Introduction

Because of their significance in fundamental science and commercial applications, superhydrophobic surfaces have sparked great interest [1,2,3,4,5]. Superhydrophobic surfaces have applications in diverse fields, such as anti-icing [6], oil–water separation [7], corrosion resistance [8], self-cleaning [9], drag reduction [10], and antifouling technologies [11]. Superhydrophobic surfaces have been fabricated using various techniques, such as electrospinning [12], etching [6], sol–gel process [13], electrodeposition [14], laser fabrication [15], and electrochemical anodization [16]. However, most of these approaches require extreme conditions that restrict their practical aspects, such as complicated chemical treatments, high-cost materials, and multi-step manufacturing. Due to its low cost, flexibility, ambient temperature operation, and ability to monitor electrodeposition parameters, electrodeposition is an excellent technique for creating artificial superhydrophobic surfaces. Thus far, the commercialization of superhydrophobic surfaces has been significantly reduced due to their low surface chemical and mechanical stabilities and the fragility of their microscopic nanostructures characteristics [17]. The mechanical strength and durability of superhydrophobic coatings have been a major focus of recent research [17, 18]. It is essential to improve the mechanical abrasion resistance and chemical stability of superhydrophobic surfaces in order to use them in industrial applications.

Cobalt (Co) is commonly used in aerospace, shipbuilding wear-resistant, automotive, corrosion resistance, high-strength alloys, catalysis, solar energy absorption and magnetic recording [19, 20]. The material’s internal characteristics and morphology decide these different characteristics [21, 22]. As a result, controlling the development of unique cobalt nanostructures has become a critical issue in the materials fabrication industry. Because of its strength, single atomic layer thickness, chemical inertness, and impermeability to most gases, graphene is thought to be a good material for coatings [23,24,25,26]. Graphene can be processed as nanoplatelets, nanosheets, and functionalized graphene using various methodologies, such as chemical or electrochemical exfoliation, chemical vapor deposition, and single-crystal SiC crystal cleavage and annealing [27, 28]. Carbon-based films typically show low substrate bonding strength and low hydrophobic efficiency, which decrease their applications significantly. Fortunately, doping with metals or non-metals also enhanced substrate adhesion and uniformity; moreover, some novel characteristics can be found based on retaining the original excellent efficiency [29].

Steel materials are used in a wide range of industries because of their high mechanical strength. However, steel structures have poor corrosion resistance due to their thermodynamic instability when exposed to extreme temperature, humidity, and pH [30]. Many techniques have been used to protect the steel surfaces; an important of them is the fabrication of superhydrophobic films, which greatly improve the corrosion resistance of steel [31]. Superhydrophobic coatings were fabricated on different substrates using different techniques [32,33,34,35,36,37]. Philip et al. studied the fabrication of robust superhydrophobic coating on ferritic steel with the self-cleaning ability and superior corrosion resistance using a template-free one-step electrodeposition method [32]. Siddaiah et al. studied the steel coatings by nickel (Ni) as well as Ni–graphene (Ni–Gr) [36]. On a Si substrate, Yan et al. created carbon-based films with excellent self-cleaning and corrosion resistance [37]. Dong et al. [4] studied the fabrication of Ni–B4C superhydrophobic composite coatings at Q235 steel by electrodeposition and investigated its corrosion performance. Zhang et al. [38] investigated the preparation and performance of a biomimetic flower-like superhydrophobic coating on X80 pipeline steel using a static self-assembly method. Abd-El-Nabey et al. [14] studied the construction of robust superhydrophobic films on steel surfaces and studied their corrosion performance, mechanical and chemical stability.

This study aims to fabricate Co film and Co film doped with in situ prepared graphene by electrochemical exfoliation method on the steel surface. Then, the as-prepared films were modified with stearic acid to fabricate superhydrophobic surfaces. To the best of our knowledge, this is the first study on constructing a superhydrophobic Co film doped with in situ produced graphene. The effect of graphene doping on the wettability, mechanical and chemical stability, long-term stability in 0.5 M NaCl, and corrosion resistance properties of a superhydrophobic cobalt film modified by stearic acid were investigated.

Experimental

Materials

Sodium hydroxide, anhydrous ethanol, sodium chloride, sulfuric acid, cobalt chloride, boric acid, sodium carbonate, sodium dodecyl sulfate and stearic acid were purchased from Sigma-Aldrich.

