corrosion properties of glassy mg70al15ga15 in 0.1m nacl solution

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Intermetallics xxx (2009) 1–7

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Intermetallics

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Corrosion properties of glassy Mg70Al15Ga15 in 0.1 M NaCl solution

D.I. Uhlenhaut, A. Furrer, P.J. Uggowitzer, J.F. Loffler*

Laboratory of Metal Physics and Technology, Department of Materials, ETH Zurich, Wolfgang-Pauli-Str. 10, 8093 Zurich, Switzerland

a r t i c l e i n f o

Article history:Received 25 July 2008Received in revised form4 March 2009Accepted 10 March 2009Available online xxx

Keywords:B. Glasses, metallicC. Rapid solidification processingB. CorrosionB. Surface propertiesF. Electrochemical characterization

* Corresponding author. Tel.: þ41 44 632 2565; faxE-mail address: [email protected] (J.F. Lof

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The corrosion behavior of glassy Mg70Al15Ga15 was investigated in 0.1 M NaCl solutions of various pHvalues. Potentiodynamic polarization measurements confirm a pronounced passivity for 4� pH� 12.Corrosion currents in neutral solutions are found to be as low as 10�6 A/cm2. Optical microscopy andAuger electron spectroscopy were performed on samples immersed under open-circuit conditions inneutral, acidic and alkaline 0.1 M NaCl solutions to observe their time-dependent passive film formation.A significant incorporation of aluminum in the surface layers was found. The alloys show an unusualstability against dissolution in chloride-containing solutions, raising expectations of a highly corrosion-resistant Mg-based alloy. The time-dependent limitations of this stability are discussed in context withthe surface films observed.

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1. Introduction

A spontaneously-forming and self-healing passive film onmetallic materials in aqueous solutions is an essential requirementfor engineering alloy applications. One of the most importantdevelopments in this field was the discovery of stainless steel,a class of iron-based alloys with amounts of at least 12 wt.-%chromium in solid solution. Here the spontaneous formation ofa very stable (Fe-)Cr oxide film protects the alloy from dissolution,and the high solute content of chromium allows the immediaterepassivation of scratched surfaces where the metallic solid wasexposed [1,2]. Similarly, significant passivity has also beenconfirmed for glassy Fe–Cr–metalloid alloys even in very aggressivesolutions such as 0.1 M HCl [3].

A similar but intrinsic effect can be observed e.g. in titanium,where the element itself spontaneously forms an oxide layer whichis stable between slightly alkaline and not-too-acidic solutionscontaining aggressive anions such as Cl�. This makes alloys basedon such elements very interesting for structural applications,because both their specific mechanical and their electrochemicalproperties are unique in those density classes. In the case ofaluminum, such oxide films are formed, but are found to besusceptible to pitting initiated by chlorides (see, for example, [4]).

Magnesium, as one of the lightest metals, is not able to protectitself from corrosion: in contact with water it forms Mg(OH)2 and

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ut DI, et al., Corrosion prope

hydrogen. The magnesium dihydroxide equilibrium is at approxi-mately pH 11, which means that it is destabilized by an acid-typeattack in aqueous media of pH values of less than 10 [5]. Pitting canbe easily initiated by the presence of even low amounts of anionssuch as Cl�, CO3

2� or SO42� [6].

On the other hand, it was shown by Hehmann et al. [7] thatsupersaturated solutions (obtained by rapid quenching) containingAl contents beyond the equilibrium solubility in Mg increase thecorrosion resistance significantly. When the Al content exceeded30 wt.-% (corresponding to about 28 at.-%), it was found thata completely passive film formed in aerated 0.001 M NaCl solution.From observations of the ions dissolved, the authors concluded thatan Al-rich film had formed during the first few minutes ofimmersion.

In the course of the development of metastable alloys, bulkmetallic glasses, including those in Mg-based systems, began toemerge some years ago [8], yielding materials of outstandingmechanical properties [9]. It is generally assumed that the elec-trochemical properties of these metallic alloys are superior to thoseof their polycrystalline counterparts, because of the homogeneousstructure of glasses and thus the absence of gradients in chemicalpotential on the metallic surfaces [10].

