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Journal of Non-Crystalline Solids

journal homepage: www.elsevier.com/locate/jnoncrysol

Effect of Si addition on the electrochemical corrosion and passivationbehavior of Fe-Cr-Mo-C-B-Ni-P metallic glasses

Shuang Zhenga,b,c, Jiawei Lib,c,⁎⁎, Jijun Zhangb,c, Kemin Jiangd, Xincai Liua,⁎, Chuntao Changb,c,Xinmin Wangb,c

a Faculty of Materials Science and Chemical Engineering, Ningbo University, Ningbo 315211, Chinab Key Laboratory of Magnetic Materials and Devices & Zhejiang Province, Ningbo Institute of Materials Technology & Engineering, Chinese Academy of Sciences, Ningbo,Zhejiang 315201, Chinac Key Laboratory of Magnetic Materials and Application Technology, Ningbo Institute of Materials Technology & Engineering, Chinese Academy of Sciences, Ningbo,Zhejiang 315201, Chinad Center for Analysis and Measurements, Ningbo Institute of Materials Technology & Engineering, Chinese Academy of Sciences, Ningbo, Zhejiang 315201, China

A R T I C L E I N F O

Keywords:Fe-Cr-based metallic glassesThermal stabilityCorrosionPassivation behaviorSi

A B S T R A C T

Fe-Cr-based metallic glasses (MGs) have attracted extensive attention for industrial applications due to corrosionresistance that is superior to that of their crystalline counterparts. In this work, the effect of Si concentration onthe electrochemical corrosion and passivation behavior of Fe43Cr15Mo14C10−xB5Ni3P10Six (x=0, 2, 4, 6 at.%)MGs in a 3.5 wt% NaCl solution has been investigated by performing potentiodynamic polarization measure-ments, electrochemical impedance spectroscopy, cathodic polarization, the Mott-Schottky approach, and X-rayphotoelectron spectroscopy (XPS). The addition of a small amount of Si (≤4 at.%) is found to have a beneficialeffect on improving the thermal stability and corrosion resistance for this Fe-Cr-based MG system. However,higher Si amounts deteriorate the thermal stability due to precipitation of a (Fe, Cr, Mo)23(C,B)6 phase. Also, allelectrochemical measurements show that 4 at.% Si is high enough to improve the passivity stability of thesystem, whereas higher amounts of Si do not significantly improve the corrosion behavior. The improved pas-sivation ability and corrosion resistance of the Si-doped MGs can be attributed to the formation of a dense andstable passive film rich in Si- and Cr-oxides.

1. Introduction

Metal corrosion is an urgent issue in numerous industrial applica-tions where it causes significant economic losses and serious safetyincidents. Unlike transitional crystalline materials, metallic glasses(MGs) with long-range disordered amorphous structure have no defectssuch as grain boundaries and dislocations. Therefore, MGs with corro-sion-resistant components usually show higher corrosion resistancethan their crystalline counterparts. Extensive studies have been carriedout to explore Fe-Cr-based MGs due to their excellent corrosion re-sistance, high wear resistance, and relatively low material cost [1–4]. Inparticular, the Fe-Cr-Mo-C-B MG system has attracted increasing at-tention owing to its high thermal stability and extreme low corrosionrate in aggressive solutions [4–15]. It was found that the corrosionresistance can be notably improved with appropriate B [4,7], Cr[5,8,10,14], Nb [6], W [10], Mo [1,10,14], Y [11], Al [12], Ni [12], Co

