reduced graphene oxide (rGO) nanocomposite coating

Diamond & Related Materials 101 (2020) 107655

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Enhanced protective properties of hydrothermally synthesized Ni(OH)2YSZ/reduced graphene oxide (rGO) nanocomposite coating

T

Delaram Salehzadeha, Zahra Sadeghianb, , Pirooz Marashia ⁎

a b

Department of Materials and Metallurgical Engineering, Amirkabir University of Technology, P.O. Box: 15875-4413, Tehran, Iran Research Institute of Petroleum Industry (RIPI), P.O. Box: 14857-3311, Tehran, Iran

ARTICLE INFO

ABSTRACT

Keywords: Ni(OH)2-YSZ rGO Electrophoretic Microstructure Polarization EIS

The hydrothermally synthesized Ni(OH)2-YSZ composite reinforced with reduced graphene oxide (rGO), was applied for corrosion protection of 316L stainless steel (SS) by electrophoretic deposition. The effect of rGO introduction on corrosion properties of Ni(OH)2-YSZ coating was investigated. The morphology and structures were characterized by X-ray diffraction (XRD) and field emission scanning electron microscopy (FESEM). The corrosion properties of specimens were determined by Tafel polarization and electrochemical impedance spectroscopy (EIS) analysis in 3.5 wt% NaCl corrosive solution. The results showed that the corrosion protection properties were remarkably improved by the introduction of rGO into Ni(OH)2-YSZ composite film. The Ni (OH)2-YSZ/rGO coated SS showed a higher charge transfer resistance (1.85 × 105 Ω cm2) compared to Ni(OH)2YSZ coated SS (6.12 × 104 Ω cm2) and bare 316L SS (4.55 × 104 Ω cm2). The addition of rGO to Ni(OH)2-YSZ composites efficaciously improved the dispersion of nanoparticles and also reduced their size. Consequently, the electrochemical enhancement caused by rGO introduction was attributed to the nano-sized particles, compaction, and uniformity of the coating, which efficiently prevented the steel substrate from corrosion attack.

1. Introduction Applying protective coatings can effectively improve the corrosion resistance of metal surfaces such as stainless steel bipolar plate in proton exchange membrane fuel cells (PEMFCs), which is one of instances susceptible to the corrosion issue [1–35]. Among the various surface engineering techniques, electrophoretic deposition (EPD) is known as a simple and cost-effective method for applying various coatings. EPD is a process that is usually carried out in a two-electrode cell. When direct current (DC) electric field is applied, particles become charged in the solvent and move toward the oppositely charged electrode and create a homogeneous film. The thickness and morphology of coatings could be optimized by applying different voltages and deposition times [2]. Introduction of some particles such as Ni(OH)2 [3], ZrO2 [4,5], Al2O3 [6], SiC [7] and NiO [8] and carbon materials (graphene [9], graphene oxide [10], CNT [11]) into the coating systems were experimented to effectively improve the corrosion protective performances. Among the ceramic materials, yttria-stabilized zirconia (YSZ) has received great attention in the industry due to its high mechanical strength and tribological properties as well as corrosion resistance and non-toxicity [12]. It is also found that homogeneous nickel hydroxide with a high surface area can be used for improvement in the ⁎

performance of protective coatings. Another attempt to improve the corrosion behavior came to a culmination with the notion of introducing graphene-based materials into the coating matrix which could act as a barrier against the attack of the corrosive environment [9]. Graphene oxide (GO), as a main derivative of graphene, possesses a great catalytic activity due to its numerous oxygen functional groups. GO's high reactivity makes it simple to react with other reagents. Thus, reduced graphene oxide (rGO) is an appropriate alternative owing to more specific surface area and the higher chemical stability [13,14]. Reduced graphene oxide (rGO) as an unconventional type of soft material has attracted great attention because of its superior physical and chemical stability properties. The effects of graphene or reduced graphene oxide (rGO) on improvement in corrosion resistance have been widely reported [15–18]. One of the most important methods for materials preparation is hydrothermal synthesis, which is a chemical reaction in the presence of a solvent above the room temperature and at pressures higher than 1 atm in a stainless steel autoclave. This method is used because of its soft chemical route for generating nano-sized crystalline powders with weakly bonded agglomerates [19]. In the present study, nickel ions first react with ammonia as alkaline and complexing agent, and then the formed complex decomposes and releases nickel ions. Nickel ions and

Corresponding author. E-mail address: [email protected] (Z. Sadeghian).

https://doi.org/10.1016/j.diamond.2019.107655 Received 1 September 2019; Received in revised form 2 November 2019; Accepted 1 December 2019 Available online 03 December 2019 0925-9635/ © 2019 Elsevier B.V. All rights reserved.

