12.1 Introduction

Every corrosive protection effort may be lost or become more costly if the mechanisms are effective but not efficient and/or applied in an area ill-prepared or under nonideal conditions. Traditional coatings are designed to passively protect the substrate to which they are applied by providing a barrier between the surface and the environment. More advanced coatings contain a small percentage of a functional additive that enables the coatings to provide some increased functionality. Other coatings have some functionality incorporated into the resin itself. The functionality in these materials is constant and is determined solely by the initial coating formulation.¹

Smart coatings go much further, and, in order to be considered intelligent, they must be able to sense a change in conditions in the environment and respond to that change in a predictable and noticeable manner. Challener states that “smart coatings combine functionality with design to provide a system that offers simultaneous multifunctional and multidimensional beneficial effects. They offer over and above the normal functions of a coating, specifically protection and decoration, and also some unique, unusual functional properties which involve the intelligent selection between various types of responses to a given environmental stimulus.”¹

So it is necessary to develop new “avant-garde” design schemes to achieve effective protection. Currently, a smart coating applied using nanotechnology promises solutions to this problem and benefits everything from new applications to existing structures under more efficient solution. Storage of the inhibitor is based on the use of particles, which can play the role of storage devices for corrosion inhibitors, adsorbed inside.^2–5

One of the most employed methods to mitigate corrosion is the use of inhibitor substances that, when added in small amounts to an aggressive media, reduce markedly the corrosion rate. In terms of corrosion, inhibitors occupy a special place because of their specific protection and widespread application. Most inhibitor applications are in aqueous medium (natural water, acid solutions for pickling) and partial water systems (primary and secondary processing of petroleum and refining plants), and protection from atmospheric corrosion.

When selecting inhibitors, it should be taken into account what metal is to be protected, in which environment it is to be exposed (temperature, pressure, flow, etc.), and also its efficiency, availability, toxicity, and cost-effectiveness. Various mechanisms of action are considered, such as interface inhibition, electrolyte layer inhibition, membrane inhibition, and passivation.

A new generation of anticorrosion coatings that respond to changes in the environment has sparked great interest from material scientists, because corrosion is one of the most important causes of destruction of structures that involves the loss of material, and prevention is paramount in protecting investments. This type of protection is intended to retard corrosion of the metal substrate and/or control it. These anticorrosion coatings possess both passive matrix functionality and active response to changes in the local environment. Corrosion involves metal loss, and active corrosion protection aims to restore material properties (functionality) when the passive coating matrix is broken and corrosion of substrate has started. The coating has to release the active and repairing material substance within a short time after the coating integrity has been breached. This acts as a local trigger for the mechanism that heals the defect.

The oldest types of self-repairing coatings are polymer-based coatings. The concept of these coatings is as follows. Micrometer-scale containers, filled with monomers similar in nature to the polymer matrix and an appropriate catalyst or ultraviolet (UV)-sensitive agent to initiate the polymerization of the monomer when released at the damaged spot of the polymer coating, are incorporated inside the coating matrix. When these microcontainers become mechanically deformed, they release the monomer and catalyst and thus seal the defect.^3,4 Repair of macroscopic adhesion defects formed throughout the service life of composites, was carried out by filling hollow fibers with a polymer resin; these fibers would then fracture under excessive loading of the structure.^5,6 The sealing component could be different methyl compounds for fixing adhesion of the polypropylene matrix.^7,8 Polyelectrolyte-based aqueous poly-(l-lysine) poly-(ethylene glycol) was employed as the self-healing agent in an oxide-based tribo-system.⁹ The barrier properties of a damaged coating can also be recuperated by a simple blocking of the defects with insoluble precipitates.

Another approach to hybrid self-healing coatings is based on the use of inhibitors that can be released from the coating system. However, the direct introduction of inhibitor components into protective coatings very often leads to the deactivation of a corrosion inhibitor and degradation of the polymer matrix.⁸ To overcome this, several systems entrapping the inhibitor and preventing its direct interaction with the coating matrix have been developed. One quite simple approach to inhibitor entrapment is based on the complexation of organic molecules by cyclodextrin⁹; another is based on the use of oxide nanoparticles, which can play the role of nanocarriers for corrosion inhibitors adsorbed on their surface. Immobilization of Ce^3 + ions on the surface of ZrO₂ nanoparticles was achieved during the synthesis of a nano-sol by the controlled hydrolyzation of the precursor with Ce ions in an aqueous solution.¹⁰ The inhibition through inorganic ions can also be incorporated by exchangeable ions associated with cation and anion-exchange solids. Ca(II) and Ce(III) cation-exchanged bentonite anticorrosion pigments are prepared by the exhaustive exchange of naturally occurring bentonite.¹¹ For anion-exchange solids, the release of inhibitor anions can be provoked by aggressive corrosive chloride ions.^12,13

The most important aspect in the design of new active coatings is to make nanocontainers that have good compatibility with the matrix components, that can encapsulate and maintain the active material, and that possess a shell with permeability properties that can be controlled by external stimuli. In order to develop functionalized micro- and nanocontainers, several properties in the shell structure and composition have to be combined.

