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Sol-Gel Coatings with Nanocontainers of Corrosion Inhibitors for Active Corrosion Protection of Metallic Materials

  • K. A. YasakauEmail author
  • M. G. S. Ferreira
  • M. L. Zheludkevich
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Abstract

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.

Keywords

Corrosion Inhibitor Corrosion Protection Metal Alkoxide Mesoporous Silica Nanoparticles Organic Inhibitor 
These keywords were added by machine and not by the authors. This process is experimental and the keywords may be updated as the learning algorithm improves.

Introduction

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.

Sol-Gel Coatings

Chemistry of Sol-Gel

The sol-gel process consists of chemical reactions at which silicon and/or metal-containing precursors are combined to produce inorganic or organically modified metal oxide materials with tailored properties. This process implies the evolution of precursor solution through the colloidal system (sol) followed by a highly branched system called gel. Sols are dispersions of colloidal particles with diameters of 1–100 nm (Hench and West 1990) or 1–1000 nm (Brinker and Scherrer 1990) in a liquid. Gel is an interconnected rigid network with pores of sub-micrometer dimensions and polymeric chains whose average length is greater than a micrometer. The precursors for the sol-gel process can be different metal alkoxides with general formula (M(OR)z), where M can be (Ti, Zr, Ce, Sn, Al, Si), OR is an alkoxy group, and R is an alkyl ligand. The first step in sol-gel synthesis is the hydrolysis or hydroxylation which can be described by the common reaction for different metal alkoxides (Brinker and Scherrer 1990):
$$ M{(OR)}_4+{H}_2 O\to HO- M{(OR)}_3+ ROH $$
(1)
The alkoxy group is hydrolyzed and substituted by the hydroxyl group. Subsequently, two partially hydrolyzed precursors can link together during the condensation reactions :
$$ {(RO)}_3 M- O H+ HO- M{(OR)}_3\to {(RO)}_3 M- O- M{(OR)}_3+{H}_2 O $$
(2)
$$ {(RO)}_3 M- O R+ HO- M{(OR)}_3\to {(RO)}_3 M- O- M{(OR)}_3+ ROH $$
(3)

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.

Sol-gel systems as pretreatments must fulfill several important requirements. First of all they should offer a good adhesion to the metal/sol-gel interface (Fig. 1a). The adhesion is greatly facilitated because of chemical interactions between the metal and the sol-gel network. In humid environment a cleaned metal surface contains many hydroxyl groups . Upon contact with sol-gel solution, pre-hydrolyzed siloxane molecules are attracted to the metal surface by van der Waals and electrostatic forces. During the sol-gel film curing, stable covalent bonds form between the metal surface and hydrolyzed silane molecules. Thermodynamic calculations made by Schmidt et al. show that the enthalpy of the formation of covalent bonds between alumina and silica, for example, is much lower compared to boehmite that is produced during the oxidation of aluminum by water (Schmidt et al. 1997). Such covalent bonds formed between the metal and sol-gel greatly improve the stability of the interface.

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).
Fig. 1

A scheme of chemical bond formation between a metal, a sol-gel, and a paint

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.

Incorporation of inhibiting compounds to the sol-gel matrix can improve the corrosion protection performance of the sol-gel coatings providing an additional active corrosion protection . During last decade a number of works was published on the sol-gel coatings loaded with various inhibitors aiming at the improvement of corrosion protection performance via active protection functionality. However, in many cases a direct incorporation of corrosion inhibitors into sol-gel formulations does provide a desirable effect or even leads to opposite results. There are several factors which play a critical role in determining the efficiency of inhibitor-containing sol-gel coatings. These factors include:
  • 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.

Inorganic Inhibitors

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 Inhibitors

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.

Many examples of sol-gel corrosion protective systems employing various approaches for inhibitor incorporation in nanocontainers exist in literature. Such approaches can be categorized into the following groups:
  • Container layers

  • Inorganic nanocontainers

  • Layer-by-layer assembled nanocontainers

  • Organic 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.

