3.1. Surface Morphology of the Coatings
The representative SEM micrographs of the electrospun PS with different concentrations of Al
2O
3 nanoparticles are shown in
Figure 1 and
Figure 2, respectively. Different morphologies of PS in absence and presence of Al
2O
3 nanoparticles including beaded fibers and free beads are obtained by varying the concentration of PS and Al
2O
3 and/or the operational parameters of the electrospinning process.
Table 2 shows the average nanofiber diameter of the different prepared coatings before and after the addition of Al
2O
3 nanoparticles.
The morphologies of the pure PS polymer shown in
Figure 1 reveal the formation of a beaded-fiber structure. At low applied voltages, the bead shape is spindle-like,
Figure 1a,b. However, increasing the applied voltage and/or the PS concentration leads to a decrease in the number of beads and changes them to a spherical-like shape,
Figure 1c,d, due to the increase of the jet stretching. The bead formation can be demonstrated by the insufficient swift stretching through the flagellation of the jet [
42]. This observation is inconsistent with the previous work [
34]. Moreover, at low PS concentration, the elasticity is too small to supply enough resistance to tolerate the stretching, which results from the electrostatic force, leading to the formation of a large number of beads. However, by increasing the PS concentration from 2.5 to 5 wt %, the ultrafine PS fibers are formed with a small number of beads per unit area, demonstrating that the elastic properties of PS solution are more significant at high concentrations, as shown in
Figure 1d.
The SEM micrographs shown in
Figure 2 display the effect of adding Al
2O
3 nanoparticles on the surface morphology of an electrospun PS polymer. The SEM micrographs show the formation of different sizes of beaded fibers without a significant change in the surface morphology.
Table 2 shows a significant variation in the fiber diameter as the concentrations of PS polymer and Al
2O
3 nanoparticles, as well as the electrospinning parameters, change. It can be observed that the increase in the applied voltage from 20 to 25 kV, while other parameters are kept constant, leads to a decrease in the average nanofiber diameters of PS1 and PS3 to 340 ± 27 and 320 ± 21 nm, respectively. Increasing the applied voltage results in a large elongation of the solution owing to the greater columbic forces in the jet, in addition to the high electric field, which leads to the formation of thinner fibers [
43,
44]. However, as the concentration of PS is increased from 2.5 g (PS3) to 5 g (PS4), the average nanofiber diameter is increased from 320 ± 21 to 433 ± 15 nm, respectively.
The higher the quantity of polymer chains in the solution, the easier the capability of the electrospinning jet to elongate the solution, while the solvent molecules are dispersed between the polymer chains [
21,
34,
45]. In addition, with an increase of the wt % of PS, the increased viscoelastic force can hinder the jet segment from being stretched by the constant columbic force, yielding fibers with large diameters [
34]. On the other hand, it was found that the fiber diameter of PS2 and PS3 diminished to 328 ± 29 nm and 320 ± 21 nm, respectively, by reducing the flow rate from 2 to 1.5 mL·h
−1. By increasing the flow rate, the quantity of polymer flowing through the tip of the needle increases, which in turn produces thick fibers. Moreover, at very high flow rates, the polymeric jet was unsettled and tended to electrospray owing to the influence of the gravitational force.
Table 2 shows the formation of fibers with different diameters after the addition of Al
2O
3 nanoparticles to the solutions with different PS concentration. It worth mentioning that addition of Al
2O
3 nanoparticles leads to a significant decrease in the diameter of the fibers in comparison with the pure PS. However, further addition of alumina nanoparticles decreases the fiber diameter from 320 ± 21 of PS3 to 290 ± 34 nm of PA3. These results are inconsistent with a previous report [
46].
Figure 3a,b shows the SEM/EDX mapping of PA1 and PA3 SHCs, which verifies the presence and the uniform dispersion of the alumina nanoparticles on the beaded fiber structure.
3.2. Wettability
The effect of changing the compositional ratio between PS and Al
2O
3 and the different electrospinning parameters on the wettability of the PS/Al
2O
3 nanocomposite is investigated.
Table 3 reveals the measured values of WCA and SA for both PS and PS/Al
2O
3 nanocomposite coatings using the sessile droplet method at ambient temperature.
Figure 4 shows snapshots for the measured WCA after and before the addition of Al
2O
3 nanoparticles. The WCA of the uncoated Al substrate was around 87° ± 2.2°. However, WCAs values of the PS coatings vary with the processing parameters of electrospinning, due to the variation of the fibers diameters. The highest WCA and lowest CAH of the PS–coated materials is 147° ± 1° and 14° ± 3°, respectively, for PS3, which is formed at a flow rate of 1.5 mL·h
−1 and an applied voltage of 25 kV. Nevertheless, the addition of Al
2O
3 nanoparticles to PS polymer improved the hydrophobicity of the nanocomposite coating. The highest WCA and lowest CAH average values are 155.2° ± 1.9° and 3° ± 4.2°, respectively, for PA3.
