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Effects of WC on microstructure and corrosion resistance of directional structure Ni60 coatings
Journal Pre-proof Effects of WC on microstructure and corrosion resistance of directional structure Ni60 coatings
Xiaotian Yang, Xiuqian Li, Qiangbin Yang, Hengli Wei, Xiaoyue Fu, Wensheng Li PII:
S0257-8972(20)30028-1
DOI:
https://doi.org/10.1016/j.surfcoat.2020.125359
Reference:
SCT 125359
To appear in:
Surface & Coatings Technology
Received date:
17 October 2019
Revised date:
8 January 2020
Accepted date:
10 January 2020
Please cite this article as: X. Yang, X. Li, Q. Yang, et al., Effects of WC on microstructure and corrosion resistance of directional structure Ni60 coatings, Surface & Coatings Technology (2020), https://doi.org/10.1016/j.surfcoat.2020.125359
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© 2020 Published by Elsevier.
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Effects of WC on microstructure and corrosion resistance of directional structure Ni60 coatings Xiaotian Yanga,b,*, Xiuqian Lia, Qiangbin Yangc, Hengli Weia, Xiaoyue Fua, & Wensheng Lia,b a. School of Materials Science and Engineering, State Key Laboratory of Advanced Processing and Recycling of Nonferrous Metals, Lanzhou University of Technology, Lanzhou, 730050, China;
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b. Wenzhou Engineering Institute of Pump & Valve, Lanzhou University of Technology, Wenzhou,
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325105, China;
c. Chongqing Key Laboratory of Environmental Materials & Remediation Technologies, Chongqing
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Corresponding author: Xiaotian Yang
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Email: [email protected] Tel: (86)-13919318192
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University of Arts and Sciences, Chongqing, 402160, China;
Journal Pre-proof ABSTRACT To enhance the corrosion resistance of directional structure coating of Ni60 alloy, the directional structure composite coating of Ni60 alloy reinforced with WC was prepared on the S45C steel by the induction remelting and forced cooling method. The immersion corrosion behaviors of coatings in 10% H2SO4 solution for 168 h and its electrochemical corrosions were investigated to analyze its corrosion resistance. The
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results showed that WC changed the grain boundary density and grain boundary type,
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having important effect on corrosion resistance and corrosion mechanism of directional
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structure Ni60 coating. The corrosion potentials of directional structure Ni60 coating
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and Ni60/WC coating is -0.395 and -0.112 V, respectively. The corrosion resistance of
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directional structure Ni60/WC coating is better than that of the coating without WC.
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The corrosion mechanism of directional structure Ni60/WC coating is slight intracrystalline corrosion, while the coating without WC is the pitting corrosion and the
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microgalvance corrosion.
Keywords: induction remelting, directional structure, corrosion, composite coating
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1. Induction
Universally, the surface of industrial components working under aggressive conditions are susceptible to failure owning to corrosion, wear and fatigue. In order to overcome the above problems, researchers pay much attention to surface technology during recent decades [1-2]. Among the various kinds of coating materials, the
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nickel-based alloys exhibit outstanding corrosion resistance and wear resistance
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properties. Hence, the Ni-based coatings have been widely used due to corresponding
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outstanding corrosion resistance in various conditions [3-5].
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The corrosion behaviors of various Ni-based alloy coatings in corrosive solution
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were investigated in order to evaluate their corrosion resistance. It was demonstrated that Ni-based coatings deposited by the spray and fuse technique had a more positive
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potential than substrate in a 3.5% NaCl solution [6]. Moreover, the effect of vacuum
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re-melting on the corrosion resistance of plasma sprayed Ni60-NiCrMoY alloy coatings was studied, which indicated that re-melting coatings exhibited better corrosion resistance compared with as-sprayed coatings [7]. It could be concluded in the literature [8] that the corrosion resistance of the Ni/Cr3C2-NiCr composite coating achieved by electrochemical deposition method was more comparable to the pure nickel coating in the 3.5% NaCl solution. Bolelli et al. [9,10] have shown that the as-sprayed and heat-treated High Velocity Oxy-fuel (HVOF) deposited Ni-Mo-Cr-Si coatings exhibited high current density ~10-4 A/cm2 in the passive-like stage in 0.1 M HCl solution. We proposed to prepare the directional structure Ni60 coating, which had outstanding wear resistance [11]. However, its corrosion property is not good. In this
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paper, WC was added to Ni60 self-fluxing alloy powder to prepare directional structure Ni60/WC composite coating, and the effects of WC on their microstructures, immersion corrosion and electrochemical corrosion behaviors of directional structure Ni60 coatings in 10% H2SO4 solution were investigated to analyze the mechanism of corrosion resistance.
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2. Experimental section
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2.1 Prefabrication of directional structure Ni60/WC composite coating
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The substrate material used in this work was the S45C steel. The feedstock
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powders were commercial Ni60 self-fluxing alloy powder and WC powder mixed with a weight ratio of 4:1. The nominal chemical composition of the Ni60 in mass fraction
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was C 0.5~1.1%, B 2.5~3.5%, Si 3.5~5.5%, Cr 15~20%, Fe 0~5% and Ni in balance.
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The size ranges of Ni60 powder and WC powder are -150 ~ +300 mesh and -140 ~ +325 mesh, respectively. According to XRD analysis, the detected phases of Ni60 powder are Ni, Ni3Fe, Cr23C6, Cr2B, Ni3B and Ni2Si and WC particles exhibit WC/W2C phases (Fig. 1). The as-sprayed coatings with the thickness of approximately 0.7 mm were prefabricated by the flame spraying, and then the coatings were remelted and treated by forced cooling to form directional structure. The cooling water flow rate was 1.886 mL/(min·mm2), which means the tap water of 80 L/h to the sample from the direction of the matrix.
2.2 Characterization methods
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By using Scanning Electronic Microscopy (SEM, QuantaFEG450) and Energy Dispersive X-ray EDS technique, the surface morphology, cross-sectional morphology, and chemical composition were analyzed. The EDS was running in three different modes: mapping mode, line mode and pointing mode. Phase compositions of the raw powder, as-sprayed coating, and the directional
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structure coating surface before and after immersion corrosion for 168 h were studied using X-ray diffraction (XRD, D/MAX2500 type PC) in the 10° ≤ 2θ ≤ 100° range.
