A Novel Simple Fabrication Method for Mechanically Robust Superhydrophobic 2024 Aluminum Alloy Surfaces

: The mechanical durability of a superhydrophobic aluminum alloy surface is an important indicator of its practical use. Herein, we propose a strategy to prepare a superhydrophobic 2024 aluminum alloy surface with highly enhanced mechanical durability by using a two-step chemical etching method, using a NaOH solution as the etchant in step one and a Na 2 CO 3 solution as the etchant in step two. Robust mechanical durability was studied by static contact angle tests before and after an abrasion test, potentiodynamic polarization measurements after an abrasion test and electrochemical impedance spectroscopy tests after an abrasion test. Furthermore, the mechanism for enhanced mechanical durability was investigated through scanning of electron microscopy images, energy-dispersive X-ray spectra, Fourier transform infrared spectra and X-ray photoelectron spectra. The testing results indicate that a hierarchical rough surface consisting of regular micro-scale dents and some nano-scale ﬁbers in the micro-scale dents, obtained with the two-step chemical etching method, contributes to highly enhanced mechanical durability. Meanwhile, the as-prepared super-hydrophobic 2024 aluminum alloy surface retained a silvery color instead of the black shown on the superhydrophobic 2024 aluminum alloy surface prepared by a conventional one-step chemical etching method using NaOH solution as the etchant.


Introduction
Due to the advantages of low specific weight, high specific strength, excellent thermal conductivity and low cost, aluminum alloys have been widely applied in many fields, such as architecture, traffic, industry and household items [1]. However, aluminum alloys are easily corroded in environments containing chloride ions (for example, seawater) because their corrosion resistance becomes poor in those environments [2]. To enhance corrosion resistance of aluminum alloys, many types of surface modification methods based on aluminum-alloy substrates have been developed [3]. Preparation of superhydrophobic surfaces on aluminum-alloy substrates is a promising surface modification method to enhance corrosion resistance of aluminum alloys because those superhydrophobic surfaces can minimize the contact area between the corrosive solution and the surface of aluminum alloys [4,5]. However, it is difficult to obtain a mechanically robust superhydrophobic surface based on aluminum-alloy substrates. Therefore, research about enhancing mechanical durability of superhydrophobic aluminum alloy surfaces attracts great attention [5][6][7][8][9][10][11][12][13][14][15].

Preparation of the Superhydrophobic 2024 Aluminum Alloy Surface
Superhydrophobic 2024 aluminum alloy surfaces were acquired using a two-step methodology including fabrication of a rough surface using a two-step chemical etching method and chemical modification with stearic acid. A schematic diagram of preparation of a superhydrophobic 2024 aluminum alloy surface is shown in Figure 1. First, a pretreatment was conducted. The 2024 aluminum alloy sheets were polished using SiC abrasive papers of 600# and 1000#. The polished sheets were ultrasonically cleaned using ethanol and degreased using acetone. Second, a two-step chemical etching method was carried out to create a rough surface with hierarchical micro/nano structures. In order to create micrometer scale structures on the 2024 aluminum alloy surface, the degreasedsheets were immersed into a NaOH solution of 1 mol L −1 for 30 min at 25 • C. Then, the sheets were washed using 1 mol L −1 HCl solution and deionized water, respectively. To create nano structures, the primary etched sheets were immersed into a Na 2 CO 3 solution of 0.1 mol L −1 for 2 min at 25 • C. Finally, chemical modification of sheets with rough surfaces was conducted. The etched sheets were immersed in an ethanol solution with 0.05 mol L −1 stearic acid for 30 min at 25 • C. Afterwards, the chemical modified sheets were dried at 80 • C in an oven.

