Microstructural evolution and corrosion resistance of

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Surface & Coatings Technology 283 (2015) 337–346

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Microstructural evolution and corrosion resistance of super-hydrophobic electrodeposited nickel films S. Esmailzadeh, S. Khorsand ⁎, K. Raeissi, F. Ashrafizadeh Department of Materials Engineering, Isfahan University of Technology, Isfahan 84156-83111, Iran

a r t i c l e

i n f o

Article history: Received 9 September 2015 Revised 29 October 2015 Accepted in revised form 2 November 2015 Available online 9 November 2015 Keywords: Superhydrophobic Electrodeposition Nickel Surface morphology Hierarchical Corrosion resistance

a b s t r a c t Superhydrophobic coatings have become a hot research topic in recent years due to their excellent properties and wide practical applications. In the present work, hierarchical nickel films having intrinsic super-hydrophilic property were fabricated on copper substrate by two-step electrodeposition process. The surface structure and composition were characterized by means of scanning electron microscopy (SEM), X-ray diffraction pattern (XRD) and atomic force microscopy (AFM). The contact angle of water droplets on the hierarchical structure of nickel films increased over time, eventually becoming large enough to classify the surface as superhydrophobic. The surface morphology of nickel film at micro/nano scale was characterized at different deposition current densities (10–70 mA cm−2). Results showed that the value of roughness and the size of micro/nano cones were decreased by increasing the current density. The nickel film deposited at 20 mA cm−2 displayed the highest superhydrophobicity with water contact angle of 155°, which could be attributed to its pine conelike structure. Electrochemical measurements and long-term immersion test showed that the superhydrophobic nickel films greatly enhanced the corrosion resistance of copper substrate in neutral 3.5 wt.% NaCl solution. The corrosion protection of superhydrophobic films was attributed to small area of real contact with the aggressive solution. © 2015 Elsevier B.V. All rights reserved.

1. Introduction Wettability, as an important aspect of a surface chemistry, is defined by a contact angle of a droplet on a solid surface [1,2]. In recent years, superhydrophobic surface has received much attention due to their unique properties, such as anti-snow adhesion [3], corrosion protection [4] and self-cleaning [5]. It can also be employed for low flow resistance coatings in micro fluidic systems [6]. It has been found that the high roughness with low surface energy of a solid surface leads to apparent water contact angles higher than 150° [7,8]. In fact, the low surface energy is the major factor and the roughness just serves to enhance water contact angle. By increasing the roughness, the surface area of the solid is increased and thus the surface energy is decreased [9]. Techniques to make superhydrophobic surfaces can be simply divided into two categories: making a rough surface from a low surface energy material or modifying a rough surface with a material of low surface energy. Many methods have been developed to produce rough surfaces, including plasma etching [10], anodic oxidization [11], chemical vapor deposition [12], sol–gel method [13], phase separation [14] and electrodeposition [15]. Most of these methods involved multi-step procedures and harsh conditions, or require specialized reagents and

⁎ Corresponding author. E-mail address: [email protected] (S. Khorsand).

http://dx.doi.org/10.1016/j.surfcoat.2015.11.005 0257-8972/© 2015 Elsevier B.V. All rights reserved.

equipment [8,16]. In contrast, electrodeposition has been used as a one step, simple and economic method to fabricate superhydrophobic surface on different substrates [6,8]. Copper has properties such as low cost, high thermal and electrical conductivity, but it is corroded easily. So, creating a superhydrophobic surface is one of the methods used for modifying its surface [17]. Xi et al. [15] prepared a superhydrophobic surface on hydrophilic copper substrate via electroplating at large current density to produce various degrees of roughness without chemical modifications. Xu et al. [18] improved the corrosion resistance of copper by fabricating a suprehydrophobic composite coating. Up to now, a few papers have been published on superhydrophobic nickel films produced on copper substrate by electrodeposition without applying low surface energy materials. These researches have usually studied the morphology and wetting behavior of the coatings. However, there is no systematic study on the effect of deposition current density on physical properties, wetting behavior and corrosion resistance of super-hydrophobic nickel films. In the present work, fabrication of superhydrophobic nickel film with micro-nano cone array on copper substrate was done only by electrodeposition without applying any low surface energy materials. Surface morphology, super-hydrophobicity characteristics and corrosion resistance of nickel films electrodeposited at different current densities were investigated by scanning electron microscopy, water contact angle measurement and the electrochemical technique.