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The roughness of surface morphologies of as-sprayed coatings and directional
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structure coatings were observed by a laser scanning confocal microscopy (LSCM,
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2.3 Electrochemical tests
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LSM 800).
The electrochemical performance test was conducted on a CHI660E type
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electrochemical workstation with the conventional three-electrode system, working temperature of 25 ± 1 ℃, and the platinum plate and mercurous sulfate were as the counter electrode and reference electrode, respectively. The coating surface with the area of 1 cm2 was exposed in 10% H2SO4 solution as the working electrode, and the rest were coated. First, the specimen was immersed in electrolytes for 1 h to obtain the open circuit potential (Eocp). Then, the electrochemical impedance spectrum was measured at Eocp in the frequency range of 105 to 0.005 Hz. Finally, the potentiodynamic polarization curve was recorded with the scan rate of 0.001 mV/s. All measurements
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were taken three times to obtain accurate results and ensure good repeatability and reproducibility.
2.4 Immersion corrosion tests
The coatings were immersed in 10% H2SO4 solution for 168 h at the room
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temperature. After the immersion corrosion test, the coatings were washed with the
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distilled water and naturally dried. The corrosive morphologies were observed by SEM.
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3. Results and discussion
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3.1 Microstructures of as-sprayed coatings
Fig. 2 presents the cross-section and surface morphologies of as-sprayed Ni60
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coating and Ni60/WC composite coating. It can be observed from Fig. 2(a and b)
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that the sprayed Ni60 coating and Ni60/WC coating are typical lamellar structure with the mechanical bonding between the coating and the substrate. Internal porosity and defects are all found in both coatings.
3.2 In situ characterization by LSCM
Fig. 3 shows the 3D model of coating after spraying and induction remelting and forced cooling. The sink of the pits on the actual surface corresponds to the bulges in the 3D images by a reversal treatment of the surface topography. The 3D images reveal that the depth and number of the pits decrease from Fig. 2(a) to Fig. 2(d). When the as-sprayed coating is remelted and treated by forced cooling, there
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were only few shallow pits on the surface. The number of the pits of as-sprayed Ni60 coating is more than that of as-sprayed Ni60/WC coating. The average roughnesses of as-sprayed Ni60 coating, Ni60/WC coating, directional structure Ni60 coating and directional structure Ni60/WC coating are Ra=8.51 μm, Ra=5.2 μm, Ra=0.837 μm, Ra=0.279 μm, respectively.
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3.3 Influence of WC on microstructure of directional structure coating
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The cross-section and surface morphologies of directional structure Ni60 coating
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and Ni60/WC composite coating are shown in Fig. 4. It can be seen that the
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microstructures of two kinds of coatings are all mainly planar crystal, cellular crystals,
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columnar crystals and dendritic crystals with directional structural feature. The
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cross–section is impact, without obvious cracks. Although the directional structure Ni60 coatings display much finer microstructure than Ni60/WC coatings, but it mainly
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presents dendritic morphology. However, the Ni60/WC composite coating mainly presents cellular and columnar crystals morphology, it has fewer penetrating grain boundaries than Ni60 coating. In addition, the point scan analysis results for white regions at the grain boundary (point A in Fig. 4(g)) are as follows: W 34.9%, C 6.2%, B 14.1%, Cr 8.4%, Fe 25.4% and Ni 8.9%. It is demonstrated that the white regions of grain boundaries are full of W and C and the added WC gathered at the grain boundary, which will block the corrosion channel and improve the coating corrosion resistance. The linear scanning results showed the contents of W and Cr at grain boundaries increase (as marked by rectangle in Fig. 5). In other words, the elements of W and Cr
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gather at grain boundaries, which further proved that the hard reinforcement phase blocked the corrosion channel. From the surface morphologies of directional structure Ni60 coating and Ni60/WC composite coating presented in Fig. 4(c and f), it can be observed that the grain boundary of Ni60 coating is island and fibrous eutectic tissue, but that of Ni60/WC
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composite coating is regular lamellar eutectic structure with WC participation (as shown in Fig. 4(f) by arrows). This again shows that WC change the grain boundary density
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and grain boundary type, which may improve corrosion resistance of Ni60 coating.
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The results of plane scan analysis of surface elements of directional structure Ni60
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coating and Ni60/WC coating are shown in Fig. 6 and Fig. 7, respectively. It can be
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clearly observed that the element Ni is mainly distributed on the grains in two coatings.
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However, additive WC makes Ni more dispersed in Ni60/WC coating compared with that of directional structure Ni60 coating. The point scan analysis mentioned in Table 1
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shows a variation in the composition. The Ni element–poor zones are the Cr element–rich zones. The element Cr is mainly distributed at the grain boundaries in the two coatings (as marked by rectangle in Fig. 6(a and c) and Fig. 7(a and c)), while after the addition of WC, the distribution of the element Cr is more dispersed, which is the same as the change of Ni. Moreover, the element Si is uniformly distributed on the Ni60 coating surface, but the element Si is enriched on the small range and mainly located in the grain boundaries in Ni60/WC coating (as marked by rectangle in Fig. 7(a and f)). As a whole, the C, Fe are evenly distributed on the surface of two coatings, but the element Fe is poor in the W enrichment region in grain boundaries (as marked by rectangle in
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Fig. 7(a and i). These show that the addition of tungsten carbide mainly affected the distribution of Ni, Cr, Si and Fe, and WC is distributed at the grain boundaries on the directional structure Ni60/WC coatings, which is in line with the result of line scan analysis (Fig. 5).
3.4 X-ray analysis
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The XRD patterns in the 2θ range from 10° to 100° for as-sprayed coatings and
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directional structure coatings are shown in Fig. 8. The phases of as-sprayed Ni60
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coating are almost the same as that of Ni60 alloy powder(Fig. 1(a)). The phases are
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mainly Ni, Ni3Fe, Cr23C6, CrB, Ni2Si, Ni3B. And WC and W2C phases are detected in
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the as-sprayed Ni60/WC coating. It can be seen that the phase γ-(Fe, Ni), CrB, Cr23C6
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and Ni2Si can be found as the main phase in two kinds of directional structure coatings. However, additive WC makes the content of the phase γ-Ni decrease, and the phases
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Ni2.9Cr0.7Fe0.36, Ni2B and FeB are converted to Cr0.19Fe0.7Ni0.11, NiB and Fe2B phases, respectively. In addition, the WC and W2C phases are detected in the directional structure Ni60/WC coating. It can be concluded that the addition of WC promotes the transformation of some phases and has effect on the phase constituent of the composite coatings. For instance, the Ni is at the main peak in the directional structure Ni60 coating, while the main peak has no Ni when adding WC.