Sample preparation

A steel plate with dimensions of 2.0 × 2.0 × 0.1 cm and chemical composition of (wt%): S, 0.04; C, 0.21; Mn, 2.5; Si, 0.35; P, 0.04 and Fe, 96.86 was used as a working electrode. The substrate was rubbed with emery paper of various grades before electrodeposition, beginning with coarse (150 grade) and progressing in steps to the finest (1200 grade). Then it was degreased for 30.0 min in an aqueous solution containing 20 g L−1 Na2CO3, 15 g L−1 NaOH and 8 g L−1 sodium dodecyl sulfate, followed by pickling and oxide removal in 2.0 M H2SO4 for 1.0 min, and finally rinsing with distilled water and ethanol. The electrodeposition was done in an aqueous solution containing CoCl2 (200 g L−1) and H3BO3 (30 g L−1) using a potential of 3.0 V and deposition time equals 20.0 min; then, the potential was increased to 7.0 V for a time of 1.0 min. The two-step electrodeposition was used to fabricate various superhydrophobic films [39,40,41]. In the first deposition step (at low potential), the crystal growth rate of cobalt is faster than the nucleation rate, so a coarse deposit of Co is formed. In the second electrodeposition step (at high potential), the nucleation rate of cobalt is faster than the crystal growth rate, so a nano deposit of Co is formed. For the manufacture of Co film, a platinum rod was used as the anode, and for the manufacture of Co film doped with graphene, Co-G film, a graphite rod was used as the anode, where the graphite is electrochemically exfoliated, giving graphene, which incorporated into the deposited layer of Co [42, 43].

After electrodeposition, the Co and Co-G films were washed with distilled water and dried for 24.0 h at room temperature, immersed in 0.01 M ethanolic solution of stearic acid for 30 min, and then removed from the solution and left to dry at room conditions. After that, the as-prepared Co film modified by stearic acid, Co-SA, and the Co-G film modified by stearic acid, Co-G-SA, were exposed to different evaluations and characterization techniques.

Characterization

The surface morphology of prepared films was studied using a scanning electron microscope, SEM (model JSM-IT 200). A Fourier transform infrared spectrophotometer, FTIR (FTIR LX 18–5255 Perkin Elmer), was used to investigate the chemical composition of the prepared films. The spectra were recorded in the wave number range of 4000–500 cm−1. X-ray diffraction (XRD) was done with monochromatic Cu K radiation (λ = 0.154056 nm) by an X-ray diffractometer (Bruker D2 phaser). An optical contact angle meter (OCALS plus) was used to calculate the water contact angle, CA, and sliding angle, SA, with 5 µl water droplets. The recorded CAs and SAs are the averages of three measurements taken at various locations on the sample’s surface. A coating thickness gauge (FN Type CT-100) measured the thickness of the as-prepared superhydrophobic films. The reported film thickness is the average of three measurements done at different positions of the prepared sample.

Mechanical abrasion

The abrasion test has been used to analyze the mechanical stability of the as-prepared films. Sandpaper (1800 mesh and length of 30 mm) has been used as an abrasion surface. The prepared superhydrophobic film is oriented to face the sand surface, and a pressure of 5.0 kPa was applied to the superhydrophobic film.

Chemical stability

A water droplet with various pH values (pH = 1–13) was placed on the prepared superhydrophobic surfaces, and the CAs and SAs at each pH were measured. Sodium hydroxide and sulphuric acid have been used to control the pH value of the water droplet.

Corrosion tests

All electrochemical experiments were carried out in a three-electrode cell containing a 0.5 M NaCl aqueous solution at room temperature using a frequency response analyzer potentiostat (PARSTAT, USA). The reference and counter electrodes were an Ag/AgCl electrode and a platinum rod. The bare steel and steel covered by superhydrophobic Co-SA and Co-G-SA films have been used as working electrodes. The operating procedure was given in previous work [44]. The working electrode was placed in a cell containing 0.5 M NaCl solution that was opened to the environment at room temperature and left for 30 min before electrochemical measurements were taken to establish the equilibrium potential. The frequency range of the electrochemical impedance spectroscopy (EIS) measurements was 0.01 ≤ f ≤ 1.0 × 105 with an applied potential signal amplitude of 10 mV around the equilibrium potential. The polarization curves were measured starting from cathodic potential (-300 mV) to anodic potential (+ 450 mV) around the equilibrium potential at a 0.5 mV/sec scan rate.

