Stability Mechanism of Laser-induced Fluorinated Super-hydrophobic Coating in Alkaline Solution

Great attention has been focused on super-hydrophobic surfaces due to their fantastic applications. Fluoride chemicals are widely used to fabricate super-hydrophobic surfaces due to their convenience, simplicity, and high efficiency. Previous research has made extensively efforts on corrosion resistance of fluorinated super-hydrophobic surfaces in corrosive media. Nevertheless, rare papers focused on the underlying reasons of anticorrosion property and stability mechanism on the fluorinated super-hydrophobic coatings in alkaline solution. Therefore, this work aims to reveal these mechanisms of fluorinated super-hydrophobic copper samples in strong alkaline solution (pH 13). Through the characterization of surface wettability and surface morphology, the laser-induced super-hydrophobic surface retained excellent stability after soaking in alkaline solution for 4 h. Through measurement of chemical compositions, the anticorrosion mechanism and stability mechanism of the fluorinated super-hydrophobic surface were proposed. Importantly, the hydroxyl ion (OH−) can further promote the hydrolysis reaction to improve the density and bonding strength of the fluoride molecules. Finally, the electrochemical experiments (PDP and EIS tests) were conducted to validate the rationality of our proposed conclusions.


Introduction
Functional surfaces with super-hydrophobic property have aroused great attention from academic to industrial because of their broad ranges of applications, including self-cleaning [1], anti-icing [2], oil-separation [3], anti-bacteria [4], fluid drag reduction [5], and even energy storage devices [6]. The inspiration for artificial super-hydrophobic surfaces is derived from natural organisms, such as lotus leaves [7] and desert beetle [8]. It is well known that the formation of super-hydrophobicity is based on the combination of hierarchical micro/nano-structures and low surface energy [9]. The bio-inspired super-hydrophobic surfaces have been successfully fabricated by previous studies including electrodeposition/electroplating [10,11], sol-gel [12], wet chemical etching [13], laser ablation [14,15], electrospinning [16], electrochemical etching [17], Chemical Vapor Deposition (CVD) [18], and so on. Among these methods, laser treatment has been identified as a promising technique due to its advantages including non-contact and simple process, comparatively cheaper, and in-situ functionalization [19]. However, direct laser processing of hierarchical structure converts the metallic surfaces to be hydrophilic rather than the super-hydrophobic state. To expedite hydrophobicity of laser-induced surface, the post-surface chemical modification will be required to lower surface free energy using long-chain fatty acids [20], alkoxy silanes [21], alkoxy polymer [22] and fluoride chemicals [23]. Particularly, the fluoride reagents are widely used to manufacture laser-induced super-hydrophobic surfaces due to their outstanding merits of facile manipulation, high efficiency, and high content of fluorosilanized carbon groups (-CF x ) with extremely low surface energy [24].
In addition, severe metallic degradations will occur under chemical and electrochemical reactions, resulting in detrimental consequences of environmental contamination, potential safety issues, and global financial losses. As a result, the study into alternative and efficient coatings on 1 3 copper substrates for anti-corrosion resistance has become a popular research topic. It has been demonstrated that the fluorinated super-hydrophobic surfaces have superior corrosion resistance capability because of their ability to minimize the contact area between substrates and liquid media when being immersed underwater [25]. For example, Xiang et al. used a two-step method to obtain the super-hydrophobic surface on steel sheets, and the result showed that the as-prepared surface could achieve a corrosion inhibition rate of 96.3% [26]. Recently, the stability and durability of fluorinated super-hydrophobic coatings in corrosive liquids (acid, salt, and alkaline solution) have been explored to determine their possible applications in specific working environment. What's more, as the most common corrosive media in daily life, the stability of alkaline solutions has also attracted the attentions of researchers and the corrosion resistant performance can be evaluated by the measurement of contact angle. For instance, the fluorinated nickel coating has been successfully fabricated on stainless steel in Liang's research group, and the corresponding results proved that such super-hydrophobic nickel film possessed satisfied stability and durability after being immersed into strong acid solution (pH 2) and alkaline solution (pH 13) [27]. However, this research only demonstrated that the received super-hydrophobic coating could maintain excellent stability within 100 h, but the corresponding anticorrosion mechanism was absent. Besides, Yang et al. investigated the stability of fluorinated super-hydrophobic silicon surfaces in alkaline solution, and the results clearly showed that after half hour, the alkaline-corroded samples were seriously damaged, losing super-hydrophobic property [28]. On the contrary, Pu et al. reported that the Ti-6Al-4V substrates with fluorinated super-hydrophobic coating still exhibited robust super-hydrophobicity after immersion into alkaline solution for several hours [29]. Unfortunately, the protective mechanism and failure reason were not revealed in these two studies from the perspective of physical changes and chemical reactions when the fluorinated super-hydrophobic surfaces were soaked into corrosive alkaline media. To author's best knowledge, most of previous literature just declared the fluorinated super-hydrophobic surface had excellent corrosion resistance by measuring contact angles before/after corrosive immersion. Rare research is available to in-depth analyze the stability mechanism of super-hydrophobicity and bonding conditions between substrate and fluoride molecules. For practical applications of the fluorinated superhydrophobic surfaces, it is absolutely not sufficient to only describe the experimental phenomenon. It is therefore urgent to systematically explore the underlying reasons to elucidate the interaction mechanisms between the fluorinated superhydrophobic coatings and corrosive media.
Herein, this article employed nanosecond laser irradiation and following chemical modification to obtain fluorinated super-hydrophobic surfaces. Subsequently, the fresh fabricated super-hydrophobic sample was soaked in alkaline solution with pH 13 for 4 h. The main aim of this work was to investigate the anticorrosion property and stability mechanism of fluorinated super-hydrophobic surfaces in alkaline solution, through analyzing surface morphology and surface chemical compositions. Finally, electrochemical experiments were conducted to further prove the rationality of our proposed conclusions.