Preparation of the Superhydrophobic 2024 Aluminum Alloy Surface
Superhydrophobic 2024 aluminum alloy surfaces were acquired using a two-ste methodology including fabrication of a rough surface using a two-step chemical etchin method and chemical modification with stearic acid. A schematic diagram of preparatio of a superhydrophobic 2024 aluminum alloy surface is shown in Figure 1. First, pre-treatment was conducted. The 2024 aluminum alloy sheets were polished using Si abrasive papers of 600# and 1000#. The polished sheets were ultrasonically cleaned usin ethanol and degreased using acetone. Second, a two-step chemical etching method wa carried out to create a rough surface with hierarchical micro/nano structures. In order t create micrometer scale structures on the 2024 aluminum alloy surface, the degreased sheets were immersed into a NaOH solution of 1 mol L −1 for 30 min at 25 °C. Then, th sheets were washed using 1 mol L −1 HCl solution and deionized water, respectively. T create nano structures, the primary etched sheets were immersed into a Na2CO3 solutio of 0.1 mol L −1 for 2 min at 25 °C. Finally, chemical modification of sheets with roug surfaces was conducted. The etched sheets were immersed in an ethanol solution wit 0.05 mol L −1 stearic acid for 30 min at 25 °C. Afterwards, the chemical modified shee were dried at 80 °C in an oven. For comparison, another type of superhydrophobic 2024 aluminum alloy surfac was obtained using a two-step method including fabrication of a rough surface using one-step chemical etching method and chemical modification with stearic acid. First, pre-treatment was conducted. The 2024 aluminum alloy sheets were polished using Si abrasive papers of 600# and 1000#. The polished sheets were ultrasonically cleaned usin ethanol and degreased using acetone. Second, a one-step chemical etching method wa carried out to create rough surfaces. The degreased sheets were immersed into a NaOH solution of 0.25 mol L −1 for 5 min at 90 °C. Then, the sheets were washed using deionize water. Finally, chemical modification of sheets with rough surfaces was conducted. Th etched sheets were immersed into an ethanol solution with 0.05 mol L −1 stearic acid for 3 min at 25 °C. Afterwards, the chemical modified sheets were dried at 80 °C in an oven.

Characterisation of As-Prepared Superhydrophobic Surface
Water contact angles were obtained through the sessile drop method using a conta angle goniometer (DSA100, Kruss, Germany). Morphologies of samples were observe on a scanning electron microscope (Gemini SEM 300, Zeiss, Oberkochen, Germany Compositions of samples were studied using energy-dispersive X-ray (EDX) spectra ob tained with an EDX spectrometer (Aztec X-MAX80, Oxford, UK), Fourier transform in frared (FTIR) spectra obtained with an FTIR spectrophotometer (Nicolet Is5, Therm Fisher Scientific, Waltham, MA, USA) in the range 500 to 4000 cm −1 and X-ray photoele tron spectra obtained with an X-ray photoelectron spectrometer (Escalab 250Xi, Therm For comparison, another type of superhydrophobic 2024 aluminum alloy surface was obtained using a two-step method including fabrication of a rough surface using a one-step chemical etching method and chemical modification with stearic acid. First, a pre-treatment was conducted. The 2024 aluminum alloy sheets were polished using SiC abrasive papers of 600# and 1000#. The polished sheets were ultrasonically cleaned using ethanol and degreased using acetone. Second, a one-step chemical etching method was carried out to create rough surfaces. The degreased sheets were immersed into a NaOH solution of 0.25 mol L −1 for 5 min at 90 • C. Then, the sheets were washed using deionized water. Finally, chemical modification of sheets with rough surfaces was conducted. The etched sheets were immersed into an ethanol solution with 0.05 mol L −1 stearic acid for 30 min at 25 • C. Afterwards, the chemical modified sheets were dried at 80 • C in an oven.

Characterisation of As-Prepared Superhydrophobic Surface
Water contact angles were obtained through the sessile drop method using a contact angle goniometer (DSA100, Kruss, Germany). Morphologies of samples were observed on a scanning electron microscope (Gemini SEM 300, Zeiss, Oberkochen, Germany). Compositions of samples were studied using energy-dispersive X-ray (EDX) spectra obtained with an EDX spectrometer (Aztec X-MAX80, Oxford, UK), Fourier transform infrared (FTIR) spectra obtained with an FTIR spectrophotometer (Nicolet Is5, Thermo Fisher Scientific, Waltham, MA, USA) in the range 500 to 4000 cm −1 and X-ray photoelectron spectra obtained with an X-ray photoelectron spectrometer (Escalab 250Xi, Thermo Fisher Scientific, Waltham, MA, USA). Overview spectra (from 0 to 1350 eV) were recorded using Al Kα radiation (hv = 1486.6 eV) and binding energies were calibrated based on 284.8 eV.