Recently, the goal of coating technology is to formulate smart coatings as structured systems providing selective response to physical, physicochemical or biochemical external stimulus such as temperature, stress, strain, and corrosion. Their smart response is a combination of coating properties and the incorporation of unique nanotechnology material properties.¹⁴ Ideally, a smart corrosion-inhibitor system coating will generate or release an inhibitor only when demanded and required by the corrosion initiation process. In this respect, different types of smart coatings have been proposed: paints (in particular waterborne without solvents detrimental to the environment) formulated with conductive polymers, self-healing coatings with ion exchange, and more.^1,10,14

Depending on the nature of the “smart” materials (e.g., polymers, nanoparticles, or hybrids) introduced into the container shell, various stimuli can induce reversible and irreversible shell modifications. The different responses that can be observed vary from fine effects like tunable permeability, to more profound ones like total rupture of the container shell.^13,14

Several approaches have been developed so far to fabricate micro-, meso-, and nanocontainers.^11,13 One approach is based on the self-assembly of lipid molecules or block copolymers into spherically closed bilayer structures.^13–16 These relatively unstable structures then undergo cross-linking to stabilize the nanocontainer shell. A second approach is to use hyperbranched polymers as nanocontainers.^17–19 However, in this case the particle preparation is a rather costly and time-consuming procedure, limiting any possible applications and scaling up. A third procedure involves suspension and emulsion polymerization around latex particles to form a cross-linked polymer shell. This method allows the obtaining of hollow nanoshells from as small as 100 nm in a simple one-step procedure.^18,19

The approaches described above provide the general route for shell formation. The next step in the fabrication of nanocontainers suitable for self-repairing anticorrosion coatings is to make the nanocontainer shell sensitive to the corrosion process itself. Nanocontainers with a shell that possesses controlled release properties can be used to fabricate a new family of active coatings that can respond quickly to changes in the coating environment or the coating’s integrity.

The release of corrosion inhibitors encapsulated within nanocontainers is triggered by the corrosion process, which prevents the spontaneous leakage of the corrosion inhibitor out of the coating. Moreover, if different types of nanocontainers loaded with the corresponding active agents are incorporated simultaneously into a coating matrix, the coating can act in several different ways (e.g., antibacterial, anticorrosion, and antistatic).

This task can be achieved by employing the layer-by-layer (LbL) assembly approach in the formation of the nanocontainer shell.^19–21 These polyelectrolyte films are able to change their chemical composition with pH because the degree of dissociation of them is pH sensitive. The films can be made to be richer in one polymer than in the other by working in a pH regime in which one of the polymers is weakly charged, while the other is strongly charged. Active species deposited as a component of this kind of film are able to be released on demand. The shell of the resulting containers is semipermeable and sensitive to a variety of physical and chemical conditions (mechanical impact, change in pH) in the surrounding medium, enabling it to regulate the release of the entrapped inhibitor species. Nanocontainers able to regulate the storage/release of an inhibitor can thus be constructed with nanometer-scale precision.

Another novel and seldom reported^22,23 approach is to use polymer nylon pellets or electrospun fibers and its properties as storage/carriers in combination with inhibitors and conventional coatings as a possible corrosion protection smart coating system.

12.2 Rebar Concrete Application

A simple approach was attempted in rebar concrete to inhibit corrosion that happens when steel is in contact with the electrolytic medium that promotes their physical and chemical degradation, and also affecting its practical application. In rebar concrete for construction developments, steel used plays an important role as structural element of buildings and civil works. One way to inhibit the chemical degradation of steel in concrete consists in absorbing an inhibitor onto “activated” nylon particles acting as storage/carrier, which are added as aggregates in the concrete mixture.

Reinforced concrete is exposed during their lifetimes to aggressive environments and sometimes is subject to stress, which attacks concrete or steel reinforcement. Concrete is alkaline with a 12-14 pH, promoting a protective passive layer preventing corrosion from occurring. The passive film is not indestructible, and it can be damaged by chemical and/or mechanical breakdown. Proper design and preparation of concrete would guarantee long-term durability to the system. Nevertheless some damage may occur during the service life and preventive measures are often used to further protect the structures. These include the use of cathodic protection, chloride removal, penetrating sealers, preventive and curative inhibitors, and coatings to control rebar corrosion and the improvement of the mechanical properties of concrete. These will extend the service life of the structure.