Diamond & Related Materials 101 (2020) 107655

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Table 1 Chemical composition (wt%) of AISI 316L stainless steel. Element

Cr

Ni

Mo

Mn

S

Si

C

P

Fe

wt%

16–18

10–14

2–3

2↓

0.03↓

0.75↓

0.03↓

0.045↓

Balance

Fig. 2. FTIR spectra of GO, rGO, and the Ni(OH)2-YSZ/rGO nanocomposite.

powders were synthesized using Zr(OC4H9)4 (Sigma Aldrich) and Y (NO3)3·6H2O (Merck) as the YSZ precursor, Ni(NO3)2·6H2O (Merck) as the Ni(OH)2 precursor and graphene oxide (GO) as the rGO precursor. GO was prepared from natural graphite powder by the modified Hummers' method [22]. GO sheets were dispersed in deionized water and homogenized by magnetic stirrer (MR Hei-Tec, Heidolph) for 2 h. Zr(OC4H9)4, Y(NO3)3·6H2O and Ni(NO3)2·6H2O were separately dispersed in ethanol as the solvent and then all three solutions were mixed together [23]. The weight ratio of added GO to Ni(OH)2-YSZ main precursors was considered to be 1:100. The mixed solution was added into the stirred aqueous solution of GO and was sonicated for 30 min. Ammonia solution 25% (Merck) was then added slowly as a reducing agent that increased the pH to 10 [24]. Finally, the mixture was transferred to a stainless steel autoclave. The hydrothermal reaction was carried out at 180 °C for 12 h. The separated precipitations were washed with acetone and deionized water repeatedly to ensure the removal of residual NH3 and other impurities. Centrifugation (ROTOFIX 32A, Hettich) was carried out at an angular velocity of 3000 rpm for 10 min. For the preparation of Ni(OH)2-YSZ nanocomposite powder, all the steps were exactly repeated except the addition of GO solution. Obtained powders were dried at 70 °C for 12 h.

Fig. 1. XRD patterns of rGO, GO, YSZ, Ni(OH)2-YSZ and Ni(OH)2-YSZ/rGO powders. Table 2 Corresponding planes and crystallites size using Scherrer formula. Powder

YSZ

Ni(OH)2-YSZ

Ni(OH)2-YSZ/rGO

Crystallite

YSZ

Ni(OH)2

YSZ

Ni(OH)2

YSZ

hkl Crystallite size (nm)

101 20

001 25

101 29

001 20

101 18

2.2. Thin-film preparation

sedimentation agents react at the molecular level and two-dimensional nickel hydroxide plates eventually form. This method is easy and does not require expensive raw materials and equipment [20,21]. The aim of this work is to investigate the effect of rGO introduction on morphology, microstructure and electrochemical properties of Ni(OH)2-YSZ coatings applied by electrophoretic deposition. The microstructure of powders was characterized by X-ray diffraction (XRD) analysis. Afterward, the films were deposited on 316L SS substrates by electrophoretic deposition. Field emission scanning electron microscopy (FESEM) was used to study the morphology of synthesized powders and deposited films. In addition, the effect of rGO introduction on corrosion resistance of the Ni(OH)2-YSZ coating was examined in 3.5 wt% NaCl aqueous solution using electrochemical impedance spectroscopy (EIS) and potentiodynamic polarization measurements.

AISI 316L austenitic stainless steel sheets with dimensions of 20 × 10 × 1 mm3 were used as substrates for the electrophoretic deposition. The chemical composition of 316L stainless steel is shown in Table 1 [25]. Before the coating process, steel substrates were polished using SiC abrasive papers (320–1000 grit) and ultrasonically degreased in acetone. Investigations have shown that there is no need to use any additives such as charging agents or dispersants while using acetylacetone as the EPD solvent [2]. The 10 g/L stable suspensions of Ni (OH)2-YSZ/rGO and Ni(OH)2-YSZ nanoparticles in acetylacetone were prepared by stirring for 2 h. Although generated particles and agglomerates through hydrothermal synthesis are weakly bonded, a 30 min sonication was carried out before the deposition process for breaking up the weak agglomerates and providing a homogeneous colloidal suspension. Two parallel steel electrodes were placed in the suspension with a distance of 1 cm. Depositions were performed at a constant voltage of 25 V and a deposition time of 1 min using a DC power supply (RXN-302D, Zhaoxin). The particles were attached to the negative electrode during the deposition process, consequently demonstrating the particles were positively charged during the cathodic EPD process. After drying at room temperature, the specimens were