Abrasion tests of as-prepared samples were performed with the following steps: The testing sample was placed on a SiC abrasive paper of 800#. The superhydrophobic surface of the testing sample faced the abrasive paper. Meanwhile, an iron block was loaded on the testing sample. The testing sample, with a load of 0.03 kg cm −2 , was moved in one direction with 5 mm s −1 at two strokes of 15 cm.
Potentiodynamic polarization measurements were carried out on an electrochemical workstation (CHI660D, Shanghai CH Instruments, Shanghai, China) using a three-electrode system at room temperature. The as-prepared sample was used as a working electrode. A platinum sheet was used as a counter electrode. An Ag/AgCl/saturated KCl electrode was used as a reference electrode. To realize stable open-circuit potential before collection of potentiodynamic polarization data, three electrodes were immersed in 3.5 wt% NaCl solution for 30 min. Potentiodynamic polarization curves were achieved at stable open-circuit potential and at a scanning rate of 1 mV s −1 . Electrochemical impedance spectroscopy (EIS) tests were also carried out on the CHI660D electrochemical workstation. EIS curves were acquired at frequencies in a range of 100 kHz to 10 mHz.

Results and Discussion
Because water repellency was not the main focus in this study, only static water contact angles were obtained. SEM images and static water contact angles of the samples treated at different conditions are demonstrated in Figure 2a-g,a -g . Figure 2a-e demonstrates morphologies and contact angles of samples treated at various conditions using the twostep chemical etching method. As shown in Figure 2b,b , the NaOH-etched sample at 25 • C presented a rough surface with irregular micro-scale particles, cracks and irregular nano-scale fibers on the surface of the micro-scale particles, which is in accord with the results in the previous literature [60,62]. Furthermore, as shown in the inset of Figure 2b, the NaOH-etched sample at 25 • C showed a black surface, which was not observed in other types of NaOH-solution-etched aluminum alloy surfaces or documented in the previous literature [15,[54][55][56][57][58][59][60][61][62][63][64]. As shown in Figure 2c, the NaOH-etched aluminum alloy at 25 • C after HCl washing presented a relatively regular rough surface with regular micro-scale dents. The inset of Figure 2c presents a silvery white surface, indicating removal of the black substance. As shown in Figure 2d,d , after Na 2 CO 3 -etching, the Na 2 CO 3 -etched sample presented a relatively regular rough surface with regular micro-scale dents and some nano-scale fibers in the micro-scale dents. The inset of Figure 2d also shows a silvery white surface. As shown in Figure 2e,e , the stearic-acid-treated sample presented a similar, relatively regular rough surface with regular micro-scale dents. The nano-scale fibers in Figure 2d turned into nano-scale grids. The inset of Figure 2e also shows a silvery white surface. Figure 2f,f ,g,g demonstrates the morphologies of the samples prepared using the one-step chemical etching method. As shown in Figure 2f,g, samples prepared using the one-step chemical etching method presented a rough surface with irregular micro-scale particles and cracks, which is in accord with the results in the previous literature [57,60]. However, as shown in the insets of Figure 2f,g, the samples showed a black surface: a result that was not documented in the previous literature [15,[54][55][56][57][58][59][60][61][62][63][64]. As shown in the insets of Figure 2e,e ,g,g and Table 2, both stearic-acid-treated samples had good superhydrophobicity, with a contact angle of over 160 • . The regular rough surface of the sample prepared using the two-step chemical etching method was expected to facilitate robust mechanical durability for the as-prepared superhydrophobic aluminum alloy surface. Table 2. Static water contact angles for (a) untreated aluminum alloy, (b) NaOH-etched aluminum alloy at 25 • C after HCl washing, (c) HCl-washed aluminum alloy after Na 2 CO 3 etching, (d) Na 2 CO 3 -etched aluminum alloy at 25 • C, (e) stearic-acid-treated aluminum alloy after Na 2 CO 3 etching, (f) NaOH-etched aluminum alloy at 90 • C, (g) stearic-acid-treated aluminum alloy after NaOH etching at 90 • C, (h) stearic-acid-treated sample after Na 2 CO 3 etching after abrasion test and (i) stearic-acid-treated sample after NaOH etching at 90 • C after abrasion test.   