The static contact angle is measured according to the Wenzel and Cassie–Baxter models. The hypothesis developed by Wenzel is described in Equation (1) [
47,
48].
in which
r is the ratio between the true surface area and the apparent one (for a rough surface
r > 1 and =1 for a smooth one).
is the apparent macroscopic WCA and
is the intrinsic contact angle for the droplet on a corresponding flat surface obtained by the Young’s equation (as defined for an ideal surface). On the other hand, Cassie-Baxter regime suggested that the water droplet is suspended on the rough surfaces and is not penetrated into the channels among the rough surface, resulting in heterogeneous structures consisting of air and solids. Therefore, the contact angle in terms of the Cassie-Baxter hypothesis is given in Equation (2) [
49]:
in which
and
represent the contact angle on the rough nanocomposite coating (PS/Al
2O
3) and the smooth PS surfaces, respectively.
and
are the area fractions of the solid and air on the surface in which
. Given that
is 151° ± 2.8°, 152° ± 1.2° and 155° ± 1.9° for PA1, PA2, and PA3 nanocomposite coatings, respectively, and
for the PS before addition of Al
2O
3 equals 141° ± 2°, 143° ± 3°, and 147° ± 1° for PS1, PS2, and PS3, respectively, then the estimated values of
f2 are 0.43, 0.42, and 0.47, respectively. These calculations indicate that air occupies about 43%, 42%, and 47% of the contact regions area for the as-prepared PA1, PA2, and PA3 nanocomposite coating, respectively. This confirms that the reason behind the superhydrophobicity of the PS/Al
2O
3 composite coating is mainly attributed to the multiscale (hierarchical) roughness of the surface, which is induced by the micro-/nano-sized fibers or beads and the nanoparticles incorporated in the polymer solution.
CAH is an important parameter to be measured besides the WCA, as it gives information about the stickiness of the surface. The high WCA of the coating does not necessarily indicate a low CAH, due to the effect of chemical heterogeneity [
50]. The CAHs of PS coatings before the addition of Al
2O
3 nanoparticles are 24° ± 3°, 20° ± 4°, 14° ± 3°, and 32° ± 6° for PS1, PS2, PS3, and PS4 respectively, as shown in
Table 3. The increase of the nanofiber diameter at high PS concentration showed a low WCA and a high CAH for PS4. After the addition of Al
2O
3 nanoparticles, the CAH decreased to 7.1 ± 2.3, 5 ± 3.1, and 3 ± 4.2 for PA1, PA2, and PA3, respectively. These results reveal a low adhesion force between the prepared PS/Al
2O
3 nanocomposite surface and water droplets, which allows water droplets to slide easily from the substrate’s surface.
In addition to the thickness of the fiber and the electrospinning parameters, the size of the nanoparticles may have an effect on the wettability. According to Conti et al. [
51], a combination of silica nanoparticles of two different sizes (70 and 100 nm) increased the WCA to 147° and lowered the WCAH to 10°. However, when the size of SiO
2 nanoparticles was increased to 150 nm, the WCA was decreased drastically to 124°. On the other hand, Karapanagiotis et al. [
52] used Al
2O
3 nanoparticles with different sizes (25, 35, and 150 nm) in their coatings, and they found that the coating wettability is independent of the size of the nanoparticles. Then, in a more recent study, they [
53] found an effect for the size of the nanoparticles on the wettability of their coatings, only if the concentration of the nanoparticle is high.
Figure 5 shows the FTIR spectra of pure PS and PS/Al
2O
3 nanocomposite coatings at different electrospinning parameters and concentrations of PS and Al
2O
3. The spectra for PS before and after the addition of Al
2O
3 with different electrospinning parameters are very similar. No remarkable changes are observed before and after addition of Al
2O
3. This is probably because of the small concentration of the Al
2O
3 used compared with the PS amount. The FTIR of Al
2O
3,
Figure 5a, shows a transmission band located at about 3400 cm
−1 that is assigned to hydrogen–bonded –O–H of the adsorbed water, while the band at 400–1000 cm
−1 can be attributed to the bending stretching band of Al–O [
54]. PS/Al
2O
3 nanocomposite and PS show the same spectra between the 700–3100 cm
−1. Bands of the IR spectrum for PS correspond to their functional groups. The observed peaks at 3000–3100 cm
−1 are attributed to the aromatic C–H stretching vibration and the C–H group on the PS side chain. However, the absorbance bands at 2922 and 2854 cm
−1 correspond to the C–H stretching vibration of the –CH
2 and –CH groups on the main PS chain. The C–C aromatic stretch and the vibration of the aromatic ring are noticed at 1494 and 1091 cm
−1, respectively. The strong peaks located at 700 and 760 cm
−1 are ascribed to –CH
2 rocking mode and C–H out-of-plane bend, respectively. The inorganic segment of the Al–O–Al band can be noticed at 400–800 cm
−1, and its absorbance intensity increases with increasing the Al
2O
3 content [
55]. However, it is noticed that there no new bands appear after adding Al
2O
3 nanoparticles to the PS, which proves that (i) Al
2O
3 only interacted physically with PS [
56], and (ii) the electrospinning in this case does not affect the chemical structure of the polymer.
