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Journal of Alloys and Compounds 483 (2009) 514–518

Contents lists available at ScienceDirect

Journal of Alloys and Compounds

journa l homepage: www.e lsev ier .com/ locate / ja l l com

queous corrosion behaviour of Fe–Ni–B metal glasses

. Lekatou ∗, A. Marinou, P. Patsalas, M.A. Karakassidesniversity of Ioannina, Department of Materials Science and Engineering, Ioannina 45110, Greece

r t i c l e i n f o

rticle history:eceived 30 August 2007eceived in revised form 2 July 2008ccepted 22 July 2008

a b s t r a c t

The corrosion performance of Fe–40Ni–20B (at%) amorphous ribbons in 7N NaOH and 3.5% NaCl, at 25 ◦C,is investigated by potentiodynamic polarization, chronoamperometry and Raman spectroscopy. Immer-sion in 7N NaOH induces a multiple stage passivation/pseudopassivation, which is associated with thesuccessive formation of (Ni,Fe) compounds based on Fe3O4, Ni(OH)2/NiO, FeOOH/Fe2O3, Ni(OH)2·2NiOOH

vailable online 14 November 2008

eywords:etallic glasses

orrosionassivity

and �NiOOH. Fe3O4, NiO, Fe2O3 and �NiOOH are responsible for true passivity. Although carbonate ionsfrom the environment have penetrated the passive film, the glass is not susceptible to pitting in 7N NaOH.Despite amorphicity, the glass behaviour in 7N NaOH is governed by the behaviour of its crystallinemetallic constituents. The alloy exhibits a slight susceptibility to localized corrosion, in 3.5% NaCl.

© 2008 Elsevier B.V. All rights reserved.

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. Introduction

Metallic glasses are rapidly quenched alloys with metals as theain constituents. Metalloid elements (B, C, P, Si) are indispens-

ble components of the metallic glasses, as they largely contributeo the amorphous phase formation [1]. The Fe–Ni–B glasses areromising materials for various industrial applications, since theyombine interesting properties with low material cost [1,2]. As such,he subject of their corrosion resistance is of great importance.

Amorphous alloys are expected to have higher corrosion resis-ance than their metallic constituents, since they have no grainoundaries, segregates or crystalline defects that may act as cor-osion initiation sites. For example, bulk amorphous Ni–Nb–Ta–Plloys exhibited spontaneous passivity and immunity to pitting,ven in 12 M HCl [3]. Fe–Ni–Cr–W amorphous sputter depositedlloys showed excellent pitting resistance to neutral and acidicolutions, which was preserved after crystallization heat treatment4]. Mg–Ni–Nd glasses presented low corrosion rates in 0.01 MaCl, due to NiO and Nd2O3 incorporation in the passive film [5].ebert et al. [6] attributed the improved corrosion properties ofmorphous Mg65Cu25Y10 in a highly alkaline electrolyte to the for-

ation of a homogeneous and dense surface film. Magrini andatteazzi [7] reported that the corrosion resistance of amorphous

e–Ni–Mo–B alloys is enhanced with Ni content, due to an increasen the alloy disorder. Metalloid elements influence the corrosion

∗ Corresponding author. Tel.: +30 26510 97309; fax: +30 26510 97034.E-mail address: alekatou@cc.uoi.gr (A. Lekatou).

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925-8388/$ – see front matter © 2008 Elsevier B.V. All rights reserved.oi:10.1016/j.jallcom.2008.07.217

ehaviour of metal glasses: the corrosion resistance of amorphouse–Cr alloys progressively increased in the order Si, B, C and P, in.1N H2SO4, and in the order B, Si, C, P, in 3% NaCl [8].

The present work is part of a wider study aiming at determininghether Fe–Ni–B glasses have satisfactory corrosion properties and

lucidating on the corrosion mechanisms, in alkaline, neutral andcid environments.

