Sol-Gel Coatings with Nanocontainers of Corrosion Inhibitors for Active Corrosion Protection of Metallic Materials
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This chapter provides an overview of corrosion protection strategies using sol-gel coatings. The chapter starts by acknowledging current state of corrosion engineering and main directions in corrosion protection of metals specifically addressing how sol-gel systems differ from other coatings types. The examples of application of sol-gel coatings as barrier pretreatments have been presented. With that some preliminary conclusions have been made emphasizing the important need for active corrosion protection using inhibitors. Simple approaches presenting inorganic and organic corrosion inhibitors incorporation to hybrid sol-gel coatings are described. The corrosion performance and associated drawbacks of such approach are presented. The chapter also critically reviews different smart protection systems with controlled inhibitor release based on “smart” micro- and nanocontainers. The inhibitor loading strategies to nanocontainers, types of containers, performance, and associated protection mechanisms are critically discussed. The chapter concludes with final remarks regarding the utilization of sol-gel protective coatings combined with corrosion inhibitors.
KeywordsCorrosion Inhibitor Corrosion Protection Metal Alkoxide Mesoporous Silica Nanoparticles Organic Inhibitor
Application of protective coatings is one of the main approaches to combat the corrosive degradation of different metallic systems. The protective coatings can be composed by metallic, organic polymer, inorganic, or hybrid polymer layers applied using various techniques such as spraying or electrodeposition, just to mention a few. The main function of the protective coatings is to ensure a physical barrier between the metallic surface and the environment limiting penetration of the corrosive species. Since over two decades, sol-gel-based systems are also considered as potential candidates to be used in corrosion protection. Sol-gel technologies allow variation of the coating composition across a wide range from fully inorganic layers to hybrid polymers with high content of organic constituents. Such flexibility makes the sol-gel process a versatile tool to design protective and functional coatings.
An important advantage of sol-gel layers is their high adhesion to the metallic surfaces ensured by formation of Met1-O-Met2 or Si-O-Met bonds. However, the purely inorganic sol-gel layers are normally not able to ensure a good barrier and in most of the cases can be used only as a pretreatment before application of polymer barrier layers. Hybrid sol-gel coatings are much more versatile in this sense. They can be applied as very thin submicron films as well as thick, over 100 μm, coatings. The main function of thin films normally is still to provide an improved adhesion between the top paint layer and the metal. The presence of organofunctional groups in the hybrid sol-gel plays a crucial role in this case conferring possibility for covalent bonding sol-gel/paint interface. Thus, the multilayer protective coating systems can be fully covalently bonded on both interfaces. The thicker hybrid sol-gel layers can perform as stand-alone protective coatings with good barrier properties. There is already a wide range of sol-gel products as pretreatments and one layer full coatings available in the market. BoegelTM is one of the examples of successfully implemented thin hybrid sol-gel layers as pretreatment for aeronautical applications.
The conventional sol-gel coatings can provide only a passive corrosion protection as other paint systems. However, during the service life of coated metallic structures, the rapture of barrier is unavoidable as a result of external mechanical impacts or due to formation of pinholes and cracks. The corrosion processes can significantly accelerate and become uncontrollable in such defects. Therefore an active corrosion protection in the defects is needed in order to provide effective long-term protection. Suppressing the corrosion activity in the defects by the coating itself can be considered as a self-healing effect. There are different ways how to create such active protection coatings employing different self-healing mechanisms . One of the approaches is based on the mechanical healing of the defects in the polymer layers by a reversible polymerization of the broken bonds. The concepts based on thermo-activated reactions such as Diels-Alder/retro Diels-Alder or on polymers with weak reversible bonds are often suggested. Polymerizing the monomer released after rupture of the capsules introduced to the polymer coating matrix is another possibility. In the case of corrosion protective coatings, a full recovery of mechanical integrity is often not achievable especially when wide defects appear in relatively thin films. Different active protection mechanisms based on trapping the corrosive species or hydrophobization of the defects were recently reported. However, the main mechanism for self-healing coatings is still grounded on the implementation of corrosion inhibition concept.
Combining several active protective mechanisms in the same coating was also recently suggested as a multilevel active corrosion protection concept which can ensure a long-term effective protection of coated metallic structures.
This chapter is giving a brief overview of the protective sol-gel coatings with a special emphasis on the hybrid layers containing corrosion inhibitors. Different approaches for incorporation of the corrosion inhibitors into the sol-gel coatings are addressed. The self-healing properties based on the nanocontainers of corrosion inhibitors are in the main focus.
Chemistry of Sol-Gel
The rate of hydrolysis reaction is commonly higher than the rate of condensation process. The reactions (2 and 3) can continue building branched network with –O–M–O– bridges by the process of polymerization until gel (branched network), colloid, or precipitates form. Such structures can be obtained by controlling the parameters of the sol-gel process. The most important parameters that influence the hydrolysis and condensation reaction are water-to-alkoxide ratio r, pH, temperature, concentration of reagents, complexing agents, and nature of the alkoxide groups. The variation of such parameters can influence not only the kinetics of the sol-gel process but the micro- and nanostructure of the final product (Brinker and Scherrer 1990).
The main basic aspects of hydrolysis and condensation processes and the use of inhibitors for metal alkoxide hydrolysis were discussed in (Schubert et al. 1995; Judeinstein and Sanchez 1996). The reactivity of metal alkoxides depends on the properties of the metal (M) and nature of the alkoxy group. The hydrolysis reaction involves a nucleophilic attack of the M atom. Therefore, the electronegativity of an M atom and the coordinative unsaturation degree are crucially important parameters for controlling the reaction kinetics. Metal alkoxides are more reactive when M has lower electronegativity and higher coordination unsaturation. As Si has high electronegativity and no coordination unsaturation in the Si-alkoxide molecule, thus it has a very low reactivity. The reactivity toward the hydrolysis increases in the following order: Si(OR)4 ≪ Ti(OR)4 < Zr(OR)4 < Ce(OR)4. An additional catalysis is required to accelerate the hydrolysis of Si containing metal alkoxides. Acetic acid, different mineral acids, amines, alkali metal hydroxides, and Lewis bases can be used for this purpose.
On the other hand, transition metal alkoxides are very reactive; thus, their reaction kinetics must be controlled by inhibitors. Complexing agents such as different β-diketones, carboxylic acids, glycols, and others can act as inhibitors. Nucleophilic agents (inhibitors) in most cases react with metal alkoxides via SN mechanism. However, when alkoxides are coordinatively unsaturated, the reaction of an inhibitor with the metal alkoxide can proceed via AN mechanism. The resulting product of the metal alkoxide with the complexing agent is more stable toward the hydrolysis or condensation reactions. In excess of water, the (Me–O–R) bonds are slowly broken, thus allowing a higher control over the hydrolysis reaction.
