Corrosion Science 82 (2014) 115–124

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Corrosion Science

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Effect of applied potential on passivation and erosion–corrosionof a Fe-based amorphous metallic coating under slurry impingement

0010-938X/$ - see front matter � 2014 Elsevier Ltd. All rights reserved.http://dx.doi.org/10.1016/j.corsci.2014.01.004

⇑ Corresponding author. Tel.: +86 24 23928381; fax: +86 24 23894149.E-mail address: ygzheng@imr.ac.cn (Y.G. Zheng).

Z.B. Zheng a, Y.G. Zheng a,⇑, W.H. Sun b, J.Q. Wang b

a State Key Laboratory for Corrosion and Protection, Institute of Metal Research, Chinese Academy of Sciences, 62 Wencui Road, Shenyang 110016, PR Chinab Shenyang National Laboratory for Materials Science, Institute of Metal Research, Chinese Academy of Sciences, 72 Wenhua Road, Shenyang 110016, PR China

a r t i c l e i n f o a b s t r a c t

Article history:Received 25 October 2013Accepted 11 January 2014Available online 21 January 2014

Keywords:A. Metal coatingsB. ErosionB. PolarisationB. PotentiostaticB. XPSC. Passive films

The passive behaviour and erosion–corrosion behaviour of a HVOF sprayed Fe-based amorphous metalliccoating have been investigated in 3.5 wt.% NaCl solution by using potentiostatic polarisation, X-ray pho-toelectron spectroscopy and Mott–Schottky analysis. The fact that passive current density increased withrising potential might result from the preferential dissolution of high-valence oxides and the existence ofmore chlorides at a higher potential. The critical flow velocity decreased with rising potential because ofthe lower resistance of passive film at a higher potential. The reason why passive current density changedunder jet impingement was discussed by a simple formula.

� 2014 Elsevier Ltd. All rights reserved.

1. Introduction

Most Fe-based metals and alloys owe their outstanding corro-sion resistances to the formation of thin and stable passive filmson the metallic surfaces. Passivity is defined as a decrease in thecorrosion rate of a metal or alloy by forming a thin and tenaciousprotective film due to the oxidation of the material [1]. The protec-tiveness of passive film is affected by many factors such as semi-conducting property, chemical composition and thickness. Thefactors are all influenced by environmental conditions. One ofthem is the applied potential. According to the Point Defect Model(PDM), a linear dependence between steady-state thickness (Lss)and formation potential (E) can be drawn for the cases where nochange in the oxidation state occurs [2,3]. Corresponding resultsare found in studies [4–7] which were focused on passivations ofdifferent alloys in different solutions. The semiconducting proper-ties of passive films are usually studied by Mott–Schottky (M–S)analysis. Cheng et al. [8] investigated the passive film formed oniron in borate buffer solution and concluded that the donor densityof the film decreased exponentially with the increasing film forma-tion potential. Fattah-alhosseini et al. [9] discovered a similar rela-tionship between the donor density and the applied potential in aresearch focused on the passive films on AISI 316L and AISI 321L inH2SO4 solution. While Qiao et al. [10] studied the passive film of ahigh nitrogen stainless steel in acidic solution and found that the

acceptor/donor density increased with the rising applied poten-tials. An analogous result was presented by Wang et al. [11]. Differ-ent results also existed in the semiconductor type of passive filmsas described by Fernández-Domene et al. [1], who detected thatthe passive film formed on Alloy 31 could be p-type and/or n-typein electronic character depending on the film formation potential.The composition, which was regarded as the most important prop-erty of passive film, changed a lot by the shifty film formation po-tential. The oxidation states of oxides were different as a result ofapplied potential [12–14]. For some Fe–Cr alloys, enrichment ofchromium disappeared in the high passive region [7,15]. In a word,the film formation potential affects the structure and feature ofpassive films a lot and is a key role in the protective barrier tocorrosion.

