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Role of passive film in dominating the electrochemical corrosion behavior of FeCrMoCBY amorphous coating
Journal of Alloys and Compounds 811 (2019) 151962
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Journal of Alloys and Compounds journal homepage: http://www.elsevier.com/locate/jalcom
Role of passive film in dominating the electrochemical corrosion behavior of FeCrMoCBY amorphous coating Miqi Wang*, Zehua Zhou**, Qijie Wang, Zehua Wang, Xin Zhang, Yuying Liu College of Mechanics and Materials, Hohai University, Nanjing, 211100, PR China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 3 July 2019 Received in revised form 20 August 2019 Accepted 21 August 2019 Available online 23 August 2019
FeCrMoCBY amorphous coating with a high volume fraction of amorphous phase (90%) was prepared on Q235 steel by atmospheric plasma spraying. Influence of passivation potential on the corrosion resistance of passive films on the coating surface was estimated by electrochemical measurements, capacitance analysis, AFM and XPS technique. The results revealed that increasing potential promoted the growth of a more compact and thicker film due to the formation of more bounded water and oxides. Reduction in hydroxides further decreased point defects density significantly in the passive layer when passivated at a higher potential. A low diffusivity (1.67 1015 cm2 s1) corresponding to point defects could suppress both the growth and degradation processes of passive film in chloride containing electrolyte, and thus enhance the film resistance to local thinning. © 2019 Elsevier B.V. All rights reserved.
Keywords: Fe-based amorphous coating Passive film Passivation potential Point defects Semiconducting structure
1. Introduction Fe-based amorphous coatings, inheriting the exceptional amorphous nature from Fe-based metallic glasses (BMGs), exhibit remarkable corrosion resistance, high hardness, superior wear behavior and a relatively low cost [1e3]. Application of thermal spraying technique has expanded the industrial production and practical application of amorphous materials. In particular, Febased amorphous coatings are of great potential for marine metal protection under harsh environment. Theoretically, Fe-based amorphous coatings should manifest extraordinary resistance to corrosive medium in view of the absence of grain boundaries and dislocations [4,5]. In essence, the corrosion resistance of Fe-based amorphous coatings is dependent on their passivity in the electrolyte. Previous studies have pointed out that Cr-rich passive film is conductive to hamper the evolution of corrosion process [6,7]. In addition, Mo element could augment pitting resistance by hindering the dissolution of Cr oxide species, and thus promote the chemical homogeneity and stabilize the protective film. In contrast, pores and micro-cracks during deposition procedure will initiate Cr-depletion zone and alter the film
* Corresponding author. ** Corresponding author. E-mail addresses: [email protected] (M. Wang), [email protected] (Z. Zhou). https://doi.org/10.1016/j.jallcom.2019.151962 0925-8388/© 2019 Elsevier B.V. All rights reserved.
composition homogeneity [8e10]. It can be concluded that chemical composition analysis is vital for evaluation of corrosion mechanism of Fe-based amorphous coatings. Recently, some scholars also proposed that electronic structure and semiconducting property of passive films on stainless steels are closely related to the barrier performance of oxide layers [11e14]. From this excitation, this work intends to figure out the influence of topographies, electronic structure and cationic species of oxide films on the electrochemical characteristics of Fe-based amorphous coatings systematically. This work may offer new perspectives into the growth and rupture features of passive films formed on amorphous coatings. 2. Experimental 2.1. Coating preparation Water atomized Fe48Cr15Mo14C15B6Y2 (at. %) amorphous powder shown in Fig. 1 ranging from 100 to 200 mm was used as feedstock. Q235 steel as the substrate was cleaned in acetone, dried in air and sand blasted prior to spraying. Coating deposition process was carried out on PRAXAIR 37l0 type atmospheric plasma spraying system. Spraying parameters were elaborately tailored as: 650 A of spraying current, 49.6 L min1 of primary gas flow velocity, 7.5 L min1 of carrier gas flow velocity, 5.6 L min1 of secondary gas flow velocity, 120 mm of stand-off distance and 27 g min1 of
2
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force microscopy (AFM, NT-MDT Prima). 3. Results and discussions 3.1. Microstructure of the coating
Fig. 1. Morphology of FeCrMoCBY amorphous powder by water atomization method.
powder feed rate. 2.2. Coating characterization Microstructure of amorphous powder and cross-sectional morphology of as-sprayed coating were observed by scanning electron microscopy (SEM, Hitachi S3400 N). X-ray diffraction (XRD, D8 Advance) technique with Cu Ka radiation was applied to detect phase structure of the powder and coating. The amorphous volume fraction of the coating was determined by Pseudo-Voight function in Jade 6.0 software using Verdon method [15]. 2.3. Electrochemical measurements Coating surface was firstly mechanically abraded by SiC papers and then polished to mirror-like face before electrochemical test. A conventional three-electrode system, consisting of a platinum plate (counter electrode), standard calomel electrode (SCE, reference electrode) and a working electrode (coating sample), was connected to CHI 660E electrochemical workstation. The working electrode with 1 cm2 exposure area was initially polarized at 1.5 VSCE for 600 s for air-formed oxides removal in 3.5 wt. % NaCl solution. Immediately, the coating was immersed in the solution until reaching a steady open circuit potential (OCP). Potentiodynamic polarization curve was recorded from 1.4 to 1.5 VSCE at a scanning rate of 1 mV s1. According to the passive region DE derived from the potentiodynamic polarization curve, potentiostatic polarization was performed at 0.2, 0.4, 0.6 and 0.8 VSCE for 3600 s, separately. Then electrochemical impedance spectroscopy (EIS) was documented ranging from 0.01 Hz to 10 kHz at OCP state. EIS data were simulated and analyzed using ZSimpwin software. Mott-Schottky measurement was conducted at 1000 Hz for capacitance analysis including the semiconductor type and point defects density in the film. All the mentioned tests were triplicated for data reliability. 2.4. Surface evaluation of passive film Chemical composition and oxidation state of passive films were characterized by X-ray photoelectron spectroscopy (XPS, Thermo ESCALAB 250XI) approach and Al Ka was used as the excitation source. The binding energies corresponding to Fe 2p3/2, Cr 2p3/2, Mo 3d and O 1s were calibrated by C 1s standard peak (284.6 eV) and referred from NIST XPS data library online. XPS results were analyzed by means of XPSPeak software. Topographic observation of passive films at different potentials was supported by atomic