Liquid sol-gels can be applied on substrates using different techniques including dip coating, spin coating, spray coating, brushing, rolling, and others followed by a drying/curing step. The application methods, humidity, temperature, and time of drying/curing may affect the quality of the final coated product.
Sol-Gel Pretreatments for Metals
Sol-gel technique can be successfully applied for preparation of different types of materials such as powders, bulk glasses, xerogels, membranes, ceramic composites fibers, and others (Brinker and Scherrer 1990). There are many fields where sol-gel-based material applications were found. Hybrid sol-gels are also actively used for production of advanced materials employed in optics, electronics, biomaterials, and chemical engineering. Among those, corrosion protection technology uses sol-gel materials mostly as coatings for corrosion protection of various metals and alloys (Guglielmi 1997).
Organic coatings have been one of the primary corrosion protection systems for metals (Gordon Bierwagen 1996). A thick and undamaged polymeric film provides a significant barrier to the transport of ions or charge. Polymeric coatings are used as a part of general corrosion protection scheme for different metals. In general a metal surface has to be properly pretreated in order to afford a good bonding strength to an organic coating layer. A multilayer protection scheme is typically used in more demanding applications, e.g., aerospace industry, where corrosion protection is crucial. This scheme consists of several levels of protection including a pretreatment, an organic primer coating, and a thick polymeric coating. The primer coating often includes inhibiting additives and provides a better link between the pretreated surface and the organic coating. Sol-gel-based coatings provide important advantages as pretreatments for different metals among the existing different inorganic pretreatments (Critchlow and Brewis 1996; Twite and Bierwagen 1998). Some of the sol-gel pretreatments are successfully commercialized and applied in aeronautical industry. Boegel™ is a good example.
Secondly, sol-gel coatings may offer good adhesion to organic paints leading to higher barrier properties and corrosion protection of the entire protective system (Fig. 1b). In the aeronautical industry, as an example, many paints consist of epoxy-functionalized components cross-linked by amine-containing agents. To ensure good adhesion, the sol-gel coating must be chemically bonded to the paint layer. Sol-gel solutions prepared with only metal alkoxide precursors produce purely inorganic coating matrix. Purely inorganic coatings may suffer from the extensive stress upon drying which leads to cracks formation during drying. Cracks may be produced during the curing process due to shrinkage and thermal expansion mismatches between the metal and inorganic layer. Moreover, extensive capillary forces building up during drying may rupture stiff inorganic matrix . Hybrid sol-gel films in contrast to pure inorganic ones combine properties of organic polymeric material and properties of ceramic. The inorganic components mostly contribute to the increase of scratch resistance, durability, and adhesion to the metal substrate. The organic component increases density, flexibility, and functional compatibility with organic paint systems. Sol-gel hybrid materials containing both inorganic and organic groups are often called organically modified silicates (ORMOSILS) (Schmidt 1985).
There are two major classes of hybrid materials which can be used for surface applications (Judeinstein and Sanchez 1996; Schubert et al. 1995). In the first class, inorganic sol-gel matrix is physically mixed with the organic matrix. The formed composite possesses properties of the both phases. The interactions between these phases are weak. In the second class, chemical bonds are formed between the inorganic and organic parts by means of functional groups or organomodified precursors (mostly by organomodified silanes). Such silanes can be represented by a formula (RO) 4-n -Si-(R1-Y) n where RO- is an alkoxy group, R1 is an organic part, and Y is an organic functional group . Depending on the nature of the organic group Y, the second class of materials can be subdivided to cases in which the Y group is nonreactive (inert) and reactive. In the latter case, the organic functional groups Y can react between themselves in the organic matrix or with other organic functional groups such as those present in paints. Different alkyl groups are typical nonfunctional organic groups present in nonreactive hybrid networks (Sheffer et al. 2003; Metroke et al. 2004). Commonly used organofunctional groups for building hybrid materials are epoxy (Hoebbel et al. 1998; Metroke et al. 2002) and methacrylic (Chou et al. 2003; Smarsly et al. 2003). Such groups are used for synthesis of hybrid organo-inorganic materials since they can provide additional polymerization and cross-linking of hybrid sol-gel matrix using appropriate initiators or cross-linking agents. Functional groups that are also used in organofunctional precursors are phenyl (Sheffer et al. 2003), amino (Mayrand et al. 1998), and alkyd (Ballard et al. 2001). The covalent bonding at metal and organic paint interfaces is often provided by the effective bonding agents and adhesion promoters on the basis of siloxanes, e.g., Oxsilan®.
Sol-Gel Layers for Passive Corrosion Protection
The main advantages of using sol-gel coatings are good adhesion , dense matrix, tailored surface properties, chemical resistance, etc. Sol-gel synthesis can employ various precursors which allow producing a broad range of inorganic and hybrid materials. The simplest purely inorganic sol-gel coatings can be prepared using only metal-organic alkoxides. Such inorganic coatings are normally sintered at high temperatures, e.g., >300 °C at which the matrix cross-linking is improved and the inorganic matrix becomes denser. However, the intact and uniform coatings can be only obtained with a thickness of less than a few hundred nm. At higher thickness the coatings usually contain many cracks and are significantly damaged. Such damage is caused by the shrinkage of sol-gel matrix during the drying step and by different thermal expansion coefficients between the substrate and the coating during heating/cooling. Inorganic sol-gel coatings are mostly used for metal protection against high-temperature oxidation or other aggressive conditions (Vasconcelos et al. 2000; Miyazawa et al. 1995).
In contrast to purely inorganic coatings, hybrid sol-gel coatings are typically cured at temperatures below 200 °C. A lower temperature or even room conditions are often used since it does not lead to excessive organic functional group oxidation. Moreover, some alloys do not tolerate high temperature which may affect the microstructure of the alloy’s matrix and lead to weakening of mechanical properties. Hybrid sol-gels may contain organically modified silanes or a mixture of silanes with metal alkoxides. Using the right proportions of precursors is crucial in forming a flexible yet compact hybrid network which is able to withstand mechanical stresses during the drying process. Therefore thick and crack-free coatings can be prepared using hybrid sol-gels (Ono et al. 2004). Moreover, hybrid sol-gel coatings have better mechanical properties and adhesion to organic paints compared to the sol-gel coatings based on nonfunctional organosilanes (Joshua et al. 2001). Hybrid coatings may show lower hardness than purely inorganic ones due to excessive amount of organic component. Such drawback can be eliminated, if needed, by incorporating different oxide nanoparticles to the sol-gel matrix. Nanoparticles of silica, zirconia, ceria, and others were successfully used as fillers in sol-gel coatings applied onto different metallic substrates (Conde et al. 2003; Schem et al. 2009; Chen et al. 1998).