There is no such thing as a static passive film [16]. The thin filmis getting more and more attention in the erosion–corrosionbehaviour of passive alloys [17,18]. It is well known that ero-sion–corrosion consists of two main effects: electrochemical corro-sion and mechanical damage. Although the effect of pure corrosioncontributes little to the total material loss in the flowing slurriessometimes, it promotes erosion observably [19–22]. By detectingelectrochemical transients generated by erosion–corrosion,researchers [23–25] found increases of current density. It derivedfrom the degradation or rupture of passive film by the impinge-ment of solid particles. The rise of corrosion current density accel-erated the erosion rate markedly. Lu et al. [26,27] studied theerosion–enhanced corrosion of stainless steel and carbon steel atpassive states and successfully developed a theoretical model on

116 Z.B. Zheng et al. / Corrosion Science 82 (2014) 115–124

electrochemical response of passive metals to the solid particleimpingement. The model they built was used to describe the cor-relation between the corrosion current density and the erosionrate. As concluded by Burstein, erosion by the slurry causeddepassivation of the metal surface when the energy of the solidparticles in the slurry exceeded a threshold kinetic energy[28,29]. The threshold kinetic energy was a function of the massof particles and their impact velocities. The critical parametersare getting more and more attention. Hu and Neville [30,31] stud-ied the critical solid loading and the critical flow velocity of stain-less steel by electrochemical responses. Other studies [32–34] alsorelated to critical parameters have been reported. It is believed thatthe critical parameters are associated with the depassivation–repassivation balance on the surface of passive metal. Depassiva-tion dominates in the competition when the parameters are be-yond the critical values and it results in an irreparable damage ofpassive film under erosion–corrosion. Although the explanationof critical parameters has been accepted by some researchers,more investigations on the correlation between passivation andthe critical parameters should be carried out.

A newly developed Fe-based amorphous metal coating (AMC)draws much attention for its high corrosion resistance [35,36]and wear resistance [37]. The potential application under ero-sion–corrosion condition of AMC has been discussed in our earlypaper [38]. The erosion–corrosion resistance of AMC was foundbetter than 304 stainless steel and the behaviour of passive filmsunder jet impingement was connected to the critical velocity[39]. However, the passive behaviour of AMC was different fromthe traditional passivation and the reason remained unknown.The aim of the present paper was to investigate the passive behav-iour of the Fe–Cr–Mo–Mn–W–B–C–Si AMC by studying the passivefilms at different film formation potentials in 3.5 wt.% NaCl solu-tion. Mott–Schottky (M–S) analyses and X-ray photoelectron spec-troscopy (XPS) were used to evaluate the properties of passivefilms. In order to emphasise the important role that passive filmplayed under impingement in sand-containing NaCl solution, thecorrelation between passivation and erosion–corrosion behaviourof AMC was studied by discussing the effect of applied potentialon the critical flow velocity.

Fig. 1. Schematic diagram of the jet impingement apparatus for erosion–corrosion.(1) Control cabinet; (2) frequency converter; (3) motor; (4) lobular pump; (5)stirring pump; (6,7) valves; (8) screw elevator; (9) thermocouple I; (10) impinge-ment cabinet; (11) impingement angle meter; (12) electromagnetic flowmeter; (13)nozzle; (14) reference electrode; (15) counter electrode; (16) pH meter probe; (17)sample (working electrode); (18) valve; (19) thermocouple II; (20) overflow tube;(21) slurry container; (22) cooling water; (23) heater; (24) electrochemicalworkstation; (25) computer.

2. Experimental

The Fe54.2Cr18.3Mo13.7Mn2.0W6.0B3.3C1.1Si1.4 (wt.%) AMC basedon 304 austenitic stainless steel in this work was prepared byhigh-velocity oxy-fuel thermal spray technology. The detail pro-cess was described in our previous work [40]. The test mediumfor the passivation of AMC was 3.5 wt.% NaCl. The test mediumfor impingement (to simulate the sand-containing seawater at Qin-shan China [38]) was a slurry composed of 2 wt.% silica sand parti-cles with the sizes of 75–150 mesh and 3.5 wt.% NaCl.

Electrochemical measurements were conducted using a Poten-tiostat/Galvanostat (PARSTAT 2273). Before the tests, sampleswere sealed carefully and then ground with 240, 400, 600 and1000 grid abrasive papers in sequence, degreased in alcohol,washed in distilled water and dried in air. During the tests, AMCwas used as working electrode (WE), a saturated calomel electrode(SCE) was the reference electrode and a platinum plate was thecounter electrode. Anodic polarisation was swept from 0 to+1.5 V versus the open-circuit potential at a scan rate of 0.5 mV s�1

after immersing the specimens in the solution for 1 h to keep theopen-circuit potential stable. Before the potentiostatic passivitytests, the surface of specimens were initially pretreated cathodi-cally at �1.0 V versus the open-circuit potential for 5 min to cleanthe surface. Similar treatment has been previously reported[41–45]. The effectiveness of this treatment might be due to the