Combined with elemental
distribution discussed above (Fig. 4 and Fig. 5), WC promotes element redistribution, phase changes and distribution, which can effectively improve the corrosion resistance of the directional structure Ni60 coating.
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3.5 Electrochemical measurements
Fig. 9 presents the results of electrochemical experiments. The electrochemical parameters of coatings and substrate such as corrosion potential (Ecorr) and corrosion current density (Icorr) were obtained from the polarization curves (Fig. 9a). The corrosion potentials of S45C steel, directional structure Ni60 coating, and directional
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structure Ni60/WC composite coating were -0.412V, -0.395V, and -0.112 V, respectively.
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The corrosion potential of directional structure Ni60/WC composite coating shifted
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positively, indicating that it had the better corrosion resistance. The current densities of
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directional structure Ni60 coating and directional structure Ni60/WC composite coating
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were 2.858×10-5, 3.002×10-6 A/cm2, respectively, which were lower than 1.748×10-3 A/cm2 of S45C steel. Generally, the higher Ecorr meant better chemical stability, and
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the lower Icorr meant lower corrosion rate [12]. The value of current density of the
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S45C steel was the largest, showing the coatings can well protect the substrate, and the directional structure Ni60/WC composite coating has the higher Ecorr and the lower Icorr than directional structure Ni60 coating, indicating its better corrosion resistance. In addition, the passivation phenomenon could be observed in both kinds of coatings. It can be observed from Fig. 9b that Nyquist plots are semicircle, the radius of directional structure Ni60/WC composite coating is greater than that of Ni60 coating. As well known, the larger the radius of the capacitive impedance loop is, the better the corrosion resistance is [13]. Thus, WC can largely improve the corrosion resistance of directional structure Ni60 coatings. Due to displaying the highest phase angle (Fig. 9c), the directional structure Ni60/WC composite coating own the best anti-corrosion
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properties. In addition, the values of |Z|0.01
Hz
of directional structure Ni60/WC
composite coatings is largest (Fig. 9d), showing that the directional structure Ni60/WC coatings own excellent corrosion resistance [14]. W-rich phase in the coating formed by W diffusion will increase the corrosion resistance of coating, and WC will be oxidized in the solution on the coating surface, which is also beneficial to block the corrosion
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solution penetration. The formation of tungsten oxide in sulfuric acid is reported by S. Sutthiruangwong et al. [15], and the formation formula is as follows [16]: WC + 5H2O
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→WO3 + CO2 + 10 H+ + 10e-.
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3.6 The surface morphologies and phase compositions after immersion corrosion
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Two kinds of coating surfaces were corroded through immersion in a 10% H2SO4
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solution. Fig. 10(a and c) and Fig. 10(b and d) show the corrosion morphologies of the directional structure Ni60 coating and Ni60/WC composite coating, respectively. After
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immersion for 168 h, several grain boundary cracks and certain pits appeared on the directional structure Ni60 coating surface, the holes or pits provided the path for the migration of H+ and SO42- and caused a further attack to the uncorroded region [17]. The corrosion mechanism of Ni60 coating is mainly the microgalvance corrosion and the pitting corrosion. However, there is no obvious pitting corrosion phenomena in the directional structure Ni60/WC coating. The areas enriched WC in the directional structure Ni60/WC coating have no corrosion, while the intracrystalline locations without WC show slight corrosion. From higher magnification images (Fig. 10(c and d)), it can be clearly observed that the corrosion phenomena are the same as the above
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statement. Usually, the corrosion is easy to take place at the grain boundary location [18], combined with Fig. 4, It can be seen from Fig. 10 that the corrosion of Ni60 coating without WC mainly occurs at the grain boundary, while the corrosion of Ni60/WC composite coating with WC is slight, no obvious corrosion occurs at the grain boundary, indicating that the addition of WC can block the grain boundary and improve
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the corrosion resistance of the coating, which is beneficial to improve the immersion corrosion resistance.
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The XRD patterns of directional structure coatings after the immersion corrosion
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test for 168 h are shown in Fig. 11. The detected phases of directional structure Ni60
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coatings are composed of Ni, γ-(Fe,Ni), Cr23C6, CrB, Ni2Si, Ni2B and FeB. Compared
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with those in Fig. 8(c), Ni2.9Cr0.7Fe0.36 is not detected and there was no new phases
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formed, indicating that their phase compositions were nearly stable. Moreover, the phases of directional structure Ni60/WC coatings have no obvious change after the
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immersion corrosion, too. The XRD results showed that the effect of phase compositions on the corrosion resistance was small.
4. Conclusions
(1) The WC addition has an obvious effect on the microstructure of directional structure Ni60 coating. WC is distributed at the grain boundaries of directional structure Ni60/WC coating. WC changes the element distribution, partial phase constituent and the peak position of some phases. (2) The electrochemical tests show that the directional structure Ni60/WC coating
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has lower corrosion current density, higher corrosion potential, and larger radius of the capacitive impedance loop, phase angle and the values of |Z|0.01
Hz
than directional
structure Ni60 coating, indicating that directional structure Ni60/WC composite coating has better corrosion resistance. (3) In immersion corrosion tests, directional structure Ni60 coating displays the
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pitting corrosion and the microgalvance corrosion, while Ni60/WC composite coating presents the slight corrosion on the intracrystalline locations without WC and no
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obvious pitting corrosion phenomenon. WC blocks the corrosion channel at the grain
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boundaries and changes the grain boundary density and grain boundary type, which is
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good for improving the corrosion resistance.
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Acknowledge
This work was supported by National Natural Science Foundation of China (No.
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51365024); and Zhejiang Provincial Natural Science Foundation of China (No. LGG19E010003).
Disclosure statement
No potential conflict of interest was reported by the authors.
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Table 1 Chemical composition of the grains in Figs. 6(a) and 7(a) (wt%)
Ni
Cr
Fe
Si
B
C
W
A
50.3
6.1
30.3
2.6
0.0
10.1
-
B
31.1
4.2
46.0
1.2
6.6
3.5
4.9
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Point no.
