Tetraethylorthosilicate (TEOS, >96%), 3-Glycidoxypropyltrimethoxysilane (GPTMS, >97%), hydrochloric acid (HCl, 37%), ethanol (>98%), tetra (n-butyl orthotitanate, >98%) (TBT), acetylacetone (AcAc, >98%), Isopropanol (>99%) and nitric acid (HNO₃, 65%) were purchased from Merck and used as received. ¹H, ¹H,²H,²H-Perfluorooctyltriethoxysilane (PFOTES, >98%) and itaconic acid catalyst were obtained from Sigma-Aldrich. Polyol resin (3186) was purchased from Gohar Resin Company, and Isocyanates Aliphatic (N75) hardener was obtained from Desmodur Company.

The hybrid sol was prepared by admixing silica precursors (GPTMS, TEOS. GPTMS, and TEOS) which were mixed (3:1 molar ratio) with 12.3 cc of ethanol in a beaker while being stirred using a magnetic stirrer at 200 rpm for 1 hour at ambient temperature. The resultant two-phase solution was then vigorously stirred at 240 rpm for 4 h. With its pH adjusted at 2, water served as the acidic catalyst. Hydrolysis and condensation of the silane were conducted at stoichiometric and 50% sub-stoichiometric water pH = 2, ethanol ratio in the presence of an acid catalyst. Once finished with hydrolysis and condensation, the sol was stirred at room temperature for 24 hours [16] [17].

TBT and Isopropanol (1:23 molar ratio) were mixed and stirred by a magnetic stirrer (200 rpm) at room temperature for 2 h. Then, 0.31 cc of AcAc was added into the mixture to control the rate of the hydrolysis reaction. The mixture was subsequently stirred at the same rate for 1 h to obtain a transparent solution. Next, 2.16 CC of distilled water (pH = 2, from HNO₃) was added to the above solution in a drop-wise fashion, and the resulting solution was stirred at ambient temperature for 4 h [16] [17].

To prepare nanocomposite coatings for different tests, first, iron substrates were cleaned as follows:

The iron sheets were cleaned with water and soap and washed by acetone. Then, they were washed with 5% w/w NaOH solution at 60˚C for 3 times and then washed with distilled water. Next, the above substrates were washed 3 times with 5% w/w HCl solution and then washed with distilled water. Lastly, the substrates were washed with ethanol and dried at room temperate. Cleaned aluminum iron substrates were dip-coated in the as-prepared mixture and dried at ambient temperature for 24 hours and post cured at 130˚C for 90 minutes [16] [17].

Polyurethane (PU) resin has been employed as polymeric matrix and N75 hardener as a curing agent to obtain low energy surface coating. TiO₂, SiO₂, PDMS and PFOTES nanoparticles with different proportion has been added to PU matrix in each sample (Table 1) to a 50 ml beaker, at room temperature for 15 min stirred with speed of 200 rpm, then diluted with diluent (xylene or toluene) and stirring continued with same speed for 5 min. The data of Table 1 are selected based on previous works in this area [7] [8] [9] [10] [11]. Hardener, silane, and TEOS were added to the obtained solution, using an Ultrasonic probe were placed in a vessel with the rated output power of 100 W and sonicated for 2 min.

Infrared spectra were recorded using objective FTIR on an Equinox 55 spectrometer (Bruker, Germany) in the range of 400 and 4000 cm⁻¹. A field emission scanning electron microscope (FE-SEM) was used to investigate the surface morphology and microstructure of the nanocomposites. The FE-SEM instrument was a Tescan Mira3 (Czech Republic) with 15.0 kV accelerating voltage in

the secondary electron imaging mode. Mapping of the distribution of Si, Ti and F constituting the specimen was obtained through EDAX (INCA, Oxford Instruments, England) in coated aluminum substrates. Contact angle and tilt angle measurements were performed with an optical contact angle meter (OCA40 Micro, Dataphysics, Germany). The wettability of the film was determined by measuring the contact angles of two probe liquids, water (g = 72.8 mN/m), and milk (g = 52.5 mN/m). The hydrophobicity and the oleophilicity were measured respectively with 5 mL water and 5 mL milk. An average of five measurements taken at different positions on each sample was made to determine the hydrophobicity and oleophilicity of the film.

