
==== Front
Heliyon
Heliyon
Heliyon
2405-8440
Elsevier

S2405-8440(24)13373-7
10.1016/j.heliyon.2024.e37342
e37342
Research Article
Electrodeposition of Ce-Zr myristate superhydrophobic coating on copper substrate
Behniya Sara
Yousefpour Mardali myousefpor@semnan.ac.ir
⁎
Faculty of Materials and Metallurgical Engineering, Semnan University, Semnan, Iran
⁎ Corresponding author. Faculty of Materials and Metallurgical Engineering, Semnan University, Semnan, 35131-19111, Iran. myousefpor@semnan.ac.ir
05 9 2024
15 9 2024
05 9 2024
10 17 e3734214 5 2024
31 8 2024
2 9 2024
© 2024 The Authors
2024
https://creativecommons.org/licenses/by-nc-nd/4.0/ This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).
Copper and its alloys due to favorable properties such as ductility, high electrical and thermal conductivity are very important in various industries. The coating of rare earth elements and intermediate elements is a suitable method to form a super-hydrophobic coating on copper substrate. The aim of this research is to fabricate a controlled super-hydrophobic coating of cerium-zirconium myristate on the copper base using the electrochemical deposition process and to prevent the corrosive solution penetration and reaching to the copper substrate due to removing the corrosive solution from the surface of the coating. The variables parameters in this process are the change of deposition time, the different concentrations of cerium chloride, and zirconium chloride salts and is necessary to investigate their effect of them on the structural morphology, wettability and corrosion properties. According to the results of Field Emission Scanning Electron Microscope (FE-SEM), the surface morphology of the coatings is consisted of lotus-like nano and micro leaves. Furthermore, the wettability of the prepared coatings was analyzed and observed by measuring the contact angle of the coating with water droplet that the fabricated coating from the electrolyte containing 0.056 M of cerium chloride, 0.014 M of zirconium chloride, 0.1 M of myristic acid, 10 ml of hydrochloric acid and ethanol solvent has super-hydrophobic properties and the contact angle with water droplet is measured at 161°. The FT-IR and EDS analysis showed that the chemical composition of lotus-like leaves is cerium myristate and zirconium myristate. As a result, it can be inferred that the type of morphology and surface roughness play an important role in inducing the super-hydrophobic properties and has the most effect on the corrosion resistance of the coating due to the formation air pockets and then to prevents the corrosive solution penetration through the cross section of the coating and reaches to the copper substrate.

Graphical abstract

Effect of Roughness on the contact Angle.Image 1

Highlights

• Super-hydrophobic coating with excellent adhesion properties was fabricated on the copper substrate using the electrodeposition process.

• The chemical composition of electrolyte and duration time of electrodeposition process were influenced on the microstructure of coating.

• With control of ratio Ce+3/Zr+4 in the electrolyte, the superhydrophobic and corrosion resistant of hybrid coating were optimized.

Keywords

Cerium-zirconium myristate
Super-hydrophobic
Electrochemical deposition
Corrosion resistance
Copper
Wettability
==== Body
pmc1 Introduction