dissolution of the metallic oxide at the potential region, and thegeneration of hydrogen by the hydrogen evolution reactions whichmight cause mechanical stripping of the oxide film. Afterwards,AMCs were polarised at different film formation potentials (0,0.2, 0.4, 0.6 and 0.8 VSCE) for 2 h to form passive films. M–S testswere carried out to evaluate the semiconducting properties ofthe passive films obtained above. The measurement frequency inthe M–S experiments was 1 kHz. The potential was swept from 0to 0.8 VSCE at a scan step rate of 10 mV [46,47]. The high scanningrate in M–S tests was used to avoid electroreduction of the passivefilm and changes in film thickness during the measurements and toprevent the defect density from being affected by potential. Alltests were conducted at ambient temperature (around 18 �C).

The surface morphologies of the coating were detected by scan-ning electron microscopy (SEM) (FEI-Inspect F). X-ray photoelec-tron spectroscopy (XPS) was employed by using ESCALAB 250photoelectron spectrometer with Al Ka excitation (hc = 1486.6 eV)to study the compositions of passive films formed at 0, 0.4 and0.8 VSCE. The test spot was selected randomly on the surface witha diameter of 0.5 mm. Binding energies were calibrated using car-bon contamination with C1s peak value of 284.6 eV. The standardbinding energies of Fe 2p, Cr 2p, Mo 3d, Mn 2p, W4f, O 1s and Cl 2pcore levels were referred from the NIST XPS database. XPS ionbeam sputtering with argon was used to analyse the depth profilesof the surfaces and the sputter rate was 0.1 nm s�1. The sputtertime was decided by a rule that the content of the main elementsof AMC were nearly constant.

In order to study the effect of passivation on erosion–corrosionbehaviour, potentiostatic measurement was used to learn the crit-ical flow velocity of AMC under slurry impingement by sand-con-taining NaCl solution. The equipment for erosion–corrosion wasan impinging jet apparatus as shown in Fig. 1 and was describedin our previous work [48]. A liquid jet from a nozzle of 3 mm indiameter impacted onto the test sample which was closely sealedwith epoxy resin. The exposed area of the sample was 1 cm2. Thestand-off distance between the jet nozzle exit and sample surfacewas kept at 5 mm and the impingement angle was 90�. The flowvelocity applied in the tests varied by time as shown in Fig. 2. Fivepotentials (0, 0.2, 0.4, 0.6 and 0.8 VSCE) were used to carry out thepotentiostatic measurements under impingement. At least tworeplicate tests were carried out to determine the experimentalerrors.

Fig. 2. Impact velocity of samples corresponding to time curves.

Z.B. Zheng et al. / Corrosion Science 82 (2014) 115–124 117

3. Results and discussion

3.1. Potentiodynamic polarisation

Fig. 3 shows the anodic polarisation of Fe-based AMC in 3.5 wt.%NaCl solution. The current density was observed to increases withpotential. It is also observed that the increase in current density isrelatively slow at potential region between 0 and 1.0 VSCE. Similarobservation has been previously reported [35,49,50]. The regionwas regarded as a passive region although the performance inthe region differed a little from the traditional passivation whichexpressed a decrease in current density. Some transient peakswere observed in the polarisation curve. Compared to some Refs.[12,42], the transient peaks might be metastable pits. With the in-crease of potential, pits could initiate and propagate although theycould not grow and repassivated after a while below the pitting po-tential. It could be drawn from the phenomenon that passive filmon AMC was relative stable with the change of potential. The pas-sivity of AMC contributes to its high corrosion resistance both inCl� containing neutral solutions and in acidic solutions. Further in-sight into the special passive behaviour of AMC is presented belowand the reason for the increase in current density in the passive re-gion is discussed via the changes in properties or compositions ofthe passive films at different potentials.

3.2. Mott–Schottky analysis

Passive films formed on most metals and alloys exhibit semi-conducting behaviour, which can be described by Mott–Schottky(M–S) analysis. For n-type seminconductor, M–S relationship canbe shown as [41,51,52]:

Fig. 3. Anodic polarisation curve of Fe-based AMC in 3.5 wt.% NaCl.