2. Experimental details 2.1. Composite powder synthesis Ni(OH)2-YSZ (60:40 wt%) composite reinforced with reduced graphene oxide was prepared by the hydrothermal method. The composite 2

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Fig. 3. (a) Low and (b) high magnification FE-SEM images of synthesized Ni(OH)2-YSZ composite powder.

3. Results and discussion 3.1. Structural and surface morphological characterization The XRD patterns of rGO, GO, YSZ, Ni(OH)2-YSZ and Ni(OH)2-YSZ/ rGO powders are shown in Fig. 1. The XRD diffraction peaks of YSZ, Ni (OH)2-YSZ and Ni(OH)2-YSZ/rGO powders at approximately 30, 50 and 60° correspond to tetragonal YSZ. The crystallized β-Ni(OH)2 peaks are displayed at 19, 33 and 38°. Three strongest peaks of YSZ and Ni(OH)2 have been used for measurement of the average crystallites size which is calculated by Scherrer equation (Eq. (1)) [26]:

D=

K cos

(1)

where D is the crystallite size, K is the shape factor (K = 0.9), θ is the Bragg diffraction angle, λ is the wavelength of the incident radiation (λ = 1.54 Å) and β is the peak width at half-maximum intensity (FWHM). In XRD patterns of composite powders, peak shifts and intensity changes were observed compared to the standard pattern. Theoretically, the introduction of other atoms into a structure will lead to both peak shifts and intensity changes [27]. As can be seen in the XRD pattern of Ni(OH)2-YSZ composite powder, the YSZ peak at 30° has been shifted to 28°. XRD peak shifts are mostly related to strain which causes stress in the lattice. A peak shift to a lower angle demonstrates the extension of d-spacing in the out-of-plane direction, which illustrates that the lattice is under the compression [28]. It means that after adding Ni(OH)2, a strain has been created in the lattice. However, by the introduction of rGO into Ni(OH)2-YSZ, this YSZ peak shift is not observed anymore, representing the reduction of lattice strain. The XRD pattern of GO revealed a sharp peak at 10.8° corresponding to the (001) plane. For rGO, the peak at 24° corresponds to the (002) plane. However, no peak at angle 10.8° was seen in the XRD pattern of Ni(OH)2YSZ/rGO powder, which indicates that GO was reduced during the hydrothermal process. The crystallite size of YSZ in the (101) plane, is respectively 20, 29 and 18 nm for YSZ, Ni(OH)2-YSZ and Ni(OH)2-YSZ/ rGO powders. As can be seen in Table 2, the crystallite size of YSZ was increased after adding Ni(OH)2. The crystallites size of β-Ni(OH)2 with a hexagonal structure in the plane (001), are 25 and 20 nm for Ni(OH)2YSZ and Ni(OH)2-YSZ/rGO powders, respectively. FTIR spectra of the GO, rGO and Ni(OH)2-YSZ/rGO are depicted in Fig. 2. The reduction of GO to rGO and variations in the chemical compositions were evaluated by FTIR analysis. A broad peak at around 3420 cm-1 is observed in all three spectra, which corresponds to the adsorbed OeH stretching vibration. According to GO spectra, some peaks at 1621 cm-1 (skeletal vibration of unoxidized graphitic

Fig. 4. SEM images of graphene oxide (GO) (a) and reduced graphene oxide (rGO) obtained by hydrothermal synthesis (b).

dried at 120 °C for 2 h. 2.3. Characterization techniques The phase structures were analyzed by X-ray diffraction (Equinox 3000, Inel) in the 2θ range of 0–80° with Cu-Kα radiation (λ = 1.54 Å). Fourier transform infrared (FTIR) spectroscopy (Vertex 70, Bruker) was carried out in the 4000–500 cm−1 wavenumber at a resolution of 4 cm−1. The morphology of thin films was observed using field emission scanning electron microscopy (Mira 3 XMU, Tescan). The corrosion behavior of specimens was evaluated by potentiodynamic polarization and electrochemical impedance spectroscopy (EIS) (VersaSTAT 3, Princeton Applied Research) in 3.5 wt% NaCl solution. The samples were immersed in electrolyte for 30 min to reach a stable open circuit potential. Polarization tests were performed from −0.5 V to 1 V from OCP at a scan rate of 1 mV/s. The polarization resistance of specimens was determined by VersaStudio software. For the EIS test, the frequency range from 105 to 10-2 Hz and an amplitude of 10 mV were considered under open-circuit conditions.

