**3. Results and discussion**

A number of interesting results have been obtained for novel epoxy-silica and PMMA-silica hybrid coatings, concerning their nanostructural properties, modified by the variation of synthesis conditions or by addition of nanofillers, in form of lignin, carbon nanotubes, and graphene oxide. The main purpose of this work was to relate these properties with the barrier characteristics, in terms of corrosion resistance and durability in aggressive environments and to compare the obtained results with those reported for a variety of hybrid coating systems. For the fine tuning of the performance of both coating systems toward an efficient and stable anticorrosive barrier, it is crucial to obtain detailed information on the formation process of the hybrid network and the structural and compositional properties of the nanocomposites.

#### **3.1. Epoxy-silica hybrid**

Bisphenol stands for a group of chemical compounds with two hydroxyphenyl functionalities. There is a wide diversity of bisphenol molecules; however, the most common are the Bisphenol A (BPA) and the Bisphenol F (BPF) (**Figure 5**). Epoxy resins can be produced from the combination of bisphenol, such as bisphenol A, with epichlorohydrin (IUPAC name: 2-(chloromethyl)oxirane)) to give, for example, bisphenol A diglycidyl ether (**Figure 6**). The epoxy resins present in general poor thermal, mechanical, and chemical stability, properties which are however significantly improved when a curing agent is added. Most curing agents are composed of nitrogen-containing molecules that have a functionality equal or superior of three (f ≥ 3), which provides cross-linking between the bisphenol segments. The functionality is the number of available bonding sites, such as f = 4 for diamino diphenyl methane (4 hydrogens prone to provide bond), f = 6 for triethylene tetraamine, and f = 5 for diethylenetriamine (**Figure 7**). Curing reactions by DETA proceed by SN2 nucleophilic attack of the curing agent to the less-substituted carbon in the oxirane ring, resulting in its opening and formation of an OH group. The nitrogen of the amine group can attack another epoxy ring resulting in a highly branched polymer system, known as a thermoset, which presents high thermal stability and mechanical resistance [23]. This second nucleophilic attack of nitrogen can occur at the epoxy group of the resin or another molecule containing epoxy group, such

**Figure 5.** Common epoxy resin precursors.

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

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,

significant drop of the impedance modulus was observed, indicating the failure of the coating.

A number of interesting results have been obtained for novel epoxy-silica and PMMA-silica hybrid coatings, concerning their nanostructural properties, modified by the variation of synthesis conditions or by addition of nanofillers, in form of lignin, carbon nanotubes, and graphene oxide. The main purpose of this work was to relate these properties with the barrier characteristics, in terms of corrosion resistance and durability in aggressive environments and to compare the obtained results with those reported for a variety of hybrid coating systems. For the fine tuning of the performance of both coating systems toward an efficient and stable anticorrosive barrier, it is crucial to obtain detailed information on the formation process of the hybrid network and the structural and compositional properties of

Bisphenol stands for a group of chemical compounds with two hydroxyphenyl functionalities. There is a wide diversity of bisphenol molecules; however, the most common are

SO4

+ 0.05-mol L−1 NaCl), until a

in saline (3.5% NaCl) or saline/acid solutions (0.05-mol L−1 H2

**3. Results and discussion**

the nanocomposites.

**3.1. Epoxy-silica hybrid**

mL min−1.

28 New Technologies in Protective Coatings

**Figure 6.** Bisphenol A diglycidyl ether.

**Figure 7.** Common curing agents.

as (3-glycidoxypropyl)trimethoxysilane (GPTMS) to produce an organic-inorganic hybrid structure (**Figure 8**).

Simultaneously to the curing reaction, the sol-gel reactions of hydrolysis and condensation take place to produce the silica inorganic phase. GPTMS and TEOS Si–O–R groups, in presence of acidified water, become Si–OH through the hydrolysis reaction, and posteriorly, the Si–OH groups can condense with another Si–OH group or an initial Si–O–R group, forming Si–O–Si bonding and eliminating water or alcohol, respectively.

The surface characterization of the epoxy-silica hybrids deposited on carbon steel has shown that the coatings are uniform, transparent, smooth, and crack free (**Figure 9**). AFM images

**Figure 8.** Molecular structure of the epoxy-silica hybrid.

**Figure 9.** Representative image (a), and AFM image (b), of T1/G1 epoxy-silica hybrid coating deposited on carbon steel.

with an area of 1 μm2 were used to obtain the surface roughness of the coatings. **Table 2** summarized all RMS surface roughness and thickness values determined for epoxy-silica hybrids of T-series (TEOS variation) and G-series (GPTMS variation). With increasing GPTMS and TEOS fraction, a significant increment of the surface roughness can be observed. The data suggest that increasing concentration of TEOS has a larger impact on the surface roughness than that of GPTMS, probably due to the formation of silica domains of larger size. Measurements of the films thickness indicate for all samples of the G-series a constant value of about 1.7 μm, while for films of the T-series the thickness varies from 2 to 3 μm, except for the T1.5 sample having 6.7 μm.

as (3-glycidoxypropyl)trimethoxysilane (GPTMS) to produce an organic-inorganic hybrid

Simultaneously to the curing reaction, the sol-gel reactions of hydrolysis and condensation take place to produce the silica inorganic phase. GPTMS and TEOS Si–O–R groups, in presence of acidified water, become Si–OH through the hydrolysis reaction, and posteriorly, the Si–OH groups can condense with another Si–OH group or an initial Si–O–R group, forming

The surface characterization of the epoxy-silica hybrids deposited on carbon steel has shown that the coatings are uniform, transparent, smooth, and crack free (**Figure 9**). AFM images

**Figure 9.** Representative image (a), and AFM image (b), of T1/G1 epoxy-silica hybrid coating deposited on carbon steel.

Si–O–Si bonding and eliminating water or alcohol, respectively.

**Figure 8.** Molecular structure of the epoxy-silica hybrid.

structure (**Figure 8**).

30 New Technologies in Protective Coatings


**Table 2.** Properties of epoxy-silica hybrids: film thickness (optical interferometry); surface roughness (AFM); degree of polycondensation, Cd, (29Si-NMR); Porod coefficient, α, radius of gyration, R<sup>g</sup> , and correlation distance, d, (SAXS); temperature of the limit of thermal stability T0 in N2 atmosphere (TGA); and coating lifetime in 3.5% NaCl (EIS).


**Table 3.** Comparison between XPS and calculated nominal atomic concentrations for epoxy-silica coatings.

**Table 3** shows that results of the quantitative XPS analysis are in good agreement with those obtained for the nominal composition for both series of samples. As expected, the data show an increase of silicon and oxygen atomic concentration for the G and T-series, while nitrogen content increases slightly only for the G-series due to the higher DETA content. As the structure of GPTMS contains also carbon atoms, its addition leads to a less pronounced increase of the Si content. As a consequence, for the G-series the decrease of the C/Si ratio from 14.1 (G0.5) to 10.0 (G2.5) was smaller than that observed for the T-series from 15.9 (T0.5) to 8.3 (T2.5).