In recent years, the use of inhibitors has increased significantly being these chemical substances that decrease the corrosion rate of rebar steel in the corrosion system. The inhibitors are chemical substances organic or inorganic, which act under different concentrations and mechanisms, diminishing corrosion. Organic inhibitors may form a barrier over the metal surfaces and can affect the anodic, the cathodic, or both reactions. Inorganic inhibitors promote oxidation and formation of a protective passive film. A good review on the subject has been presented recently.^24,25

Concrete modification by using polymeric materials has been studied for the last four decades. Nevertheless, in certain applications these kinds of materials had failed, and it is necessary to use other complex and expensive technologies for specific applications. In general, the reinforcement with fibers or particles of brittle building materials has been known for long time, as well as, synthetic materials as polyvinyl alcohol, polypropylene, polyethylene and polyamides.^26–29

Micro- or macro-synthetic nylon fibers have been in use since early 1980s for secondary temperature-shrinkage reinforcement in concrete. In the microcase monofilaments and fibrillated shapes are used. Because these fibers are very thin, their number per weight (fiber count) is in the range of millions per kilogram of concrete.³⁰ Several improvements can occur when adding micro-synthetic nylon fibers to the concrete, typically at dosage rates of 0.6-0.9 kg/m³ of mixture:

a) Reduction in cracking, plastic shrinkage cracking, and plastic settlement. The reduction in cracking and settlement prior to setting produces concrete with improved long-term durability.

b) Improvement in high impact resistance by lowering the extent of stretching and pull-out of the fibers which occurs at large strains, resulting in failure of the matrix at relatively low loads.

c) Creation of a 3-D network of reinforcement with superior fiber/mix bonding. The integration of fibers disperses stress evenly throughout the reinforcement network, modifies the micro-macro cracking mechanism and enhances durability.

d) Reduction the number of bleeding channels and thus the bleed, resulting in less water migration to the concrete surface. This action helps to control the water/cement ratio and produces a concrete with less permeability, greater strength, and improved toughness.

The materials used so far to reinforce the concrete matrix are adhering only by physical interactions and not by primary chemical bonds. Other procedures used such as chemical attack or thermal treatment, are costly and time consuming.

The aggregates in concrete take up 75% of the total volume: clay, lime, organic matter or chemical salts, and so on. ASTM standards define the shape and size of aggregates.²⁸ Concrete manufactured from Portland cement is the most utilized due to its range of applications (structures, blocks, pavement, etc.) and the resulting properties including durability and plasticity.

Generally, simple concrete shows high compressive but low tensile strength. Thus, it is necessary to add various aggregates. The problem of adhesion of added polymers has already noticed. The polymer is surrounded by sand and gravel, facilitating the development of cracks when subjected to mechanical stress. Formation of cracks along with alternative mechanisms of polymer response under load, have been thoroughly reviewed.³¹

Improvement of concrete by using polymeric materials has been attempted before, Given the properties of concrete without such a reinforcement, the main objectives of adding them are high compressive and tensile strength, high impact and abrasive resistance, service in adverse environments (wind, moisture, etc.), lower weight, and lower costs.^32,33

The reinforced concrete structural element (embedded steel) is gradually facing electrolyte medium leakage due to contaminated water, which will foster corrosion and a long-term metal pulverization, mechanically unprotecting the structural element. Therefore, there is currently an arduous work for development of new construction material, new coatings, or the use of new inhibitors, to monitor the corrosion in the different structural elements and thus avoid major economic losses.

The possibility of using construction materials that decrease the embedded steel corrosion and harmonize together with this particular structure without affecting it mechanically has set the pace to study the use of polymer aggregates as a “smart inhibitor storage/carrier system” (smart system). An alternative explored was to activate the polymer surface by the use of gamma radiation or chemical attack, to have at least a chance to make it compatible with the brittle concrete matrix, and to “pack” the inhibitor within the cavities formed.

The smart system will be able to release the corrosion inhibitor at the time when oxidation reactions do occur; the inhibitor will move due to capillary action or other through the porous structure till reaching the metal to be protected and electrochemically reduced or, alternatively, promoting oxidation and passive film formation. In both cases it is estimated that with the proper release of the inhibitor and timely transportation of it, the steel corrosion rate will be significantly reduced. Also the presence of polymeric material within the concrete mixture as an additive, improves the mechanical properties of the concrete.³²

Figure 12.1 compares the compression resistance (%) as a function of concrete curing time, for concrete slabs without (f_c = 219.91 kg/cm²) and with nylon 6,6 pellets (f_c = 225.40 kg/cm²) as additives in the concrete matrix. An improvement of the mechanical properties of concrete was reached with the addition and compares favorably with reported results.³³

12.2.1 Polymer selection

The material additive used should not corrode, and it has to be able to store the inhibitor and should be compression resistant in order to harmonize with the concrete structure. With these considerations, the use of a polyamide such as nylon 6,6 is proposed. The commercial presentation of nylon may be fibers, blocks, bars, or pellets.

