Microstructural evolution and corrosion properties of Ni-based alloy coatings fabricated by multi-layer laser cladding on cast iron

Journal of Alloys and Compounds 822 (2020) 153708

Contents lists available at ScienceDirect

Journal of Alloys and Compounds journal homepage: http://www.elsevier.com/locate/jalcom

Microstructural evolution and corrosion properties of Ni-based alloy coatings fabricated by multi-layer laser cladding on cast iron Jian Liu a, b, Hao Liu a, b, *, Xianhua Tian a, Haifeng Yang a, Jingbin Hao a a

School of Mechanical and Electrical Engineering, China University of Mining and Technology, Xuzhou, China Jiangsu Engineering Technology Research Center on Intelligent Equipment for Fully Mining and Excavating, China University of Mining & Technology, Xuzhou, China

b

a r t i c l e i n f o

a b s t r a c t

Article history: Received 26 July 2019 Received in revised form 18 December 2019 Accepted 6 January 2020 Available online 7 January 2020

To solve the problem of brittleness at the bonding interface, the multi-layer laser cladding method was carried out to fabricate Ni-based alloy coating on the compacted graphite cast iron. The effect of dilution rate on the microstructure and corrosion behavior of the coatings were investigated. The results showed that the volume fraction and the secondary dendrite arm spacing of the dendrite increase significantly with the increasing number of the deposited layers. The dendritic phase in the coatings changed from gFe phase to g-(Fe, Ni) phase, and the interdendritic phase transformed from martensite phase to M7C3 (M ¼ Fe, Cr) phase with the decrease of the dilution rate. The lattice parameter of the dendritic phase decreased gradually, as the increase of Ni content. The corrosion tests show that the coating with six deposited layers exhibited the best corrosion resistance at room temperature in 3.5% NaCl solution. The corrosion mechanism changed from severe intragranular corrosion to pitting corrosion and intergranular corrosion. According to the analysis results of electrochemical impedance spectroscopy curves, MottSchottky curves, and point defect model, the improvement of the corrosion resistance was attributed to the high compactness and resistance of the passive film, which provided fewer oxygen vacancies in the corrosion process due to the increase of Ni and Cr concentration of the coating. © 2020 Elsevier B.V. All rights reserved.

Keywords: Multi-layer laser cladding Cast iron Microstructural evolution Corrosion resistance Passive film

1. Introduction Laser cladding is an effective technique for preparing thick coatings with metallurgical bonding properties [1,2]. In general, the coating with a different material prepared by laser cladding can improve the surface properties of the substrate purposefully, such as wear resistance [3,4], corrosion resistance [5,6] and oxidation resistance [7]. Additionally, laser cladding process has been applied to numerous kinds of metal surfaces, such as stainless steel [8], carbon steel [9] and titanium alloy [10], etc. Cast iron with excellent mechanical properties and desirable castability is extensively used as the engineering material [11]. Since demands for reliability and durability of the cast iron components are continuously on the rise, it becomes essential to improve the surface performance of the cast iron by the laser cladding coatings. Unfortunately, there are some undesirable situations when the coating is deposited on the surface

* Corresponding author. School of Mechanical and Electrical Engineering, China University of Mining and Technology, Xuzhou, China. E-mail address: [email protected] (H. Liu). https://doi.org/10.1016/j.jallcom.2020.153708 0925-8388/© 2020 Elsevier B.V. All rights reserved.

of cast iron. It is found that a hard and brittle layer, which is composed of martensite, retained austenite, undissolved graphite, and a small amount of ledeburite, is formed between the coating and the substrate due to the high carbon content in cast iron in the laser cladding process [11,12]. Such a brittle and hard layer where micro-cracks probably initiate under the residual stresses or external forces, remarkably weakens the bonding strength between the coating and the substrate [13]. In our previous work, an approach of superimposed scanning pattern with relatively low powder feed rate, called multi-layer laser cladding, was proposed in the process of laser cladding on the cast iron. It was proved that the hard and brittle layer in the vicinity of the bonding interface could be eliminated by the multilayer deposition method because the martensite was gradually transformed into the tempered sorbite by the heat of the subsequent layers [14]. Consequently, the crack susceptibility was reduced, and the bonding strength was improved by the multilayer cladding. However, a potential problem is that the dilution rate of Ni-based alloy coatings, which refers to the degree of variability in the composition of the coating due to the mixing of the melted substrate, decreases when the number of deposited layers

