Electrophoretic deposited Ni(OH)2-YSZ and NiO-YSZ nanocomposite coatings, microstructural and electrochemical evaluation

Journal Pre-proof Electrophoretic deposited Ni(OH)2-YSZ and NiO-YSZ nanocomposite coatings, microstructural and electrochemical evaluation

Delaram Salehzadeh, Pirooz Marashi, Zahra Sadeghian PII:

S0257-8972(19)31146-6

DOI:

https://doi.org/10.1016/j.surfcoat.2019.125155

Reference:

SCT 125155

To appear in:

Surface & Coatings Technology

Received date:

18 December 2018

Revised date:

19 September 2019

Accepted date:

9 November 2019

Please cite this article as: D. Salehzadeh, P. Marashi and Z. Sadeghian, Electrophoretic deposited Ni(OH)2-YSZ and NiO-YSZ nanocomposite coatings, microstructural and electrochemical evaluation, Surface & Coatings Technology (2018), https://doi.org/ 10.1016/j.surfcoat.2019.125155

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© 2018 Published by Elsevier.

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Electrophoretic deposited Ni(OH)2-YSZ and NiO-YSZ nanocomposite coatings, microstructural and electrochemical evaluation Delaram Salehzadeha, Pirooz Marashi*a, Zahra Sadeghianb Mining and Metallurgical Engineering, Amirkabir University of Technology, Tehran, Iran,

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a

Research Institute of Petroleum Industry (RIPI), Tehran, Iran, P.O. Box: 14857‐3311

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b

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P.O. Box: 15875-4413

*

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Corresponding author:

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Email: [email protected] Tel: +982164542910

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Abstract

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Fax: +982166405846

In this research, Ni(OH)2-YSZ and NiO-YSZ composite powders were synthesized with the mass ratio of 60%Ni-40%YSZ using the hydrothermal method. The crystalline structure of nanocomposite powders was examined by X-ray diffraction analysis (XRD). Nanocomposite powders were coated on 316L stainless steel by Electrophoretic deposition. The surface morphology of coatings was investigated using scanning electron microscope (SEM). NiO-YSZ coating provided a more uniform particle size distribution. The corrosion behavior of samples in 3.5 wt.% NaCl solution was determined by potentiodynamic polarization and electrochemical impedance spectroscopy (EIS) tests. The corrosion resistance of bare 316L

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Journal Pre-proof stainless steel, 1.01×104 Ω cm2, increased by applying Ni(OH)2-YSZ and NiO-YSZ coatings to about 1.88×104 Ω cm2 and 1.16×105 Ω cm2, respectively. From the Nyquist plots, NiO-YSZ coated 316L SS showed a larger arc compared to Ni(OH)2-YSZ coated SS. Thus, NiO-YSZ coating provided a more appreciable barrier in the corrosive medium and improved the corrosion resistance of 316L stainless steel.

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

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1. Introduction

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Corrosion is one of the most prevalent phenomena of passive metals, that generally occurs

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in aggressive environments, and can lead to significant economic costs and severe damages [1, 2]. Bipolar plate in proton exchange membrane fuel cell (PEMFC) applications

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is considered as one of the cases which intensely suffers from the corrosion issue. The ideal

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bipolar plate materials should own distinguished characteristics such as great corrosion resistance, supreme mechanical strength, and low cost and weight. In this regard, stainless steel (SS) was proposed by many researchers as an excellent candidate for its low gas permeability, high electrical and thermal conductivity and good strength [3-6]. Applying protective coatings on the metal surface is a suitable method for inhibition of the corrosion process. To enhance the corrosion properties of bare stainless steel, different surface treatments have been suggested for the fabrication of ceramic coatings, such as sol-gel, spin coating, dip coating and electrophoretic deposition (EPD) [7-11]. Electrophoretic deposition (EPD) is considered as a versatile, simple and cost-effective method for applying various coatings. In the EPD process, particles become charged in a solvent in the presence of a direct

