Eco-Friendly Approach for the Construction of Superhydrophobic Coating on Stainless Steel Metal Based on Biological Metal–Organic Framework and Its Corrosion Resistance Performance

In this paper, we present a sustainable approach for the creation of superhydrophobic (SP) coating on a stainless-steel substrate based on a biological metal–organic framework (MOF). The MOF was synthesized using aspartic acid as a linker and copper ions as a core metal. Two SP coatings were well constructed on stainless steel utilizing electrodeposition of nickel (Ni) and nickel altered by MOF (Ni@Bio-MOF) coatings followed by soaking in a solution of stearic acid in ethanol. The results of Fourier transform infrared spectroscopy demonstrate that the stearic acid-grafted nickel coating (Ni@SA) and the stearic acid-grafted Ni@Bio-MOF composite (Ni@Bio-MOF@SA), were effectively deposited on the stainless steel. The wettability findings displayed that the water contact angle of Ni@SA and Ni@Cu-As MOF@SA are 160° ± 1.1°, and 168° ± 1.2°, respectively. The prepared SP coating was also found to be chemically and mechanically stable. The results show that the Ni@SA coating maintains SP characteristics in a pH range of 3–11 while the Ni@Cu-As MOF@SA coating retained SP characteristics in a pH range of 1–13. Additionally, the superhydrophobic Ni@SA coating demonstrated SP characteristics up to a length of abrasion equal to 1300 mm, while the Ni@Cu-As MOF@SA coating exhibited SP characteristics up to a length of abrasion equal to 2700 mm. Furthermore, the Ni@SA and Ni@Cu-As MOF@SA coatings exhibited significantly improved corrosion protection in a 0.5 M NaCl solution compared with bare stainless steel, with protection efficiencies of approximately 94% and 99%, respectively. The results of this study demonstrate that the proposed approach is a promising method for the fabrication of eco-friendly and corrosion-resistant SP coatings on stainless steel substrate.


Introduction
Extremely non-wettable surfaces, also known as superhydrophobic (SP) surfaces are surfaces that have a contact angle greater than 150 degrees and are highly resistant to water [1]. These surfaces have attracted interest due to their potential applications in various fields such as antifouling technologies, microfluidic devices, biomedical, solar cells, sensors, drag reduction, oil-water separation, and corrosion resistance [2][3][4][5]. However, creating SP surfaces can be difficult and most methods require extreme conditions, especially when environmental issues are present. There are a variety of techniques used to create SP surfaces, including electrodeposition, spraying, anodization, electrospinning, sol-gel, and chemical vapor deposition [6,7]. Electrodeposition is a technique that is relatively simple, electrodeposition, and nickel. Stearic acid was utilized as a low-surface energy compound due to it being an eco-friendly substance that is more cost-effective in comparison with other options, such as toxic fluorinated polymers and silanes. The prepared SP coatings' wettability, mechanical and chemical stability, and corrosion resistance in a solution of 0.5 M NaCl were evaluated.

The Construction of Cu-As MOF
The process of creating a copper aspartic acid metal-organic framework (Cu-As MOF) involved polishing a copper sheet using 200 and 1200 grit sandpaper to eliminate the oxide layer on its surface, followed by a rinsing of it with distilled water and ethanol. An electrochemical bath made up of 100 mL aspartic acid at a concentration of 4.5 gL −1 was used, with the copper sheet serving as both the anode and cathode. A direct current was applied at a current density of 0.05 A/cm 2 for 1 h (an investigation into the effects of varying current densities over different time intervals was conducted, and the reported current density value and time in this study represents the optimal condition determined through our experimentation) at a temperature of 55 ± 1.5 • C [1]. As the process continued, a precipitate of blue color is deposited at the anode. The precipitate was then scratched and dried at room temperature. The MOF was further dried at 100 • C in an oven for 2 h to yield the final product, the Cu-As MOF.