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Fig. 1 XRD analysis of Ni60 and WC powders: (a) Ni60 powder; (b) WC powder Fig. 2 Back-scattered SEM images of as-sprayed coating cross-section and surface: cross-section;
(b) Ni60/WC composite coating cross-section;
(a) Ni60 coating
(c) Ni60 coating surface;
(d) Ni60/WC composite coating surface Fig. 3 3D model by LSCM at the same position of samples: (a) as-sprayed Ni60 coating, (b) as-sprayed
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Ni60/WC coating, (c) directional structure Ni60 coating, (d) directional structure Ni60/WC coating
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Fig. 4 Back-scattered SEM images of directional structure coating cross-section and surface: (a) the bottom of cross-section, (b) the top of cross-section and (c) surface of Ni60 coatings; (d) the bottom of
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cross-section, (e) the top of cross-section and (f) surface of Ni60/WC composite coating; (g) Partial
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enlarged detail of Fig. 4 d
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Fig. 5 Line scan analysis of directional structure Ni60/WC composite coating cross-section
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Fig. 6 The result of plane scan analysis of directional structure Ni60 coating surface Fig. 7 The result of plane scan analysis of directional structure Ni60/WC coating surface
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Fig. 8 XRD patterns of as-sprayed and directional structure coatings: (a) as-sprayed Ni60 coating, (b) as-sprayed Ni60/WC coating, (c) directional structure Ni60 coating, (d) directional structure Ni60/WC coating
Fig. 9 Electrochemical results of substrate and two kinds of coatings:
(a) Tafel polarization curves,
(b) Nyquist plots, (c) phase angle, and (d) bode plots Fig. 10 SEM images of directional structure coating surface after corrosion: surface;
(a and c) Ni60 coating
(b and d) Ni60/WC composite coating surface
Fig.11 XRD analysis results of directional structure coatings after immersion corrosion test: (a) Ni60 coating; (b) Ni60/WC composite coating
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(b)
(a) 1 2 3 4
1 W2 C
2 Ni3Fe
2 WC
3 Cr23C6
Intensity(a.u.)
Intensity(a.u.)
4 Cr2B 5 Ni2Si 6 Ni3B
1 2 3
5 6 3 6 3
1 2 3
3
30
40
50
2
1 1 1
60
70
80
90
2
2
100
10
20
30
40
50
60
2/degree
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Figure 1
1
2 2 1 1
2
2/degree
1
1
1
1
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20
2
2
1 4
4
10
1
Ni
70
80
2 1
90
100
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of
Figure 2
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na
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re
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Figure 3
+
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Figure 4
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Figure 5
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(b) Ni
(c) Cr
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(a) Plane scanned position
(e) B
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(d) C
(g) Fe
(h) O
Figure 6
(f) Si
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(b) Ni
(c) Cr
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(a) Plane scanned position
(e) B
(f) Si
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(d) C
(g) Fe
(h) O
Figure 7
(i) W
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(a)
1 2 3
(b)
Ni
4 CrB 5 Ni2Si
4
10
20
30
6 Ni3B
4 5 6 6 4
3 4 6 6 6
40
1 2 3
50
3 Cr23C6 5 Ni2Si 6 Ni3B
3 6 8 4
1
4
2 Ni3Fe 4 Cr2B
Intensity(a.u.)
Intensity(a.u.)
3 Cr23C6
1 2 3
Ni
1 2 3 4
2 Ni3Fe
5 6
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7
1
60
7 WC 8 W2 C
1 2 8
4
100
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2/degree
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(d) Ni 2 -(Fe, Ni)
3 Ni2.9Cr0.7Fe0.36
7
4 Ni2B 5 FeB 6 CrB 7 Cr23C6 8 Ni2Si
40
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9 9
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Figure 8
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2 4 356 5 68 7 47
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1
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Ni 2 -(Fe, Ni)
2 3 7
Intensity(a.u.)
Intensity(a.u.)
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80
2/degree
(c) 1
70
1
30
1 4 5 2 6 10 378
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4 NiB 5 Fe2B 6 CrB 7 Cr23C6 8 Ni2Si 9 WC 10 W2C
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(a)
Directional structure Ni60 coatings Directional structure Ni60/WC coatings The S45C steel
0.4
Potential(V)
0.2 0.0 -0.2 -0.4 -0.6 -0.8
-7
-6
-5
-4
-3
-2
-1
log(current density(A/cm2))
(d )4.0
Directional structure Ni60 coatings Directional structure Ni60/WC coatings The S45C steel
3.5 3.0
log(Z(ohm))
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2.5 2.0
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-20
1.5 1.0
0
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0.5 0.0
20 -4
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log(Frequency(HZ))
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Figure 9
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Phase()
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Directional structure Ni60 coatings Directional structure Ni60/WC coatings The S45C steel
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(c) -80
-1
0
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log(Frequency(HZ))
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Figure 10
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(b)
(a) Ni 2 -(Fe, Ni)
3 Cr23C6
4 Cr23C6
Intensity(a.u.)
Intensity(a.u.)
6 Ni2B 7 FeB
3
40
2 3 4 8
5 CrB 6 Ni2Si 7 Ni2B 8 FeB 9 WC 10 W2C
94
2 3 10
2 3 10
5 16 7 9
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2/degree
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2/degree
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Figure 11
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30
2 4 15 76 63
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2 3 4
3 Cr0.19Fe0.7Ni0.11
4 CrB 5 Ni2Si
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Ni 2 -(Fe, Ni)
1 2 3
80
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Credit Author Statement Xiaotian Yang and Xiuqian Li conceived and designed the study. Xiuqian Li and Hengli Wei performed the experiments. Xiaotian Yang and Xiuqian Li wrote the paper. Xiaotian Yang, Xiuqian Li, Qiangbin Yang, Xiaoyue Fu and Wensheng Li reviewed and
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edited the manuscript. All authors read and approved the manuscript.
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Highlights 1. The directional structure coatings of Ni60 and Ni60/WC were prepared, respectively. 2. WC resulted in the microstructural evolution of directional structure Ni60 coating.
channel. WC
addition
improved
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structure Ni60 coating.
corrosion
resistance
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4.