The electrochemical properties of various Mg-based glassycompositions have been investigated [11–20]. All compositionstested so far show no strong passivity in neutral or acidic solutionswith significant amounts of aggressive anions such as chloride.Moreover, the comparison between crystalline and glassy compo-sitions of equivalent stoichiometric compositions confirmed the

rties of glassy Mg70Al15Ga15 in 0.1 M NaCl..., Intermetallics (2009),

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assumption that the presence of precipitates of varying electro-chemical potential deteriorates the performance of the alloy inaqueous solution.

In the search for alloy systems which generate ‘stainlessmagnesium’ by producing an extensive passivity as suggested inRef. [21], we developed glassy Mg–Al–Ga alloys on the basis of twoternary eutectic compositions in the Mg-rich corner [22], allowingan extensive amount of homogeneously-distributed Al and Ga inthe solid. The glass-forming ability and compositional range of therapidly-quenched alloys found in the course of our work agree wellwith the findings of an earlier publication on this system [23],where electronic transport properties were investigated.Mg70Al15Ga15 alloys were chosen for our study due to their lowamounts of the heavy Ga required for glass formation. They weremelt-spun to amorphous ribbons and characterized by potentio-dynamic polarization as well as immersion in 0.1 M NaCl solutionsof various pH values (adjusted by NaOH and HCl). The surfaces ofimmersed samples were also characterized by X-ray diffraction andAuger depth profiling. The composition is expected to showa significant passivity in neutral to acidic media due to the highamount of Al, and possibly even extended corrosion resistance dueto the presence of Ga. Little is known about the electrochemicalproperties of Ga [24] or of Mg–Ga alloys. Improved corrosionresistance due to the presence of Ga in the alloy is, however,expected, as the Pourbaix diagrams of Ga and Al are similar [25],and both elements form trioxides.

2. Experimental procedure

2.1. Alloys

High-purity elements (Mg 99.95% and Al 99.999% from Cerac Inc.,USA, and Ga 99.99% from Alfa Aesar, USA) were alloyed in a graphitecrucible in a Ti-gettered Ar atmosphere of 99.999% purity. Theingots were then quenched by melt spinning in a 99.999% heliumatmosphere using graphite nozzles. The wheel of the melt spinnerhad surface speeds of 25–30 m/s, generating ribbons of 10 mmwidth, 0.05 mm thickness and several meters length. A calibratedwavelength-dispersive X-ray spectrometer (JEOL Ltd., JP) was usedto verify the composition of the ribbons, which was found to accordwith the nominal concentrations within the measurement’s preci-sion. For the corrosion testing, the ribbons were used in theas-quenched state without further surface treatment.

2.2. Microstructural and thermal characterization

The fully amorphous state of the ribbons was confirmed by X-ray diffraction (XRD), using monochromated Cu-Ka radiation torecord a 2q-range of 20–120 degrees (Panalytical X’Pert PRO, NL)with 0.002 degree steps and a step rate of 0.01 degrees/s. Thermalstability was verified using a differential scanning calorimeter(DSC220CU, Seiko Instruments, JP) operated at a heating rate of20 K/min under a constant Ar flow of 50 ml/min. The alloy showeda crystallization onset of 394 K and a liquidus temperature of 637 K.