Results and discussion

Chemical composition and morphology of surfaces

SEM and wettability results

One of the most outstanding parameters for investigating superhydrophobic properties is the surface morphology, so the SEM technique has been used to study the topography of the prepared superhydrophobic films on the steel substrate. Figure 1a shows the micrograph of steel coated by Co; it is clear that the electrodeposited cobalt has micro-nanostructures of papillae structure. Figure 1b shows the micrograph of the magnified papillae structure, which has a hierarchical roughness. Figure 1c shows the micrograph of steel coated by Co-SA; it shows stearic acid's white structures covering the electrodeposited micro-nano-papillae structure.

Figure 1
figure 1

SEM images of the prepared films a Co film, b magnified image for papillae structure in Co film, c Co-SA film, d Co-G film and e Co-G-SA film

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Figure 1d shows the micrograph of steel coated by Co-G; it is clear that the graphene forms a network layer of micro-nano-papillae structure; the porosity of the network gives a higher roughness, hierarchical roughness, to the formed layer. Figure 1e depicts the micrograph of steel coated by Co-G-SA; it is clear that stearic acid, white structures, graft the prepared film. The values of the contact angles of bare steel and steel coated with Co and Co-G films are 58°, 21° and 15°, respectively. The value of contact and sliding angles for Co-SA film are 155° and 3°, respectively, while the contact and sliding angles for Co-G-SA are 158° and 2°, respectively. The contact angle image of the water droplet on the prepared superhydrophobic surfaces is shown in Fig. 2.

Figure 2
figure 2

Contact angle graphs of steel coated with a Co-SA film, and b Co-G-SA film

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These results indicate that the grafting of the Co film at the steel surfaces leads to an enhancement of the hydrophilic character of the bare steel, and the contact angle is decreased. While, when the Co film is doped with graphene, Co-G film, the contact angle shows a greater decrease reflecting higher hydrophilic characteristics. When the prepared Co and Co-G films are modified with stearic acid, superhydrophobic surfaces were obtained with greater superhydrophobic characteristics of Co-G-SA film due to the presence of graphene, which enhances the surface roughness. These results can be discussed on the basis that as the surface roughness increases, the hydrophilic surfaces become more hydrophilic, while the hydrophobic surfaces show higher hydrophobicity [45].

The thickness of the Co-SA and Co-G-SA films are 19 µm and 25 µm, respectively. These results demonstrate that doping of Co film with graphene increases the thickness of the prepared film and improves the roughness and so shows higher superhydrophobicity after modification with the stearic acid as a low surface energy material. Based on the Cassie–Baxter state [46], much air can be easily stored in the micro-nanostructures of the Co-G-SA film. Furthermore, the wettability of the superhydrophobic coated steel by Co-G-SA is superior to several previously recorded values [47, 48].

FTIR and XRD results

Figure 3 and Table 1 show the FTIR spectra and band assignments of stearic acid powder and steel coated by Co, Co-SA, Co-G, and Co-G-SA. The spectrum for steel coated with cobalt shows a peak at 576 cm−1 characteristics of the Co–O stretching vibration. In addition, the weak absorptions peaks at 3342 and 1660 cm−1 correspond to the stretching and bending vibration modes of the –OH group; this will prove the absorption of water by the deposited nanostructure of cobalt at the steel surface[49, 50]. The spectrum for the powder of stearic acid shows the characteristic peaks of the stearic acid. The peaks at 2866 cm−1 and 2944 cm−1 are assigned to the stretching vibrations of –CH2 groups. The peak at 1719 cm−1 is due to the stretching vibration of the carbonyl group of stearic acid. The peak at 1314 cm−1 is attributed to the bending vibration of -OH, while the peak at 1477 cm−1 is due to the bending vibration of C–H [51, 52]. The spectrum for steel coated by Co-G depicts the characteristic peaks of the graphene. The peak at 3363 cm−1 is attributed to the bond tension vibration of O–H; the peak at 1151 cm−1 corresponds to the hydroxyl groups of the graphene. The peak at 1651 cm−1 is characteristic of the double bond's graphene ring. The peak at 1078 cm−1 is attributed to the C–O–C group [53].