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3. The added WC was distributed at the grain boundary and blocked the corrosion
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directional
Xiaotian Yang, Xiuqian Li, Qiangbin Yang, Hengli Wei, Xiaoyue Fu, Wensheng Li PII:
S0257-8972(20)30028-1
DOI:
https://doi.org/10.1016/j.surfcoat.2020.125359
Reference:
SCT 125359
To appear in:
Surface & Coatings Technology
Received date:
17 October 2019
Revised date:
8 January 2020
Accepted date:
10 January 2020
Please cite this article as: X. Yang, X. Li, Q. Yang, et al., Effects of WC on microstructure and corrosion resistance of directional structure Ni60 coatings, Surface & Coatings Technology (2020), https://doi.org/10.1016/j.surfcoat.2020.125359
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© 2020 Published by Elsevier.
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Effects of WC on microstructure and corrosion resistance of directional structure Ni60 coatings Xiaotian Yanga,b,*, Xiuqian Lia, Qiangbin Yangc, Hengli Weia, Xiaoyue Fua, & Wensheng Lia,b a. School of Materials Science and Engineering, State Key Laboratory of Advanced Processing and Recycling of Nonferrous Metals, Lanzhou University of Technology, Lanzhou, 730050, China;
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b. Wenzhou Engineering Institute of Pump & Valve, Lanzhou University of Technology, Wenzhou,
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325105, China;
c. Chongqing Key Laboratory of Environmental Materials & Remediation Technologies, Chongqing
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Corresponding author: Xiaotian Yang
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Email: [email protected] Tel: (86)-13919318192
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University of Arts and Sciences, Chongqing, 402160, China;
Journal Pre-proof ABSTRACT To enhance the corrosion resistance of directional structure coating of Ni60 alloy, the directional structure composite coating of Ni60 alloy reinforced with WC was prepared on the S45C steel by the induction remelting and forced cooling method. The immersion corrosion behaviors of coatings in 10% H2SO4 solution for 168 h and its electrochemical corrosions were investigated to analyze its corrosion resistance. The
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results showed that WC changed the grain boundary density and grain boundary type,
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having important effect on corrosion resistance and corrosion mechanism of directional
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structure Ni60 coating. The corrosion potentials of directional structure Ni60 coating
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and Ni60/WC coating is -0.395 and -0.112 V, respectively. The corrosion resistance of
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directional structure Ni60/WC coating is better than that of the coating without WC.
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The corrosion mechanism of directional structure Ni60/WC coating is slight intracrystalline corrosion, while the coating without WC is the pitting corrosion and the
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microgalvance corrosion.
Keywords: induction remelting, directional structure, corrosion, composite coating
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1. Induction
Universally, the surface of industrial components working under aggressive conditions are susceptible to failure owning to corrosion, wear and fatigue. In order to overcome the above problems, researchers pay much attention to surface technology during recent decades [1-2]. Among the various kinds of coating materials, the
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nickel-based alloys exhibit outstanding corrosion resistance and wear resistance
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properties. Hence, the Ni-based coatings have been widely used due to corresponding
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outstanding corrosion resistance in various conditions [3-5].
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The corrosion behaviors of various Ni-based alloy coatings in corrosive solution
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were investigated in order to evaluate their corrosion resistance. It was demonstrated that Ni-based coatings deposited by the spray and fuse technique had a more positive
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potential than substrate in a 3.5% NaCl solution [6]. Moreover, the effect of vacuum
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re-melting on the corrosion resistance of plasma sprayed Ni60-NiCrMoY alloy coatings was studied, which indicated that re-melting coatings exhibited better corrosion resistance compared with as-sprayed coatings [7]. It could be concluded in the literature [8] that the corrosion resistance of the Ni/Cr3C2-NiCr composite coating achieved by electrochemical deposition method was more comparable to the pure nickel coating in the 3.5% NaCl solution. Bolelli et al. [9,10] have shown that the as-sprayed and heat-treated High Velocity Oxy-fuel (HVOF) deposited Ni-Mo-Cr-Si coatings exhibited high current density ~10-4 A/cm2 in the passive-like stage in 0.1 M HCl solution. We proposed to prepare the directional structure Ni60 coating, which had outstanding wear resistance [11]. However, its corrosion property is not good. In this
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paper, WC was added to Ni60 self-fluxing alloy powder to prepare directional structure Ni60/WC composite coating, and the effects of WC on their microstructures, immersion corrosion and electrochemical corrosion behaviors of directional structure Ni60 coatings in 10% H2SO4 solution were investigated to analyze the mechanism of corrosion resistance.
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2. Experimental section
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2.1 Prefabrication of directional structure Ni60/WC composite coating
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The substrate material used in this work was the S45C steel. The feedstock
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powders were commercial Ni60 self-fluxing alloy powder and WC powder mixed with a weight ratio of 4:1. The nominal chemical composition of the Ni60 in mass fraction
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was C 0.5~1.1%, B 2.5~3.5%, Si 3.5~5.5%, Cr 15~20%, Fe 0~5% and Ni in balance.
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The size ranges of Ni60 powder and WC powder are -150 ~ +300 mesh and -140 ~ +325 mesh, respectively. According to XRD analysis, the detected phases of Ni60 powder are Ni, Ni3Fe, Cr23C6, Cr2B, Ni3B and Ni2Si and WC particles exhibit WC/W2C phases (Fig. 1). The as-sprayed coatings with the thickness of approximately 0.7 mm were prefabricated by the flame spraying, and then the coatings were remelted and treated by forced cooling to form directional structure. The cooling water flow rate was 1.886 mL/(min·mm2), which means the tap water of 80 L/h to the sample from the direction of the matrix.
2.2 Characterization methods
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By using Scanning Electronic Microscopy (SEM, QuantaFEG450) and Energy Dispersive X-ray EDS technique, the surface morphology, cross-sectional morphology, and chemical composition were analyzed. The EDS was running in three different modes: mapping mode, line mode and pointing mode. Phase compositions of the raw powder, as-sprayed coating, and the directional
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structure coating surface before and after immersion corrosion for 168 h were studied using X-ray diffraction (XRD, D/MAX2500 type PC) in the 10° ≤ 2θ ≤ 100° range.
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The roughness of surface morphologies of as-sprayed coatings and directional
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structure coatings were observed by a laser scanning confocal microscopy (LSCM,
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2.3 Electrochemical tests
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LSM 800).