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Journal Pre-proof current (DC) electrical field. The thickness and morphology of films can be controlled by applying different voltages and deposition times [12-15]. Many composite coatings including the particles such as ZrO 2 [16, 17], Al2O3 [18], SiC [19, 20], NiO [21], Ni(OH)2 [1] have been introduced for metal corrosion protection. The presence of reinforcing particles in the matrix especially in the grain boundaries, which are the pathways for transmitting the corrosive materials, is effective in blocking the diffusion

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of corrosive medium and improves the corrosion resistance [22]. Based on the findings of

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Fabijanic et al. [23], the reaction rate of hydrogen formation in defects, such as grain

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boundaries and dislocations, is the main reason for accelerating cathodic reaction kinetics. The influence of yttria-stabilized zirconia (YSZ) films on the electrochemical behavior of

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316L SS in NaCl solution was investigated by Bačić et al. [17]. Their results revealed that

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YSZ coated samples possess lower corrosion current density and higher corrosion resistance

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in comparison with bare 316L SS. Nickel oxide is considered as a suitable material for anticorrosion applications [24]. It is known as an effective material in the industry for preventing

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oxidation of metals surface. Using stable and inactive hydroxides such as Ni(OH)2 is another appropriate option for protection of metals against corrosion. Homogeneous nickel hydroxide powder with a high surface area can be applied for better performance of protective coatings. Several methods have been developed for the preparation of this material. The most important method is the precipitation from a solution. In one of the most effective precipitation methods, nickel ions first react with ammonia as alkaline and complexing agent, and then by changing the physical conditions of the solution, such as concentration or temperature, the complex decomposes and releases nickel ions. Since nickel ions and sedimentation agent are homogeneously distributed in the solution, they can react at the molecular level and, as a result, nickel hydroxide particles are formed in nanoscale 3

Journal Pre-proof dimensions. According to previous studies [25, 26], Ni(OH)2 tends to form a two-dimensional layer structure. This method is easy and does not require expensive raw materials and equipment. It is capable of industrialization and can be extended to produce other oxide and hydroxide nanocrystals [27]. The major focus of this study is the evaluation to assess the possibility of performing the Ni(OH)2-YSZ and NiO-YSZ composite coatings, for enhancing the corrosion resistance of 316L SS. The composite powders were

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synthesized by hydrothermal method and subsequently coated by electrophoretic deposition.

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The phase structure of powders was characterized using X-ray diffraction (XRD) analysis.

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Scanning electron microscopy (SEM) was used for evaluating the morphology of synthesized

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powders and deposited films. The corrosion behavior of bare 316L SS substrate with and without the coatings was examined in 3.5 wt.% NaCl aqueous solution by potentiodynamic

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2. Experimental procedure

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polarization and electrochemical impedance spectroscopy (EIS) measurements.

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2.1. Composite Powders Synthesis

Ni(OH)2-YSZ composite powders with the mass ratio of 60%Ni-40%YSZ, were synthesized using stoichiometric amounts of Zr(OC4H9)4 (Sigma Aldrich), Y(NO3)3 6H2O (Merck) and Ni(NO3)2 6H2O (Merck) as precursors and (CH3)2CHOH as the solvent [28]. Then, it was homogenized for 2 h by a magnetic stirrer (MR Hei-Tec, Heidolph) and subsequently subjected to sonication for 30 min. For precipitation, the ammonia solution (25%, Merck) was added to increase the pH to 10 [29]. The solution was poured into the autoclave and placed in an oven, heating to 180 °C for 12 hours. The chemical reaction or the separation process was occurred in hydrothermal conditions [30-32]. Eventually, it reached the over saturation range and new compounds began to precipitate. 4

Journal Pre-proof After the hydrothermal treatment, the solution was transferred to the tubes and was placed in a centrifuge (ROTOFIX 32A, Hettich). Centrifugation was performed with ethanol and deionized water at an angular velocity of 3000 rpm for 10 min. The separated precipitates were washed by acetone. This process was repeated 2 more times to ensure the removal of residual NH3 and other impurities. The powders were then dried in an oven at 70 °C for 12 hours. For the preparation of NiO-YSZ composite powders, the Ni(OH)2-YSZ production process was exactly repeated, then the final powders were calcined at 1000 °C for 2 hours. Finally, nickel hydroxide was transformed to nickel oxide and NiO-YSZ composite powders