Destruction of metals in the corrosive environments has brought irreparable results in the ecological and economic system of the world, and has affected on the lifetime and performance of them [[1], [2], [3]]. In order to prevent above problems, super-hydrophobic surfaces have been used in most cases [4]. In recent years, the super-hydrophobic property has been noticed due to its high application potential in overcoming the challenges of the real world. Because, the super-hydrophobic property is related to the specific surface area (SSA), pore structure, nano and microstructure, and low surface energy [5]. The synthetic super-hydrophobic surfaces have been fabricated by controlling and studying the natural systems [6]. Because, some plants and insects, including lotus leaves, rose petals, cactus, peanut and sycamore tree leaves, butterfly wings, desert beetles, spider silk, mosquito compound eyes, domestic lizard legs, and water striders have naturally super-hydrophobic and self-cleaning property [7]. The mechanism behind this property remained a mystery until the development of the scanning electron microscope (SEM) in the mid-1960s. The studies on the leaves of natural plants during the last decade revealed that the macroscopically smooth surface usually consists of microscopic roughness with different length scales and the formation of micro-nano structure on the surface along with epicuticular wax caused hydrophobicity. This discovery is considered as a great progress in the field of making super-hydrophobic surfaces and finally fabricating a controlled super-hydrophobic coating [8]. The surfaces with a contact angle greater than 150° and a slipping angle below 10° are called super-hydrophobic. In the super-hydrophobic surfaces, water droplets can roll off from the surface. Therefore, the roll off and repelling of the water droplets from surface is very important in wetting behavior, and thus it will be depended on the surface roughness and surface energy. According to the mentioned mean, to get the super-hydrophobic behavior is necessary to creating the surfaces with a high surface roughness and low surface energy. As a result, the water droplet on the surface of the super-hydrophobic coating forms with spherical shape and is difficult to stay on [9]. The super-hydrophobic surfaces in the engineering applications including; self-cleaning, anti-fouling, anti-corrosion, bio-medicine and textile industry, electrical industry, etc. have been used [10]. Furthermore, they have many applications in various fields such as selective adsorption and separation of water and oil, controlled drug delivery, antibacterial and anti-corrosion compounds [11]. These surfaces are desirable features for industrial and biological applications such as anti-stain paints for boats, anti-snow adhesion to antennas and windows, self-cleaning of car windows, metal polishing, anti-stain fabrics and anti-dust and water-resistant architectural coatings which are considered for the surface of the earthing rods [[12], [13], [14], [15]]. Copper is one of the main non-ferrous metals and is the most used after aluminum in the group of non-ferrous metals. Due to its high electrical and thermal conductivity, unique workability and flexibility, machinability and low chemical reactivity, is widely used in the world industry, [16]. In this case, copper is used to fabricate the microelectronic materials. However, they can easily adsorbed the water vapor from environment, which it is contained the humid conditions during packaging field, storage, and transportation. The adsorbed water vapor can be adsorbed onto the surface of substrate and converted to the water droplets under low temperature, which will be reduced the corrosion resistant of the substrate. Hence, inhibiting the water droplet formation, self-cleaning, and anti-biofouling on the copper surface is needed by creation the super-hyrophobic coating of rare earth metals and fatty acids [17,18]. To obtain the super-hydrophobic property, the addition of rare earth elements and transition-metal oxides to the coatings [19], with micro-nanoparticle sizes in the fatty acids matrix have an important role in anti-corrosion coatings, antibiofouling activity, multifunctional super-hydrophobic property, and catalysts [[20], [21], [22]]. Additionally, in the rare earth elements, the small energy difference between the inner 4f electron and the outer valence electrons gives them the stability of variable valence, and water is a polar molecule. Therefore, the polarity of the coating surface plays an essential role in the hydrophobicity of the surface. since the 4f bond does not have a tendency to interact with the 4 d and 5p electrons. The probability of hydrogen bonding with water interface molecules is very low. As a result, rare earth elements in the coatings create super-hydrophobic behavior by depositing on the copper substrate. Hence, super-hydrophobicity behavior is depended on design an especial micro and nano-structure with low surface energy [23,24]. Because, micro and nano-structure has a rough honeycomb structure, which induces it has an excellent anti-water droplet formation behavior [25]. Furthermore, the micro and nano-structure of super-hydrophobic coating trapes air and then changes the contact mode of coating from solid-liquid to solid-gas-liquid state. The blocked air remarkably enhances the buoyancy of super-hydrophobic coating and reduces the floating capacity [26,27]. In this regard, cerium and erythium are famous among the rare earth metals, which are used to fabricate the super-hydrophobic coating [28]. Because, the rare earth metals have a rapid redox conversion between different chemical states, Various methods have been used to make super-hydrophobic surfaces, including sol-gel, plasma, chemical etching, electrical deposition, laser, lithography, electrospinning, chemical vapor deposition, etc., [28,29]. The most of the used methods are expensive and also may not be used on a large scale substrate due to the complex multi-stage processes and time-consuming coating [30]. In contrast, electrodeposition is easy and has advantages in terms of low price and easy operation conditions for mass production [31]. Meanwhile, Liu et al. [32], had made super-hydrophobic and corrosion resistant cerium coating on the brass sheets (20 % zn- 80 % cu) by a one-step process of electrodeposition, and reported that the formation of the super-hydrophobicity coating due to the formation of myristate with the micro/nanostructure of the flower-like microstructure. Also, Liu et al. [33], had developed a super-hydrophobic and self-cleaning coating with controlled adhesion by electrodeposition method in an electrolyte solution containing cerium chloride salt, myristic acid and ethanol on a copper substrate. And they realized that the formation of coagulation of nanostructures in the microstructure of the coating with low surface energy plays a very important role in the formation of super-hydrophobic surfaces. In addition, Hussain et al. [4], had fabricated a super-hydrophobic corrosion-resistant coating of cerium myristate with cauliflower-like morphology by one-step electrodeposition method on AISI 316L austenitic stainless steel. They observed the cerium myristate superhydrophobic coating with a water contact angle of 167.7° and with a cauliflower-like morphology, and showed significant corrosion resistance in a 3.5 wt% NaCl solution. The reason for that was the entrapment and formation of air cushions trapped in the coating and as a result, it had prevented the salt solution entering from the coating. In follow, Liu et al. [34], had prepared the super-hydrophobic coating of cerium myristate to improve the anti-corrosion property of magnesium alloy through a single step process of electrodeposition and they found that this super-hydrophobic coating with a water contact angle equal to 159.8° and slip angle of 1°, which had a flower-like morphology and a hierarchical structure on a nano scale and had greatly improved the corrosion resistance of the magnesium substrate. Furthermore, Chen et al. [35], had produced the super-hydrophobic of nickel myristate surfaces with a cauliflower-like structure on copper by electrodeposition method from an electrolyte solution including nickel chloride, myristic acid and ethanol. They had identified the formation of the cauliflower-like morphology of nickel myristate. Moreover, Zhang et al. [36], had created a super-hydrophobic and corrosion resistant coating with the chemical composition of cerium palmitate by adding a certain amount of palmitic acid and cerium nitrate to the electrolyte on aluminum foils using the electrodeposition method. Fan et al. [37], also had made a super-hydrophobic zirconium palmitate film with a mound shaped structure on carbon fiber by one step electrodeposition technique. In addition, Farid et al. [38], had studied the super = hydrophobic zirconium based thin films by an electrodeposition method on 6061 aluminum alloy substrates and reported that the super-hydrophobic coating of zirconium and fatty acid, that improved the corrosion resistance of the aluminum substrate about 93 % and the maximum efficiency of corrosion protection for zirconium-base super-hydrophobic thin film had achieved, which was beneficial for expanding the efficiency of aluminum and its alloys in industrial fields. Setare et al. [39], had produced the nanocrystalline zirconia coating with a thickness of 1.5 μm by electrical deposition method under pulse current and direct current, under annealing operation on 316L austenitic stainless steel. They reported that the zirconia coating showed a suitable corrosion resistant against 3.5 wt% NaCl solution and acted as an inert layer. Another fatty acid in chemistry is a type of carboxylic acid with a long aliphatic chain length, which can be saturated or unsaturated. The most of fatty acids are unbranched and contain the number of carbon atoms. The number of these atoms are from 4 to 28 [40]. The fatty acids are provided outstanding super-hydrophobic property on the metal substrate due to having the functional groups [41]. Therefore, the mentioned acids can be sued to obtain an excellent corrosion resistance and antifouling properties [[41], [42], [43]]. Then, Badreh et al. [44], had prepared superh-ydrophobic surfaces of nanostructured zinc oxide layers using fatty acid molecules. In this study, they achieved a contact angle of 167° using stearic fatty acid. Wang et al. [45], had been investigated micro-nanostructured hierarchical structure films on copper by electrochemical deposition method and reported that after surface modification with long-chain fatty acid, super-hydrophobicity showed that these characteristics were effective for oil and water separation. They had also concluded that fatty acid with long chains had a significant effect on the wettability of films, and the participation of two factors, the hierarchical structure and fatty acid led to the unique property of super-hydrophobicity. In addition, Chen et al. [46], had fabricated a multifunctional lanthanum super hydrophobic coating on carbon steel. They resulted that the reason for the super-hydrophobicity of the lanthanum coating was the formation of lanthanum palmitate with papilla-shaped cluster morphology, which is formed by the reaction between palmitic acid and lanthanum nitrate. In addition, Abirami et al. [47], had been created a cerium stearate super0hydrophobic coating with a wetting angle of about 165° by electrodeposition method on AA2024-T3 aluminum alloy and reported that the above super-hydrophobic coating provided corrosion protection. Because, it was active and had the property of self-healing, which was due to the release of Ce3+ ions [48]. Furthermore, Kai et al. [49], had improved a ceramic based super-hydrophobic corrosion resistant coating with a single step electrodeposition method on aluminum alloy and reported that the coating's inhibitory property was due to the active inhibition of cerium. Through, reviewing the related articles, it is found out that the effect of the simultaneous presence of cerium and zirconium salts in the electrolyte on the fabrication of super-hydrophobic properties in the coating prepared by the electrodeposition process has not been investigated and published any reports in this area. As a result therefore, in this research, an attempt was made to investigate the parameters affecting the microstructure and properties of the coating by creating a double hybrid coating on the copper substrate using an electrodeposition method. In order to achieve this goal, it was necessary to investigate the morphology of the double hybrid coating, the contact angle with water, and the corrosion resistance of the coating. Therefore, the relationship between the main parameters of the coating process, including the concentration of salts and deposition time and the effect of morphology on the wettability was evaluated.

2 Materials and methods

2.1 Sample preparation and precursors materials

Anhydrous ethanol, myristic acid (Merck, Chemical purity), hydrochloric acid (36–38 %), cerium chloride (CeCl3.6H2O, (Merck, Chemical purity)) and zirconium chloride (ZrCl4, (Merck, Chemical purity)) were used. Furthermore, copper sheet with chemical purity of 99.99 % with dimensions of 3 × 10 × 50 mm3 were used as anode and cathode. In follow, the copper sheets were polished using the mechanical polishing, and using the silicon carbide sandpaper from 400 to 3500 grade, respectively. Then, after washing several times with distilled water and ethanol, they were dried in the normal air conditions.