Sol-gel-derived coatings have been suggested for corrosion protection of different metals and alloys (Metroke et al. 2001; Zheludkevich et al. 2005c; Wang and Bierwagen 2009, Bera et al. 2015). Many works reported that corrosion resistance of metals coated by hybrid sol-gel coatings can be significantly improved. Corrosion protection properties of sol-gel coatings were studied in application to different metals such as aluminum and aluminum alloys (Conde et al. 2003; Metroke et al. 2002, 2004; Bera et al. 2015) (Parkhill et al. 2001; Chen et al. 1998), steel (Ono et al. 2004; Chou et al. 2001, 2003;), zinc (Bera et al. 2016), copper and copper-containing alloys (Bescher and Mackenzie 2003; Rao et al. 2011; Li et al. 2009), and magnesium alloys (Tamar and Mandler 2008; Zomorodian et al. 2012; Tan et al. 2005). Sol-gel coatings may include various inorganic precursors and organic functional groups (Chen et al. 1998). The presence of amino groups in the sol-gel matrix in some cases may deteriorate barrier properties of the sol-gel coatings (Bera et al. 2015). On the other side, various cross-linkers can also increase the polymerization degree of the sol-gel matrix giving consequent positive impact on the barrier properties of the layers. Mayrand et al. (1998) reported that amino groups present in the sol-gel layer improved both corrosion protection of electrogalvanized steel and adhesion between the sol-gel and primer coatings. Similarly, corrosion protection properties of the sol-gel coatings deposited on AA2024 substrates were improved by impregnation of the epoxy-SiOx sol-gel matrix with different amines (Khramov et al. 2003). The influence of the nature of alkyl radicals in organically modified sol-gel-based coatings on corrosion protection was investigated by Metroke et al. (2004). It was found that the higher alkyl chain size contributes to the higher film’s thickness and barrier protection. In general, sol-gel coatings which possess high hydrophobic nature offer the highest degree of corrosion protection to metallic substrates (Sheffer et al. 2003; Metroke et al. 2004). Nevertheless, sol-gel pretreatments without specific additives cannot achieve active corrosion protection and self-healing of corrosion on metals.
Hybrid Sol-Gel Coatings for Active Corrosion Protection
In spite of relatively good barrier properties at even reasonably small thickness (ca. 1 μm), the sol-gel layers cannot serve as a long-term protection solution for demanding applications where integrity of the polymer can be disrupted by mechanical or other environmental impacts. This is due to the fact that sol-gel coatings provide only passive protection acting as a barrier against corrosion species. In reality even immediately after application, any coating matrix contains intrinsic defects and imperfections such as micropores, cracks, and areas of low cross-linking density which provide pathways for diffusion of corrosive species. Thus, sooner or later corrosion process starts in the defects initially present in the coating or induced during the service. One of the approaches to control the corrosion processes in such defects is to utilize corrosion inhibitors.
Potential chemical interactions between inhibiting species and the sol-gel components
Effect of inhibitor on adhesion
Inhibitor concentration in sol-gel
Kinetics of inhibitor release
Solubility of inhibitors in corrosive electrolyte
Firstly, the inhibitor concentration in coating matrix must be in a certain range. This range is restricted by the minimal effective concentration of the inhibitor needed to provide the inhibition action and the maximal concentration of the inhibitor in the coating matrix at which barrier protection or adhesion is not significantly affected. Inhibitors with low effective concentration are more efficient in this case since active corrosion protection can be achieved even after a small amount of inhibitor is released from the matrix into a coating defect .
Secondly, the choice whether an inhibitor is suitable for incorporation into sol-gel should be based on the analysis of possible chemical interactions between the inhibitor molecules and the matrix. There are several methods to incorporate inhibitors into sol-gel coatings (Osborne et al. 2001). Adding the inhibitor directly into a sol-gel matrix is the simplest way. It can result in a physical mixing with no chemical interactions between the inhibitor molecules and the sol-gel network. In this case the sol-gel matrix acts as a container for inhibitor which leaches out in contact with the corrosive environment. In other cases the inhibitor can chemically interact with the components of the sol-gel formulation. The hybrid polymer matrix becomes chemically bound to the inhibitor molecules through inorganic or organic functional groups . Organic inhibitor molecules can also develop weak chemical interactions with functional groups like phenyl, vinyl, and mercapto. Such weaker interactions do not significantly limit an inhibitor’s ability to leach out in contrast to the case when an inhibitor forms chemical bonds.
Using different chemistry of the sol-gel matrix and the inhibitor, it is possible to control the kinetics of release of active species to the corrosive media. The concentration of inhibiting compounds in the corrosive environment in confined defects should be maintained at a level sufficient to provide the inhibition action. At very low concentrations, the inhibitor simply does not ensure an effective inhibiting action. On the other hand, a fast release of the inhibitor from the matrix can lead to its fast depletion limiting the active protection effect only to initial stages of service. Adding a highly water-soluble inhibitor to a sol-gel coating can lead to the excessive osmotic pressure causing blister formation and fast destruction of the barrier layer . Thus, different factors must be carefully weighted and studied when developing active corrosion protective sol-gel coatings loaded with corrosion inhibitors.
Despite the potential obstacles associated with the direct addition of inhibitors to sol-gel coatings, such coating systems have been extensively studied for protection of various metallic substrates. Below different examples of sol-gel-based protective systems with incorporated inorganic and organic inhibitors are briefly addressed.