2.3. Potentiodynamic measurements

Linear potentiodynamic polarization curves were measuredusing an Autolab PGSTATS 302 device (Eco Chemie B.V., TheNetherlands). A 3-electrode setup was deployed, with a saturatedKCl/Hg2Cl2 calomel electrode as reference (with the standardhydrogen electrode potential E(SHE)¼ 240 mV), connected to thecell by means of a Haber-Luggin capillary, and a platinum mesh asa counter electrode. The open-circuit potential (OCP, or corrosionpotential Ecorr) was measured for 30 min before each

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measurement; usually scan rates of 0.001 V/s were applied, whilesome measurements at 0.0001 V/s were performed to investigatethe influence of the scan rate. Polarization was recorded fromOCP� 0.5 V to OCPþ 1 V, or in a few cases starting directly from themeasured OCP, to investigate the influence of cathodic prepolari-zation. The as-prepared ribbons were attached to conductingBakelite and sealed at the edges with silicon sealant, which lefta sample surface of approximately 0.8 cm2. All experiments werecarried out at room temperature in 0.1 M NaCl solution, the pH ofwhich ranged between a value of 12 and 2 for the differentmeasurements, adjusted by the appropriate addition of HCl orNaOH as verified by a calibrated pH meter.

2.4. Auger electron spectroscopy (AES)

Samples of approximately 0.25 cm2 in size were attached toglass discs with silicon sealant and immersed in 0.1 M NaCl solutionof pH values of 3.5, 7 and 12, for time spans of 3 h, 72 h and 600 h,under true open-circuit conditions. After immersion, the sampleswere briefly rinsed with deionised water and dried with pressedair. Following this, X-ray diffraction was performed on the corrodedsurfaces.

Auger electron spectroscopy on the immersed samples wascarried out on a PHI 4300 Scanning Auger Microscope (Perkin–Elmer, Massachusetts, USA) using a cylindrical mirror analyzer(CMA). Its electron gun was equipped with an LaB6 filament andoperated at a voltage of 5 keV. The sample current for themeasurements was slightly below 100 nA. Depth profiles wererecorded with the help of an argon ion gun, operated at a voltage of4 keV. The sputter rate was calibrated with Ta2O5, leading to a slightunderestimation of sputter depths. The ion beam covered an area of4� 4 mm2 for the briefly-immersed samples and 2� 2 mm2 forthose immersed for longer, generating corresponding sputter ratesof approximately 5 and 25 nm/min, respectively. Detail scans wereperformed on oxygen and the metallic elements of interest,including a sufficiently wide energy range. The results wereanalyzed with the help of ‘PHI Multipak’ Version 6.1A, using thepeak-to-peak method to determine the intensities [26]. The valuesobtained were subsequently factored with the correspondingsensitivity factors [27] of pure elements and used for semi-quantitative calculation of the atomic concentrations.

3. Experimental results and discussion

3.1. Potentiodynamic measurements

Potentiodynamic polarization scans were performed to obtainqualitative and quantitative information on the ability of theMg70Al15Ga15 alloys to form a passive layer in the conditionsapplied. Comparative scans deploying equal measurement condi-tions were performed on the commercial Mg-based alloy WE43(containing 4 wt.-% Y, 3 wt.-% Nd and 0.5 wt.-% Zr), pure Mg, pureAl, and the well-known metallic glass-former Mg65Cu25Y10 [8]From Fig. 1, it can be seen that the cathodic currents ofMg70Al15Ga15 in neutral 0.1 M NaCl solution are rather small,indicating a strongly hindered cathodic reaction, apparentlyresulting from the formation of passive films on the surface. TheOCP is found at significantly lower values than those of the refer-ence alloys and pure Al. The anodic polarization of theMg70Al15Ga15 alloy produces current densities comparable to thoseof pure Al, again at much lower potentials. Once the pittingpotential of Mg70Al15Ga15 is reached, immediate and strong acti-vation occurs, while only in pure Al the steps indicate spontaneousrepassivation. From the corrosion currents at OCP and from thepassive current densities, it can be concluded that the alloy

erties of glassy Mg70Al15Ga15 in 0.1 M NaCl..., Intermetallics (2009),

Fig. 1. Potentiodynamic polarization curves of Mg70Al15Ga15, WE43, pure Mg, pure Aland Mg65Cu25Y10 in neutral 0.1 M NaCl solutions.