Fig. 2 (a) shows the cross-sectional morphology of as-sprayed FeCrMoCBY amorphous coating. It is seen that the coating is closely packed and rather dense without obvious splat interfaces. However, several visible pores and micro-cracks are still detected as dark regions in the coating. These defective flaws are inevitable due to partially melted powders and junctions of well-flattened splats [4,8]. A wide halo peak with no crystalline peak appears in the XRD patterns in Fig. 2 (b). It indicates that both the powder and coating are almost full of amorphous phase [16]. In addition, amorphous volume fraction of the coating is calculated as 90%, demonstrating that the coating could retain its amorphous essence from amorphous powder even under a extremely high temperature (over 10000 K) of plasma flame. 3.2. Potentiodynamic polarization curve Potentiodynamic polarization curve of the coating is presented in Fig. 3. It is clear that this coating can passivate spontaneously across a wide range from 0 to 1.0 VSCE and the passive region DE is about 0.92 VSCE. Accordingly, an approximation of breakdown potential Eb [17] is 0.97 VSCE. These critical parameters prove a relatively strong passivation ability of FeCrMoCBY amorphous coating. The obtained passive current density Ip approaches to 1.25 103 Acm2, which is usually related to the microstructural defects in the coating as indicated in Fig. 2 (a). Further, the corrosion current density Icorr is confirmed to be 8.24 106 Acm2, nearly three magnitudes lower than Ip. Hence, it can be speculated that the coating is more sensitive to localized corrosion. 3.3. Electrochemical impedance spectroscopy Fig. 4 exhibits the Nyquist plots and phase angle transformation of passive films polarized at different potentials. Noting that all the capacitive loops are incomplete semi-circles over the whole frequency ranges, suggesting that passive films show similar features at various passivation potentials. The diameter of capacitive loop rises with increasing potential, proving the enhancement of corrosion resistance. As indicated in Fig. 4 (b), the enlarging phase angle and expanding plateau in the medium frequency further confirm that a more protective oxide layer is generated when the potential shifts to a positive trend. A simple Randle's model in Fig. 4 (c) composed of electrolyte resistance Rs, double layer capacitance Qdl and passive film resistance Rf, is used to simulate the EIS data since that only a time constant can be observed from EIS plots. The fitting results are summarized in Table 1. Evidently, Rf raises from 1846 to 3545 Ucm2 as the potential shifts from 0.2 to 0.8 VSCE, indicating that the electrochemical corrosion resistance is gradually enhanced. Meanwhile, reduction of Qdl proves that a thicker passive film is formed at a higher potential [18,19]. 3.4. Capacitance analysis of passive film It has been assumed that passive film resistance to corrosive medium is relevant with its semiconducting distribution and electronic structure. Consequently, Mott- Schottky measurement was conducted and the space charge capacitance CSC corresponding to n-type semiconductors is associated with potential E [20]:
M. Wang et al. / Journal of Alloys and Compounds 811 (2019) 151962
3
Fig. 2. (a) Microstructure and (b) XRD patterns of FeCrMoCBY amorphous powder and plasma sprayed FeCrMoCBY amorphous coating.
indicates that less chloride ions could occupy the positions of vacancies in the film. Some metal ions such as Cr from coating will react with chloride ions and produce metal chlorides (MClx), leading to the volume expansion and further mechanical failure of passive film [23e25]. As a consequence, a prompt penetration of pitting nucleation into the coating surface will be easily triggered. It can be concluded that increasing potential reduces the vacancy sites for chloride ions, and this is beneficial for the film to resist local thinning. A decreased donor density is also in good agreement with the enhanced film resistance from EIS data in Table 1. To further figure out the growth feature of passive film, the film thickness should be considered. The growth trend of passive film, is roughly calculated from the following equation [26]:
62εε0 6 d¼6 6 4
31 2 . E Ufb kT e7 7 7 7 eNd 5 =
2
(2)
Fig. 3. Potentiodynamic polarization curve of FeCrMoCBY amorphous coating in 3.5 wt. % NaCl solution.
C2SC ¼
2 kT E Ufb εε0 qNd q
(1)
where ε is the dielectric constant of passive film (15.6 for FeeCr alloy), ε0 represents the vacuum permittivity (8.854 1012 Fm1), q denotes the electron charge (1.602 1019 C), Nd means the donor density, Ufb signifies the flat band potential, k stands for the Boltzmann constant (1.38 1023 J K1) and T is the absolute temperature (298 K in this work). Thus, the donor density Nd can be easily attained from the positive slope within the linear region between CSC 2 and E. Fig. 5 shows Mott-Schottky curves of passive films as a function of potential. In the range of 0e0.5 VSCE, n-type semiconducting property can be ascertained due to the positive and linear relationship between C2SC and E. Correspondingly, the donor densities Nd at 0.2, 0.4, 0.6 and 0.8 VSCE are 2.19 1022,8.89 1021,6.56 1021, 6.13 1021 cm3, respectively. Obviously, an increased potential drops the donor density in the passive film significantly. A lower donor density means less defective point defects in the film according to the point defect model (PDM) [21,22], and it
In this study, the flat band potential Ufb at 0.2, 0.4. 0.6 and 0.8 VSCE are 0.96, 0.25, 0.32, 0.18 VSCE, respectively. Accordingly, the film thicknesses are 0.76, 0.88, 1.23 and 1.32 nm, corresponding to 0.2, 0.4, 0.6 and 0.8 VSCE, separately. It can be inferred that a thicker film is generated at a higher potential, offering better resistance to aggressive ions penetrating into the layer. Another critical parameter to describe the growth and breakdown kinetic of passive film is the diffusion coefficient D of point defects [27]. The relationship between donor density Nd and passivation potential E is expressed as [28]:
Nd ¼ u1 expðbEÞ þ u2
(3)
where u1 , u2 and b are constants acquired from the plot of Nd versus E in Fig. 6 (a). Nd decreases as an exponential function of E, which coincides with the relationship revealed in Eq. (3). In this case, u2 is 6.044 1021 cm3 as a significant factor for evaluating the diffusion coefficient D of point defects according to the following formula [28]:
D¼
Ip RT 4qFεL u2
(4)
where Ip is the steady passive current density (1.25 103 A cm2),
4
M. Wang et al. / Journal of Alloys and Compounds 811 (2019) 151962
Fig. 4. (a) Nyquist diagrams, (b) Bode phase angles and (c) equivalent electrical circuit of passive films of FeCrMoCBY amorphous coating at different formation potentials after 1 h polarization in 3.5 wt. % NaCl solution.