Nano-porous TiO2 layer was proposed as a reservoir for incorporation of benzotriazole corrosion inhibitor (Lamaka et al. 2006, 2007b). The layer was applied onto 2024 aluminum alloy substrate by dipping into the solution of hydrolyzed Ti(IV) alkoxide mixed with a templating agent. The coating was dried and cured at an elevated temperature. Then, the metallic substrate covered with TiO2 porous layer was immersed in ethanol solution of benzotriazole and dried to load the pores with inhibitor. Finally it was coated with a hybrid sol-gel film. Figure 2 shows AFM topography maps of the porous TiO2 coating and the bare alloy surface. The porous structure of the inorganic oxide layer permits incorporation of inhibitor which stays close to the metal substrate. Figure 2c, d shows the evolution of impedance of the coated samples during immersion in 0.05 M NaCl solution. The self-healing ability of the coating with incorporated inhibitor in TiO2 layer was reflected in the periodic recovery of impedance during immersion, while the impedance of the reference coating was continuously decreasing.
Fig. 2

AFM topography maps of the uncoated (a) and TiO2-coated metallic substrate (b); Nyquist plots obtained during immersion of the sol-gel-coated AA2024 substrate with TiO2 sub- layer (c) and reference sol-gel-coated AA2024 substrate (d) in 0.05 M NaCl solution (Adapted from Lamaka et al. 2006)

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.

The application of natural polymer materials as reservoirs for environment-friendly corrosion inhibitors offers an attractive and relatively unexplored approach for the development of “green” self-healing coatings . One of such materials can be chitosan. It is nontoxic and chemically compatible with various organic matrices biopolymer which can be used as a “green” reservoir layer for storage and release of various chemicals. It offers good film-forming properties and superior adhesion to metallic surfaces. One of important features of chitosan is its high ability to form complexes with corrosion inhibitors in a reversible manner. Quite interesting approach of corrosion inhibitor immobilization in chitosan layers has been suggested in (Zheludkevich et al. 2011). Cerium (III) nitrate was employed as the corrosion inhibitor for 2024 aluminum alloy. Chitosan layers were applied on AA2024 plates from the solution of chitosan in ethanol with or without Ce(NO3)3 and then dried. Afterward, the samples were coated by a sol-gel film and cured in an oven at high temperature. The final bilayer structure is shown on SEM micrograph (Fig. 3a). It can be seen that Ce(III) preferentially stays in the inner chitosan layer. Electrochemical tests explored self-healing corrosion protective properties of the obtained coatings using SVET equipment. Figures 3b, c, d, e show two artificial defects formed on the coated AA2024 surface and ionic current maps measured during immersion in NaCl solution. The local ionic current was decreasing during immersion which was correlated with well-defined active inhibition and self-healing of the corrosion reactions in the artificial defects.
Fig. 3

(a) SEM cross section of the coated AA2024; (b) optical photograph of a coating with two artificial defects; (c, d, e) ionic currents maps acquired after 5 h (a), 40 h (b), and 70 h (c) of immersion in NaCl solutions (Adapted from Zheludkevich et al. 2011)

Layer-by-layer (LbL) polyelectrolyte films have attained much interest due to their unique chemical properties such as a response to pH changes and the ability to incorporate charged species between the layers (Andreeva et al. 2008). The ability of LbL films to incorporate corrosion inhibitor has been successfully explored. Such films were studied as stand-alone corrosion protective system. Although active corrosion protection was improved, such coatings could not provide long-term corrosion protection. The approach presented by Lamaka et al. (2008) explored a bilayer corrosion protective system which included a sol-gel coating and polyelectrolyte films which were loaded with inhibitor. In the first approach, a polyelectrolyte film consisting of poly(ethyleneimine) (PEI) and poly(sodium styrenesulfonate) (PSS) layers was deposited directly on top of AA2024 and then loaded with 8-hydroxyquiniline corrosion inhibitor. Then the film was coated by a hybrid sol-gel coating. In the second approach, a sol-gel coating was deposited directly on AA2024, and then the coating was coated with polyelectrolyte multilayers which were loaded with the corrosion inhibitor. Corrosion protection properties were evaluated using electrochemical impedance spectroscopy (EIS) and scanning vibrating electrode technique (SVET) on the two previously mentioned systems and the third control system containing only one sol-gel coating (blank sample). It was a surprise that the system containing LbL film with the corrosion inhibitor directly in contact with the alloy surface showed the worst performance in both corrosion tests. However, the rational explanation for such phenomenon would be the lack of adhesion strength between the alloy surface and the sol-gel coating. The adhesion is primarily important for successful barrier protective properties of any coating. In this case it was significantly compromised by an LbL layer. Contrastingly, the other system having the LbL layer with inhibitor on the top of the sol-gel coating demonstrated active corrosion protection and self-healing during immersion in NaCl solution. The mechanism of the active corrosion protection in the latter case is presented on a scheme (Fig. 4). The inhibitor is entrapped inside the polyelectrolyte net deposited atop the sol-gel film. The corrosion process starting in the microdefect changes local pH due to generation of OH (cathodic reaction) and H+ (anodic reaction). The shells of the polyelectrolyte reservoir opened and the inhibitor is released. This leads to suppression of the corrosion process.
Fig. 4

Schematic representation of effects of inhibitor-doped polyelectrolyte multilayers with the SEM/EDS structure of the AA2024 substrate (Lamaka et al. 2008)

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.