1C2

sc

¼ 2eNDeoer

E� Efb �kTe

� �ð1Þ

where Csc is the space charge capacitance, e is the electron charge(�1.602 � 10�19 C), ND is the donor density for n-type semiconduc-tor, eo is the vacuum permissivity constant (8.85 � 10�14 F cm�1), er

is the dielectric constant of the film (for Fe–Cr alloy, er = 15.6 [53]), Eis the applied potential, Efb is the flat band potential, k is the Boltz-mann constant (1.38 � 10�23 J K�1), and T is the temperature in Kel-vin. It is notable that M–S analysis is based on the assumption thatthe capacitance of the space charge layer is much smaller than thedouble layer (Helmholtz) capacitance.

The M–S curves of the passive films pre-passivated at variousfilm formation potentials are shown in Fig. 4. Linear relationshipscan be observed between C�2 and E for different passive films inthe potential region between 0 and 0.6 VSCE. The positive slopesindicate that the passive films formed on AMC are n-type semicon-ductors. At the potential between 0.6 and 0.8 VSCE, the type ofsemiconductor changes to p-type. The inversion from n-type top-type occurs commonly for the passive films formed on stainlesssteel or other alloys [11,46,54]. It is probably caused by thechanges in the compositions and structures of the passive films.Different oxide layers formed in these alloys exhibit various semi-conducting properties according to the main defects in the films[52]. Oxides such as Cr2O3 and MoO2 are usually considered as p-type semiconductor for their properties with a deficiency in metalions or excess with cation vacancies. While oxides like FeO3 andMoO3 are regarded as n-type semiconductor by anion diffusioninto the metal or by cation transport through interstitial diffusion[55]. Taking the results into consideration, it is reasonable to drawan inference that the main oxides of the passive film on AMCchange with the increasing test potential.

3.3. Composition of passive films

The composition of passive film affects their protectiveness. Inthe AMC, Cr is the main element to improve the passivation abilityof AMC, followed by Mo, W and Mn [56]. It is necessary to analysethe oxides of these elements in the passive films in order to com-prehend the effect of applied film formation potential on the pro-tectiveness of passive films.

Samples were passivated at 0, 0.4 and 0.8 VSCE in 3.5 wt.% NaClsolutions for 2 h and the passive films formed on AMCs were inves-tigated by X-ray photoelectron spectroscopy (XPS). Fig. 5 showsthe normalized XPS survey spectra of the three samples after Arions sputtering for 20 s. Some main peaks are marked in thespectra which indicate the presences of Fe, Cr, Mo, Mn, W, C, Oand Cl. Although these curves are almost the same: peaks appear

Fig. 4. Mott–Schottky curves of the passive films pre-passivated at various filmformation potentials in 3.5 wt.% NaCl.

Fig. 5. Wide scan XPS spectra for Fe-based AMC polarised at 0, 0.4 and 0.8 VSCE in3.5 wt.% NaCl.

Fig. 6. XPS spectra of (a) Fe 2p, (b) Cr 2p, (c) Mo 2d, (d) Mn 2p, (e) W 4f and (f) O

118 Z.B. Zheng et al. / Corrosion Science 82 (2014) 115–124

in similar positions in the three curves, and differences exist inboth the intensities and the precise positions of some of the ele-ments. The differences result from the composition of the passivefilms formed at various applied potentials. In order to identifythe differences, high resolution XPS spectra of main elements wereextracted as shown in Fig. 6.

Fig. 6a shows the peaks of Fe 2p3/2. Some positions of referencespectra are marked with dashed lines. For the passive film formedat 0 VSCE, the centre of the Fe 2p3/2 spectrum is located at 707.0 eV,confirming the presence of Fe metal (706.8 eV) as the main compo-sition in the passive film. There are also some FeO (709.4 eV) andFe2O3 (710.9 eV) for the change of curve in the corresponding posi-tions. For the film formed at 0.4 VSCE, the centre of the Fe 2p3/2

spectrum shifts to a higher binding energy which is about710.0 eV. It is notable that the rises near the reference spectra ofFe metal still exists, indicating that the Fe metal, FeO and Fe2O3

are all in the passive film formed at 0.4 VSCE. When the potential

1s for Fe-based AMC polarised at 0, 0.4 and 0.8 VSCE after 2 h in 3.5 wt.% NaCl.

Z.B. Zheng et al. / Corrosion Science 82 (2014) 115–124 119

increases to 0.8 VSCE, the peak around 707.0 eV almost disappears,denoting that the main compositions are converted to FeO andFe2O3.