The polyamide has a smooth surface, for such reason the surface has to be exposed to chemical (different NaOH concentrations) or gamma irradiation (argon or air atmospheres) activation so it can be able to turn the surface porous or rugged to promote the adsorption of the corrosion inhibitor, as seen in Figure 12.2. The effect of chemical treatment in a sodium hydroxide solution at 80 °C and gamma irradiation are clearly visible over the surface of nylon 6,6 pellets, rendering them active forming mesopores over the surface.²³ The detailed procedure has been reported elsewhere.^34–36

Adsorption of the passivation inhibitor onto the nylon 6,6 pellets active surface is obtained filling up the nylon pellets, in acetone with up to 8% (80 mg/ml) of ferric nitrate [Fe(NO₃)₃]. Activated nylon 6,6 pellets are immersed in cycles to achieve maximum particles weight gain by the presence of the inhibitor.²³ The formation of seals on the ends of the activated polymer is based on the reaction between the inhibitor loaded and the transition Fe metal ions that brings insoluble complexes of ferric nitrate distributed at the ends of the smart system acting as seals or stoppers, forming in this way the smart system.

Surface roughness of nylon 6,6 is determined after activation through image processing and fractal dimension analysis.^37–42 Figure 12.3 presents the fractal dimension for gamma irradiated as a function of dose and chemical treatment as a function of time of immersion samples, and the roughest surface conditions were obtained. Chemically treated pellets were selected for inhibitor adsorption and further rebar concrete additives.

The choice of the Fe(NO₃)₃ passivation inhibitor concentration is performed based on a combination of different parameters such as inhibitor activity for corrosion protection of the steel, corrosion potential, pitting potential, passivation current density obtained using potentiodynamic polarization measurements.²⁴ They are performed in a saturated basic Ca(OH)₂ and CaCl₂ 1:1 solution (pH 12) simulating the concrete in solution environment, in the absence and presence of nylon 6,6 pellets containing the adsorbed inhibitor in different concentrations, as shown in Figure 12.4. More positive corrosion potentials are obtained as a function of inhibitor concentration. Also, the passive current is smaller for lower overpolarization, but the pitting potential became more negative, as shown in Figure 12.4a. The free corrosion potential and the noise current density as a function of time of immersion in chloride solution for rebar concrete, with and without the smart system as aggregates, are presented in Figure 12.4b. Both potentials increased similarly to noble values as a function of time, but noise current densities observed for the two systems did not. These results demonstrate the effect of the inhibitor release and corrosion protection activity.

The release of the adsorbed corrosion inhibitor may be triggered by the corrosion process itself, which prevents the spontaneous leakage of such inhibitor or the slow release from the smart system providing a response to changes in the service environment.⁴³

12.3 Electrospun Smart Coating

As far as metallic monuments, statues, and works of art are concerned, they account for the vast majority of existing and cultural heritage objects. Copper and its alloys play an important role as a base metallic material used by artists and architects. Corrosion processes is a complex phenomenon involving corrosion products or oxide film formation on copper and alloys. These products are formed by a number of brittle oxide and hydroxide surface layers, in many cases of different colors and textures, some of calcareous origin, including silicates, and all of them formed over the copper oxidation patina (typically cuprite).^44–48 Copper, commonly used in structures and sculptures, is usually covered with layers of corrosion products providing its aesthetic value and, at the same time, protecting the metallic substrate. Due to the increase in atmospheric pollution, these layers commonly dissolve when exposed to polluted environments.

Polymeric materials’ efficiency as corrosion protection coatings for metals increases when polymeric ligands are modified using corrosion inhibitors.⁴⁹ Nanocarriers with storage properties, liberating inhibitors in a controlled way, can be used to manufacture a new family of hybrid coatings that respond to changes within the coating environment or its integrity. Liberation of encapsulated inhibitors within nanostorage/carriers is activated by the corrosion process itself, preventing spontaneous leaking out of the inhibitor from the coating. With this restriction, it is possible to reach the double goal of corrosion protection by a barrier and inhibitor mechanism of metallic elements.