2.2 Preparation of sample coatings

Electrolyte solutions with the addition of 0.019, 0.038 and 0.056 M of cerium chloride, 0.1 M of myristic acid, 10 ml of hydrochloric acid and different amounts of zirconium chloride including 0.007, 0.012, 0.014, 0.018, 0.024 and 0.028 M were prepared under the constant stirring rate in ethanol. And stirring treatment is continued until to obtain a uniform solution of 150 ml at room temperature. In order to investigate the effect of zirconium chloride concentration on the morphology of the coatings, electrolyte solutions with different amounts of zirconium chloride were used to make samples (C1:0, C2:0.007, C3:0.0014, C3:0.0024). C4: 0.038 and C5: 0.0028 mol L−1 were prepared in fixed concentrations of cerium chloride salt, 0.1 M of myristic acid and 10 ml of hydrochloric acid. Furthermore, the effect of cerium chloride concentration on the morphology of the coatings, electrolyte solutions with different amounts of cerium chloride were used to make samples (C3:0.038, C6:0.019, 0.056, and C7: 0.014 M zirconium chloride salt, 0.1 M myristic acid and 10 ml hydrochloric acid. In addition, to study the effect of deposition time on the morphology of the coatings, electrolyte solutions in fixed concentrations of 0.056 M of cerium chloride salt, 0.014 M of zirconium chloride salt, 0.1 M of myristic acid and 10 ml of hydrochloric acid was prepared to make samples (C7, C8, C9 and C10). It should be noted that myristic acid is added to the electrolyte solution in order to reduce the surface energy of the copper substrate to prevent contamination and drops of corrosive substances from the copper substrate. The electrolyte solution was stirred with a magnetic stirrer at 300 rpm for homogenization during the coating process. Since, the solutions containing myristic acid had weak electrical conductivity, thus 10 ml of hydrochloric acid was added to the electrolyte solution to increase the conductivity. Moreover, in during coating, a Teflon cover was used on the beaker due to creating the toxic vaporous of the electrolyte solution. Furthermore, GPS3306D direct current power supply was used to perform the electrical process. The coating process with copper sheet anode and copper sheet cathode were used as the cathode in the electrolytic cell and 30 V was applied on the two electrodes with a distance of 2 cm. Then, after a period of 5, 10 [32], 15 and 30 min, the working electrodes were placed in the cathode and after the deposition, the coated samples were washed several times with distilled water and ethanol, respectively. And then were dried in atmospheric conditions.

2.3 Characterization of coatings

The water contact angle of all the samples was measured at room temperature with a Jikan CAG-20 contact angle measuring device equipped with a camera for taking pictures. Water droplets with a volume of 5 μL were dripped onto the surface of each sample with an injection speed of less than 1 μL/s. All contact angles were measured at three different points in dynamic mode and then reported the average of them. The surface morphology of the prepared samples was examined with a field emission scanning electron microscope (FE-SEM, Model TESCAN MIRA3 at 15 kV). FT-IR spectrophotometer (Model, S 8400) was used to identify the functional groups and the chemical composition of the coating (Sample C7). The test was conducted under the conditions of 1 atm and 25 °C. In addition, spectroscopy energy dispersive, weight percentage and mass percentage of coating elements were determined by EDS test at a random point on the surface of the sample. Furthermore, the EDS test was conducted in order to check the linear distribution of the elements in the cross section of the coating. In the next step, the topography of the coating surface and the roughness of the coating surface were investigated using the atomic force microscope (AFM, Vecco model in contact mode). In addition, to study the thickness of the coatings, their cross-sections were prepared by mechanical polishing with sandpaper with 80–2500 grades and then polishing with 0.05 μm alumina powders, and scanning electron microscope (SEM, FEI Quanta model) was used to determine the thickness. At the end, the adhesion quality of the coating was evaluated by the Cross-Cut Tape according to the ASTM D 3359 standard. In this method, a cross cut cutting tool or any other cutting tool such as a sharp blade of a carpet cutter or a knife, two cross cutting grids are marked. Then, with the help of a soft brush, it is cleaned on the grid cuts. And, a special glue is completely located on the surface of coating and after a few seconds, the glue is quickly removed from the surface of the coating. Finally, the cutting site is evaluated with the degree of separation of the coating from the substrate surface. Adhesion test acceptance criterion is determined by mode B4 and B5. Furthermore, electrochemical corrosion behavior was investigated by OGF500 potentiostat device made in France with polarization tests in a three-electrode cell (Saturated Kalmel electrode(reference electrode), platinum electrode (Auxiliary electrode), and coating sample (Working electrode)). The tests were performed at room temperature and inside the salt solution with 3.5 wt%. All the samples were immersed in salt solution for 30 min before the electrochemical test in order to reach to the stable state. The polarization curves were recorded at the scan rate (0.5 mV s−1) from −250 to 250 mV, relative to the open circuit potential (OCP).

3 Results and discussion

3.1 Surface morphology

In order to investigate the effect of zirconium chloride concentration on the morphology of the coatings, electrolyte solutions with different amounts of zirconium chloride were used to make samples (C1:0, C2:0.007, C3:0.0014, C3:0.0024). C4 and C5: 0.0028 mol L−1) in a constant concentration of 0.038 M of cerium chloride salt. Fig. 1(a–e) shows the FE-SEM images of the coatings obtained from the mentioned electrolyte solution. The electrical deposition process for all samples has been carried out under the required current density and a voltage of 30 V at 10 min.Fig. 1 FE-SEM images of cerium-zirconium coatings prepared from electrolyte solution with different amounts of zirconium chloride with different magnifications, a. Morphology of samples C1, b, c, d and e. Morphology of samples C2, C3, C4 and C5, respectively.