Figure 3
figure 3

FTIR spectra of stearic acid powder and steel coated by Co, Co-G, Co-G-SA, and Co-SA

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Table 1 FTIR band assignments for stearic acid powder and steel coated by Co, Co-G, Co-SA, and Co-G –SA films
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The steel coated by Co-SA spectrum shows two peaks at 2919 cm−1 and 2850 cm−1 attributed to asymmetry and symmetry vibration of –CH2– of the stearic acid. The stretching vibration of C=O is responsible for the peak at 1702 cm−1, while the bending vibration of C-H is responsible for the peak at 1468 cm−1. The bending vibration absorption peak of –OH is at 1330 cm−1. The stearic acid peaks at around 2922, 2852, and 1703 cm−1 in the spectrum for steel coated with Co-G-SA suggest that the deposited graphene is modified by stearic acid.

The composition and crystal orientation of steel coated with Co-SA and Co-G-SA films were determined using the XRD technique. The XRD patterns of Co-SA and Co-G-SA films are depicted in Fig. 4. The diffraction peaks imply that the deposited film is Co3O4, and its sharpness indicates that the deposited cobalt has good crystallinity [54, 55]. For Co-SA coating, there are six diffraction peaks at 2θ values of 30.7°, 36.4°, 38.1°, 44.5°, 59.1° and 64.9°, which correspond to faced cubic centered, fcc, of Co3O4 (JCPDS No. 00-042-1467)[56]. The (311) plane has the highest intensity of the three peaks, indicating the preferred crystal orientation, with higher periodicity than the other orientations [57]. The Co-G-SA film has the same six diffraction peaks as the Co-SA film, with one greater diffraction peak at 2θ values of 24.2°, corresponding to graphene [58]. The graphene peak is broad, showing that graphene has a small particle size.

Figure 4
figure 4

XRD patterns of steel coated by a Co-SA film and b Co-G-SA film

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Mechanical abrasion resistance

The superhydrophobic surfaces are susceptible to mechanical abrasion. Improvement of superhydrophobic coatings’ abrasion resistance has become the primary concern for their industrial applications [59].

Abrasion testing was used to assess the resistance of the prepared superhydrophobic films to mechanical abrasion. The variations in water contact and sliding angles of the prepared superhydrophobic films as a function of the number of abrasion cycles are shown in Fig. 5.

Figure 5
figure 5

CAs and SAs as a function of the number of abrasion cycles for coated steel by a Co-SA film, and b Co-G-SA film

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Increased abrasion cycles result in a decrease in contact angle values and an increase in sliding angle values, as seen in the graph. The superhydrophobic Co-SA film exhibits superhydrophobicity until 500 abrasion cycles; however, the superhydrophobic Co-G-SA film maintains superhydrophobicity until 900 abrasion cycles, which may be due to the higher adhesion of the low surface energy stearic acid to the rough structure of the Co-G composite than that of the Co film alone.

Figure 6 shows the SEM micrographs of steel coated with Co-SA and Co-G-SA films after the abrasion test. The micro-nano-papillae structure was destroyed, and the density of stearic acid (white structures) on the surface was decreased, so the surface lost its superhydrophobic characteristics. The contact angle shape of the water droplet on the prepared superhydrophobic surfaces after the abrasion test is shown in Fig. 7. The abrasion resistance of the superhydrophobic coated steel by Co-G-SA is superior to several previously recorded values [32,33,34,35, 47, 60, 61].

Figure 6
figure 6

SEM images of the superhydrophobic films a Co-SA film and b Co-G-SA film after abrasion test

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Figure 7
figure 7

Contact angle images of steel coated with a Co-SA film and b Co-G-SA film after abrasion test

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Chemical stability

To demonstrate that the prepared superhydrophobic film can be used in the industrial sector, a chemical stability test must be conducted. Figure 8 shows the relationship between each contact and sliding angles and the pH of the water droplets.

Figure 8
figure 8

Variation of pH values of a water droplet with the CAs and SAs of the coated steel by a Co-SA film and b Co-G-SA film

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The Co-SA film has superhydrophobicity only in the pH range 3–11, while the Co-G-SA film has superhydrophobicity in the pH range 1–13, where the CAs are always greater than 150, and the SAs are less than 10. The two essential factors necessary for the fabrication of superhydrophobic films are low surface energy and surface roughness. So, the aggressive acidic and basic liquids could reduce the density of hydrophobic groups on the surface and destroys the micro/nanostructures of the surface, and so the surface loses its superhydrophobic characteristics [9, 45, 62, 63]. The chemical stability of the superhydrophobic coated steel by Co-G-SA is superior to several previously recorded values [47, 64].