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performs significantly better than pure Mg or the WE43 alloy,which is expected to be one of the more corrosion-resistantconventional Mg-based alloys because of its high Y content [18].The Mg65Cu25Y10 glass was measured for comparison, because the10 at.-% Y might assist passivation at least in neutral or alkalinesolutions. A small passive transition can be observed, but pittingoccurs rather quickly; the high OCP can be explained by the high Cucontent.

Fig. 2 shows a series of potentiodynamic scans of Mg70Al15Ga15

in 0.1 M NaCl solution for pH values between 2 and 12. A distinctpassive plateau can be observed in the anodic range for samplesmeasured in solutions of pH� 4. Passive currents are around3�10�6 A/cm2, independent of the pH. In contrast, the samplesmeasured at pH 2 and 2.5 are active in these solutions. A significantvariation in OCP values was observed. For the samples measuredbetween 4� pH� 7.7, the OCP values decreased linearly from�1.6 V to�1.76 V, while the corresponding corrosion currents werefound to be constant and little below 10�6 A/cm2. With increasingpotentials the slope of the cathodic reactions flattened. Measure-ments obtained in solutions above or below the pH range of4� pH� 7.7 did not follow this trend. Here the corresponding

Fig. 2. Potentiodynamic polarization curves of amorphous Mg70Al15Ga15 ribbons in0.1 M NaCl solution at adjusted pH values as indicated. The applied scan rate was0.001 V/s.

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slopes of the cathodic reactions suggest different reduction kineticsof the participating species (hydrogen for pH< 4, oxygen forpH> 8) compared to the above-mentioned systematics. Pittingpotentials were typically found at approximately 0.25–0.35 V abovethe OCP, while these values did not show a particular trend with pH.The variance of these values could not be related to any of thecontrolled experimental parameters and is assumed to originatefrom the unconditioned surface state of the ribbons.

The ribbons were additionally measured at a lower scan speedand without cathodic prepolarization in a neutral solution of 0.1 MNaCl, to investigate the influence of the scan speed on the currentdensities. A speed of 0.001 V/s was applied during the measure-ments shown above, while here a rate of 0.0001 V/s was used forcomparison. The result is shown in Fig. 3, together with theexperiments where the potentiodynamic polarization was startedfrom the OCP. The reduction of the scan speed by one order ofmagnitude influenced both the cathodic and the anodic reactionsignificantly in the case of cathodically pre-polarized sweeps, whilewithout prepolarization the influence of scan speed was negligible.In the first case the cathodic polarization slope was slightly steeperfor lower speeds, while the passive current density in the anodicsegment dropped to about 4�10�7 A/cm2. The OCP shifted to evenlower potentials, while the interval between OCP and pittingpotential remained comparable for both measurements. Appar-ently the passive current density of the sample measured at lowerspeed decreased with increasing potential (and time), while theopposite was observed for the higher scan rate. The samplesmeasured from the OCP to anodic potentials showed higher andincreasing current densities, indicating a passivation layer on themetallic surface of lower protective quality. The generally low OCPvalues found for the Mg70Al15Ga15 alloy, establishing at significantlylower potentials than those of the pure elements, may be related tothe negative difference effect (NDE) observed in pure Mg [28], forwhich an explanation was given by the presence of unipositive Mgþ

species [29]. The investigation performed in this work does,however, not allow conclusions towards the origin of this effect.

From the potentiodynamic measurements it can be concludedthat the Mg70Al15Ga15 alloy investigated in this work shows verylow current densities both at the corrosion potential and in thepassive range. A lower polarization speed strongly reduces passivecurrent densities after cathodic polarization. The decrease incurrent density (and corrosion rate) during low-speed scanningthrough the anodic passivity range may be related to a growing or

Fig. 3. Potentiodynamic measurements in neutral 0.1 M NaCl solution at scan rates of0.001 V/s and 0.0001 V/s, with and without cathodic prepolarization.



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densifying passive layer during the measurement. In addition, theoverall lower current density may be related to the time availablefor the alloy to build a passive layer. This is supported by the factthat higher passive currents (without a significant dependence onthe scan rate) have been observed in measurements withoutcathodic prepolarization.