Superhydrophobic Coating Manufacture
The SS was prepared for electrodeposition by first polishing it with sandpaper of different grades, starting with a rough grade (200) and gradually moving to a smoother grade (1200). The SS was then cleaned by immersing it in the soap for 10 min, rinsing it with distilled water, and submerging it in 2.0 M H 2 SO 4 for 1 min. After this preparation, an area of 2.0 cm × 1.0 cm of SS was immersed in the bath for electrodeposition. The parameters for this process, which involved coating the SS with Ni and a modified form of Ni by Bio-MOF (Ni@Bio-MOF), are listed in Table 1. The used parameters in the table (concentrations of NiCl 2 .6H 2 O, NiSO 4, and H 3 BO 3 as well as the time and potential of deposition) were based on previous studies [27]. Different quantities of Bio-MOF (0.1, 0.2, 0.3, 0.4 and 0.5 gL −1 ) were studied and the reported quantity is the optimum which gives the higher superhydrophobicity. The SS acted as the cathode and was parted from the platinum anode of the same area by 2.0 cm. Once the coatings were applied, the distilled water was used to clean the coated substrates and left to air dry overnight. The coatings were then modified by submersion in an ethanolic solution of 0.01 M SA for 15 min, washing them with ethanol, and allowing them to dry for 24 h. The coated SS with Ni modified with SA (Ni@SA) and the coated SS with Ni@ Bio-MOF modified with SA (Ni@Cu-As MOF@SA) were subjected to various characterizations processes.

Surface Characterization
Thermogravimetric analysis was utilized to assess the thermal stability of the Cu-As MOF (TGA-Shimadzu-50, Shimadzu Corp., Kyoto, Japan). With the aid of a Bruker Tensor 37 FTIR Fourier transform infrared spectrophotometer (Bruker, Singapore), the surfaces' chemical compositions were investigated. The morphology of the surface of the SP coatings was inspected using an SEM (JSM-200 IT, JEOL, Tokyo, Japan) via optical contact angle goniometer (model 190-F2) and 5 µL water droplet, the water contact angle (CA) and water sliding angle (SA) were calculated. The CA and SA readings that were presented were calculated as the average of three tests made at various points on the substrate.

Chemical Stability
The SP films were put into different solutions with pH levels ranging from 1.0 to 13 and were left there for 1 h. The CA and SA were measured after each pH change. To alter the pH of the solution, sulfuric acid and sodium hydroxide were utilized. The chemical stability results are based on an average of two tests undertaken on different substrates.

Mechanical Abrasion
To evaluate the mechanical properties of the SP films, two experiments were conducted: a sand impact test and an abrasion test. In the abrasion test, the SP film was applied to an 800 grade SiC paper and a pressure of 5.0 kPa was applied to it. The CA and SA of a water droplet were measured for every 100 mm of abrasion. In the sand impact experiment, 50 g of sand was dropped from a height of 60 cm onto SP-coated SS. The CAs and SAs were measured for every 50 g of sand that hit the SP surface to determine the material's water superhydrophobicity. To make a further assessment of the mechanical properties of the SP coatings, CAs and SAs were measured after every 50 g of sand was dropped onto the SP surface. The existing data are an average of two tests taken on different substrates.

Corrosion Test
With a three-electrode cell and a Pt sheet serving as the auxiliary electrode and an Ag/AgCl electrode serving as the reference electrode, the electrochemical tests were conducted using an ACM frequency response analyzer. The used SS samples, working electrode, were bare and SP-coated SS with Ni@SA and Ni@Cu-As MOF@SA films, which were then coated with an epoxy layer except for a 1 cm 2 area was left bare for the test solution. Prior to performing electrochemical tests, the SS samples were submerged in a 0.5 M NaCl solution at room conditions for 30 min to establish a rest potential. The electrochemical impedance spectroscopy tests used a frequency range between 0.01 and 1.0 × 10 4 . The potentiodynamic polarization tests were performed within a potential range of ±250 mV around the rest potential. The tests were repeated to ensure accuracy with an error margin of 2%.

Thermogravimetric Results of the Prepared Cu-As MOF
The thermogravimetric results of Cu-aspartic acid MOF, shown in Figure 1, would likely center around the three distinct regions observed in the graph, and the changes in weight that occur in each region. The first region, between 32 and 101 • C, may be characterized by a relatively low rate of weight loss as the MOF loses adsorbed water or other weakly bound species. The second region, between 101 and 218 • C, may be characterized by a more rapid weight loss as the MOF loses more strongly bound species or undergoes structural changes. The third region, between 218 to 288 • C, may be characterized by a slower rate of weight loss, as the MOF reaches its maximum decomposition temperature. This region may indicate that the MOF is losing its structural integrity.

Thermogravimetric Results of the Prepared Cu-As MOF
The thermogravimetric results of Cu-aspartic acid MOF, shown in Figure 1, would likely center around the three distinct regions observed in the graph, and the changes in weight that occur in each region. The first region, between 32 and 101 °C, may be characterized by a relatively low rate of weight loss as the MOF loses adsorbed water or other weakly bound species. The second region, between 101 and 218 °C, may be characterized by a more rapid weight loss as the MOF loses more strongly bound species or undergoes structural changes. The third region, between 218 to 288 °C, may be characterized by a slower rate of weight loss, as the MOF reaches its maximum decomposition temperature. This region may indicate that the MOF is losing its structural integrity.