The chemical bonding structure of the inorganic network can be characterized according to the proportion of different Si species having a fixed number of oxygen bridging silicon atoms bonded to one (central) silicon atom. A common notation is Qj for orthosilicates (0 ≤ j ≤ 4), such as TEOS, and Tj for organically modified silicates (0 ≤ j ≤ 3), such as GPTMS, where j gives the number of Si–O–Si bridges attached to the silicon atom.

**Figure 10** shows the 29Si NMR spectra, fitted with Gaussian components, used to extract the proportion of Qj and Tj species. It can be observed that the Q4 and T3 peaks (−107 ppm and −62 ppm, respectively) have the highest intensities in relation to the other components related to lower network connectivity. The degree of connectivity of the inorganic phase, the so-called degree of polycondensation, Cd, has been determined from the fitted 29Si NMR spectra using the following equation:

**Figure 10.** 29Si-NMR spectra obtained for epoxy-silica hybrids. Inset: schematic representation of the TJ and QJ species, where 'R' indicates OH or OCH<sup>3</sup> or OCH2 CH3 groups.

$$C\_d = \frac{T^4 + 2 \cdot T^2 + 3 \cdot T^3}{3} + \frac{Q^4 + 2 \cdot Q^2 + 3 \cdot Q^3 + 4 \cdot Q^4}{4} \times 100\tag{4}$$

The Cd values of **Table 3** show a high connectivity of the inorganic network with a clear predominance of a tetra-substituted TEOS and a tri-substituted GPTMS sites. Furthermore, it seems that an increase of GPTMS favors the Q4 and T3 structures, yielding a highly crosslinked inorganic network reaching about 95% connectivity for G1.5 sample, while the variation of TEOS does not change the Cd values significantly, remaining in the range of 85–88%. More information on the structure and size of the inorganic domains was obtained by small angle X-ray scattering (SAXS) measurements.

**Table 3** shows that results of the quantitative XPS analysis are in good agreement with those obtained for the nominal composition for both series of samples. As expected, the data show an increase of silicon and oxygen atomic concentration for the G and T-series, while nitrogen content increases slightly only for the G-series due to the higher DETA content. As the structure of GPTMS contains also carbon atoms, its addition leads to a less pronounced increase of the Si content. As a consequence, for the G-series the decrease of the C/Si ratio from 14.1 (G0.5) to 10.0 (G2.5) was smaller than that observed for the T-series from 15.9 (T0.5) to 8.3 (T2.5).

The chemical bonding structure of the inorganic network can be characterized according to the proportion of different Si species having a fixed number of oxygen bridging silicon atoms

**Figure 10** shows the 29Si NMR spectra, fitted with Gaussian components, used to extract the

ppm, respectively) have the highest intensities in relation to the other components related to lower network connectivity. The degree of connectivity of the inorganic phase, the so-called degree of polycondensation, Cd, has been determined from the fitted 29Si NMR spectra using

species. It can be observed that the Q4

**Figure 10.** 29Si-NMR spectra obtained for epoxy-silica hybrids. Inset: schematic representation of the TJ

groups.

or OCH2

CH3

for organically modified silicates (0 ≤ j ≤ 3), such as GPTMS, where j gives the

and T3

for orthosilicates (0 ≤ j ≤ 4), such

peaks (−107 ppm and −62

and QJ

species,

bonded to one (central) silicon atom. A common notation is Qj

number of Si–O–Si bridges attached to the silicon atom.

and Tj

as TEOS, and Tj

32 New Technologies in Protective Coatings

proportion of Qj

the following equation:

where 'R' indicates OH or OCH<sup>3</sup>

The SAXS technique allows to access the nanostructural characteristics of the inorganic network due to the higher electronic density of silica compared that of the polymeric matrix. The log-log plots of scattering intensities I (q) recorded for different fractions of GPTMS and TEOS (**Figure 11**) show three main characteristics: a linear decay located at low q values, corresponding to the Porod region; a Gaussian decay in the mid q-range, corresponding to the Guinier regime; and a broad correlation peak superimposed to the Guinier region, observed only for T0.0 and T0.5 samples. The former feature, in the mid q-range, is characteristic of a diluted set scatters, while the latter is the result of the interferences of the scattered X-ray caused by the concentrated set of nano-objects.

These scattering patterns have been already observed for other silica-polymer hybrids [24, 25] and attributed to a hierarchical organization of silica nano-domains. Accordingly, we propose that the nanostructure of the hybrid can be described by a two-level hierarchical model, corresponding to a diluted or concentrated (T0.0 and T0.5) set of silica nanoparticles inside the aggregation zones embedded in the polymer matrix. In the case of the diluted system, the size of the smaller particles was determined, in terms of the radius of the gyration, Rg , by fitting the Gaussian decay observed in the mid q-range using the

**Figure 11.** SAXS curves of T-series (a), and G-series (b), where the black lines represents the fits used to calculate the Porod coefficient (α) for q < 1 (except for the G0.5 sample, which presents α at q>1), and the radius of the gyration (Rg ) for q >1. (The intensities were shifted to obtain a better visualization of the curves).

Guinier model: I(q) = I0 exp (−R<sup>g</sup> q2 /3), where I0 is the scaling factor. However, this was only possible for scattering curves, which did not present an overlapping correlation peak. Therefore, values for Rg and those for the correlation distance, d ≈ 2π/q, have been obtained only for a restricted number of samples (**Table 2**). Except for T0.0, the form of the scattering objects was determined by fitting the curves using the Porod model: I ∝ q−α, where α is the Porod exponent. α ≈ 4 indicates a bi-phase system formed by set of nearly isometric scattering objects with a smooth surface, while for smaller values, a rough surface (fractal) is expected.

The results indicate that the inorganic phase consists of aggregates with relatively smooth surface and an average spacing of several nanometers (d ≈ 4 nm). These domains have been formed by agglomeration of smaller silica particles with a size of about 1 nm (0.3 < R<sup>g</sup> < 1.5 nm).

Some clear correlations between these parameters and the increasing silica concentration of the G- and T-series could be established. For the G-series, the evolution of the SAXS pattern evidences the role of GPTMS in controlling the size of primary silica particles and they aggregation. The power law decay over a decade and α = 3.8 observed for the hybrid prepared only with TEOS (G0.0) characterizes the scattering by the surface of very large silica particles (>30 nm). The addition of a small amount of GPTMS (G0.5 sample) reduces the size of silica particles more than ten times (Rg = 1.5 nm) and prevents further aggregation, as evidenced by extended plateau at q < 0.1 nm. These unique features suggest for G0.5 sample an elevated nanostructural homogeneity, which might be responsible for the superior corrosion protection performance of this material (**Table 2**). In the case of the T-series, the correlation peak disappears for higher TEOS content and the linear decay shifts to higher q-values. These features evidence that TEOS addition favors the formation of more open aggregates, leading to a less compact nanostructure.