The spectra of Cr are shown in Fig. 6b. The centre of the Cr 2p3/2

spectrum shifts to higher binding energies with increasing appliedpotential. In the film formed at 0 VSCE, the main composition is Crmetal (574.3 eV), followed by Cr2O3 (576.8 eV) and CrO3

(578.3 eV). Although the intensity of the curve at 0.4 VSCE is notstrong, some undulations can be distinguished. The centre of theCr 2p3/2 spectrum locates at 576.8 eV when the film is formed at0.8 VSCE, showing that the main oxide is Cr2O3.

The spectra of Mo 2d5/2, Mn 2p3/2 and W4f7/2 are shown inFig. 6c, d, e, respectively. As the potential increases, the centresof Mo 2d5/2 and W4f7/2 spectra change a lot. MoO3 (232.6 eV)and WO3 (35.6 eV) replace Mo metal (227.8 eV) and W metal(31 eV), becoming the main compositions in the passive filmformed at high applied potential. The intensity of Mn metal(638.8 eV) also decreases with the increasing applied potential.

Fig. 6f shows the peak of O 1s spectra. It is clear that the centreof the spectrum for passive film formed at 0 VSCE is in the middle ofreference spectra of OH� (531.5 eV) and O2� (530.2 eV), indicatinga similar amount of OH� and O2�. The peak moves toward thedashed line which stands for the position of O2� as the applied po-tential increases, indicating an increase in the quantity of oxidesand a decrease of hydroxides.

The above results show a relationship between oxidation statesof metallic elements and the applied potential. For the filmsformed on the surface of AMC in 3.5 wt.% NaCl solution at a lowfilm formation potential like 0 VSCE, metallic states of elementsare detected and the oxides of different oxidation states are notmuch. While at high applied potentials such as 0.4 and 0.8 VSCE,it is apparent that the compositions of metallic states reduce shar-ply and the oxides increase. For some elements, oxides in higher

Fig. 7. The detailed XPS spectra of Fe 2p3/2 and Mn 2p3/2 of the passive films form

oxidation states act as the primary components such as MoO3

and WO3. Oxide of low valence state like WO2 (32.5 eV) is rarelyseen in the passive film formed at 0.8 VSCE by the low intensityin the referential position. It is obvious that unlike the oxides ofMo and W, Cr2O3 is detected as the main oxidation state insteadof CrO3 even at high film formation potential. The spectra of Fe2p3/2 and Mn 2p3/2 were taken out to conduct a deep study becauseit was not easy to point out the main oxidation states of Fe and Mnat 0.4 and 0.8 VSCE from Figs. 6a, and d. After background subtrac-tion, the XPS results were separated into contributions of differentoxidation states as shown in Fig. 7 by a fit procedure which was de-scribed in previous paper [14]. Meanwhile, fractions of oxides suchas FeO, Fe2O3, MnO (641.2 eV) and MnO2 (642.2 eV) in the passivefilms formed at 0.4 and 0.8 VSCE were figured out in Fig. 8. It isapparent that the fractions of sub-oxides like FeO and MnO in-crease when the applied film formation potential increases andthe fractions of oxides in higher oxidation states like Fe2O3 andMnO2 decrease, which are similar to the obvious situation for oxi-des of Cr as mentioned above.

Wang et al. [57] studied the passive film formed on an yttriumcontaining Fe-based bulk metallic glass in HCl solution and foundout a bi-layer structure with a defective outer layer of high-valencespecies and an inner layer of low-valence species. The defectiveouter layer of high-valence species was easily dissolved. Selectivedissolution of Fe and enrichment of Cr were also found in the filmon the surface of 316L stainless steel [58]. It is believed that theenrichment of the low-valence species in the film can be ascribedto the preferential dissolution of high-valence oxides [59]. In thepresent study, the fractions of Fe2O3, MnO2 and CrO3 detected de-crease at high film formation potential, leaving more stable oxidessuch as FeO, MnO and Cr2O3 in the films. The excess dissolution ofhigh-valence species raises the anodic current density and resultsin the strange increase of passive current density in the passive

ed on Fe-based AMC polarised at 0.4 and 0.8 VSCE after 2 h in 3.5 wt.% NaCl.

Fig. 8. Fractions of iron oxides and manganese oxides in the passive films formed on Fe-based AMC at 0.4 and 0.8 VSCE after 2 h in 3.5 wt.% NaCl.