FTIR Results
The FTIR spectra of coated SS with Ni@Cu-As MOF, Ni@Cu-As MOF@SA, and Ni@SA are presented in Figure 2. The FTIR results for the coated SS with Ni@Cu-As MOF likely indicate the presence of several functional groups in the material. The band at 3463 cm −1 , and 3111 cm −1 may be due to the N-H2 stretch of aspartic acid [27]. The bands at 2980 cm −1 , and 2899 cm −1 may be due to the presence of C-H symmetry and a symmetry vibration of -CH2groups [1]. The band at 1724 cm −1 is due to the stretching vibration of C=O and the band at 1439 cm −1 may be due to the presence of C-N stretching vibrations, indicating the presence of amine groups [1]. The band at 864 cm −1 is due to the stretching of C-H bonds in an aspartic acid compound [1]. The band at 728 cm −1 is due to the presence of Ni(OH)2 bending in the coating while the band at 506 cm −1 is due to the metal-oxygen stretching vibrations and the band at 429 cm −1 is due to the metal-oxygen bending vibrations, indicating the coordination of the copper ions with the oxygen atoms of the carboxylate and amine groups [1].

FTIR Results
The FTIR spectra of coated SS with Ni@Cu-As MOF, Ni@Cu-As MOF@SA, and Ni@SA are presented in Figure 2. The FTIR results for the coated SS with Ni@Cu-As MOF likely indicate the presence of several functional groups in the material. The band at 3463 cm −1 , and 3111 cm −1 may be due to the N-H 2 stretch of aspartic acid [27]. The bands at 2980 cm −1 , and 2899 cm −1 may be due to the presence of C-H symmetry and a symmetry vibration of -CH 2 -groups [1]. The band at 1724 cm −1 is due to the stretching vibration of C=O and the band at 1439 cm −1 may be due to the presence of C-N stretching vibrations, indicating the presence of amine groups [1]. The band at 864 cm −1 is due to the stretching of C-H bonds in an aspartic acid compound [1]. The band at 728 cm −1 is due to the presence of Ni(OH) 2 bending in the coating while the band at 506 cm −1 is due to the metal-oxygen stretching vibrations and the band at 429 cm −1 is due to the metal-oxygen bending vibrations, indicating the coordination of the copper ions with the oxygen atoms of the carboxylate and amine groups [1]. Materials 2023, 16, x FOR PEER REVIEW 6 of 15 The spectrum of the SS coated with Ni@Cu-As MOF@SA displays similar bands to that of the Cu-As MOF, but with slight changes in the position of the band of the C=O stretch and the N-H2 stretch band, which appear at 1733 cm −1 and 3294 cm −1 , respectively. This suggests that the Cu-As MOF has been doped with SA [27].
The spectrum of the coated SS with Ni@SA coat displays a band at 3530 cm −1 which is likely due to the presence of the hydroxyl groups of stearic acid [27]. The bands at 2932 cm −1 , and 2894 cm −1 may be due to the presence of C-H symmetry and a symmetry vibration of -CH2-groups [27]. The band at 1698 cm −1 is associated with the stretching vibrations of C=O in the stearic acid and the band at 1454 cm −1 is likely due to the bending vibrations of the CH2 groups in the stearic acid [29]. The bands at 1255 and 971 cm −1 are due to CH stretch. The band at 689 cm −1 is likely attributed to the presence of Ni(OH)2 bending in the coating [30].