3.2. Surface characterization of spontaneously oxidized andimmersed samples

In order to investigate the initial state of the samples’ surfacebefore immersion, Auger electron spectroscopy (AES) was per-formed in combination with Ar sputtering on samples stored in dryair for several weeks. An outermost oxide layer of about 5 nmthickness was found which consisted solely of oxidic Mg and oxygen.Underneath, a transient, oxygen-free zone of approximately 20 nmcould be observed, within which the aluminum and galliumcontents increased with depth, before reaching equilibrium valuesresulting from the ‘bulk’ composition. Here the contents weredetermined to be 50 at.-% Mg, 35 at.-% Al and 15 at.-% Ga.The difference compared to the nominal composition is believedto originate from matrix effects, i.e. the electrons backscattered fromthe heavier elements (Ga) in the alloy generate a higher Auger-electron emission than a matrix solely constituted of light elements.

Fig. 4 shows an overview of the various sample surfaces afterimmersion. Severe corrosion on the surface was already found aftera few days, ranging from holes in the cases of pH 3.5 and 7 to

Fig. 4. Optical microscopy images of immersed Mg70Al15Ga15 samples in 0.1 M


speckles and inhomogeneous color changes for the solution at pH12. No complete metallic luster was preserved by these alloys in anyof the salty aqueous solutions applied, but the samples kept at pH 7and 12 showed no visible corrosion during the first 24 h. After 5–6days of immersion, however, white powdery corrosion productsand holes were found on parts of the samples’ surface.

Several samples immersed in neutral solution initially builtiridescent surface layers. From this, a film thickness of at least65 nm was estimated, as the required travel distance of light for aniridescent phase shift needs to be at least a quarter of the light’swavelength. Such oxide layers are commonly amorphous. XRDmeasurements for samples immersed in neutral solutions for 3 h(see Fig. 5 (top)) show no Bragg peaks, which can be understood toconfirm the amorphous state, but is most likely related to aninsufficient film thickness for measurable diffraction intensity. Thesample after 72 h (see Fig. 4(d)) already revealed heavy pitformation and a large number of electrochemically-active sites.XRD on these samples (see Fig. 5 (bottom)) confirmed the presenceof crystalline phases, which were partially matched with MgO,Mg(Al,Ga)2O4, Al2O3, and the intermetallic compound Ga2Mg.

Fig. 6 shows the depth- and concentration-corrected profiles (inatomic percent) of the immersed samples, the optical images ofwhich are shown in Fig. 4. It can be seen that the thicknesses of thecorrosion layers were between 50 nm ((d), pH 7, 3 h), and 750 nm((i), pH 12, 600 h), while the samples in neutral or acidic solutionswere completely corroded after times of 600 h. In the latter cases,no metallic components were found. Metallic gallium was usually

NaCl solutions at pH 3.5, 7, and 12, and for times of 3 h, 72 h, and 600 h.


Fig. 5. Patterns of samples immersed in neutral 0.1 M NaCl solution for 3 h (top) and72 h (bottom); for the partially crystalline sample partial matches with crystallinephases are indicated.


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only found below the oxidation layer, and oxidic gallium was neverobserved. The metallic compositions found below the oxidationlayer deviated strongly from the nominal composition used toprepare the alloys, a fact that can be explained by the neglectedmatrix effect, as sensitivity factors of pure elements were used forthe data analysis.

The sample immersed in pH 3.5 solution for 3 h (Fig. 6(a))showed an oxidic layer of approximately 120 nm. The ratio of Mg toAl in the first 30 nm of the film was smaller than in the rangebetween 50 nm and 100 nm. After 72 h (Fig. 6(b)), the surface layerthickness was only slightly larger compared to the one after 3 h,

a

b

c

d

e

f

Fig. 6. Schematic depth profiles for metallic and oxidized components as ob


and the Mg content exceeded the Al content in the outermostlayers. In the metallic solid, the Ga content was largest, while the Alcontent was found to be very small. When increasing the time to600 h (Fig. 6(c)), no metallic solid was found, and a Mg-rich oxide orhydroxide remained, containing some Al.