Long term stability

The manufacture of surfaces with long-term stable superhydrophobicity remains a major problem that restricts superhydrophobic surface industrial applications. By measuring the contact angle every 2 days for 30 days, the long-term stability of the prepared Co-SA and Co-G-SA superhydrophobic films on steel substrate in 0.5 M NaCl solution was investigated. Figure 9 shows that the Co-SA film exhibit superhydrophobicity and has a contact angle greater than 150°; after immersion for 20 days, the contact angle becomes smaller than 150°, and the surface loses the superhydrophobic property.

Figure 9
figure 9

CAs and SAs as a function of immersion time of coated steel by a Co-SA film, and b Co-G-SA film

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The Figure demonstrates that the Co-G-SA film retains superhydrophobicity after 30 days in a 0.5 M NaCl solution, implying that doping the Co-SA film with graphene improves the long-term stability of the superhydrophobic film prepared. The prepared superhydrophobic films will lose their superhydrophobic characteristics after a definite immersion time in a 0.5 M NaCl solution as the Cl− ions attack the films and could decrease the hydrophobic groups' density on the surface and destroy the micro/nanostructures of the surface, and so the surface loses its superhydrophobicity. The enhanced mechanical, chemical, and long-term stability of Co-G-SA layer in a 0.5 M NaCl solution is due to the synergistic effect of superhydrophobicity and the high chemical and mechanical stability, impermeability, hydrophobicity, and chemical inertness of graphene [58, 65,66,67,68,69]. The long-term stability of the superhydrophobic coated steel by Co-G-SA is superior to several previously recorded values [5, 18, 70, 71].

Corrosion resistance behaviour

Potentiodynamic polarization results

The corrosion resistance of bare steel and superhydrophobic coated steel by Co-SA and Co-G-SA has been investigated using the potentiodynamic polarization technique. Figure 10 shows the potentiodynamic polarization curves of bare steel and superhydrophobic coated steel in a 0.5 M NaCl aqueous solution.

Figure 10
figure 10

Potentiodynamic polarization curves for bare steel and superhydrophobic coated steel in 0.5 M NaCl solution

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It is obvious that the cathodic polarization plots show a limiting diffusion current, IL, due to the reduction of oxygen according to Eq. (1).

$${\text{O}}_{2} + 2{\text{H}}_{2} {\text{O}} + 4{\text{e}} \to 4{\text{OH}}^{ - }$$
(1)

Thus, the cathodic process is controlled by mass transport. The rapid formation of corrosion products, in the case of bare steel, or formation of passive layer, in the case of the prepared superhydrophobic coated steel, on the electrode surface hinders the development of an ideal anodic Tafel region [72, 73].

Table 2 displays the result of the bare steel and superhydrophobic coated steel's potentiodynamic polarization parameters, including corrosion current density, icorr., corrosion potential, Ecorr., and protection efficiency, % P. Equation 2 is used to measure the protection efficiency [74]

$$\% P = \, \left[ {\left( {i^{o}_{{{\text{corr}}{.}}} - i_{{{\text{corr}}{.}}} } \right)\backslash i^{o}_{{{\text{corr}}{.}}} } \right] \, \times \, 100$$
(2)
Table 2 The potentiodynamic polarization parameters for the bare steel and the prepared superhydrophobic coated steel in 0.5 M NaCl solution
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iocorr. and icorr. are the corrosion current density for bare steel and superhydrophobic coated steel.

It is clear that the icorr. value for steel coated with Co-SA (0.7094 µA) is lower than that of bare steel (0.1457 mA); this can be attributed to coated steel's superhydrophobic behaviour. The trapped air around the microstructures can reduce the contact area between the coated steel and the solution, resulting in a significant reduction in the icorr [75]. The steel coated with Co-G-SA film has a greater reduction in both the contact area between the coated steel and the medium and the icorr. value (0.1732 µA) because the presence of graphene increases the superhydrophobicity of the prepared Co-G-SA film as well as the high mechanical and chemical stability, hydrophobicity, impermeability, and chemical inertness of graphene [58, 65,66,67,68,69]. So, the inhibition efficiency of steel coated by Co-G-SA is higher than that of Co-SA. The value of Ecorr for steel coated by Co-G-SA is nobler than Co-SA, which is extremely noble than bare steel.

