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Electrochimica Acta 62 (2012) 305– 312

Contents lists available at SciVerse ScienceDirect

Electrochimica Acta

jou rn al hom epa ge: www.elsev ier .com/ locate /e lec tac ta

ffect of silicon and germanium alloying additions on the passivationharacteristics of Fe3Al intermetallic in sulphuric acid solution

. Rosalbinoa,∗, R. Carlinib,c, R. Parodid, G. Zanicchib,c

Dipartimento di Scienza dei Materiali e Ingegneria Chimica, Politecnico di Torino, Corso Duca degli Abruzzi 24 – 10129 Torino, ItalyDipartimento di Chimica e Chimica Industriale, Università di Genova, Via Dodecaneso 31 – 16146 Genova, ItalyGenoa Research Unit of the National Consortium of Materials Science and Technology (INSTM), ItalyINFN – Dipartimento di Fisica, Università di Genova, Via Dodecaneso 33 – 16146 Genova, Italy

r t i c l e i n f o

rticle history:eceived 28 July 2011eceived in revised form 9 December 2011ccepted 10 December 2011vailable online 19 December 2011

a b s t r a c t

The effect of Si and Ge on the passivation properties of Fe75Al25 intermetallic compound has been eval-uated in 0.25 M H2SO4 solution by means of potentiodynamic polarization, potentiostatic transients andelectrochemical impedance spectroscopy (EIS) techniques. Fe75Al25 and Fe70Al25M5 (M = Si, Ge) inter-metallic compounds were prepared by arc melting the stoichiometric amounts of pure elements in argonatmosphere. All the investigated iron aluminides exhibit a single-phase microstructure. Electrochemical

eywords:ron aluminidesotentiodynamic polarizationlectrochemical impedance spectroscopyEIS)assive film

studies reveal an improvement in the protective properties of passive films formed on Fe70Al25Si5 andFe70Al25Ge5 intermetallics as compared to Fe75Al25. XPS analysis of the surface layers anodically grownon ternary iron aluminides indicate that the passive films are composed of Al2O3, Fe2O3 and SiO2 or GeO2,thus explaining the better passivation characteristics of Fe70Al25Si5 and Fe70Al25Ge5 intermetallics withrespect to Fe75Al25.

© 2011 Elsevier Ltd. All rights reserved.

PS

. Introduction

Ordered iron aluminide intermetallics with the compositione3Al and FeAl possess unique properties of high mechanicaltrength whose weight ratio provide excellent resistance to highemperature oxidation and sulphidation [1,2]. Though iron alu-

inides are mainly developed for the structural applications atlevated temperatures, a number of studies have been concen-rated on the aqueous corrosion behaviour of these materials3–10]. There are primarily two reasons for this. First, to test dura-ility of these alloys when they are either stored ideally or duringorking conditions in a corrosive atmosphere. Second, being low

ost compared to conventional stainless steels, these materials arelso considered for room application in place of stainless steels in,or example, pipes and tubes for heating elements, and in distilla-ion and desalination plants. In addition to this, due to the fact thathe use of strategic elements such as Ni and Cr can be avoided, these

aterials are also considered for biomedical applications, such as

one joints and surgical instruments. Studies on passivation of ironluminide intermetallics in acid sulphate solutions have demon-trated that these materials possess a greater tendency to passivate

∗ Corresponding author. Tel.: +39 110904641; fax: +39 110904699.E-mail address: francesco.rosalbino@polito.it (F. Rosalbino).