The thermal properties of the hybrids were studied by thermogravimetry under nitrogen flow. **Table 2** shows the temperature of the limit of thermal stability, T0 , for all epoxysilica hybrids, defined as the temperature of 5% weight loss during the annealing process. The hybrids presented a thermal stability of about 300°C, relatively high values compared with those of other polymeric and hybrid materials [18, 22]. This advantageous property comes from the highly cross-linked structure provided by the curing agent (DETA) combined with the high polycondensation degree of the silica phase, as revealed by 29Si NMR.

The anticorrosive performance of the hybrids was assessed by EIS measurements, in a 3.5% NaCl saline solution at 25°C. The hybrid coatings deposited on carbon steel were attached to an electrochemical cell, and after verifying a constant value of the open-circuit potential, the impedance measurements were performed as a function of time until a significative drop of the impedance modulus occurred. This time period was defined as lifetime of the coating, listed in **Table 2**. The impedance modulus at low frequency of the Bode plot is generally used as an indicator of the anticorrosive performance of the coating, with values higher than 0.1 GΩ cm−2 typically considered an excellent protection. The corrosion resistance of the films generally decreases with time, caused by the penetration of electrolyte into the protective layer through zones of residual porosity and defects.

Organic-Inorganic Hybrid Coatings for Corrosion Protection of Metallic Surfaces http://dx.doi.org/10.5772/67909 35

Guinier model: I(q) = I0

34 New Technologies in Protective Coatings

Therefore, values for Rg

particles more than ten times (Rg

a less compact nanostructure.

revealed by 29Si NMR.

layer through zones of residual porosity and defects.

is expected.

exp (−R<sup>g</sup>

q2

/3), where I0

only possible for scattering curves, which did not present an overlapping correlation peak.

only for a restricted number of samples (**Table 2**). Except for T0.0, the form of the scattering objects was determined by fitting the curves using the Porod model: I ∝ q−α, where α is the Porod exponent. α ≈ 4 indicates a bi-phase system formed by set of nearly isometric scattering objects with a smooth surface, while for smaller values, a rough surface (fractal)

The results indicate that the inorganic phase consists of aggregates with relatively smooth surface and an average spacing of several nanometers (d ≈ 4 nm). These domains have been formed

Some clear correlations between these parameters and the increasing silica concentration of the G- and T-series could be established. For the G-series, the evolution of the SAXS pattern evidences the role of GPTMS in controlling the size of primary silica particles and they aggregation. The power law decay over a decade and α = 3.8 observed for the hybrid prepared only with TEOS (G0.0) characterizes the scattering by the surface of very large silica particles (>30 nm). The addition of a small amount of GPTMS (G0.5 sample) reduces the size of silica

by extended plateau at q < 0.1 nm. These unique features suggest for G0.5 sample an elevated nanostructural homogeneity, which might be responsible for the superior corrosion protection performance of this material (**Table 2**). In the case of the T-series, the correlation peak disappears for higher TEOS content and the linear decay shifts to higher q-values. These features evidence that TEOS addition favors the formation of more open aggregates, leading to

The thermal properties of the hybrids were studied by thermogravimetry under nitrogen

silica hybrids, defined as the temperature of 5% weight loss during the annealing process. The hybrids presented a thermal stability of about 300°C, relatively high values compared with those of other polymeric and hybrid materials [18, 22]. This advantageous property comes from the highly cross-linked structure provided by the curing agent (DETA) combined with the high polycondensation degree of the silica phase, as

The anticorrosive performance of the hybrids was assessed by EIS measurements, in a 3.5% NaCl saline solution at 25°C. The hybrid coatings deposited on carbon steel were attached to an electrochemical cell, and after verifying a constant value of the open-circuit potential, the impedance measurements were performed as a function of time until a significative drop of the impedance modulus occurred. This time period was defined as lifetime of the coating, listed in **Table 2**. The impedance modulus at low frequency of the Bode plot is generally used as an indicator of the anticorrosive performance of the coating, with values higher than 0.1 GΩ cm−2 typically considered an excellent protection. The corrosion resistance of the films generally decreases with time, caused by the penetration of electrolyte into the protective

flow. **Table 2** shows the temperature of the limit of thermal stability, T0

by agglomeration of smaller silica particles with a size of about 1 nm (0.3 < R<sup>g</sup>

and those for the correlation distance, d ≈ 2π/q, have been obtained

= 1.5 nm) and prevents further aggregation, as evidenced

is the scaling factor. However, this was

< 1.5 nm).

, for all epoxy-

**Figure 12.** Nyquist and Bode plots of (a) series T and (b) series G of the epoxy-silica coatings deposited on carbon steel, compared to those of bare carbon steel, after 1 h of immersion in 3.5% NaCl solution.

The Nyquist and Bode plots obtained after 1 day of immersion in 3.5% NaCl solution are presented in **Figure 12**. It can be observed that two samples containing intermediate TEOS to GPTMS ratios (T1.5 and G0.5) presented the highest impedance modulus of 0.9 and 0.2 GΩ cm<sup>2</sup> , respectively, and showed also the longest lifetime of several weeks (**Table 2**). These coatings show at higher frequencies (>1 Hz), a capacitive behavior with a phase angle higher than −80° extending over a range of 4 decades, characteristic for an efficient anticorrosive barrier layer. In contrast, for formulations with excess of TEOS or GPTMS, both the corrosion resistance and lifetime values show considerably lower values.

The results of the structural analysis indicate that the excellent barrier properties, found for coatings with intermediate TEOS to GPTMS ratio, result from a highly reticulated hybrid structure combining a number of favorable properties, such as a high polycondensation degree of the inorganic phase, a extremely smooth surface, indicating a very homogeneous distribution of silica nanodomains, high thermal stability, as well as an adequate quantity of the silica phase which ensures a good adhesion of the film to the metallic substrate. Although the corrosion protection efficiency of the best epoxy-silica coatings, reported so far [7, 26, 27], is comparable with results presented in this work, it may profit from their 10–100 times higher thickness.