[12], N [12], or P [6,15] additions in this system. Metalloids B, P and Care primarily added to improve the corrosion resistance of Fe-Cr-basedMGs, because they can accelerate the active dissolution of MGs, andaccordingly lead to the rapid formation of a passive film and the en-richment of Cr in the passive film [1]. A few studies have reported thatSi addition can also improve the corrosion resistance of Fe-Cr-basedMGs. For example, Fe62Cr10Ni8Si20 MG showed better corrosion re-sistance than Fe62Cr10Ni8P20 MG due to the formation of a protectivefilm mainly consisting of Cr oxides, contrary to the case of P-containingMG which showed a mixture of Fe and Ni oxides [16]. In addition, forFe-Cr-13P-7X (X=B, C, Si) MGs submitted to 3 wt% NaCl solution, Siaddition led to the lowest corrosion rate when compared with B and Cadditions [17]. The ability to resist corrosion is mainly determined bythe stability of the passive film formed on a material's surface [18,19].Although some previous studies point to an enhancement of the cor-rosion resistance for alloys containing Si, the effect of Si content on the

https://doi.org/10.1016/j.jnoncrysol.2018.04.036Received 9 January 2018; Received in revised form 14 April 2018; Accepted 22 April 2018

⁎ Corresponding author.⁎⁎ Correspondence to: J. Li, Key Laboratory of Magnetic Materials and Devices & Zhejiang Province, Ningbo Institute of Materials Technology & Engineering, Chinese Academy of

Sciences, Ningbo, Zhejiang 315201, China.E-mail addresses: lijw@nimte.ac.cn (J. Li), liuxincai@nbu.edu.cn (X. Liu).

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0022-3093/ © 2018 Published by Elsevier B.V.

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corrosion and passivation behavior is not completely clear. As well, it isnot yet well understood how Si improves the corrosion resistance of Fe-Cr-based MGs.

Recently, the authors reported a Fe63Cr8Mo3.5C4B4Ni5P10Si2.5 MGand a thermal spray coating made of this MG material. They showcorrosion resistance comparable to that of the well-known amorphoussteel SAM1651 (Fe48Mo14Cr15C15B6Y2) despite a very low Cr and Mocontent [5,18,20]. It was interesting to note that the corrosion re-sistance of the Fe-Cr-Mo-C-B-Ni-P-Si system was not significantly im-proved with further increase in Cr, Mo, or Ni content. Moreover, the Si-free Fe-Cr-Mo-C-B-Ni-P MGs showed poor pitting resistance (un-published data). In consideration of potential applications for refining,shipping and tunnel-boring, etc., it is important to study the role of Sion the corrosion behavior of this system. In the present work, theelectrochemical corrosion and passivation behavior ofFe43Cr15Mo14C10−xB5Ni3P10Six (x=0, 2, 4, 6 at.%) MGs have beeninvestigated by potentiodynamic polarization measurements, electro-chemical impedance spectroscopy, cathodic polarization, the Mott-Schottky approach and XPS. This work will provide important in-formation for the design of low-cost Fe-Cr-based MGs with improvedcorrosion resistance and high thermal stability.

2. Experimental procedure

Alloys ingots with nominal compositions ofFe43Cr15Mo14C10−xB5Ni3P10Six (x=0, 2, 4, 6 at.%) were prepared byinduction melting the mixtures of pure Fe, Cr, Mo, Ni metals, pure B, C,and Si crystals, and pre-alloyed Fe-P ingots under an argon atmosphere.MG ribbons with a cross-section of about 1.5× 0.03mm2 were pre-pared by single roller melt-spinning method in an argon atmosphere ata surface speed of 30m/s. Thermal stability of the MGs was measuredby differential scanning calorimetry (DSC, NETZSCH 404C) at a heatingrate of 0.67 k/s. Amorphous nature of the MGs was examined by X-ray

