Experimental error at all ph values of the water

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experimental error at all pH values of the water droplet, ranging from 1 to 14. SA values always remain below 5 ° , which shows the low adhesion of the as-prepared super- hydrophobic silica coatings. We conclude that the super- hydrophobic coatings show good chemical stability to acid droplets, basic droplets and some salt aqueous solutions. The corrosion resistance of the superhydrophobic sur- faces is a key determinant in the superhydrophobic surfaces practical applications. The corrosion resistance of the superhydrophobic silica coatings was tested in a 3.5 wt% NaCl aqueous solution by electrochemical workstation. Figure 3 shows potentiodynamic polarization curves of a bare Al substrate after immersion in the 3.5 wt% NaCl aqueous solution and the superhydrophobic silica coatings formed on an Al substrate after immersion in the 3.5 wt% NaCl aqueous solution for 3 h at room temperature. As given in Table 1 , the electrochemical parameters of the corrosion current density ( I corr ) for bare Al and the silica coatings were obtained using the Tafel extrapolation from the potentiodynamic polarization curves. It should be noted that I corr of the superhydrophobic coatings on the Al sub- strate immersed in a 3.5 wt% NaCl corrosive solution for 3 h decreased by 2 orders of magnitude compared to that of the bare Al substrate. In a typical polarization curve, a lower corrosion current density usually indicates a lower corrosion rate and a better corrosion resistance [ 30 , 31 ]. The superhydrophobic coatings on the Al substrate provide very effective protection of Al from corrosion. The surface composed of some particles agglomerate and interval was appropriate for Cassie’s state with a mass of trapped air [ 32 , 33 ]. Studies have shown that the air was the most important element for corrosion resistance. Thus, we could conclude from analysis of Fig. 3 that the superhydrophobic coatings are effective for improving the corrosion resis- tance of the Al surface. EIS is an effective and revealing method for the corro- sion characterization of coated metals. In our study, the EIS measurements were performed under open-circuit potential in 3.5 wt% NaCl corrosive solutions under a working fre- quency range from 10 mHz to 100 kHz using an ac per- turbation of 5 mV. Figure 4 presents the evolution of the impedance spectra of the superhydrophobic silica coatings formed on an Al surface after immersion in a 3.5 wt% NaCl solution for 3 h and bare Al for 2 h. Figure 4 a shows that the Nyquist plot of bare Al is composed of a capacitive loop at high frequency range and a straight line in low frequency range. As shown in Fig. 4 a, the superhy- drophobic coatings have bigger loops than the bare Al substrate. The diameter size of capacitive loops represents the stand or fall of anticorrosion. The capacitive loops are from the trapped air, and the line presents a diffusion process [ 34 , 35 ]. The conclusion which is consistent with the result forms the Bode plot of impedance modulus | Z | as a function of frequency in Fig. 4 b. The impedance modulus Fig. 2
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  • Fall '19
  • Surface tension, Contact angle, superhydrophobic surface, superhydrophobic coatings

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