Table 1 EIS parameters obtained from the fitting results for FeCrMoCBY amorphous coating passivated in 3.5 wt. % NaCl solution. a1
Potential (VSCE)
Rs (U cm )
Qdl (F s
0.2 0.4 0.6 0.8
9.41 9.20 11.08 11.64
0.000921 0.000534 0.0000878 0.0000838
2
cm
2
)
n
Rf (U cm )
chi
0.607 0.742 0.847 0.861
1846 2180 2554 3545
9.37e-04 3.40e-04 7.12e-04 5.35e-04
2
d¼
1 ð1 aÞE þ b εL
(5)
2
where a ¼ 0.5 represents the polarization level of the film/electrolyte interface and b is a constant. Fig. 6 (b) shows the plot of d versus E and the extracted results indicate that εL is approximately 4.95 106 V cm1. Based on the above analysis, the corresponding diffusivity D of point defects in the passive film regarding to FeCrMoCBY amorphous coating in 3.5 wt. % NaCl solution is about 1.67 1015 cm2 s1, which is comparable to the results reported on bulk alloys [28,29]. This value is relatively low and it indicates that the transportation rate of vacancies involving the growth and breakdown processes are not fast sufficiently. Namely, a moderate balance between the growth near the film/coating interface and dissolution close to film/electrolyte interface can be created [30], which restrains the local thinning of passive film to some extent.
3.5. Topographies of passive film
Fig. 5. Mott-Schottky plots of passive films formed on FeCrMoCBY amorphous coating at different formation potentials in 3.5 wt. % NaCl solution.
R is the universal gas constant (8.314 J mol1 K1), T is the absolute temperature (298 K), F is the Faraday constant (96500 C mol1) and εL is the average electric field in the film. The value of εL is accomplished through the linear relationship between film thickness d and applied potential E [28]:
AFM topographic profiles of the coating surface at varied potentials for 3600 s are displayed in Fig. 7. No obvious oxides but distinct grooves paralleling to the surface appear on the coating surface at 0.2 VSCE. Only a tiny but discontinuous film can be observed. After passivation at 0.4 VSCE, grooves still exist but more integral film is generated. In contrast, passive film at 0.6 VSCE covers almost the coating surface with several amount of pores. Furthermore, the film evolves into a denser layer without evident flaws at 0.8 VSCE. It is also clear that substantial rod-like trips are shown in Fig. 7 (c) and (d). But the film seems to be refined in smaller size at 0.8 VSCE, which profits the corrosion resistance of the coating. Thus, the increased potential contributes to the formation of a more compact and resistive film in chloride containing solution.
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5
Fig. 6. (a) Donor densities, (b) film thicknesses of passive films formed on FeCrMoCBY amorphous coating as a function of formation potential in 3.5 wt. % NaCl solution.
Fig. 7. AFM topography of passive films of FeCrMoCBY amorphous coating polarized at (a) 0.2 VSCE, (b) 0.4 VSCE (c) 0.6 VSCE and (d) 0.8 VSCE, respectively in 3.5 wt. % NaCl solution.
3.6. XPS analysis of passive film Fig. 8 exhibits the high resolution results of Fe 2p3/2, Cr 2p3/2, Mo 3d and O 1s of passive films at diverse potentials. Fe 2p3/2 peaks are
separated into Fe0 (706.5 eV), Fe3O4 (708.3 eV), FeO (709.5 eV), Fe2O3 (710.6 eV) and FeOOH (711.5 eV), indicating of five component states of Fe element. Four species representing metallic state Cr0 (574.2 eV), Cr2O3 (575.6 eV), Cr(OH)3 (577 eV) and CrO3 (578 eV)
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Fig. 8. XPS spectra for (a) Fe 2p3/2 (b) Cr 2p3/2 (c) Mo 3d and (d) O 1s of passive films on FeCrMoCBY amorphous coating at various potentials in 3.5 wt. % NaCl solution, respectively.
M. Wang et al. / Journal of Alloys and Compounds 811 (2019) 151962
Fig. 8. (continued).