Electrospinning is a recognized technique to create polymer fibers with diameters ranging from 40 to 2000 nm.⁴⁸ Fibers can be electrospun direct from solution or from the fused material state, controlling the diameter size through adjustment of the surface tension, solution concentration, conductivity, and so forth.^48,50,51 Electrospinning occurs when the electric force of the solution surface overcomes the surface tension and triggers an electric spark provoking the solution to be expelled from the containing device (syringe) and the jet flow impacts, deposits, and is collected in a metal screen. When the expelled material dries out or solidifies, it forms an electric charged fiber, and this could be directed or speeded up by electric forces.^44–52 As the jet stretches and dries, radial electrical forces cause it to splash repeatedly, and the fibers dry and solidify.^53–55 The improvements in the electrospinning technique allow production of liquid crystals or other tailored systems and even thinner fibers.⁵⁶ The thermodynamic compatibility between polymers and corrosion inhibitors allows their combination as an integral material system, creating quasi-compatible compounds. This is the direction in which polymer-inhibitor materials science is progressing.⁵⁷

Inhibitors are compounds capable of diminishing metal corrosion even at very low concentrations in aggressive media. They change the electrochemical reaction kinetics producing corrosion, and the rate is significantly slowed down.⁴⁹ Corrosion processes include, in most cases, water molecules, and inhibitors are used in aqueous systems and in atmospheric corrosion protection. Nowadays, corrosion inhibitors do exist such as chromates, nitrites, benzotriazole, and other organic and inorganic compounds. Benzotriazole (BTAH) is an organic compound of low molecular weight with metallic corrosion-inhibitor properties, for copper in particular.

The next step in the fabrication of nanostorage/carriers by the authors,^53,54 suitable for self-repairing anticorrosion coatings, is to make them sensitive to the corrosion process, or another external trigger, in order to activate the release of the stored inhibitor species. The mechanism of these storage/carriers is the smart self-healing process, being able to regulate the storage/release of an inhibitor.^19,21

The direct introduction of components of the protective coating inhibitor often leads to the deactivation of the corrosion-inhibitor and polymer matrix degradation. An alternative approach is to produce a corrosion protection hybrid coating containing a commercial polymer varnish and electrospun nylon 6,6 nanofibers acting as a BTAH inhibitor storage/carrier and the whole as a “smart coating.” The objective is the possible application of this corrosion protection system to statues or works of art of cultural value, commonly made of copper or its alloys and exposed to atmospheric corrosion conditions.

To achieve these, the authors studied several system conditions to electrospin and trap the inhibitor. The concept of this smart coating is as follows: the scale of the storage/carriers is of nanometer dimensions, so these are matched with the corrosion inhibitor. When these storage/carriers are, for instance, mechanically deformed or the metal surface is corroding, the inhibitor is released slowing it down as a passive layer formed over the surface as a consequence of the changes in the chemical environment. Also, the intricate electrospun nylon 6,6/benzotriazole (Ny-BTAH) inhibitor film formed in combination with a polymer coating acts as a smart coating and barrier and tortuous path for the eventual diffusion of aggressive species reaching the metal surface.^53,54

Once coating films are elaborated, they have to be characterized by infrared spectroscopy (FTIR), where nylon 6,6, solid BTAH, and Ny-BTAH 20% electrospun fibers spectra are presented, as shown in Figure 12.5a. Absorption peaks for BTAH inhibitor and for nylon 6,6 are observed. The spectrum of Ny-BTAH 20%, presents peaks corresponding to the ones observed in the other spectra, indicative of the merging of nylon and BTAH in the film formed. The electrospun fibers were observed under SEM and the merged Ny-BTAH present a transparent quality, as observed in Figure 12.5b.⁵⁴ Afterwards, a waterborne alkyd varnish was painted over the electrospun nylon inhibitor (smart) coating, drying at ambient temperature for 24 h. Good adhesion properties are determined after testing.

Once coated copper electrodes are prepared, electrochemical evaluation is realized, and the results are presented below. Figure 12.6a shows the free corrosion potential as a function of time of immersion, comparing bare copper (blank), copper varnish coated, and copper smart coating immersed in a chloride-sulfate solution. The noblest potential is observed for copper smart coating, followed by the copper varnish coated presenting similar behavior to the bare copper sample, as seen in Figure 12.6a. At about 350 h of immersion the decreasing potential of the smart coating sample reached the steady state values of the other two. This behavior could be ascribed to the difficulty of aggressive anions reaching the metal surface due to limitations in the diffusion path promoted by the nylon/inhibitor fibers and varnish coating action.

To test the “intelligence” of the smart coating produced after 500 h immersion, a mechanical coating simulating an in-service coating damage was produced scribing the coating surface with the tip of a cutter. A change in the noble direction was registered, returning back to more active potential values but increasing again, therefore showing better anticorrosion performance. The total impedance value acts accordingly.