diffraction (XRD, Bruker D8 Advance) with Cu-kα radiation and high-resolution transmission electron microscopy (HRTEM, FEI Tecnai F20).Corrosion resistance measurements were conducted by electrochemicalwork station (ZAHNER ZENNIUM) in 3.5 wt% NaCl solution at roomtemperature. A three-electrode cell was employed, with the samplesacting as the working electrode, the saturated calomel electrode (SCE)as the reference electrode, and a platinum sheet as the counter elec-trode. Before electrochemical tests, all samples were polished withmetallographic sandpaper of 2000 grain size, washed in ethanol, anddried in air for 24 h. The experiments were started after immersing thesamples into 3.5 wt% NaCl solution for 20min. Potentiodynamic po-larization curves were measured at a scanning rate of 1mV/s. Current-time transient measurements were carried out at four applied polar-ization potentials (0, 0.3, 0.5, and 0.7 V SCE) for 1800 s after immersingthe samples into 3.5 wt% NaCl solution for 20min. Electrochemicalimpedance spectroscopy (EIS) measurements were performed aftercurrent-time measurements. The EIS method was used in the frequencyranging from 104 to 0.01 Hz and involved the perturbation amplitude of10mV. Mott-Schottky (M-S) plots were carried out to evaluate thesemiconductor properties of passive films. The passive films for the M-Sanalysis were fabricated by immersing the MGs into 3.5 wt% NaCl so-lution for 1800 s at the given applied potentials. The measurementpotential in the M-S experiments was range from −0.6 to 0.8 V.Potential-time curve was tested by immersing the MGs into 3.5 wt%NaCl solution for 600 s at an applied current of 2 μA. All electro-chemical measurements were repeated 3–5 times to determine therandom error and systematic error. Composition of the passive filmswas analyzed by X-ray photoelectron spectroscopy (XPS, AXIS UltraDLD) with a Mg Kα X-ray source (hν=1253.6 kV).

3. Results and discussion

DSC traces of Fe43Cr15Mo14C10−xB5Ni3P10Six (x=0, 2, 4, 6 at.%)

Fig. 1. DSC traces (a) and XRD patterns (b) of the Fe43Cr15Mo14C10−xB5Ni3P10Six (x=0, 2, 4, 6 at.%) MGs. (c) and (d) show HRTEM images and the correspondingSAED patterns of the Si4 and Si6.

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Table 1Thermal and electrochemical parameters obtained from the DSC traces and electrochemical polarization curves of the Fe43Cr15Mo14C10−xB5Ni3P10Six (x=0, 2, 4,6 at.%) MGs. Tg, Tx, ΔTx, Icorr, Ipass, Ecorr, and Epit represent the glass transition temperature, crystallization temperature, supercooled liquid region, corrosion currentdensity, passivation current density at the applied potential of 0.3 V, corrosion potential, and pitting potential.

Thermal parameters Electrochemical parameters

Tg(K)

Tx(K)

ΔTx(K)

Icorr(×10−7 A/cm2)

Ipass(×10−6 A/cm2)

Ecorr(V)

Epit(V)

x=0 818 879 61 3.90 ± 0.25 4.91 ± 0.42 −0.20 ± 0.009 0.55 ± 0.002x=2 828 900 72 2.60 ± 0.11 3.36 ± 0.15 −0.17 ± 0.001 0.98 ± 0.010x=4 830 891 61 1.45 ± 0.10 1.39 ± 0.05 −0.21 ± 0.005 0.99 ± 0.012x=6 / 905 / 1.41 ± 0.13 1.72 ± 0.07 −0.16 ± 0.006 1.04 ± 0.002

Fig. 2. Potentiodynamic polarization curves in 3.5 wt% NaCl solution at room temperature for: (a) Fe43Cr15Mo14C10−xB5Ni3P10Six (x=0, 2, 4, 6 at.%) MGs, (b) Si4(Fe45.75Cr14.86Mo25.59C1.37B1.03Ni3.36P5.9Si2.14 wt%), amorphous steel SAM1651 (Fe51.29Cr14.92Mo25.7C3.45B1.24Y3.4 wt%), super duplex stainless steel S32750(Fe62.99Cr25.15Mo3.43C0.02Ni6.74P0.03Si0.55N0.27Mn0.69Cu0.13 wt%), austenitic stainless steels AISI 316L (Fe65.16Cr17.54Mo2.47C0.03Ni12.3P0.02Si0.44Mn1.84 wt%), and 304(Fe72.87Cr18.11C0.06Ni8.59P0.02Si0.35Mn0.59 wt%). The chemical compositions are given in mass percent for comparison.

Fig. 3. Potentiostatic plots of the Fe43Cr15Mo14C10−xB5Ni3P10Six (x=0, 2, 4, 6 at.%) MGs in 3.5 wt% NaCl solution at applied potentials of 0 V (a), 0.3 V (b), 0.5 V(c), and 0.7 V (d).