The electrochemical performance test was conducted on a CHI660E type
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electrochemical workstation with the conventional three-electrode system, working temperature of 25 ± 1 ℃, and the platinum plate and mercurous sulfate were as the counter electrode and reference electrode, respectively. The coating surface with the area of 1 cm2 was exposed in 10% H2SO4 solution as the working electrode, and the rest were coated. First, the specimen was immersed in electrolytes for 1 h to obtain the open circuit potential (Eocp). Then, the electrochemical impedance spectrum was measured at Eocp in the frequency range of 105 to 0.005 Hz. Finally, the potentiodynamic polarization curve was recorded with the scan rate of 0.001 mV/s. All measurements
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were taken three times to obtain accurate results and ensure good repeatability and reproducibility.
2.4 Immersion corrosion tests
The coatings were immersed in 10% H2SO4 solution for 168 h at the room
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temperature. After the immersion corrosion test, the coatings were washed with the
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distilled water and naturally dried. The corrosive morphologies were observed by SEM.
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3. Results and discussion
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3.1 Microstructures of as-sprayed coatings
Fig. 2 presents the cross-section and surface morphologies of as-sprayed Ni60
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coating and Ni60/WC composite coating. It can be observed from Fig. 2(a and b)
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that the sprayed Ni60 coating and Ni60/WC coating are typical lamellar structure with the mechanical bonding between the coating and the substrate. Internal porosity and defects are all found in both coatings.
3.2 In situ characterization by LSCM
Fig. 3 shows the 3D model of coating after spraying and induction remelting and forced cooling. The sink of the pits on the actual surface corresponds to the bulges in the 3D images by a reversal treatment of the surface topography. The 3D images reveal that the depth and number of the pits decrease from Fig. 2(a) to Fig. 2(d). When the as-sprayed coating is remelted and treated by forced cooling, there
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were only few shallow pits on the surface. The number of the pits of as-sprayed Ni60 coating is more than that of as-sprayed Ni60/WC coating. The average roughnesses of as-sprayed Ni60 coating, Ni60/WC coating, directional structure Ni60 coating and directional structure Ni60/WC coating are Ra=8.51 μm, Ra=5.2 μm, Ra=0.837 μm, Ra=0.279 μm, respectively.
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3.3 Influence of WC on microstructure of directional structure coating
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The cross-section and surface morphologies of directional structure Ni60 coating
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and Ni60/WC composite coating are shown in Fig. 4. It can be seen that the
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microstructures of two kinds of coatings are all mainly planar crystal, cellular crystals,
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columnar crystals and dendritic crystals with directional structural feature. The
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cross–section is impact, without obvious cracks. Although the directional structure Ni60 coatings display much finer microstructure than Ni60/WC coatings, but it mainly
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presents dendritic morphology. However, the Ni60/WC composite coating mainly presents cellular and columnar crystals morphology, it has fewer penetrating grain boundaries than Ni60 coating. In addition, the point scan analysis results for white regions at the grain boundary (point A in Fig. 4(g)) are as follows: W 34.9%, C 6.2%, B 14.1%, Cr 8.4%, Fe 25.4% and Ni 8.9%. It is demonstrated that the white regions of grain boundaries are full of W and C and the added WC gathered at the grain boundary, which will block the corrosion channel and improve the coating corrosion resistance. The linear scanning results showed the contents of W and Cr at grain boundaries increase (as marked by rectangle in Fig. 5). In other words, the elements of W and Cr
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gather at grain boundaries, which further proved that the hard reinforcement phase blocked the corrosion channel. From the surface morphologies of directional structure Ni60 coating and Ni60/WC composite coating presented in Fig. 4(c and f), it can be observed that the grain boundary of Ni60 coating is island and fibrous eutectic tissue, but that of Ni60/WC
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composite coating is regular lamellar eutectic structure with WC participation (as shown in Fig. 4(f) by arrows). This again shows that WC change the grain boundary density
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and grain boundary type, which may improve corrosion resistance of Ni60 coating.
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The results of plane scan analysis of surface elements of directional structure Ni60
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coating and Ni60/WC coating are shown in Fig. 6 and Fig. 7, respectively. It can be
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clearly observed that the element Ni is mainly distributed on the grains in two coatings.
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However, additive WC makes Ni more dispersed in Ni60/WC coating compared with that of directional structure Ni60 coating. The point scan analysis mentioned in Table 1
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shows a variation in the composition. The Ni element–poor zones are the Cr element–rich zones. The element Cr is mainly distributed at the grain boundaries in the two coatings (as marked by rectangle in Fig. 6(a and c) and Fig. 7(a and c)), while after the addition of WC, the distribution of the element Cr is more dispersed, which is the same as the change of Ni. Moreover, the element Si is uniformly distributed on the Ni60 coating surface, but the element Si is enriched on the small range and mainly located in the grain boundaries in Ni60/WC coating (as marked by rectangle in Fig. 7(a and f)). As a whole, the C, Fe are evenly distributed on the surface of two coatings, but the element Fe is poor in the W enrichment region in grain boundaries (as marked by rectangle in
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Fig. 7(a and i). These show that the addition of tungsten carbide mainly affected the distribution of Ni, Cr, Si and Fe, and WC is distributed at the grain boundaries on the directional structure Ni60/WC coatings, which is in line with the result of line scan analysis (Fig. 5).
3.4 X-ray analysis
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The XRD patterns in the 2θ range from 10° to 100° for as-sprayed coatings and
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directional structure coatings are shown in Fig. 8. The phases of as-sprayed Ni60
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coating are almost the same as that of Ni60 alloy powder(Fig. 1(a)). The phases are
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mainly Ni, Ni3Fe, Cr23C6, CrB, Ni2Si, Ni3B. And WC and W2C phases are detected in
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the as-sprayed Ni60/WC coating. It can be seen that the phase γ-(Fe, Ni), CrB, Cr23C6
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and Ni2Si can be found as the main phase in two kinds of directional structure coatings. However, additive WC makes the content of the phase γ-Ni decrease, and the phases
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Ni2.9Cr0.7Fe0.36, Ni2B and FeB are converted to Cr0.19Fe0.7Ni0.11, NiB and Fe2B phases, respectively. In addition, the WC and W2C phases are detected in the directional structure Ni60/WC coating. It can be concluded that the addition of WC promotes the transformation of some phases and has effect on the phase constituent of the composite coatings. For instance, the Ni is at the main peak in the directional structure Ni60 coating, while the main peak has no Ni when adding WC.