2

J. Liu et al. / Journal of Alloys and Compounds 822 (2020) 153708

increases. A great deal of literature showed that the unique dilution effect of laser cladding is a key factor affecting the microstructure and phase composition of coatings [15e17]. The dilution rate of the CoCrBFeNiSi high-entropy alloy amorphous coating could be affected by laser power in the process of laser cladding [15]. The chemical composition of coatings varied with the dilution rate. Thus, the critical cooling rate of the coatings was altered, resulting in the microstructure evolution. Driving or restraining the convective movement of the molten pool through an electric field or a magnetic field was considered as another way to control the dilution rate of the coatings [16]. The dilution rate of the NiCrBSi coating decreased (10.8%) under a steady electromagnetic field. This led to the increase of the volume fraction of the CrB phase and the improvement of the hardness and corrosion resistance of the coating [17]. As for Ni-base alloy, it is consisted of g (g-Ni or g-Co (Ni, Cr)) solid solution phase with a FCC lattice and b (b-AlNi or b-(Co, Ni)Al) intermetallic phase with a B2 structure [18,19], and possess outstanding corrosion resistance and excellent wear resistance [20,21]. It is speculated that the microstructure and properties of Ni-based coatings will be affected by the change of dilution rate caused by the multi-layer cladding process. In this paper, the effects of dilution rate on the microstructure, phase composition, and corrosion behavior of the Ni-based alloy coatings prepared by laser cladding were investigated. The primary purpose of this paper is to explore the microstructural evolution and surface properties change, especially corrosion property, of the coatings with different deposited layers. 2. Material and methods 2.1. Coating preparation The Ni-based alloy metal powders with nominal compositions of 3.4 wt% Al, 18.6 wt% Cr, 2.6 wt% Co, 0.42 wt% Y and Ni in balance were used as raw materials. The powders were dried in a DZF-6020 type electric vacuum drying oven for 6 h to ensure the excellent fluidity. The Ni-based alloy coatings were deposited on the surface of compacted graphite cast iron (CGI) using laser cladding. The substrate was pre-treated with alcohol for oil removal and 400-grit SiC abrasive paper for surface smoothing. The coatings preparation was executed by a YLS-400-CTTC-Y11 type fiber laser system equipped with a DPSF-2 type paraxial powder feeding system. The high purity argon was selected as shielding gas and carrier gas for powder transportation. The detailed parameters of laser cladding used in this work were listed in Table 1. The variety of dilution rate was obtained by multi-layer deposition since the diffusion distance of elements is limited in the case of rapid solidification. In this work, the coatings were fabricated in one, three, and six deposition layers, respectively. Hereafter, the corresponding samples were referred as coating L1, L3, and L6, respectively. 2.2. Microstructural characterization and phase analysis The microstructure and phase composition analysis of all specimens were based on the top layer of the multilayered coatings. The microstructure of the multilayered coatings etched by aqua regia was revealed using scanning electron microscopy (SEM;

Quanta 250). The chemical compositions in different morphologies of the coatings were investigated by energy dispersive spectrometer (EDS). The phase composition of the coatings was analyzed using X-ray diffraction (XRD; D8 ADVANCE) with a Cu-Ka (l ¼ 1.5406 Å) radiation at a voltage of 40 kV. The lattice parameters and interplanar spacings of the phase contained in the coatings were calculated according to the Bragg’s diffraction law. The nanoprecipitated phases were further characterized by transmission electron microscopy (TEM). Selected area electron diffraction (SAED) was executed to explore the crystal lattices and lattice parameters furtherly, which were used to verify the correctness of the XRD analysis results. 2.3. Electrochemical tests The electrochemical tests were performed in 3.5 ± 0.2% NaCl solution at room temperature using a CHI660D type electrochemical workstation with a typical three-electrode system, where the measured coating was specified as working electrodes, the platinum electrode was used as auxiliary electrode, and the saturated calomel electrode (SCE) was functioned as reference electrode. As a sensitive area, scratches may be corroded preferentially in the process of corrosion, which can affect the authenticity of corrosion resistance test results of the coatings. Therefore, the surfaces of the samples were ground with SiC abrasive papers from 500-grit to 3000-grit, followed by graded polishing finish using the diamond paste with particle size ranging from 1.5 mm to 0.1 mm. The surfaces of the coatings, as exposed facets, contacted directly with the NaCl solution, while the non-test surfaces of the samples were sealed with epoxy resin. The samples were immersed in the solution for 60 min to ensure the steady-state of the exposed facets in NaCl solution. The steady-state potential of the electrode system was verified by open circuit potential (OCP)-time curve for 60 min prior to the electrochemical testing. Potentiodynamic polarization tests were conducted from an initial potential of 0.8 V to a final potential of 0.9 V at a scan rate of 0.1 mV/s. The morphologies of the corroded surfaces were observed using SEM. Before the electrochemical impedance spectroscopy (EIS) tests, the samples were immersed in 3.5 ± 0.2% NaCl solution for 60 min at room temperature. The EIS tests were implemented with a frequency ranging from 0.01 Hz to 100 kHz. The applied AC amplitude was selected as 10 mV. The Mott-Schottky plots were recorded and fitted at a frequency of 1000 HZ, an initial potential of 1.0 V and a final potential of 0.2 V. All tests were carried out at least three times to ensure the accuracy of the test results. 3. Results and discussion 3.1. Microstructure characterization The cross-section SEM images of the single-track samples of the coatings L1, L3, and L6 are shown in Fig. 1(a), (b) and (c), respectively. Defects, such as cracks and voids, are not found in the dense coatings. The dilution rate of the coatings deposited in various layers can be calculated by the geometric characteristics of the cross-sections. Based on our previous work, the dilution rate of L1, L3, and L6 is calculated to be about 67.7%, 53.9%, and 36.4%, respectively [14]. It is obvious that the dilution rate decreases with

Table 1 Detailed parameters of laser cladding. Laser power (W)

Powder feed rate (g/min)

Spot diameter (mm)

Scanning velocity (mm/s)

Gas flow rate (L/min)

Overlap rate

900

4.3

2

10

5

50%

J. Liu et al. / Journal of Alloys and Compounds 822 (2020) 153708

3

Fig. 1. Cross-section images of the single-track samples: (a) Coating L1; (b) Coating L3; (c) Coating L6.