#### **3.2. PMMA-silica hybrids**

Poly(methyl methacrylate), also known as acrylic and Pexiglas®, is a rigid, low cost, nontoxic, transparent and colorless thermoplastic polymer, extensively used as optical lenses, protective coatings, optical fibers, and as an alternative to glass in windows as well as a variety of household appliances. The introduction of an inorganic component, such as silica, improves the thermal stability, mechanical strength, and the adhesion to metallic substrates, the latter property being an essential feature for a high-efficiency coatings. The covalent bond between the PMMA and the silica phase can be achieved by the addition of 3-(trimethoxysilyl)propyl methacrylate (MPTS), a coupling agent formed by an alkoxy-silane group attached by a nonhydrolysable Si–C bond to the acrylic tail, which polymerizes with PMMA chains, while the inorganic part reacts with the silica precursor (TEOS), yielding an organic-inorganic hybrid structure, shown in **Figure 13**.

A variety of PMMA-silica hybrids have been studied, changing the organic/inorganic phase proportion, the amount of thermal initiator, the synthesis temperature and time, as well as the ethanol-to-water ratio. Furthermore, cerium salt has been added to the PMMA-silica matrix as corrosion inhibitor, and lignin, carbon nanotubes, and graphene oxide as fillers. The main results, found for pure PMMA-silica hybrids, are summarized in **Table 4**, those obtained using additives will be discussed in the following sections.

Similar to epoxy-silica coatings, PMMA-silica hybrids deposited on metallic substrates were homogeneous, transparent and had a very smooth surface. Structural analysis of PMMAsilica hybrids, performed by AFM, SAXS, NMR and XPS, has shown that the nanostructure is formed by a dense amorphous network of ramified silica-siloxane cross-link nodes, covalently interconnected by short PMMA chain segments [10, 22]. Varying the MMA/MPTS molar ratio from 2 to 10, NMR and SAXS results have shown that the M8 sample (MMA/MPTS = 8) presented the highest degree of polycondensation (83.9%) of the silica nanoparticles with an average radius of 0.8 nm, spaced by PMMA segments over an average distance of 4.6 nm. This coating exhibited also an excellent adhesion to the substrate (detachment force > 3.5 MPa) and the best anti-corrosion performance [10, 22]. EIS and potentiodynamic polarization results have shown that the M8 coating deposited on carbon steel acts as a very efficient corrosion barrier, increasing the total corrosion resistance by almost 6 orders of magnitude (>1 GΩ cm<sup>2</sup> ) and reducing the current densities by more than 4 orders of magnitude (<0.1 nA cm−2), compared to the bare steel substrate [22]. Furthermore, XPS analysis confirmed that no corrosioninduced changes had occurred after 18 days of immersion in 3.5% NaCl solution [22].

Increasing the synthesis temperature from 70 to 80°C and the time of reaction from 2 to 4 h (sample M8\_4h), an increase in the amount of polymeric phase was detected yielding a more compact and durable coating (56 days) [18, 22]. After optimizing also the ethanol-to-water ratio of the inorganic phase to a value of 0.2, the corrosion resistance and lifetime were further increase to 196 days in 3.5% NaCl (M8\_4h\_E0.2 coating, **Table 4**) [10]. Other important finding was the improvement of the corrosion resistance by hot deposition (M8\_T80B0.01), which enhances the reaction between the inorganic phase and the metal substrate, improving the coating adhesion. This sample has been also deposited on Al2024 substrate and tested in saline (**Figure 14a**) and saline/acid environment (**Figure 14b**). This coating highlights a Organic-Inorganic Hybrid Coatings for Corrosion Protection of Metallic Surfaces http://dx.doi.org/10.5772/67909 37

**Figure 13.** Molecular structure of the PMMA-silica hybrid.

**3.2. PMMA-silica hybrids**

36 New Technologies in Protective Coatings

structure, shown in **Figure 13**.

using additives will be discussed in the following sections.

Poly(methyl methacrylate), also known as acrylic and Pexiglas®, is a rigid, low cost, nontoxic, transparent and colorless thermoplastic polymer, extensively used as optical lenses, protective coatings, optical fibers, and as an alternative to glass in windows as well as a variety of household appliances. The introduction of an inorganic component, such as silica, improves the thermal stability, mechanical strength, and the adhesion to metallic substrates, the latter property being an essential feature for a high-efficiency coatings. The covalent bond between the PMMA and the silica phase can be achieved by the addition of 3-(trimethoxysilyl)propyl methacrylate (MPTS), a coupling agent formed by an alkoxy-silane group attached by a nonhydrolysable Si–C bond to the acrylic tail, which polymerizes with PMMA chains, while the inorganic part reacts with the silica precursor (TEOS), yielding an organic-inorganic hybrid

A variety of PMMA-silica hybrids have been studied, changing the organic/inorganic phase proportion, the amount of thermal initiator, the synthesis temperature and time, as well as the ethanol-to-water ratio. Furthermore, cerium salt has been added to the PMMA-silica matrix as corrosion inhibitor, and lignin, carbon nanotubes, and graphene oxide as fillers. The main results, found for pure PMMA-silica hybrids, are summarized in **Table 4**, those obtained

Similar to epoxy-silica coatings, PMMA-silica hybrids deposited on metallic substrates were homogeneous, transparent and had a very smooth surface. Structural analysis of PMMAsilica hybrids, performed by AFM, SAXS, NMR and XPS, has shown that the nanostructure is formed by a dense amorphous network of ramified silica-siloxane cross-link nodes, covalently interconnected by short PMMA chain segments [10, 22]. Varying the MMA/MPTS molar ratio from 2 to 10, NMR and SAXS results have shown that the M8 sample (MMA/MPTS = 8) presented the highest degree of polycondensation (83.9%) of the silica nanoparticles with an average radius of 0.8 nm, spaced by PMMA segments over an average distance of 4.6 nm. This coating exhibited also an excellent adhesion to the substrate (detachment force > 3.5 MPa) and the best anti-corrosion performance [10, 22]. EIS and potentiodynamic polarization results have shown that the M8 coating deposited on carbon steel acts as a very efficient corrosion barrier, increasing the total corrosion resistance by almost 6 orders of magnitude (>1 GΩ cm<sup>2</sup>

and reducing the current densities by more than 4 orders of magnitude (<0.1 nA cm−2), compared to the bare steel substrate [22]. Furthermore, XPS analysis confirmed that no corrosion-

Increasing the synthesis temperature from 70 to 80°C and the time of reaction from 2 to 4 h (sample M8\_4h), an increase in the amount of polymeric phase was detected yielding a more compact and durable coating (56 days) [18, 22]. After optimizing also the ethanol-to-water ratio of the inorganic phase to a value of 0.2, the corrosion resistance and lifetime were further increase to 196 days in 3.5% NaCl (M8\_4h\_E0.2 coating, **Table 4**) [10]. Other important finding was the improvement of the corrosion resistance by hot deposition (M8\_T80B0.01), which enhances the reaction between the inorganic phase and the metal substrate, improving the coating adhesion. This sample has been also deposited on Al2024 substrate and tested in saline (**Figure 14a**) and saline/acid environment (**Figure 14b**). This coating highlights a

induced changes had occurred after 18 days of immersion in 3.5% NaCl solution [22].