Fig. 1

Fig. 1(a–e), is related to the microstructure of the coating surface with different amounts of zirconium chloride in the electrolyte. The microstructure of the coating consists of large needle leaves and semi-spherical leaves. The average width of needle-shaped leaves is about micrometer and the average width of semi-spherical leaves is less than 1 μm. The average width of semi-spherical leaves are approximately several hundred nanometers. By checking the images of the microstructures of the coating with different amounts of zirconium, it can be seen that the average size of the needle-shaped leaves is greatly reduced. Furthermore, the average width of the semi-spherical leaves is smaller and decreases to less than one hundred nanometers. In general, it can be concluded that the higher concentration ratio of zirconium chloride to cerium chloride, the number and size of needle-shaped leaves will be reduced, and as a result, the number of semi-spherical leaves will increase, however at the same time, the average size of the semi-spherical leaves are reduced. And, according to the microstructure images, they are reduced to less than one hundred nanometers. Therefore, it can be seen that with the increase in the concentration of zirconium chloride in the electrolyte, the conditions for the formation of zirconium myristate are accelerated, and then thus the growth of needle-shaped leaves is reduced, their width is increased, and their size is reduced to approximately a few hundred nanometers. On the other hand, with the limitation of the growth of needle-shaped leaves, the number of semi-spherical leaves increased, and as a result, their approximate size also decreased and reached to an average size of less than one hundred nanometers. Furthermore, the effect of cerium chloride concentration at a constant concentration of zirconium chloride on the microstructure of the coatings, solutions with different amounts of cerium chloride were used to make samples (C3:0.038, C6:0.019, 0.056, C7: 0.014 M of zirconium chloride salt). The electrical deposition process for all the samples has been done by applying the necessary current density under the voltage of 30 V at 10 min. Fig. 2(a–d) shows the field emission electron microscopic images of the obtained coatings. According to Fig. 2(a–d), the pictures show the microstructure of coatings with a constant concentration of zirconium chloride and a variable concentration of cerium chloride. It can be seen by studying the pictures that at a constant concentration of zirconium chloride and a small amount of cerium chloride in the electrolyte, after applying the coating, the microstructure of the coating consists of fine needle-shaped leaves with an average size of width less than a micrometer, with semi-spherical leaves with an average size of one hundred nanometers. Therefore, by evaluation the microstructure images, it can be seemed that the size of the needle-shaped leaves does not change much with increasing the concentration of cerium chloride in the electrolyte, but their number is increasing. Thus, with increasing the concentration of the mentioned salt in the electrolyte, the average size of needle-shaped leaves does not change. As a result, it can be observed that the effect of concentration of the salt on the shape and size of the leaves is effective to a certain amount, if the concentration is chosen more than this certain limit, it will not has much effect on the average size of needle-shaped leaves with semi-spherical leaves. The cause of this behavior can be analyzed in the competitive growth effect of simultaneous presence of zirconium chloride and cerium chloride salts in the electrolyte. In general, it can be concluded that the average size of the width of needle-shaped leaves and semi-spherical nano-leaves depends on the concentration of cerium chloride salt during the deposition process.Fig. 2 FE-SEM images of cerium-zirconium coatings prepared from solutions with different amounts of cerium chloride with different magnifications, a. Morphological image of sample C6, b, and c. Morphological images of samples C3, C7, respectively, and d. High magnification of (c).

Fig. 2

A needle-shaped leaves and semi-spherical nano-leaves was connected in an irregular porous micro–nanostructure. The porous microstructure was considerably small and more intensive. This behavior enhanced the surface hydrophobic properties, which was evaluated by measuring the static contact angle. As shown in Fig. 1(a–e) and Fig. 2(a–d), with the changed the ratio of Ce+3/Zr+4, the needle-shaped leaves and semi-spherical leaves structure becomes more porous. At the starting, the electrochemical reaction between electrolyte and cathode copper surface produced a large number of hydrogen molecules and induced to the generation of porous microstructure. Moreover, Fig. 1, Fig. 2 show that less than 1 μm of the needle-shaped leaves and semi-spherical leaves formed on the copper cathode with a rough surface (Fig. 13(b)) after electrodeposition. Therefore, the surface of coating converted to the surface with hierarchical micro and nano structure, such as the microstructure surfaces of lotus leave. The formation of nanostructure can considerably limit the Solid-liquid contact area. With the enhance in the ratio of Ce+3/Zr+4, the variation of the morphology caused in the transition of the wetting model from the high adhesive (Petal effect) to the low adhesive (Lotus effect). As a result, it can clearly note the formation of the complex of micro and nano structure of the surface, which are two important terms for fabrication the super-hydrophobic, rough, with low surface energy. The formation of rough surface with low sorfaace energy can improvec the super-hydrophobic behavior [5].

3.2 Chemical composition evaluation

The results of FTIR analysis of the coatings prepared on the copper substrate are shown in Fig. 3.Fig. 3 FTIR diagram related to the coating prepared with an electrodeposition process on the copper substrate (Sample C7).

Fig. 3

According to Fig. 3, in the low frequency region of the FTIR diagram of myristic acid, the peak corresponding to the carboxylate group (-COO-) forms at the 1701 Cm-1. However, in this coating fabricated with copper cathode, the absorption peak corresponding to this wavelength has disappeared, and instead of it, two absorption peaks have been observed at the 1421.73 and 1640 Cm-1. This behavior observed due to the formation of symmetric and asymmetric stretching peak (-COO-) related to the formation of coordination bond-COO-moieties. Furthermore, two absorption peaks have shown in the high frequency region at 2871.8 and 2916. 71 Cm-1, which indicate symmetric and asymmetric stretching vibrations (-CH-), respectively [13,14]. Hence, it can be concluded that zirconium myristate ((Ce[CH3(CH2)12COO]4)) and cerium myristate((Ce[CH3(CH2)12COO]3)) are formed in the super-hydrophobic coating [32,33,50,51].

Considering the findings of FTIR, it can be concluded that the compounds of cerium myristate and zirconium myristate on the surface of the coating in the desired wave numbers of the infrared spectroscopy diagrams, which are formed due to achieving of Equations (3)–(1) and (3-2) [[52], [53], [54], [55], [56]].Equation (3-1) Ce3+ + 3CH3(CH2)4COOH … … Ce[CH3(CH2)14COO]3 +3H+

Equation (3-2) Zr4+ + 4CH3(CH2)4COOH … … Ce[CH3(CH2)14COO]4 +4H+

Therefore, it can also be said that the needle-shaped leaves and semi-spherical leaves consist of cerium myristate and zirconium myristate compounds. In addition, EDS analysis has been used to confirm the presence of carboxylate compounds related to cerium and zirconium elements in the super-hydrophobic coating of sample (C7). Fig. 4 shows the graph of X-ray spectroscopy analysis of the coating (C7), which was obtained at a random point from lotus nano leaves on the surface of the coating. According to the results, the type of coating elements, weight percentage and atomic percentage of each of them can be seen in Table 1.Fig. 4 EDS results at a random point on the surface of the coating corresponding to sample(C7).

Fig. 4

Table 1 Atomic and weight percent of the coating elements related to sample (C7).

Table 1Element	Wt%±0.01	at%±0.01	
Cu K-series	82.98	49.27	
C K-series	13.92	43.73	
O K-series	2.93	6.92	
Cl K-series	0.04	0.03	
Ce L-series	0.10	0.05	
Zr L-series	0.05	0.02	

According to Table 1, it can be confirmed that the compounds of cerium myristate and zirconium myristate are formed due to the presence of cerium, zirconium and carbon elements on the coating surface due to occur Equations (3)–(1) and Equations (3)–(2). Therefore, it can also be concluded that the microstructure of the surface of the electrodeposited coating is composed of cerium myristate and zirconium myristate compounds.