013-4686/$ – see front matter © 2011 Elsevier Ltd. All rights reserved.oi:10.1016/j.electacta.2011.12.031

than metallic iron as a result of the formation of a more protec-tive passive film in which Al is the predominant cation [8,9,11].However, it is well known that iron aluminides undergo hydro-gen embrittlement due to the interaction of atmospheric moisturewith Al as per 2Al + 3H2O → Al2O3 + 6H [12]. Hydrogen embrittle-ment can be minimized by providing an oxide film on the ironaluminide surface [13,14]. The role of surface films in the hydrogenembrittlement of iron aluminide intermetallics has been reviewed[15]. In this respect, the addition of passivity-inducing elements(Cr, Ti, Mo, Ta, Nb, Zr, W, Si, Ge) should modify surface conditionssuch that hydrogen diffusion into the material is hindered, therebyreducing the propensity to hydrogen embrittlement. Though theelectrochemical corrosion behaviour of Fe3Al and FeAl intermetal-lic compounds in acid sulphate solutions has been studied by anumber of researchers [3,9,16] the influence of alloying elementson their passivation behaviour has scarcely been explored [17].

In this first work, the passivation behaviour of Si and Ge alloyedFe3Al intermetallic in sulphuric acid solution has been investigated.The aim of the present study was to assess the effect of Si and Geon the passivity of Fe3Al intermetallic compound.

2. Experimental

Fe75Al25 and Fe70Al25M5 (M = Si, Ge) intermetallic compoundswere prepared by arc melting the stoichiometric amounts of pure

3 himic

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tcmpapp

dswtdaafPite

bcweTetsi−Im1EpaSlvt(

fiuowprspsqcnaEcur

with the same crystal structure. Lattice parameter a values, deter-mined by X-ray diffraction analysis, are reported in Table 2.

Table 1Composition of the investigated intermetallic compounds determined by spectro-chemical analysis.

Sample Fe (at.%) Al (at.%) Si (at.%) Ge (at.%)

Fe75Al25 75.4 24.6Fe70Al25Si5 70.3 24.4 5.3

06 F. Rosalbino et al. / Electroc

lements (Fe and Al, 99.999 mass% purity, both supplied by MaTeckmbH, Julich, Germany; Si and Ge, 99.99 mass% purity, both sup-lied by Newmet Koch, Waltham Abbey, England) under a purifiedr-gettered argon atmosphere. Samples were tested in the as-castondition.

Chemical composition of samples was determined by spec-rometric analysis employing an argon microwave plasma torchoupled to spark ablation (Spectro Analytical Instruments). Speci-ens were ablated by a medium voltage spark (450 V, 370 Hz) in a

oint-to-plane configuration (spark times: 125 s) and swept into 100-W, 2.45-GHz argon microwave discharge. The microwavelasma was observed end-on and the radiation analysed with aolychromator.

Scanning electron microscopy (SEM) equipped and X-rayiffraction analysis were used to examine the microstructure ofpecimens. A scanning electron microscope EVO 40 (Carl Zeiss)as employed. Smooth surfaces for microscopic observation before

he electrochemical tests were prepared by using SiC papers andiamond pastes with grain size down to 1 �m. X-ray diffractionnalysis was used in order to identify the phase crystal structuresnd to determine lattice parameters. The measurements were per-ormed by means of a vertical diffractometer (Philip X’Pert model).hases were structurally identified and their diffraction peaks werendexed by using the program PowderCell [18]. Lattice parame-ers were determined by means of LCLSQ software [19] using thexperimentally obtained 2� values as input data.

Electrochemical measurements were carried out in a round-ottomed electrochemical cell using a standard three-electrodeonfiguration: saturated calomel electrode (SCE) as a referenceith a platinum electrode as counter and a sample as the working

lectrode. The surface area exposed to the test solution was 0.5 cm2.he specimens were given a metallographic polishing prior to eachxperiment, followed by washing with distilled water and ace-one. All the experiments were performed in aerated 0.25 M H2SO4olution at room temperature (25 ± 0.1 ◦C). Potentiodynamic polar-zation curves were recorded after a cathodic pre-treatment at1000 mV/SCE for 300 s followed by potential sweep at 0.5 mV s−1.