)


**Table 4.** Properties of PMMA-silica hybrid coatings: film thickness (optical interferometry); surface roughness (AFM); degree of polycondensation, Cd, (29Si-NMR); limit of thermal stability T0 in N2 atmosphere (TGA); impedance modulus |Z|, after 1 day exposure to 3.5% NaCl solution (EIS); and coating lifetime in 3.5% NaCl (EIS).

corrosion resistance of up to 50 GΩ cm<sup>2</sup> , in saline environment, showing only a small performance decrease to 0.1 GΩ cm<sup>2</sup> after 560 days exposure. This is to our best knowledge the highest durability, obtained so far for hybrid coatings in standard saline solution. Also in contact with a saline/acid solution (0.05 mol L−1 H2 SO4 + 0.05 mol L−1 NaCl), this about 3-μm thick coating presented a high corrosion resistance (20 GΩ cm<sup>2</sup> ), remaining almost unchanged during its lifetime of 87 days. It is interesting to note that the phase angle dependence has a capacitive behavior (θ ≈ −90°), over a frequency range of 6 decades, a behavior close to that of an ideal capacitor, highlighting the extraordinary performance of this coating.

PMMA-silica hybrids have been also prepared at different synthesis temperatures of the organic precursor (80–100°C) and different BPO/MMA molar ratio (0.01–0.1), using the wellestablished MMA/MPTS molar ratio of 8 [22]. The increase in the synthesis temperature did not influence significantly the structure, the thermal properties and the corrosion resistance, however, the increase of the BPO amount led to an increase of the polymerization degree, thermal stability of 40°C (BPO0.05), and also of the anticorrosive efficiency (**Table 4**). The sample M8\_BPO0.05 and M8\_BPO0.10 presented an impedance modulus of 10 GΩ cm<sup>2</sup> in a saline medium (3.5% NaCl), remaining essentially unchanged during more than 6 months of exposure (M8\_BPO0.10).

**Figure 14.** Time dependence of Nyquist and Bode plots of the M8\_T80BPO0.01 PMMA-silica coating deposited on Al2024 substrate in contact with a) in 3.5% NaCl solution and b) 0.05 mol L−1 NaCl + 0.05 mol L−1 H2 SO4 solution.

### **3.3. PMMA-silica hybrid modified with Ce(IV) salt for self-healing ability**

After identifying the optimum proportion of polymeric and silica phase for the formation of a highly ramified structure (MMA/MPTS/TEOS molar ratio = 8/1/2), increasing molar percentage of Ce(IV) ions (0.1% < Ce/Si < 5%) have been added to the inorganic precursor to enhance the passivating character of the films [11].

NMR, XPS, and SAXS results, summarized in **Table 5**, have revealed the active role of Ce(IV) in the PMMA-silica matrix. The correlation of XPS and NMR data evidenced that the Ce(IV) concentration is directly related to the polycondensation degree (Cd) and the degree of Ce(IV) reduction, both decreasing with increasing cerium concentration. Low concentrations of cerium lead to an enhanced polycondensation of the siloxane/silica phase, with connectivity of the inorganic phase up to 87%. For low doping levels of Ce/Si < 0.7%, SAXS results have revealed increasing values of radius of gyration, Rg , suggesting an active role of Ce(IV) as oxidation agent in the enhanced growth of a cross-linked and polycondensed inorganic phase. Detailed investigation of the structural effects of cerium species has shown that reduction of Ce(IV) ions not only catalyzes a higher connectivity of the silica phase, but also enhances the polymerization of organic moieties. The resulting enhancement of the overall network connectivity leads to an improvement of the thermal stability of the hybrids, as evidenced by the results of the thermogravimetric analysis [11].

The electrochemical assays, performed by EIS and potentiodynamic polarization curves, have shown that the PMMA-silica coatings containing intermediate concentrations of cerium present a combination of high-corrosion resistance (~10 GΩ cm<sup>2</sup> ), elevated overpotential stability at low-current densities (<10−11 A), as well as excellent long-term stability of up to 304 days. Compared to the bare carbon steel substrate, the coated samples showed up to 6 orders of magnitude higher impedance modulus and up to 6 orders of magnitude lower current densities [11].

For coatings containing elevated Ce(IV) doping levels (Si/Ce = 5%), the self-healing effect was observed, induced by the formation of insoluble cerium oxides and hydroxides in corrosion affected regions. The presence of these phases in the near surface region was evidenced by XPS O 1s spectra and by scanning electron microscopy, showing the presence of nanopits (<300 nm). It was suggested that these phases were formed by reactions of Ce(III) and Ce(IV) with water and residual hydroxyl groups of the hybrid. The self-healing process prevented the progression of the corrosion process for more than 13 months keeping the corrosion resistance constant above 0.01 GΩ cm<sup>2</sup> . The excellent anticorrosive efficiency achieved by PMMAsilica coatings containing cerium can be related to a double effect of Ce(IV), combining the densification of the hybrid network with the formation of insoluble cerium species in regions affected by pitting [11].

#### **3.4. PMMA-silica hybrid reinforced with lignin, carbon nanotubes and graphene oxide**

**Figure 14.** Time dependence of Nyquist and Bode plots of the M8\_T80BPO0.01 PMMA-silica coating deposited on

SO4

, in saline environment, showing only a small per-

+ 0.05 mol L−1 NaCl), this about 3-μm

), remaining almost unchanged

in a

after 560 days exposure. This is to our best knowledge the

highest durability, obtained so far for hybrid coatings in standard saline solution. Also in

during its lifetime of 87 days. It is interesting to note that the phase angle dependence has a capacitive behavior (θ ≈ −90°), over a frequency range of 6 decades, a behavior close to that of

PMMA-silica hybrids have been also prepared at different synthesis temperatures of the organic precursor (80–100°C) and different BPO/MMA molar ratio (0.01–0.1), using the wellestablished MMA/MPTS molar ratio of 8 [22]. The increase in the synthesis temperature did not influence significantly the structure, the thermal properties and the corrosion resistance, however, the increase of the BPO amount led to an increase of the polymerization degree, thermal stability of 40°C (BPO0.05), and also of the anticorrosive efficiency (**Table 4**). The sample M8\_BPO0.05 and M8\_BPO0.10 presented an impedance modulus of 10 GΩ cm<sup>2</sup>

saline medium (3.5% NaCl), remaining essentially unchanged during more than 6 months of

an ideal capacitor, highlighting the extraordinary performance of this coating.

SO4

solution.

Al2024 substrate in contact with a) in 3.5% NaCl solution and b) 0.05 mol L−1 NaCl + 0.05 mol L−1 H2

corrosion resistance of up to 50 GΩ cm<sup>2</sup>

contact with a saline/acid solution (0.05 mol L−1 H2

thick coating presented a high corrosion resistance (20 GΩ cm<sup>2</sup>

formance decrease to 0.1 GΩ cm<sup>2</sup>

38 New Technologies in Protective Coatings

exposure (M8\_BPO0.10).