Materials
High-purity copper (99.9%) with a thickness of 0.5 mm was used as the substrate material in this study, which was provided by Kejing Material Technology Co., Ltd, Hefei, China. To ensure experimental consistency, all the copper substrates were cut into square shape with the same size of 10 mm × 10 mm. 1H, 1H, 2H, 2H-perfluorodecyltrichlorosilane [FAS, CF 3 (CF 2 ) 7 (CH 2 ) 2 Si(OC 2 H 5 ) 3 ] were obtained from Alfa Aesar Company. The analytical reagents (acetone, ethanol, hydrochloric acid, sodium hydroxide) were purchased from Beijing Chemical Works. The distilled water utilized throughout the whole experimental progresses was supplied by a commercial water purification system (ZhiAng Instruments Co., Ltd, Shanghai, China).

Preparation of the Experiments
Prior to laser treatment, the copper substrates were ultrasonically washed by the order of acetone, ethanol, and distilled water each for 10 min. Then, the substrates were soaked in the dilute hydrochloric acid (10 vol%) for 1 min to remove residual surface oxides. Finally, the cleaned substrates were flushed by distilled water again, and dried in a vacuum drying oven at the temperature of 35 °C. To reduce surface energy of laser-induced copper surfaces, FAS/ethanol solution was prepared. Specifically, to achieve the maximum hydrolysis of the FAS molecules, 1 g FAS was dropped into the mixed solution containing 125.4 mL ethanol (AR > 99%) and 38.4 mL distilled water. The pH value of alkali solution used in the paper was 13. To guarantee hydrolysis process of FAS molecules, the FAS/ethanol solution was continuously stirred by magnetic force for 12 h at room temperature.

Laser Treatment and Further Processing
The pre-prepared copper substrates were irradiated by a nanosecond Ytterbium fiber laser source (IPG photonics from Germany) with a wavelength of 1064 nm, a beam waist size of 50 μm, pulse duration of 50 ns, repetition rate up to 20 kHz, output power of 16 W, and laser fluence of 40.76 J cm −2 . The laser treatment was carried out at atmospheric conditions by both horizontal and longitudinal scanning at the scanning speed of 500 mm·s −1 and scanning interval of 80 μm. Hence, the grid-patterned structure was created on the copper substrates.
Subsequently, the laser-induced samples were chemically modified to lower surface energy by immersing into the FAS/ethanol solution for 2 h at room temperature. Finally, the soaked samples were flushed by distilled water and dried for 30 min at 80 °C in an oven to obtain the super-hydrophobic copper samples. To verify chemical stability or corrosion resistance of the copper material in alkaline solution, both the pristine copper and the as-prepared super-hydrophobic samples were immersed into NaOH solution (pH 13) for 4 h, which were abridged as Alkaline-Etched Pristine (AEP) sample and Alkaline-Etched Super-hydrophobic (AES) sample, respectively.