The compositions of the samples treated at different conditions were characterized by conduction of EDX measurements. As shown in Figure 3, along with the Cu, Mn, Mg, Fe and Si elements, the O and C element were present on the stearic-acid-treated sample after Na2CO3-etching. The presence of the C element can be ascribed to chemical modification with stearic acid. Table 3 exhibits chemical compositions of samples treated at different conditions. The content of the O element for the Na2CO3-etched sample after-NaOH etching was higher than that of the pristine sample, indicating oxidation of some metal elements. The conclusion was further verified by the following XPS analyses. The compositions of the samples treated at different conditions were characterized by conduction of EDX measurements. As shown in Figure 3, along with the Cu, Mn, Mg, Fe and Si elements, the O and C element were present on the stearic-acid-treated sample after Na 2 CO 3 -etching. The presence of the C element can be ascribed to chemical modification with stearic acid. Table 3 exhibits chemical compositions of samples treated at different conditions. The content of the O element for the Na 2 CO 3 -etched sample afterNaOH etching was higher than that of the pristine sample, indicating oxidation of some metal elements. The conclusion was further verified by the following XPS analyses.

Samples
XPS spectra were obtained to further study chemical compositions of samples treated at different conditions. Figure 4a shows the survey of XPS spectra. In XPS spectra of the stearic-acid-treated aluminum alloy after Na 2 CO 3 etching and of the stearic-acid-treated aluminum alloy after NaOH etching at 90 • C, characteristic peaks of Al and Cu for both stearic-acid-treated samples are clearly present, indicating a thin superhydrophobic film for both stearic-acid-treated samples. High-resolution spectra of the Cu and Al elements are shown in Figure 4b,c. In Figure 4b, the 933.6 eV peaks for the untreated sample and the stearic-acid-treated sample after Na 2 CO 3 etching can mainly be ascribed to 2p 3/2 peaks of Cu, which was deduced based on the results in the previous literature [64] and the silvery color of the samples. The 934.8 eV peak of the stearic-acid-treated sample after NaOH etching at 90 • C can mainly be ascribed to 2p 3/2 peaks of CuO, which was deduced based on the results in the previous literature [65] and the black color of the sample. The result of the high-resolution XPS spectra of Cu is in agreement with that of the chemical composition change detected by EDX mapping. In Figure 4c, the 72.8 eV peak of the untreated sample can be ascribed to 2p 3/2 peaks of Al, and the 74.8 eV peak can be ascribed to 2p 3/2 peaks of Al 2 O 3 [65]. The 72.8 eV peak of the stearic-acid-treated sample after Na 2 CO 3 etching can be ascribed to 2p 3/2 peaks of Al, and the 74.4 eV peak can be ascribed to 2p 3/2 peaks of Al(OH) 3 . The 74.4 eV peak of the stearic-acid-treated sample after NaOH etching at 90 • C can be ascribed to 2p 3/2 peak of Al(OH) 3 [55]. The result of the high-resolution XPS spectra of Al is also in agreement with that of the chemical composition change detected by EDX mapping.  Table 3. Chemical compositions of (a) untreated aluminum alloy, (b) NaOH-etched aluminum alloy at 25 • C after HCl washing, (c) HCl-washed aluminum alloy after Na 2 CO 3 etching, (d) Na 2 CO 3etched aluminum alloy at 25 • C, (e) stearic-acid-treated aluminum alloy after Na 2 CO 3 etching, (f) NaOH-etched aluminum alloy at 90 • C and (g) stearic-acid-treated aluminum alloy after NaOH etching at 90 • C.