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comprise of Cr 2p3/2 spectra. Meanwhile, peaks at 227.8 eV 3d5/2 and 230.8 eV 3d3/2 correspond to metallic state of Mo element, while MoO2 and MoO3 are featured by 229.8 eV 3d5/2 and 232.5 eV, respectively. In addition, O 1s ionization is composed of O2(530.4 eV), OH (531.9 eV) and bounded water (532.5 eV). The elemental distribution and cationic concentrations of oxidation species are presented in Fig. 9. Fe content is the maximum among three detected metal elements due to the fact that Fe is the primary alloying element of amorphous coating. It is observed that the concentration of Mo is always under 10 at. %, but its contribution to film stability cannot be ignorable. A suitable addition of Mo element could enhance the pitting corrosion resistance of passive film since it can promote the enrichment of Cr [31]. Especially, the generation of low valence oxide species (such as MoO2) contributes to the stability of passivation ability formed on FeeCr based amorphous materials [32]. A protective insoluble Mocontaining film can behave like an effective barrier against the diffusion of oxide species, and thus decrease the dissolution rate of passive film. In contrast, Cr proportion exceeds 20 at. % at all potentials and it rises gradually as potential increases from 0.2 to 0.8 VSCE. Previous researches have concluded that the critical concentration of Cr to form a protective Cr oxide film is nearly 15 at. %, and the corresponding film behavior will resemble to pure Cr [33]. It is supposed that Cr-containing species in this study are believed to be crucial for a resistive oxide layer to hamper chloride ions. Specially, the accumulation of Cr2O3 indicated from Fig. 9 (c) at a higher
potential accounts for the enhancement of corrosion resistance of the coating. Insoluble Cr2O3 is stable enough and less defective than other metallic oxides, which can elucidate the stronger passivity at a higher potential. It is assumed that a tenser electric strength at a higher potential as shown in Fig. 6 (b) can promote the diffusion of Cr element from inner layer to outer layer, further shaping a denser passive film. Fig. 9 (d) also reveals that hydroxides reduce progressively, whereas the total fraction of oxides and bounded water increases steadily with the increment of potential. It was reported that dissolved metal ions can be captured by bounded water [34], and thus more fresh oxide films are facilitated and generated. At the same time, hydroxides exhibiting hydrophilic surfaces [35] are deemed to deteriorate the corrosion resistance of the coating. Hydroxyl vacancies (V·OH) are generated at the coating/film interface and diffuse to and experience an annihilation at the film/electrolyte interface, which are expressed in Eqs. (6a) and (6b) [36]:
m 4 MM þ V ,OH þ xe
(6a)
V ,OH þ H2 O4MM þ OHOH þ Hþ
(6b)
where m is the amorphous coating, MM is a metal cation, V ,OH is the hydroxyl vacancies. Accordingly, the formation and annihilation reactions of hydroxyl vacancies (V·OH) underline the significance of hydration on the coating surface. Incorporation of hydroxyl ions
Fig. 9. Fractions of (a) total Fe, Cr, Mo species (b) iron oxides (c) chromium oxides (d) oxides/hydroxides, bounded water, (O2-þ H2O)/OH ratio detected from XPS results from the film surface of FeCrMoCBY amorphous coating at various potentials in 3.5 wt. % NaCl solution.
M. Wang et al. / Journal of Alloys and Compounds 811 (2019) 151962
results in the hydration-dehydration, which favors the generation of hydrated sites. Further, production of hydrogen gas at the coating/film interface will accelerate the breakdown of passive film. R.C. Sierra [36] et al. put forward that more hydroxyl vacancies are produced accompanied with the generation of more hydroxides. As a consequence, point defects in the film are induced simultaneously, resulting the increment of donor density. It means that the film resistance to local thinning is raised via shifting the potential toward a more positive tendency. 4. Conclusions (1) The electrochemical corrosion behavior of FeCrMoCBY amorphous coating was sensitive to the passivation potential, and a higher potential lead to a better corrosion resistance. (2) Formation of more insoluble Cr2O3 in the oxide layer stabilized and promoted the compactness of passive film on FeCrMoCBY amorphous coating surface. (3) Local thinning of passive film on FeCrMoCBY amorphous coating was closely related to the point defects density, which was reduced by the reduction of hydrophilic hydroxides. In addition, a relative low diffusivity of point defects in the film balanced the growth and breakdown processes, and hampered the pitting initiation. Acknowledgements This work was financially supported by the National Natural Science Foundation of China (Grant No. 51379070& No. 51909071), the Fundamental Funds for the Central Universities (Grant No. 2018B689X14), the Postgraduate Research & Practice Innovation Program of Jiangsu Province (Grant No. KYCX18_0569), China Scholarship Council (2019). We also would like to thank Liang Wang from Shiyanjia Lab (www.shiyanjia.com) for support of XPS test. References [1] P.F. Gostin, S. Oswald, L. Schultz, A. Gebert, Acid corrosion process of Fe-based bulk metallic glass, Corros. Sci. 62 (2012) 112e121. [2] S.F. Guo, K.C. Chan, S.H. Xie, P. Yu, Y.J. Huang, H.J. Zhang, Novel centimetersized Fe-based bulk metallic glass with high corrosion resistance in simulated acid rain and seawater, J. Non-Cryst. Solids 369 (2013) 29e33. [3] D. Zenebe, S. Yi, S.S. Kim, Sliding friction and wear behavior of Fe-based bulk metallic glass in 3.5% NaCl solution, J. Mater. Sci. 47 (2012) 1446e1451. [4] J. Henao, A. Concustell, I.G. Cano, N. Cinca, S. Dosta, J.M. Guilemany, Influence of cold gas spray process conditions on the microstructure of Fe based amorphous coatings, J. Alloy. Comp. 622 (2015) 995e999. [5] Z. Zhou, L. Wang, F.C. Wang, H.F. Zhang, Y.B. Liu, S.H. Xu, Formation and corrosion behavior of Fe-based amorphous metallic coatings by HVOF thermal spraying, Surf. Coat. Technol. 204 (2009) 563e570. [6] Y. Wang, S.L. Jiang, Y.G. Zheng, W. Ke, W.H. Sun, J.Q. Wang, Electrochemical behaviour of Fe-based metallic glasses in acidic and neutral solutions, Corros. Sci. 63 (2012) 159e173. [7] A.R. Newmark, U. Stimming, Photoelectrochemical studies of passive films on zirconium and amorphous iron-zirconium alloys, Langmuir 3 (1987) 905e910. [8] S.D. Zhang, J. Wu, W.B. Qi, J.Q. Wang, Effect of porosity defects on the longterm corrosion behaviour of Fe-based amorphous alloy coated mild steel, Corros. Sci. 110 (2016) 57e70. [9] J. Wu, S.D. Zhang, W.H. Sun, Y. Gao, J.Q. Wang, Enhanced corrosion resistance in Fe-based amorphous coatings through eliminating Cr-depleted zones, Corros. Sci. 136 (2018) 161e173. [10] R.Q. Guo, C. Zhang, Q. Chen, Y. Yang, N. Li, L. Liu, Study of structure and corrosion resistance of Fe-based amorphous coatings prepared by HVAF and HVOF, Corros. Sci. 53 (2011) 2351e2356.