the increase of the deposition layers. Fig. 2 shows the microstructure of the cross-section in the top layer of the coatings. The SEM image of the coating L1 (Fig. 2(a)) exhibits a typical dendritic structure. An enlarged view (Fig. 2(b)) of the interdendritic region (IR) shows a continuous sheet-like and uniform structure, while the refined dendritic region (DR) is smooth. The coatings L3 and L6 exhibit similar characteristics of microstructure ((Fig. 2(c) and (e)). Compared with the coating L1, it can be seen that the DR is coarsened remarkably, while the IR is refined. Statistics of the relative volume fractions of the DR and the IR are shown in Fig. 3. The coating L6 has the largest volume ratio of the DR (about 53.19%). In particular, the secondary dendrite

coarsening behavior can be observed. It can be found that with the increase of the number of the deposition layers, the secondary dendrite arm spacing (SDAS) increases gradually. The SDAS of the coatings L1, L3, and L6 is calculated to be 2.40 mm, 2.67 mm, and 3.75 mm, respectively. The coarsening of dendrites is related to the cooling rates of the top layers, since the cooling rate is considered to be a crucial factor in determining the size of crystalline grain [22]. A larger cooling rate generally tends to yield a set of refined grains, while a smaller one leads to the coarsening of the grains [23]. It was proved that the cooling rate at the top of the coating decreases as the deposition height ascends [24]. Compared with the coating L1, the cooling

Fig. 2. Microstructure of the cross-section in the top layer of the coatings: (a), (b) Coating L1; (c), (d) Coating L3; (e), (f) Coating L6.

4

J. Liu et al. / Journal of Alloys and Compounds 822 (2020) 153708

Fig. 3. The volume fractions of the DR and the IR.

rates of the top layer of the coating L3 and the coating L6 decrease in turn, and the dendrites are coarsened gradually. Furthermore, as the superposition of cladding layers, the heat accumulation in the cladding layer increases gradually, which significantly reduces the temperature gradient (G) of the top layer and prolongs the growth time of the secondary dendrite arm dendrite during the multilayered cladding process [25]. The chemical composition analysis results of the coatings are shown in Table 2. The effect on the chemical composition, which caused by the change of the dilution rate, is mainly manifested on Fe, Ni, and Cr. It can be observed that the concentration of Ni and Cr presents an upward tendency, and the content of Fe shows a law of decline. Also, compared with the DR, the IR shows a significant enrichment of C and a slight infertility of Ni and Fe. 3.2. Phases analysis The XRD patterns of the multilayered NiCoCrAlY coatings are displayed in Fig. 4. The coating L1 presents a dual-phases structure consisting of the martensite phase with a tetragonal lattice and gFe solid solution phase with an FCC lattice (Fig. 4(a)). Compared with the coating L1, there are transformations in the phase composition of the multilayered coatings, as shown in Fig. 4(b) and (c). The diffraction peaks of the M7C3 phase and the M23C6 phase appear in the XRD patterns of the multilayered coatings L3 and L6. Also, the initial g-Fe phase is replaced by the g-(Fe, Ni) solid solution phase. Fig. 4(d) is an enlarged view of the peaks (111) of the solid solution phase. For Fig. 4(d), the right shift of the diffraction peak (111) can be found, indicating the reduction of lattice parameters. The detailed degrees corresponding to the diffraction peaks of coatings L1, L2 and L3 are 43.57, 43.76 , and 44.16 , respectively. According to the Bragg’s diffraction law, the