SEM and Wettability
The SEM of the SS coated by Ni@Cu-As MOF@SA, and Ni@SA are presented in Figure 3. The discussion of SEM results of SP-coated SS with Ni@Cu-As MOF@SA, and Ni@SA likely centers around the differences in microstructure and surface roughness between the two coatings. The SEM micrographs likely show that the coating made with Ni@Cu-As MOF@SA has smaller circular microstructures compared with the coating made with Ni@SA. The usage of an MOF in the Ni@Cu-As MOF@SA coating may operate as a nucleation center for the electrodeposition process, speeding up the nucleation process rather than crystal growth and producing smaller structures, increasing the surface's roughness. The smaller size of the microstructures may also contribute to the SP properties of the coating, as smaller structures can lead to a more roughness, and stable water-repellent coating. The wettability of the Ni@SA, and Ni@Cu-As MOF@SA was examined by the measurement of CA. The Ni@SA has CA of 160° ± 1.1°, and an SA of 4° ± 0.1°, while Ni@Cu-As MOF@SA has a CA of 168° ± 1.2°, and an SA of 1° ± 0.1°, so the two coats showed excellent SP properties. The micrograph of the water droplet on the SP prepared with Ni@SA and Ni@Cu-As MOF@SA is shown as an inset in Figure 3. The rolling/bouncing of the water droplet on the SS coated by Ni@Cu-As MOF@SA is illustrated in Video S1. The spectrum of the SS coated with Ni@Cu-As MOF@SA displays similar bands to that of the Cu-As MOF, but with slight changes in the position of the band of the C=O stretch and the N-H 2 stretch band, which appear at 1733 cm −1 and 3294 cm −1 , respectively. This suggests that the Cu-As MOF has been doped with SA [27].
The spectrum of the coated SS with Ni@SA coat displays a band at 3530 cm −1 which is likely due to the presence of the hydroxyl groups of stearic acid [27]. The bands at 2932 cm −1 , and 2894 cm −1 may be due to the presence of C-H symmetry and a symmetry vibration of -CH 2 -groups [27]. The band at 1698 cm −1 is associated with the stretching vibrations of C=O in the stearic acid and the band at 1454 cm −1 is likely due to the bending vibrations of the CH 2 groups in the stearic acid [29]. The bands at 1255 and 971 cm −1 are due to CH stretch. The band at 689 cm −1 is likely attributed to the presence of Ni(OH) 2 bending in the coating [30].

SEM and Wettability
The SEM of the SS coated by Ni@Cu-As MOF@SA, and Ni@SA are presented in Figure 3. The discussion of SEM results of SP-coated SS with Ni@Cu-As MOF@SA, and Ni@SA likely centers around the differences in microstructure and surface roughness between the two coatings. The SEM micrographs likely show that the coating made with Ni@Cu-As MOF@SA has smaller circular microstructures compared with the coating made with Ni@SA. The usage of an MOF in the Ni@Cu-As MOF@SA coating may operate as a nucleation center for the electrodeposition process, speeding up the nucleation process rather than crystal growth and producing smaller structures, increasing the surface's roughness. The smaller size of the microstructures may also contribute to the SP properties of the coating, as smaller structures can lead to a more roughness, and stable waterrepellent coating. The wettability of the Ni@SA, and Ni@Cu-As MOF@SA was examined by the measurement of CA. The Ni@SA has CA of 160 • ± 1.1 • , and an SA of 4 • ± 0.1 • , while Ni@Cu-As MOF@SA has a CA of 168 • ± 1.2 • , and an SA of 1 • ± 0.1 • , so the two coats showed excellent SP properties. The micrograph of the water droplet on the SP prepared with Ni@SA and Ni@Cu-As MOF@SA is shown as an inset in Figure 3. The rolling/bouncing of the water droplet on the SS coated by Ni@Cu-As MOF@SA is illustrated in Video S1.

Chemical Stability
Chemical stability is considered an essential requirement for SP coatings to work well over time in harsh solution conditions. The correlations between the CAs and SAs of water droplets on the SP coatings and the solution pH are depicted in Figure 4. The shape of the water droplet on the SS coated with Ni@Cu-As MOF@SA after being immersed in a solution of pH 7 for 5 h is illustrated in Figure 5. Video S2 demonstrates the superhydrophobicity and rolling of a water droplet on the SS coated with a Ni@Cu-As MOF@SA surface after immersion in the pH 7 solution for 5 h. According to the findings, Ni@SA films are SP in the pH range of 3-11, while Ni@Cu-As MOF@SA films are SP in the pH range of 1-13, where the CAs are frequently higher than 150 o and the SAs are less than 10°. The chemical stability of SP-coated SS with Ni@Cu-As MOF@SA is higher than that of SS coated with Ni@SA because the Cu-As MOF enhances the coating superhydrophobicity and provides an additional layer of protection. The SP-coated SS with Ni@Cu-As MOF@SA has a superior chemical stability to numerous values that have been reported previously [31][32][33][34].