3

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Fig. 5. FE-SEM image of synthesized Ni(OH)2-YSZ/rGO composite powder (a) and its corresponding elemental mapping images of C, Zr, Ni and Y (b–e).

Fig. 6. FE-SEM images of the surface of Ni(OH)2-YSZ (a) and Ni(OH)2-YSZ/rGO (b) composite coatings, cross-section FE-SEM images of Ni(OH)2-YSZ (c) and Ni (OH)2-YSZ/rGO (d) composite coatings.

domains), 3395 cm-1 (CeOH stretching vibrations), 1734 cm-1 (C]O stretching vibration of –COOH), and 1410 cm-1 (C]O stretching vibration of eCOOH and CeOH bending vibration) can be observed, and the peaks at 1227 cm-1 and 1052 cm-1 are assigned to the CeO vibration of epoxide groups. For rGO spectra, the peaks attributed to oxygencontaining groups have been weakened or disappeared, representing the incomplete reduction of GO [28]. The peak at 1052 cm-1, related to CeO vibration, has been shifted to 1155 cm-1. For Ni(OH)2-YSZ/rGO nanocomposite, the peak at 509 cm-1 and the sharp peak at 3635 cm-1

are assigned to the ZreO stretching vibration and OeH stretching vibration in the crystalline Ni(OH)2, respectively. The morphology of as-prepared Ni(OH)2-YSZ and Ni(OH)2-YSZ/rGO composite powders were evaluated by FE-SEM. The surface image of Ni (OH)2-YSZ powder (Fig. 3) reveals the presence of two different morphologies: Layer-structured Ni(OH)2 and spherical YSZ particles. Ni (OH)2 layers with the thickness of 60 nm in Ni(OH)2-YSZ powder (Fig. 3) would be interconnected with each other in the electrical field during EPD process and create disordered plates with a thickness range 4

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morphology area. The elemental mapping analysis of Ni(OH)2-YSZ/rGO for C, demonstrates the presence of rGO flakes as marked in the image (Fig. 5(b)). The cross-section FE-SEM images of Ni(OH)2-YSZ composite coating are shown in Fig. 6(c) possessing a rough surface and a thickness of 15 μm deposited on 316L substrate by electrophoretic technology. Strong adhesion between the Ni(OH)2-YSZ coating and the substrate was not obtained. Accordingly, this is one of the main reasons for the weaker corrosion resistance of this coating. Ni(OH)2-YSZ film consists of Ni(OH)2 exclusive rectangular plates [20] and spherical YSZ agglomerates in the size range of 1–2 μm [8]. β-Ni(OH)2 plates have a tendency to form a two-dimensional structure [29]. According to the cross-section FE-SEM image of the uniform Ni(OH)2-YSZ/rGO coating, a lower thickness of about 5 μm was observed, resulting from the compact structure and the lower porosity of the film (Fig. 6(b, d)). The presence of rGO in Ni(OH)2-YSZ/rGO film, inhibited the grain growth of crystals and resulted in finer grains in this coating [30]. These features will improve the anti-corrosion properties of the coating, which will be considered in the electrochemical studies.

Table 3 Fitting results of the Tafel polarization curves for bare 316L SS, Ni(OH)2-YSZ and Ni(OH)2-YSZ/rGO coated SS in 3.5 wt% NaCl solution.