When the copper electrodes are immersed in the electrolyte due to the applied direct current and voltage and according to Equations (3)–(1) and Equations (3)–(2), Ce3+ and Zr4+ ions react with myristic acid near the cathode electrode and produce cerium myristate and zirconium myristate compounds. Moreover, some H+ ions adsorb on the surface of the cathode electrode and obtain the electrons and form H2 molecules [33]. And then, the formation of hydrogen molecules, that led to the formation of porous micro and nano structure (Needle-shaped leaves and semi-spherical leaves).Equation (3-3) 2H++2e … H2

The presence of Zr, Ce, Cu, O, and C on the fabricated coatings can be observed in Fig. 4 and Table 1. The obtained data indicates the incorporation of Ce and Zr on the surface of coatings. Therefore, the presence of Ce and Zr on the surface of coatings is confirmed to be in the state of Ce3+ and Zr+4 ions. Compared with uncoated copper, the amount of C and O elements, as well as the Ce and Zr element from CeCl3, and ZrCl4 were remarkably enhanced on fabricated surface of coatings, which were modified by myristic acid. This behavior concluded the formation of cerium myristate (Ce[CH3(CH2)12COO]3) and zirconium myristate (Zr[CH3(CH2)12COO]4), which are the low-energy materials. Formed surface of coatings are able to make micro and nano hierarchical structures (Needle-shaped leaves and semi-spherical leaves) with low surface energy, which are optimizing conditions for super-hydrophobicity. Therefore, the findings are confirmed the formation mechanism of electrodeposition, which the mechanism is discussed as follow. In the first step, when the copper electrode was dipped in the electrolyte solution under the applied DC voltage, some Ce3+ and Zr+4 ions near the copper cathode react with myristic acid molecules, and then formed cerium myristate, zirconium myristate and H+ ions on the copper cathode surface. In addition, free H+ ions obtained electrons and generated H2 molecules (Equations (3)–(3)). The formation of H2 agitated or disturbed the solution and increase the rate of reactions in the area of the cathode of the growing compounds, which induced the generation of the micro and nanostructure on the cathode surface. In Fig. 5, the SEM images of the cross-section of the coating related to the sample (C7) are presented with low and high magnification. Fig. 5(a), shows that the coating is not completely uniform on the surface of the substrate. The non-uniformity in the thickness of the coating on the cross-sectional surface is due to the mechanical polishing operation on the cross-sectional surface of the coating after mounting process, and the thickness of the coating on the cross-sectional surface is between 3 and 4 μm (μm). It can also be seen that the surface morphology of the cross section of the coating is consisted of the micro and nanostructure.Fig. 5 Image of the cross-section of the coating of sample (C7) with a. Low magnification and b. High magnification.

Fig. 5

In Fig. 5(b), the SEM image of the cross-section of the coating of the sample (C7) is presented with high magnification. Figure 5(b) shows that the nano-blocks are the same lotus nano-leaves that are seen as nano-scale blocks in the cross-sectional of the surface. According to the mentioned image, the average size of nano leaves of lotus leaves is about 186 nm. Furthermore, in order to check the chemical composition and percentage changes of elements in the cross-section of the coating from the substrate side to the surface of coating, the results of the EDS results are presented in Fig. 6.Fig. 6 EDS pattern from the side of the substrate to the coating.

Fig. 6

According to Fig. 5, Fig. 6, it can be said that the copper, carbon, cerium and zirconium elements are also presented in the chemical composition of the cross section of the coating. AS shown in Fig. 6, it can be seen that the copper layer is under the coating, the amount of copper is dominant and the peaks of other elements are displayed with low intensity. Because, the thickness of coating is more less than 15 (μm). The display of the peak related to gold is also due to the applying of a thin film of gold on the cross-sectional surface of the coating, which is reported with the elements of the coating.

3.3 Surface wettability behavior

In order to investigate, the effect of zirconium chloride concentration on the wettability properties of the coatings, the water contact angle test at room temperature on three different points on the surface of the samples (C1, C2, C3, C4 and C5), and the average amount of contact angles are presented in Table 2. The evaluation of the results of the contact angle in the samples of different coatings show that the addition of zirconium chloride to the electrolyte solution, the wettability properties of the prepared coatings are variable. Thus, among the coated samples, the sample (C3) shows the highest contact angle with water droplet. As a result, in constant concentrations of 0.038 M of cerium chloride salt, 0.1 M of myristic acid and 10 ml of hydrochloric acid, the most optimal concentration of zirconium chloride is 0.014 M, to reach to the super-hydrophobicity property. The reason for the effect of changing the amount of zirconium chloride on the wetting property is that the zirconium chloride is effective on the microstructure of the coating due to changing in the shape, number and average size of needle-shaped leaves and semi-spherical leaves on the surface of the coating (Fig. 1(a–e) and Fig. 13). Because, with the change of the surface morphology of the coating, the roughness of the coating surface changes, and as a result with the change of the roughness, the surface energy changes due to the change in the surface energy of the coating surface. In the following, the images related to the determination of the wetting angle of the coating with different amounts of zirconium chloride are presented in Fig. 7, Fig. 8. Also, in Fig. 9, there is a diagram of the change of the wettability angle vs the concentration of zirconium chloride.Table 2 The effect of zirconium chloride concentration on the wettability properties of coatings prepared with copper anode and hysteresis angle.

Table 2Sample	Zirconium chloride con.(mol.L−1)	Hysteresis angle (Degree)	Contact angle (Degree)	
C1	0	>60	109.3	
C2	0.007	>60	111.9	
C3	0.014	>60	147.4	
C4	0.024	>60	99.2	
C5	0.028	>60	108.3	

Fig. 7 The image of the water contact angle of sample C1 at two random points on the surface of the sample.

Fig. 7

Fig. 8 Image of the water contact angle of sample C3 at two random points on the surface of the sample.

Fig. 8

Fig. 9 The effect of zirconium chloride concentration on the water contact angle (Degree) of the coatings related to samples C1, C2, C3, C4 and C5.