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were obtained. Schematic of the hydrothermal synthesis of composite powders is illustrated in

Ni

Mo

Mn

wt. %

16-18

10-14

2-3

2↓

S

Si

C

P

Fe

0.03↓

0.75↓

0.03↓

0.045↓

Balance

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Cr

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Element

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

YSZ

Crystallite

YSZ

Ni(OH)2-YSZ YSZ

NiO

YSZ

001

101

200

101

25

29

29

32

101

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hkl Crystallite size (nm)

NiO-YSZ

Ni(OH)2

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Powder

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Table 2. Corresponding planes and crystallites size using Scherrer formula.

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

icorr (A cm-2)

βa (V dec-1)

Bare 316L SS

-0.353

1.78×10-6

0.102

0.070

1.01×104

Ni(OH)2-YSZ

-0.320

1.02×10-6

0.117

0.071

1.88×104

NiO-YSZ

-0.278

1.72×10-7

0.125

0.073

1.16×105

5

βc (V dec-1)

Rp (Ω cm2)

Journal Pre-proof Table 4. EIS fitted values of bare 316L SS, Ni(OH)2-YSZ and NiO-YSZ coated SS in 3.5 wt.% NaCl solution. Qcoat Y0

Rpore

Y0

(Ω cm2)

(F cm-2 sn-1)

(Ω cm2)

(F cm-2 sn-1)

Bare 316L SS

11.81

1.34×10-4

0.81

2.87×103

7.73×0-4

0.71

4.55×104

Ni(OH)2-YSZ

54.19

6.75×10-5

0.87

6.73×103

5.78×10-5

0.84

6.12×104

NiO-YSZ

54.57

6.24×10-5

0.89

4.76×104

4.47×10-5

0.67

1.66×105

n

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Rs

Qdl

6

n

Rct (Ω cm2)

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After calcination process, YSZ and NiO phases obtained a uniform distribution that could actually improve the corrosion protection of the coated film on the 316L stainless steel.

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No distinguishable crack was observed for deposition time of 1 min and applied voltage of 25 V.

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Tafel polarization data of NiO-YSZ coated SS represented reduced corrosion current density and the positive shift of corrosion potential which revealed its better corrosion

From the Nyquist plots, it was observed that NiO-YSZ coating had a larger arc with

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-

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resistance with the value of 1.16×105 Ω cm2.

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Rct value of 1.66×105 Ω cm2 in comparison with uncoated and Ni(OH)2-YSZ coated SS, indicating that it obviously provided a better protective layer in 3.5

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wt.% NaCl solution as the corrosion medium.

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2.2. Thin film preparation

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. 1.

Specimens (20×10×1 mm3) were cut from 316L stainless steel sheet as substrates for the

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electrophoretic deposition. Table 1 shows the elemental composition of AISI 316L stainless steel samples [5]. Before the coating process, the samples were polished using 200 grit SiC abrasive paper followed by 320, 600, 800 and 1000 grit SiC papers and degreased by acetone in an ultrasonic cleaner. For the preparation of the electrophoretic suspension with a concentration of 10 g/L, each composite powder was dissolved in acetylacetone [33]. Researches have indicated that there is no necessity to use any additives such as charging agents or dispersants when utilizing acetylacetone as the EPD solvent [34]. The suspension was stirred for 2 hours and then homogenized by ultrasonic bath for 15 min. Two 316L SS as the working and counter electrodes were placed parallel to each other with a distance of 1 cm.

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Journal Pre-proof Electrophoretic depositions were carried out using a DC power supply (RXN-302D, Zhaoxin) under applied voltages of 15, 25, 50, 100 and 200 V for 1 min. As seen in Fig. 2, some obvious pinholes existed in the sample coated under an applied voltage of 15 V. In the case of the sample coated under voltage of 50 V, the coating was very fragile and easily chipped from the edge of the substrate. For 100 V sample, the chipping from the surface and edge of the substrate was aggravated and in the 200 V sample, the cracks went all the way through

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the coating, which the crack paths were noticeably observed. Accordingly, the optimized

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voltage was considered as 25 V, which no pore or crack sign was detected in the related

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sample. After electrophoretic deposition, the resulting films were dried at room temperature

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for 24 hours and eventually heated to 120 °C for 2 hours to remove the solvents present in the

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2.3. Characterization techniques

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coatings.