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Journal of Alloys and Compounds journal homepage: http://www.elsevier.com/locate/jalcom
Role of passive film in dominating the electrochemical corrosion behavior of FeCrMoCBY amorphous coating Miqi Wang*, Zehua Zhou**, Qijie Wang, Zehua Wang, Xin Zhang, Yuying Liu College of Mechanics and Materials, Hohai University, Nanjing, 211100, PR China
a r t i c l e i n f o
a b s t r a c t
Article history: Received 3 July 2019 Received in revised form 20 August 2019 Accepted 21 August 2019 Available online 23 August 2019
FeCrMoCBY amorphous coating with a high volume fraction of amorphous phase (90%) was prepared on Q235 steel by atmospheric plasma spraying. Influence of passivation potential on the corrosion resistance of passive films on the coating surface was estimated by electrochemical measurements, capacitance analysis, AFM and XPS technique. The results revealed that increasing potential promoted the growth of a more compact and thicker film due to the formation of more bounded water and oxides. Reduction in hydroxides further decreased point defects density significantly in the passive layer when passivated at a higher potential. A low diffusivity (1.67 1015 cm2 s1) corresponding to point defects could suppress both the growth and degradation processes of passive film in chloride containing electrolyte, and thus enhance the film resistance to local thinning. © 2019 Elsevier B.V. All rights reserved.
Keywords: Fe-based amorphous coating Passive film Passivation potential Point defects Semiconducting structure
1. Introduction Fe-based amorphous coatings, inheriting the exceptional amorphous nature from Fe-based metallic glasses (BMGs), exhibit remarkable corrosion resistance, high hardness, superior wear behavior and a relatively low cost [1e3]. Application of thermal spraying technique has expanded the industrial production and practical application of amorphous materials. In particular, Febased amorphous coatings are of great potential for marine metal protection under harsh environment. Theoretically, Fe-based amorphous coatings should manifest extraordinary resistance to corrosive medium in view of the absence of grain boundaries and dislocations [4,5]. In essence, the corrosion resistance of Fe-based amorphous coatings is dependent on their passivity in the electrolyte. Previous studies have pointed out that Cr-rich passive film is conductive to hamper the evolution of corrosion process [6,7]. In addition, Mo element could augment pitting resistance by hindering the dissolution of Cr oxide species, and thus promote the chemical homogeneity and stabilize the protective film. In contrast, pores and micro-cracks during deposition procedure will initiate Cr-depletion zone and alter the film
* Corresponding author. ** Corresponding author. E-mail addresses: [email protected] (M. Wang), [email protected] (Z. Zhou). https://doi.org/10.1016/j.jallcom.2019.151962 0925-8388/© 2019 Elsevier B.V. All rights reserved.
composition homogeneity [8e10]. It can be concluded that chemical composition analysis is vital for evaluation of corrosion mechanism of Fe-based amorphous coatings. Recently, some scholars also proposed that electronic structure and semiconducting property of passive films on stainless steels are closely related to the barrier performance of oxide layers [11e14]. From this excitation, this work intends to figure out the influence of topographies, electronic structure and cationic species of oxide films on the electrochemical characteristics of Fe-based amorphous coatings systematically. This work may offer new perspectives into the growth and rupture features of passive films formed on amorphous coatings. 2. Experimental 2.1. Coating preparation Water atomized Fe48Cr15Mo14C15B6Y2 (at. %) amorphous powder shown in Fig. 1 ranging from 100 to 200 mm was used as feedstock. Q235 steel as the substrate was cleaned in acetone, dried in air and sand blasted prior to spraying. Coating deposition process was carried out on PRAXAIR 37l0 type atmospheric plasma spraying system. Spraying parameters were elaborately tailored as: 650 A of spraying current, 49.6 L min1 of primary gas flow velocity, 7.5 L min1 of carrier gas flow velocity, 5.6 L min1 of secondary gas flow velocity, 120 mm of stand-off distance and 27 g min1 of
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force microscopy (AFM, NT-MDT Prima). 3. Results and discussions 3.1. Microstructure of the coating
Fig. 1. Morphology of FeCrMoCBY amorphous powder by water atomization method.
powder feed rate. 2.2. Coating characterization Microstructure of amorphous powder and cross-sectional morphology of as-sprayed coating were observed by scanning electron microscopy (SEM, Hitachi S3400 N). X-ray diffraction (XRD, D8 Advance) technique with Cu Ka radiation was applied to detect phase structure of the powder and coating. The amorphous volume fraction of the coating was determined by Pseudo-Voight function in Jade 6.0 software using Verdon method [15]. 2.3. Electrochemical measurements Coating surface was firstly mechanically abraded by SiC papers and then polished to mirror-like face before electrochemical test. A conventional three-electrode system, consisting of a platinum plate (counter electrode), standard calomel electrode (SCE, reference electrode) and a working electrode (coating sample), was connected to CHI 660E electrochemical workstation. The working electrode with 1 cm2 exposure area was initially polarized at 1.5 VSCE for 600 s for air-formed oxides removal in 3.5 wt. % NaCl solution. Immediately, the coating was immersed in the solution until reaching a steady open circuit potential (OCP). Potentiodynamic polarization curve was recorded from 1.4 to 1.5 VSCE at a scanning rate of 1 mV s1. According to the passive region DE derived from the potentiodynamic polarization curve, potentiostatic polarization was performed at 0.2, 0.4, 0.6 and 0.8 VSCE for 3600 s, separately. Then electrochemical impedance spectroscopy (EIS) was documented ranging from 0.01 Hz to 10 kHz at OCP state. EIS data were simulated and analyzed using ZSimpwin software. Mott-Schottky measurement was conducted at 1000 Hz for capacitance analysis including the semiconductor type and point defects density in the film. All the mentioned tests were triplicated for data reliability. 2.4. Surface evaluation of passive film Chemical composition and oxidation state of passive films were characterized by X-ray photoelectron spectroscopy (XPS, Thermo ESCALAB 250XI) approach and Al Ka was used as the excitation source. The binding energies corresponding to Fe 2p3/2, Cr 2p3/2, Mo 3d and O 1s were calibrated by C 1s standard peak (284.6 eV) and referred from NIST XPS data library online. XPS results were analyzed by means of XPSPeak software. Topographic observation of passive films at different potentials was supported by atomic