Lignin is a macromolecule present in the cell walls of terrestrial plants that confers rigidity and impermeability, usually corresponding to 15–30% of the dry weight of wood (**Figure 15a**).


**Table 5.** Properties of PMMA-silica hybrids containing different amounts of Ce(IV): degree of polycondensation, Cd, ( 29Si-NMR); percentage of the Ce(IV) oxidation state (XPS Ce 3d spectra); radius of gyration, Rg , (SAXS); and impedance modulus, |Z|, after 1 day exposure to 3.5% NaCl solution (EIS).

Presently, millions of tons of lignin are generated from biodiesel and ethanol production, and most part is incinerated to generate electric energy. However, nobler applications have been found to add value to this biomass, such as the reinforcement of different classes of materials. Properties, such as low density, low abrasive character, hydrophobicity, and low cost, make lignin ideal to use as filler in polymeric and organic-inorganic hybrid matrices [20].

Carbon nanotubes (CNTs) and graphene oxide (GO) (**Figure 15b and c**) are also very interesting nanofillers for the structural reinforcement of polymeric and hybrid materials, due to their exceptional thermal, chemical, and mechanical resistance. Both present a hexagonal sp2 arrangement of carbon atoms, forming extremely stable cylindrical and monolayer structures, respectively. Graphene oxide has been obtained from oxidation and exfoliation of graphite, yielding a graphene layer containing oxygen functional groups such as epoxy, hydroxyl, and carboxyl [18].

PMMA-silica hybrids reinforced with 0.05, 0.10, 0.50 and 1.00 wt.% of lignin have been deposited on carbon steel by dip-coating, producing about 2.5 μm thick coatings (**Table 6**). Optical microscopy and optical microscopy and atomic force microscopy showed that lignin was well dispersed in the hybrid matrix, and all coatings presented a low RMS surface roughness between 0.3 and 0.4 nm. The introduction of lignin in the PMMA-silica hybrid increased the water contact angle of the film surface from 79.3° to 87.9°, the hardness from 22.9 to 30.9 HV, and the scratch resistance (critical load for delamination) from 55 to 80 mN. In addition, the thermal degradation events, obtained by thermogravimetric analysis (TGA) under nitrogen atmosphere, were shifted to higher temperatures with lignin addition, due to its phenolic structure which has the ability to trap radicals formed during the depolymerisation. Besides increasing the thermal stability of the polymeric phase, it acts also as UV stabilizer [20].

The electrochemical tests performed by EIS showed that the PMMA-silica coatings containing lignin act as efficient diffusion barriers, with corrosion resistance higher than 0.1 GΩ cm<sup>2</sup> after Organic-Inorganic Hybrid Coatings for Corrosion Protection of Metallic Surfaces http://dx.doi.org/10.5772/67909 41

**Figure 15.** Molecular structure of (a) lignin, (b) carbon nanotube, and (c) graphene oxide.

Presently, millions of tons of lignin are generated from biodiesel and ethanol production, and most part is incinerated to generate electric energy. However, nobler applications have been found to add value to this biomass, such as the reinforcement of different classes of materials. Properties, such as low density, low abrasive character, hydrophobicity, and low cost, make

**Table 5.** Properties of PMMA-silica hybrids containing different amounts of Ce(IV): degree of polycondensation, Cd,

**0 0.1 0.2 0.3 0.5 0.7 1 3 5**

– – – 28.5 ± 4 37.2 ± 3 46.4 ± 2.5 48.5 ± 2.5 55.5 ± 2 60.4 ± 1.5

~0.5 ~0.5 – – ~0.1 ~10 ~5 – ~0.5

42 85 – 96 – 304 65 48 404

Cd (%) 82.8 87.1 – 83.6 82.7 79.3 78.5 77.6 77.3

(nm) 0.9 1.1 1.8 2.3 2.3 1.9 1.9 – –

Carbon nanotubes (CNTs) and graphene oxide (GO) (**Figure 15b and c**) are also very interesting nanofillers for the structural reinforcement of polymeric and hybrid materials, due to their exceptional thermal, chemical, and mechanical resistance. Both present a hexagonal sp2 arrangement of carbon atoms, forming extremely stable cylindrical and monolayer structures, respectively. Graphene oxide has been obtained from oxidation and exfoliation of graphite, yielding a graphene layer containing oxygen functional groups such as epoxy, hydroxyl, and

PMMA-silica hybrids reinforced with 0.05, 0.10, 0.50 and 1.00 wt.% of lignin have been deposited on carbon steel by dip-coating, producing about 2.5 μm thick coatings (**Table 6**). Optical microscopy and optical microscopy and atomic force microscopy showed that lignin was well dispersed in the hybrid matrix, and all coatings presented a low RMS surface roughness between 0.3 and 0.4 nm. The introduction of lignin in the PMMA-silica hybrid increased the water contact angle of the film surface from 79.3° to 87.9°, the hardness from 22.9 to 30.9 HV, and the scratch resistance (critical load for delamination) from 55 to 80 mN. In addition, the thermal degradation events, obtained by thermogravimetric analysis (TGA) under nitrogen atmosphere, were shifted to higher temperatures with lignin addition, due to its phenolic structure which has the ability to trap radicals formed during the depolymerisation. Besides increasing the thermal stability of the polymeric phase, it acts also as

The electrochemical tests performed by EIS showed that the PMMA-silica coatings containing lignin act as efficient diffusion barriers, with corrosion resistance higher than 0.1 GΩ cm<sup>2</sup>

after

, (SAXS); and impedance

lignin ideal to use as filler in polymeric and organic-inorganic hybrid matrices [20].

29Si-NMR); percentage of the Ce(IV) oxidation state (XPS Ce 3d spectra); radius of gyration, Rg

modulus, |Z|, after 1 day exposure to 3.5% NaCl solution (EIS).

carboxyl [18].

**Properties Ce(IV)/Si molar fraction (%)**

40 New Technologies in Protective Coatings

XPS Ce(IV) fraction (%)

Rg

(


Lifetime (days)

UV stabilizer [20].


**Table 6.** Properties of PMMA-silica hybrids containing lignin, CNT or GO: film thickness (optical interferometry); limit of thermal stability T0 in N2 atmosphere (TGA); critical load for delamination (microscrach test); impedance modulus |Z|, after 1 day exposure to 3.5% NaCl solution (EIS); and coating lifetime in 3.5% NaCl (EIS).

exposure to 3.5% NaCl aqueous solution. For intermediate lignin content of 0.10 wt.% the coatings presented best results with an impedance modulus of 0.5 GΩ cm<sup>2</sup> , remaining almost unchanged after 50 days of exposure to aggressive environment [20].