R drop compensation was achieved using a current interruptionethod. Measurements were carried out employing a Solartron

286 Electrochemical Interface controlled by a personal computer.lectrochemical impedance was measured at a selected anodicotential value within the fully passive region of specimens using

Solartron 1255 Frequency response Analyzer coupled to theolartron 1286 Electrochemical Interface. The frequency range ana-ysed went from 100 kHz down to 10 mHz, with the frequencyalues spaced logarithmically (seven per decade). The width ofhe sinusoidal voltage signal applied to the system was 10 mV rmsroot-mean-square).

The electrochemical treatments used for growing the anodiclms for subsequent XPS analysis were performed in a glove boxnder a nitrogen atmosphere in order to minimize the influencef the air atmosphere on the surface composition. The specimensere first held at the free corrosion potential for 60 s before theotential was one-step switched to the desired value in the passiveegion and maintained there for 30 min. The anodically passivatedpecimens were removed from the electrolyte under the appliedotential, rinsed with de-ionized water and dried in a nitrogentream before they were mounted onto the XPS holder. Subse-uently, they were transferred from the glove box to the UHVhamber of the spectrometer using a transfer vessel filled withitrogen. XPS analyses were carried out in Ar atmosphere using

PHI ESCA 5600 Multi Technique electron spectrometer (Physical

lectronics). During the measurements the residual pressure in thehamber was always <6 × 10−10 mbar. All spectra were obtainedsing an Al anode (K� radiation 1486.6 eV, monochromatized) as X-ay source operating at 0.5–2 kV voltages and 10–100 �A currents.

a Acta 62 (2012) 305– 312

The spherical capacitor electron energy analyser was used in thefixed analyser transmission mode adjusting the pass energy to givean overall resolution of approximately 0.01 eV. XPS measurementswere performed at the take-off angle of 45◦ measured with respectto the plane of the specimen. The composite peaks were resolvedin their components by means of the Gaussian–Lorentzian formulacombined with a profile background subtraction in accordance withthe Shirley procedure [20].

3. Results and discussion

3.1. Microstructural characterization

Compositional data of the iron aluminides under study wereobtained by spectrometric analysis of bulk samples and arereported in Table 1. Good agreement between nominal and exper-imental composition can be observed.

All the investigated intermetallic compounds show a single-phase microstructure, as evidenced by scanning electronmicroscopy coupled with EDX analysis. As an example, Fig. 1reports the microstructure of the Fe70Al25Ge5 alloy after etchingwith Nital (alcoholic solution of HNO3). Grain boundaries sur-rounding iso-oriented sub-grains, formed during solidificationprocess, are easily highlighted. At the actual state of the research,the samples represent “as-cast” intermetallic compounds obtainedby arc-melting, with uncontrolled cooling rate, therefore thesize and the shape of the grains are not corresponding to adesigned process. However the metallography helps to determinethe monophasic nature of the solid solution and the absence ofprimary heterogeneity.

Iron aluminide Fe75Al25 belongs to cF16-BiF3, cubic structure,space group Fm-3m (2 2 5). The solubility of Si and Ge in Fe allowsus to obtain Fe70Al25Si5 and Fe70Al25Ge5 intermetallic compounds

Fig. 1. Optical micrograph of Fe70Al25Ge5 intermetallic compound (Nital etching).Single-phase microstructure.

F. Rosalbino et al. / Electrochimica Acta 62 (2012) 305– 312 307

Table 2Values of the lattice parameter a for the investigated intermetallic compounds.