Measurement and Characterization
The influences of laser treatment and alkaline corrosion on surface morphology as well as surface chemical compositions were analyzed by Scanning Electron Microscopy (SEM, Zeiss Sigma300, Germany) and X-ray Photoelectron Spectroscopy (XPS, Thermo Fisher Scientific, Escalab 250Xi, USA), respectively. CasaXPS software was applied to process the corresponding XPS spectra. To explore the variation of wetting property, the goniometer (AST, VCA Optima, USA) was used with 8 μL water droplet. The Water Contact Angle (WCA) and Rolling Angle (RA) were recorded by measuring three different points on each sample, and averaged to determine the final results.
To further validate the discrepancy in corrosion resistance, the Potentiodynamic Polarization (PDP) and Electrochemical Impedance Spectroscopy (EIS) tests were examined in a standard three-electrode cell configuration at room temperature. The electrochemical experiments were conducted in 3.5 wt% NaCl solution by an electrochemical workstation (CH Instruments Ins., 660E, China). The platinum electrode (10 mm × 10 mm × 0.1 mm) and the Standard Calomel Electrode (SCE) were selected as the auxiliary electrode and reference electrode, respectively. The tested samples with 1 cm 2 exposed area in corrosive NaCl solution were used as the working electrodes. Prior to electrochemical tests, the individual sample was stabilized in NaCl solution for 15-30 min before electrochemical tests. The PDP curves were obtained at a sweep rate of 1 mV·s −1 , which was utilized to calculate the corrosion potential (E corr , V), corrosion current density (I corr , A·cm −2 ) and Corrosion Rate (CR, mm·a −1 ). The EIS experiments were conducted at the frequency range from 10 5 to 10 -2 Hz with a sinusoidal amplitude of 5 mV. The received data were then fitted by the ZSimpWin software.

Surface Wettability
Surface wettability of the pristine copper, super-hydrophobic copper and their corresponding alkaline-etched samples was investigated by measuring WCAs and RAs. As can be seen in Fig. 1, the pristine copper substrate presented hydrophilic character, showing a WCA value of 83.0 ± 2.0°. However, after soaking in alkaline solution for 4 h, the WCA of AEP copper experienced a sharp decrease, and the value was only 49.2 ± 1.0°. This phenomenon implied that the strong alkaline solution had dramatically damaged the flat copper surface to generate rough micro/nano-structures, causing a huge change in surface wettability. Based on Wenzel theory, surface roughness has the amplifying effect on surface wettability, indicating that the increase of surface roughness results in smaller WCA on intrinsic hydrophilic surface or larger WCA on intrinsic hydrophobic surface [30,31].
where r > 1 represents the surface roughness factor. θ f and θ w denote the WCAs of the pristine copper and AEP copper, respectively. In this experiment, the value of r can be calculated at 5.36, demonstrating that after alkaline immersion the surface roughness of planar copper had changed to a larger extent. Hence, it can be deduced that the decrease of WCA for the pristine copper substrate was ascribed to the formation of surface micro/nano-structures, which will be further verified by surface morphology measurement in the next section.
(1) cos w = r cos f ,  It is also noted from Fig. 1 that after hybrid processes of laser texturing and FAS modification, the WCA of treated copper sample increased to 156.4 ± 1.6° with a RA of around 7°, showing a typical super-hydrophobic character. It is noteworthy that after being immersed into strong alkaline solution, the laser treated surface presented robust superhydrophobicity, exhibiting inconspicuous decrease of WCA (152.3 ± 1.5°) and imperceptible increase of RA (around 9°). The results demonstrate that although the strong alkaline solution can damage the pristine copper, the laser-induced super-hydrophobic surface could remain good condition in this corrosive solution. The alkaline solution may have the protective effect for the super-hydrophobic coating so as to inhibit the inner copper corrosion. In this work, the stability mechanism of laser-induced super-hydrophobic FAS coating in alkaline solution was systematically investigated through the chemical and molecular structure levels. In the meanwhile, the way in which pure copper surface destroyed by alkaline solution was also explored from the physical and chemical aspects.