Table 2 Chemical composition (at. %) of the coatings. Coatings

Regions

C

Al

Si

Cr

Fe

Co

Ni

L1

DR IR DR IR DR IR

46.70 52.43 40.29 48.03 37.59 55.35

0.93 0.28 1.64 0.65 2.07 0.87

2.62 1.67 2.11 1.41 1.70 1.54

1.54 3.64 3.81 11.48 6.36 16.35

38.08 36.83 28.99 26.20 18.75 12.33

0.94 0.39 1.24 0.53 1.59 0.67

9.19 4.76 21.91 11.69 31.94 12.89

L3 L6

interplanar spacings of (111) planes at the corresponding angles are calculated to be 2.07 nm, 2.06 nm, and 2.04 nm, and the lattice parameters are estimated to be 3.59 nm, 3.58 nm and 3.53 nm, respectively. The change of the lattice parameters is attributed to the increase of the Ni concentration. It is reported that the atomic radius of Ni and Fe are 1.62 Å and 1.72 Å, respectively [26]. The substitution of partial Fe atoms contained in the g-Fe phase by Ni atoms with the smaller atom radius leads to the decrease of the lattice parameters. Based on the EDS analysis results, the DR of the coating L1 can be identified as the g-Fe phase, and the IR can be recognized as the martensite phase. From the XRD analysis results (Fig. 4 (b) and (c)) of the coatings L3 and L6, it can be found that the coatings contain three phases which are g-(Fe, Ni) phase, M7C3 phase, and M23C6 phase, respectively. However, the SEM images (Fig. 2(d) and (f)) show two different kinds of structure (IR and DR) merely. The phase number obtained from XRD analysis does not coincide with the microstructure observed by SEM (Fig. 2(c)e(f)). Thus, the detailed microstructure of coating L6 is revealed by TEM, as shown in Fig. 5. The bright-field TEM (BFTEM) image (Fig. 5(a)) shows that there are two populations of precipitates, which are primarily located at the DR and the IR, respectively. The enlarged views of the two populations precipitates are shown in Fig. 5((b) and (c). The nanoprecipitates contained in the DR (Fig. 5(b)) exhibit a rod-like morphology and are disorderly embedded in the matrix (Gray area). The bulk precipitates contained in the IR (Fig. 5(c)) exhibit sub-micron grain size. The SAED results of the matrix, the nanoprecipitates, and the bulk precipitates are shown in Fig. 5(d), (e) and (f), respectively. The matrix, the nano-precipitates, and the bulk precipitates are respectively identified as g-(Fe, Ni) phase, M23C6 (M ¼ Fe, Cr) phase and M7C3 (M ¼ Fe, Cr) phase, which is consistent well with the XRD and the EDS analysis results. According to the above experimental and analytical results, it can be concluded that the superposition of cladding layers leads to the phase transformation of the top layers in the multilayered coatings. The transformation of the g-Fe phase can be attributed to the variation of chemical composition caused by the dilution effect, while the disappearance of martensite is related to the cooling rate of the top layers in the multilayered coatings. For the coating L1, though the element concentration of Ni (9.19 at. %) is relatively low, it can improve the stability of the g-Fe phase at room temperature, as well as the melting point. Thus, the g-Fe phase is crystalized as the primary phase in the form of dendrites. Owing to the high cooling rate, the diffusion of C atoms is suppressed, hence the martensite is formed in the IR. For the multilayered coating L6, the variety of the dilution rate leads to the significant rise of Ni (31.94 at. %) element concentration in the multilayered coatings, which provides more solutes for the formation of g-(Fe, Ni) solid solution phase in the initial solidification stage. Due to the similar atomic radius and the same lattice structure between Ni and Fe, Ni atoms have a higher solid solubility in the g-Fe phase, allowing that the primary phase is transformed into a substitutional solid solution referred as g-(Fe, Ni) in the process of solidification. A small amount of the C atoms dissolve into the octahedral interstice of the FCC lattice of the g-(Fe, Ni) phase, while the majority of C atoms are precipitated in the form of carbides on account of the relatively low cooling rate. 3.3. Electrochemical properties Fig. 6(a) displays the OCP curves of the three samples. These curves show a marked decreased tendency with the increasing number of the deposited layers. The potentiodynamic polarization curves of the multilayered coatings in 3.5% NaCl solution are shown in Fig. 6(b). The apparent passive behavior can be observed in the

J. Liu et al. / Journal of Alloys and Compounds 822 (2020) 153708

5

Fig. 4. XRD patterns of the coatings: (a) Coating L1; (b) Coating L3; (c) Coating L6; (d) An enlarged view of the diffraction peak (111).

Fig. 5. TEM results of the multilayered coating L6: (a) TEM image of the coating; (b) Magnified BFTEM image of the DR; (c) Magnified DFTEM image of the IR; (d) SAED pattern of the matrix; (e) SAED pattern of the nano-precipitates in the DR; (f) SAED pattern of the precipitates in the IR.

6

J. Liu et al. / Journal of Alloys and Compounds 822 (2020) 153708

Fig. 6. OCP curves and potentiodynamic polarization curves in 3.5% NaCl solution: (a) OCP curves; (b) potentiodynamic polarization curves.

anodic polarization curves of the coatings. Also, the increasing number of the deposited layers significantly extends the potential range (Epr) of the passivation zone and decreases the dimensional passive current density (Ip). The detailed parameters of the potentiodynamic polarization curves are listed in Table 2, in which Icorr is the self-corrosion current density, Ecorr is the self-corrosion potential, ba is the anode area slope, bc is the cathode area slope, EF is the activated potential, and Etr is the trans-passive potential, respectively. From Table 3, the coating L1 with an Icorr of 2.22  105 A/cm2 and an Ecorr of 0.48 V presents the worst corrosion resistance, while the coating L6 exhibits the smallest Icorr (2.22  107 A/ cm2) and the most positive Ecorr (0.42 V), implying that the coating L6 possesses the best corrosion resistance. The corrosion morphologies of the coating L1 are shown in Fig. 7. It can be found from Fig. 7(a) that the corroded coating L1 exhibits a rough appearance resulting from intragranular corrosion. The corrosion mechanisms of the coating L1 can be clearly revealed by the metallographic image of the corroded surface (Fig. 7(b)). The DR (g-Fe phase) is severely damaged, while the IR (martensite phase) remains intact. The poor corrosion resistance can be explained by the excessive Fe content caused by the high dilution rate. Fe atoms, as the productive electron donors, tend to participate in the oxidation reaction during the anodic polarization process. Furthermore, the local corrosion feature of the coating L1 is caused by the heterogeneous catalytic effect. The potential difference caused by the excessive C element concentration in the IR results in the formation of the corrosion cell between the martensite phase and the g-Fe solid solution phase. As the positive electrode in the primary battery, the g-Fe solid solution phase preferentially participates in the anodic dissolution process and accelerates the material loss in the DR. The SEM images of corroded surfaces of the coatings L3 and L6 are shown in Fig. 8. Unlike the corroded morphology of the coating L1, the corroded surfaces of the coatings L3 and L6 are smoother, as shown in Fig. 8(a) and (d). The typical pitting holes can be observed