Fig. 9

The wetting properties of fabricated coatings can be evaluated based on the effect of five petals. According to the petal effect, super-hydrophobic surfaces with high adhesion have a large hysteresis angle and water droplets stick on the surface. Therefore, these types of coatings are known as adhesive super-hydrophobic coatings. The Petal effect represents the wettability model of the saturated cup. In this wettability model, water droplets are able to maintain their shape in the position against the earth's gravity. It should be noted that super-hydrophobic coatings with low adhesion are self-cleaning and follow the Lotus effect and can be evaluated according to the Cassie-Baxter wettability model. According to this theory, the transformation of the petal effect into the lotus effect can be made possible by changing the microstructure of the surface and thus controlling the level of surface roughness. In general, the surface morphology and average size of needle-shaped leaves and average size of semi-spherical leaves and their density and ratio on the surface of the coating play a significant role in the amount of adhesion of water droplets on the surface of the coating and consequently on the value of the wetting angle leaves [[57], [58], [59], [60]]. By evaluation the data presented in Table 2, it can be said that in the coating consisting of zero molar of zirconium chloride salt (C1), the formation of cerium myristate with a surface morphology containing needle-shaped leaves and in semi-spherical leaves is happened according to Equations (3)–(1), and therefore the wetting angle of the coating surface increases to 109.3° compared to the wetting angle of copper to 76°. Addition of 0.007 M zirconium chloride salt to the electrolyte (C2), causes the formation of cerium myristate, the composition of zirconium myristate is also confirmed by the FTIR and EDS analyzes. As a result, due to the change in the microstructure of the coating surface, the wetting angle of the surface increases to 111.9°. Furthermore, in the sample (C3), by adding 0.014 M zirconium chloride, the wetting angle reaches to 142.9°. In addition, in the sample (C4), if the concentration of this salt increases to 0.024 M, the wetting angle decreases to 99.2°. Finally, in the sample (C5) with the concentration of 0.028 M zirconium chloride, the wetting angle of the surface reaches to 108.3°. The reason for this case is related to the effect of zirconium chloride concentration on the microstructure of the coating surface. Because the average size of needle-shaped leaves in the surface microstructure of the sample C1 is more than the samples (C2, C3, C4 and C5), (Fig. 1). In the sample with a concentration of 0.007 M of zirconium chloride, with the addition of zirconium chloride to the solution, the average size of the needle-shaped leaves is greatly reduced, and as a result the contact angle with the water droplet or the wetting angle of the coating surface increases. For the sample containing 0.014 M of salt, with increasing concentration of zirconium chloride, the average size of needle-shaped leaves increases and the contact angle of the surface with water droplet increases significantly. However, in the sample with 0.024 M of zirconium chloride, the average size of the needle-shaped leaves and the contact angle of the surface with the water droplet decrease. The reason for the that behavior, can be related to the change of surface roughness (Fig. 13). In the sample containing 0.028 M of salt, with the increase in the concentration of zirconium chloride, the average size of the needle-shaped leaves decreases, however the size of the surface contact angle with the water droplet increases slightly compared to the previous sample (C4). In general, according to the obtained data, it can be concluded that the formation of needle-shaped leaves with low surface energy plays a very important role in creating the super-hydrophobic property on the surface of the coating. It can also be concluded that the optimal conditions for the average size of the needle-shaped leaves and the super-hydrophobic property is related to sample (C3), which is containing 0.024 M zirconium chloride, 0.038 M cerium chloride salt, 0.1 M of myristic acid, and 10 ml of hydrochloric acid. The surface of copper metal, like most metals, is hydrophilic because of its high surface energy, and water droplets spread almost completely on its surface. Based on the research, it is reported that the contact angle of the copper metal surface in the polished state is usually between 60 and 76° [61]. To increase the hydrophobicity of the surface of copper metal, the salts of rare earth elements such as cerium, lanthanum and zirconium are recommended as additives in the coatings created on the copper surface due to the improvement of the hydrophobicity. Among these salts, cerium salts are very important due to their super-hydrophobic properties in coatings. For this reason, in order to investigate the effect of cerium chloride concentration on the wetting properties of the coatings created with copper cathode, the contact angle test with a water droplet at room temperature on three different points of the surface is determined and the average value of the wetting angle is shown in Table 3, the change of the wetting angle in Fig. 10 and the related images in Fig. 11, Fig. 12, are given.Table 3 The effect of cerium chloride concentration on the wettability properties of coatings at 0.014 M of zirconium chloride.

Table 3Sample	Cerium chloride con. (mol.L−1)	Hysteresis angle (Degree)	Contact angle (Degree)	
C3	0.038	>60	147.4	
C6	0.019	>60	120.1	
C7	0.0560	>60	161	

Fig. 10 Cerium chloride concentration vs the water contact angle (Degree) of the prepared coatings.

Fig. 10

Fig. 11 Images of water contact angle of sample C6 at two random points on the surface of coating.

Fig. 11

Fig. 12 Images of water contact angle of sample C7 at two random points on the surface of coating.

Fig. 12

Fig. 13 Atomic force microscopic images of surface a. Uncoated copper base and b. Copper base with cerium myristate-zirconium myristate coating fabricated by electrodeposition method for the sample (C7).

Fig. 13

Analyzing the results in Table 3, it can be seen that the wettability properties of the prepared coatings are varied due to adding the different amounts of cerium chloride salt to the electrolyte solution. Then, by increasing the concentration of cerium chloride salt in a constant concentration of zirconium chloride salt, the density of needle-shaped leaves in the microstructure increases. With the formation of nanostructured needle-shaped leaves due to creating air gaps on the surface of these leaves and thus they prevent the water droplets from entering to the inner areas of the coating and inhibit them from reaching to the substrate, and as a result it prevents the corrosive fluid from penetrating to the surface of the substrate. And then, the coating protects the copper surface from the penetration of corrosive solution. Therefore, with the increase in the concentration of cerium chloride salt, the contact surface angle with the water droplet of the coatings increases. In this case, among the three samples containing the coating, the sample (C7) has the highest contact surface angle with water droplet, and as a result the surface of the sample (C7) has super-hydrophobic property. In general, according to the data presented in Table 3, it can be noted that in the electrolyte with constant concentrations of 0.014 M of zirconium chloride and 0.1 M of myristic acid with increasing the concentration of cerium chloride salt, the contact surface angle with the water droplet on the surface of the applied coating on copper increases. The contact surface angle of the super-hydrophobic coating of the sample (C7) is different compared to the contact surface angle of the copper sample without the coating, which is due to the low roughness of the copper surface compared to the high surface roughness of the sample (C7), (Table 4, and Fig. 13). In Fig. 13, Atomic force microscopy images and the roughness values obtained from uncoated and coated sample (C7) is shown. According to Fig. 13, the wetting angle of the coating surface of sample (C7) is equal to 161°, which has a higher roughness compared to the uncoated copper sample with a contact surface angle of 76°. Based on the conditions of establishing the wetting model and compared to the uncoated copper sample, the saturated Cassie model has super-hydrophobic properties [57].Table 4 Comparison of surface roughness of uncoated and coated copper sample based on Atomic force.

Table 4Sample	Contact angle (Degree)	Ra (nm)	Rq (nm)	
Super hydrophobic coating surface	161.7	40.10	49.69	
Uncoated copper surface	75.2	31.03	49.14	

Moreover, the effect of deposition time on the wettability properties of the coatings and the change of the contact angle of the surface with a droplet of water at room temperature on three different points on the surface of the samples (C7, C8, C9 and C10) were studied. And their average amount was reported as the wetting angle in Table 5 and Fig. 14. By evaluation the mentioned results, it can be seen that the wettability properties of the coatings applied on copper changes with the increase in the duration time of the electrodeposition process. In this case, among the four samples under the investigation conditions compared to other samples, sample (C7) has the highest contact angle with the water droplet and the surface of the coating, which has the super-hydrophobic properties. As a result, in constant concentrations of 0.056 M of cerium chloride salt, 0.014 M of zirconium chloride, 0.1 M of myristic acid and 10 ml of hydrochloric acid, the most optimal electrolyte composition and deposition time is 10 min in electrodeposition process to making the super-hydrophobic coating of cerium myristate and zirconium myristate. It should be noted that the process of changes in the wetting properties of the coating surface with the change of deposition time is dependent on the average size of needle-shaped leaves and semi-spherical leaves in the micro and nano structure and surface roughness (Fig. 13).Table 5 The effect of deposition time on the wetting properties of coatings applied with copper cathode.