Surface Morphology of Pristine Copper and AEP Copper Samples
To investigate the wettability alternation mechanism for pristine copper before/after immersion into alkaline solution, the surface morphologies of planner copper and alkaline-etched copper substrates were detected by SEM. It is obvious from Fig. 2a that the pristine copper presented an overall flat appearance. Localized pits and scratches could also be observed, which mainly came from the surface oxides removal during the cleaning process. Nevertheless, the AEP copper surface became much rougher, showing a nodular and lumpy structure with many irregular-shape particles (as shown in Fig. 2b). To the best of authors' knowledge, the pure copper cannot directly react with alkaline solutions. However, the results indicate that original flat copper surface had been severely damaged. In this paper, the corrosion mechanism of pure copper in alkaline solution can be explained by following two main stages. can react with sodium hydroxide (NaOH) to generate soluble species, resulting the formation of a large number of pits upon copper substrate. The specific reactions can be described in detail by the following equations: Due to the dissolution of the produced copper hydroxide, pits were generated onto the copper surface to create micro/ nano-structures. Therefore, the surface roughness was significantly increased, resulting in a huge difference of surface wettability between the pristine copper and the alkaline-corroded copper substrates.

Surface Morphology of Super-hydrophobic Surface and AES Sample
As shown in Fig. 2c, a homogeneous mesh-array structure was observed on copper substrates after laser texturing treatment and FAS modification. The magnified SEM image in Fig. 2d showed that numbers of micro/nano sized particles also appeared after laser ablation. It is manifest that the micro/ nano particles were randomly deposited on the integral reticulated structure at micron size, leading to huge amounts of space inside the surface texture. Therefore, this unique hierarchal topography can contribute to minimizing the liquid-solid contact area due to the trapped air pockets underneath the liquid, which can repel the water droplet to realize super-hydrophobicity [32]. On the other hand, it is interesting to find that the AES sample showed the similar surface structure compared with the fresh super-hydrophobic substrate, as shown in Fig. 2e, f. The results demonstrate that the surface structure could keep intact even though the super-hydrophobic surface was immersed into alkaline solution for 4 h. It is inferred that the corrosive hydroxyl ions (OH − ) had litter influence on the super-hydrophobic FAS coating. The stability or anticorrosion mechanism of the laser-induced fluorinated coating in alkaline solution will be further investigated through the molecular level.

Surface Chemistry of Pristine Copper and Its Corroded Sample
Surface chemical compositions of the pristine copper and its corresponding alkaline-etched samples were detected by XPS technique, as illustrated in Fig. 3. Regarding to surface element contents, it is noted from Figs. 3a, c that the AEP copper presented much higher oxygen element (31.78%) than the pristine copper (18.98%), indicating that vigorous oxidation had occurred upon the pure copper surface after immersed into alkaline solution for 4 h. In addition, Cu 2p spectra were also analyzed and depicted in Figs. 3b, d. The pristine copper showed two peaks locating at 933.3 eV and 953.0 eV, which were related to Cu 2p3/2 and Cu 2p1/2 of Cu 0 , respectively [33]. Obviously, the AEP copper presented a series of Cu 2+ satellite peaks in the ranges of 942-949 eV and 960-966 eV for Cu 2p3/2 and Cu 2p1/2. The peaks locating at 934.8 eV and 954.6 eV represented Cu 2p3/2 and Cu 2p1/2 of Cu 2+ , respectively [34]. The results clearly validated that the pure copper had been oxidized, which would lead to the formation of rough micro/nano-structures and the change of surface wettability. Therefore, the copper material would be easily corroded in the alkaline solution.