The phase identification of synthesized powders was analyzed by X-ray diffraction (Cu-Kα

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radiation, wavelength λ = 1.54 Å) (Equinox 3000, Inel). The diffraction patterns were recorded in the 2θ range from 10 ° to 90 ° and were compared with standard patterns collected in the ICDD PDF2 database. The crystallite size of phases was calculated using Scherer equation. The surface morphology and microstructures were characterized using scanning electron microscopy (AIS2100, Seron Technology). The corrosion behavior of specimens was investigated by potentiodynamic polarization and electrochemical impedance spectroscopy (EIS) (VersaSTAT 3, Princeton Applied Research) instruments at room temperature. The specimens with an area of 1 cm2 were immersed in 3.5 wt.% NaCl solution for 30 min to reach a stable OCP (Open-Circuit Potential). The range of potential was -0.5 to 1 V from OCP. The frequency range of 105-10-2 Hz at a scan rate of 1 mV/s and amplitude of 8

Journal Pre-proof 10 mV were considered under open-circuit conditions. The polarization resistance was calculated using the Stern-Geary equation [35]. 3. Results and discussion 3.1. Structural and surface morphological characterization The Ammonia could act as a coordination agent for Ni2+ ions to form nickel hexammine

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solution. Then, the complex was dissociated. After reaching the free nickel ions to a distinct

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amount, β-Ni(OH)2 precipitates will form. The reactions are shown below [27]:

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2+ [Ni(NH3 )6 ]2+ (aq) → Ni(aq) + 6NH3

(2)

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− Ni2+ (aq) + 2OH(aq) → Ni(OH)2 (s)

(1)

follows:

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According to Liang et al. [36], the dehydroxylation reaction of the hydroxide layers is as

(3)

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Ni(OH)2 → NiO + H2 O

The XRD patterns of YSZ, Ni(OH)2-YSZ, and NiO-YSZ powders are presented in Fig. 3.

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Additionally, the Miller indices and peak positions for YSZ, Ni(OH) 2 and NiO reference samples are included for comparison. The diffraction result of YSZ powder confirmed the formation of the YSZ tetragonal phase in (101), (110), (112), (200) and (211) planes, which are in a good agreement with standard XRD data of ICDD#00-048-0224. After calcination of Ni(OH)2-YSZ powder, diffraction peaks corresponding to (001), (101) and (102) planes of βNi(OH)2 were disappeared and subsequently high intensity NiO peaks at (111), (200), (220), (311) and (222) planes were recorded. All the diffraction peaks of Ni(OH) 2 and NiO are indexed to the hexagonal nickel hydroxide and cubic nickel oxide and match very well to the standard XRD data of ICDD#00-014-0117 and ICDD#00-047-1049, respectively. It is also

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Journal Pre-proof worthy to mention that a strong increase in the intensity of the YSZ peaks occurred arising from the calcination process. The narrower reflection peaks of NiO-YSZ composite indicate the higher degree of crystallinity in comparison with of Ni(OH)2-YSZ composite. It also could be attributed to the increase in the crystallites size after calcination of powder. The strongest reflections of YSZ, Ni(OH)2 and NiO have been considered to measure the crystallites size which was calculated using Scherer equation (Eq. 4) [37]: Kλ

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(4)

βcosθ

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D=

which is made up of parameters including Bragg diffraction angle (θ), the crystallite size (D),

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shape factor (K=0.9), the wavelength of x-ray (λ=1.54 Ǻ) and FWHM (β). Corresponding

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planes and crystallites size are given in Table 2. The crystallite size of phases in

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Ni(OH)2-YSZ composite powders were identified from the tetragonal YSZ (101) and hexagonal Ni(OH)2 (001). Although tetragonal zirconia is the stable phase at high

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temperature, this structure was observed after completion of the low-temperature