Sample Lattice parameter a (nm)

Fe75Al25 0.5813

3

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ppap

ciam

flFMpEb

o(fwp

FH

20001500100050000

1

2

3

4 Fe75Al25 Fe70Al25Si5 Fe70Al25Ge5

j / m

A c

m-2

t / s

Fe70Al25Si5 0.5773Fe70Al25Ge5 0.5812

.2. Electrochemical characterization

Fig. 2 reports typical potentiodynamic polarization curves ofe75Al25, Fe70Al25Si5 and Fe70Al25Ge5 intermetallic compoundsecorded in aerated 0.25 M H2SO4 solution at 25 ◦C. With respecto Fe75Al25 intermetallic, the active dissolution range of iron alu-

inides Fe70Al25Si5 and Fe70Al25Ge5 is shifted to more nobleotentials. All samples exhibit active-passive behaviour, implyinghat there is a region where the current density is lowered on polar-zing past the zero current potential. However, they differ in naturef transition from active to passive state. Fe75Al25 intermetallichows sudden transition from active to passive state after consid-rable active dissolution, whereas iron aluminides Fe70Al25Si5 ande70Al25Ge5 display a slower decrease in anodic current density.

The passivity parameters (potential for primary passivation, Epp,otential for complete passivation, Ecp, transpassive potential, Etp,assive range, Etp − Ecp, critical current density for passivation, icrit,nd passive current density, ipass) obtained from the analysis ofolarization curves are reported in Table 3.

Electrochemical data obtained from the analysis of polarizationurves (Table 3) show that critical current density, icrit, which is anmportant parameter in assessing the dissolution tendency of anlloy, is significantly lower for Fe70Al25Si5 and Fe70Al25Ge5 inter-etallic compounds as compared to Fe75Al25.While comparing all samples in their passive regions it is

ound that iron aluminides Fe70Al25Si5 and Fe70Al25Ge5 displayower potential for primary passivation, Epp, values with respect toe75Al25 intermetallic, indicating a higher tendency to passivation.oreover, Fe70Al25Si5 and Fe70Al25Ge5 intermetallics exhibit lower

assive current density, ipass, values, and wider passive ranges,tp − Ecp, compared to Fe75Al25, indicating better passivation sta-ility in the passive regions.

In all cases, above the transpassive potential, Etp, gases arebserved to evolve on the specimen surface. Since the Etp valuesTable 3) are well above the equilibrium potential of 960 mV/SCEor oxygen evolution, i.e. 2H O ↔ O + 4H+ + 4e− equilibrium, which

2 2as calculated based on the measured pH of 0.6 and assuming O2artial pressure of 1 atm, the evolved gas could correspond to O2.

18001150500-150-8000.01

0.1

1

10

100

1000 Fe75Al25 Fe70Al25Si5 Fe70Al25Ge5

j / m

A c

m-2

E vs SCE / mV

ig. 2. Potentiodynamic polarization curves of iron aluminides in aerated 0.25 M2SO4 solution at 25 ◦C.

Fig. 3. Current–time transients for passive film formation on iron aluminides inaerated 0.25 M H2SO4 solution at 25 ◦C.

To assess the tendency to passivation of specimens potentio-static tests were carried out. Fig. 3 depicts the change of passivecurrent density over time for iron aluminides Fe70Al25Si5 andFe70Al25Ge5 compared to Fe75Al25 intermetallic during polariza-tion of samples at the anodic potential of 1000 mV/SCE. A constantdecrease of current density with increasing time is observed, indi-cating passive film formation on the specimen surface. However,the passive current of Fe70Al25Si5 and Fe70Al25Ge5 intermetallicsdecreases more rapidly, exhibiting a steady-state current of 220 and110 �A cm−2 after about 600 and 300 s, respectively. On the otherhand, Fe75Al25 attains a stationary passive current of 340 �A cm−2

after about 1300 s. On the basis that the time for achievement ofsteady-state conditions in the passive region is usually taken as anindicator of the ability of a material to passivate, this behaviourshows that the additions of silicon and germanium to Fe75Al25intermetallic enhance the formation of a passive layer.