Chemical Stability
Chemical stability is considered an essential requirement for SP coatings to work well over time in harsh solution conditions. The correlations between the CAs and SAs of water droplets on the SP coatings and the solution pH are depicted in Figure 4. The shape of the water droplet on the SS coated with Ni@Cu-As MOF@SA after being immersed in a solution of pH 7 for 5 h is illustrated in Figure 5. Video S2 demonstrates the superhydrophobicity and rolling of a water droplet on the SS coated with a Ni@Cu-As MOF@SA surface after immersion in the pH 7 solution for 5 h. According to the findings, Ni@SA films are SP in the pH range of 3-11, while Ni@Cu-As MOF@SA films are SP in the pH range of 1-13, where the CAs are frequently higher than 150 • and the SAs are less than 10 • . The chemical stability of SP-coated SS with Ni@Cu-As MOF@SA is higher than that of SS coated with Ni@SA because the Cu-As MOF enhances the coating superhydrophobicity and provides an additional layer of protection. The SP-coated SS with Ni@Cu-As MOF@SA has a superior chemical stability to numerous values that have been reported previously [31][32][33][34].

Chemical Stability
Chemical stability is considered an essential requirement for SP coatings to work well over time in harsh solution conditions. The correlations between the CAs and SAs of water droplets on the SP coatings and the solution pH are depicted in Figure 4. The shape of the water droplet on the SS coated with Ni@Cu-As MOF@SA after being immersed in a solution of pH 7 for 5 h is illustrated in Figure 5. Video S2 demonstrates the superhydrophobicity and rolling of a water droplet on the SS coated with a Ni@Cu-As MOF@SA surface after immersion in the pH 7 solution for 5 h. According to the findings, Ni@SA films are SP in the pH range of 3-11, while Ni@Cu-As MOF@SA films are SP in the pH range of 1-13, where the CAs are frequently higher than 150 o and the SAs are less than 10°. The chemical stability of SP-coated SS with Ni@Cu-As MOF@SA is higher than that of SS coated with Ni@SA because the Cu-As MOF enhances the coating superhydrophobicity and provides an additional layer of protection. The SP-coated SS with Ni@Cu-As MOF@SA has a superior chemical stability to numerous values that have been reported previously [31][32][33][34].

Mechanical Stability
SP surfaces often have limited practical applications due to their mechanical fragility. When touched with a finger, some surfaces with SP characteristics can crash [4]. The produced SP films' resistances to mechanical abrasion were assessed utilizing abrasion and sand impact tests. Figure 6 depicts the variations in CAs and SAs of the manufactured SP films with respect to the abrasion length. The Ni@SA SP film maintains its SP characteristics up to a 1300 mm abrasion length. In comparison, the SP Ni@Cu-As MOF@SA film preserves its SP characteristics up to a 2700 mm abrasion length. The SPcoated SS with Ni@Cu-As MOF@SA exhibits larger abrasion resistance than numerous stated values [27,35]. The mechanical abrasion test for SS coated with Ni@Cu-As MOF@SA film for abrasion length of 20 cm is shown in Video S3. The mechanical abrasion resistance of SP-coated SS via Ni@Cu-As MOF@SA is higher than that of SP SS coated with Ni@SA only because the MOF layer enhances the

Mechanical Stability
SP surfaces often have limited practical applications due to their mechanical fragility. When touched with a finger, some surfaces with SP characteristics can crash [4]. The produced SP films' resistances to mechanical abrasion were assessed utilizing abrasion and sand impact tests. Figure 6 depicts the variations in CAs and SAs of the manufactured SP films with respect to the abrasion length. The Ni@SA SP film maintains its SP characteristics up to a 1300 mm abrasion length. In comparison, the SP Ni@Cu-As MOF@SA film preserves its SP characteristics up to a 2700 mm abrasion length. The SP-coated SS with Ni@Cu-As MOF@SA exhibits larger abrasion resistance than numerous stated values [27,35]. The mechanical abrasion test for SS coated with Ni@Cu-As MOF@SA film for abrasion length of 20 cm is shown in Video S3.