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hydrothermal method. The decrease in the phase transformation temperature could be caused by several factors. Reduction of grain size and also the compressive stresses applied during the hydrothermal reaction, provide the stability of the high-density phases at room temperature [38]. For NiO-YSZ composite powders, the crystallites size was calculated from the tetragonal YSZ (101) and cubic NiO (200). Crystallites size of Ni(OH)2 and YSZ in Ni(OH)2-YSZ composite powders were assessed to be 25 nm and 29 nm. However, the crystallite size of NiO and YSZ in NiO-YSZ powders were 29 nm and 32 nm, respectively. This crystallite size growth is attributed to the calcination process [36]. The morphology of as-prepared Ni(OH)2-YSZ and NiO-YSZ composite powders are shown in Fig. 4. Ni(OH)2-YSZ composite coating containing a heterogeneous particle size 10

Journal Pre-proof distribution, consists of two different types of phases: spherical particles in the size range of 1-2 μm, are attributed to the YSZ [21] and exclusive rectangular plates correspond to Ni(OH)2 with a thickness range of 0.1-1 μm [25]. β-Ni(OH)2 tends to form a two-dimensional structure [36]. Clearly, a large quantity of YSZ particles was anchored onto Ni(OH) 2 plates )Fig. 4a). NiO-YSZ particles revealed a slight tendency for agglomeration )Fig. 4b). Fig. 5a-c shows the SEM images of Ni(OH)2-YSZ composite film with a rough surface deposited on

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316L SS surface by electrophoretic technology. Ni(OH) 2-YSZ composite coating containing

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heterogeneous particle size distribution, consists of two different types of phases. Spherical

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particles in the size range of 1-2 μm are attributed to the YSZ [21] and exclusive rectangular

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plates correspond to Ni(OH)2 with a thickness range of 0.1-1 μm [25]. During deposition process and exposing of samples to the electrical field, Ni(OH) 2 plates in Ni(OH)2-YSZ

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powder would be interconnected with each other and arrange disordered plates with the

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mentioned thickness range and length of several tens micrometer. As shown in Fig. 5a and d, the thickness of Ni(OH)2-YSZ and NiO-YSZ coatings are approximately 15 and 5 μm,

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respectively. It indicates the higher deposition tendency of Ni(OH) 2-YSZ powders at the same applied voltage and deposition time, resulted in creating a rougher coating. There is a difference between the particle size in the SEM image and crystallite size determined from the XRD pattern. In XRD characterization, the crystallite size can be measured. However, the whole particle is observed in the SEM image [36, 39, 40]. As shown in Fig. 5e, NiO and YSZ particles are considerably uniform and well-distributed. The surface morphology of NiO-YSZ powders shows potato sprouts like the structure of mixed particles of YSZ and NiO [21]. YSZ and NiO phases which possess uniform distribution, inhibit each other's grain growth, therefore, it favorably affects the particles size reduction. These features

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Journal Pre-proof improve the anti-corrosion ability of the film which will be discussed in the following section. 3.2. Electrochemical studies The corrosion behaviors of bare 316L SS, Ni(OH)2-YSZ and NiO-YSZ coated 316L SS were investigated using potentiodynamic tests in 3.5 wt.% NaCl solution, which evaluate the

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current as a function of the potential. The polarization curves shown in Fig. 6, represented

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active, passive and trans-passive states of corrosion. The corrosion current density (icorr) and

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corrosion potential (Ecorr) were determined by Tafel extrapolation method using VersaStudio software. The polarization resistance of coatings was calculated using the Stern-Geary

βa βc

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Rp =

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equation [35]:

2.303×icorr (βa +βc )

(5)

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where Rp is the polarization resistance, icorr is the current density and βc and βa are the

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cathodic and anodic Tafel slopes, respectively. As exhibited in Fig. 6, both coated specimens showed a decrease of corrosion current compared to bare 316L SS. The corrosion resistance of bare 316L SS which equals to 1.01×104 Ω cm2 was increased after deposition of Ni(OH)2-YSZ film reaching a value of about 1.88×104 Ω cm2. After calcination of Ni(OH)2-YSZ powders, the Tafel polarization curve of NiO-YSZ coated SS showed a positive shift of corrosion potential. The fitting data, given in Table 3, indicates a decrement in corrosion current density from 1.78×10-6 A cm-2 for bare 316L SS to 1.02×10-6 A cm-2 for Ni(OH)2-YSZ coated SS. It also should be mentioned that in comparison with NiO-YSZ coating, a higher current density was observed for Ni(OH)2-YSZ coating. According to SEM images of the films (Fig. 5),