Recent studies on the inclusion of carbon nanotubes and graphene oxide in hybrid and polymer matrices have shown excellent results in terms of increased mechanical strength, scratch and wear resistance, thermal stability, adhesion to metallic substrate, hydrophobicity, and electrical conductivity [18, 19, 28–30]. Despite all of these advances, a simultaneous improvement not only of mechanical and thermal stability but also of anticorrosive efficiency of protective coatings has been accomplished only recently by the incorporation CNT and GO in a PMMA-silica matrix [18].

To synthesize the CNT and GO modified PMMA-silica hybrids, first, the single-walled carbon nanotubes and graphene oxide were dispersed in a water/ethanol, adding in the case of CNTs dodecyl sulfate surfactant (SDS) as dispersant. Then, the carbon nanostructures were added to the inorganic precursor solution of the PMMA-silica hybrid at a carbon (CNT or GO) to silicon (TEOS and MPTS) molar ratio of 0.05% in two different matrices, prepared at BPO/MMA molar ratios of 0.01 and 0.05. As the function of BPO as a thermal initiator is to produce radicals that initiate the polymerization process of MMA, an increased BPO content results in enhanced polymerization degree in the hybrid. The transparent hybrids deposited on A1020 carbon steel substrates by dip coating presented thickness values between 2.8 and 6.6 μm (**Table 6**), a good dispersion of the carbon nanostructures, and a very smooth surface (0.3–0.5 nm RMS surface roughness) [18].

Microscratch and wear tests, performed with a spherical-conical diamond tip of 10 μm radius, confirmed for the PMMA-silica coatings that both additives, CNT and GO, improved the scratch resistance (increase of the friction coefficient by 0.1−0.2), adhesion to the metallic substrate (no delamination for M8\_BPO001\_CNT up to 240 mN) and wear resistance (smooth and shallow wear track after 50 cycles). The superior behavior of CNT containing coatings was attributed to their property to act as rigid obstacles for the scratch tip. Results of the thermogravimetric analysis have shown that the addition of CNT and GO to the BPO0.01 matrix and to a smaller extent to the BPO0.05 matrix, increased the thermal stability of the hybrids up to 70°C for GO containing samples (**Table 6**). This improvement was attributed to interaction between carbon nanostructures and macroradicals generated during the process of depolymerisation combined with a 2D barrier effect of GO, hindering molecular diffusion through the matrix and thus providing an improved thermal resistance [18].

Results of electrochemical impedance spectroscopy in 3.5% NaCl solution showed that PMMAsilica coatings reinforced with CNTs and GO had an improved anticorrosive efficiency, with impedance modulus of ~1 GΩ cm<sup>2</sup> and ~10 GΩ cm<sup>2</sup> for the BPO0.01 and BPO0.05 matrix, respectively. Besides the improved barrier property, GO containing coatings presented also a prolonged lifetime of up to 203 days. This was attributed to the two dimensionality of the GO structure that provides an enhanced barrier effect against the propagation of corrosive species. Furthermore, it was suggested that both carbon nanostructures act as densifiers of the nanocomposite and also as negatively charged repulsive agents for chloride anions, thus improving barrier property of the coating. Based on the equivalent circuit used to fit the EIS data, this notable barrier behavior was interpreted, in terms of two distinct dielectric layers, one related to a porous water uptake zone at the coating/electrolyte interface and the other corresponding to the underlying unaffected film region, having three orders of magnitude higher resistivity [18].

#### **3.5. Advances in organic-inorganic hybrid coatings for corrosion protection**

exposure to 3.5% NaCl aqueous solution. For intermediate lignin content of 0.10 wt.% the

Recent studies on the inclusion of carbon nanotubes and graphene oxide in hybrid and polymer matrices have shown excellent results in terms of increased mechanical strength, scratch and wear resistance, thermal stability, adhesion to metallic substrate, hydrophobicity, and electrical conductivity [18, 19, 28–30]. Despite all of these advances, a simultaneous improvement not only of mechanical and thermal stability but also of anticorrosive efficiency of protective coatings has been accomplished only recently by the incorporation CNT and GO in a

To synthesize the CNT and GO modified PMMA-silica hybrids, first, the single-walled carbon nanotubes and graphene oxide were dispersed in a water/ethanol, adding in the case of CNTs dodecyl sulfate surfactant (SDS) as dispersant. Then, the carbon nanostructures were added to the inorganic precursor solution of the PMMA-silica hybrid at a carbon (CNT or GO) to silicon (TEOS and MPTS) molar ratio of 0.05% in two different matrices, prepared at BPO/MMA molar ratios of 0.01 and 0.05. As the function of BPO as a thermal initiator is to produce radicals that initiate the polymerization process of MMA, an increased BPO content results in enhanced polymerization degree in the hybrid. The transparent hybrids deposited on A1020 carbon steel substrates by dip coating presented thickness values between 2.8 and 6.6 μm (**Table 6**), a good dispersion of the carbon nanostructures, and a very smooth surface

Microscratch and wear tests, performed with a spherical-conical diamond tip of 10 μm radius, confirmed for the PMMA-silica coatings that both additives, CNT and GO, improved the scratch resistance (increase of the friction coefficient by 0.1−0.2), adhesion to the metallic substrate (no delamination for M8\_BPO001\_CNT up to 240 mN) and wear resistance (smooth and shallow wear track after 50 cycles). The superior behavior of CNT containing coatings was attributed to their property to act as rigid obstacles for the scratch tip. Results of the thermogravimetric analysis have shown that the addition of CNT and GO to the BPO0.01 matrix and to a smaller extent to the BPO0.05 matrix, increased the thermal stability of the hybrids up to 70°C for GO containing samples (**Table 6**). This improvement was attributed to interaction between carbon nanostructures and macroradicals generated during the process of depolymerisation combined with a 2D barrier effect of GO, hindering molecular diffusion through

Results of electrochemical impedance spectroscopy in 3.5% NaCl solution showed that PMMAsilica coatings reinforced with CNTs and GO had an improved anticorrosive efficiency, with

respectively. Besides the improved barrier property, GO containing coatings presented also a prolonged lifetime of up to 203 days. This was attributed to the two dimensionality of the GO structure that provides an enhanced barrier effect against the propagation of corrosive species. Furthermore, it was suggested that both carbon nanostructures act as densifiers of

for the BPO0.01 and BPO0.05 matrix,

and ~10 GΩ cm<sup>2</sup>

the matrix and thus providing an improved thermal resistance [18].

, remaining almost

coatings presented best results with an impedance modulus of 0.5 GΩ cm<sup>2</sup>

unchanged after 50 days of exposure to aggressive environment [20].

PMMA-silica matrix [18].