Many different inorganic ionic species such as phosphates, vanadates , molybdates, permanganates, phosphates, rare earth metal (RE) salts, salts of some transition metals, and others are known to be reasonable corrosion inhibitors for different kinds of aluminum alloys , steels, zinc, and magnesium alloys. Among others the rare earth metal salts are effective inhibitors of the corrosion of aluminum alloys, steel, and zinc (Hinton 1992). Cerium and other (RE) cations got attention due to their “green” nature and efficient cathodic-inhibiting properties. Dozens of works have been published on the use of RE salts as active corrosion protective additives in sol-gel coatings. In general most of the studies demonstrate an additional active protection effect when RE inhibitors are added into the sol-gel matrix. Garcia-Heras et al. showed that the critical concentration of the cerium nitrate inhibitor introduced to the hybrid sol-gel matrix (3-methacryloxypropyltrimethoxysilane (MAP) + tetramethoxysilane (TMOS)) is in the range of 0.2–0.6 wt.%. A higher concentration of Ce cations led to formation of defects in the sol-gel network (Garcia-Heras et al. 2004). Rosero-Navarro et al. (2010a) studied the effect of different cerium nitrate concentrations in silica-methacrylate sol-gel network on the coating stability. This study was mainly accomplished using IR and UV-Vis spectroscopy and rheology tests. The chemical structure of the sol-gel was based on tetraethoxysilane (TEOS) and MAP. It was shown that the maximal critical concentration of Ce in the sol-gel solution was about 5 mol. % with respect to alkoxides. It is worth noting that the critical concentration of Ce inhibitor depends on the chemical composition of the sol-gel matrix and must be carefully chosen. When concentration of Ce cations in a sol is too high, it can cause fast gelation of the formulation making the sol-gel unsuitable for coating application (Rosero-Navarro et al. 2010a).
The corrosion protection efficiency can be improved when different RE mixtures are incorporated to a sol-gel coating matrix. For example, the work of Wittmar et al. 2012 demonstrated that the mixtures of different RE inhibitors such as Ce(III) and Pr(III) together with inorganic inhibiting anionic species such as PO4 2− and Zr(IV) can be beneficial for corrosion protection of aluminum alloy 2024 coated with hybrid sol-gel coating. Most probably the good inhibition is achieved in this case, thanks to the synergistic effect provided by the mixture of different inhibitors.
Voevodin et al. investigated the corrosion protection properties of inhibitor-containing sol-gel coatings applied on AA2024. The coatings contained 4–5 wt.% of different inorganic inhibitors such as Ce(NO3)3, Na2MoO4, NaVO3, and Na2Cr2O7 (Voevodin et al. 2001a). Potentiodynamic polarization measurements demonstrated a reasonable performance of the sol-gel coatings doped with the cerium salt. On the other hand, sol-gel coatings loaded with molybdate or vanadate salts did not show any adequate corrosion protection. The coatings had poor barrier properties due to the decreased sol-gel matrix stability. Moutarlier et al. has found similar tendency when incorporating permanganates and molybdates to the sol-gel coating matrix (Moutarlier et al. 2008). Sol-gel coatings prepared with highly soluble inhibitors often contain many defects. Moreover analytical measurements of the inhibitors concentration in coatings using glow discharge optical emission spectroscopy revealed the fast inhibitor release from the sol-gel coatings after immersion. As has been mentioned above, in order to control the release of inhibitor from the coating, the inhibitor molecules can be bound to the hybrid matrix. Such approach was presented by Mascia et al. (2006). Molybdate ions were bound to amine molecules and mixed with the hybrid sol-gel matrix. The inhibitor release kinetics was significantly lower in this case compared to that of the standard epoxy coating loaded with molybdate inhibitor .
The highly soluble inorganic inhibitors can have a negative impact on the stability of sol-gel coatings and lead to significant osmotic blistering. Therefore using the inorganic inhibiting compounds in a form of pigments with low solubility can be an option. The solubility of inhibitors can be decreased combining inhibiting cations with inhibiting anions which together can form a salt with limited solubility. Inhibitive compounds with low solubility are preferable candidates for incorporation into sol-gel coatings. For example, strontium aluminum polyphosphate (SAPP) is a poorly soluble inhibitor and was suggested as a potential inhibitive pigment for corrosion protection of AA2024 (Raps et al. 2009). The addition of SAPP into different sol-gel coatings provides an active corrosion protection and contributes toward increased barrier properties. Similar approach was employed in the work of Yasakau et al. (2013) in which cerium molybdate nanowires were incorporated into hybrid silica/zirconia sol-gel coatings applied onto AA2024. During immersion cerium and molybdate ions were released from the nanowires to the defects and inhibited corrosion of the alloy in places of defects.
Organic corrosion inhibitors are an abundant group of compounds which have been used in corrosion protection systems for many metals and alloys. The number of organic inhibitors which can be used in corrosion protection is significantly higher than that of the inorganic ones since there are many different classes of organic compounds and great variety of functional groups which can interact with metallic surfaces or with metal cations ensuring an inhibitive action. Many organic compounds have been studied as inhibitive additives to hybrid sol-gel protection coatings to mention a few: mercaptobenzothiazole , mercaptobenzimidazole, mercaptobenzimidazolesulfonate, aminopiperazine, aminopiperidine, 2-methyl piperidine, 8-hydroxyquinoline, tetrachloro-p-benzoquinone, benzotriazole , triazole, and their derivatives. The early works of Voevodin and Khramov explored the efficiency of different organic inhibitors in hybrid sol-gel coatings for protection of 2024 aluminum alloy against corrosion (Voevodin et al. 2003, Khramov et al. 2004, 2005). In general, these studies showed the positive effects of inhibitor addition on the corrosion performance of the sol-gel coatings. However, the performance strongly depends on the type of inhibitor and the chemistry of the sol-gel matrix.
The addition of certain organic inhibitors to a hybrid sol-gel matrix may cause unwanted chemical interactions and affect the polymerization and cross-linking of the matrix. For example, benzotriazole addition to an epoxy-based zirconia-silica sol-gel formulation drastically reduces the barrier of the coatings applied onto AA2024 (Yasakau et al. 2008). On the other hand, sol-gel coatings containing equal amounts of 8-hydroxyquinoline did not show the loss of barrier properties. Moreover the coating with such inhibitor has demonstrated an additional active corrosion protection effect. 8-Hydroxyquinoline, triethyl phosphate and 1,2,4-triazole inhibitors were found efficient when added to sol-gel coatings for AZ31 magnesium alloy as reported by Galio et al. (2010) and Supplit et al. (2007).
In spite of efficiency of organic inhibitors, a high inhibitor concentration in sol-gel formulation may cause weakening of barrier properties. For example, the addition of tetrachloro-p-benzoquinone (chloranil) at a concentration 1,2·10−4 M did not confer an adequate corrosion protection for 2017 aluminum alloy due to disorganization of the sol-gel matrix based on zirconia copolymerized with organomodified siloxanes. The matrix can be disturbed by the formation of agglomerates as a result of a low solubility of the inhibitor in the formulation. The inhibitor agglomerates create defects and voids in the sol-gel matrix which negatively influence the barrier properties of the sol-gel coating . Oppositely, at a lower concentration of chloranil, the coatings had homogeneous structure and conferred a good corrosion protection for the substrate (Quinet et al. 2007).