. Experimental

Fe–40Ni–20B (at%) ribbons were manufactured by melt-spinning. Rectangularibbon coupons were subjected to electrochemical testing in a three electrode corro-ion cell (reference electrode: Ag/AgCl (3.5 M KCl), counter electrode: Pt). They wereltrasonically cleaned and encapsulated in PTFE, leaving a surface area of ∼50 mm2,o be exposed to aerated 7N NaOH (pH 12.1) and 3.5% NaCl (pH 6.5), at roomemperature. Cyclic potentiodynamic polarization tests (scan rate: 10 mV/min) andotentiostatic polarization tests were performed using the potentiostat/galvanostatILL AC (ACM Instruments). The nature of the corrosion products was investigatedy Raman spectroscopy (RM 1000 RENISHAW).

. Results and discussion

A detailed characterization of the glass by EDS, Auger electronpectroscopy and XRD has previously been reported [9].

.1. Electrochemical testing

Fig. 1 illustrates the polarization behaviour of Fe–40Ni–20B inN NaOH (25 ◦C). A multiple stage passivity in a wide range ofotentials is observed. The large passive current densities indi-ate pseudopassive behaviour. The positive hysteresis of the reverse

A. Lekatou et al. / Journal of Alloys and Compounds 483 (2009) 514–518 515

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ig. 1. Cyclic polarization of Ni–40Fe–20B in 7N NaOH (25 ◦C); the numbers corre-pond to passive stages. Also, anodic polarization curves for Ni and Fe; current peaksenoting the onset of passivation are pointed out by circles.

can (“reverse” current densities lower than “forward” ones) showshat the alkaline electrolyte does not cause localized corrosion. (Its well known that caustic solutions do not promote pitting in crys-alline alloys. However, cyclic polarization in aqueous NaOH wasarried out, since it was observed (see Section 3.2) that the pas-ive film had been penetrated by carbonate ions, which are very

ggressive.) In an attempt to clarify the processes causing multipletage passivation in the caustic solution, potentiodynamic polar-zation tests on pure Ni and Fe were conducted. The respectiveoltammograms are also included in Fig. 1.

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Fig. 3. Chronoamperometry plots at active and passive potentials, in 7N NaOH (2

Fig. 2. Cyclic polarization of Fe–40Ni–20B in 3.5% NaCl (25 ◦C).

Fig. 2 presents the polarization behaviour of Fe–40Ni–20B in.5% NaCl (25 ◦C). The alloy shows a small susceptibility to pitorrosion, as the slightly higher currents during reverse scanninguggest. The corrosion potential in 3.5% NaCl (−320 mV vs. Ag/AgCl)s much lower than the corrosion potential in 7N NaOH (−1160 mVs. Ag/AgCl). This shows that, from a thermodynamic standpoint,he alloy is nobler in the saline solution. On the other hand, the higheactivity of the glass in the alkaline electrolyte is responsible for

ts passivity.

Passivity in 7N NaOH is further investigated by potentiostaticolarization (Fig. 3). Although, a more analytical discussion on theharacteristics of the i–t curves will follow in context with the

5 ◦C). (a) First to third passive stages and (b) fourth and fifth passive stages.

516 A. Lekatou et al. / Journal of Alloys and C

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ig. 4. Chronoamperometry plots at active potentials in 7N NaOH and 3.5% NaCl25 ◦C).

esults of Raman spectroscopy (Section 3.3), some important obser-ations can be reported in this section: (i) the i–t curve at an activeotential shows that the current density initially increases withime, as a result of active dissolution of the alloy; then, currentensity very slowly decreases with time, suggesting deposition oforrosion products on the alloy surface and/or reduction of thevailable surface for corrosion. (ii) The i–t curves at higher over-otentials present notably fluctuating transients: sharp decreases

n current (as a function of time) suggest formation and growthf a film consisting of oxidized compounds; sharp increases in cur-ent suggest film dissolution. (iii) The i–t curve morphology appearsuite different after polarization at −770 V, −50 V and 0 V: currentensities are lower than the ‘active’ ones indicating a truly passivetate; absence of fluctuation or low fluctuation of current densitys. time also supports true passivity; long stabilization trends ofurrent vs. time indicate that the passive film has attained constanthickness.