Combined with elemental
distribution discussed above (Fig. 4 and Fig. 5), WC promotes element redistribution, phase changes and distribution, which can effectively improve the corrosion resistance of the directional structure Ni60 coating.
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3.5 Electrochemical measurements
Fig. 9 presents the results of electrochemical experiments. The electrochemical parameters of coatings and substrate such as corrosion potential (Ecorr) and corrosion current density (Icorr) were obtained from the polarization curves (Fig. 9a). The corrosion potentials of S45C steel, directional structure Ni60 coating, and directional
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structure Ni60/WC composite coating were -0.412V, -0.395V, and -0.112 V, respectively.
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The corrosion potential of directional structure Ni60/WC composite coating shifted
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positively, indicating that it had the better corrosion resistance. The current densities of
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directional structure Ni60 coating and directional structure Ni60/WC composite coating
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were 2.858×10-5, 3.002×10-6 A/cm2, respectively, which were lower than 1.748×10-3 A/cm2 of S45C steel. Generally, the higher Ecorr meant better chemical stability, and
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the lower Icorr meant lower corrosion rate [12]. The value of current density of the
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S45C steel was the largest, showing the coatings can well protect the substrate, and the directional structure Ni60/WC composite coating has the higher Ecorr and the lower Icorr than directional structure Ni60 coating, indicating its better corrosion resistance. In addition, the passivation phenomenon could be observed in both kinds of coatings. It can be observed from Fig. 9b that Nyquist plots are semicircle, the radius of directional structure Ni60/WC composite coating is greater than that of Ni60 coating. As well known, the larger the radius of the capacitive impedance loop is, the better the corrosion resistance is [13]. Thus, WC can largely improve the corrosion resistance of directional structure Ni60 coatings. Due to displaying the highest phase angle (Fig. 9c), the directional structure Ni60/WC composite coating own the best anti-corrosion
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properties. In addition, the values of |Z|0.01
Hz
of directional structure Ni60/WC
composite coatings is largest (Fig. 9d), showing that the directional structure Ni60/WC coatings own excellent corrosion resistance [14]. W-rich phase in the coating formed by W diffusion will increase the corrosion resistance of coating, and WC will be oxidized in the solution on the coating surface, which is also beneficial to block the corrosion
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solution penetration. The formation of tungsten oxide in sulfuric acid is reported by S. Sutthiruangwong et al. [15], and the formation formula is as follows [16]: WC + 5H2O
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→WO3 + CO2 + 10 H+ + 10e-.
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3.6 The surface morphologies and phase compositions after immersion corrosion
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Two kinds of coating surfaces were corroded through immersion in a 10% H2SO4
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solution. Fig. 10(a and c) and Fig. 10(b and d) show the corrosion morphologies of the directional structure Ni60 coating and Ni60/WC composite coating, respectively. After
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immersion for 168 h, several grain boundary cracks and certain pits appeared on the directional structure Ni60 coating surface, the holes or pits provided the path for the migration of H+ and SO42- and caused a further attack to the uncorroded region [17]. The corrosion mechanism of Ni60 coating is mainly the microgalvance corrosion and the pitting corrosion. However, there is no obvious pitting corrosion phenomena in the directional structure Ni60/WC coating. The areas enriched WC in the directional structure Ni60/WC coating have no corrosion, while the intracrystalline locations without WC show slight corrosion. From higher magnification images (Fig. 10(c and d)), it can be clearly observed that the corrosion phenomena are the same as the above
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statement. Usually, the corrosion is easy to take place at the grain boundary location [18], combined with Fig. 4, It can be seen from Fig. 10 that the corrosion of Ni60 coating without WC mainly occurs at the grain boundary, while the corrosion of Ni60/WC composite coating with WC is slight, no obvious corrosion occurs at the grain boundary, indicating that the addition of WC can block the grain boundary and improve
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the corrosion resistance of the coating, which is beneficial to improve the immersion corrosion resistance.
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The XRD patterns of directional structure coatings after the immersion corrosion
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test for 168 h are shown in Fig. 11. The detected phases of directional structure Ni60
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coatings are composed of Ni, γ-(Fe,Ni), Cr23C6, CrB, Ni2Si, Ni2B and FeB. Compared
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with those in Fig. 8(c), Ni2.9Cr0.7Fe0.36 is not detected and there was no new phases
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formed, indicating that their phase compositions were nearly stable. Moreover, the phases of directional structure Ni60/WC coatings have no obvious change after the
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immersion corrosion, too. The XRD results showed that the effect of phase compositions on the corrosion resistance was small.
4. Conclusions
(1) The WC addition has an obvious effect on the microstructure of directional structure Ni60 coating. WC is distributed at the grain boundaries of directional structure Ni60/WC coating. WC changes the element distribution, partial phase constituent and the peak position of some phases. (2) The electrochemical tests show that the directional structure Ni60/WC coating
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has lower corrosion current density, higher corrosion potential, and larger radius of the capacitive impedance loop, phase angle and the values of |Z|0.01
Hz
than directional
structure Ni60 coating, indicating that directional structure Ni60/WC composite coating has better corrosion resistance. (3) In immersion corrosion tests, directional structure Ni60 coating displays the
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pitting corrosion and the microgalvance corrosion, while Ni60/WC composite coating presents the slight corrosion on the intracrystalline locations without WC and no
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obvious pitting corrosion phenomenon. WC blocks the corrosion channel at the grain
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boundaries and changes the grain boundary density and grain boundary type, which is
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good for improving the corrosion resistance.
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Acknowledge
This work was supported by National Natural Science Foundation of China (No.
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51365024); and Zhejiang Provincial Natural Science Foundation of China (No. LGG19E010003).
Disclosure statement
No potential conflict of interest was reported by the authors.
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Table 1 Chemical composition of the grains in Figs. 6(a) and 7(a) (wt%)
Ni
Cr
Fe
Si
B
C
W
A
50.3
6.1
30.3
2.6
0.0
10.1
-
B
31.1
4.2
46.0
1.2
6.6
3.5
4.9
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Point no.
