in the magnesium or is present as a secondphase, such as Mg2Cu, requires more detailed examination by TEM.

Enrichments of copper, tungsten and zinc have previously been de-tected beneath anodic films formed on sputtering-deposited Mg-Cu,Mg-W and Mg-Zn alloys anodized in an aqueous electrolyte in whichfilms based on magnesium oxide/hydroxide were formed.20,21 Fur-ther, these elements are also enriched beneath oxide films formed byanodizing of Al-Cu, Al-W and Al-Zn alloys.22 However, the presentfilms contain fluoride species and a dependence on the thermody-namic properties of the fluorides may be anticipated. The Gibbs freeenergies per equivalent for formation of MgF2 (s), CuF2 (s), WF6

(l) and ZnF2(s) are −536 −241, −272 and −357 kJ mol−1 respec-tively. The values suggest that copper, tungsten and zinc should beenriched based on the criterion used to explain enrichments of bi-nary aluminum. Hence, the absence of a significant enrichment of theMg-W alloy appears to be anomalous. Thus, enrichment and migra-tion processes appear to be modified for the nanocrystal-containing,fluorine-rich films on magnesium alloys compared with aluminumalloys, and are possibly more complex than in simpler amorphous,

oxide-based systems. In this respect, it has recently been found thatgold is oxidized during formation of fluoride-rich films on an Mg-Ausubstrate and gold species migrate outward at ∼0.4 times the rateof migration of magnesium.31 Further, the migrating species appearto be associated with fluorine. In contrast, for Al-Au alloys, gold isincorporated into oxide films as metal nanoparticles.32 In the case ofthe present films, further understanding of the behavior of the alloyingelements may be obtained from examination of the chemical statesof the alloying element species and the details of the distributions offluoride and oxide species within the films. From the more extensiveknowledge base for aluminum alloys,33 the enrichment of alloyingelements and the migration of alloying element species in the an-odic film do not change greatly with variation of the current densityand hence behaviors similar to those reported here are expected forfilms grown on the magnesium alloys at rates different from thoseof the present study. However, the behaviors may be modified by theuse of other electrolytes, which may result in films containing dif-ferent proportions of oxygen and fluorine species and with differentstructures.

Conclusions

1. Anodic films can be formed at high efficiency on Mg, Mg-0.75at.%Cu and Mg-1.23at.%W alloys in the present fluo-ride/glycerol electrolyte. The films form by migration of cationand anion species, with a transport number of ∼0.5 to 0.6 for theformer species.

2. The films contain nanocrystals and are rich in fluorine species,with an O:F ratio of ∼0.15 to 0.20.

3. The growth of the anodic film on the Mg-0.75at.%Cu alloy isaccompanied by enrichment of copper in the alloy up to ∼6×1015

Cu atoms cm−2. Enrichment of tungsten is negligible for the Mg-1.23at.%W alloy.

4. Copper species migrate outward in the anodic films at about twicethe rate of magnesium species. In contrast, tungsten species areimmobile.

Acknowledgments

The authors are grateful to the Engineering and Physical SciencesResearch Council (U. K.) (Programme Grant: LATEST2) for supportof this work. They also thank the European Community for financialassistance within the Integrating Activity “Support of Public and In-dustrial Research Using Ion Beam Technology (SPIRIT)”, under ECcontract no. 227012. Assistance was also provided CEITEC- Cen-tral European Institute of Technology with research infrastructuresupported by the project CZ.1.05/1.1.00/02.0068 financed from theEuropean Regional Development Fund. One of the authors (M.S.P.)gratefully acknowledges the financial support (Erasmus Placement)provided by University of Palermo.

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