Fig. 4 includes chronoamperometry plots at active potentials, forpecimens immersed in 7N NaOH and 3.5% NaCl. Initially, in bothlectrolytes, the current density sharply increases, due to activepecies dissolution; however, the much faster current increase in

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ig. 5. Raman spectra after chronoamperometry at active (−1060 mV) and passive (365 mV: goethite (FeOOH) and n: Ni(OH)2·NiOOH or NiO·Ni2O3·xH2O.

ompounds 483 (2009) 514–518

N NaOH demonstrates the much higher reactivity of the glass inhis electrolyte. Then, in 7N NaOH, the current density smoothlyecreases due to surface depositions. The eventual stabilizationrend suggests that equilibrium has been attained, where dissolu-ion and formation rates equalize. In contrast, in 3.5% NaCl, currenteeps on increasing with time, at a deminishing rate though. Theatter also shows formation of corrosion products and/or reductionf the available surface for corrosion, though at a much lower ratehan that in 7N NaOH.

.2. Nature of corrosion products

Fig. 5 includes typical Raman spectra of active and “passive”fifth passive stage) corrosion products after potensiostatic polar-zation in 7N NaOH (25 ◦C). In the majority of spectra, regardlessf corrosion state, the two strongest bands correspond to theavenumbers 536–571 cm−1 (higher intensity) and 445–473 cm−1

lower intensity). These bands characterize nickel oxides andydroxides (NiO, Ni(OH)2, NiOOH); the lower wavenumber bandenerates from the Ni–OH stretching vibration, whilst the higheravenumber band generates from the Ni–O stretching vibration

10,11]. Various small peaks in the range 220–720 cm−1 are assignedo iron oxides/hydroxides (�-Fe2O3, �-Fe2O3, Fe3O4, �-FeOOH)12].

The possibility of formation of mixed (Ni,Fe) compounds istrong, as implied by the simultaneous presence of bands gener-ted from iron and nickel oxides/hydroxides and by the fact that thee–Ni system thermodynamically approaches an ideal solid solu-ion. Indeed, previous works have shown that exposure of Fe–Nilloys to caustic and H2SO4 solutions leads to the formation of pas-ive bi-layered films. The outer layer consists of (Ni,Fe) hydroxides.he inner layer mainly consists of anhydrous oxides [13,14].

Often, Raman spectra from specimens polarized at active poten-ials have poorly defined peaks, evidence of depositions of very lowhickness. Active corrosion products in 7N NaOH are dominated byiO, �Ni(OH)2 and Ni(OH)2·NiOOH, probably incorporating iron in

olid solution with nickel. NiO and �Ni(OH)2 are formed at the earlyxidation stages of Ni [15]. �Ni(OH)2 is amorphous, hydrated and

The participation of NiO and Fe2O3 in the oxidation productsndicates that passivation is caused by the NiO/Fe2O3 inner layer.iO is generally accepted as a p-type semiconductor and shouldave a barrier character [13]. Another compound that should favour

) potentials, in 7N NaOH (25 ◦C). h: hematite (�-Fe2O3), mh: maghemite (�-Fe2O3),

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assivity is �NiOOH. �NiOOH predominates over the passive cor-osion products, in the final passive stage, whereas Ni(OH)2 isbsent. Therefore, Ni oxidation has progressed up to the Ni3+ state.NiOOH is the most stable Ni-hydroxide [17]. The (Ni,Fe)(OH)2 layeroes not contribute much to passivity, due to its low thicknessfew monolayers) in all types of electrolytes [13,17]. (It should beoted that the structure of the passivating layers is complex andighly non-stoichiometric. Therefore, the fore-mentioned chemi-al formulae are simplified forms of much more complex—perhapsolymeric—structures.)