42 New Technologies in Protective Coatings

(0.3–0.5 nm RMS surface roughness) [18].

impedance modulus of ~1 GΩ cm<sup>2</sup>

To be able to evaluate the relevance of the obtained results, it is important to place them in the context of the state of the art in the field of anticorrosive coatings. In the last decade, the concept of organic-inorganic hybrids as protective coating has been intensely investigated using different approaches involving a variety of organic and inorganic precursor reagents, resulting in a number of promising coatings systems. The most widely applied formulations for hybrid phases used to prepare high-performance anticorrosive coatings are based on epoxy-silica (**Table 7**) and acrylic-silica (**Table 8**) hybrids, and to a lower extent on polyurethane-silica, polyurethane-silica-zirconia and other epoxy systems (**Table 9**). As can be inferred from these data, the electrochemical barrier properties, obtained for different hybrid formulations, have achieved a notable performance in the last years, making these novel nanocomposites very promising candidates for efficient corrosion protection of metallic surfaces. This is justified especially when considering that a high-corrosion resistance and long durability in aggressive environments can be achieved by thin films with thicknesses of less than 10 μm, resulting in substantially reduced material costs compared to conventional high-performance coating systems. More specifically, regarding the epoxy-silica hybrid system, the results presented in this work and those listed in **Table 7** show that different compositions applied to distinct alloys can provide a very effective long-term corrosion protection [7, 26, 27]. Very promising results were also achieved for PMMA-silica coatings [8, 10, 31], with the highest observed durability of more than 560 days in 3.5% NaCl, and for hybrids containing reinforcement and inhibitor agents [11, 18]. Moreover, for some polyurethane-silica and polyurethane-zirconia-silica systems, it has been demonstrated that they also have a high potential to be used as efficient anticorrosive barrier layers [9, 32].

All these results demonstrate the wealth of possibilities to prepare nanocomposite materials based on organic-inorganic hybrids in the form of highly efficient anticorrosive coatings. The optimization of the barrier property can be achieved by the careful adjustment of the precursor proportions, including coupling agents and additives, together with the conditions of synthesis, deposition, and thermal treatment. However, the main key for this task is an in-depth knowledge of the formation mechanisms as well as the compositional and structural properties of the material. Based on this information, it was shown that a relatively simple preparation process yields highly efficient and very durable anticorrosive films. They unite three essential perquisites for an appropriate coating system: a high corrosion resistance, long-term stability, and environmental compatibility. Considering also the simplicity of the sol-gel process and the low material consumption, which scales with the film thickness, these about 5-μm thick hybrid films constitute from the economical and environmental point of view a very interesting alternative for conventional protective coating systems.


GPTMS: (3-glycidoxypropyl)trimethoxysilane; TEOS: tetraethoxysilane; APTES: aminopropyl-triethoxysilane; tetrathiol: pentaerythritol tetrakis(3-mercaptopropionate); MTEOS: methyl-triethoxysilane; SCE: standard calomel electrode.

**Table 7.** Principal preparation parameters and results reported for epoxy-silica coatings applied for corrosion protection of metallic surfaces, including corrosion resistance |Z|, current density, Icorr, and corrosion potential, Ecorr.


of the sol-gel process and the low material consumption, which scales with the film thickness, these about 5-μm thick hybrid films constitute from the economical and environmental point of view a very interesting alternative for conventional protective coating systems.

> **EIS: |Z| (GΩ cm2**

42 3.5% NaCl

~1 35 0.05 M NaCl

21 0.1 M Na2

7 3.5% NaCl

~10 30 3% NaCl

~1 350 0.5 M NaCl

~100 467 3% NaCl

38 5% NaCl

31 0.5 M NaCl

51 0.05 M NaCl

SO4

**), lifetime (days), solution** **Polarization Icorr (A cm−2) Ecorr (V), reference electrode**

\_ [33]

\_ [27]

\_ [34]

\_ [35]

\_ [26]

\_ [36]

\_ [37]

[7]

[38]

\_ −0.08 Ag/AgCl

\_ −0.65 SCE

10−10 −0.3 Ag/AgCl **Reference**

[This work]

**thickness (μm)**

Dipcoating/~12

**Hybrid Synthesis Substrate Deposition/**

44 New Technologies in Protective Coatings

Sol-gel Mg alloy

AZ31

Epoxy-APTES \_ Carbon steel Spray/125 ~100

Epoxy-SiO2 Sol-gel Mg alloy Dip-coating/- ~100

Sol-gel Carbon steel Dip-coating/6.7 ~1

Mild steel Brush

AA2024-T3

low carbon steel

AA2024-T3

AZ31

AA2024

method/70–80

Single blade/150

Air-less spray/70

Dip-coating/25 ~0.1

Dip-coating/14 ~10

Dip-coating/~8 ~1

GPTMS: (3-glycidoxypropyl)trimethoxysilane; TEOS: tetraethoxysilane; APTES: aminopropyl-triethoxysilane; tetrathiol: pentaerythritol tetrakis(3-mercaptopropionate); MTEOS: methyl-triethoxysilane; SCE: standard calomel electrode.

**Table 7.** Principal preparation parameters and results reported for epoxy-silica coatings applied for corrosion protection

of metallic surfaces, including corrosion resistance |Z|, current density, Icorr, and corrosion potential, Ecorr.

Epoxy-GPTMS-TEOS

Epoxy-APTES-ZnO

Epoxy-APTES Solution

Epoxy-APTEStetrathiol

Epoxypolysiloxane

GPTMS-MTEOS-TEOS

Epoxy-GPTMS-MTEOS/- intercalation method

Sol-gel Al alloy

Commercial Cold rolled

Sol-gel Al alloy

Sol-gel Al alloy

Epoxy-APTES \_ Mg alloy

PMMA: poly(methyl methacrylate); MPTS: 3-(trimethoxysilyl)propyl methacrylate; TEOS: tetraethoxysilane; GMA: glycidyl methacrylate; EHA: 2-ethylhexyl acrylate; GPTMS: (3-glycidoxypropyl) trimethoxysilane;CNTs: carbon nanotubes; GO:graphene oxide; SCE: standard calomel electrode.

**Table 8.** Principal preparation parameters and results reported for acrylic-silica coatings applied for corrosion protection of metallic surfaces, including corrosion resistance |Z|, current density Icorr, and corrosion potential Ecorr.


APTES: aminopropyltriethoxysilane; TEOS: tetraethoxysilane; HA: hydroxyapatite; LDH: Layered double hydroxide; HS: halloysites; CaCO3 : calcium carbonate; SCE: standard calomel electrode.

**Table 9.** Principal preparation parameters and results reported for a varied of hybrid coatings applied for corrosion protection of metallic surfaces, including corrosion resistance |Z|, current density Icorr, and corrosion potential Ecorr.