Oxide Nano-/Microparticles

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.

However, the strategy of inhibitor adsorption on inorganic nanocontainers has an important drawback associated with the low loading capacity of such carriers. This fact does not allow a long-term effect due to fast depletion of inhibitor from the nanocontainers. Moreover the necessary critical concentration of the inhibitor in a defect is not achieved even in the beginning. One of the approaches toward increased capacity of nanocarriers is to use mesoporous nanoparticles such as SiO2 (Fig. 5). These containers possess many benefits compared to conventional inorganic oxide nanoparticles. The surface of the nanocontainers can be functionalized with organic groups to improve the compatibility with the polymer matrix and offer an additional control over the inhibitor release via bonding of inhibitor molecules or creating an additional barrier on the surface of nanocontainers. Mesoporous silica nanocontainers have been extensively studied as containers for incorporation of different inhibitors by a group of Moehwald and others (Borisova et al. 2011, 2012, 2013; Ding et al. 2016; Liang et al. 2016; Arunchandran et al. 2014). Highly effective organic corrosion inhibitors, e.g., those presented in (Lamaka et al. 2007a; Zheludkevich et al. 2007) are typical candidates for incorporation to these nanocontainers.
Fig. 5

Schematic representation of a mesoporous SiO2 particle and typical stages for functionalization, inhibitor loading, and incorporation to a hybrid sol-gel system

Ding et al. (2016) have suggested an advanced approach for preparation of self-healing protective sol-gel coatings loaded with mesoporous silica nanoparticles (MSN) on magnesium alloys. At first MSN were prepared following a conventional route of alkaline hydrolysis of tetraethyl orthosilicate in the presence of a surfactant. Then the MSNs were functionalized consequently by (3-aminopropyl)trimethoxysilane, 1,6-ditosylate hexane, and 1-(6-aminohexyl)-pyridinium. After this three functionalization steps, the corrosion inhibitor 2-hydroxy-4-methoxy-acetophenone has been incorporated into the prepared silica producing modified MSN particles. The functionalization steps were performed to enhance the intelligent release ability for such nanoparticles triggered by the alkaline pH or in the presence of Mg2+. The obtained nanocontainers were then incorporated to SNAP (self-assembled nanophase particles) sol-gel coatings to obtain MSN@SNAP coatings (Fig. 6b). Another coating was created by applying 1H,1H,2H,2H-per-fluorodecyltriethoxysilane (PFDS) onto MSN@SNAP, thus giving PFDS/MSN@SNAP coating (Fig. 6c). AZ31B magnesium alloy was used as a substrate for different SNAP coating depositions. The self-healing properties of such system were analyzed by electrochemical impedance spectroscopy and scanning vibration electrode technique (SVET). The results obtained via the latter method are presented in Fig. 6. Artificial scratches were made on each of the coatings by a razor-sharp blade in order to induce local corrosion at the metal surface. While immersing the samples in 0.01 M NaCl solution, ionic currents at the surface were measured by SVET. The indicated results show well-defined self-healing properties of the SNAP coatings containing MSN modified with inhibitor (Fig. 6b, c). The scheme presented in Fig. 6d illustrates the self-healing mechanism conferred by the coatings. At first when local corrosion starts at the metal, it induces corrosive dissolution of Mg at anodic sides producing Mg2+ and consequent alkalinization at local cathodes producing OH (Fig. 6d, (a)). Consequently, these two factors trigger the inhibitor release at these anodic and cathodic zones from the MSN nanocontainers which suppress the corrosion processes in the defects (Fig. 6d(b, c)).
Fig. 6

SVET current density maps of (a) SNAP coating for immersion of (a) 5 h, (b) 24 h, and (c) 72 h. (b) MSN@SNAP for immersion of (a) 5 h, (b) 24 h, and (c) 72 h. (c) PFDS/MSN@SNAP for immersion of (a) 5 h, (b) 24 h, and (c) 72 h. (d) Schematic representation of the self-healing mechanism involving the development anodic and cathodic local zones (a), followed by the inhibitor release from the MSN (b) and efficient corrosion inhibition (c) (Reproduced from Ding et al. 2016 with permission from Royal Society of Chemistry)

Hollow Nanocontainers

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 .

