## 2 Materials and Methods  
First, 6082-T6 aluminum alloy specimens (size: 3 cm \(\times\) 2.4 cm \(\times\) 0.2 cm) obtained from a \(100\times 100\times 0.2\) cm\({}^{3}\) commercially laminated plate (chemical composition: Si 0.70-1.3%; Mg 0.60-1.2%; Fe \(\leq\) 0.50%; Cu \(\leq\) 0.10%; Mn 0.40-1.0%; Cr \(\leq\) 0.25%; Zn \(\leq\) 0.20%; Ti \(\leq\) 0.10% and balance Al) were used as substrates. Octadecyltrimethoxysilane (C\({}_{21}\)H\({}_{46}\)O\({}_{3}\)Si, 90%) and hydrofluoric acid (48% concentration) were purchased from Sigma-Aldrich. Hydrochloric acid (37% concentration) and nitric acid (60% concentration) were purchased from Carlo Erba. In addition, bi-distilled water with a resistivity of 18.0 M\(\Omega\)-cm, from Best-Chemical, Vairoo Patenora, Italy, was used throughout the experiments.  
The as-received 6082 aluminum alloy samples were ultrasonically cleaned in ethanol, acetone, and ultra-pure water for 5 min before roughening treatment. In order to create a hierarchical structure on the aluminum surface, the samples were divided into three batches: The first batch was treatedin boiling water for 5 min, the second one was chemically etched in HNO3/HCl (1:3) for 1 h, and the third one was etched in [PERSON]'s HF/HCl solution for 15 s. Then, the made specimens were ultrasonically washed and dried at 70 \({}^{\circ}\)C for 1 h. Afterward, the rough samples were immersed in 1 vol.% octadecyltrimethoxysilane/toluene solution for 10 min at room temperature and dried in an oven at 100 \({}^{\circ}\)C for 180 min for silane curing. All the parameters were optimized based on the literature [23; 24; 25] and preliminary tests performed in our laboratory. Table 1 illustrates the specimen details.  
The surface morphologies of the specimens were studied by using a focused ion dual beam/scanning electron microscope (FIB-SEM ZEISS Crossbeam 540). Moreover, the coating composition was evaluated via energy-dispersive spectroscopy (Aztec Oxford, EDS). The roughness of the surfaces was measured using the profilometer SJ-210 (Mitutoyo, Japan).  
The static water contact angles (WCAs) of the different aluminum surfaces were measured using an Attenison Theta Tensiometer (Biolin Scientific, Sweden) following the sessile drop technique (\(\approx\)3 \(\upmu\)L). The sliding angles (WSA) were recorded by tilting the platform until the water droplets rolled off the surfaces. For each treatment step, five samples were prepared in order to ensure the reproducibility of the process. Fifty replicas of water contact angle (WCA), located on the surface to obtain a regular grid, for each sample were performed, and the average values and standard errors of WSA and WCA were calculated.  
Electrochemical measurements were performed, using a BioLogicSP-300 potentiostat, at ambient temperature in simulated seawater (3.5 wt.% NaCl solution) open to air. A standard three-electrode cell, having a platinum wire as the counter electrode, a saturated Ag/AgCl electrode as the reference electrode, and the superhydrophobic sample as the working electrode, was used as the working cell. The exposed area of the working electrode was 1 cm\({}^{2}\). Prior to the test, each sample was in contact with the NaCl solution for 30 min to ensure the steady state. The potentiodynamic polarization curves were collected with a scanning rate of 0.2 mV/s.