Surface Chemistry of Super-hydrophobic and AES Surfaces
Surface chemical components of the fabricated super-hydrophobic copper sample were explored to elucidate the formation mechanism of super-hydrophobicity. As illustrated in Fig. 4a, strong signal of F 1 s and relatively weak signal of Si 2p were observed on the super-hydrophobic FAS surface. This phenomenon revealed that FAS chains with low surface energy were successfully grafted on the laser textured hierarchical structures. The high-resolution spectrum of C 1 s for the super-hydrophobic surface was depicted in Fig. 4b. The peaks locating at 284.8 eV and 286.3 eV were attributed to the functional groups of C-C(H) and C-O, respectively [35]. The peak around 288.6 eV was attributed to the functional group of O=C-O [36]. Most importantly, the fluorinated functional groups of -CF 3 and -CF 2 were also decomposed, which were located at 293.8 eV and 291.7 eV, respectively [37]. According to previous article [24], -CF 3 and -CF 2 functional groups possess extremely low surface free energy. Hence, the existence of these fluorinated functional groups can significantly reduce surface energy for the laser textured rough structures, and enable laser-induced surface to exhibit super-hydrophobic property. Besides, Cu 2p spectrum of the super-hydrophobic surface was also analyzed so as to insight into the laser-material reaction. As illustrated in Fig. 4c, no significant deviation was observed from the peaks position compared with the AEP copper surface. But for the superhydrophobic surface, it is manifested that the peak intensity of the Cu 2+ was higher than that of the AEP copper surface. This phenomenon is mainly due to the intense oxidation of copper by nanosecond laser ablation, resulting in the production of large amounts of CuO.    Surface chemistry of the AES sample was also examined to investigate surface stability of the super-hydrophobic FAS coating in strong alkaline solution. Figure 5a shows its survey spectrum and the relative atomic percentage of the studied elements. It is noted that there was only tiny difference of F element concentration between the AES sample (57.12%) and the fresh super-hydrophobic surface (60.63%). The decomposition of the C 1 s was also carried out as shown in Fig. 5b. The corresponding concentrations of each decomposed component were summarized in Table 1. The results indicate that the concentrations of -CF 2 (43.99%) and -CF 3 (8.08%) functional groups on AES surface were just slightly lower than that of the fresh super-hydrophobic surface (49.24% and 9.33%, respectively), which was consistent with variation of F element concentration. Additionally, 23.19% C-C(H) functional group was detected on the AES sample. Through the decomposed of C 1 s high resolutions, we can conclude that AES surface was dominated by the fluorinated functional groups and non-polar hydrocarbon groups, which could maintain the surface free energy at extremely low level. Therefore, the hydroxide ions lost its corrosive effect regarding to the as-prepared super-hydrophobic FAS coating, and the AES copper sample showed strong chemical stability in alkaline solution. The stability mechanism will be investigated in detail below.

Stability Mechanism
The analyses of surface morphology and surface chemistry inferred that the pristine copper was easily corroded in the strong alkaline solution, while the laser-induced superhydrophobic FAS coating presented excellent stability. To further explore the corresponding stability mechanism, the generation mechanism of super-hydrophobic surface should be clearly described. In the FAS/ethanol solution, three kinds of hydrolyzed-FAS molecules had been produced in the hydrolysis process, including -Si-(OH) 3 , -Si-(OH) 2 (OC 2 H 5 ), and -Si-(OH)(OC 2 H 5 ) 2 . While the laser-induced copper sample was soaked into FAS/ethanol solution, the CuO species would easily absorb water to generate a hydroxyl interface (Cu-OH). Under dehydration process, the Cu-OH interface would react with hydrolyzed-FAS molecules (Si-OH), leading to the attachment of fluorine-containing chains onto the laser-induced hierarchical structures. Therefore, the super-hydrophobic copper surface was successfully obtained by the synergistic effects of rough surface texture and fluorinated functional groups with low surface free energy.
Due to the inadequate hydrolysis of FAS molecules, there were many -Si-OC 2 H 5 and -Si-(OC 2 H 5 ) 2 groups upon the super-hydrophobic surface. When the fresh super-hydrophobic surface was immersed into the alkaline solution, the hydroxide ions (OH − ) acting as a catalyst could promote the hydrolysis reaction for the inadequate hydrolyzed-FAS to produce more Si-OH group. Then the Si-OH group would further react with Cu-OH under the dehydration process to absorb hydrolyzed-FAS molecules on the laser-induced structures, resulting in the improvement of bond strength between the FAS coating and the base substrate. Particularly, two adjacent Si-OH groups can dehydrate with each other and then create Si-O-Si functional group under poly-condensation effect. The schematic illustration is exhibited in Fig. 6   hydrolysis reaction to improve density and strength of the FAS molecules, and the fluorinated super-hydrophobic surface can maintain excellent stability in strong alkaline solution.
To validate the stability mechanism, the O 1 s spectra of fresh super-hydrophobic surface and AES surface were decomposed, as shown in Figs. 7a, b. The O 1 s spectra were fitted into four functional groups, at bonding energy around 530.3 eV, 532.5 eV, 531.5 eV, and 533.4 eV, corresponding to Cu-O, Si-O-C, Si-OH, and Si-O-Si, respectively [38,39]. The concentrations of each component are summarized in Table 2. It is obvious that the concentration of Si-O-C functional group (44.05%) on fresh superhydrophobic surface was greater than that of AES surfaces (25.76%). Meanwhile, the concentrations of Si-O-Si and Si-OH groups for the fresh super-hydrophobic surface were 8.18% and 28.87%, which were less than that of AES surface (11.05% and 46.28%). More Si-OH group on the AES surface implied that further hydrolysis reaction had occurred on the inadequate hydrolyzed-FAS molecules to consume