In the work presented by Chen and Fu (2012), the hollow inorganic silica nanocapsules modified with organic complexes were employed as intelligent alkaline- and acid-responsive nanocontainers for benzotriazole corrosion inhibitor. Hollow silica nanocontainers (HMSNs) were prepared by covering hematite nanoparticles with silica layer at alkaline pH and then dissolving the hematite template in acid. The obtained containers are presented in Fig. 7. The outer shell of the nanocontainers was modified by 3-chloromethyl triethoxysilane/1,4-butanediamine or N-phenylaminomethyltriethoxysilane which induced pH-responsive abilities to the nanocontainers. The corrosion inhibitor benzotriazole was incorporated into the containers from the saturated solution in acetone. Then the loaded containers were sealed by cucurbit[6]uril and α-cyclodextrin and dried under vacuum. Capping the nanocontainers with cucurbit[6]uril ensured an enhanced release kinetics at alkaline pH. On the other hand, capping the nanocontainers with α-cyclodextri n offered acid-responsive behavior. The modified HMSNs were successfully incorporated into hybrid epoxy-modified zirconia-silica sol-gel coating and applied on AA2024 substrate. Corrosion protective performance of the coatings was evaluated by electrochemical impedance spectroscopy (EIS). Usually the addition of nanocontainers to a sol-gel matrix causes negative effects such as sol-gel cracking compromising the barrier properties. In this case the pH-responsive HMSNs displayed a good compatibility with the sol-gel coating and good barrier protection which was monitored by EIS. Moreover, the coatings containing HMSNs with inhibitor showed an active corrosion protection compared to the blank coating in EIS tests and on artificially scratched surface.
Fig. 7

TEM images of HMSNs (Reproduced from Zhao et al. 2009 with permission from the Royal Society of Chemistry)

Ion-Exchange Containers

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).

Figure 8 schematically presents the triggered release of inhibitor molecules from the LDH structure containing inhibitor anions (Tedim et al. 2010). In this case, the release of anionic inhibitors is triggered by the anion-exchange mechanism with chloride anions. Thereby, LDH plays a double role, at first entrapping corrosive chlorides and releasing the inhibitor in response (Fig. 8).
Fig. 8

Scheme of inhibitor release from LDH (I). The release of inhibitors (Inh-) is triggered by the presence of anions in solution (Cl) (II). The aggressive anions are entrapped from the environment by LDHs (Tedim et al. 2010)

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.

Ferrer et al. (2014) presented an intelligent approach for synergistic corrosion inhibition of AA2024 using zeolite containers doped with a mixture of inhibitors. NaY zeolites were double loaded in a two-step process with cerium and diethyldithiocarbamate (DEDTC) as these are known efficient corrosion inhibitors for AA2024 -T3. At first zeolite was immersed in 0.1 M solution of cerium nitrate producing NaY-Ce zeolite. At the second step, the modified zeolites were immersed in 0.15 M DEDTC solution under constant stirring producing NaY-Ce-DEDTC-modified zeolite. After each step all samples were vacuum filtered and washed with water and dried. The sol-gel solution was prepared by mixing tetraethyl orthosilicate and 3-glycidoxypropyltrimethoxysilane and 0.05 M nitric acid (1.14:2.62:1 weight ratio). In the prepared solution, 10 wt. % of zeolite powder was introduced and then shear mixed at 2500 rpm prior to application to abraded AA2024 panels. The double corrosion inhibition concept relies on a fast release of the inhibitor from the outside of container particle when the carrier is exposed to a certain corrosive media. The second inhibitor remains inside the container until a slower release is triggered when the corrosion process advances. Such double loading approach was implemented for building anticorrosive self-healing sol-gel coatings with zeolites. The self-healing ability of such coating systems with an artificial defect was evaluated by electrochemical impedance spectroscopy in 0.05 M NaCl solution. Figure 9 displays the impedance variation with the immersion time for the three protective coatings, namely, sol-gel with 10% of NaY, sol-gel with 10% of NaY-Ce, and sol-gel with 10% of NaY-DEDTC. The periodic recovery of the impedance values, which can be associated with the self-healing events, can be observed in the case of the sol-gel coating loaded with zeolites. The system containing double inhibitors clearly demonstrated an improved protection which was attributed to the synergistic inhibiting effect .
Fig. 9

Variation of the impedance with the immersion time of different scratched hybrid sol-gel-coated AA2024-T3 samples in 0.05 M NaCl. Arrows indicate increase or decrease of impedance between immersion times (Reproduced from Ferrer et al. 2014 with permission from Elsevier)

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).