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MGs are shown in Fig. 1(a). Thermal parameters obtained from the DSCtraces are summarized in Table 1. The MGs with x=0, 2, 4, and 6 areconveniently labeled as Si0, Si2, Si4, and Si6, respectively. It is seen thatthe glass transition temperature (Tg) moves to a higher temperaturewhen the Si content increases from 0 to 4 at.%. The glass transition,however, disappears for Si6. Also, an exothermic peak can be observedbefore the main crystallization peak of the Si4 and Si6, decreasing thecrystallization temperature (Tx) of the MGs. Therefore, the supercooledliquid region (ΔTx= Tx− Tg) increases from 61 K for Si0 to 72 K for Si2,and then decreases to 61 K again for Si4, but finally vanishes when theSi content reaches 6 at.%. If a MG has high Tg and ΔTx, the high solid/liquid interfacial energy is good for inhibiting crystal nucleation andgrowth [21]. Therefore, the thermal stability of the Fe-Cr-Mo-C-B-Ni-Psystem can be effectively improved by doping a small of amount of Si(≤4 at.%), while it decreases with higher amounts of added Si. Fig. 1(b)shows the XRD patterns of the MGs. Apart from the Si6, only a broadhalo peak without a crystalline peak is detected at the resolution ofXRD. To analyze the reasons for the decrease of thermal stability, wecarefully examined high-resolution TEM images (HRTEM) and thecorresponding selected area electron diffraction (SEAD) patterns of Si4

and Si6. As shown in Fig. 1(c–d), only short-range/medium-range or-dered clusters and a halo pattern can be found in the HRTEM image ofSi4, indicating the fully amorphous structure of this alloy. On the otherhand, very small amounts of (Fe, Cr, Mo)23(C,B)6 crystalline phase areembedded in the amorphous matrix of Si6, which is a common pre-cipitate phase in the Fe-Cr-Mo-C-B system [22]. These results suggestthat a higher amount of added Si is favorable for the nucleation of the(Fe, Cr, Mo)23(C,B)6 phase, decreasing the thermal stability of thepresent system.

Fig. 2(a) presents the potentiodynamic polarization curves ofFe43Cr15Mo14C10−xB5Ni3P10Six (x=0, 2, 4, 6 at.%) MGs in 3.5 wt%NaCl solution at room temperature. The corrosion current density(Icorr), passivation current density (Ipass), corrosion potential (Ecorr) andpitting potential (Epit) are listed in Table 1. It is clear that the Si-con-taining MGs exhibit lower Ipass and higher Epit than the Si-free MG,implying that the addition of Si is effective in improving the corrosionresistance of the MGs. However, MGs with>4 at.% Si have similarpolarization plots. Specifically, the Si2 and Si4 exhibit nearly the sameEpit value. The Ecorr does not change significantly within experimentalerror. This trend suggests that the addition of a small amount of Si

Fig. 4. Nyquist plots of the Fe43Cr15Mo14C10−xB5Ni3P10Six (x=0, 2, 4, 6 at.%) MGs in 3.5 wt% NaCl solution at applied potentials of 0 V (a), 0.3 V (b), 0.5 V (c), and0.7 V (d). (e) Equivalent circuit for fitting the impedance spectra. (f) Resistance for ion transfer as a function of the applied potential.

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(≤4 at.%) is sufficient to improve the corrosion and pitting resistanceof the MG system. However, more Si than this does not notably enhancethe corrosion resistance, due to the precipitation of a (Fe, Cr,Mo)23(C,B)6 crystalline phase. Because the formation of a (Fe, Cr,Mo)23(C,B)6 crystalline phase gives rise to Cr-depleted regions, wherethe content of Cr is not enough to form a stability passive film [23].Moreover, the (Fe, Cr, Mo)23(C,B)6 phase preferentially dissolves owingto the defects such as grain boundaries and dislocations. To furtherevaluate the electrochemical properties of the Si-alloyed MGs, po-tentiodynamic polarization curves of the Si4, amorphous steelSAM1651, super duplex stainless steel S32750, as well as austeniticstainless steels AISI 316 L and 304 are shown for comparison. As shownin Fig. 2(b), all the alloys are spontaneously passivated with a very lowIcorr on the order of 10−7 A/cm2. However, the passive region of the Si4is more stable and wider than that of the typical stainless steels. Also,Si4 exhibits lower Icorr and Ipass as well as higher Epit than the SAM1651,illustrating the outstanding corrosion resistance of the MG. The polar-ization results imply that the partial substitution of high-cost elements,such as Y, by lower-cost Si may not decrease the protective capability ofthe passive film in the Fe-Cr-based MGs.