To better understand the effect of nanoparticle addition on PS, AFM is used to document the surface topography of both PS and PS/Al
2O
3 nanocomposite coatings. AFM images in
Figure 6 and
Table 4 show the 3D height and the roughness values (
Ra), respectively. It is clear from
Table 4 that the
Ra of the prepared PS coatings before the addition of the Al
2O
3 nanoparticles (PS1, PS2, PS3, and PS4) are slightly close to each other. In the images a–d of
Figure 6, it can be noticed that the PS surface is relatively smooth and small bumps appeared on the surface, which may happen due to the beaded fibers structure. However, after the addition of Al
2O
3 nanoparticles, the
Ra values significantly increase as shown in
Table 4 and in Images e–h in
Figure 6.
The Ra values of PS1, PS2, PS3, and PS4 before the addition of the Al2O3 nanoparticles are 22 ± 2, 26 ± 4, 32 ± 3, and 17 ± 4 nm, respectively. However after the addition of the nanoparticles, the surface roughness values increases to 53 ± 3, 61 ± 4, 82 ± 2, and 27 ± 3 nm for PA1, PA2, PA3, and PA4, respectively. This explains why PA3 has the highest WCAs. The AFM measurements verified the effect of the composition ratio and the electrospinning operational parameters on the surface roughness of superhydrophobic nanocomposite coatings. Generally, Al2O3 and the combination of micro beads/nano fiber structure increase the roughness, which in turn increases the WCA in comparison with the pure PS polymer. It also can be noticed that increasing the concentration of PS or the flow rate decreases the surface roughness and consequently lowers the WCA. It is worth mentioning that lowering the operational potential from 25 to 20 kV also decreases the surface roughness and consequently lowers the WCA.
3.3. Anticorrosion Performance of the As-Prepared Nanocomposite Coatings
As a non-destructive technique, EIS is usually used to study the corrosion mechanism and the corrosion resistance of SHCs.
Figure 7 and
Figure 8 show the Nyquist and Bode plots of the measured EIS data, respectively. The dots are the measured data, while the solid lines are the fitted data using the electrical equivalent circuit shown in
Figure 9 that has two time constants.
Rpo and
Rct refer to the coating pore resistance and charge transfer resistance, respectively. On the other hand,
Cdl1 and
Cdl2 are the double layer capacitances of the constant phase elements representing the coating/solution and metal/solution interfaces, respectively. The high-frequency intercept |Z
100 kHz| refers to the solution resistance (
Rs), and the low-frequency intercept |Z
0.01 Hz| represents the sum of the solution and charge transfer resistances. Simply, the larger the diameter of the semicircle (charge transfer resistance), the lower the corrosion rate is.
Table 5 exhibits the fitted data that were attained from Gamry Echem Analyst software. The capacitances are substituted by constant phase elements (
Cdl1 and
Cdl2) to give a more precise fit to the experimental outcomes. The exponent (0 ≤
n ≤ 1) represents deviation from an ideal dielectric behavior; if
n = 0, it shows an ideal resistor, and if
n = 1, it acts as an ideal capacitor [
57,
58,
59,
60,
61,
62,
63,
64]. It is noticed that the
Rpo and
Rct significantly increase as the WCA increases. Increasing the WCA form 141° ± 2° to 147° ± 1° of the hydrophobic coatings has a significant influence on increasing the
Rct from 102 kΩ·cm
2 of PS1 to 140 kΩ·cm
2 of PS3. However, increasing the WCA to 151° ± 2.8° and 155° ± 1.9° and decreasing the CAH to 7 ± 2.3 and 3 ± 4.2 leads to a remarkable increase of the
Rct to three orders of magnitude (8 and 18 × 106 kΩ·cm
2) in comparison to the uncoated Al, respectively. On the other hand,
Cdl1 and
Cdl2 of the SHCs distinctly is decreased to very low values in comparison to the uncoated and PS-coated Al substrates (PS1 and PS3), which indicates a low permeation of the electrolyte to the SHC/metal surface and a polarization resistance. It is worth mentioning that the shape of the phase angle of the uncoated and the coated Al (see
Figure 8a,c), is not changed, which could be ascribed to the electrolyte permeation through the coating defects. The high corrosion resistance of the as prepared SHCs can be attributed to the trapped air in the disproportions of the heterogeneous surface, which prevents the aggressive ions from easily attacking the underlying surface of Al alloy. Moreover, the hydrophobic PS is non-polar and very good electrical insulator; therefore, aggressive ions can hardly transport through such coatings and reach the metal surface, thereby showing a superior corrosion resistance via physical modification of its surface.