on the corroded surfaces of the coating L3 and L6, which indicates that the pitting corrosion occurs. Moreover, the distribution density of pitting holes on the surface of the coating L6 is much less than that of the coating L3, suggesting that the coating L6 has better pitting resistance. The enlarged views of pitting holes are shown in Fig. 8(b) and (e). The diameter of pitting holes on the corroded surface of the coating L3 ranges from 60 mm to 70 mm, while that on the corroded surface of the coating L6 ranges from 80 mm to 90 mm. The enlarged images of the non-pitted region of the coating L3 and L6 are displayed in Fig. 8(c) and (f), respectively. This shows that the coating L6 effectively inhibits the formation and growth of the pitting holes under the same corrosion conditions. The EDS mappings of the corroded surface of the coating L3 is shown in Fig. 9. The elements exhibit a homogeneous distribution basically, implying that the formation of the pitting holes and the excellent pitting resistance of the coating are not associated with the distribution of elements. Therefore, the excellent pitting resistance of the coating L6 is considered to be the contribution of passive film formed on its corroded surface during the polarization process. Additionally, the DR is intact on the corroded surfaces, while the IR is damaged with varying degrees, indicating that there is intergranular corrosion behavior in the process of anode polarization as well. Compared the morphologies of the IR (Fig. 8(c) and (f)), it can be found that the intergranular corrosion resistance of the coating L6 is enhanced obviously due to the decrease of the volume fraction of the IR. The Nyquist plots are recorded and fitted to characterize the performance of the passive films formed on the surfaces of the multilayered coatings in 3.5% NaCl solution, as shown in Fig. 10. The corrosion resistance of the tested samples can be proved by the radius of the fitted Nyquist plots. The larger radius commonly symbolizes better corrosion resistance [27]. It is easy to observe that the radius of the Nyquist plots presents an increasing tendency with the increasing number of the deposited layers, implying that the corrosion resistance of the coating is improved. Furthermore,

Table 3 The detailed parameters of the potentiodynamic polarization curves. Samples L1 L3 L6

Icorr A/cm2 5

2.22  10 7.38  107 2.22  107

Ecorr/V 0.48 0.46 0.42

ba V/dec 25.30 31.66 43.53

bc V/dec 31.00 33.83 28.47

Ip A/cm2 4

1.51  10 1.70  105 7.41  106

EF/V

Etr/V

Epr/V

0.43 0.40 0.37

0.28 0.04 0.07

0.15 0.36 0.44

J. Liu et al. / Journal of Alloys and Compounds 822 (2020) 153708

7

Fig. 7. Corrosion morphologies of the coating L1: (a) SEM image of the coating L1 corroded surface; (b) Metallographic image of the coating L1 corroded surface.

Fig. 8. Corrosion morphologies of the coatings L3 and L6: (aec) SEM image of the coating L3 corroded surface; (def) Magnified view of the coating L6 corroded surface.

the electrochemical parameters of the equivalent resistance (Rt) and the equivalent capacitance (Cdl) can be used to identify the characteristics of passive films, where Rt and Cdl signify the chargetransfer resistance and the interfacial capacitance, respectively [28,29]. The passive film with a larger Rt and a smaller Cdl is more conducive to the corrosion resistance of the coating. The electrochemical parameters are listed in Table 4. The multilayered coating L6 possesses the largest Rt (96226.00 U/cm2) and the least Cdl (2.03  104 F/cm2), which suggest that the passive film formed on the corroded surface of the coating L6 has the best-passivated property. The performance of the passive films of the coatings could be explained by Mott-Schottky theory [30]. The Mott-Schottky plots of the coatings are shown in Fig. 11. An interval approximating a straight line (0.55 V to 0.00 V), selected as a linear fitting part, can be observed in the Mott-Schottky curves. The results of linear fitting are listed in the table at the upper left of Fig. 11. The fitting results show that the slope of the linear region is positive, meaning that the passive films formed on the surface of the coatings present the properties of n-type semiconductors. The space charge capacitance of n-type semiconductors can be expressed by Eq. (1) [31].

C 2 ¼


2 KT E  EFB  εr ε0 eND e

(1)

where ε0 represents the vacuum permittivity (8.85  1014 F cm1),

ND is the donor density of the n-type semiconductors, E represents the applied potential, EFB is the flat band potential, K represents the Boltzmann constant (1.38  1023 J K1), T represents the absolute temperature, e represents the electron charge (1.6  1019 C) and εr, which is selected as 12 [30], is the relative dielectric constant of Ni oxide. ND can be used to characterize the conductivity of the passive film. A lower ND means higher resistance of the passive film, which implies that the passive film plays a better role to protect the coating and improve the corrosion resistance of the coating. According to Eq. (2), ND can be expressed as follows:

2 dC 2 ND ¼ εr ε0 e dE

!1 (2)

The ND of passive film formed on the coatings L1, L3 and L6 is calculated to be 6.06  1021 cm3 and 5.60  1021 cm3 and 4.99  1021 cm3, respectively. The coating L6 shows the lowest ND of 4.99  1021 cm3, and ND presents a declining trend with the increasing number of deposited layers. Therefore, the passive film formed on the surface of the coating L6 can protect the coating and slow down the damage process more effectively, which is consistent with the results of EIS. Further, the high concentration of Ni contributes to the performance improvement of the passive film formed on the multilayered coatings in 3.5% NaCl solution, which could be revealed by the point defect model (PDM) [32]. According to the PDM, carriers in the passive film with n-type

8

J. Liu et al. / Journal of Alloys and Compounds 822 (2020) 153708

Fig. 9. The EDS mappings of the corroded surface of the coating L3.