Fig. 4 displays complex plane impedance plots of Fe75Al25,Fe70Al25Si5 and Fe70Al25Ge5 iron aluminides in the passive stateafter potentiostatic transients at an applied anodic potential of1000 mV/SCE for 30 min. As can be seen, the response of the systemin the Nyquist complex plane is a semicircle for all materials, butthe diameter of the capacitive loop for Fe75Al25 is smaller than thatobtained for Fe70Al25Si5 and Fe70Al25Ge5 intermetallics.

In mathematical analysis of impedance diagrams the constantphase element (CPE), was used instead of an “ideal” capacitorto account for the deviations observed as capacitive loops aredepressed. The impedance, ZCPE, of CPE is described by the expres-sion [21,22]:

ZCPE = [Q (jω)n]−1

with n and Q constants, which can be used in data fitting wheninterpreting frequency dispersion. When the value of n is close tounity, the CPE behaves as an ideal capacitance. The equivalent cir-cuit, shown in detail in Fig. 5, was used to fit the experimentaldata. Rel represents the resistance of electrolyte, Rf is the resistancedue to electric conduction in the passive film and Q is the capac-itance due to the dielectric properties of the passive film (n ≈ 1;Q ≈ C). The agreement between experimental and simulated results(Fig. 4) indicates that the experimental results are well fitted tothe proposed equivalent circuit. A standard deviation �-quadratewas in the order of 10−5, and the relative error was less than5%. The parameters of the equivalent circuit Rel, Rf and Q wereevaluated using a simple least square fit procedure and are pre-

sented in Table 4. As can be seen, iron aluminides Fe70Al25Si5 andFe70Al25Ge5 exhibit significantly higher Rf values with respect toFe75Al25 intermetallic. This indicates that the addition of Si andGe influences the nature of the surface layer giving rise to passive

308 F. Rosalbino et al. / Electrochimica Acta 62 (2012) 305– 312

Table 3Passivation parameters obtained from the potentiodynamic polarization curves of Fe75Al25, Fe70Al25Si5 and Fe70Al25Ge5 intermetallic compounds recorded in aerated 0.25 MH2SO4 solution at 25 ◦C.

Sample Epp (mV/SCE) Ecp (mV/SCE) Etp (mV/SCE) Etp − Ecp (mV) icrit (mA cm−2) ipass (mA cm−2)

Fe75Al25 320 645 1560 915 100–245 0.350Fe70Al25Si5 −275 615 1590 975 34–35 0.190Fe70Al25Ge5 −75 595 1670 1075 2.4–2.5 0.120

500004000030000200001000000

6000

12000

18000

24000

30000Fe75Al25 Experimental

Fitting

-Z(Im

) / Ω

cm

2

1000008000060000400002000000

14000

28000

42000

56000

70000Fe70Al25Si5 Experimental

Fitting

-Z(Im

) / Ω

cm

2

140000105000700003500000

25000

50000

75000

100000Fe70Al25Ge5 Experimental

Fitting

-Z(Im

) / Ω

cm

2

Z(Re) / Ω cm2

Z(Re) / Ω cm2 Z(Re) / Ω cm2

Fig. 4. Nyquist plots of iron aluminides in the passive state after potentiostatic transientfor 30 min.

Solution

Metal Passive Film

Q

Rs

Rf

Q

Rs

Rf

Fig. 5. Equivalent circuit used to fit the experimental impedance data.

s in aerated 0.25 M H2SO4 solution at an applied anodic potential of 1000 mV/SCE

films with improved properties. Moreover, analysis of potentiody-namic polarization curves shows that Fe70Al25Si5 and Fe70Al25Ge5intermetallics display lower passive current density, ipass, valuesand wider passive ranges, Etp − Ecp, compared to Fe75Al25 (Table 3).Therefore, it is reasonable to conclude that Si and Ge alloying addi-tions lead to the formation of passive films with higher protectivecapabilities.

3.3. Surface characterization

In order to examine the composition of passive films formedon the exposed surfaces of iron aluminides Fe75Al25, Fe70Al25Si5and Fe70Al25Ge5, samples of these intermetallic compounds were

Table 4Electrical parameters of equivalent circuit obtained by fitting the experimentalresults of EIS.