To determine the pitting behavior, potentiostatic polarizationmeasurements were performed. Fig. 3 shows current-time curves ofFe43Cr15Mo14C10−xB5Ni3P10Six (x=0, 2, 4, 6 at.%) MGs at constantpotentials of 0, 0.3, 0.5, and 0.7 V. As shown in Fig. 3(a)–(c), the cur-rent density decreases rapidly at first, and then stays relatively steadywith time due to the formation of a passive film on the surface of theMGs [24,25]. The current densities of the Si-containing MGs achievestability faster than those of the Si-free MG, especially for the Si4, whichexhibits the lowest current density value among these MGs. No sharpcurrent transients are observed for the Si4 and Si6 at high potentials.However, the Si0 is very unstable and accompanied by obvious fluc-tuation in the curve (Fig. 3(d)), which results from metastable corrosionpits caused by local breakdown of the passive film [18,26]. Therefore,Si is good for accelerating the formation of a stable passive film [18],leading to the superior pitting resistance and protection performance ofthe MGs.

Fig. 4(a)–(d) displays the electrochemical impedance spectroscopy(EIS) results for the passive films developed by potentiostatic polar-ization for 30min at 0, 0.3, 0.5, and 0.7 V. It can be seen that theNyquist plots of the Si-containing MGs show larger semicircle diameter

Fig. 5. Mott-Schottky plots of the Fe43Cr15Mo14C10−xB5Ni3P10Six (x=0, 2, 4, 6 at.%) MGs in 3.5 wt% NaCl solution at applied potentials of 0 V (a), 0.3 V (b), 0.5 V(c), and 0.7 V (d). (e) Donor density (ND) as a function of the applied potential. (f) Potential-time curves during cathodic polarization.

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than the Si-free MG, suggesting that the Si addition increases the chargetransfer resistance of the passive film [5]. To fit the impedance spectra,an equivalent circuit describing the electrochemical reaction, Rs(RtQ),is shown in Fig. 4(e), where Rs is the solution resistance, Rt is the chargetransfer resistance, and Q represents the possibility of a non-ideal ca-pacitance. It is known that the Rt simulated by the equivalent electricalcircuit model increases with the semicircle diameter of the Nyquistplots, and the larger the Rt, the better the corrosion resistance [27]. Asshown in Fig. 4(f), the Rt values of the Si-containing MGs are higherthan those of the Si-free MG, indicating that the passive films of the Si-containing MGs are a more effective barrier against corrosive ions. Inaddition, the Rt obtained at 0.3 V increases notably with minor Si ad-dition and reaches its highest value for the Si4, leading to the lowestcurrent density value of the alloy. Thus the EIS data agree well with thetrend of the potentiodynamic and potentiostatic polarization curves.

Generally, the passive film can be considered to be a semiconductorstructure [11,25,28,29]. To understand the electronic/ionic transportproperties of the passive films, Mott-Schottky (M-S) analysis based onthe measurement of apparent capacitance vs. potential, was in-vestigated. M-S curves of the passive films formed on theFe43Cr15Mo14C10−xB5Ni3P10Six (x=0, 2, 4, 6 at.%) MGs at differentapplied potentials are shown in Fig. 5(a)–(d). All plots exhibit a similarshape with a linear region in the range of 0 to 0.4 V. At the low po-tential region, the nonlinear area is affected by the outermost layer ofthe passive film, whereas it is associated with the protective structure ofthe passive film at high potential. Each plot has a positive slope in thelinear region, indicating that the passive film is an n-type semi-conductor. The n-type semiconductor will cause local corrosion bypromoting the penetration of corrosive ions (such as Cl−) into thepassive film [29]. Based on the model of point defect, the main carrierof an n-type semiconductor is vacancy oxygen (donor) generated at theinterface of the passive film and the alloy substrate. The transportation