Fig. 10. Nyquist plots of the coatings in 3.5% NaCl solution: (a) Nyquist plots; (b) Magnified view of the Nyquist plot of the coating L1.

semiconductor characteristics are considered to be mainly oxygen vacancies. The defect reactions in the passive film are provided in ·· Fig. 12, where MFe’’ (M ¼ Fe, Ni, Al, Cr) is the cation interstitials, VO is the oxygen vacancies, and V··M (M ¼ Fe, Ni, Al, Cr) is cation vacancies. It is believed that the increase of the Ni concentration in the coating

could promote the progress of the reaction (3), while the progress of the reaction (2) will be retard due to the decrease of Fe content. Consequently, the increase of Ni concentration caused by the decrease of dilution rate leads to a lower oxygen vacancies concentration in the passive film formed on the coating L6 than other

J. Liu et al. / Journal of Alloys and Compounds 822 (2020) 153708 Table 4 Electrochemical parameters obtained from the Nyquist plots. Samples

Rt U/cm2

Cdl F/cm2

L1 L3 L6

5010.60 38548.00 96226.00

3.89  103 5.58  104 2.03  104

9

coatings. The compactness improvement of the passive film leads to an increase in its resistance, which is consistent with the EIS and Mott-Schottky curves analysis results. The improvement of pitting resistance of the coating L6 is related to the lower concentration of oxygen vacancies [33,34]. Fig. 13 presents the model of pitting corrosion induced by oxygen vacancies. As is shown in Fig. 13 (a), the passivation film is formed by the deposition of oxides generated by the cathodic polarization reaction during the polarization process. The accumulation area of oxygen vacancies tends to dissolve preferentially in the anode planning process due to the existence of defects. Accordingly, the anodic dissolution rate in this region is significantly higher than that in the surrounding area, which leads to the formation of the hollows, as shown in Fig. 13 (b). The autocatalytic effect caused by the difficulty of ion diffusion in the hollows leads to the formation of pitting holes (Fig. 13(c)). Therefore, the decrease of oxygen vacancies concentration in the coating L6 contributes to inhibit the formation of pitting holes, thus improving the pitting resistance of the multilayered coating. It has also been proved that Ni addition can effectively improve the corrosion resistance of the Fe-based alloy coating by forming the NiO passive film [35]. Also, the increase of Cr concentration is conducive to the improvement of corrosion resistance of the coating L6 [36]. Therefore, the reduction of the dilution rate is beneficial to enhance the compactness of the passive film and improve the corrosion resistance of the Ni-based coating.

Fig. 11. Mott-Schottky plots of the coatings in 3.5% NaCl solution.

Fig. 12. Schematic diagram of defect reaction in 3.5% NaCl solution.

Fig. 13. The model of pitting corrosion induced by oxygen vacancies.

10

J. Liu et al. / Journal of Alloys and Compounds 822 (2020) 153708

4. Conclusions

250).

The Ni-based alloy coatings well bonded to the compacted graphite cast iron substrate were successfully fabricated using the multi-layer laser cladding method. The microstructure and corrosion behavior of the coatings varying with the number of the deposited layer and the dilution rate were studied. The main experimental results and conclusions were as follows:

References

(1) The volume fraction and the SDAS of the dendrite increase significantly with the increase of cladding layers. The coating L6 exhibits the largest volume ratio of the DR (about 53.19%). The SDAS of the coatings L1, L3, and L6 is calculated to be 2.40 mm, 2.67 mm, and 3.75 mm, respectively. (2) The dendritic phase in the coatings changed from g-Fe phase to g-(Fe, Ni) phase. The interdendritic phase transforms from the martensite phase to the M7C3 (M ¼ Fe, Cr) phase as the increase of the deposited layers. Also, the reduction of dilution rate is beneficial to the dissolution of Ni atoms in the gFe phase, which results in the decrease of the lattice parameter of the g-Fe phase. (3) The decrease of dilution rate is beneficial to the passivation behavior of coatings during the anodic polarization process. The coating L6 shows the best corrosion resistance with the smallest Icorr (2.22  107 A/cm2) and the most positive Ecorr (0.42 V) (4) The corrosion mechanism of Ni-based coating is closely related to its phase composition and microstructure. The corrosion form of coating L1 is severe intragranular corrosion, while that of the coatings L3 and L6 with the same phase composition are mainly pitting corrosion and intergranular corrosion. (5) The increase of Ni concentration in the multi-layered coating is instrumental in decreasing the concentration of oxygen vacancies containing in the passive film. Compared with the coating L1 and L3, the excellent corrosion resistance of the coating L6 is attributed to the high compactness and resistance of the passive film.