Sample Rel (� cm2) Q (F cm−2 sn−1) n Rf (k� cm2)

Fe75Al25 21.76 8.54 × 10−6 0.93 47.65Fe70Al25Si5 21.68 5.76 × 10−6 0.95 96.01Fe70Al25Ge5 21.59 4.34 × 10−6 0.96 128.45

F. Rosalbino et al. / Electrochimica Acta 62 (2012) 305– 312 309

Fig. 6. Deconvoluted XPS spectrum of (a) Al 2p, (b) Fe 2p, and (c) O 1s for Fe75Al25 intermetallic compound after passivation in 0.25 M H2SO4 solution at 1000 mV/SCE for3

a3

ft

mi[sfia[lp5BF[

0 min.

nodically polarized in 0.25 M H2SO4 solution at 1000 mV/SCE for0 min.

Figs. 6–8 reports experimental XPS spectra of passive filmormed on Fe75Al25, Fe70Al25Si5 and Fe70Al25Ge5 after removal ofhe 5 nm top layer by Ar ions etching.

On deconvolution, XPS spectrum of Al 2p for Fe75Al25 inter-etallic, Fig. 6a, reveals the presence of two peaks. The low

ntensity peak at BE 72.1 eV corresponds to 2p level of metallic Al23], while the high intensity peak at BE 74.8 eV corresponds to 2ppectrum of Al2O3 [23]. In Fig. 6b, deconvoluted Fe 2p spectrumor Fe75Al25 intermetallic consists of two peaks, the low bind-ng energy peak at 706.3 eV assigned to metallic state (Fe0) [24]nd the broad peak appearing at 709.9 eV corresponds to Fe2O324]. Deconvoluted XPS spectrum of O 1 s for Fe75Al25 intermetal-ic, Fig. 6c, reveals the presence of three peaks. The high intensityeak at BE 531.59 eV and the intermediate intensity peak at BE

30.46 eV correspond to Al2O3 [25], while the low intensity peak atE 529.40 eV is assigned to Fe2O3 [26]. The formation of Al2O3 ande2O3 can be explained by the following electrochemical reactions27,28]:

2Al3+ + 3H2O → Al2O3 + 6H+

2Fe3+ + 3H2O → Fe2O3 + 6H+

Deconvoluted XPS spectrum of Al 2p for Fe70Al25Si5 intermetal-lic, Fig. 7a, reveals the presence of two peaks. The low intensitypeak at BE 72.0 eV corresponds to 2p level of metallic Al [23], whilethe high intensity peak at BE 74.3 eV corresponds to 2p spectrum ofAl2O3 [23]. In Fig. 7b, deconvoluted Fe 2p spectrum for Fe70Al25Si5intermetallic consists of two peaks, the low binding energy peakat 706.1 eV assigned to metallic state (Fe0) [24] and the broad peakappearing at 709.9 eV corresponds to Fe2O3 [24]. Deconvoluted XPSspectrum of Si 2p for Fe70Al25Si5 intermetallic, Fig. 7c, reveals thepresence of two peaks. The low binding energy peak at 99.8 eVcorresponds to elemental Si [29] while the high binding energypeak at 103.3 eV corresponds to SiO2 [29]. In Fig. 7d, deconvo-

luted XPS spectrum of O 1s for Fe70Al25Si5 intermetallic consistsof three peaks. The peaks at BE 530.30 and 532.00 eV are assignedto Fe2O3 [30,31] and Al2O3 [32,33], respectively, while the peak atBE 533.80 eV corresponds to SiO2 [29].

310 F. Rosalbino et al. / Electrochimica Acta 62 (2012) 305– 312

Fig. 7. Deconvoluted XPS spectrum of (a) Al 2p, (b) Fe 2p, (c) Si 2p and (d) O 1s for Fe70Al25Si5 intermetallic compound after passivation in 0.25 M H2SO4 solution at1

lptAi7aspcpXtct[

000 mV/SCE for 30 min.