Samples
Cu  Figure 4c, the 72.8 eV peak of the untreated sample can be ascribed to 2p 3/2 peaks of Al, and the 74.8 eV peak can be ascribed to 2p 3/2 peaks of Al2O3 [65]. The 72.8 eV peak of the stearic-acid-treated sample after Na2CO3 etching can be ascribed to 2p 3/2 peaks of Al, and the 74.4 eV peak can be ascribed to 2p 3/2 peaks of Al(OH)3. The 74.4 eV peak of the stearic-acid-treated sample after NaOH etching at 90 °C can be ascribed to 2p 3/2 peak of Al(OH)3 [55]. The result of the high-resolution XPS spectra of Al is also in agreement with that of the chemical composition change detected by EDX mapping. Deduced from analyses of SEM images, SEM-EDX mapping and XPS spectra, the chemical etching mechanism of the two-step chemical etching method can be described as the following equations. The NaOH solution etching process can be described by Equations (2)-(6). The HCl solution washing process can be described by Equations (7) and (8).The Na2CO3 solution etching process can be described by Equations (9)- (11). Due to the generation of CO2 in the Na2CO3 solution etching process, the reaction between copper and NaOH was restrained. Therefore, the Na2CO3-etched aluminum alloy and the stearic-acid-treated aluminum alloy after Na2CO3 etching still showed a silvery white surface.
2Al + 2NaOH + 2H 2 O → 2NaAlO 2 + 3H 2 (1) FTIR spectra were obtained to investigate the chemical functional group and chemical structure of samples treated at different conditions. Figure 5 shows the FTIR spectra of the untreated aluminum alloy, the stearic-acid-treated aluminum alloy after Na 2 CO 3 etching and the stearic-acid-treated aluminum alloy after NaOH etching at 90 • C. The peaks in the FTIR spectrum of the stearic-acid-treated sample after Na 2 CO 3 etching are much weaker than those in the FTIR spectrum of the stearic-acid-treated sample after NaOH etching at 90 • C, which indicates a thinner superhydrophobic film on the stearicacid-treated sample after Na 2 CO 3 etching. This result is in accord with that observed in the SEM images in Figure 2e,e ,g,g . The broad peak at 3400 cm −1 can be ascribed to the -OH band of Al(OH) 3 [54]. The peaks at 2915 and 2847 cm −1 are attributed to the C-H and -CH 2 bands [58,62], respectively. The peak at 1586 cm −1 is attributed to the -COOAl band [58]. The presence of -OH, C-H, -CH 2 and -COOAl bands verifies successful chemical modification with stearic acid on etched aluminum-alloy substrates. The peaks at 1410 and 1360 cm −1 are attributed to the -AlO band [62]. Furthermore, the peaks at 717, 554 and 405 cm −1 are attributed to the Al-O band [58,62]. The weak peaks at 3400, 717, 554 and 405 cm −1 for stearic-acid-treated aluminum alloy after Na 2 CO 3 etching indicate a thin Al(OH) 3 film, which is also observed in the SEM image in Figure 2d,d .
-CH2 bands [58,62], respectively. The peak at 1586 cm −1 is attributed to [58]. The presence of -OH, C-H, -CH2 and -COOAl bands verifies s modification with stearic acid on etched aluminum-alloy substrates. and 1360 cm −1 are attributed to the -AlO band [62]. Furthermore, th and 405 cm −1 are attributed to the Al-O band [58,62]. The weak peak and 405 cm −1 for stearic-acid-treated aluminum alloy after Na2CO3 etch Al(OH)3 film, which is also observed in the SEM image in Figure 2d,d′ Figure 5. FTIR spectra of (a) untreated aluminum alloy, (b) stearic-acid-trea after Na2CO3 etching and (c) stearic-acid-treated aluminum alloy after NaOH Digital images of surfaces on stearic-acid-treated samples after N stearic-acid-treated samples after NaOH etching at 90 °C before and were obtained to investigate mechanical durability. As shown in Fig  ric-acid-treated sample after Na2CO3 etching presented a silvery c ric-acid-treated sample after Na2CO3 etching after the abrasion test color. However, as shown in Figure 6c,d, the stearic-acid-treated sa etching at 90 °C presented a black color and the stearic-acid-treated s etching at 90 °C after the abrasion test showed a light gray color. Fig  images of surfaces on the stearic-acid-treated sample after Na2CO abrasion test and on the stearic-acid-treated sample