Declaration of competing interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. CRediT authorship contribution statement Jian Liu: Methodology, Formal analysis, Investigation, Writing original draft. Hao Liu: Conceptualization, Validation, Writing review & editing, Supervision, Funding acquisition. Xianhua Tian: Funding acquisition. Haifeng Yang: Resources. Jingbin Hao: Resources. Acknowledgements This work was supported by the National Natural Science Foundation of China (Grant No. 51905534), the Natural Science Foundation of Jiangsu Province (BK20170286), the China Postdoctoral Science Foundation (Grant No. 2015M581881), and the Priority Academic Program Development of Jiangsu Higher Education Institutions (PAPD). The authors thank the support of Advanced Analysis & Computation Center, China University of Mining and Technology for providing the X-ray diffraction (Bruker, D8 Advance) and the scanning electron microscope (FEI, Quanta TM

[1] X. He, R.G. Song, D.J. Kong, Microstructures and properties of Ni/TiC/La2O3 reinforced Al based composite coatings by laser cladding, Opt. Laser Technol. 117 (2019) 18e27. [2] Q. Chao, T.T. Guo, T. Jarvis, X.H. Wu, P. Hodgson, D. Fabijanic, Direct laser deposition cladding of AlxCoCrFeNi high entropy alloys on a hightemperature stainless steel, Surf. Coat. Technol. 332 (2017) 440e451. [3] J.Z. Lu, J. Cao, H.F. Lu, L.Y. Zhang, K.Y. Luo, Wear properties and microstructure analyses of Fe-based coatings with various WC contents on H13 die steel by laser cladding, Surf. Coat. Technol. 369 (2019) 228e237. [4] X. Li, C.H. Zhang, S. Zhang, C.L. Wu, Y. Liu, J.B. Zhang, M.B. Shahzad, Manufacturing of Ti3SiC2 lubricated Co-based coatings using laser cladding technology, Opt. Laser Technol. 114 (2019) 209e215. [5] J. Liu, H. Liu, P.J. Chen, J.B. Hao, Microstructural characterization and corrosion behaviour of AlCoCrFeNiTix high-entropy alloy coatings fabricated by laser cladding, Surf. Coat. Technol. 361 (2019) 63e74. [6] Y.Q. Jiang, J. Li, Y.F. Juan, Z.J. Lu, W.L. Jia, Evolution in microstructure and corrosion behavior of AlCoCrxFeNi high-entropy alloy coatings fabricated by laser cladding, J. Alloy. Comp. 775 (2019) 1e14. [7] F. Chang, B.J. Cai, C. Zhang, B.A. Huang, S. Li, P.Q. Dai, Thermal stability and oxidation resistance of FeCrxCoNiB high-entropy alloys by laser cladding, Surf. Coat. Technol. 359 (2019) 132e140. [8] X.F. Li, Y.H. Feng, B. Liu, D.H. Yi, X.H. Yang, W.D. Zhang, G. Chen, Y. Liu, P.K. Bai, Influence of NbC particles on the microstructure and mechanical properties of AlCoCrFeNi high-entropy alloy coatings prepared by laser cladding, J. Alloy. Comp. 788 (2019) 485e494. [9] S. Yang, W.J. Liu, M.L. Zhong, Z.J. Wang, H. Kokawa, Fabrication of in-situ synthesized TiC particles reinforced composite coating by power feeding laser cladding, J. Mater. Sci. 40 (2005) 2751e2754. [10] Q.S. Gao, H. Yao, Y. Qin, P.L. Zhang, J.L. Guo, Z.F. Chen, Z.S. Yu, Laser cladding Ti-Ni/TiN/TiWþTiS/WS2 self-lubricating wear resistance composite coating on Ti-6Al-4V alloy, Opt. Laser Technol. 113 (2019) 182e191. [11] R.A. Jeshvaghani, M. Jaberzadeh, H. Zohdi, M. Shamanian, Microstructure study and wear behavior of ductile iron surface alloy by Inconel 617, Mater. Des. 54 (2014) 491e497. [12] G.F. Sun, R. Zhou, P. Li, A.X. Feng, Y.K. Zhang, Laser surface alloying of C-B-WCr powers on nodular cast iron rolls, Surf. Coat. Technol. 205 (2011) 2747e2754. [13] Z. Lestan, M. Milfelner, J. Balic, M. Brezocnik, I. Karabegovic, Laser deposition of Metco 15E, Colmony 88 and VIM CRU 20 powders on cast iron and low carbon steel, Int. J. Adv. Manuf. Technol. 66 (2013) 2023e2028. [14] H. Liu, J.B. Hao, Z.T. Han, G. Yu, X.L. He, H.F. Yang, Microstructural evolution and bonding characteristic in multi-layer laser cladding of NiCoCr alloy on compacted graphite cast iron, J. Mater. Process. Technol. 232 (2016) 153e164. [15] F.Y. Shu, B.L. Zhang, T. Liu, S.H. Sui, Y.X. Liu, P. He, B. Liu, B.S. Xu, Effect of laser power on microstructure and properties of laser cladded CoCrBFeNiSi highentropy alloy amorphous coatings, Surf. Coat. Technol. 358 (2019) 667e675. [16] L.L. Zhai, C.Y. Ban, J.W. Zhang, X.Y. Yao, Characteristics of dilution and microstructure in laser cladding Ni-Cr-B-Si coating assisted by electromagnetic compound field, Mater. Lett. 243 (2019) 195e198. [17] L.L. Zhai, C.Y. Ban, J.W. Zhang, Microstructure, microhardness and corrosion resistance of NiCrBSi coatings under electromagnetic field auxiliary laser cladding, Surf. Coat. Technol. 358 (2019) 531e538. [18] J.C. Pereira, J.C. Zambrano, M.J. Tobar, A. Amigo, High temperature oxidation behavior of laser cladding MCrAlY coatings on austenitic stainless steel, Surf. Coat. Technol. 270 (2015) 243e248. [19] A. Feizabadi, M.S. Doolabi, S.K. Sadrnezhaad, M. Rezaei, Cyclic oxidation characteristics of HVOF thermal-sprayed NiCoCrAlY and CoNiCrAlY coatings at 1000 degrees C, J. Alloy. Comp. 746 (2018) 509e519. [20] J.Q. Yang, S.Z. Wang, Y.H. Li, X.Y. Tang, Y.Z. Wang, D.H. Xu, Y. Guo, Effect of salt deposit on corrosion behavior of Ni-based alloys and chock for stainless steels in supercritical water, J. Supercrit. Fluids 152 (2019). [21] E.K. Hao, Y.L. An, X.Q. Zhao, H.D. Zhou, J.M. Chen, NiCoCrAlYTa coatings on nickel-base superalloy substrate: deposition by high velocity oxy-fuel spraying as well as investigation of mechanical properties and wear resistance in relation to heat-treatment duration, Appl. Surf. Sci. 462 (2019) 194e206. [22] H.S. Tran, J.T. Tchuindjang, H. Paydas, A. Mertens, R.T. Jardin, L. Duchene, R. Carrus, J. Lecomte-Beckers, A.M. Habraken, 3D thermal finite element analysis of laser cladding processed Ti-6Al-4V part with microstructural correlations, Mater. Des. 128 (2017) 130e142. [23] H.M. Li, M.B. Li, X.P. Qin, S. Huang, F. Hong, Numerical simulation and experimental analysis of wide-beam laser cladding, Int. J. Adv. Manuf. Technol. 100 (2019) 237e249. [24] R.T. Jardin, J.T. Tchuindjang, L. Duchene, H.S. Tran, N. Hashem, R. Carrus, A. Mertens, A.M. Habraken, Thermal histories and microstructures in direct energy deposition of a high speed steel thick deposit, Mater. Lett. 236 (2019) 42e45. [25] C. Lu, W.L. Wang, J. Zeng, C.Y. Zhu, J. Chang, Effect of naturally deposited film on the sub-rapid solidification of medium manganese steel by using droplet solidification technique, Metall. Mater. Trans. B 20 (2019) 77e85. [26] X.W. Qiu, C.G. Liu, Microstructure and properties of Al2CrFeCoCuTiNix high-