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Table 1 Operating conditions for electrodeposition of nickel films. Specimen Micro-nickel film

Miro-nano nickel film

Deposition— step 1

Deposition— step 1

Deposition— step 2

Current density (mA cm−2)

Time (s)

Current density (mA cm−2)

Time (s)

Current density (mA cm−2)

Time (s)

10 20 30 50 70

1200 600 400 240 171

10 20 30 50 70

1200 600 400 240 171

50 50 50 50 50

60 60 60 60 60

2. Experimental procedure Nickel coatings were prepared by electrodeposition on the copper substrate. Before plating, copper substrates in disk shape with a surface area of 1.53 cm2 were mechanically polished down to 2400 grit size using abrasive SiC papers and then polished with 0.05 μm alumina. The specimens were cleaned ultrasonically in ethanol for 30 min, electropolished at 20 mA cm− 2 for 1 min in a solution containing 70 g L−1 Na2CO3, 10 g L−1 KOH and 10 g L−1 sodium dodecyl sulfate (C12H25NaO4S) and then activated in 20% HCl for 20 s. After that, they were washed with deionized water and immediately placed in an electrodeposition bath. The bath consisted of NiCl2.6H2O (1 mol L−1) as ion source, H3BO3 (0.5 mol L−1) as pH buffer and ethylenediammonium dichloride; C2H10Cl2N2 (1.5 mol L− 1), as a crystal modifier, was dissolved in deionized water. The solution temperature was kept constant at 60 °C and pH 4 (adjusted by NH4OH). A digital coulometer (model BHP 2050) was used to produce a micro-nano hierarchical structure. The electrodeposition process was carried out in two steps. In the first step, electroplating process was performed at different current densities

and deposition times. The optimized operating conditions including current density and time are given in Table 1. The constant current density of 50 mA cm−2 was applied for 60 s in the second step of deposition. For convenience in identifying the specimens, labels FNF10, FNF20, FNF30, FNF50 and FNF70 were selected for fresh Ni films (superhydrophilic) deposited at the current densities of 10, 20, 30, 50 and 70 mA cm− 2 and at deposition times of 1200, 600, 400, 240 and 171 s, respectively. Labels SNF10, SNF20, SNF30, SNF50 and SNF70 were also selected for Ni films exposed in air (superhydrophobic) at their corresponding conditions. The counter electrode was a platinum wire (as the anode electrode). The electrochemical cell was connected to an EG&G (model 263A) computer-controlled potentiostat/ galvanostat. An EG&G ac responser (model 1025) was coupled with the mentioned potentiostat/galvanostat to read the ac impedance data. The reference electrode was an Ag/AgCl saturated in KCl (SSE). The test solution for the corrosion investigations was 3.5 wt.% NaCl solution at room temperature. Potentiodynamic polarization curves were recorded after 30 min of immersion in the test solution. The potential range for these measurements was fixed from −250 mV, below the open circuit (OCP) value in the cathodic regime, to 500 mV, above OCP value in the anodic regime, at a scan rate of 1 mV s−1. The corrosion current density (icorr) for the specimens was determined by extrapolating the anodic and cathodic branches. Electrochemical impedance spectroscopy (EIS) measurements were performed from an initial frequency of 100 kHz to a final frequency of 10 mHz, using an AC sine wave with the amplitude of 10 mV. The impedance spectra for specimen produced at the current density of 20 mA cm−2 were gathered after different immersion times (1, 3, 6, 9 and 15 days). The surface morphology of Ni films was studied using a scanning electron microscopy (SEM) (model Philips XL30). The SEM images were then utilized for measuring the cones size on films surface by processing the images using Image J software. Water contact angle of the films was measured by a 4 μL distilled water droplet at ambient temperature using Theta Attension optical tensiometer (KSV instrument) with

Fig. 1. SEM images of nickel films deposited at current density of 20 mA cm−2 at different magnifications, (a, b) micro nickel film, and (c, d) micro-nano nickel film.