Weak bands at 805–820 cm−1, 1210 cm−1 and 1260 cm−1 aressigned to vibrations of boroxol rings [18,19]. In this work, boronas been found to participate in both passive and active corrosionroducts. Boron has been reported to be present in the passive filmf Ni-metallic glasses as borate. It is considered to negatively affectassivity, by hindering NiO formation in Ni-glasses [20]. Whether

ts contribution to the corrosion behaviour of the studied glasses ismportant, is a matter of a future investigation.

Intense peaks in the range 1044–1074 cm−1 are ascribed tohe stretching vibrations of CO3

2−[21]. The peak at 1004 cm−1 isssigned to the stretching vibrations of HCO3

−. The presence ofarbonate ions makes corrosion attack even more severe, due tohe easy penetration of the hydroxide films by carbonate ions andubsequent formation of carbonate salts. The latter are readilyccommodated in the (Ni,Fe)-hydroxide structure.

Bands in the range 1340–1440 cm−1 and 1550–1650 cm−1 areypical of the carboxylate ion (–COO(H)), corresponding to thentisymmetric and symmetric stretching vibrations of the –COO–roup, respectively [22].

.3. Passivity in 7N NaOH

The less noble nature of Fe compared to Ni and the higheroncentration of iron over nickel at the ribbon surface [9] indi-ate that iron oxidation precedes nickel oxidation. The five currentimiting zones during forward polarization (Fig. 1), suggest a pas-ivation mechanism progressing via distinct stages. These stageselate to the succession of oxidation reactions in the systemsi–H2O, Fe–H2O and Fe–Ni–H2O. In Fig. 1, the Ni anodic voltam-ogram shows three current peaks, each one denoting the onset

f a passivating process. According to the Pourbaix diagram [23],or pH 12, these peaks correspond to the successive formationf Ni(OH)2 (NiO·H2O), Ni(OH)2·2NiOOH (NiO·Ni2O3·2H2O) andNiOOH (Ni2O3·H2O). Similarly, the Fe voltammogram shows

wo current peaks, which correspond to the successive forma-ion of Fe(OH)2·2FeOOH (Fe3O4·2H2O) and Fe(OH)3 (Fe2O3·3H2Or FeOOH·H2O). Therefore, based on the Pourbaix diagrams, theolarization tests on Ni and Fe in 7N NaOH and relevant litera-ure [15,24,25], it is possible to ascribe the passive/pseudopassiveegions in the anodic curve of Fe–40Ni–20B (Fig. 1), to the growthf the following current limiting depositions:

Stage 1: Fe(OH)2·2FeOOH and Fe3O4.Stage 2: Ni(OH)2 and NiO.Stage 3: FeOOH and Fe2O3.Stage 4: Ni(OH)2·2NiOOH and NiO·Ni2O3.Stage 5: �NiOOH and Ni2O3.

The above formulae are simplified forms of much more complexompounds generated from a series of complicated oxidation reac-

ions.)

The chronoamperometry plots in Fig. 3 support the above suc-ession: current densities at the lower critical passivation potential−770 mV: active → passive stage 1) are smaller than the active cur-ent densities, reflecting a transpassive state towards true passivity;