Mechanical Stability
SP surfaces often have limited practical applications due to their mechanical fragility. When touched with a finger, some surfaces with SP characteristics can crash [4]. The produced SP films' resistances to mechanical abrasion were assessed utilizing abrasion and sand impact tests. Figure 6 depicts the variations in CAs and SAs of the manufactured SP films with respect to the abrasion length. The Ni@SA SP film maintains its SP characteristics up to a 1300 mm abrasion length. In comparison, the SP Ni@Cu-As MOF@SA film preserves its SP characteristics up to a 2700 mm abrasion length. The SPcoated SS with Ni@Cu-As MOF@SA exhibits larger abrasion resistance than numerous stated values [27,35]. The mechanical abrasion test for SS coated with Ni@Cu-As MOF@SA film for abrasion length of 20 cm is shown in Video S3. The mechanical abrasion resistance of SP-coated SS via Ni@Cu-As MOF@SA is higher than that of SP SS coated with Ni@SA only because the MOF layer enhances the The mechanical abrasion resistance of SP-coated SS via Ni@Cu-As MOF@SA is higher than that of SP SS coated with Ni@SA only because the MOF layer enhances the superhy- drophobicity and provides an additional layer of protection, resulting in a more durable and abrasion-resistant coating [36,37].
As seen in Figure 7, the sand abrasion assessments were undertaken to evaluate the mechanical performance of the SP coatings. The Ni@SA film maintains SP characteristics up to 10 sand impact cycles, while the Ni@Cu-As MOF@SA film exhibits superhydrophobicity up to 20 sand impact cycles. Ni@Cu-As MOF@SA exhibit a sand impact resistance larger than several previously stated values [33,38]. superhydrophobicity and provides an additional layer of protection, resulting in a more durable and abrasion-resistant coating [36,37]. As seen in Figure 7, the sand abrasion assessments were undertaken to evaluate the mechanical performance of the SP coatings. The Ni@SA film maintains SP characteristics up to 10 sand impact cycles, while the Ni@Cu-As MOF@SA film exhibits superhydrophobicity up to 20 sand impact cycles. Ni@Cu-As MOF@SA exhibit a sand impact resistance larger than several previously stated values [33,38].

Potentiodynamic Polarization Results
The corrosion behaviors of uncoated and SP-coated SS by Ni@SA, Ni@Cu-As MOF@SA were studied using the potentiodynamic polarization technique. The potentiodynamic polarization plots of bare and SP-coated SS in 0.5 M NaCl are shown in Figure 8. The observation of limited diffusion currents during cathodic polarization suggests that the cathodic process is governed by the transfer of oxygen gas from the bulk to the electrode surface. Pitting corrosion for bare SS or the development of a passive layer for SS cured with an SP coating prevent the formation of an ideal anodic Tafel area [39,40].

