**2. Experimental**

electrochemical parameters such as coating capacitance, pore resistance, double layer capacitance, charge transfer resistance, water uptake, diffusivity, among others. Additional methods like salt spray test and immersion techniques are used according to different norms for the qualitative and quantitative evaluation of corrosion zones, pitting, and for the determination of corrosion rates. To evaluate the electrochemical performance of the protective system for a given corrosive environment and coating thickness, the most important criteria are (i) the magnitude of the initial impedance modulus obtained by EIS at low frequency, defined as corrosion resistance; (ii) the values of the open circuit potential, obtained by chronopotentiometry; and (iii) the time evolution of both parameters, to evaluate the long-term stability of the coatings. For industrial application, another important aspects have to be considered such as the simplicity of the synthesis process, low costs of reagents, and their environmental

One example of an efficient corrosion protection of mild steel was recently reported for a hybrid system combining an epoxy-siloxane topcoat with an epoxy primer containing micaceous iron oxide and zinc phosphate pigments [7]. The electrochemical measurements

1 year in contact with 3% NaCl solution. The authors attribute the excellent protection to the high resistance of the coating against water uptake provided by suitable epoxy/primer combination and the relatively high thickness (~140 μm) of this coating system. In another recent study, Ammar et al. [8] report on high-performance hybrid coatings based on

a thickness of 75 μm by brush coating. EIS measurements confirmed the high-corrosion

decade after 90 days of immersion in 3.5% NaCl solution. Visuet et al. [9] obtained similar

stand 263 days, in 3.5% NaCl solution, with almost unaltered corrosion resistance of about

The above results demonstrate that elevated anticorrosive performance is usually achieved for sophisticated barrier coatings with an average thickness in the order of dozens to hundreds micrometers. For the market, however, which aims on economic and efficient solutions, elevated thickness, and complexity of the coating system, implies elevated material costs and weight increase, issues that are hardly to be accepted, especially by the aerospace industry. In this regard, dos Santos and coauthors [10] have successfully prepared highly efficient PMMAsilica coatings having a thickness of only ~2 μm, which were able to withstand aggressive

SO4

The excellent performance of the primer free coating was explained by the high connectivity of reticulated sub-nanometric silica domains densely interconnected by short PMMA chain segments. Another results that confirmed the viability of thin hybrid films as efficient corrosion barrier have been reported in the study of Harb et al. [11]. The authors showed that the addition of cerium (IV) salt into PMMA-silica system results in a further improvement

surfaces, thus enhancing the barrier property of the coating against electrolyte uptake.

105 and 196 days, respectively, maintaining the corrosion resistance in the GΩ cm<sup>2</sup>

protection efficiency with an impedance modulus of more than 10 GΩ cm<sup>2</sup>

results for polyurethane/polysiloxane hybrid coatings containing TiO2

, remaining stable for more than

, decreasing one

range.

as pigment. The EIS

nanoparticles, applied to mild steel with

pigment works as a charge (ionic) storage

) and 3.5% NaCl environments for up to

(75 μm thick) were able to with-

showed a high-impedance modulus of up to 100 GΩ cm<sup>2</sup>

analysis showed that coatings loaded with 10-wt% TiO2

. Their model proposes that the TiO2

acrylic-silica polymeric matrix reinforced by SiO2

saline/acid (0.05 mol L−1 NaCl + 0.05 mol L−1 H2

compatibility.

22 New Technologies in Protective Coatings

100 GΩ cm<sup>2</sup>

### **2.1. Epoxy-silica and PMMA-silica hybrid synthesis**

All reagents used to epoxy-silica and PMMA-silica hybrids synthesis were purchased from Sigma-Aldrich and used as received, apart from the methyl methacrylate (MMA) monomer, which had been distilled before use to remove the polymerization inhibitor. The molecular structures of the epoxy-silica and PMMA-silica hybrid precursors are presented in **Figures 1** and **2**, respectively, and the synthesis procedures are summarized in **Figure 3**.