Electrochemical impedance spectroscopy results

The Nyquist and Bode plots of bare steel and superhydrophobic coated steel in 0.5 M NaCl solution are shown in Fig. 11. At high frequency, the Nyquist plots show a depressed capacitive semicircle, accompanied by a diffusion tail at low frequency. The interfacial charge transfer reaction is responsible for the depressed capacitive semicircle of the Nyquist plots at high frequencies [76]. The diffusion tail at low frequency is due to the mass transport process. These results indicate that steel coated by Co-SA, which shows high charge transfer resistance compared to bare steel, has larger charge transfer resistance due to the presence of a protective superhydrophobic layer. Steel coated by Co-G-SA shows the highest capacitive semicircle, so it has the highest protection efficiency. The superhydrophobic coated steel blocks the active corrosion sites and limits the diffusion of the corrosive species, such as Cl− and H2O, into the surface of steel metal.

Figure 11
figure 11

Nyquist and Bode plots of bare steel and superhydrophobic coated steel in 0.5 M NaCl solution

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According to Fig. 11b, the Bode plots for prepared superhydrophobic coated steel in 0.5 M NaCl solution show higher impedance magnitudes at the low frequency than bare steel. This indicates the protective action of the prepared superhydrophobic coats on the steel substrate. The phase angle plot, Fig. 11c, shows two times constant at low and moderate frequencies. The time constant appearing in the low-frequency range was due to the protective superhydrophobic film or the unprotective corrosion products in the case of bare steel. The time constant appearing at the moderate frequency was attributed to the electrical double layer [77,78,79].

The impedance parameters were determined using the Zsimpwin software to fit the Nyquist plots to the equivalent circuit shown in Fig. 12. The equivalent circuit includes solution resistance, Rs, film resistance, Rf, film constant phase element, CPEf, charge transfer resistance, Rct, and double-layer constant phase element, CPEdl. Table 3 shows the EIS parameters of bare steel and superhydrophobic coated steel. Equation (3) is used to calculate the protection efficiency [74]

$$\% P = \, \left[ {\left( {R_{{{\text{ct}}}} - R_{{{\text{ct}}}}^{o} } \right)/R_{{{\text{ct}}}} } \right] \, \times \, 100$$
(3)
Figure 12
figure 12

The equivalent circuit model used to fit the experimental Nyquist plots for steel in 0.5 M NaCl solution

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Table 3 The impedance parameters for the bare steel and superhydrophobic coated steel in 0.5 M NaCl solution
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Rcto and Rct are the charge transfer resistance for the bare steel and superhydrophobic coated steel. It is clear that each of Rct, and %P increase in the following order, bare steel < steel + Co-SA < steel + Co-G-SA, and so increasing the corrosion resistance in the same order. The corrosion resistance of the superhydrophobic coated steel by Co-G-SA composite is superior to several previously reported values [5, 71, 80].

Mechanism of anti-corrosion performance

Bare steel freely interacts with surrounding water molecules; the water molecules can be adsorbed to the steel surface. Along with water molecules, chloride ions can also get adsorbed to the steel surface and form [FeClOH]−, which will lead to severe corrosion of the uncoated steel. So, water and Cl− ions easily reach the metal surface and initiate corrosion [32].

On the contrary, the steel coated by superhydrophobic films has a micro-nanostructure covered by adsorbed hydrophobic material. The roughness of the superhydrophobic coatings allows air to be trapped easily within the valleys between the peaks of the rough surface. Consequently, the aggressive ion species such as Cl− in the electrolyte or corrosive environments can rarely attack the underlying surface due to trapped air's obstructive influence [32, 45]. In fact, the air trapped on the superhydrophobic surface acts as a passivation layer between the substrate and the corrosive environment. The enhanced corrosion resistance for steel coated by Co-G-SA films is due to the synergistic effect of superhydrophobicity and the high mechanical and chemical stability, impermeability, hydrophobicity, and chemical inertness of graphene. The schematic representation of the proposed mechanism for corrosion protection of the prepared superhydrophobic films is shown in Fig. 13.

Figure 13
figure 13

Schematic representation of the proposed mechanism for corrosion protection of the prepared superhydrophobic films

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Conclusion

  1. 1.

    superhydrophobic Co-SA and Co-G-SA films were fabricated on the steel substrate.

  2. 2.

    The doping of the superhydrophobic Co film with graphene greatly enhances the superhydrophobicity, and the contact angle increases from 155° to 158°.

  3. 3.

    The doping of the superhydrophobic Co film with graphene also greatly improves steel’s chemical, mechanical, long-term stability and corrosion resistance behaviour in 0.5 M NaCl solution.