The averaged total electrochemical impedance modulus observed for bare copper, copper varnish, as well as for the copper smart coating samples as a function of immersion are presented in Figure 12.6b. Higher total impedance values are observed, for the smart coated, as compared to the bare copper and copper varnish samples. Bear in mind the scratch made after 500 h, where the impedance drops markedly for the smart coating sample, increasing again afterwards till the end of the time of immersion. The potential and total impedance change suggests that corrosion and oxide film formation is taking place over copper exposed by the damaged area and promoted by the action of the BTAH inhibitor release.⁵⁴ In general it can be said that the total impedance values remained higher because of the inhibitor action and are considered as good impedance coating after 900 h in the aggressive conditions, confirming the “intelligent” action of the smart coating.

To observe the damaged scribed surface morphology, SEM micrographs are obtained and presented in Figure 12.7a, and the elemental chemical analysis performed to the sample is shown in Figure 12.7b, presenting corrosion products with elements associated to the electrolyte such as sodium, chloride, and sulfate. Elements revealing the presence of the polymer and inhibitor like carbon, oxygen, and nitrogen are also present. Also copper as metallic substrate and particles of iron associated to the cutter used to cause the artificial damage over the polymer coating is observed. All these suggest the probable formation of a layer of a polymeric compound from the ligand action of BTAH inhibitor, as reported.^58,59

The results obtained support the following mechanism for copper-BTAH inhibitor film formation in chloride solution⁵⁸

$4\text{Cu}+8{\text{Cl}}^{-}+{\text{O}}_2+2{\text{H}}_2\text{O}\Rightarrow 4{\text{CuCl}}_2+4{\text{OH}}^{-}$

${\text{CuCl}}_2+n\text{BTAH}\Rightarrow {\left[–\text{CuBTA}–\right]}_n+n{\text{H}}^{+}+2n{\text{Cl}}^{-}$

The possible formation of this polymer will further protect the damaged area, passivating the surface with the help of the smart coating system proposed to act in aggressive atmospheric conditions.

12.4 Sol-Gel Coatings for Corrosion Control

The sol-gel route for processing inorganic materials can be tracked to the beginning of the twentieth century with studies based on silica gels carried out mainly by Graham and Ebelmen (1842).⁶⁰ These research results led to the conclusion that the use of tetraethyl-orthosilicate (TEOS) as a precursor in an acidic environment formed a glass-like material, which was a viscous gel with different applications (optical, ceramic) and considered a composite. These research results led to the conclusion that the hydrolysis and polycondensation of silicon precursors, such as the tetraethyl-orthosilicate (TEOS) in an acidic or basic environment. The growing of the inorganic network renders first a colloidal suspension or so and, the continuity of the condensation reactions increases the size and complexity of the network until a glasslike material or gel is formed. The gel consists of a solid network with remnant liquid trapped in its pores or a viscous gel (optical, ceramic) because this new material is considered a composite. Waterless gels present cracks, thus making them difficult for other applications.⁶¹ However; the inorganic network is susceptible to different applications after the proper chemical, thermal, or physicochemical treatment. These procedures have been named the sol-gel method, and because the hydrolysis and polycondensation reactions occur under relative soft chemical and thermal conditions, it was considered a soft chemistry, or chemi deuce.^62,63

In the 1970s the new formulations were synthesized because the former gels had presented defects. In 1971 the precursor TEOS was synthesized in conjunction with cationic surfactants, which led to the production process named low-bulk density silica sol-gel.⁶⁴ Since then, many scientists^65–68 have synthesized new formulations in order to enhance the sol-gel properties. From that moment, the synthesis of organic-inorganic hybrid sol-gels began to reveal wider applications than those observed with the former sol-gels.

Since that discovery, different applications in the form of coatings have been produced using this sol-gel route. The sol-gel coatings present numerous advantages as compared with other processing technologies, such as a high purity and homogeneous matrix, and low curing temperatures of processing (< 500 °C). Furthermore, the soft conditions usually employed in the sol-gel process allow the physical trapping of diverse inorganic, organic,^69–71 biochemical,^72,73 or biological^74,75 species inside the pores of the network.

This fact maintains the base substrate properties unaltered mainly in metallic materials. During the past two decades, the sol-gel route has extensively enhanced its applications because of its versatility in combination with other materials that lead to more uniform and more resistant coatings. Furthermore, this new synthesis route prepares a new family of ceramic glass coatings which are applicable mainly in electronics, optics, ceramics, automotive, biotechnology, and corrosion.^76,77

Hybrid sol-gel coatings from the standpoint of corrosion resistance offer wide possibilities because brittleness and high temperatures are avoided. The single inorganic oxide sol-gel layers usually present cracks and voids after the curing treatment. Therefore, hybrid sol-gel coatings, which have low viscosity, allow easy casting into a mold during the deposition procedure regardless of the technique used for this purpose.^78–80 Dip-coating and spin-coating, which are the two most common deposition methods,^81,82 offer advantages as compared to spraying or electrodeposition procedures. These advantages are homogeneous thickness and low superficial defects such as pores (see below).