Table 5Sample	Deposition time (min)	Contact Angle Hysteresis (Degree)	Contact Angle (Degree)	
C7	10	>60	161	
C8	5	>60	116.7	
C9	15	>60	145.6	
C10	30	>60	118.3	

Fig. 14 The graph of deposition duration (min) vs water contact angle (Degree) of the surface coatings.

Fig. 14

On the other hand, the coating consisting of cerium myristate and zirconium myristate on copper, there are kinds of rose petals [59], chives, and garlic that show super-hydrophobic properties and have a high hysteresis contact angle (greater than 60°). Since the water droplets easily roll on the surface of them. This phenomenon has not occur on these super-hydrophobic surfaces, but the water droplets stick on these surfaces. It is seen that the chemical composition of the surface plays a significant role in the wetting behavior of the surface [60]. The wetting properties of prepared coatings can be checked based on the effect of five petals. According to the petal effect, super-hydrophobic surfaces with high adhesion have a large hysteresis angle and water droplets stick on the surface. Therefore, these types of coatings are known as sticky super-hydrophilic coatings. The Petal effect represents the wettability model of the saturated Cassit model. In this wettability model, the water droplets are able to maintain their shape in the position against the earth's gravity. It should be noted that super-hydrophobic coatings with low adhesion are self-cleaning and follow the Lotus effect and can be evaluated according to the Cassie-Baxter wettability model. The transformation of the petal effect into the lotus effect can be made possible by changing the size of the surface microstructure. In general, the morphology and size of needle-shaped leaves and average size of semi-spherical leaves on the surface play a significant role in the state of wettability (Fig. 1(a–e), Fig. 2(a–d), and Fig. 14). Liu et al. reported that increasing the duration of deposition time leads to changes in the structural morphology of the coating and the wettability transformation of petal effect to lotus effect occurs [61]. Similar to the mentioned results, it is observed the chemical composition of coating and duration time process have the most important effect on the nano and micro structural of the coating. Therefore, it can be inferred that the porous micro and nanostructure with low surface energy has important role in the super-hydrophobic behavior. When the Ce+3/Zr+4 ratio is increase in the solution, the contact angle is enhanced to 161.7°. In addition, when the duration time of deposition becomes 10 min, the contact angle of 161.7° is the maximum amount. This finding confirms that the super-hydrophobic surface has the self-cleaning behavior. The enhance in contact angle can be related to the generation of the heterogeneous structure (Needle-shaped leaves and semi-spherical leaves, (Fig. 1(a–e) and Fig. 2(a–d)). When, the deposition time is varied from 5 min to 30 min, the contact angle of the surface is changed from 161 to 118°, and then the super-hydrophobic behavior is slightly decreased. Moreover, with further increase of the duration time of electrodeposition process from 15 to 30 min, the contact angle amount has decreased a little, however the surface coating has the super-hydrophobicity behavior still. Therefore, when a water droplet on the surface of the copper is spherical, that cannot roll off, and thus the surface tension force between the copper structure and water droplet induces to the adhesive behavior. However, when a water droplet contacts with the super-hydrophobic coating can roll off from it without keeping any contamination, that was shown the water droplet contact angle on the super-hydrophobic surface coating in macroscopic scale [32]. This behavior is defined the lotus effect, which is led to the self-cleaning property. The surface chemistry and the surface morphology are controlled the wettability and the contact angle of water droplets with the surface coating. Compared to the non-coated surface and the super-hydrophobic coated surface, there are the same factors of the liquid vapor interface energy and the critical nucleation radius. Thus, it can be seemed that the contact angle of the water droplets increases with the increase of the rough surface [25].

The SEM study and the contact angle determine indicate that the chemical compositions and surface microstructure changes on the copper surface are important parameters on the surface wettability with optimized adhesion. To study the issue of the resulted super-hydrophobicity, the contact angle is evaluated using the Cassie–Baxter model [62].

3.4 Adhesive quality

Since among all the prepared samples, only the sample (C7) contains a composite coating with a surface contact angle equal to 161° and has super-hydrophobic properties. Therefore, the adhesion quality of the mentioned coating to the substrate in the sample (C7) with the cross-cut test, has been evaluated. According to the description in ASTM D3359 [60], the location of the cross cuts in the cross-cut test is evaluated in the state of the degree of separation of the coating from the surface of the substrate, and as a result it can be said that if the adhesion of the coating to the substrate is higher than B5, therefore it is to the optimal conditions in the state of the adhesion quality of the coating on the substrate. Furthermore, it can also be concluded that the separation rate of the coating from the substrate in B5 state, which shows the almost zero percent separation and the adhesion quality of the coating to the substrate is satisfactory (Fig. 15).Fig. 15 A macro view of the surface of the coating before the cross-cut test (Left) and a macro view of the separation of the coating from the substrate after the cross-cut test (Right).

Fig. 15

3.5 Corrosion behavior

In order to investigate the electrochemical behavior of the coatings, the potentiodynamic polarization test was used after 30 min immersion of the coatings in 3.5 wt% sodium chloride solution. And then, the polarization curves of the coatings with different contact angles with water droplet are shown in Fig. 16. Based on Fig. 16 and Table 6, it is seen the change in the corrosion current density vs the potential in the deposited coatings, and also it is observed that the contact angle of the surface with the water droplet with different values under different conditions is related to the concentration of the initial salts and the duration of deposition. Since, corrosion potential (Ecorr), corrosion current density (icorr), and polarization resistance (Rp) are important for the ultimate corrosion resistance of coatings. Therefore corrosion parameters including corrosion potential (Ecorr), corrosion current density (icorr) from the intersection point of two cathodic branches (βc) and anodic branch (βa) are determined and shown in Table 6. And, the values of corrosion parameters for the coatings are reported as well.Fig. 16 Corrosion current density vs the potential for uncoated copper and coated copper under different conditions in a 3.5 wt% of sodium chloride solution under the open circuit potential for 15 min with a sweep rate of 0.5 mV s−1 in the potential range of −250 to 250 mV in compared to the open circuit potential.

Fig. 16

Table 6 Corrosion potential values and corrosion current density and other corrosion parameters related to the coatings.