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Fig. 1 XRD analysis of Ni60 and WC powders: (a) Ni60 powder; (b) WC powder Fig. 2 Back-scattered SEM images of as-sprayed coating cross-section and surface: cross-section;
(b) Ni60/WC composite coating cross-section;
(a) Ni60 coating
(c) Ni60 coating surface;
(d) Ni60/WC composite coating surface Fig. 3 3D model by LSCM at the same position of samples: (a) as-sprayed Ni60 coating, (b) as-sprayed
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Ni60/WC coating, (c) directional structure Ni60 coating, (d) directional structure Ni60/WC coating
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Fig. 4 Back-scattered SEM images of directional structure coating cross-section and surface: (a) the bottom of cross-section, (b) the top of cross-section and (c) surface of Ni60 coatings; (d) the bottom of
-p
cross-section, (e) the top of cross-section and (f) surface of Ni60/WC composite coating; (g) Partial
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enlarged detail of Fig. 4 d
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Fig. 5 Line scan analysis of directional structure Ni60/WC composite coating cross-section
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Fig. 6 The result of plane scan analysis of directional structure Ni60 coating surface Fig. 7 The result of plane scan analysis of directional structure Ni60/WC coating surface
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Fig. 8 XRD patterns of as-sprayed and directional structure coatings: (a) as-sprayed Ni60 coating, (b) as-sprayed Ni60/WC coating, (c) directional structure Ni60 coating, (d) directional structure Ni60/WC coating
Fig. 9 Electrochemical results of substrate and two kinds of coatings:
(a) Tafel polarization curves,
(b) Nyquist plots, (c) phase angle, and (d) bode plots Fig. 10 SEM images of directional structure coating surface after corrosion: surface;
(a and c) Ni60 coating
(b and d) Ni60/WC composite coating surface
Fig.11 XRD analysis results of directional structure coatings after immersion corrosion test: (a) Ni60 coating; (b) Ni60/WC composite coating
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(b)
(a) 1 2 3 4
1 W2 C
2 Ni3Fe
2 WC
3 Cr23C6
Intensity(a.u.)
Intensity(a.u.)
4 Cr2B 5 Ni2Si 6 Ni3B
1 2 3
5 6 3 6 3
1 2 3
3
30
40
50
2
1 1 1
60
70
80
90
2
2
100
10
20
30
40
50
60
2/degree
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-p
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Figure 1
1
2 2 1 1
2
2/degree
1
1
1
1
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20
2
2
1 4
4
10
1
Ni
70
80
2 1
90
100
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-p
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of
Figure 2
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Figure 3
+
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Figure 4
of
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Figure 5
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(b) Ni
(c) Cr
-p
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(a) Plane scanned position
(e) B
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(d) C
(g) Fe
(h) O
Figure 6
(f) Si
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(b) Ni
(c) Cr
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(a) Plane scanned position
(e) B
(f) Si
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(d) C
(g) Fe
(h) O
Figure 7
(i) W
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(a)
1 2 3
(b)
Ni
4 CrB 5 Ni2Si
4
10
20
30
6 Ni3B
4 5 6 6 4
3 4 6 6 6
40
1 2 3
50
3 Cr23C6 5 Ni2Si 6 Ni3B
3 6 8 4
1
4
2 Ni3Fe 4 Cr2B
Intensity(a.u.)
Intensity(a.u.)
3 Cr23C6
1 2 3
Ni
1 2 3 4
2 Ni3Fe
5 6
70
80
90
1 2 7
7
1
60
7 WC 8 W2 C
1 2 8
4
100
10
20
30
40
2/degree
50
60
(d) Ni 2 -(Fe, Ni)
3 Ni2.9Cr0.7Fe0.36
7
4 Ni2B 5 FeB 6 CrB 7 Cr23C6 8 Ni2Si
40
50
60
70
80
90
9 9
100
10
20
-p
30
2/degree
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Figure 8
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20
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1 2 3 8
2 4 356 5 68 7 47
10
100
3 Cr0.19Fe0.7Ni0.11
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3
1
90
Ni 2 -(Fe, Ni)
2 3 7
Intensity(a.u.)
Intensity(a.u.)
2
80
2/degree
(c) 1
70
1
30
1 4 5 2 6 10 378
40
50
60
2/degree
4 NiB 5 Fe2B 6 CrB 7 Cr23C6 8 Ni2Si 9 WC 10 W2C
70
80
1 2 3
90
100
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(a)
Directional structure Ni60 coatings Directional structure Ni60/WC coatings The S45C steel
0.4
Potential(V)
0.2 0.0 -0.2 -0.4 -0.6 -0.8
-7
-6
-5
-4
-3
-2
-1
log(current density(A/cm2))
(d )4.0
Directional structure Ni60 coatings Directional structure Ni60/WC coatings The S45C steel
3.5 3.0
log(Z(ohm))
-60
2.5 2.0
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-20
1.5 1.0
0
-p
0.5 0.0
20 -4
-3
-2
-1
0
1
2
3
4
5
-2
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log(Frequency(HZ))
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Figure 9
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-5
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Phase()
-40
Directional structure Ni60 coatings Directional structure Ni60/WC coatings The S45C steel
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(c) -80
-1
0
1
2
3
log(Frequency(HZ))
4
5
6
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Figure 10
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(b)
(a) Ni 2 -(Fe, Ni)
3 Cr23C6
4 Cr23C6
Intensity(a.u.)
Intensity(a.u.)
6 Ni2B 7 FeB
3
40
2 3 4 8
5 CrB 6 Ni2Si 7 Ni2B 8 FeB 9 WC 10 W2C
94
2 3 10
2 3 10
5 16 7 9
50
60
70
80
90
100
10
20
30
2/degree
40
50
60
70
2/degree
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Figure 11
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30
2 4 15 76 63
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20
2 3 4
3 Cr0.19Fe0.7Ni0.11
4 CrB 5 Ni2Si
10
Ni 2 -(Fe, Ni)
1 2 3
80
90
100
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Credit Author Statement Xiaotian Yang and Xiuqian Li conceived and designed the study. Xiuqian Li and Hengli Wei performed the experiments. Xiaotian Yang and Xiuqian Li wrote the paper. Xiaotian Yang, Xiuqian Li, Qiangbin Yang, Xiaoyue Fu and Wensheng Li reviewed and
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edited the manuscript. All authors read and approved the manuscript.
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Highlights 1. The directional structure coatings of Ni60 and Ni60/WC were prepared, respectively. 2. WC resulted in the microstructural evolution of directional structure Ni60 coating.
channel. WC
addition
improved
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-p
structure Ni60 coating.
corrosion
resistance
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4.
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3. The added WC was distributed at the grain boundary and blocked the corrosion
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directional