It is also important that the chemical interactions of inhibitors with the sol-gel matrix can sufficiently influence the release kinetics . The charged species can have a stronger tendency for interacting with sol-gel matrix because of the electrostatic interactions (Vreugdenhil and Woods 2005). For example, Khramov et al. (2005) showed that ionizable inhibitors like mercaptobenzimidazolesulfonate and thiosalicylic acid present a weaker corrosion protective effect than nonionizable inhibitors, mercaptobenzothiazole and mercaptobenzimidazole. In another example the inhibiting properties of phenylphosphonic acid incorporated in hybrid sol-gel coatings with phenyl functional groups were explored (Sheffer et al. 2004). The inhibitor was entrapped inside the sol-gel matrix due to specific π-π interactions between the phenyl rings. Sol-gel coatings with phenylphosphonic acid deposited on aluminum substrates demonstrated the enhanced corrosion protection attributed to the prolonged release of phenylphosphonic molecules. Phenylphosphonate molecules adsorb on the aluminum surface and displace Cl- anions thereby increasing the pitting potential of aluminum.
Mechanical properties of the sol-gel coatings may be affected by the incorporated inhibitor. Recent studies by Roussi et al. (2013) envisaged the correlation between inhibitor addition to the sol-gel and mechanical behavior of the obtained coatings. The studied sol-gel system was based on hybrid epoxy-functionalized silica network cross-linked with poly(ethylene imine). Two organic inhibitors were explored, namely, 2-mercaptobenzothiazole (MBT) and 2-mercaptobenzimidazole (MBI). The incorporated inhibitors certainly improved the corrosion protective properties of the sol-gel coatings. However, the inhibitors also affected the mechanical properties of the sol-gel coatings. In particular the additions of MBI decreased the hardness and increased the flexibility of the sol-gel coatings. For the MBT addition, such effects were somewhat lower.
It becomes evident from the discussed examples that the performance of a sol-gel coating depends on the compatibility between the polymer matrix and the inhibitor. The strategy of the direct addition of inorganic and organic inhibitors is associated with important limitations discussed above, e.g., the critical concentration of inhibitors in a sol-gel, the high solubility of inhibitors, the chemical interactions between a sol-gel matrix and inhibitors, and the uncontrolled release of inhibitors. In order to build a successful protection coating system for a metal, all these limitations must be considered. Often an efficient inhibitor causes a significant damage to the sol-gel matrix rendering the coatings to be useless in corrosion protection. Therefore, stepping up to a higher level of complexity, more sophisticated corrosion protective systems have been designed. These systems rely on different reservoirs, nanocontainers , carriers, and capsules which can be used as potential storage units for inhibitive compounds . Different concepts and practical examples of various sol-gel systems with different nanocontainers/reservoirs are discussed in the following section.
Self-Healing Sol-Gel Coatings with Encapsulated Inhibitors
The strategies of direct incorporation of inorganic and organic inhibitors into sol-gel coatings were briefly addressed above. However, the associated drawbacks such as deactivation of corrosion inhibitors and weakening the adhesion and the barrier properties of hybrid polymers stimulate the development of different encapsulation approaches which can allow preventing the interaction between the inhibiting species and the polymer matrix. Various ways of inhibitor immobilization are briefly described below with special emphasis on functional nanocontainers which allow controllable release of corrosion on demand under effect of different corrosion-relevant triggers.
A nanocontainer can be an object with a submicron size having a confined space restricted by a shell or a large surface area, which permits storage and release of incorporated or adsorbed chemical species. The storage ability can be provided either by chemical, electrostatic, and van der Waals interactions between the surface of the nanocontainer and the chemical species or by enclosing the species inside of the container shell. In order to incorporate nanocontainers into coatings, the latter have to have an appropriate thickness. Sol-gel coatings are very convenient candidates for storage of nanocontainers since they can have various thicknesses varied between about 100 nm and 10 μm. Such coatings allow incorporation of different nanocontainers in the sol-gel matrix without significantly disturbing the coating’s barrier properties.
Nanocontainers of inhibitors distributed in a coating matrix confer a controllable or, at least, prolonged inhibitor release contributing to longer corrosion protection properties of the coating. In general, the inhibitor release is required when the coating has lost its barrier protection due to defects which cause the development of corrosion on metal. Inhibitor release mechanisms from nanocontainers can be different and depend on the nanocontainer type. Many nanocontainer designs employ the triggering mechanism of inhibitor release by external stimuli . Such stimuli can be mechanical damage, light irradiation, changes of pH, ionic strength, etc. Upon the action of a stimulus, the nanocontainers can supply the inhibitor needed for “healing” of metal surface subjected to corrosion processes. Such trigger-response behavior of nanocontainers is successfully used for development of “intelligent” protective systems.
Layer-by-layer assembled nanocontainers
In the sections below, different cases of nanocontainers will be overviewed in more detail giving a particular attention to the corrosion protection efficiency of the protective hybrid sol-gel coatings with nanocontainers of inhibitors and inhibitor release mechanisms.
Sol-Gel Coatings with Inhibitor-Loaded Inorganic Layers
The main reason of inhibitor incorporation to hybrid sol-gel coatings systems is to enhance the coating’s active corrosion protective properties. After being released from the coating, the inhibitor has to diffuse and reach the metal surface. The distance that the inhibitor has to cover must be as small as possible in order to provide rapid and enhanced protection. Otherwise, active corrosion protection can be delayed because of increased diffusion path of the entrapped inhibitor. Inorganic porous reservoir layers were reported as promising containers for inhibitor storage and release in different works. The inhibitor is stored in such layers very close to the metal surface. When defect appears in the coating, the inhibitor can be quickly delivered to the corrosion zone.
In another work by Rosero-Navarro et al. (2010b), a multilayer approach for inhibitor storage in sol-gel coatings was used for corrosion protection of the same substrate. The coatings were based on tetraethoxysilane, methacryloxypropyl trimethoxysilane, colloidal SiO2, and methacrylate-based monomers. Two types of coated samples were prepared. The uninhibited sample consisted of two layers of sol-gel coatings. The inhibited sample consisted of three layers where the intermediate one contained 5 mol. % of Ce(NO3)3 inhibitor. Glow discharge optical emission spectroscopy was used to measure Ce species distribution in the coatings after different times of immersion. Decrease of Ce concentration in the intermediate layer was associated with the release and diffusion of Ce species to the corroding substrate.