**Figure 1.** Molecular structures of the epoxy-silica hybrid precursors.

**Figure 2.** Molecular structures of the PMMA-silica hybrid precursors.

Epoxy-silica hybrids were prepared from the curing reaction of poly(bisphenol A-coepichlorohydrin), glycidyl end-capped (DGEBA, Mn = 377 g/mol) with diethyltriamine (DETA) as hardener, and (3-glycidoxypropyl)methyltriethoxysilane (GPTMS), as coupling agent between the organic and inorganic phase, combined with the sol-gel hydrolysis and condensation reactions of tetraethoxysilane (TEOS) and GPTMS. In the first step, DGEBA and GPTMS were mixed with DETA in tetrahydrofuran (THF) solvent during 4 h at 70°C and 25 min at 25°C, under constant stirring in a reflux flask. In the next step, TEOS, ethanol, and acidified water (pH 1 using nitric acid) were added to the reflux system at room temperature and stirred for an additional 1 h. At this stage, the sol-gel reactions take place, as shown below, where the alkoxide precursors (TEOS and GPTMS) are hydrolyzed, forming Si–OH groups, Eq. (1), which subsequently condense with an initial alkoxide molecule, Eq. (2), or another Si–OH group, Eq. (3), yielding Si–O–Si bond and eliminating alcohol or water, respectively.

**Figure 3.** Synthesis procedures used to prepare epoxy-silica and PMMA-silica hybrids.

**Figure 2.** Molecular structures of the PMMA-silica hybrid precursors.

**Figure 1.** Molecular structures of the epoxy-silica hybrid precursors.

24 New Technologies in Protective Coatings

Epoxy-silica hybrids were prepared from the curing reaction of poly(bisphenol A-coepichlorohydrin), glycidyl end-capped (DGEBA, Mn = 377 g/mol) with diethyltriamine (DETA) as hardener, and (3-glycidoxypropyl)methyltriethoxysilane (GPTMS), as coupling agent between the organic and inorganic phase, combined with the sol-gel hydrolysis and condensation reactions of tetraethoxysilane (TEOS) and GPTMS. In the first step, DGEBA and GPTMS were mixed with DETA in tetrahydrofuran (THF) solvent during 4 h at 70°C and 25 min at 25°C, under constant stirring in a reflux flask. In the next step, TEOS, ethanol, and acidified water (pH 1 using nitric acid) were added to the reflux system at room temperature and stirred for an additional 1 h. At this stage, the sol-gel reactions take place, as shown below, where the alkoxide precursors (TEOS and GPTMS) are hydrolyzed, forming Si–OH groups, Eq. (1), which subsequently condense with an initial alkoxide molecule, Eq. (2), or another Si–OH group, Eq. (3), yielding Si–O–Si bond and eliminating alcohol or water, respectively. The homogeneous and transparent sols were used for the film deposition by dip-coating onto A1020 carbon steel.

$$\text{\#Si-OR} + \text{H}\_2\text{O} \not\equiv \text{\#Si-OH} + \text{ROH} \tag{1}$$

$$\text{\#Si-OR} + \text{HO-Si} \rightleftharpoons \text{\#i-O-Si} + \text{ROH} \tag{2}$$

$$\text{\#Si-OH} + \text{HO-Si\#} \rightleftharpoons \text{\#Si-O-Si\#} + \text{H}\_2\text{O} \tag{3}$$

Two series of epoxy-silica hybrids were prepared, varying the amount of GPTMS or TEOS and keeping the molar concentrations of other compounds constant (**Figure 4**). In order to ensure a fully cured thermosetting, DETA was added in a proportion that resulted in one oxirane group for each hydrogen atom of the amine groups.