Potentiodynamic Polarization Results
The corrosion behaviors of uncoated and SP-coated SS by Ni@SA, Ni@Cu-As MOF@SA were studied using the potentiodynamic polarization technique. The potentiodynamic polarization plots of bare and SP-coated SS in 0.5 M NaCl are shown in Figure 8. The observation of limited diffusion currents during cathodic polarization suggests that the cathodic process is governed by the transfer of oxygen gas from the bulk to the electrode surface. Pitting corrosion for bare SS or the development of a passive layer for SS cured with an SP coating prevent the formation of an ideal anodic Tafel area [39,40]. Table 2 shows the potentiodynamic polarization variables for bare and SP-coated SS, containing corrosion potential (E corr ), protection efficiency (%P), and corrosion current density (i corr ). The %P was determined utilizing Equation (2) [41].
where, i o and i are the corrosion current densities of the bare SS and the SP-coated SS, respectively. The i corr value for coated SS with Ni@SA is lower than that for bare SS due to the superhydrophobicity of the coated SS. Air trapped in the SP coating microstructures can diminish the surface area between the solution and SS, which causes the i corr value to fall more quickly [42]. The doping of the superhydrophobic Ni@SA coat with MOF enhances the SP property, leading to a greater decline in the contact area between the medium and SS. Therefore, SS coated with Ni@Cu-As MOF@SA has a higher protection efficiency than SS coated with Ni@SA.  Table 2 shows the potentiodynamic polarization variables for bare and SP-coated SS, containing corrosion potential (Ecorr), protection efficiency (%P), and corrosion current density (icorr). The %P was determined utilizing Equation (2 ) [41].
where, io and i are the corrosion current densities of the bare SS and the SP-coated SS, respectively. The icorr value for coated SS with Ni@SA is lower than that for bare SS due to the superhydrophobicity of the coated SS. Air trapped in the SP coating microstructures can diminish the surface area between the solution and SS, which causes the icorr value to fall more quickly [42]. The doping of the superhydrophobic Ni@SA coat with MOF enhances the SP property, leading to a greater decline in the contact area between the medium and SS. Therefore, SS coated with Ni@Cu-As MOF@SA has a higher protection efficiency than SS coated with Ni@SA. Nyquist, Bode and Theta plots of the bare and SP-coated SS in a 0.5 M NaCl solution are presented in Figure 9. The Nyquist plots, shown in Figure 9a, exhibit a diffusion tail at low frequency and a depressed capacitive semicircle at high frequency. The depressed capacitive semicircle observed at high frequencies in the Nyquist plots is due to the interfacial charge transfer reaction [43]. The diffusion tails observed at low frequencies are attributed to mass transfer. Based on these observations, it can be inferred that the  Nyquist, Bode and Theta plots of the bare and SP-coated SS in a 0.5 M NaCl solution are presented in Figure 9. The Nyquist plots, shown in Figure 9a, exhibit a diffusion tail at low frequency and a depressed capacitive semicircle at high frequency. The depressed capacitive semicircle observed at high frequencies in the Nyquist plots is due to the interfacial charge transfer reaction [43]. The diffusion tails observed at low frequencies are attributed to mass transfer. Based on these observations, it can be inferred that the improved charge transfer resistance of SS coated with Ni@SA compared with bare SS is attributed to the presence of a protective SP layer. The SS coated with Ni@Cu-Asp MOF@SA exhibits the largest capacitive semicircle, suggesting that it provides the highest level of protection. The incorporation of MOF onto Ni@SA enhances the superhydrophobicity of the surface, making the superhydrophobic Ni@Cu-Asp MOF@SA coating more effective in restricting the diffusion of corrosive species such as Cl − and H 2 O into the SS substrate.
improved charge transfer resistance of SS coated with Ni@SA compared with bare SS is attributed to the presence of a protective SP layer. The SS coated with Ni@Cu-Asp MOF@SA exhibits the largest capacitive semicircle, suggesting that it provides the highest level of protection. The incorporation of MOF onto Ni@SA enhances the superhydrophobicity of the surface, making the superhydrophobic Ni@Cu-Asp MOF@SA coating more effective in restricting the diffusion of corrosive species such as Cl − and H2O into the SS substrate. When the superhydrophobic-coated SS was immersed in a 0.5 M NaCl solution, it demonstrated significantly higher impedance magnitudes at lower frequencies on the Bode plots, shown in Figure 9b, compared with bare steel. This clearly indicates that the created SP coatings have successfully protected the SS substrate. The phase angle plot, shown in Figure 9c, shows two time constants at low and intermediate frequencies.
The time constant observed at the low-frequency region is attributed to the unprotective corrosion products of bare SS or the protective SP coating. On the other hand, the time constant observed at the moderate frequency is attributed to the electrical double layer. The theta angle, which is approximately 45 degrees at a moderate frequency, indicates that the corrosion process is under diffusion control.
The equivalent circuit displayed in Figure 10 was utilized to fit the results of the electrochemical impedance spectroscopy experiment and the Zsimpwin program was utilized to calculate the impedance parameters. The components of the equivalent circuit include the solution resistance (Rs), the charge transfer resistance (Rct), the double-layer When the superhydrophobic-coated SS was immersed in a 0.5 M NaCl solution, it demonstrated significantly higher impedance magnitudes at lower frequencies on the Bode plots, shown in Figure 9b, compared with bare steel. This clearly indicates that the created SP coatings have successfully protected the SS substrate. The phase angle plot, shown in Figure 9c, shows two time constants at low and intermediate frequencies.
The time constant observed at the low-frequency region is attributed to the unprotective corrosion products of bare SS or the protective SP coating. On the other hand, the time constant observed at the moderate frequency is attributed to the electrical double layer. The theta angle, which is approximately 45 degrees at a moderate frequency, indicates that the corrosion process is under diffusion control.
The equivalent circuit displayed in Figure 10 was utilized to fit the results of the electrochemical impedance spectroscopy experiment and the Zsimpwin program was utilized to calculate the impedance parameters. The components of the equivalent circuit include the solution resistance (R s ), the charge transfer resistance (R ct ), the double-layer constant phase element (CPE dl ), and the Warburg element (W). Table 3 demonstrates the electrochemical impedance spectroscopy parameters for both bare SS and SP-coated SS. Equation (2) was utilized to estimate the %P [27]: where the charge transfer resistances for uncoated and SP-coated SS are R cto and R ct , respectively. Table 3 presents the obtained impedance parameters. It is clear that, each of R ct and %P of the bare SS < SS + Ni @SA < SS + Ni@Cu-As MOF@SA. The corrosion performance of the SP-coated SS by Ni@Cu-As MOF@SA is greater than many previously documented values [44][45][46][47] and lower than other previously reported values [48,49].
constant phase element (CPEdl), and the Warburg element (W). Table 3 demonstrates the electrochemical impedance spectroscopy parameters for both bare SS and SP-coated SS. Equation (2) was utilized to estimate the %P [27]: where the charge transfer resistances for uncoated and SP-coated SS are Rcto and Rct, respectively. Table 3 presents the obtained impedance parameters. It is clear that, each of Rct and %P of the bare SS < SS + Ni @SA < SS + Ni@Cu-As MOF@SA. The corrosion performance of the SP-coated SS by Ni@Cu-As MOF@SA is greater than many previously documented values [44][45][46][47] and lower than other previously reported values [48,49]. Figure 10. The equivalent circuit model. The corrosion resistance of SP-coated SS with a Ni@Cu-As MOF@SA coating is higher than that of SS coated with Ni@SA. This may be due to the way in which the Cu-As MOF increases the superhydrophobicity and forms a protective barrier against corrosive agents such as water and oxygen, thus slowing down the corrosion process and enhancing the corrosion resistance of the coating.