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Fig. 2. SEM images of micro-nano nickel films deposited at current density of: (a, b) 10 mA cm−2, (c, d) 30 mA cm−2, (e, f) 50 mA cm−2, and (g, h) 70 mA cm−2.

automatic multi-liquid dispenser, monochromatic cold light source and the accompanying software. The initial contact angle values reported the mean values from five measurements made on different locations of the sample surface. The X-ray diffraction patterns of Ni films were obtained using Philips X'pert MPD diffractometer. The diffractometer was operated using Cu-Kα at an accelerating voltage of 40 kV and a current of 40 mA. A standard Nano scope III atomic force microscope (AFM) was employed in the contact mode to observe the surface topography of Ni films obtained at different current densities.

Table 2 Structural parameters on different nickel films. Process condition −2

10 mA cm 20 mA cm−2 30 mA cm−2 50 mA cm−2 70 mA cm−2

Cone height (μm)

Cone width (μm)

1.14 ± 0.18 0.72 ± 0.05 0.47 ± 0.12 0.32 ± 0.07 0.28 ± 0.05

0.67 ± 0.12 0.42 ± 0.07 0.32 ± 0.06 0.2 ± 0.05 0.17 ± 0.03

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3. Results and discussion

surface roughness was not varied significantly, probably due to the homogenous structure of micro and nano cones on the surface of coating.

3.1. Morphology and microstructure of nickel films 3.2. Wettability characterization Fig. 1a and b shows SEM images of Ni film electrodeposited in one step at 20 mA cm−2 for 600 s at 8000 and 16,000 ×, respectively. According to Fig. 1a and b, the surface of Ni film was covered with a micro-cone array structure. Fig. 1c and d shows the morphology of Ni film electrodeposited at two steps: first, at 20 mA cm−2 for 600 s and second, at 50 mA cm−2 for 60 s. It can be seen that by applying the second current density, nano-cones were randomly deposited on to the surface of former micro-cones (Fig. 1c). Careful inspection of the surface indicated that the micro-particles were in the shape of pinecone-like micro clusters (Fig. 1d). The surface of pinecone-like micro clusters was covered by nano-cones with different sizes. The results showed that the Ni film fabricated at two-step electrodeposition had a hierarchical micro-nano structure. In order to investigate the morphology more clearly, the surface morphology against increasing current density was studied (Fig. 2). It was found that the physical properties of films fabricated by electrodeposition were strongly affected by deposition parameters such as current density, concentration of electrolyte, pH value of the electrolyte, deposition time and bath temperature [19,20]. Among them, the current density, as a key factor, specifies the physical properties of electrodeposited films during the growth process [19]. Therefore, in this study, the pH value, the temperature and the concentration of electrolyte were kept constant to investigate the net effect of current density. In order to keep the thickness of the film constant, the deposition time was also changed corresponding to the current density to keep constant the past Coulombs. The average height and width of each cone at half of the maximum height were measured at each current density by Image J software as listed in Table 2. According to Fig. 2 and the results shown in Table 2, the cone size became smaller by increasing the current density. Liang et al. [21] have reported that at low electrodeposition current density, the effect of cathodic polarization is negligible and the deposit growth rate is faster than the nucleation rate. Therefore, it causes the coarser morphology of Ni deposits. Different current densities applied in the first step of electrodeposition caused different surface morphologies. Among the Ni films with micro-nano cone structures, the Ni film electrodeposited at 20 mA cm−2 had pine-cone like protrusions with the average diameter of 1 μm and the surface of each protrusion was covered by numerous irregular nano-cones structures. It has been reported that the morphology of Ni films electrodeposited at different current densities greatly influenced by the crystal modifier (C2H10Cl2N2). The exact role of the crystal modifier in electrodeposition process is not clear, but one possible function of it, is to kinetically control the growth rates of different crystalline faces of nickel by interacting with these faces through adsorption and desorption processes [6,7]. Fig. 3 shows X-ray diffraction patterns of the Ni films at different deposition current densities. Four diffraction peaks at the approximate angles 44, 52, 77 and 93° could be indexed as face-centered cubic Ni with lattice constant being a =3.5°A (JCPDS file card #04-0831). The copper strong peaks were also found in XRD pattern, indicating that the Ni films were very thin [6]. In order to estimate the roughness of the coatings, AFM analysis was performed. For instance, 2D and 3D images of three Ni films at 10, 20 and 50 mA cm−2 are shown in Fig. 4 and the roughness numbers including average roughness (Ra), root mean square (RMS) and maximum height (Rz) obtained by profilometry are summarized in Table 3. The cone structure of coatings can be easily seen in the 3D image. The results showed that the surface roughness of coatings was decreased by increasing the current density (Table 3). This was in agreement with the findings obtained by Zamanzad et al. [23], but as can be seen, the change in the roughness was not so significant. Despite the reduction in the size of micro-cones obtained by increasing the current density (Fig. 2), the