PMMA-silica hybrids have been prepared by the radical polymerization of methyl methacrylate (MMA) and 3-(trimethoxysilyl)propyl methacrylate (MPTS, also known as TMSM) using benzoyl peroxide (BPO), as thermal initiator of the polymerization, and tetrahydrofuran (THF) as solvent. The sol-gel route has been used to perform the hydrolytic condensation of tetraethoxysilane (TEOS) and MPTS, using ethanol and acidified water (pH 1 using nitric acid), during 1 h at room temperature. In the presence of acidified water, alkoxide precursors (TEOS and MPTS) are hydrolyzed and subsequently condensed to form Si–O–Si bonds. After mixing the organic and inorganic precursor, the obtained transparent and homogeneous sols were used to deposit few micrometer thick films onto A1020 carbon steel or AA2024 aluminum alloy substrates.

**Figure 4.** Epoxy-silica hybrids sample names and compositions.

To investigate the relation between structure and barrier properties, the hybrids films were prepared at different synthesis conditions (**Table 1**), varying the ratio between the organic to inorganic phase (MMA/TEOS), the temperature (80–100°C) and time (2–4 h) of the organic precursor reaction, as well as the BPO/MMA molar ratio (0.01–0.1). The molar ratios of H2 O/ Si = 3.5 and ethanol/H2 O = 0.5 were kept constant. Ce(IV) salt (ammonium cerium nitrate), lignin, carbon nanotube (CNT), and graphene oxide (GO) were added separately as modifier to the inorganic precursor of the PMMA-silica hybrid.

Carbon steel 1020 (25 mm × 20 mm × 5 mm), a ferrous alloy with low carbon content, and 2024 aluminum alloy (20 mm × 20 mm × 1 mm) have been used as substrates. Low-carbon steels are produced in large quantities at relatively low costs and widely used in automobilist, construction, oil industries, etc [21]. Although the use of ferrous alloys is economically viable due to the low cost and versatility, corrosion is the great obstacle when it comes to the durability of these materials that undergo severe corrosion in contact with humid environments, low amounts of chloride ions and acid solutions in general. The 2000 and 7000 series of aluminum alloys, containing roughly 4.3–4.5% copper, 0.5–0.6% manganese, 1.3–1.5% magnesium, are widely used in the aerospace industry due to their improved mechanical properties; however, they are susceptible to enhanced corrosion especially at the grain boundaries. Prior to deposition, all substrates had been sanded with 100, 300, 600, and 1500 grit emery paper, washed with isopropanol for 10 min in an ultrasound bath and dried under a nitrogen stream. The deposition of the hybrids coatings was performed by dip-coating (Microchemistry—MQCTL2000MP) at a rate of 14 cm min−1, with 1 min of immersion and air-drying during 10 min at room temperature. This procedure was performed three times for each sample. The coated substrates and Organic-Inorganic Hybrid Coatings for Corrosion Protection of Metallic Surfaces http://dx.doi.org/10.5772/67909 27


**Table 1.** PMMA-silica hybrid preparation conditions.

the remaining solution, placed in Teflon holders, were cured for 24 h at 60°C and then 3 h at 160°C to ensure the liberation of all volatile species and the densification of the hybrid matrix.

#### **2.2. Characterization techniques**

To investigate the relation between structure and barrier properties, the hybrids films were prepared at different synthesis conditions (**Table 1**), varying the ratio between the organic to inorganic phase (MMA/TEOS), the temperature (80–100°C) and time (2–4 h) of the organic precursor reaction, as well as the BPO/MMA molar ratio (0.01–0.1). The molar ratios of H2

lignin, carbon nanotube (CNT), and graphene oxide (GO) were added separately as modifier

Carbon steel 1020 (25 mm × 20 mm × 5 mm), a ferrous alloy with low carbon content, and 2024 aluminum alloy (20 mm × 20 mm × 1 mm) have been used as substrates. Low-carbon steels are produced in large quantities at relatively low costs and widely used in automobilist, construction, oil industries, etc [21]. Although the use of ferrous alloys is economically viable due to the low cost and versatility, corrosion is the great obstacle when it comes to the durability of these materials that undergo severe corrosion in contact with humid environments, low amounts of chloride ions and acid solutions in general. The 2000 and 7000 series of aluminum alloys, containing roughly 4.3–4.5% copper, 0.5–0.6% manganese, 1.3–1.5% magnesium, are widely used in the aerospace industry due to their improved mechanical properties; however, they are susceptible to enhanced corrosion especially at the grain boundaries. Prior to deposition, all substrates had been sanded with 100, 300, 600, and 1500 grit emery paper, washed with isopropanol for 10 min in an ultrasound bath and dried under a nitrogen stream. The deposition of the hybrids coatings was performed by dip-coating (Microchemistry—MQCTL2000MP) at a rate of 14 cm min−1, with 1 min of immersion and air-drying during 10 min at room temperature. This procedure was performed three times for each sample. The coated substrates and