Electrochemical measurements were performed under extreme environmental conditions, consisting of aqueous, air-exposed sodium chloride (5% NaCl) solution. Each sample was sealed with waterproof tape in order to prevent premature corrosion along the edges of the substrate. A 1 cm × 1 cm area within the center of each sample was exposed to the solution during testing. Corrosion analysis of bare and coated substrates was done using an AutolabPGSTAT30 potentiostat system connected to a corrosion analysis software program. Polarization measurements were carried out potentiostatically at room temperature using an Ag/AgCl/Cl⁻ (0.222 V) reference electrode and a platinum counter electrode. The potentiodynamic measurements were taken within the range of −2000 to 2000 mV versus Ag/AgCl/Clat a rate of 5 mV/s. Before the measurements, in order to reach steady potential, the electrodes were kept in the working solutions for at least 30 min [17].

Figure 3 and Figure 4 show morphological features of SiO₂ and TiO₂ hybrid nanocomposite coatings as recorded via field and a point EDAX technique. These figures indicate the uniform spherical distribution of nanoparticles within

hybrid coatings with no aggregation observed, and both titania and silica particles are spherical [16] [17]. In Figure 3 EDAX images mapping which shows a map of Ti elements distribution in TiO₂ verifies well distribution of titania particles in the coating. Pointed EDAX of a spherical particle, confirmed that spherical particles are synthesized TiO₂. Mohseni-Salehi et al. have somehow reported the same results (The presence of well-distributed particles) about TiO₂ nanoparticle [21] [22]. It can be seen spherical particles of silica in Figure 4. It is shown the presence of Si elements similar before the discussion.

FESEM of SiO₂ and TiO₂ hybrid coating showed uniform, crack-free, and smooth morphology with a uniform dispersion of nanoparticles throughout the surface of the hybrid coating without any aggregation (Figure 5). No cracks and fracture are observed on the coating surface. Figure 5(a) shows a small diameter size of TiO₂ particles for example from 19.45 to 15.36 nm in comparison with SiO₂ diameter size Figure 5(b) from 7.85 nm to 15.36.

In general, materials with a contact angle greater than 90˚ can be called a hydrophobic material. They have two characteristics; one was low surface energy, and the other was a rough structure. Because PFOTES contained a large number of hydrophobic groups, it had low surface energy and a specific self-cleaning effect (Figure 6).

When hydrophilic TiO₂ was added to the PFOTES, the coatings with micro-nano rough structure had a more significant amount of air, which blocks the entry of water, influencing the CA of the surface. On the other hand, the hydrophilic properties of the TiO₂ coating would decrease the CA. When the content of TiO₂ was low, the CA of the coating was decreased from 102 to 98. The less content of TiO₂ on the surface, the less number of micro-nano structures. Besides, the addition of TiO₂ would increase the roughness of the composite coating, thereby increasing the CA. When the content of TiO₂ was high (8 and 10 wt%), it formed more micro-nano structures which changed the CA of the coating. The rough structure of the coating surface played a major role, and the CA of the coating was increased from 98˚ to 108˚. If the content of TiO₂ was too high, the uniformity of TiO₂ in the coating would be destroyed, causing a decreased CA of composite coatings [8] [9].