The wettability of Ni films at different current densities was evaluated by measuring the contact angle of water on the surface. As shown in Fig. 5a and b, the water contact angle of freshly deposited Ni film with smooth surface (deposited in the absence of the crystal modifier), was about 79±0.5°. After exposure in air for two weeks, the water contact angle was increased to 98±0.3°. For Ni films with micro-nano hierarchical structures, when they were freshly deposited, the water droplets were speared out on the surface and water contact angles were around 0° (Fig. 5c). The water contact angle rose to about 150° and over for Ni films fabricated at the first deposition current densities of 10, 20, 30 and 50 mA cm−2 after exposing to air for two weeks (Fig. 5d). However, for the film fabricated at 70 mA cm− 2, the contact angle was about 138 ±0.8° after two weeks of exposure (Fig. 5e). With respect to the type of morphology, the cones in small size could not provide the superhydrophobic property. On the other hand, all Ni films deposited at different current densities showed the superhydrophilicity behavior when freshly prepared, but after storage in air for two weeks, only Ni films produced at current densities lower than 70 mA cm−2 showed the superhydrophobicity characteristic (Table 4). In this case, the pinecone-like structure of Ni film produced at 20 mA cm−2 showed the best superhydrophobicity (CA = 155°, SA = 5°) (Fig. 5f). The change of hydrophilicity to superhydrophilicity could be attributed to micro-nano cones as explained by Wenzel equation [21,24]. cosθ ¼ rcosθ0

ð1Þ

Where θ refers to the apparent contact angle of water on a rough surface, θ0 refers to the water contact angle on flat Ni surface and r represents the roughness factor. According to Wenzel equation, the roughness decreases the wettability of hydrophobic surfaces (θ0 N 90) and increases the wettability for hydrophilic surfaces (θ0 b 90) [25]. Since metals are usually hydrophilic [7], the presence of a micro-nano cone

Fig. 3. XRD patterns of superhydrophobic nickel films deposited on copper substrate.

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Fig. 4. AFM micrograph of nickel films deposited at current density of: (a) 10 mA cm−2, (b) 20 mA cm−2, and (c) 50 mA cm−2.

array on the surface of Ni coatings facilitated the transition of hydrophilicity to superhydrophilicity. Two factors necessary to obtain a superhydrophobic surface are low surface energy and surface roughness [26]. Eq. (2) shows Cassie–Baxter equation [25]:

Table 3 The roughness numbers calculated from AFM analysis. Condition −2

10 mA cm , 1200 s 20 mA cm−2, 600 s 30 mA cm−2, 400 s 50 mA cm−2, 240 s 70 mA cm−2, 171 s

Ra(nm)

Rz(μm)

RMS (μm)