Bare 316L SS Ni(OH)2-YSZ Ni(OH)2-YSZ/ rGO

Ecorr (V)

icorr (A cm-2)

βa (V dec-1)

βc (V dec-1)

Rp (Ω cm2)

−0.353 −0.320 −0.286

1.78 × 10-6 1.02 × 10-6 1.25 × 10-7

0.102 0.117 0.130

0.070 0.071 0.076

1.01 × 104 1.88 × 104 1.66 × 105

3.2. Electrochemical studies The corrosion current density (icorr) and corrosion potential (Ecorr) of bare 316L SS, Ni(OH)2-YSZ and Ni(OH)2-YSZ/rGO coated 316L SS were investigated by Tafel extrapolation method using VersaStudio software. The polarization resistance of specimens was calculated using the Stern-Geary equation [31]:

Rp = Fig. 7. Tafel polarization plots of Ni(OH)2-YSZ, Ni(OH)2-YSZ/rGO and bare 316L SS in 3.5 wt% NaCl solution at room temperature.

a c

2.303 × i corr (

a

+

c)

(2)

where Rp is the polarization resistance, icorr is the current density and βc and βa are the cathodic and anodic Tafel slopes. Both coated samples demonstrated a decrease of corrosion current compared to bare 316L SS. The corrosion resistance of bare 316L SS (1.01 × 104 Ω cm2) increased after deposition of Ni(OH)2-YSZ coating reaching a value of about 1.88 × 104 Ω cm2. The effect of rGO introduction on the corrosion resistance of Ni(OH)2-YSZ coated SS was investigated. The fitting data given in Table 3, showed that the corrosion current density of Ni (OH)2-YSZ coated SS with a value of 1.02 × 10-6 A cm-2 was decreased to 1.25 × 10-7 A cm-2 value for Ni(OH)2-YSZ/rGO coated SS. Qiu et al. [3] investigated Ni(OH)2-GO coatings and reported that the combination of Ni(OH)2 and GO in the composite coating can create a good barrier against the penetration of corrosive solution (such as the gas molecules, H2O, electrolyte ions, etc.). As shown in Fig. 7, the corrosion potential (Ecorr) of bare 316L SS and Ni(OH)2-YSZ were shifted to more positive potentials in Ni(OH)2-YSZ/rGO. From the results of the polarization curves, it can be concluded that rGO in the coating acts as a strong layer against ion penetration and corrosion process. The great surface area and high electrical conductivity of graphene influence advantageously on anti-corrosion features. Graphene increases the corrosion resistance of the coating by three main mechanisms. First, graphene could create a twisty pathway for the corrosive medium diffusion. Second, the excellent impermeability of graphene provides a supreme protective barrier against the corrosive materials. Third, the electrical conductivity of graphene is higher compared to steel. During the corrosion process in the metal-coating interface, the produced electrons by anodic reaction migrate from metal to a cathodic site. On the other hand, while graphene is introduced in the coating, another electron pathway would be substituted. Thus, reaching of the generated electrons to a cathodic site is not likely to happen [32]. The reduced corrosion current density and the positive shift of corrosion potential of Ni(OH)2-YSZ/rGO coated SS reveals its superior corrosion resistance. In addition, corrosion resistance of Ni(OH)2-YSZ/rGO coated sample (1.66 × 105 Ω cm2) was one order of magnitude higher than bare 316L SS. As a consequence, the film consisting of rGO is a protective layer which can increase the barrier effect of the coating and improve the

Fig. 8. The schematic representation of corrosion protection mechanism of Ni (OH)2-YSZ and Ni(OH)2-YSZ-rGO coatings.

Fig. 9. Equivalent circuit model for electrochemical impedance analysis.

of 0.1–1 μm and several tens micrometer length (Fig. 6(a)). The SEM image of ultrathin rGO flakes acquired by the chemical reduction of GO is presented in Fig. 4. The layer-structured rGO nanosheets consist of many wrinkles. Fig. 5 presents the FE-SEM image of Ni(OH)2-YSZ/rGO composite powder. As can be seen, some of Ni(OH)2 and YSZ particles in the size range of 50–60 nm anchored on the rGO due to its lager 5

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Table 4 EIS fitted values of bare 316L SS, Ni(OH)2-YSZ and Ni(OH)2-YSZ/rGO coated SS in 3.5 wt% NaCl solution. Rs (Ω cm2)

Y0 (F cm-2 sn-1) Bare 316L SS Ni(OH)2-YSZ Ni(OH)2-YSZ/rGO

11.81 54.19 41.21

Rpore (Ω cm2)

Qcoat

-4

1.34 × 10 6.75 × 10-5 3.59 × 10-5

Y0 (F cm-2 sn-1)

n 3

0.81 0.87 0.91

2.87 × 10 6.73 × 103 8.50 × 104

Rct (Ω cm2)