after NaOH etchin abrasion test. As shown in Figure 6e,f, the stearic-acid-treated sample ing at 90 °C after the abrasion test showed a severely broken surface. A the static contact angle changed from 164.7° to 133.9° after the abrasio the water contact angle for the stearic-acid-treated sample after NaO was 30.8°. In contrast to the severely broken surface of the stearic-a after NaOH etching at 90 °C after the abrasion test, the stearic-acid-tr Na2CO3 etching after the abrasion test showed a relatively slightly bro consisted of some slight broken micro-scale dents and intact nano-sca cro-scale dents. As listed in Table 2, the static contact angle changed fr Digital images of surfaces on stearic-acid-treated samples after Na 2 CO 3 etching and stearic-acid-treated samples after NaOH etching at 90 • C before and after abrasion tests were obtained to investigate mechanical durability. As shown in Figure 6a,b, the stearicacid-treated sample after Na 2 CO 3 etching presented a silvery color, and the stearic-acidtreated sample after Na 2 CO 3 etching after the abrasion test showed a similar color. However, as shown in Figure 6c,d, the stearic-acid-treated sample after NaOH etching at 90 • C presented a black color and the stearic-acid-treated sample after NaOH etching at 90 • C after the abrasion test showed a light gray color. Figure 6e,f show SEM images of surfaces on the stearic-acid-treated sample after Na 2 CO 3 -etching after the abrasion test and on the stearic-acid-treated sample after NaOH etching at 90 • C after the abrasion test. As shown in Figure 6e,f, the stearic-acid-treated sample after NaOH etching at 90 • C after the abrasion test showed a severely broken surface. As listed in Table 2, the static contact angle changed from 164.7 • to 133.9 • after the abrasion test. Variation of the water contact angle for the stearic-acid-treated sample after NaOH etching at 90 • C was 30.8 • . In contrast to the severely broken surface of the stearic-acid-treated sample after NaOH etching at 90 • C after the abrasion test, the stearic-acid-treated sample after Na 2 CO 3 etching after the abrasion test showed a relatively slightly broken surface, which consisted of some slight broken micro-scale dents and intact nano-scale fibers in the micro-scale dents. As listed in Table 2, the static contact angle changed from 161.7 • to 144.6 • after the abrasion test. Variation of the water contact angle for the stearic-acid-treated sample after Na 2 CO 3 etching was 17.1 • , which is bigger than the 10.7 • variation of water contact angle for superhydrophobic surfaces on 1060 aluminum-alloy substrates in the previous literature [66]. This can be ascribed to the different aluminum-alloy substrates. In contrast to the severely broken surface and big variation of the static contact angle for the stearic-acid-treated sample after NaOH etching at 90 • C, the slightly broken surface and smaller variation of the static contact angle for the stearic-acid-treated sample after Na 2 CO 3 etching indicated highly improved mechanical durability of the stearic-acid-treated sample after Na 2 CO 3 etching. previous literature [66]. This can be ascribed to the different aluminum-alloy substrates. In contrast to the severely broken surface and big variation of the static contact angle for the stearic-acid-treated sample after NaOH etching at 90 °C, the slightly broken surface and smaller variation of the static contact angle for the stearic-acid-treated sample after Na2CO3 etching indicated highly improved mechanical durability of the stearic-acid-treated sample after Na2CO3 etching. Figure 6. Digital images of (a) stearic-acid-treated sample after Na2CO3 etching, (b) stearic-acid-treated sample after Na2CO3 etching after abrasion test, (c) stearic-acid-treated sample after NaOH etching at 90 °C and (d) stearic-acid-treated sample after NaOH etching at 90 °C after abrasion test. SEM images of (e) stearic-acid-treated sample after Na2CO3 etching after abrasion test, (f) stearic-acid-treated sample after NaOH etching at 90 °C after abrasion test. The inset is the image of the contact angle.