96 ± 10 93 ± 8 88 ± 5 89 ± 7 82 ± 9

0.612 ± 0.06 0.579 ± 0.06 0.543 ± 0.04 0.575 ± 0.03 0.483 ± 0.06

1792.46 ± 0.03 1792.46 ± 0.02 1792.47 ± 0.03 1792.46 ± 0.03 1792.44 ± 0.02

cos θ ¼ r ƒ s cosθ0  ð1  ƒs Þ

ð2Þ

Where θ represents the contact angle of water on a rough surface, θ0 is the intrinsic contact angle on a flat surface, r represents the roughness factor, ƒ is the area fraction of micro-nano cones and 1-ƒ refers to the

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Fig. 5. Photographs of water droplet on: (a) freshly smoothed nickel film, (b) smooth nickel film after two weeks, (c) freshly prepared micro-nano nickel film at 20 mA cm−2, (d) micronano nickel film prepared at 20 mA cm−2 after two weeks, (e) micro-nano nickel film prepared at 70 mA cm−2 after two weeks and (f) a snapshot of water droplet rolling off the nickel film prepared at 20 mA cm−2 after 90 days.

area fraction of air on the film surface. Air could be trapped in the grooves of the film surface with micro-nano structures (Fig. 2). Therefore, the increase of air trapped in the grooves, i.e. (1-ƒ), could produce large water contact angle (θ) and prevent the penetration of water droplets into the surface roughness. Furthermore, regarding the wettability transition of micro-nano structured metal surfaces over time, Khorsand et al. [27] indicated that the main reason for

Table 4 The water contact angle for superhydrophobic nickel films.

superhydrophilicity to superhydrophobicity transition of one and two step electrodeposition Ni–Co coatings is adsorption of organic hydrocarbons on coating surface. They revealed by XPS analysis that the adsorption of non polar bond (C–C(H)) on the surface reduced the surface energy. Long et al. [28] observed that the wettability transition for Aluminum coating was accelerated in atmosphere that was rich in organic compounds. Therefore they proposed that the change in wettability was attributed to the adsorption of the organic compounds from the surrounding atmosphere on the oxide aluminum surface. 3.3. Electrochemical measurements

Specimen

CA right(°)

CA left(°)

SNF10 SNF20 SNF30 SNF50 SNF70

152 ± 1.8 155.2 ± 3.2 149.3 ± 2.1 153.5 ± 0.9 139.2 ± 1.4

151 ± 1.6 155.8 ± 3.3 149 ± 1.7 152.04 ± 0.8 138.3 ± 1.2

Potentiodynamic polarization plots of copper substrate, the freshly electrodeposited Ni films (superhydrophil) and air exposed Ni films (superhydrophob) in 3.5 wt.% NaCl solution are presented in Fig. 6. Corrosion current density (icorr) and corrosion potential (Ecorr) extracted from the intercept of Tafel slopes are summarized in Table 5.

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Fig. 6. Potentiodynamic polarization curves of copper bare substrate, freshly micro-nano nickel films and the superhydrophobic nickel films in neutral 3.5 wt.% NaCl solution.

Table 5 The corrosion parameters extracted from polarization plots in Fig.6. Specimen

Ecorr (VSSE)

icorr (A cm−210−6)

Cu substrate FNF 10 SNF 10 FNF 20 SNF 20 FNF 30 SNF 30 FNF 50 SNF 50 FNF 70 SNF 70

−0.24 −0.44 −0.23 −0.38 −0.22 −0.48 −0.28 −0.48 −0.27 −0.49 −0.27

4.30 21.50 0.33 27 0.26 20.60 1.22 22 1.17 19.20 2.20

The potentiodynamic curves (Fig. 6) and their analyses (Table 5) indicated that the superhydrophobic coatings significantly improved the corrosion resistance of the substrate. Also, the comparison of potentiodynamic curves convincingly demonstrated the higher stability of superhydrophobic coatings. The positive shift of Ecorr and the decrease of icorr, as compared to the bare copper substrate, indicated a significant