Finally, the corrosion behavior of the sol-gel hybrid coatings was also analyzed

using electrochemical impedance spectroscopy (EIS) in 5% NaCl aqueous solution. EIS measurements were done to investigate the anti-corrosion performance of the synthesized coatings. As complementary experiments, potentiodynamic polarization curves were plotted (Figure 7 and Figure 8) under extreme environmental conditions developed by aqueous, air-exposed sodium chloride (5% NaCl) solution. The values of each component of the equivalent circuit were similarly obtained when the specimen was immersed in a salt solution for different time durations, and the Rhealing of the PFOTES-based self-healing coating as a function of immersion time was plotted in Figure 5(b). It was seen that generally, the Rhealing of the self-healing coating exhibited an increasing trend. The healing increased from about 1.0 × 103 to 2.2 × 103 Ω when the immersion time increased from 2 to 24 h [16] [20] [23] [24] [25] [26].

The potentiodynamic polarization curves in Figure 8 show that the destructive behavior of the alloys with a coating containing nanoparticles (SiO₂/TiO₂, PFOTES/TiO₂, PFOTES/SiO₂, and SiO₂/TiO₂/PFOTES) is very different to the

observed for the alloy without protective coating (identified as “bare” in Figure 7 and Figure 8). The protected alloys display very resistive behaviors with shallow current density values (in the range from 10 – 14 to 5 × 10 – 11 A cm^–2), which are typical of non-conductive materials, like the ceramic coatings. Note that the polarization curves of the alloys with a coating containing nanoparticles were carried out in an extensive range of potentials, between –1.4 and 0.6 V versus SCE (2.5 V), which is higher than the commonly used range as observed for the bare alloy. A decrease in corrosion current density is observed in the samples coated with SiO₂/TiO₂, SiO₂/PFOTES, TiO₂/PFOTES, and PFOTES/SiO₂/TiO₂ coatings; this is important when it comes to corrosion protection for the alloy. Addition of TiO₂, SiO₂ and PFOTES nanoparticles to the hybrid coatings was seen to increase the hardness of coatings and improve their corrosion resistance. This effect is a result of a particular size and extreme aspect ratio of these nanoparticles which reduced the space between the particles and coating to far less than that of larger particles. It may also be attributed to good dispersion of nanoparticles which enhances the density of the coating films containing nanoparticles, thereby increasing the transport paths for the corrosive electrolyte to pass through the coating and reduce the extent of corrosion process consequently [16] [20] [23] [24] [25] [26].

For SiO₂/PFOTES and TiO₂/PFOTES coatings, samples loaded with TiO₂NPs show a higher impedance value in comparison with the samples without TiO₂NPs (bare sample and SiO₂/PFOTES coating). The presence of these TiO₂ NPs has a beneficial impact on the sol-gel hybrid coating resistance, since the impedance values at low frequency are increased, and as a result, an enhancement of the barrier properties. PFOTES containing coatings showed a good anticorrosive performance. The experimental results indicate a direct relationship between both the hydrophobic property and corrosion resistance (Figure 7). The highest hydrophobicity is related to PFOTES /SiO₂/TiO₂ coating (the highest WCA value) in comparison with the sol-gel coated samples without PFOTES precursor. Presence of a hydrophobic layer in the outer surface acts as a physical diffusion barrier due to the entrapment of air between the roughness protrusions, so the coated sample shows a good anti-corrosion property. EIS experimental results clearly demonstrate that the combination of both metal oxide nanoparticles (TiO₂ NPs) and the use of PFOTES precursor in the outer surface for increasing the hydrophobicity allows the design of novel coatings, which can considerably reduce the corrosion susceptibility, while showing very good mechanical properties due to the presence of these metal oxide nanoparticles inside the sol-gel hybrid coatings. However, the impedance values were lower for the bare sample, indicating that the coating was beginning to delaminate, which is more likely to happen within the scribed area as a result of corrosion attack at the coating/substrate interface (Figure 8).

Table 2 showed corrosion rates of uncoated samples and samples coated with nanoparticles which indicated that‏ coating containing PFOTES and TiO₂ provides

better protection than SiO₂. The impedance values for the‏ bare sample rapidly decrease indicating that‏ coating is starting to delaminate, which is more likely to happen within the scribed area as a result of corrosion‏ attack to the coating/substrate interface [10] [11] [12].