In some cases the inorganic layer acts not only as a reservoir for a corrosion inhibitor but as an additional protective layer. High corrosion susceptibility of magnesium and some of its alloys requires measures which would prevent the contact of the metal surface with a corrosion solution. Plasma electrolytic oxidation (PEO) is one of the perspective methods for growth of an inorganic mixed oxide layer on Mg alloy surface. However, such a layer cannot completely shield Mg from corrosion because of its intrinsic porosity. The corrosion protection system presented in (Ivanou et al. 2016) provided the efficient corrosion protection for ZE41 magnesium alloy by a PEO layer with incorporated corrosion inhibitor 1,2,4-triazole and sealed by a hybrid hydrophobic sol-gel coating. It is worth noting that the stand-alone sol-gel coating applied onto the Mg alloy could not provide an efficient and long-term corrosion protection. Therefore PEO layer significantly enhanced the protective efficiency of the sol-gel coating. Self-healing properties of the double layer coatings were studied by scanning vibrating electrode technique (SVET). The coating system containing the inhibitor revealed lower local corrosion currents from artificial defects. Filiform corrosion was delayed by the inhibitor-containing coating compared to the uninhibited one. Thus the porous inorganic pre-layers have proven themselves to be efficient depots for corrosion inhibitors when combined with thin barrier sol-gel films.
Sol-Gel Coatings with Inorganic Micro-/Nanocontainers of Inhibitors
Inorganic micro- or nanoparticles have been widely used as pigments and fillers in paints, lacquers, and plastics. Nanoparticle additives to sol-gel can improve mechanical and barrier properties of the coatings (Conde et al. 2003; Zheludkevich et al. 2006). One of the properties of such inorganic structures, namely, high absorption capacity, has been thoroughly explored in the last decade. Inorganic particles can be good storage units for incorporation of various functional species like inhibitors or other active compounds. Literature offers many works where different approaches of inhibitors incorporation have been developed. Among the abundant knowledge on synthesis and application of nanocontainers of inhibitors, one can highlight the following main nanocontainer classes: oxide nano-/microparticles, hollow nanocontainers, and ion-exchange containers which have been successfully used in corrosion protective sol-gel coatings.
In some cases the addition of nanoparticles only is sufficient to impart additional corrosion protection to a sol-gel-coated metal. The approach presented by Schem et al. (Schem et al. 2009) employed CeO2 nanoparticles having a size about 5 nm as the inhibitive pigment to the epoxy-based sol-gel coatings applied to AA2024. The modified coatings demonstrated the superior behavior in salt spray corrosion tests compared with the unfilled coating. The main approach of this work was based on the assumption that amorphous nanoparticles can confer an increased solubility when compared to the crystalline particles.
In situ-synthesized ZrO2 oxide nanoparticle carriers were also explored as potential storage units for cerium (III) inhibitor in sol-gel-based coatings (Zheludkevich et al. 2005a, b). Zirconia nanoparticles were synthesized in a solution of zirconium (IV) tetrapropoxide in 2-propanol with acidified water addition. The nanoparticles were modified with Ce(III) ions in situ during the ultrasonic agitation. The prepared sol was mixed with a sol containing pre-hydrolyzed tetraethoxysilane/epoxy-modified silane, and then the final mixture was applied to AA2024 . The corrosion protection efficiency of the obtained coatings was evaluated by electrochemical impedance technique. The tests displayed the enhanced protection capability of the coatings with nanoparticles modified with Ce(III) ions compared to that of the coatings which did not have Ce(III) or zirconia-modified Ce(III) nanoparticles. The obtained results indicated that the corrosion protection performance of the sol-gel coatings was enhanced as a result of inhibiting effect provided by the Ce cations released from the zirconia nanoparticles.
Commercially available nanoparticles offer an easy and effective way of incorporation of inhibitors. The studies of corrosion protective properties of silane-based and hybrid sol-gel coatings with lanthanide inhibitors immobilized on commercially available SiO2 and CeO2 oxide nanoparticles have been presented in different works (Montemor and Ferreira 2007; Zandi Zand et al. 2014; Balan et al. 2016). In general, these studies indicate that the sol-gel protective coatings loaded with nanocontainers containing adsorbed Ce(III) or La(III) inhibitors offer benefits in terms of the longer corrosion protection and better coating barrier.
Another strategy to achieve a higher inhibitor loading is to use hollow inorganic structures such as halloysite. Halloysite is a naturally occurring clay material which has a tubular structure similar to kaolinite. Although halloysite has different morphologies, a tubular form presents more interest to be used as a delivery carrier. First attempts of intercalation of inorganic salts and organic species to halloysite are dated back to several decades ago (Bradley 1945; MacEwan 1948). More recently a corrosion protective system for 2024 aluminum alloy has been developed on the basis of sol-gel coatings loaded with halloysite nanotubes which acted as carriers of corrosion inhibitors like benzotriazole or 8-hydroxyquinoline (Fix et al. 2009). The measurements of local corrosion currents in the defects using SVET demonstrated a superior corrosion protection of the sol-gel coatings containing hallosite@benzotriazole or hallosite@8-hydroxyquinoline when compared to a blank coating system.
The approach of inhibitor incorporation into halloysite was later refined by Abdullayev et al. (2009). Although the halloysite allows loading of chemicals, it also permits their quite fast release in aqueous solutions. Abdullayev suggested using a “stopper” at both ends of halloysite tubes in order to control the release kinetics. Copper ions were used as a stopper for halloysite loaded with benzotriazole inhibitor. The idea behind using the stopper was based on the formation of Cu-benzotriazole hardly soluble complexes. After loading halloysite tubes with the inhibitor, the both sides of the tubes were clogged due to reaction between the copper ions and benzotriazole. The sol-gel coatings loaded with such modified halloysite nanocarriers were tested on copper and AA2024 metal substrates. Corrosion testing methods demonstrated a successful corrosion inhibition in micro-confined defects on the coated substrates.
The approach presented by Arunchandran et al. (2012) is based on TiO2 nanotubes as nanocontainers of benzotriazole corrosion inhibitor. The nanocontainers were synthesized on Ti foil by anodization in the HClO4 electrolyte. The anodization was continued until the Ti foil completely transformed into TiO2 nanotube powder. The powder underwent a calcination process at 400 °C in order to obtain the anatase crystalline modification of TiO2. The corrosion inhibitor was mixed with TiO2 powder in ethanol solution after which the powder was carefully washed in ethanol and then dried. The sol-gel consisted of a mixture of (3-glycidyloxypropyl)trimethoxysilane and titanium isopropoxide which was hydrolyzed in the presence of acid. The coatings were applied to 9Cr-1Mo steel by dip coating and cured at 120 °C. The electrochemical impedance spectroscopy was used to characterize the corrosion protective properties of such coatings during immersion in 0.05 M NaCl solution. Overall the results indicated a certain improvement of corrosion protection of the steel by the coating with nanocontainers .