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Journal Pre-proof Ni(OH)2-YSZ coating possesses a higher inhomogeneity and there are so many holes around the particles, degrading the corrosion resistance of the coating. Another reason could be attributed to the defective sites, which may be generated on the film due to its inhomogeneity and

higher

thickness.

Obviously,

corrosion

resistance

of NiO-YSZ

coated SS

(1.16×105 Ω cm2) was one order of magnitude higher compared to bare 316L SS. From the results of the polarization curves, it can be deduced that NiO-YSZ film acts as a strong layer

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against ion penetration and corrosion process. At last, the positive shift of corrosion potential

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together with the reduced corrosion current density of NiO-YSZ composite coating reveals its

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better corrosion resistance.

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Electrochemical Impedance Spectroscopy (EIS) was applied to evaluate the corrosion resistance of 316L SS with and without coatings. The appropriate equivalent circuit was

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considered (Fig. 7), which is made up of elements including solution resistance (Rs), charge

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transfer resistance (Rct), pore resistance of coating (Rpore), double layer capacitance (Qdl) and coating capacitance (Qcoat) [17]. Equivalent circuit parameters were fitted using ZsimpWin

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software and the results are given in Table 4. To better fit the experimental data, the capacitance was replaced by the constant phase element (CPE, Q) [41]. The impedance of the constant phase element is given as the equation below [42, 43]: 𝑍𝐶𝑃𝐸 = 1/𝑌0 (𝑗𝜔)𝑛

(6)

where the element Y0 is admittance numerical value at an angle (ω) of 1 rad/s and n (0
The qualitative data can be swiftly

extracted by the visual exploration of the Nyquist plots. As can be seen from Nyquist plots in Fig. 8a, the curves have resembling shapes but various sizes. This is ascribed to the same 13

Journal Pre-proof corrosion process in all specimens, but the diverse effective area. The corrosion behavior of metal substrate and also characteristics of the film in corrosive electrolyte could be illustrated by the time constant. In the Phase Bode plot of Ni(OH)2-YSZ coated SS (Fig. 8b, c) two-time constant appeared. The peak at higher frequencies is related to the processes happening through pores, that could be explained by the coating capacitance and pore resistance of the coating. The second peak at lower frequencies is relevant to the corrosion process and could

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be explained by the double-layer capacitance and charge transfer resistance [17].

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Although two constant phase elements exist in the equivalent circuit of NiO YSZ coated

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SS, one peak was observed in the Phase Bode plot. The reason is two overlapping peaks

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which were merged and appeared as one peak. It is attributed to the great barrier properties of this coating and premier corrosion resistance property. The Nyquist plots indicate that

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NiO-YSZ coated SS has a higher curvature radius and subsequently a higher corrosion

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resistance than Ni(OH)2-YSZ coated SS, as evident from comparison of the arcs with charge transfer resistance (Rct) values of 6.12×104 and 1.66×105 Ω cm2 for Ni(OH)2-YSZ and

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NiO-YSZ coated SS, respectively.

There are some reasons responsible for the superior corrosion resistance properties of NiOYSZ coated SS. As obvious from the SEM images of NiO-YSZ powder and coating (Fig. 4b and 5e), the monotonous distribution of NiO and YSZ particles in the powder results in a uniform coating, which acts as a potent barrier against the permeation of corrosive electrolyte to the metal substrate. Moreover, the low porosity level of the film enhances its density and creates a more compact film. This hinders the diffusion of electrolyte into the coating, resulting in a lower deterioration of the coating. The relevance between capacitance values and the coating's morphology is noticed as another remarkable concept. Higher capacitance value illustrates the higher active surface sites of the coating. A comparison between the Cdl 14

Journal Pre-proof values, indicates that the Ni(OH)2-YSZ film has more active surface sites and thereupon more pits and voids, which was clearly displayed in SEM images of the films. As a consequence, NiO-YSZ film with less porosity and defects and also a higher R ct, provided better protection in NaCl solution compared to the Ni(OH) 2-YSZ film. The fitted results of EIS test were properly in agreement with the results obtained from the Tafel polarization technique.