Electrochemical Measurements
To prove the rationality of the stability mechanism and corrosion resistant property, electrochemical experiments (including PDP and EIS measurements) were carried out on the pristine copper, fresh super-hydrophobic copper, AEP, and AES copper substrates:

Potentiodynamic Polarization Curves (PDP)
The potentiodynamic polarization tests were conducted in 3.5 wt% aqueous NaCl solution, and the corresponding curves were depicted in Fig. 8. The corrosion current density (I corr ) and corrosion potential (E corr ) were analyzed and calculated by the extrapolation method of linear Tafel segments, which was summarized in Table 3. For electrochemical tests, the measurement of PDP curves mainly consisted of two processes: (i) the generation of Cu + during the initial potential corrosion resulting in the decline of current density; (ii) the combination of Cu + with Cl − in the electrolyte resulting in the rise of current density. Therefore, a positively increased corrosion potential combined with a smaller corrosion current density means the excellent anticorrosion property [40]. It is obvious from Fig. 8 that after being corroded in strong alkaline solution, the E corr of the copper substrate dropped rapidly from − 0.530 to − 0.917 V, and the corresponding I corr rose from 3.91 × 10 -5 to 3.95 × 10 -4 A·m −2 .
The results indicate that the AEP copper substrate presented relatively weak corrosion resistance compared with the pristine copper. On the contrary, the E corr and I corr of the both fresh super-hydrophobic and AES samples only showed a slight change, even though the laser-induced super-hydrophobic surfaces were immersed into alkaline solution for 4 h. The results reveal that the super-hydrophobic FAS coating presented excellent stability and anticorrosive property to prevent the copper from alkaline corrosion. The corrosion rate (CR) was also calculated to provide a straightforward demonstration of the corrosion resistance for the studied substrates. The formula for calculating CR value is shown as follow [41]: where M and ρ represent the molar mess and the density of copper material. d represents the valence state of copper. K is a constant with a value of 3450. It is noted from Table 3 that CR value of the AEP sample was almost 10 times larger than that of the pristine sample, indicating that the copper material was more prone to corrosion after immersed into alkaline solution for 4 h. As expected, the fresh super-hydrophobic sample presented excellent corrosion resistance due to the existence of FAS coating, showing the smallest CR value (1.24 × 10 -1 mm·a −1 ) among the investigated samples. In contrast, the CR value of the AES surface was on the same order of magnitude with the fresh super-hydrophobic surface, only demonstrating a negligible variation (1.33 × 10 -1 mm·a −1 ). Therefore, the Tafel curves were surprisingly consistent with the characterization of surface morphology and surface chemistry, which can further prove that the laser-induced super-hydrophobic FAS coating exhibited excellent stability in alkaline solution.