J. Liu et al. / Journal of Alloys and Compounds 822 (2020) 153708

[27]

[28]

[29]

[30] [31]

entropy alloys prepared by laser cladding, J. Alloy. Comp. 553 (2013) 216e220. Y.Q. Jiang, J. Li, Y.F. Juan, Z.J. Lu, W.L. Jia, Evolution in microstructure and corrosion behavior of AlCoCrxFeNi high-entropy alloy coatings fabricated by laser cladding, J. Alloy. Comp. 775 (2019) 1e14. M. Hosseini, L. Fotouhi, A. Ehsani, M. Naseri, Enhancement of corrosion resistance of polypyrrole using metal oxide nanoparticles: potentiodynamic and electrochemical impedance spectroscopy study, J. Colloid Interface Sci. 505 (2017) 213e219. E. Kowsari, S.Y. Arman, M.H. Shahini, H. Zandi, A. Ehsani, R. Naderi, A. PourghasemiHanza, M. Mehdipour, In situ synthesis, electrochemical and quantum chemical analysis of an amino acid-derived ionic liquid inhibitor for corrosion protection of mild steel in 1M HCl solution, Corros. Sci. 112 (2016) 73e85. J.L. Lv, Effect of grain size on mechanical property and corrosion resistance of the Ni-based alloy 690, J. Mater. Sci. Technol. 34 (2018) 1685e1691. L.M. Zhang, S.D. Zhang, A.L. Ma, A.J. Umoh, H.X. Hu, Y.G. Zheng, B.J. Yang,

[32] [33]

[34]

[35]

[36]

11

J.Q. Wang, Influence of cerium content on the corrosion behavior of Al-Co-Ce amorphous alloy in 0.6 M NaCl solution, J. Mater. Sci. Technol. 35 (2019) 1378e1387. D.D. Macdonald, A. Sun, An electrochemical impedance spectroscopic study of the passive state on Alloy-22, Electrochim. Acta 51 (2006) 1767e1779. Y. Li, X.G. Zhang, Y.X. Cui, H.Y. Wang, J.X. Wang, Anti-corrosion enhancement of superhydrophobic coating utilizing oxygen vacancy modified potassium titanate whisker, Chem. Eng. J. 374 (2019) 1326e1336. J.L. Lv, W.L. Guo, T.X. Liang, The effect of pre-deformation on corrosion resistance of the passive film formed on 2205 duplex stainless steel, J. Alloy. Comp. 686 (2019) 176e183. J.L. Zhou, D.J. Kong, Effects of Ni addition on corrosion behaviors of laser cladded FeSiBNi coating in 3.5% NaCl solution, J. Alloy. Comp. 795 (2019) 416e425. H. Luo, Z.M. Li, A.M. Mingers, D. Raabe, Corrosion behavior of an equiatomic CoCrFeMnNi high-entropy alloy compared with 304 stainless steel in sulfuric acid solution, Corros. Sci. 134 (2018) 131e139.