Fig. 2 (a) shows the cross-sectional morphology of as-sprayed FeCrMoCBY amorphous coating. It is seen that the coating is closely packed and rather dense without obvious splat interfaces. However, several visible pores and micro-cracks are still detected as dark regions in the coating. These defective flaws are inevitable due to partially melted powders and junctions of well-flattened splats [4,8]. A wide halo peak with no crystalline peak appears in the XRD patterns in Fig. 2 (b). It indicates that both the powder and coating are almost full of amorphous phase [16]. In addition, amorphous volume fraction of the coating is calculated as 90%, demonstrating that the coating could retain its amorphous essence from amorphous powder even under a extremely high temperature (over 10000 K) of plasma flame. 3.2. Potentiodynamic polarization curve Potentiodynamic polarization curve of the coating is presented in Fig. 3. It is clear that this coating can passivate spontaneously across a wide range from 0 to 1.0 VSCE and the passive region DE is about 0.92 VSCE. Accordingly, an approximation of breakdown potential Eb [17] is 0.97 VSCE. These critical parameters prove a relatively strong passivation ability of FeCrMoCBY amorphous coating. The obtained passive current density Ip approaches to 1.25 103 Acm2, which is usually related to the microstructural defects in the coating as indicated in Fig. 2 (a). Further, the corrosion current density Icorr is confirmed to be 8.24 106 Acm2, nearly three magnitudes lower than Ip. Hence, it can be speculated that the coating is more sensitive to localized corrosion. 3.3. Electrochemical impedance spectroscopy Fig. 4 exhibits the Nyquist plots and phase angle transformation of passive films polarized at different potentials. Noting that all the capacitive loops are incomplete semi-circles over the whole frequency ranges, suggesting that passive films show similar features at various passivation potentials. The diameter of capacitive loop rises with increasing potential, proving the enhancement of corrosion resistance. As indicated in Fig. 4 (b), the enlarging phase angle and expanding plateau in the medium frequency further confirm that a more protective oxide layer is generated when the potential shifts to a positive trend. A simple Randle's model in Fig. 4 (c) composed of electrolyte resistance Rs, double layer capacitance Qdl and passive film resistance Rf, is used to simulate the EIS data since that only a time constant can be observed from EIS plots. The fitting results are summarized in Table 1. Evidently, Rf raises from 1846 to 3545 Ucm2 as the potential shifts from 0.2 to 0.8 VSCE, indicating that the electrochemical corrosion resistance is gradually enhanced. Meanwhile, reduction of Qdl proves that a thicker passive film is formed at a higher potential [18,19]. 3.4. Capacitance analysis of passive film It has been assumed that passive film resistance to corrosive medium is relevant with its semiconducting distribution and electronic structure. Consequently, Mott- Schottky measurement was conducted and the space charge capacitance CSC corresponding to n-type semiconductors is associated with potential E [20]:
M. Wang et al. / Journal of Alloys and Compounds 811 (2019) 151962
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Fig. 2. (a) Microstructure and (b) XRD patterns of FeCrMoCBY amorphous powder and plasma sprayed FeCrMoCBY amorphous coating.
indicates that less chloride ions could occupy the positions of vacancies in the film. Some metal ions such as Cr from coating will react with chloride ions and produce metal chlorides (MClx), leading to the volume expansion and further mechanical failure of passive film [23e25]. As a consequence, a prompt penetration of pitting nucleation into the coating surface will be easily triggered. It can be concluded that increasing potential reduces the vacancy sites for chloride ions, and this is beneficial for the film to resist local thinning. A decreased donor density is also in good agreement with the enhanced film resistance from EIS data in Table 1. To further figure out the growth feature of passive film, the film thickness should be considered. The growth trend of passive film, is roughly calculated from the following equation [26]:
62εε0 6 d¼6 6 4
31 2 . E Ufb kT e7 7 7 7 eNd 5 =
2
(2)
Fig. 3. Potentiodynamic polarization curve of FeCrMoCBY amorphous coating in 3.5 wt. % NaCl solution.
C2SC ¼
2 kT E Ufb εε0 qNd q
(1)
where ε is the dielectric constant of passive film (15.6 for FeeCr alloy), ε0 represents the vacuum permittivity (8.854 1012 Fm1), q denotes the electron charge (1.602 1019 C), Nd means the donor density, Ufb signifies the flat band potential, k stands for the Boltzmann constant (1.38 1023 J K1) and T is the absolute temperature (298 K in this work). Thus, the donor density Nd can be easily attained from the positive slope within the linear region between CSC 2 and E. Fig. 5 shows Mott-Schottky curves of passive films as a function of potential. In the range of 0e0.5 VSCE, n-type semiconducting property can be ascertained due to the positive and linear relationship between C2SC and E. Correspondingly, the donor densities Nd at 0.2, 0.4, 0.6 and 0.8 VSCE are 2.19 1022,8.89 1021,6.56 1021, 6.13 1021 cm3, respectively. Obviously, an increased potential drops the donor density in the passive film significantly. A lower donor density means less defective point defects in the film according to the point defect model (PDM) [21,22], and it
In this study, the flat band potential Ufb at 0.2, 0.4. 0.6 and 0.8 VSCE are 0.96, 0.25, 0.32, 0.18 VSCE, respectively. Accordingly, the film thicknesses are 0.76, 0.88, 1.23 and 1.32 nm, corresponding to 0.2, 0.4, 0.6 and 0.8 VSCE, separately. It can be inferred that a thicker film is generated at a higher potential, offering better resistance to aggressive ions penetrating into the layer. Another critical parameter to describe the growth and breakdown kinetic of passive film is the diffusion coefficient D of point defects [27]. The relationship between donor density Nd and passivation potential E is expressed as [28]:
Nd ¼ u1 expðbEÞ þ u2
(3)
where u1 , u2 and b are constants acquired from the plot of Nd versus E in Fig. 6 (a). Nd decreases as an exponential function of E, which coincides with the relationship revealed in Eq. (3). In this case, u2 is 6.044 1021 cm3 as a significant factor for evaluating the diffusion coefficient D of point defects according to the following formula [28]:
D¼
Ip RT 4qFεL u2
(4)
where Ip is the steady passive current density (1.25 103 A cm2),
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Fig. 4. (a) Nyquist diagrams, (b) Bode phase angles and (c) equivalent electrical circuit of passive films of FeCrMoCBY amorphous coating at different formation potentials after 1 h polarization in 3.5 wt. % NaCl solution.