O = 0.5 were kept constant. Ce(IV) salt (ammonium cerium nitrate),

Si = 3.5 and ethanol/H2

26 New Technologies in Protective Coatings

to the inorganic precursor of the PMMA-silica hybrid.

**Figure 4.** Epoxy-silica hybrids sample names and compositions.

O/

Structural and morphological characteristics have been investigated using nuclear magnetic resonance spectroscopy (NMR), X-ray photoelectron spectroscopy (XPS), small angle X-ray scattering (SAXS), atomic force microscopy (AFM) and thermogravimetric analysis (TGA). The anticorrosive properties of coated samples were evaluated by exposure of the coated samples to standard 3.5% saline and saline/acid solutions, using electrochemical impedance spectroscopy (EIS).

The thickness of the coatings was determined using a Filmetrics F3-CS optical interference system. An Agilent Technologies Model 5500 atomic force microscope was used to obtain AFM topography images, in tapping mode, with 1 × 1 μm, of the hybrid coatings deposited on the metallic substrates. 29Si nuclear magnetic resonance spectroscopy (29Si-NMR) measurements of the hybrid powders were performed in a 300-MHz Varian Inova spectrometer, using a Larmor frequency of 59.59 Hz and tetramethyl silane (TMS) as an external standard. The CasaXPS processing software was used for spectral deconvolution using Gauss profiles. XPS was carried out in a UNI-SPECS UHV surface analysis system, using the Mg Kα radiation (hν = 1253.6 eV) and pass energy of 10 eV to record the high-resolution spectra. The near surface composition was determined from relative peak intensities of carbon (C 1s), oxygen (O 1s) and silicon (Si 2p) corrected by Scofield's atomic sensitivity factor of the corresponding elements. To study the oxidation state of Ce (Ce 3d) and the local bonding structure of carbon (C 1s), oxygen (O 1s), and silicon (Si 2p) of the coatings, the spectra were deconvoluted applying Voigt profiles and Shirley's background subtraction using the CasaXPS software. SAXS experiments were carried out at the SAXS-1 beamline in the National Synchrotron Light Laboratory (LNLS, Campinas, Brazil) to determine the nanostructural characteristics of the hybrids. The scattering intensity I(q) was recorded as a function of the modulus of the scattering wave vector q = (4π/λ) sin θ, θ being half of the scattering angle. The SAXS beamline uses a monochromatic X-ray beam (λ = 1.548 Å) and a 2D detector, Dectris Pilatus 300k, positioned 0.9 m away from the sample holder. Thermogravimetric analysis of unsupported hybrids films was performed in a TA Instruments STD Q600 analyzer, under a nitrogen flow of 100 mL min−1.

The anticorrosive performance of hybrid coatings, deposited on A1020 carbon steel or Al2024 aluminum alloy, was investigated by electrochemical impedance spectroscopy (EIS) with a Gamry Potentiostat Reference 600, using 10 points per decade and RMS amplitude of 10 mV in a frequency range of 50 mHz–100 kHz. The electrochemical cell consisted of an Ag|AgCl|KClsat reference electrode, a platinum mesh counter electrode, a platinum electrode connected to the reference electrode through a 0.1-μF capacitor and the working electrode of either coated or uncoated metal substrate. The measurements were performed once a week, in saline (3.5% NaCl) or saline/acid solutions (0.05-mol L−1 H2 SO4 + 0.05-mol L−1 NaCl), until a significant drop of the impedance modulus was observed, indicating the failure of the coating.