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corrosion inhibition. As shown in Fig. 6, all films deposited at 10, 20, 30 and 50 mA cm−2 could be almost superhydrophobic after exposing to air, but exhibited a different corrosion behavior. The film deposited at the current density of 20 mA cm− 2 showed a better corrosion protection. In general, it can be said that the inhibition mechanism for superhydrophobic coatings seems to be due to a very low portion of real area which is in contact with the aggressive solution and also, the shift of Ecorr toward positive values [22,29,30]. Electrochemical impedance spectroscopy (EIS) was carried out for copper substrate and Ni films deposited at different current densities in the cases of freshly deposited (superhydrophil) and after 14 days of air exposure (superhydrophob), respectively (Fig. 7). According to Nyquist and Bode plots of copper substrate, two time constants can be distinguished within the testing frequency range. It is probably related to the formation of the corrosion product layer of CuCl at higher frequencies and corrosion interface at lower frequencies [24,30]. So, an equivalent electrical circuit represented in Fig. 8a can be considered for the corrosion of substrate, where Rs is the solution resistance, Rct is the charge transfer resistance, Rf is the film resistance due to corrosion products, and CPEdl and CPEf are constant phase elements related to the double layer capacity and corrosion products film capacity, respectively. W refers to the Warburg diffusion too. The impedance plots for Ni films deposited at different current densities exhibited a capacitive loop by applying the frequency range [7]. Accordingly, EIS results could be analyzed with the equivalent electrical circuit shown in Fig. 8b, where Rs is the resistance solution, Rct is the charge transfer resistance and CPEdl is constant phase element modeling double layer capacity at solution- film interface. The impedance of CPE is described below:  1 ZCPE ¼ Y0 ðjωÞn

ð3Þ

where ZCPE represents CPE impedance, Y0 is CPE constant, ω is angular frequency and n is the exponent of the CPE, which varies between 0 and 1 [1,31]. The fitted impedance spectra were in a good agreement with the impedance spectra results (Fig. 7). The results obtained from equivalent electrical circuits are summarized in Tables 6 and 7. It is well known that the diameter of the capacitive loop in Nyquist plots refers to the polarization resistance of specimen [22]. Therefore, a comparison between Fig. 7a and b exhibited that the superhydrophobic films had the highest polarization resistance. Also, it could be seen that the double layer

Fig. 7. Nyquist plots of (a) freshly nickel films deposited at different conditions, and (b) copper substrate and superhydrophobic nickel films at different conditions in 3.5 wt.% NaCl solution.

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Fig. 8. Equivalent electrical circuits of: (a) copper substrate, and (b) freshly prepared nickel films and superhydrophobic nickel films.

capacity for superhydrophobic specimens had been decreased considerably. Therefore, superhydrophobic films could provide an excellent and successful protective effect for the substrate. As mentioned before, air layer is stabilized within the grooves of super-hydrophobic films, considerably reducing the penetration of the corrosive ions such as Cl− and improving the corrosion resistance [29,31]. It is also worth noting that the diameter of capacitive loop for the superhydrophobic film deposited at 20 mA cm−2 was larger than that of others (Fig. 7b). This indicated that this film had the highest corrosion resistance. To find the reason for this behavior, it is necessary to consider the deposit property which affects the corrosion resistance. For the superhydrophobic specimens, according to Cassi–Baxter model, surface morphology and roughness can be more effective on trapping the amount of air in the grooves [24]. In this study, the roughness of superhydrophobic coatings was approximately similar to each other. The higher water contact angle and excellent corrosion protection of the superhydrophobic film deposited at the current density of 20 mA cm−2 can be referred to as coating morphology. The hierarchical pine cone-like structure of Ni film deposited at 20 mA cm−2 can probably generate numerous grooves in which air may be trapped easily.

Fig. 9. Nyquist plots of superhydrophobic nickel films at 20 mA cm−2 with different immersion times in 3.5 wt.% NaCl solution.

3.4. Long-term stability of superhydrophobic nickel film In order to estimate the long-term stability of superhydrophobic film fabricated at 20 mA cm−2, EIS measurements were done at different immersion times in 3.5 wt.% NaCl solution. The results are presented in the form of Nyquist plots in Fig. 9 and the fitted parameters are shown in Table 8. It is obvious that the capacitive loop unexpectedly became larger for a few days after immersion in corrosive solution. The capacitive loop gradually became larger until 6 days and then it started to decrease.