Qdl

-4

7.73 × 10 5.78 × 10-5 3.25 × 10-5

n 0.71 0.84 1

4.55 × 104 6.12 × 104 1.85 × 105

corrosion resistance. It is due to the lower defects of Ni(OH)2-YSZ-rGO coating, which was clearly observed in the SEM images of Ni(OH)2-YSZ and Ni(OH)2-YSZ-rGO coatings. Also, this improvement in corrosion resistance is due to the formation of twisty pathways for the corrosive medium diffusion within the coating contains rGO in comparison with the coating without rGO. The corrosion protection mechanism of Ni (OH)2-YSZ-rGO coating is illustrated by the schematic representation shown in Fig. 8. Electrochemical impedance spectroscopy is a potent tool to study the corrosion behavior of coated or uncoated metallic specimens. A proper equivalent circuit model was used to fit the spectra (Fig. 9), made up parameters including solution resistance (Rs), pore resistance (Rpore), charge transfer resistance (Rct), double layer capacitance (Qdl) and coating capacitance (Qcoat) [5]. These parameters were obtained using ZsimpWin software and the results are shown in Table 4. The use of constant phase element (CPE, Q) instead of the capacitance, results in a better fitting with minimum error [33]. The impedance of CPE is given as the formula below [34]:

ZCPE = 1/Y0 (j )n

(3)

where Y0 is admittance value at an angle (ω) of 1 rad/s and n (0 < n < 1) is index number. The n value < 1, indicates the presence of porosities and heterogeneities in the passive film. EIS results can be demonstrated by Nyquist and Bode plots (Fig. 10). As shown in Nyquist plots, the curves have similar shapes but different sizes. This is attributed to the same corrosion process in all samples, but the various effective area in each specimen [35]. In the phase Bode plots of all samples, a two-time constant have been seen. The peak at high frequencies expresses the actions happening through the pores and another peak at lower frequencies is related to the corrosion process. It is obvious from the data table that charge transfer resistance (Rct) of Ni (OH)2-YSZ/rGO coating was enhanced to 1.85 × 105 Ω cm2 in comparison with Ni(OH)2-YSZ coating (6.12 × 104 Ω cm2). The Cdl values express that the Ni(OH)2-YSZ film has more pits and voids. As a consequence of rGO introducing, Ni(OH)2-YSZ/rGO film with less porosity and higher Rct, provides better protection in NaCl solution compared to the film without rGO. 4. Conclusions Ni(OH)2-YSZ and Ni(OH)2-YSZ/rGO composite powders were synthesized by the hydrothermal method and were successfully deposited on 316L SS substrate by electrophoretic deposition. In order to improve the corrosion resistance of Ni(OH)2-YSZ coating, rGO was used. Uniform Ni(OH)2-YSZ/rGO coating revealed a lower thickness, demonstrating the compact structure and the lower porosity of the coating. Tafel polarization data of Ni(OH)2-YSZ/rGO coated SS represented a positive shift of corrosion potential and corrosion current density reduction in comparison with Ni(OH)2-YSZ. The corrosion resistance of Ni(OH)2-YSZ coated SS was about 1.88 × 104 Ω cm2 whereas for Ni(OH)2-YSZ/rGO it reached to 1.66 × 105 Ω cm2. From the Nyquist plots, it was observed that Ni(OH)2-YSZ/rGO coated SS had a larger arc with Rct value of 1.85 × 105 Ω cm2. Thus, we can consider Ni(OH)2-YSZ/rGO composite coating as a premiere protective film in 3.5 wt% NaCl solution. The nano-sized partic Table 1, 2, 3 and 4 les and its coating uniformity improve the anti-corrosion properties of this coating and the service life-time of the substrates.

Fig. 10. Nyquist (a) and Bode (b, c) plots of Ni(OH)2-YSZ, Ni(OH)2-YSZ/rGO and bare 316L SS in 3.5 wt% NaCl solution at room temperature. Symbols demonstrate measured data and solid lines are fitted data. 6

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Declaration of competing interest

• All authors have participated in (a) conception and design, or ana• •

[14]

lysis and interpretation of the data; (b) drafting the article or revising it critically for important intellectual content; and (c) approval of the final version. This manuscript has not been submitted to, nor is under review at, another journal or other publishing venue. The authors have no affiliation with any organization with a direct or indirect financial interest in the subject matter discussed in the manuscript.

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Acknowledgement

[19]

This study was financially supported by Amirkabir University of Technology.

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