An example of the influence of the corrosion properties as a function of the deposition technique, (Figure 12.8a and b) shows an aluminum-coated sample immersed in NaCl 0.1 M after 48 h. As can be seen the best resistance properties is given by the spin-coating and the dip-coating techniques with final impedance (resistance) at low frequencies up to 10⁵ Ω-cm². The paint brush and the spray technique offered the lowest resistance properties. All samples had similar average thickness around 4-5 μm thus it is clear from these results that dip-coating and spray painting techniques increased the number of defects in the coating.

The hybrid sol-gel coatings also offer additional advantages after the coating deposition, which are the use of a low processing temperature and the final characteristics of a hydrophobic/hydrophilic matrix. The inorganic part generally controls brittleness, hardness, and a degree of transparency after the curing time process while the organic counterpart governs density, thermal stability, and superficial properties such as porosity.⁸³ If the organic part or molecules are solely entrenched within the inorganic matrix, weak bonds such as Van-der Walls forces or electrostatic bonds are formed. On the other hand, when the organic and inorganic parts establish strong interactions among all molecules, the formation of covalent bonds is the main bonding force.

From the early 1990s, some other materials have been synthesized from concentric nanoparticle multilayers to improve the properties of the final substrate. Thus, the synthesis of particles with useful dual properties, for instance magnetic and luminescent, is a very attractive possibility. As a consequence new methodologies for the synthesis of nanoparticles composed of two or more materials, named core/shell nanoparticles, have been developed.^84–87 The sol-gel method allows the possible synthesis of a network of transition metals oxides such as TiO₂, ZrO₂, ZnO, and Al₂O₃.⁶¹

Additionally, these hybrid materials can also be employed when fibers, nanoparticles, or nanocontainers are incorporated into the network. Depending on the nature of the species in the coatings, different categories of components can be described according to their chemical bond with the inorganic/organic matrix.⁸⁸ In order to obtain a gel, different basic steps are needed for this process^68,76: mixing, gelation, aging, drying, and sintering. The application of the sol-gel process for the synthesis of new hybrid materials, and production of catalyst, biosensors, or hybrid materials for drug or anticorrosion delivery, are modern emerging interdisciplinary areas.

12.4.1 Corrosion applications

Sol-gel methodology intended to protect substrates against the corrosion process has grown in the last 15 years. The former inorganic coatings containing silica-oxides were evaluated as protective layers because of their high chemical stability and excellent wear-erosion resistance.⁸⁹ The SiO₂, ZrO₂, Al₂O₃, and TiO₂ based coatings are generally excellent layers for the prevention of oxygen diffusion in metal substrates.^90,91 Bi-ceramic coatings based on TiO₂-SiO₂ formulations have provided high resistance to metal oxidation for 15 h at 800 °C. However, their oxidation resistance in acidic conditions (H₂SO₄) was poor. The ZrO₂ coatings are improved with the incorporation of Y₂O₃ because the yttrium has a great affinity with oxygen to form a stable layer. Nevertheless, the corrosion resistance is a function of the internal or superficial defects formed after a high sintering temperature.⁷⁹ The differences between thermal expansion coefficients of the coating and the substrate impeded the formation of thicker layers without cracks. For this reason, hybrid sol-gel coatings are formulated in order to prevent high curing temperatures leading to thicker layers (micrometers from single layer deposition). Thus, superficial defect-free coatings with better adherence and flexibility properties can be achieved.^92,93

As previously mentioned, the incorporation of TEOS has led to a less viscous gel with wider applications. The combined use of TEOS with an organo-modified alkoxide such as the methyl-triethoxysylane (MTES) in a molar ratio of 40/60 allows free-crack coatings up to 2 micrometers in thickness to be obtained from a single layer deposition.⁹⁴ The alkaline conditions in which the TEOS/MTES proceeds promote the formation of nanoparticles of diameter sizes below 20 nm, which behave as dense particulates. These conditions induced a high hydrolysis reaction rate, which needed water content in order to promote the slow formation and growth of the agglomerates within the gel network. This type of gel has successfully been employed in the corrosion protection of galvanized steel.⁹⁵ Other studies have demonstrated that the electrophoretic deposition of particulate sols with basic catalyst allows thickness coatings between 2 and 10 μm. This route promotes not only thicker coatings, but also denser layers because of the electric field that easily attracts the colloidal particles to the metal substrate, thus reducing the porosity of the coating and improving the corrosion resistance in stainless steel.⁹⁶