Fig. 9. XPS spectra of Cl 2p for Fe-based AMC polarised at 0, 0.4 and 0.8 VSCE after2 h in 3.5 wt.% NaCl.

120 Z.B. Zheng et al. / Corrosion Science 82 (2014) 115–124

region as shown in anodic polarisation. It is interesting to find outthat the dissolution is selective because the high-valence speciessuch as MoO3 and WO3 are still the main compositions in the pas-sive film at 0.8 VSCE. The differences in binding energy may be thereason why Fe2O3, MnO2 and CrO3 dissolved first.

The Cl 2p XPS spectra of the passive films formed on Fe-basedAMC at different film formation potentials are shown in Fig. 9.The binding energies of ClO�2 (203.4 eV) and some possible Cl�

(199.9, 198.9 and 198.6 eV) were labeled. It is clear that the inten-sities of the peaks at 0.4 and 0.8 VSCE are stronger than that at0 VSCE, indicating more chlorides participate in the formation ofpassive films at these conditions. The effect of chlorides will be dis-cussed in next section.

3.4. Depth profiles of the passive films investigated by X-rayphotoelectron spectroscopy

The depth profiles generated by Ar+ ion sputtering of Fe 2p, Cr2p, Mo 3d, Mn 2p, W 4f, O 1s and Cl 2p signals in the passive filmson Fe-based AMC at different potentials are shown in Fig. 10. Thesputtering distance at which the oxygen content decreases to half,relative to that at the surface, is taken to calculate the thickness ofpassive film.

It is apparent that for passive films formed on Fe-based AMC,thicknesses of passive films at different potentials are different.In accordance with the rule above, the thicknesses of passive filmsat 0, 0.4 and 0.8 VSCE are about 3, 8 and 50 nm, respectively. Thethickness increases with increasing potential. In the outer layerof these three passive films, the contents of Cr and Mo remainnearly constant. Although the contents of Cr and Mo are relativelysmall, the stability of the contents suggests that they are the main

compositions in the films. When the sputtering time increases, thecontent of Fe is stable. It indicates that the main oxides in the innerlayer of passive films are oxides of iron. Similar structures are re-ported in the passive films on 304 stainless steel [60] and other al-loy [61].

PDM proposes a linear relationship between the steady-statethickness and the film formation potential. However, the resultsobtained in this study shows a deviation from PDM, especiallythe passive film formed at 0.8 VSCE. This deviation is not strangeas changes in the oxidation states of the cations released fromthe barrier layer were observed as shown in Section 3.3 and theabsolute steady state of the passive films is unknown. Besides,the appearance of chloride may affect the performance of passivefilms a lot.

It is not surprising that chloride ions took part in the formationof passive film. Several researches have shown the phenomenon onFe–Cr alloys [62,63] and Fe-based amorphous alloys [38,64]. Theincorporation of chloride in the passive film can be detrimentalto film stability and may result in pit initiation. In chloride solu-tion, chloride penetrates into the passive film because of the exis-tence of concentration gradient between solution and film. For n-type semiconductor, oxygen vacancies which generate at the me-tal/oxide interface are the donors. Chloride ions penetrate intothe passive film and occupy the position of O2� [56]. Metallic ele-ments like Fe or Cr are amiable to chloride ions rather than oxygenanions, leading to the formation of solid metal chloride. The occur-rence of solid metal chloride causes volume expansion and abruptmechanical breakdown of the oxide film [65]. As shown in Fig. 10.The percentages of chloride in passive films formed at 0.4 and0.8 VSCE are larger than the percentage at 0 VSCE. Thus, it may be de-rived from the acceleration of applied potential. At high potential,ions are more active and the transfers of charge carriers are faster.More solid metal chlorides are formed and the passive films areless stable, leading to the dissolution of local films. Pits penetraterapidly into the films and the metal substrate, which may causelocalized corrosion. An unexpected aggregation of chloride hap-pens in the inner layer (from about 50 nm) of the passive filmgrown at 0.8 VSCE as shown in Fig. 10c. The reason is not clearbut a conjecture is presented here. In the preparation of the HVOFFe-based amorphous metallic coating, pores were left in the coat-ing. As shown in Fig. 11, on both the surface and cross section mor-phologies of the coating, pores exist even after potentiostatic testsat different applied potentials. When the coating was polarised,chloride ions penetrated into the passive films and came across apore. The pore became a place where chloride ions assembled,demonstrating the strange concentration distribution of chlorideion in Fig. 10c. The dissolution of passive films caused by chlorideions at high film formation potential may be another reason for theincrease in passive current density as shown in the anodic

Fig. 10. XPS depth profiles of the films formed on Fe-based AMC for 2 h in 3.5 wt.% NaCl at different potentials: (a) 0, (b) 0.4 VSCE and (c) 0.8 VSCE. The enlarged view of a part inthe box of (c) is shown in (d).