Table 1 EIS parameters obtained from the fitting results for FeCrMoCBY amorphous coating passivated in 3.5 wt. % NaCl solution. a1
Potential (VSCE)
Rs (U cm )
Qdl (F s
0.2 0.4 0.6 0.8
9.41 9.20 11.08 11.64
0.000921 0.000534 0.0000878 0.0000838
2
cm
2
)
n
Rf (U cm )
chi
0.607 0.742 0.847 0.861
1846 2180 2554 3545
9.37e-04 3.40e-04 7.12e-04 5.35e-04
2
d¼
1 ð1 aÞE þ b εL
(5)
2
where a ¼ 0.5 represents the polarization level of the film/electrolyte interface and b is a constant. Fig. 6 (b) shows the plot of d versus E and the extracted results indicate that εL is approximately 4.95 106 V cm1. Based on the above analysis, the corresponding diffusivity D of point defects in the passive film regarding to FeCrMoCBY amorphous coating in 3.5 wt. % NaCl solution is about 1.67 1015 cm2 s1, which is comparable to the results reported on bulk alloys [28,29]. This value is relatively low and it indicates that the transportation rate of vacancies involving the growth and breakdown processes are not fast sufficiently. Namely, a moderate balance between the growth near the film/coating interface and dissolution close to film/electrolyte interface can be created [30], which restrains the local thinning of passive film to some extent.
3.5. Topographies of passive film
Fig. 5. Mott-Schottky plots of passive films formed on FeCrMoCBY amorphous coating at different formation potentials in 3.5 wt. % NaCl solution.
R is the universal gas constant (8.314 J mol1 K1), T is the absolute temperature (298 K), F is the Faraday constant (96500 C mol1) and εL is the average electric field in the film. The value of εL is accomplished through the linear relationship between film thickness d and applied potential E [28]:
AFM topographic profiles of the coating surface at varied potentials for 3600 s are displayed in Fig. 7. No obvious oxides but distinct grooves paralleling to the surface appear on the coating surface at 0.2 VSCE. Only a tiny but discontinuous film can be observed. After passivation at 0.4 VSCE, grooves still exist but more integral film is generated. In contrast, passive film at 0.6 VSCE covers almost the coating surface with several amount of pores. Furthermore, the film evolves into a denser layer without evident flaws at 0.8 VSCE. It is also clear that substantial rod-like trips are shown in Fig. 7 (c) and (d). But the film seems to be refined in smaller size at 0.8 VSCE, which profits the corrosion resistance of the coating. Thus, the increased potential contributes to the formation of a more compact and resistive film in chloride containing solution.
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Fig. 6. (a) Donor densities, (b) film thicknesses of passive films formed on FeCrMoCBY amorphous coating as a function of formation potential in 3.5 wt. % NaCl solution.
Fig. 7. AFM topography of passive films of FeCrMoCBY amorphous coating polarized at (a) 0.2 VSCE, (b) 0.4 VSCE (c) 0.6 VSCE and (d) 0.8 VSCE, respectively in 3.5 wt. % NaCl solution.
3.6. XPS analysis of passive film Fig. 8 exhibits the high resolution results of Fe 2p3/2, Cr 2p3/2, Mo 3d and O 1s of passive films at diverse potentials. Fe 2p3/2 peaks are
separated into Fe0 (706.5 eV), Fe3O4 (708.3 eV), FeO (709.5 eV), Fe2O3 (710.6 eV) and FeOOH (711.5 eV), indicating of five component states of Fe element. Four species representing metallic state Cr0 (574.2 eV), Cr2O3 (575.6 eV), Cr(OH)3 (577 eV) and CrO3 (578 eV)
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Fig. 8. XPS spectra for (a) Fe 2p3/2 (b) Cr 2p3/2 (c) Mo 3d and (d) O 1s of passive films on FeCrMoCBY amorphous coating at various potentials in 3.5 wt. % NaCl solution, respectively.
M. Wang et al. / Journal of Alloys and Compounds 811 (2019) 151962
Fig. 8. (continued).