After brief immersion in neutral solution (pH 7, 3 h, shown inFig. 6(d)), an oxidized surface layer of approximately 40 nm wasobserved. The deviation in composition of the metallic solidcorresponds qualitatively to that observed for the same immersiontime at pH 3.5. Here, in the outermost 20 nm of the oxide film, theAl content decreased with increasing sputter depth, while the Mgcontent increased. A longer immersion time in the same solution(72 h, Fig. 6(e)) produced a significantly greater film thickness ofapproximately 450 nm. Here a very distinct outer surface layer ofroughly 20 nm was found where the Al content exceeded the Mgcontent, covering a Mg-dominated rich intermediate compositionalstage to about 350 nm before reaching the transient regionbetween corroded layer and metallic substrate. If the immersiontime was increased to 600 h (Fig. 6(f)), no metallic substrate wasfound. Here a small Ga-rich region was observed directly at thesurface, being metallic in nature.

In strongly alkaline solutions and at short times (pH 12, 3 h,shown in Fig. 6(g)) the surface layer formed was about 70 nmthick, while the transient region of oxygen and magnesium wassmeared: oxidic Mg was still found at depths where otherelements were metallic. This may originate from a MgO particle,presenting a higher resistance to Ar sputtering. The Mg content inthe outermost surface exceeded the value of Al, and increasedfrom the surface with increasing sputter depth. After 72 h(Fig. 6(h)), the oxide layer grew to about 200 nm and wasMg-dominated. If the time was increased to 600 h (Fig. 6(i)) theoxide layer became very thick, reaching values of 750 nm.

g

h

i

tained by AES measurements, for the immersion conditions indicated.



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From the depth information, it can be seen that increasingimmersion time leads to an increase in film thickness. The filmthicknesses after immersion in pH 12 roughly follows a square-rootpower law with time, while the film formed after 3 h in neutralsolution was thinner than those in acidic or alkaline solution. After3 days, the film thicknesses of samples immersed in neutral solu-tion were significantly stronger than the others.

3.3. Interpretation of AES results

The variation in oxygen content of the corrosion layers can beexplained easily by the different oxidation states of Mg (2þ) and Al(3þ), a correlation consistent for all samples immersed up to 72 h.Oxides and hydroxides could not, however, be distinguished by themethod applied.

In neutral solutions for times up to 72 h, or acidic solutions forshort times of about 3 h, the outermost films showed a tendency tobe aluminum-rich in comparison. This effect is particularlypronounced for 72 h immersion in pH 7.

The corrosion films in strongly alkaline solution were found to beMg-rich, corresponding to the stability of this element in the givenconditions. Immersed for 600 h, the sample showed an increasingMg content during the first 500 nm sputter depth. This can beexplained by a re-dissolution of Mg into aqueous media aftera continuous decrease in pH, e.g. caused by the formation of acidicdihydrogen carbonate (H2CO3) from CO2 dissolving into the solution.

The sample at pH 3.5/72 h showed a Ga content in the solidwhich exceeded all other elements. The resistance of the element tooxidation, and the assumed ‘insolubility’ in the aqueous solution(as no oxidized Ga was found), may explain the presence of excessGa below the continuously growing oxide layer. The existence oflocally-high amounts of metallic Ga can therefore be assumed.Further, a deviation from the stoichiometric composition of themetallic residues might be caused by the progressively growingsurface layer, changing the elemental composition of the under-lying substrate and possibly causing crystallization, leading to evenstronger elemental gradients.

In summary, it can be said that passive layers of comparablylarge Al content on some of the outermost sample surfaces wereconfirmed by AES, particularly for short immersion times in the pHrange where aluminum oxides are stable. The surface stability islimited by time and the pH value of the solution. In all pH ranges, nosignificantly stable passivity was observed, as indicated by verythick and continuously growing films.