Deconvoluted XPS spectrum of Al 2p for Fe70Al25Ge5 intermetal-ic, Fig. 8a, reveals the presence of two peaks. The low intensityeak at BE 72.2 eV corresponds to 2p level of metallic Al [23], whilehe high intensity peak at BE 74.3 eV corresponds to 2p spectrum ofl2O3 [23]. In Fig. 8b, deconvoluted Fe 2p spectrum for Fe70Al25Ge5

ntermetallic consists of two peaks, the low binding energy peak at06.4 eV assigned to metallic state (Fe0) [24] and the broad peakppearing at 709.9 eV corresponds to Fe2O3 [24]. Deconvoluted XPSpectrum of Ge 2p for Fe70Al25Ge5 intermetallic, Fig. 8c, reveals theresence of two peaks. The low binding energy peak at 1217.6 eVorresponds to elemental Ge [29] while the high binding energyeak at 1220.2 eV corresponds to GeO2 [29]. In Fig. 8d, deconvolutedPS spectrum of O 1s for Fe70Al25Ge5 intermetallic, Fig. 8d, reveals

he presence of three peaks. The peaks at BE 530.30 and 532.20 eVorrespond to Fe2O3 [30,31] and Al2O3 [32,33], respectively, whilehe intermediate intensity peak at BE 533.60 eV is assigned to GeO229].

The following electrochemical reactions can be considered inorder to explain the formation of SiO2 and GeO2 oxides:

Si4+ + 2H2O → SiO2 + 4H+

Ge4+ + 2H2O → GeO2 + 4H+

Based on XPS analysis, the passive film formed on the sur-face of Fe75Al25 intermetallic polarized in 0.25 M H2SO4 solutionat 1000 mV/SCE is thought to consist of mixed Al and Fe oxides[34 and references therein]. On the other hand, XPS analysis per-formed on Fe70Al25Cr5 intermetallic compound passivated in 1 M

Na2SO4 solution at 1000 mV/SCE for 45 min revealed the presenceof a surface layer composed by a mixed oxide of Al2O3, Fe2O3 andCr2O3 [35]. Such a passive film is considered to be responsiblefor the improved corrosion behaviour of Fe70Al25Cr5 with respect

F. Rosalbino et al. / Electrochimica Acta 62 (2012) 305– 312 311

F for F1

t5

sshStTrsc

stettl

ig. 8. Deconvoluted XPS spectrum of (a) Al 2p, (b) Fe 2p, (c) Ge 2p and (d) O 1s000 mV/SCE for 30 min.

o Fe75Al25 intermetallic in sodium sulphate solution containing00 ppm Cl− ions [35].

In the present work, on the basis of XPS results, it is reasonable touppose that for iron aluminides Fe70Al25Si5 and Fe70Al25Ge5 pas-ivity must take place when a surface layer composed of a mixedomogeneous Al, Fe and Si or Ge oxide builds up. Besides, sinceiO2 and GeO2 are oxides with high dielectric characteristics [36,37]heir presence enhances the protective properties of passive films.his may explain the experimental findings showing the beneficialole of Si and Ge not only in accelerating the formation of the pas-ive layer, but also by providing a film with improved protectiveapabilities.