To further confirm highly improved mechanical durability of the stearic-acid-treated sample after Na2CO3 etching, the anti-corrosive properties of the stearic-acid-treated sample after Na2CO3-etching after the abrasion test and of the stearic-acid-treated sample after NaOH etching at 90 °C after the abrasion test were investigated via potentiodynamic polarization curves as well as corrosion current density (icorr), corrosion potential (Ecorr) and polarization resistance (Rp). Potentiodynamic polarization curves are shown in Figure 7. Corresponding data are listed in Table 4. As listed in Table 4, the values of icorr and Rp for the stearic-acid-treated sample after Na2CO3 etching after the abrasion test were 0.15 μA cm −2 and 268.7 kΩ cm 2 , respectively. However, the values of icorr and Rp for the stearic-acid-treated sample after NaOH etching at 90 °C after the abrasion test were 0.68 μA cm −2 and 35.2 kΩ cm 2 , respectively. The smaller icorr value and bigger Rp value for the stearic-acid-treated sample after Na2CO3 etching after the abrasion test indicate better Figure 6. Digital images of (a) stearic-acid-treated sample after Na 2 CO 3 etching, (b) stearic-acidtreated sample after Na 2 CO 3 etching after abrasion test, (c) stearic-acid-treated sample after NaOH etching at 90 • C and (d) stearic-acid-treated sample after NaOH etching at 90 • C after abrasion test. SEM images of (e) stearic-acid-treated sample after Na 2 CO 3 etching after abrasion test, (f) stearic-acid-treated sample after NaOH etching at 90 • C after abrasion test. The inset is the image of the contact angle.
To further confirm highly improved mechanical durability of the stearic-acid-treated sample after Na 2 CO 3 etching, the anti-corrosive properties of the stearic-acid-treated sample after Na 2 CO 3 -etching after the abrasion test and of the stearic-acid-treated sample after NaOH etching at 90 • C after the abrasion test were investigated via potentiodynamic polarization curves as well as corrosion current density (i corr ), corrosion potential (E corr ) and polarization resistance (R p ). Potentiodynamic polarization curves are shown in Figure 7. Corresponding data are listed in Table 4. As listed in Table 4, the values of i corr and R p for the stearic-acid-treated sample after Na 2 CO 3 etching after the abrasion test were 0.15 µA cm −2 and 268.7 kΩ cm 2 , respectively. However, the values of i corr and R p for the stearic-acid-treated sample after NaOH etching at 90 • C after the abrasion test were 0.68 µA cm −2 and 35.2 kΩ cm 2 , respectively. The smaller i corr value and bigger R p value for the stearic-acid-treated sample after Na 2 CO 3 etching after the abrasion test indicate better corrosion resistance, which verifies highly improved mechanical durability of the stearic-acid-treated sample after Na 2 CO 3 etching. of (a) stearic-acid-treated sample after Na2CO3 etching after abrasion test and (b) ste ric-acid-treated sample after NaOH etching at 90 °C after abrasion test.

Sample
Ecorr (mV vs. To further confirm highly improved mechanical durability of the stearic-acid-treate sample after Na2CO3 etching, EIS measurements were taken to study the anti-corrosiv properties of the stearic-acid-treated sample after Na2CO3-etching after the abrasion te and of the stearic-acid-treated sample after NaOH etching at 90 °C after the abrasion te Nyquist plots, presenting two semicircles, are shown in Figure 8a. The smaller semicirc at high frequency represents resistance of the superhydrophobic thin films (RSH). Th second large semicircle represents charge-transfer resistance (RctSH) of the double layer the interface between the superhydrophobic surface and the salt solution. The Nyqui plots can be fitted to the equivalent electrical circuit with two CPEs shown in Figure 8 [58,62]. In the equivalent electrical circuit, Rs is resistance of the solution. RSH represen resistance of the superhydrophobic thin films. CPESH is the constant phase element ass ciated with the dielectric nature of the superhydrophobic film. RctSH represen charge-transfer resistance of the double layer at the interface between the superhydr phobic surface and the salt solution. CPEctSH is the constant phase element associate with the double layer at the interface between the superhydrophobic film surface an the salt solution. The RSH and RctSH values of the stearic-acid-treated sample after Na2CO etching after the abrasion test were 28.32 and 112.06 kΩ cm 2 , respectively. Nevertheles the RSH and RctSH values of the stearic-acid-treated sample after NaOH etching at 90 ° after the abrasion test were 12.77 and 45.49 kΩ cm 2 , respectively. The much larger RSH an RctSH values of the stearic-acid-treated sample after Na2CO3 etching after the abrasion te indicate a better anti-corrosive property. This result is in agreement with that obtaine from potentiodynamic polarization measurements and is shown in Figure 7 and listed Table 4.  