of vacancy oxygen will lead to the destruction of passive film [30,31].The higher the donor density, the more susceptible a passive film is topitting corrosion [11,25]. Since the donor density has a great influenceon the stability of passive film, it is necessary to study the donor densityof the n-type passive film (ND), which can be deduced from the slope ofthe linearly fitted plots. The expression of ND is as follows [18]:

⎜ ⎟= ⎛⎝

⎞⎠

− −

Nε ε e

CE

2 ddD

r 0

SC2 1

(1)

where εr is the dielectric constant of the film (for the Fe-Cr alloy,εr = 15.6 [25]), ε0 is the vacuum permittivity (8.85×1012 Fm−1), e isthe charge of an electron, E is the applied potential, CSC is the spacecharge capacitance. Thereby, the larger the slope of M-S curve, thesmaller the donor density of passive film.

Fig. 5(e) shows the donor density as a function of the applied po-tential obtained from Fig. 5(a)–(d) and Eq. (1). It is observed that theND decreases from 0 to 0.3 V, and then increases with an increase inapplied potential. This trend suggests that the passive film formed at0.3 V has better stability than those formed at other applied potentials,which is consistent with the results of the potentiodynamic polarization(Fig. 2) and current-time transient measurements (Fig. 3). Moreover,the ND values of the Si-containing MGs are lower than those of the Si-free MG, implying a more stable and compact passive film formed onthe Si-containing MGs. To investigate the formation process of thepassive films, potential-time curve of the MGs were recorded as afunction of the immersion time. As shown in Fig. 5(f), the potentials ofall MGs move rapidly toward a more positive value due to the initialformation and growth of passive film, and then reach a relatively sta-tionary value. With an increase in Si content, the time taken to reach asteady potential becomes shorter, indicating a faster growth rate of thepassive films formed on the surface of the Si-containing MGs [32]. Thepotential increases with an increase of Si content, and the Si-containing

Fig. 6. XPS spectra of Fe 2p (a), Cr 2p (b), Mo 3d (c), and Si 2p (d) recorded from the surfaces of Si0 and Si4 with Ar-ion sputtering for 60 s. For comparison, thespectra are normalized to the same height at their maximum peaks and moved up and down as necessary. Superscript m: metallic state.

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MGs have higher potentials than the Si-free MG. A higher stable po-tential suggests the passive films are thicker or more compact [33]. Itshould be noted that the Si4 shows lower ND value than the Si6 does atapplied potentials of 0.3 and 0.5 V (Fig. 5(e)), and exhibits a potential-time curve similar to that of the Si6 (Fig. 5(f)). Therefore, 2–4 at.% Si ishigh enough to improve the stability of the passive film, resulting inexcellent corrosion resistance of the MGs.

To better understand the influence of Si addition on the corrosionbehavior, XPS analysis was performed to characterize the structure andchemical composition of the passive films formed on the surfaces of theSi0 and Si4. Fig. 6 depicts the spectra of Fe 2p, Cr 2p, Mo 3d, and Si 2p inthe Si0 and Si4 MGs with Ar-ion sputtering for 60 s. The Fe 2p spectrumrepresents the metallic state (Fem), Fe2+, and Fe3+ oxide states(Fig. 6(a)). The Cr 2p peaks include the metallic state (Crm), Cr3+, andCr6+ oxide states (Fig. 6(b)). The Mo 3d peaks are composed of metallicstate (Mom), Mo4+, and Mo6+ oxide states (Fig. 6(c)). It is seen that thepeak intensity and total area of the Fem, Crm, and Mom in the Si4 aresmaller than those of the Si0. Also, it is consistent with the expectationthat the Si0 has no Si 2p peak. However, the Si4 shows two peaks at 99and 101.9 eV, corresponding to the metallic state (Sim) in the

underlying surface and the Si4+ oxide state in the passive film, re-spectively (Fig. 6(d)).