According to Tables 7 and 8, at the beginning (30 min) of the immersion the charge transfer resistance (Rct) was 2100 kΩ cm2 but reached 1946 k Ω cm2 on the first day and then increased. This behavior confirmed the formation of stable passive layer on the surface of Ni film after one day immersion. This indicated that the superhydrophobic Ni film had good chemical stability in corrosive media. Fig. 10 shows the SEM images of superhydrophobic film deposited at 20 mA cm−2 after 15 days of immersion. It can be seen that the morphology of Ni film was not changed

Table 6 The impedance parameters extracted from Nyquist plots in Fig. 7a.

Table 8 The impedance parameters extracted from Nyquist plots in Fig. 9.

Specimen

Rs (Ω cm2)

Rct (kΩ cm2)

CPE (μF cm−2)

N

Time (day)

Rs (Ω cm2)

Rct (kΩ cm2)

CPEdl (μF cm−2)

ndl

FNF10 FNF 20 FNF30 FNF 50 FNF 70

7.4 7.3 6.5 7.7 7.1

0.6 2.7 4.1 0.8 1.1

1900 980 300 800 2860

0.8 0.8 0.9 0.8 0.7

1 3 6 9 15

13.1 12.7 11.5 11.7 10.9

1946 3233 15,546 2860 340.5

33.5 33.6 37.9 26.9 44.8

0.9 0.9 0.9 0.8 0.8

Table 7 The impedance parameters extracted from Nyquist plots in Fig. 7b. Specimen

Rs (Ω cm2)

Rct (kΩ cm2)

CPEdl (μF cm−2)

ndl

Rf (Ω cm2)

CPEf (μF cm−2)

nf

Zw (Ω−1 s0.5 cm−210−3)

Cu substrate SNF10 SNF 20 SNF30 SNF 50 SNF 70

4.9 9.1 13.1 10.3 11.7 13.6

0.9 61.3 2100 35.3 32.6 15.5

1440 85.3 7.9 89 124 59

0.6 0.9 0.9 0.9 0.8 0.8

70.1 – – – – –

197 – – – – –

0.7 – – – – –

2.6 – – – – –

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Fig. 10. SEM images of micro-nano nickel film deposited at 20 mA cm−2 after immersion in 3.5 wt.% NaCl solution for 15 days.

considerably (Fig. 1c) and only the tips of nano-cones were corroded, causing the round tops and some parts of the film covered by corrosion products. It could be assumed that the corrosion products filled the grooves of the coating surface and decreased the penetration of corrosive solution. Therefore, the formation of the passive layer and partial blocking of the surface could be considered as the reasons for the increase of charge transfer resistance observed at the initial times of immersion [32,33]. 4. Conclusions To summarize, corrosion resistant and superhydrophobic nickel films with a hierarchical structure were synthesized by directional electrodeposition process. The relationship between the wettability and surface morphology was studied using different deposition current densities. Also, corrosion resistance of different nickel films was established. The following conclusions can be drawn: (1) The superhydrophobic nickel films showed better corrosion resistance than copper substrate and fresh micro-nano structure nickel films. The corrosion resistance of the superhydrophobic surface was closely related to their wettability. (2) The Ni film deposited at 20 mA cm−2 exhibited the best superhydrophobicity with a contact angle of 155°. The superhydrophobicity of this coating was derived from its pine cone-like structure and adsorption of organic hydrocabons. (3) It was confirmed that the superhydrophobic Ni film fabricated at 20 mA cm−2exhibited high stability when immersed in 3.5 wt.% NaCl solution after several days. At the initial times of immersion, the charge transfer resistance was raised because the formation of passive layer on the surface of Ni film. (4) The inhibition mechanism of hierarchical superhydrophobic nickel films, as shown from electrochemical measurements, was the result of a very low wetted area obtained on the solid surface immersed in the aggressive solution and the shift of corrosion potential toward positive values.

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