To further confirm highly improved mechanical durability of the stearic-acid-treated sample after Na 2 CO 3 etching, EIS measurements were taken to study the anti-corrosive properties of the stearic-acid-treated sample after Na 2 CO 3 -etching after the abrasion test and of the stearic-acid-treated sample after NaOH etching at 90 • C after the abrasion test. Nyquist plots, presenting two semicircles, are shown in Figure 8a. The smaller semicircle at high frequency represents resistance of the superhydrophobic thin films (R SH ). The second large semicircle represents charge-transfer resistance (R ctSH ) of the double layer at the interface between the superhydrophobic surface and the salt solution. The Nyquist plots can be fitted to the equivalent electrical circuit with two CPEs shown in Figure 8b [58,62]. In the equivalent electrical circuit, R s is resistance of the solution. R SH represents resistance of the superhydrophobic thin films. CPE SH is the constant phase element associated with the dielectric nature of the superhydrophobic film. R ctSH represents charge-transfer resistance of the double layer at the interface between the superhydrophobic surface and the salt solution. CPE ctSH is the constant phase element associated with the double layer at the interface between the superhydrophobic film surface and the salt solution. The R SH and R ctSH values of the stearic-acid-treated sample after Na 2 CO 3 etching after the abrasion test were 28.32 and 112.06 kΩ cm 2 , respectively. Nevertheless, the R SH and R ctSH values of the stearic-acid-treated sample after NaOH etching at 90 • C after the abrasion test were 12.77 and 45.49 kΩ cm 2 , respectively. The much larger R SH and R ctSH values of the stearic-acid-treated sample after Na 2 CO 3 etching after the abrasion test indicate a better anticorrosive property. This result is in agreement with that obtained from potentiodynamic polarization measurements and is shown in Figure 7 and listed in Table 4. Coatings 2022, 12, x FOR PEER REVIEW 11 of 14 Figure 8. EIS spectra and electrical equivalent circuit of (a) stearic-acid-treated sample after Na2CO3 etching after abrasion test and (b) stearic-acid-treated sample after NaOH etching at 90 °C after abrasion test.

Conclusions
In this work, amechanically robust non-black superhydrophobic 2024 aluminum alloy surface was successfully prepared by a two-step chemical etching method, using NaOH solution as the etchant of step one and Na2CO3 solution as the etchant of step two. The variation of static contact angles for the as-prepared non-black superhydrophobic 2024 aluminum alloy surface before and after abrasion test was 17.1° while that of the black superhydrophobic 2024 aluminum alloy surface prepared by the conventional one-step chemical etching method using NaOH solution as the etchant was 30.8°. The improved mechanical durability of the as-prepared non-black superhydrophobic 2024 aluminum alloy surface can be ascribed to the rough surface with relatively regular micro-scale dents and some nano-scale fibers in the micro-scale dents, which was confirmed by SEM observation.

Data Availability Statement:
The data that support the findings of this study are available from the corresponding authors, L.-M.S., upon reasonable request.

Conflicts of Interest:
The authors declare no conflict of interest.

Conclusions
In this work, amechanically robust non-black superhydrophobic 2024 aluminum alloy surface was successfully prepared by a two-step chemical etching method, using NaOH solution as the etchant of step one and Na 2 CO 3 solution as the etchant of step two. The variation of static contact angles for the as-prepared non-black superhydrophobic 2024 aluminum alloy surface before and after abrasion test was 17.1 • while that of the black superhydrophobic 2024 aluminum alloy surface prepared by the conventional one-step chemical etching method using NaOH solution as the etchant was 30.8 • . The improved mechanical durability of the as-prepared non-black superhydrophobic 2024 aluminum alloy surface can be ascribed to the rough surface with relatively regular micro-scale dents and some nano-scale fibers in the micro-scale dents, which was confirmed by SEM observation.