2

ompounds 483 (2009) 514–518 517

he latter is attained by the formation of a Fe3O4-based layer. Athe second transpassive stage (−500 mV: passive 1 → passive 2),he current initially increases with time, due to Fe3O4 oxidation.he sharp current fall after ∼8600 s corresponds to Ni(OH)2 andiO formation. The high current values are owing to the unsta-le nature of �Ni(OH)2. The exponential decrease in current, at aotential in the beginning of the third passive stage (−350 mV),uggests passive film formation; the high currents though, indi-ate pseudopassivity. The third passive stage has been ascribed tohe formation of FeOOH and Fe2O3. Fe2O3 induces passivity. Theigh currents probably originate from �Ni(OH)2 (in solid solu-ion with Fe(OH)2), which is heavily hydrated. The i–t curve inhe third transpassive stage (−200 mV: passive 3 → passive 4) isharacterized by an initially increasing current vs. time, caused byi(OH)2 dissolution. The large decrease in current after ∼5000 s isttributed to the formation of Ni(OH)2·2NiOOH or NiO·Ni2O3. Thehronoamperometry plots at the fourth passivation stage (−50 mV,mV) are typical of a true passive state, corresponding to theuilt-up of Ni(OH)2·2NiOOH/NiO·Ni2O3. In particular, true passiv-

ty is mostly credited to compounds based on mixed (Ni,Fe) oxides.he Ni(OH)2·2NiOOH species may also contribute to true passivity,ince their dissolution rate is about half of that of Ni(OH)2 (though,n acidic electrolytes) [26]. The i–t curve at a potential close tohe breakaway potential (365 mV: 5th passive stage) presents theeneral trend of passivation. However, the oscillating zones andhe high values of current density are due to the transpassive dis-olution of the passive layers. The final transpassive dissolutionrobably occurs by the hydroxidation of the passivity inducingxides.

.4. Effect of amorphicity on the corrosion behaviour

The homogeneous single glass phase (free of localized defects)ould be expected to allow the growth of a uniform protective film.f course, the alloy chemistry is also a principal factor for passi-ation; both, iron and nickel are passivity inducing metals. A thirdontrolling factor for metal glass passivation, is the presence of met-lloids, which may accelerate active alloy dissolution causing rapidormation of a passive film [8].

In this work, the second factor appears to be the dominatingarameter in the corrosion performance of the Fe–Ni–B glass. This

s seen in Fig. 1: the anodic curve shapes of Fe–Ni–B, Ni and Feppear too similar. The passive current density values follow therder Ni < Fe–Ni–B < Fe; the corrosion potential values follow therder Ni > Fe–Ni–B > Fe. The above sequences are expected for arystalline Fe–Ni alloy. However, the passive range of potentials ine–Ni–B appears somewhat greater than the passive range in Nind Fe (to about 200–300 mV). This can be attributed to the glassytate. As metastable materials, metallic glasses have greater reac-ivity than their crystalline analogues. This reactivity can lead tohe rapid formation of a uniform surface film.

. Conclusions

The main conclusions drawn from investigating the behaviourf amorphous Fe–40Ni–20B (at%) ribbons in aerated 7N NaOH and.5% NaCl, at 25 ◦C are

1. Immersion in 7N NaOH induces multiple stage passiva-

tion/pseudopassivation, associated with the consecutive for-mation of compounds based on Fe(OH)2·2FeOOH and Fe3O4,Ni(OH)2 and NiO, FeOOH and Fe2O3, Ni(OH)2·2NiOOH, �NiOOH.

. The main corrosion products are (Ni,Fe) oxides and hydroxides.The main active corrosion products are based on �Ni(OH)2, NiO

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and Ni(OH)2·2NiOOH. The main passive corrosion products arebased on NiO and �NiOOH. Fe2O3, Fe3O4 and �FeOOH are alsoamong the active and passive corrosion depositions. Fe3O4, NiO,Fe2O3 and �NiOOH are responsible for true passivity.

. The glass is not susceptible to pitting in 7N NaOH, despite theincorporation of carbonate ions from the environment in its pas-sive film. The glass is slightly susceptible to pitting in 3.5% NaCl.

. Boron participates in the oxidized compounds, but its role in thecorrosion mechanism has not been clarified in this investigation.

. Despite amorphicity, the corrosion behaviour of the glass in thecaustic solution follows the behaviour of its crystalline metallicconstituents. However, a greater range of passive potentials isattained during glass polarization, as compared to pure Ni andFe.

cknowledgement

The authors would like to thank Prof. A.R. Yavari for providinghe glass ribbons.

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