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comprise of Cr 2p3/2 spectra. Meanwhile, peaks at 227.8 eV 3d5/2 and 230.8 eV 3d3/2 correspond to metallic state of Mo element, while MoO2 and MoO3 are featured by 229.8 eV 3d5/2 and 232.5 eV, respectively. In addition, O 1s ionization is composed of O2(530.4 eV), OH (531.9 eV) and bounded water (532.5 eV). The elemental distribution and cationic concentrations of oxidation species are presented in Fig. 9. Fe content is the maximum among three detected metal elements due to the fact that Fe is the primary alloying element of amorphous coating. It is observed that the concentration of Mo is always under 10 at. %, but its contribution to film stability cannot be ignorable. A suitable addition of Mo element could enhance the pitting corrosion resistance of passive film since it can promote the enrichment of Cr [31]. Especially, the generation of low valence oxide species (such as MoO2) contributes to the stability of passivation ability formed on FeeCr based amorphous materials [32]. A protective insoluble Mocontaining film can behave like an effective barrier against the diffusion of oxide species, and thus decrease the dissolution rate of passive film. In contrast, Cr proportion exceeds 20 at. % at all potentials and it rises gradually as potential increases from 0.2 to 0.8 VSCE. Previous researches have concluded that the critical concentration of Cr to form a protective Cr oxide film is nearly 15 at. %, and the corresponding film behavior will resemble to pure Cr [33]. It is supposed that Cr-containing species in this study are believed to be crucial for a resistive oxide layer to hamper chloride ions. Specially, the accumulation of Cr2O3 indicated from Fig. 9 (c) at a higher
potential accounts for the enhancement of corrosion resistance of the coating. Insoluble Cr2O3 is stable enough and less defective than other metallic oxides, which can elucidate the stronger passivity at a higher potential. It is assumed that a tenser electric strength at a higher potential as shown in Fig. 6 (b) can promote the diffusion of Cr element from inner layer to outer layer, further shaping a denser passive film. Fig. 9 (d) also reveals that hydroxides reduce progressively, whereas the total fraction of oxides and bounded water increases steadily with the increment of potential. It was reported that dissolved metal ions can be captured by bounded water [34], and thus more fresh oxide films are facilitated and generated. At the same time, hydroxides exhibiting hydrophilic surfaces [35] are deemed to deteriorate the corrosion resistance of the coating. Hydroxyl vacancies (V·OH) are generated at the coating/film interface and diffuse to and experience an annihilation at the film/electrolyte interface, which are expressed in Eqs. (6a) and (6b) [36]:
m 4 MM þ V ,OH þ xe
(6a)
V ,OH þ H2 O4MM þ OHOH þ Hþ
(6b)
where m is the amorphous coating, MM is a metal cation, V ,OH is the hydroxyl vacancies. Accordingly, the formation and annihilation reactions of hydroxyl vacancies (V·OH) underline the significance of hydration on the coating surface. Incorporation of hydroxyl ions
Fig. 9. Fractions of (a) total Fe, Cr, Mo species (b) iron oxides (c) chromium oxides (d) oxides/hydroxides, bounded water, (O2-þ H2O)/OH ratio detected from XPS results from the film surface of FeCrMoCBY amorphous coating at various potentials in 3.5 wt. % NaCl solution.
M. Wang et al. / Journal of Alloys and Compounds 811 (2019) 151962
results in the hydration-dehydration, which favors the generation of hydrated sites. Further, production of hydrogen gas at the coating/film interface will accelerate the breakdown of passive film. R.C. Sierra [36] et al. put forward that more hydroxyl vacancies are produced accompanied with the generation of more hydroxides. As a consequence, point defects in the film are induced simultaneously, resulting the increment of donor density. It means that the film resistance to local thinning is raised via shifting the potential toward a more positive tendency. 4. Conclusions (1) The electrochemical corrosion behavior of FeCrMoCBY amorphous coating was sensitive to the passivation potential, and a higher potential lead to a better corrosion resistance. (2) Formation of more insoluble Cr2O3 in the oxide layer stabilized and promoted the compactness of passive film on FeCrMoCBY amorphous coating surface. (3) Local thinning of passive film on FeCrMoCBY amorphous coating was closely related to the point defects density, which was reduced by the reduction of hydrophilic hydroxides. In addition, a relative low diffusivity of point defects in the film balanced the growth and breakdown processes, and hampered the pitting initiation. Acknowledgements This work was financially supported by the National Natural Science Foundation of China (Grant No. 51379070& No. 51909071), the Fundamental Funds for the Central Universities (Grant No. 2018B689X14), the Postgraduate Research & Practice Innovation Program of Jiangsu Province (Grant No. KYCX18_0569), China Scholarship Council (2019). We also would like to thank Liang Wang from Shiyanjia Lab (www.shiyanjia.com) for support of XPS test. References [1] P.F. Gostin, S. Oswald, L. Schultz, A. Gebert, Acid corrosion process of Fe-based bulk metallic glass, Corros. Sci. 62 (2012) 112e121. [2] S.F. Guo, K.C. Chan, S.H. Xie, P. Yu, Y.J. Huang, H.J. Zhang, Novel centimetersized Fe-based bulk metallic glass with high corrosion resistance in simulated acid rain and seawater, J. Non-Cryst. Solids 369 (2013) 29e33. [3] D. Zenebe, S. Yi, S.S. Kim, Sliding friction and wear behavior of Fe-based bulk metallic glass in 3.5% NaCl solution, J. Mater. Sci. 47 (2012) 1446e1451. [4] J. Henao, A. Concustell, I.G. Cano, N. Cinca, S. Dosta, J.M. Guilemany, Influence of cold gas spray process conditions on the microstructure of Fe based amorphous coatings, J. Alloy. Comp. 622 (2015) 995e999. [5] Z. Zhou, L. Wang, F.C. Wang, H.F. Zhang, Y.B. Liu, S.H. Xu, Formation and corrosion behavior of Fe-based amorphous metallic coatings by HVOF thermal spraying, Surf. Coat. Technol. 204 (2009) 563e570. [6] Y. Wang, S.L. Jiang, Y.G. Zheng, W. Ke, W.H. Sun, J.Q. Wang, Electrochemical behaviour of Fe-based metallic glasses in acidic and neutral solutions, Corros. Sci. 63 (2012) 159e173. [7] A.R. Newmark, U. Stimming, Photoelectrochemical studies of passive films on zirconium and amorphous iron-zirconium alloys, Langmuir 3 (1987) 905e910. [8] S.D. Zhang, J. Wu, W.B. Qi, J.Q. Wang, Effect of porosity defects on the longterm corrosion behaviour of Fe-based amorphous alloy coated mild steel, Corros. Sci. 110 (2016) 57e70. [9] J. Wu, S.D. Zhang, W.H. Sun, Y. Gao, J.Q. Wang, Enhanced corrosion resistance in Fe-based amorphous coatings through eliminating Cr-depleted zones, Corros. Sci. 136 (2018) 161e173. [10] R.Q. Guo, C. Zhang, Q. Chen, Y. Yang, N. Li, L. Liu, Study of structure and corrosion resistance of Fe-based amorphous coatings prepared by HVAF and HVOF, Corros. Sci. 53 (2011) 2351e2356.
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