Electrochemical Impedance Spectroscopy (EIS)
As a comprehensive and systematic measurement of electrochemical resistance, the EIS experiments were also investigated to validate the hypothesis we proposed. The Nyquist plots and their corresponding equivalent circuit models were clearly depicted in Fig. 9. Generally, the capacitance arc on the medium and high frequencies depends on charge transfer resistance, which is positively correlated with the corrosion resistant property of the working electrode [42]. As shown in Fig. 9a-c, although the super-hydrophobic surface was corroded by strong alkaline solution for 4 h, the loop radius of the AES sample still significantly larger than that of pristine surface. This was mainly due to the fact that the super-hydrophobic surface remained superior stable property even though it was corroded by the alkaline solution. Nevertheless, compared with the pristine copper, the Nyquist plot of AEP copper changed dramatically in the low frequency stage, which was caused by the presence of Warburg impedance. According to previous article [43], the Warburg impedance mainly derived from the diffusion of copper from the electrode surface to the anode in solution, resulting in the formation of additional corrosive layer with large amount of CuCl 2 − . Therefore, it is concluded that the pristine copper was severely damaged in alkaline solution, showing remarkable conformity with the failure of surface morphology and the change of surface chemical compositions.
To numerically quantify the corrosion resistance of the investigated samples, the fitting impedance parameters were obtained based on the equivalent circuit models, and then summarized in Table 4. R s represents the electrolyte resistance. R f is the film resistance, which is mainly derived from super-hydrophobic coating or corrosive layer. R ct denotes the charge transfer resistance, which is generally used to measure the corrosion resistance of tested samples. W means the Warburg impedance, and the Constant Phase Element (CPE) is employed to simulate the electrical double layer capacitance for the best-fit plots. CPE impedance can be described by the following formula [44]: Super-hydrophobic AES Fitting Fig. 9 The Nyquist plots of a pristine copper; b AEP copper; c fresh super-hydrophobic and AES samples. The corresponding equivalent circuit models of d pristine copper; e AEP copper; f the super-hydrophobic surfaces and AES samples In this formula, the Y 0 is the proportionality factor. i and n represent the imaginary unit and the exponential coefficient, respectively. Obviously, the value of R ct for the AEP sample (123 Ω·cm 2 ) witnessed a decrease trend compared to the pristine sample (415 Ω·cm 2 ), which was ascribed to the enhancement of surface hydrophilicity and the loosening of corrosion layer. As a result, the corrosive ions (Cl − ) in the electrolyte could easily reach the copper substrate to etch its surface. However, only tiny difference of R ct values was observed between the super-hydrophobic surface (3.10 × 10 4 Ω·cm 2 ) and the AES surface (2.38 × 10 4 Ω·cm 2 ). This phenomenon indicated that the super-hydrophobic FAS coating could effectively protect copper material from corrosion, and further validated that the fluorinated superhydrophobic coating could stabilize in the alkaline solution for 4 h.
In addition, Bode plots were also analyzed in this study, as shown in Fig. 10. In general, the impedance modulus |Z| in low frequency is positively related with the anticorrosion capability. It can be seen from Fig. 10a that the impedance modulus |Z| of super-hydrophobic and AES surfaces was almost at the same level, which was far higher than that of pristine copper. This was attributed to the fact that numerous air pockets were trapped by the super-hydrophobic surface to repel the corrosive ions (Cl − ) and improve surface corrosion resistance. Interestingly, the impedance modulus value of AEP sample was a little higher than that of pristine copper at low frequency. This was mainly due to the presence of a large amount of CuCl 2 − on the AEP surface, resulting in a larger Warburg impedance. Through the measurements of surface morphology and surface wettability, it is concluded that the pristine copper was prone to be corroded in alkaline solution, while the super-hydrophobic FAS coating can effectively protect the copper material from corrosion. Besides, the fluorinated super-hydrophobic coating can (6) Z CPE = Y −1 0 (iw) −n , maintain excellent stability in alkaline solution. Based on the measurements of surface chemical compositions, the stability mechanism of super-hydrophobicity was proposed and further validated by the electrochemical tests (PDP and EIS).

Conclusion
In this work, super-hydrophobic copper surface was fabricated by nanosecond laser ablation and following FAS modification. The surface morphology and wettability measurements indicated that the pristine copper can be easily corroded in alkaline solution, but the fluorinated superhydrophobic coating can protect the copper material from alkaline corrosion. According to the surface chemistry measurements, the anticorrosion mechanism and stability mechanism of the fluorinated super-hydrophobic surface were proposed. Particularly, hydroxyl ion (OH − ) can further promote the hydrolysis reaction to improve the density and bond strength for the FAS molecules. Additionally, the proposed mechanisms were also verified by the electrochemical tests (PDP and EIS) to prove their rationalities. The authors believe that these findings can provide the significant guidance for the specific working environments in industry of fluorinated super-hydrophobic coatings so as to protect metallic materials from corrosion, and to improve the lifespan of metallic components. What's more, the fluorinated super-hydrophobic coatings will be fabricated upon the copper pipes, and then immersed in alkaline waste solutions to check the time limitations. The stability of superhydrophobicity will be further derived.