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4. Conclusions

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Ni(OH)2-YSZ and NiO-YSZ composites were synthesized by hydrothermal process and

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were successfully deposited on 316L SS substrate by electrophoretic deposition. After the

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calcination process, Ni(OH)2 with the hexagonal structure was decomposed to cubic NiO and

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also the XRD intensity of tetragonal YSZ peaks was increased. According to SEM images, Ni(OH)2-YSZ nanocomposite powder with a heterogeneous particle size distribution, was

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made up of two types of particles: spherical YSZ particles in the size range of 1-2 μm and

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exclusive rectangular plates of Ni(OH)2 with a thickness range of 0.1-1 μm. After calcination process, YSZ and NiO phases obtained a uniform distribution that could actually improve the corrosion protection of the film. The thickness of Ni(OH)2-YSZ and NiO-YSZ coating were nearly 15 and 5 μm. It indicated the higher deposition rate of Ni(OH) 2-YSZ powder at the same applied voltage and deposition time, resulted in creating a rougher coating. No distinguishable crack was observed for the deposition time of 1 min and applied voltage of 25 V. The higher applied voltages resulted in the creation of some pores or cracks on the surface of the coating. Tafel polarization data of NiO-YSZ coated SS represented less corrosion current density and the positive shift of corrosion potential revealed its better corrosion resistance. The

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Journal Pre-proof corrosion resistance of bare 316L SS which equals to 1.01×104 Ω cm2 was increased after deposition of Ni(OH)2-YSZ coating reaching a value of about 1.88×104 Ω cm2 and the polarization resistance of NiO-YSZ coated SS which equals to 1.16×105 Ω cm2, was increased one order of magnitude compared to that of 316L SS. From the Nyquist plots, it was observed that NiO-YSZ coated SS has a larger arc in comparison with the Ni(OH)2-YSZ coated SS, indicating that it obviously provided a better protective layer in 3.5 wt.% NaCl

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solution as the corrosion medium. The uniform distribution of NiO and YSZ particles and

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also the lower defective sites, lead to the formation of a more monotonous coating which

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could act as a strong blocking layer against the permeation of corrosive electrolyte and

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enhance the life-time of the stainless steel substrate.

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Acknowledgement

References

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This study was financially supported by Amirkabir University of Technology.

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Fig. 1. Schematic representation of hydrothermal synthesis of Ni(OH) 2-YSZ and NiO-YSZ

Fig. 2. Coated samples fabricated by electrophoretic deposition under applied voltages of 15 (a), 25 (b), 50 (c), 100 (d) and 200 V for 1 min.

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Fig. 3. XRD patterns of YSZ, Ni(OH)2-YSZ and NiO-YSZ powders. The Miller indices and peak positions for the tetragonal YSZ, hexagonal Ni(OH) 2 and cubic NiO reference samples are included for comparison of hkl values of the composites.

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Fig. 4. SEM images of the (a) Ni(OH)2-YSZ and (b) NiO-YSZ composite powders synthesized by hydrothermal method.

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Fig. 5. SEM images of the (a-c) Ni(OH)2-YSZ and (d-e) NiO-YSZ composite coatings.

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Fig. 6. Tafel polarization curves of Ni(OH)2-YSZ coated SS (a), NiO-YSZ coated SS (b) and

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bare 316L SS (c) in 3.5 wt.% NaCl solution at room temperature.

Fig. 7. Equivalent circuit model for electrochemical impedance analysis.

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Fig. 8. Nyquist (a) and Bode (b, c) plots of Ni(OH) 2-YSZ, NiO-YSZ and bare 316L SS in 3.5 wt.% NaCl solution at room temperature. Symbols demonstrate measured data and solid lines are fitted data. 25

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