Inorganic compounds with ion-exchange properties have found applications in waste water treatment, catalysis, and ion-exchange systems (Rives 2001; Pabalan and Bertetti 2001). Such compounds exist in nature in the form of minerals like zeolites/bentonites and hydrotalcites or layered double hydroxides (LDH). Zeolite and bentonite minerals possess cationic exchange abilities, whereas hydrotalcites possess anionic exchange abilities. Zeolites are aluminosilicate minerals which have porous structure capable of incorporating different metal cations. Bentonite is an aluminum phyllosilicate clay consisting of parallel sheets of silicate tetrahedrals. The latter two mineral types have found applications in corrosion protection of metals (Buchheit et al. 2002; Bohm et al. 2001). However, layered double hydroxide-type (hydrotalcites ) materials have gained more attention for their properties and especially due to the relatively easy synthesis of LDH. Essentially the structure of hydrotalcites consists of brucite-type layers in which the (Mg, Zn) cations are partially substituted by (III) valent metal cations, e.g., Al, Fe, and Cr. The structure is composed of stacks of charged mixed hydroxide layers which are separated by inner space containing water and anions in order to keep electroneutrality. Different anionic species like inorganic or organic inhibitors can be incorporated into LDH structure. Due to such remarkable properties, these materials have been suggested as containers for storage and release of inhibitors for corrosion protection of metals (Buchheit et al. 2003; McMurray and Williams 2004).
Application of cation-exchange compounds (zeolites ) for corrosion protection of metals coated by sol-gel films has been studied in several works (Dias et al. 2012, 2013; Ferrer et al. 2014). Typically ion-exchange materials should contain high efficient corrosion inhibitors. In the case of cation-exchange materials (zeolites), cerium (III) cations are mainly used as a known cathodic inhibitor which suppresses the oxygen reduction process. Ce-substituted zeolites in general impart active corrosion protective properties to the sol-gel coatings applied onto the aluminum alloys. On the other hand, Na-containing zeolites negatively affect sol-gel matrix stability. This is mostly related to the raise of pH due to Na-zeolite interaction with water. The high pH negatively affects the stability of the metal-sol-gel interface and reduces barrier protection.
Without decreasing the significance of zeolite-type containers, they have a drawback associated with the cationic exchange properties of this material. Among the cationic species, there are not as many efficient inhibitors. This restricts the number of inhibitors which can be incorporated in zeolite. On the other hand, LDH possess anionic exchange ability. This opens opportunities for many organic and inorganic anionic inhibitive species to be incorporated in LDH. There are number of reports on the self-healing protective coatings based on the LDH nanocontainers. This type of nanocontainers was used in both the polymer organic coatings and in the hybrid sol-gel systems (Tedim et al. 2010; Zheludkevich et al. 2010). The complex system including a sol-gel pretreatment layer and primer/topcoat coatings were also addressed. The sol-gel and primer layers were loaded with LDH nanocontainers having different corrosion inhibitors. Different LDH systems containing intercalated vanadate (V2O7), phosphate, and mercaptobenzothiazole (MBT) anionic species were prepared. The main aim of these studies was to determine how different LDH additives influence barrier and active corrosion protection of the multilayer coating systems. It was shown that LDH with intercalated vanadate species imparts active corrosion protection being incorporated to the primer coating. On the contrary, LDH loaded with MBT species negatively affected the barrier properties of the primer. A double layer protective system made of a sol-gel coating and a primer demonstrated the best performance when the sol-gel coating contained LDH-MBT and the primer coating contained LDH-V2O7. The study emphasized an important point related to the compatibility of LDHs with different coatings.
Yasakau et al. (2014) have also explored the effectiveness of LDH-V2O7 particles or LDH-V2O7 layers in corrosion protection of sol-gel-pretreated AA2024. The incorporation of the LDH powder into the sol-gel matrix caused the formation of LDH agglomerates which could decrease the barrier properties of the layer. Another strategy was employed in order to decrease such a drawback. The LDH structures were directly grown on the alloy surface and then overcoated with a hybrid sol-gel layer. In both cases, however, the corrosion performance of the coatings with LDH-V2O7 was superior to that of the control (blank) sol-gel coatings as was evidenced by EIS measurements. Therefore active corrosion protection was clearly demonstrated for the systems containing of LDH-V2O7 inhibitive species.
Sol-Gel Coating with LbL-Assembled Nanocontainers
Layer-by-layer (LbL) technique is a versatile approach which has been used for many applications, e.g., biocompatibility, chemical sensing, multicomposite films, coatings, photonic devices, etc. (Decher 2003). The simplified concept behind the LbL process is based on the adsorption of a charged polyion polymeric molecule onto an oppositely charged substrate. The interactions between the substrate and the molecule have electrostatic nature. Upon such adsorption process, the charge of the substrate changes its sign. The procedure can be repeated by depositing different oppositely charged layers on the substrate.
LbL technique has found applications in corrosion protection of metals a decade ago. It allows fabricating nanocontainers coated by LbL process which have tailored permeability and intelligent release properties of the incorporated chemical species. The shell of the resulting polyelectrolyte containers is semipermeable and sensitive to a variety of physical and chemical conditions, e.g., mechanical impact and change in pH in the surrounding medium, enabling it to regulate the release of the entrapped inhibitor species. The intelligent release property is crucially important for any delivery system since the incorporated molecules will sit tight inside the container and will be released only under a certain “trigger” action. When combined with the coating system, LbL-coated nanocontainers can drastically improve the corrosion performance of the entire system (Kachurina et al. 2004).
The mechanism of inhibiting action is based on the local change of pH in the damaged area due to the corrosion processes. When corrosion starts two electrochemical processes occur on the metal surface, namely, the anodic process of metal dissolution and the cathodic process of oxygen reduction. The pH value is changed in the neighboring area due to these processes, which opens the polyelectrolyte shells of the nanocontainers in a local area with the following release of benzotriazole. Then, the released inhibitor suppresses the corrosion activity and the pH value recovers, leading to closure of the polyelectrolyte shell of nanocontainers, thus preventing further inhibitor release (Zheludkevich et al. 2007).