Table 6Sample	(i = 0) Ecorr (mV)	icorr (A. cm−2)	βa (mV)	βc (mV)	Rp (kΩ.cm−2)	Contact angle (Degree)	
Cu-pure	−276	0.000132	359.2	−247.3	4.81	75.2	
C1	−274.1	0.00001	76.5	−40.6	1.07	109.3	
C3	−220.6	0.000001	92.7	−11.4	4.29	147.4	
C6	−252.8	0.000014	102	−98.1	3.16	120.1	
C7	−34.2	0.00000002	211.4	−270.1	103 × 2.41	161	
C8	−268.4	0.000011	88.1	−366.6	2.65	116.7	
C9	−224.1	0.00000008	161.0	−220.8	20.17 × 103	145.6	

As can be seen in Table 6, the value of corrosion potential from sample C1 to C6 has led to more positive values compared to the copper corrosion potential, which has confirmed the formation of the layer on the copper substrate. Meanwhile, in the sample (C7), the corrosion potential value has changed to a positive value compared to the corrosion potential of copper and other coatings, which shows that the surface of that coating acts as a nobel metal and the hydrogen reduction reaction occurs on it. Furthermore, it has been observed in during the formation of the coating, the formation of hydrogen bubbles and the release of hydrogen gas were visible on the surface of the coating. On the other hand, the rate of hydrogen release was slower. In addition, by study the corrosion rate of coated samples compared to the copper corrosion rate without coating, it has been greatly reduced, which indicates the formation of a corrosion-resistant coating (2.41 × 103 kΩ cm−2) on copper (4.81 kΩ cm−2). Moreover, it can be concluded that super-hydrophobic coating has better corrosion resistant in the corrosive solution than the other coatings and uncoated copper. The reason of this phenomenon can be explained as the formation of cerium myristate and zirconium myristate components in the coating (C7) with super-hydrophobic properties with a wetting angle of more than 161° due to the fabrication of a microstructure similar to the surface of lotus leaves, which has needle-shaped leaves and semi-spherical leaves and the mentioned type of microstructures easily trap the outside air in their structure and lock them in themselves. And as a result, they form the air pockets between the corrosive environment and the substrate. Due to the occurrence of such an air gap, they prevent the penetration of the corrosive solution containing destructive ions such as chlorine ions (Cl−) and water droplets toward the substrate. Furthermore, due to the super-hydrophobic property of the surface, water and electrolyte droplets fall from the surface of the coating and are prevented from long-term contact with the surface and the kinetic conditions of corrosion reaction are disturbed. Fig. 17, shows the mechanism of formation of air pockets on the rough surface of the coating (Fig. 13-b)).Fig. 17 Schematic of the presence of an air gap between the corrosive substances and the pores of the super-hydrophobic coating copper substrate.

Fig. 17

Another factor is the effect of capillary force on super-hydrophobic surfaces. The capillary effect leads to the convexity of the surface between the liquid-air interface [62]. The air trapped in the micro and nano porous of the super-hydrophobic coating due to the generation Laplace pressure (LP) pushes the corrosive solution to the outside, and thus prevents the penetration of corrosive solution toward the copper substrate [[57], [58], [59], [60], [61]]. Fig. 17 indicates the schematic of a water droplet on surface coating when a droplet of water is in contact with the surface of coating, in that state the air is filled with gaps and inhibited a water droplet from penetrating onto the copper substrate. The formation of needle-shaped leaves and semi-spherical leaves as the porous micro and nanostructure is one of the most important parameters of a little adhesive, that this behavior induces the negative pressure due to the trapped air in the air pockets, which is a main term in the super-hydrophobic property [62]. Because, water droplet can penetrate and wet the rough surface (Rod-like structure), however can not wet the porous structure (Needle-shaped leaves and semi-spherical leaves), which is related to the capillary theory of the micro and nano structure. The capillary force is very strong, hence the water droplet can be blocked on the surface coating, if the surface coating is bended to certain angle or even backed upside and down. Therefore, it is seemed the large capillary force generates the obtained super-hydrophobic coating with higher adhesion, which causes the water droplet to cling to the surface coating strongly. As a result, the controlled adhesion wetting behavior is affected by changing the morphology and size of the micro-nanostructure. The morphology of gas-liquid interface is evaluated by the pressure difference between the outside and inside of the gas - liquid interface, and it is affected on the gas-liquid interface morphology and stability due to changing the pressure difference between the outside and inside of the gas – liquid interface. Furthermore, hydrostatic pressure is depended on the stability of the gas - liquid interface. When, the gas – liquid interface trapped, underwater droplet for a long duration, the air in the air layer is suddenly dissolve into the water droplet until it obtains a dynamic equilibrium. The concentration exchange of material between the two sides of the gas - liquid interface influences the pressure difference between the two sides of gas – liquid interface, which changes the morphology of the gas - liquid interface and the stability of the gas – liquid interface. Both the gas concentration and pressure in the air gap due to diffusing gas into the air gap induces the super-hydrophobicity property and gets the stable for a long time. Therefore, the super-hydrophobicity surface induced the satisfactory water repellency. The formation of the surface roughness structure proves to improve the performance of the super-hydrophobicity surface, and enhances the roughness structure of the micron and nano structure coating, and better radicalizes the alkali salt and acid resistance of the super-hydrophobic coating on the copper substrate. Therefore, it is noted that the micro and nano structure of coating surface decreases the liquid – solid contact area and gets a high roughness, which can trap more air between the air gap and inhibiting the penetration the corrosive solution corrosion on the copper substrate. And, the surface roughness morphology has a relatively good state, thus the hydrophobic angle of the coating is more than 161° [27]. On the other hand, on the surface of super-hyrophobic coating, the water droplets adsorbed, according to the special super-hydrophobic behavior, the water droplets due to rapid rolling off, the pollutants dissolved in the water droplets rolling direction were carried away from the durface coating. Due to adding of water droplet, the pollutants continuously were cleaned without putting any liquid residue, which was shown the superior self-cleaning property. The self-cleaning behavior of surface coating could be increased the corrosion resistant of the substrate metals [48].

4 Conclusion

In this research, a super-hydrophobic coating of cerium myristate and zirconium myristate was fabricated on the copper substrate using the electro-deposition process, and the various properties of coatings were investigated. The important results of this research can be summarized as follows:

The formation of cerium myristate and zirconium myristate components in the chemical composition of the coating causes the formation of a microstructure of needle-shaped leaves and semi-spherical leaves on the copper substrate, which due to having a lotus-like microstructure, the coating in optimum conditions and to having a wetting angle of more than 161°, has super-hydrophobic properties. Furthermore, the duration of deposition time (10 min) has an effect on the average size of needle-shaped leaves and the average size of semi-spherical leaves in the microstructure of the coating. In addition, the ratio of the concentration of cerium chloride to zirconium chloride has an obvious effect on the microstructure of the coating. Therefore, with an increase in the concentration of zirconium chloride in the electrolyte with a constant concentration of cerium salt, the formation of needle-shaped leaves compared to semi-spherical leaves increases. Finally, by evaluation the wetting behavior of the coatings, it was found that the coating applied on copper with the choice of electrolyte including 0.014 M zirconium chloride and 0.056 M cerium chloride, the contact angle of the surface with the water droplet is equal to 161°, which has super water repellency property. Moreover, the investigation of the polarization curves showed that the same coating has a much lower corrosion current density 0.0000002 A/cm2 compared to the copper substrate 0.000132 A/cm2 and other coatings due to maintaining the super-hydrophobic properties.

Data and code availability

The raw/processed data required to reproduce these findings will be made available on request.

CRediT authorship contribution statement

Sara Behniya: Writing – review & editing, Writing – original draft, Supervision, Methodology, Data curation, Conceptualization. Mardali Yousefpour: Data curation.

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.

Acknowledgments

The authors would like to acknowledge the 10.13039/501100007103 Semnan university of Iran and Nirou research center of Iran.
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