4. Summary and conclusions

The Mg70Al15Ga15 ribbons were investigated in aqueous solu-tions to obtain information about their electrochemical stability. Itwas expected that the large Al and Ga content in the alloy wouldincrease its resistance to corrosion significantly.

Potentiodynamic polarization measurements show that thealloys can build passive layers, and an observed current density ofas low as 10�6 A/cm2 suggests a comparably good electrochemicalperformance in aqueous solutions of 0.1 M NaCl and pH valuesgreater than 4.

Immersion testing of the Mg70Al15Ga15 alloys shows that anoxide layer resistant against pitting is not built under any of thetesting conditions investigated, i.e. the surface layers formed duringthe initial corrosion were not able to protect the underlyingmaterial for extended times from reacting further with thesurrounding medium. The samples’ surfaces lost their metallicluster, at least locally, after a few days, while this degradationobviously occurred faster at lower pH. Optical micrographs showsevere pitting after a few days, and the formation of thick films can


be understood from iridescent color effects. For the Mg-basedcompositions investigated, the films keep growing after an initialpassivation, allowing a later destabilization of the film. Thisdestabilization may be caused by local attacks, or by the formationof cracks as sometimes observed.

XRD experiments on the immersed samples confirm theformation of primary amorphous surface oxides or hydroxides aftersome hours of immersion in neutral solution, and at least partiallycrystalline oxidic phases on the surfaces after several days. Theidentification of the phases was ambiguous, but partial matchessuggest the formation of Al2O3, MgO and spinel-type oxides(MgAl2O4). The results from AES were used to limit the amount ofphases and elements expected on the surface, but as the penetra-tion depth of X-rays typically exceeds the thickness of our film,underlying phases such as Ga2Mg resulting from devitrification dueto the dissolution of elements stabilizing the glassy phase wereobserved. The observation of intermetallic phases suggests that thecommonly-assumed homogeneity of glasses is not valid forcorroded samples due to the preferential dissolution of certainelements, leading to a deviation from the glass-forming composi-tion and consequently to the formation of crystalline phases, whichthen are able to act as local galvanic elements. The macroscopicappearance of samples immersed for longer times similarly impliesthat the assumed homogeneity cannot be maintained duringcontinuous growth of the corrosion layer.

AES deep profiling shows that an aluminum-rich oxide was onlyfound at the outermost surface of the sample corroded in neutralsolution; deeper sections of the corroded surfaces revealed filmsdominated by oxidized Mg, which is not stable in aqueous solutionsof pH< 11.

Apparently Ga is not involved in the formation of the passivelayer, despite its expected good electrochemical properties in neutralsolution [25]. AES depth profiles suggest a deposition of metallic Gabelow the corrosion layer, confirmed by the XRD results. A dissolu-tion into the aqueous media might explain the limited stability of thesurface formed, as it has been observed that Ga3þ activates thepassive layers on Al [30]. However, this seems unlikely because if itwere so the species should have been found by AES.

In final conclusions, a Mg-based alloy system (Mg70Al15Ga15) hasbeen found which shows significantly-reduced corrosion rates inpotentiodynamic polarization experiments. The spontaneouspassivation observed was seen to be connected to the presence ofaluminum oxides in the surface layer. In the experiments carried out,the alloy performs significantly better in alkaline to acidic NaClsolutions than other crystalline (WE43) or amorphous (Mg65Cu25Y10)Mg-based compositions found in literature. The use of gallium in thealloys appears not to be beneficial to the corrosion resistance, as it wasnot present in the passive layers. Nevertheless, it is possible thatfurther alloy development using similar compositions (with greaterAl content) will lead to the discovery of bulk glassy alloys withexcellent electrochemical and high specific mechanical properties.

Acknowledgements

This work was supported by ETH Research Grant TH 21/04-2.

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corrosion properties of glassy mg70al15ga15 in 0.1m nacl solution

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