In the previous approach, the corrosion inhibitor was stored in between the layers of polyelectrolytes. In other approaches LbL process was used for sealing porous nanocontainers filled with corrosion inhibitors (Shchukin et al. 2008). In this study polyelectrolyte multilayers were used not only for storage and release of inhibitor but for sealing porous containers such as the halloysite. A corrosion inhibitor 2-mercaptobenzothiazole (MBT) was loaded to halloysite nanotubes from ethanol solution under vacuum, and then the suspension was centrifuged, washed with water, and dried. The maximal reported loading capacity of MBT in halloysite nanotubes was about 5 wt. %. To attain controlled release properties to the halloysite nanotubes, the surface was modified by LbL deposition process using poly(styrene sulfonate)(PSS)/poly(allylamine hydrochloride) (PAH) polyelectrolytes. The final nanocontainers had a halloysite (inhibitor)//PAH/PSS/PAH/PSS layer structure. Release properties of such nanocontainers show a noticeable inhibitor release in water solutions with pH 10. On the contrary, in solutions with pH 6.5 the release was very slow. Such controlled inhibitor release on demand can prolong the self-healing action of the containers during corrosion protection of AA2024 aluminum alloy by sol-gel coatings. Hybrid sol-gel coatings (SiOx/ZrO2) were used as a matrix for incorporation of halloysite nanocontainers. The electrochemical impedance measurements showed the improved performance of the coating with modified halloysite nanocontainers.
Quite interesting approach to the corrosion protection using light-sensitive nanocontainers was presented by (Skorb et al. 2009b). The main idea was to use a mediator which under the action of light radiation would induce chemical changes to the surrounding nanocontainers with inhibitors leading to the release of the inhibitor from the containers. A porous TiO2 host structure was used as the container in which the corrosion inhibitor benzotriazole was incorporated. TiO2 nanoparticles were sealed by LbL polyelectroly te films in order to prevent the inhibitor escape. The polyelectrolyte shells with silver nanoparticles were used as the mediator for the absorption of the light (infrared radiation). The nanocontainers were incorporated to the sol-gel films based on hydrolyzed epoxy functional silane and zirconium (IV) propoxide precursors. The concept of the intelligent inhibitor release was tested on the coated AA2024 samples. The samples were immersed in NaCl solution and eventually displayed some corrosion activity. After irradiation by IR light source, the corrosion activity monitored by SVET ceased on the sample which contained the modified nanoparticles (Skorb et al. 2009b). Similar study has been reported by Andreeva and Shchukin (2008). However, instead of the IR source, a UV light source was used to activate the TiO2 nanocontainers . After local UV irradiation, the ongoing corrosion activity was suppressed. No such UV-stimulated fast healing was observed in the case of hybrid sol-gel films without the containers. Thus, the use of IR or UV light irradiation as an external trigger has a potential in mitigating corrosion processes at the defects in sol-gel coatings using intelligent TiO2-based nanocontainers of inhibitors.
Sol-Gel Coating with Organic Nanocontainers
Organic polymer containers present an interesting type of materials which were actively used in development of intrinsic self-healing polymeric coatings where a polymer bulk material (coating) contains a microencapsulated healing agent that releases upon the rupture of the container (White et al. 2001). Then, the healing agent polymerizes after the contact with the embedded catalyst thereby sealing the crack faces and recovering the material toughness. Albeit the outstanding results were obtained, the containers, which are usually made of the polymeric shells, were usually too large. It is reasonable to have a large size of containers since they can accommodate a large quantity of monomer agent. However, it is not possible to incorporate such large containers in the thin sol-gel films without compromising the coating’s barrier properties.
The concept of inhibitor storage in organic containers for corrosion protection of metals was explored more than a decade ago by Khramov and coworkers (Khramov et al. 2004; Khramov et al. 2005). Different inhibitors such as mercaptobenzothiazole , mercaptobenzimidazole, mercaptobenzothiazole, mercaptobenzimidazole, mercaptobenzimidazolesulfonate, and thiosalicylic acid were incorporated to ß-cyclodextrin which served as a container. Then the containers were added to the sol-gel coating matrix. Active corrosion protection properties were evaluated on scratched samples. It was shown that the complexation with ß-cyclodextrin resulted in a slower release of the inhibitor and its continuing delivery to the corrosion sites ensuring certain self-healing effect.
Another possibility was reported in the work of Maia (Maia et al. 2016) where polymer macrocapsules were used as containers to introduce the corrosion inhibitor to the hybrid sol-gel coating. In this work polyurethane microcapsules (PU-MC) were synthesized following an oil-in-water microemulsion route using diethylenetriamine (DETA) and 2,4-toluene diisocyanate (TDI) as building units for the capsule shell. The corrosion inhibitor 2-mercaptobenzothiazole (MBT) was introduced into the oil phase during the synthesis step and in this way was incorporated in the capsules (MBT@PU-MC). The prepared capsules had the mean size about 300 nm, though there was also some fraction of capsules in the range of 1–2 μm. The prepared capsules were incorporated to the hybrid epoxy (SiOx/ZrOx) sol-gel matrix and applied onto AA2024 alloy substrates. EIS and SVET results clearly demonstrated active corrosion protection provided by the coating with MBT@PU-MC. The performance was sufficiently better than in the case of direct addition of MBT inhibitor to the hybrid matrix. These results confirm a well-known fact that the direct inhibitor addition to the reactive sol-gel matrix is in most cases unwelcome and causes the loss of barrier properties of the coatings. However, it should be noted that the addition of polymeric capsules to the sol-gel may also cause formation of agglomerates. Certainly, the capsule-sol-gel interface must be specifically “tuned” in a way to provide a good compatibility.
This chapter has covered a considerable body of work on active protective sol-gel coatings. Most of the approaches used to achieve a self-healing effect in the sol-gel coatings are based on addition of corrosion inhibitors. There are two main ways of integration of corrosion inhibitors within the hybrid matrix, namely, the direct addition or using the encapsulation in various nanocontainers. The direct addition of soluble corrosion inhibitors into formulations during the sol-gel synthesis is the simplest approach. However, normally only a limited range of inhibitors and at relatively low concentrations can be added because of potential adverse effects. Encapsulation of the corrosion inhibitors can significantly improve their compatibility with sol-gel coatings opening a door for significantly higher loading of corrosion inhibitors. Moreover the nanocontainers of corrosion inhibitors can provide a controllable release of inhibitors on demand only under action of corrosion-relevant triggers.
However, for uptake of active protective sol-gel coatings by industry, the following several important points should be tackled: finding more efficient “green” corrosion inhibitors, increasing the load capacity of the nanocontainers, improving compatibility of different types of nanocontainers with sol-gel formulations, and increasing the shelf life of nanocontainer-based formulations.
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