Fig. 11. SEM morphologies of the coating: (a) untreated, (b) polarised at 0 VSCE, (c) polarised at 0.4 VSCE, (d) polarised at 0.8 VSCE and cross section morphologies of (e)untreated, (f) polarised at 0 VSCE, (g) polarised at 0.4 VSCE and (h) polarised at 0.8 VSCE after 2 h in 3.5 wt.% NaCl.

Z.B. Zheng et al. / Corrosion Science 82 (2014) 115–124 121

polarisation. It may therefore be concluded that the participationof chloride ions reduces the stability of the passive film and makesthe film less protective.

3.5. Effect of applied potential on the critical flow velocity of the Fe-based AMC under slurry impingement

Combinations of jet impingement apparatus and electrochemi-cal workstation were used to investigate the erosion–corrosionbehaviour of Fe-based AMC. Our early paper [38] has proved thatthe Fe-based AMC exhibits better erosion–corrosion resistance than304 stainless steel under erosion–corrosion condition and may be agood candidate for the application under similar service environ-ment. In the latest study [39], the critical flow velocities of Fe-basedAMC and 304 stainless steel have been studied under impingementat 0 VSCE in 2 wt.% sand containing 3.5 wt.% NaCl solution. An inter-esting relationship between critical flow velocity and thicknesses ofpassive films formed on AMC has been detected. The critical flow

velocity is defined as a velocity beyond which the current densitystarts to jump for the first time in the potentiostatic test under jetimpingement. A further research on the effect of applied potentialon critical flow velocity of Fe-based AMC is carried out here.

Fig. 12 shows the potentiostatic tests at different applied poten-tials under responses of changing velocities during erosion–corro-sion. The potentials are all in the passive region of AMC. It isapparent that all current densities display a similar variation ten-dency. When the impingement is off at the beginning, the currentdensity decreases gradually and tends to be stable. It is a typicalprocess of film growth in the potentiostatic test. The current den-sity is larger when the applied potential is higher, which is in linewith the result of anodic polarisation. When the impingement isturned on, the current density decreases suddenly. The currentdensity keeps on decreasing or stays at a low level when theimpingement speeds up. Once the velocity is beyond a thresholdvalue like 14 m/s in Fig. 12a, the current density expresses anabrupt increase and then keeps on increasing or stays at a higher

122 Z.B. Zheng et al. / Corrosion Science 82 (2014) 115–124

level than before. The special value is taken out and considered asthe critical flow velocity of this material under impingement in thecertain environment. During the test, the early sudden drop of cur-rent density is attributed to the acceleration of the passivation,which is derived from increasing the mass transfer rate of dis-solved oxygen under erosion–corrosion [66]. Among all anodicprocesses such as particle impingement, passivation, depassivationand work hardening of the surface, passivation is the dominatedone at low flow velocities. Therefore, in this region, when thevelocity increases, the acceleration is enlarged, leading to the fur-ther decrease of passive current density. Although passivationdominates, the mechanical damages by the liquid–solid two-phaseflow cannot be neglected. In present study, the fluctuations of cur-rent density under impingement may display the processes of pitsgeneration and repassivation. It is observed from Fig. 12 that thebasic level of current density under a certain velocity is easily dis-tinguished and the fluctuations of current density are around thelevel. When the velocity is larger than the critical value, the higher

Fig. 12. Potentiostatic tests of AMC under impingement of different impact velocities achanges of the calculated average current densities by the impact velocities.