Conclusions
In our study, an eco-friendly approach to the construction of SP coatings on SS metal based on biological metal-organic frameworks (MOFs) was developed. The MOF was synthesized using aspartic acid as the organic linker and copper ions as the metal center. The water contact angles of SS coated with Ni@SA and Ni@Cu-As MOF@SA are 160° ± 1.1°, and 168° ± 1.2°, respectively.
The chemical stability results show that Ni @SA coating maintains SP characteristics in the pH range of 3-11, whereas the Ni@Cu-As MOF@SA coating retains SP characteristics in the pH range of 1-13. The mechanical stability results show that the created SP Ni@SA coating demonstrates SP characteristics up to an abrasion length of 1300 mm, while the Ni@Cu-As MOF@SA coating exhibits SP characteristics up to an abrasion length of 2700 mm. The corrosion resistance of the coated SS was also significantly improved. The corrosion current density of the coated SS with Ni@SA is 0.0041915 µA/cm 2 , and Ni@Cu-As MOF@SA is 0.0041915 µA/cm 2 , which was much lower than that of the bare SS (0.0710569 µA/cm 2 ). The results of this study demonstrate that the proposed approach is a promising method for the construction of eco-friendly SP coatings with excellent corrosion resistance.  The corrosion resistance of SP-coated SS with a Ni@Cu-As MOF@SA coating is higher than that of SS coated with Ni@SA. This may be due to the way in which the Cu-As MOF increases the superhydrophobicity and forms a protective barrier against corrosive agents such as water and oxygen, thus slowing down the corrosion process and enhancing the corrosion resistance of the coating.

Conclusions
In our study, an eco-friendly approach to the construction of SP coatings on SS metal based on biological metal-organic frameworks (MOFs) was developed. The MOF was synthesized using aspartic acid as the organic linker and copper ions as the metal center. The water contact angles of SS coated with Ni@SA and Ni@Cu-As MOF@SA are 160 • ± 1.1 • , and 168 • ± 1.2 • , respectively.
The chemical stability results show that Ni @SA coating maintains SP characteristics in the pH range of 3-11, whereas the Ni@Cu-As MOF@SA coating retains SP characteristics in the pH range of 1-13. The mechanical stability results show that the created SP Ni@SA coating demonstrates SP characteristics up to an abrasion length of 1300 mm, while the Ni@Cu-As MOF@SA coating exhibits SP characteristics up to an abrasion length of 2700 mm. The corrosion resistance of the coated SS was also significantly improved. The corrosion current density of the coated SS with Ni@SA is 0.0041915 µA/cm 2 , and Ni@Cu-As MOF@SA is 0.0041915 µA/cm 2 , which was much lower than that of the bare SS (0.0710569 µA/cm 2 ). The results of this study demonstrate that the proposed approach is a promising method for the construction of eco-friendly SP coatings with excellent corrosion resistance.
Supplementary Materials: The following supporting information can be downloaded at: https://www.mdpi.com/article/10.3390/ma16134728/s1, Video S1. The rolling of a water droplet on the superhydrophobic coated stainless steel by Ni@Cu-Asp MOF@SA film. Video S2. The superhydrophobicity and rolling of a water droplet on the coated stainless steel by Ni@Cu-Asp MOF@SA film after immersion in pH 7 solution for 5 h. Video S3. The mechanical abrasion test for SS coated with Ni@Cu-Asp MOF@SA film for abrasion length of 20 cm.