Fig. 7(a)–(d) shows depth profiles of the film's constituent elementsobtained through XPS analysis. For comparison, the cationic fractionsof the oxide species and the atomic fractions of the metallic species areshown in Fig. 7(e)–(f). It is observed that the concentrations of Fem,Crm, and Mom in the Si4 are smaller than those in the Si0 during theentire etching process. The atomic percentage of O in the Si4 is higherthan that in the Si0, indicating that the surface of the Si4 forms a thickerpassive film. In general, a thick passive film helps to prevent the cor-rosion ions from entering the alloy matrix, and thus improves the cor-rosion resistance of the alloy. On the other hand, both FeOx and MoOx

exhibit nearly the same constant concentration values throughout thepassive film thickness. Thus, the addition of Si does not reduce the FeOx

concentration, nor does it increase the MoOx concentration. In contrast,the concentration of CrOx first increases then decreases with etchingtime, indicating that Cr cations are mainly concentrated in the surfaceof the passive film. The Cr-rich region of the Si4 is broader than that ofthe Si0, which improves the stability of the passive film [5,8,14]. Fur-thermore, it is clearly seen that 2–4 at.% Si cations are dispersed

Fig. 7. Depth profiles of Fe and O (a), Cr (b), Mo (c), and Si (d) detected by XPS from the passive films of Si0 and Si4. (e) and (f) show the cationic fractions of theoxide species and the atomic fractions of the metallic species on the surface of Si0 and Si4, respectively. Superscript m: metallic state. Superscript Ox: metal oxidestate. FeOx (Fe2++ Fe3+); CrOx (Cr3++Cr6+); MoOx (Mo4++Mo6+); SiOx (Si4+).

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uniformly in the passive film. The presence of Si cations indicates thepossible formation of a uniform and continuous SiO2 film, which canmake the passive film more protective [1]. The formation of SiO2 can beexpressed as follows:

+ → + ++ −Si 2H O SiO 4H 4e2 2 (2)

It is reported that the addition of Si can reduce the surface oxygenconcentration toward the Si/SiO2 equilibrium, and thus promotes theenrichment of CrOx in the passive film [16,34]. Moreover, oxygencompetition between Cr and Si in the outer layer of the passive film caninhibit the partial oxidation of other elements [34]. Therefore, the sy-nergistic effect of CrOx and SiOx significantly improves the stability ofthe passive film, leading to excellent corrosion resistance of Fe-Cr-Mo-C-B-Ni-P-Si MGs.

4. Conclusions

The thermal stability, corrosion resistance and passivation behaviorin Si-alloyed Fe43Cr15Mo14C10−xB5Ni3P10Six (x=0, 2, 4, 6 at.%) MGshas been studied in order to investigate the effectiveness of Si additionin Fe-Cr-based MGs and determine the optimal amount of Si needed toachieve good corrosion properties while maintaining high thermalstability. The substitution of 2–4 at.% C by Si is found to be effective inimproving the thermal stability and pitting resistance for this Fe-Cr-based MGs system. Electrochemical measurements and XPS experi-ments show that the addition of Si favors the fast formation of a thickand stable passive film with low density of vacancy‑oxygen density,large charge transfer resistance, and high passivity stability. Moreover,a minor amount of Si is enough to promote the formation of a densepassive film containing SiO2, and also increases the thickness of the Cr-rich region. In view of these results, Fe-Cr-based MGs with high thermalstability, and good corrosion and pitting resistance can be obtained byadding 2–4 at.% Si, but the addition of a higher amount of Si decreasesthermal stability and does not noticeably enhance the corrosion beha-vior due to the precipitation of a (Fe, Cr, Mo)23(C,B)6 phase.

Acknowledgements

This work was supported by the National Natural ScienceFoundation of China (Grant No. 51771215, 51501210 and 51571207)and Ningbo Municipal Nature Science Foundation (Grant No.2017A610034).

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