level announces the victory of depassivation during the competi-tion between depassivation and repassivation. It indicates thatthe passive films are easily destroyed by the sand and cannot re-pair themselves well under the impingement at a high impactvelocity. While the passive films can protect the material effi-ciently under impingement at a velocity smaller than the criticalvalue and the level of current density remains low. It is noticedthat some of the current densities do not return to pre-test staticflow case levels. The probable reasons may be that irreparabledamages have taken place on the surface of sample at some appliedpotentials, leaving the surface different from the original one. Fur-ther researches will be carried out here.The results shown inFig. 12 confirm the relationship between critical flow velocityand the protectiveness of passive film. As introduced above, thecritical flow velocities of AMC under impingement at the appliedpotentials of 0, 0.2, 0.4, 0.6 and 0.8 VSCE are 14, 11, 14, 9 and5 m/s, respectively. In general, the critical flow velocity at highapplied potential is smaller than that at low film formation poten-

t applied potentials: (a) 0, (b) 0.2 VSCE, (c) 0.4 VSCE, (d) 0.6 VSCE, (e) 0.8 VSCE and (f)

Z.B. Zheng et al. / Corrosion Science 82 (2014) 115–124 123

tial. In order to compare the changes of current density underimpingement at different potentials, the average current densityof each level was calculated as shown in Fig. 12f. It is clear thatthe variations of current density at higher potentials are larger,indicating that the sands affect the passive film harder at higherpotentials. Especially for the situation at 0.8 VSCE, the current den-sity keeps on increasing all the time by the increasing velocity afterthe impingement at 5 m/s. Lu and Luo [26,27] built a theoreticalmodel that can be used to formulate the average anodic currentdensity (�i) of passive metals in flowing slurry as follows:

�i ¼ is þ k � ipeak ð2Þ

where is is the stable current density free of particle impingement,ipeak is the peak value of the local current density response to theimpingement and k is a parameter which is related to the decayconstant and the generating rate of the fresh surface caused bythe particle impingement. According to the discussion above, thecurrent density of passive metals in flowing slurry can be describedas follows:

�i ¼ is þ iimpact � ire ð3Þ

where iimpact is the current density caused by sand impingementand ire is the current density which results from repassivation.The parameter iimpact is affected by the condition of impingementsuch as velocity, sand concentration, size or shape of the damagedsurface and others. Parameter ire is influenced by the passivationof material, the applied potential and the solution. To analyse eachparameter clearly needs a lot of tests and will be a further work. Thesimple formula displays a qualitative analysis of a competition be-tween repassivation and depassivation under particle impingement.From the formula (3), it is obvious that depassivation increases theanodic current density and the repassivation decreases it. Presentstudy demonstrates that �i depends on the film formation potential.�i is larger at higher potential as shown in anodic polarisation. It isexplained by XPS analysis in Sections 3.3 and 3.4. Besides, the poorprotectiveness of passive films at high potential relates to the lowability of repassivation, which leads to a smaller ire. As a result,the anodic current density �i is larger at higher potential and in-creases by the rising iimpact (when the intensity of impact increases).In other words, the relatively lower ability of repassivation and theworse protective passive films formed on Fe-based AMC at higherpotential lead to a smaller critical flow velocity under slurryimpingement (see Fig. 12).

4. Conclusions

(1) Passive current density of the Fe-based amorphous metal-lic coating increases with the increase of applied potentialin anodic polarisation.

(2) Mott–Schottky analyses of passive films formed at differentfilm formation potentials show that the passive filmsformed on Fe-based AMC present n-type semiconductingproperties in the potential region between 0 and 0.6 VSCE

and change to p-type semiconducting properties after0.6 VSCE.

(3) On the basis of XPS studies, oxidation states of Fe, Cr, Mo,Mn and W in the passive film increase before 0.4 VSCE, anddecrease with increase in film formation potential. Thepreferential dissolution of high-valence oxides such asFe2O3, MnO2 and CrO3 results in a larger passive currentdensity at a higher potential.

(4) The thicknesses of films formed on Fe-based AMC at 0, 0.4and 0.8 VSCE are about 3, 8 and 50 nm. More chloride ionsin the film at higher potential cause the dissolution of pas-sive film, leading to the increase in passive current density.

(5) The critical flow velocities of Fe-based AMC underimpingement at the applied potentials of 0, 0.2, 0.4, 0.6and 0.8 VSCE are 14, 11, 14, 9 and 5 m/s, respectively. Itsuggests that the critical flow velocity depends on the pro-tectiveness of the passive film.

Acknowledgements

This work was supported by the National Natural Science Foun-dation of China (No. 51131008). The authors would like to expressthanks for the help and assistance on the experiments from S.Y. He.We are also grateful to Dr. P.C. Okafor for his useful suggestions ofEnglish.

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