However, despite all the modified routes, the synthesis of silica hybrid sol-gel coatings still persist as limitations for corrosion protection because these coatings do not provide good barrier properties in solutions of acidic or neutral pH. To overcome this limitation, the incorporation of other oxides such as zirconium, titanium, and aluminum has enhanced the corrosion applications from basic to neutral media. The development of ZrO₂/SiO₂ coatings through hydrolysis and polycondensation reactions of a zirconium tetrabutoxide solution with MTES at different molar ratios proved to be effective in the formation of Si-O-Zr bonds at sintering temperatures below 700 °C. Examples of low sintering temperatures (400 °C-500 °C) are those obtained from the TEOS/MTES synthesis and those synthesized by a polymethyl methacrylate (PMMA) dispersion in a zircon sol which sintered at 200 °C during a period of 30 min, thus improving the corrosion resistance of stainless steel. Nevertheless, these developments were dependent on the amount of organic phase during the synthesis process because high organic proportion could initiate segregation of phases and then coating delamination.

The 3-glycidyloxypropyl-trimethoxysilane (GPTMS), which belongs to organo-modified silicon precursors, RSi(OR)₃, where R represents organic groups such as epoxy, vinyl, metacryl, in conjunction with SiO₂ solution and acid catalyst, has permitted the formulation of new coatings for aluminum alloys with sintering temperatures below 200 °C.^96–98 These coatings have had a nominal thickness between 5 and 10 μm. From this standpoint, numerous combinations have been proposed in order to improve the corrosion protection of aluminum alloys employing the sol-gel route. Our research group has made hybrid coatings of tetra-n-propoxyzirconium [Zr(PrOⁿ)₄ or TPOZ] and 3-glycidyloxypropyl-trimethoxysilane on an AA2024-T2 aluminum alloy.

Coating protection behavior is analyzed in a 0.1 M NaCl solution by means of electrochemical impedance spectroscopy at room temperature. The hybrid sol-gel coatings are doped by the incorporation of 1% or 5% cerium nitrate as a corrosion inhibitor. In addition, nanocontainers SBA15 (Santa Barbara Amorphous Material 15) saturated with a cerium compound are also loaded into the sol-gel formulation in order to compare their protection against corrosion with that obtained from the addition of free Ce^3 + coatings. The cerium compounds that belong to the green inhibitors to enhance corrosion resistance combine barrier properties with an active inhibition mechanism in a damaged coating. According to Figure 12.9a and b, the sol-gel coatings loaded with saturated SBA nanocontainers exhibited the best dielectric properties after 48 h. Therefore, cerium containing coatings combined during the synthesis process do not provide better corrosion resistance on Al 2024-T3 alloy; hence, in order to provide a better inhibition effect against the corrosion process, the inorganic inhibitor works better when it is released slowly.

The sol-gel development has led to new protective coatings focused on new hybrid organic-inorganic systems incorporating not only modified alkoxides, but also nanostructured particles with the blocking effects of the aggressive species. Our research group has also incorporated nylon fibers into the sol-gel system through the electrospinning technique. The results of the electrochemical properties carried out with the EIS technique are presented in Figure 12.10a and b, where the effect of the electrospinning time on the fiber density can be seen (Figure 12.11a and b). From the Bode plots it is noted that the highest dielectric properties are ascribed to electrospun nylon for 2.5 min with a low frequency impedance modulus, in the order of 10⁵ Ω-cm², which remains unchanged after 45 days of immersion. The average coating thickness is around 4 μm, and its micrograph corresponds to that presented in Figure 12.11a. As can be seen, the nylon fibers created a web completely covered by the sol-gel whereas the fibers shown in the micrograph ascribed to electrospun nylon for 5 min do not cover the entire surface. These results suggest that the sol-gel needed a specific anchorage profile to be deposited onto the nylon fibers. The high density of nylon fibers obtained at larger electrospin times seemed to decrease the adherence properties, thus decreasing the protective properties as well.

Consequently, the adequate selection of precursors, the molar ratio between organic/inorganic phases, a proper catalyst agent, and synthesis-condition parameters will allow denser and thicker coatings with low sintering temperatures capable of being used in other substrates such as the low-melting temperature alloys of aluminum and magnesium.^99,100

12.5 Conclusion

In this chapter, novel applications of nylon-activated particles or electrospun fibers in combination with inhibitors, polymer, and sol-gel coatings are presented. These examples form what are considered smart coatings and systems. Good promise can be envisage in these examples, as to the development of more efficient systems of this type.