Fe3Al intermetallic compound possesses relatively high specifictrength and suitable mechanical properties at elevated tempera-ures in aggressive environments [1,2]. It has, therefore, undergone

xtensive development, in the recent past, exclusively for highemperature applications. However, its potential use as a struc-ural material at elevated temperatures has been hindered byimited ductility at room temperature and sharp drop in strength

e70Al25Ge5 intermetallic compound after passivation in 0.25 M H2SO4 solution at

above 600 ◦C. It is well established that the poor room temperatureductility of Fe3Al intermetallic is due to hydrogen embrittle-ment [2,14], with the hydrogen causing embrittlement arisingout of the interaction of atmospheric moisture with Al as per2Al + 3H2O → Al2O3 + 6H [12]. Addition of passivity-inducing alloy-ing elements is one of the most important methods proposed toimprove ductility of Fe3Al intermetallic compound [38,39]. Thepassive layer would act as a barrier to the diffusion of hydrogeninto the material, thereby reducing the propensity to hydrogenembrittlement. In the present work, electrochemical results high-lighted the enhanced passivity of iron aluminides Fe70Al25Si5and Fe70Al25Ge5 with respect to Fe75Al25 intermetallic. As aconsequence, Si and Ge alloying additions significantly reducethe corrosion rate of Fe3Al intermetallic compound leading to adecrease in the hydrogen reduction reaction rate. This reduced

hydrogen reduction rate lowers the amount of nascent hydrogen inthe material and hence decreases hydrogen embrittlement, therebyimproving the room temperature ductility of iron aluminideFe3Al.

3 himic

da

4

ittn

tpF

1Sli

iFdir

A

Pam

R

[[[[[[[[

[[

[[[[[[[[

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[[[[[[

Perform. 16 (2007) 779.[36] M.E. Striefler, G.R. Barsch, Phys. Status Solidi 67 (1975) 143.[37] S.T. Pantelides, W.A. Harrison, Phys. Rev. B 13 (1976) 2667.

12 F. Rosalbino et al. / Electroc

Further investigation is planned with the aim of obtaining aeeper understanding of the corrosion properties of Fe70Al25Si5nd Fe70Al25Ge5 intermetallic compounds.

. Conclusions

The influence of Si and Ge on the passive behaviour of Fe75Al25ntermetallic compound has been assessed in 0.25 M H2SO4 solu-ion by means of potentiodynamic polarization, potentiostaticransients and electrochemical impedance spectroscopy (EIS) tech-iques.

Si and Ge exercise a beneficial role in the passivating charac-eristics of Fe75Al25 as evidenced by comparing potentiodynamicolarization curves and current–time transients of Fe70Al25Si5 ande70Al25Ge5 intermetallics with those of Fe75Al25.

EIS measurements performed at the anodic potential of000 mV/SCE in the fully passive region of specimens showed thati and Ge alloying additions give rise to the formation of a surfaceayer with improved protective properties as compared to Fe75Al25ntermetallic.

XPS studies on the surface layers anodically grown on ternaryron aluminides indicated that the passive films are composed ofe, Al and Si or Ge mixed oxides, with SiO2 and GeO2 being highlyielectric oxides, thus explaining the better passivation character-

stics of Fe70Al25Si5 and Fe70Al25Ge5 intermetallic compounds withespect to Fe75Al25.

cknowledgements

The authors would like to thank prof. M.G. Ienco and prof. P.iccardo (University of Genoa, Metallurgical Section of Chemistrynd Industrial Chemistry Department) for their assistance in theicrostructural characterization of samples.

eferences

[1] C.T. Liu, J.O. Stiegler, F.H. Froes, Metals Handbook, vol. 2, 10th Ed., ASM, MetalsPark, USA, 1990.

[[

a Acta 62 (2012) 305– 312

[2] C.T. Liu, K.S. Kumar, J. Metals 45 (1993) 38.[3] S. Frangini, Corrosion 55 (1999) 89.[4] H.C. Choe, H.S. Kim, D.C. Choi, K.H. Kim, J. Mater. Sci. 32 (1997) 1221.[5] N. De Cristofaro, S. Frangini, A. Mignone, Corros. Sci. 38 (1996) 307.[6] R. Balasubramaniam, Scripta Mater. 34 (1996) 127.[7] S. Frangini, J. Lascovich, N. De Cristofaro, Mater. Sci. Forum 185–188 (1995)

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