Shchukin and Zheludkevich presented a novel approach of inhibitor storage in silica nanocontainers (Shchukin et al. 2006; Zheludkevich et al. 2007). Silica nanoparticles were used as a template and coated by polyelectrolyte molecules using LbL deposition procedure. The LbL assembly was performed step by step from the solutions congaing positively charged poly(ethylene imine) (PEI) and negatively charged poly(styrene sulfonate) (PSS) polymeric molecules. Since SiO2 nanoparticles bear negative charge, the first deposition step involved PEI deposition followed by the second deposition of PSS. Deposition of the third inhibitor layer (third) was accomplished in acidic media (pH 3) from a 10 mg/mL solution of benzotriazole . The last two deposition steps (PSS and benzotriazole) were repeated one more time to ensure the highest inhibitor loading in the final LbL structure. The resulting nanocontainers had a five-layer structure: SiO2/PEI/PSS/benzotriazole/PSS/benzotriazole. Sol-gel hybrid composite coatings were synthesized in order to evaluate corrosion protective capabilities of the nanocontainers for corrosion protection of AA2024 substrate. The sol-gel was synthesized by a controlled hydrolysis of zirconium propoxide and organofunctional siloxane. During the sol-gel synthesis, either benzotriazole or SiO2/LbL nanocontainers were dispersed in the sol-gel to ascertain the influence of such additives on the corrosion protection performance of the coatings. AA2024 samples were dip coated into the final sols then dried in air and baked at 120 °C. Corrosion protective properties and self-healing ability of the coatings was accessed by electrochemical impedance spectroscopy (EIS) and scanning vibrating electrode technique (SVET). Benzotriazole was found to negatively affect barrier properties of the sol-gel matrix sufficiently reducing the corrosion protection. On the other hand, both experimental approaches demonstrate that local corrosion activity triggers the release of a portion of benzotriazole from the nanocontainers suppressing the corrosion process in the defective area of the coating. Such a “smart” self-healing effect can originate from the active feedback between the coating and the localized corrosion processes as was presented on a scheme in Fig. 10.
Fig. 10

Scheme of the controllable release of the inhibitor from the nanocontainers distributed in the sol-gel matrix and the “smart self-healing” process (Adapted from Zheludkevich et al. 2007)

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.

In spite of high potential as carriers in delivery systems, natural halloysite nanotubes have some drawbacks associated with its particle size, the small size of its inner hollow lumen, and the presence of 10–20% of impurities in the commercial supply. The small size of the inner lumen does not permit to incorporate a sufficient amount of inhibitor to the containers. To overcome these obstacles, one can use mesoporous and microporous particles as hosts for the entrapped inhibitor (Skorb et al. 2009a). Nano-SiO2 particles were prepared with control chemical modification using the Stöber process using ammonia-catalyzed hydrolysis of TEOS in a batch reactor. The starting reagents were tetraethoxysilane, ethanol, and water, and cetyltrimethylammonium bromide as the structure-directing agent. The containers had an average pore size of about 6–7 nm determined from BET adsorption isotherms. The 2-(benzothiazol-2-ylsulfanyl)-succinic acid corrosion inhibitor was loaded inside the pores of the SiO2 containers. In order to prevent a spontaneous inhibitor release, the containers were coated by polyethyleneimine (PEI) and PSS polyelectrolyte layers. Self-healing abilities of the sol-gel coatings containing the nanocontainers were investigated on AA2024 substrates with the help of a scanning vibrating electrode technique (SVET). Artificial scratches were made on sol-gel-coated substrates without and with SiO2 nanocontainers. SVET monitored the ionic current densities at the surface with scratches. A kinetics of ionic current change can be seen in Fig. 11 (1-A and 2-A for the coatings without and with SiO2 nanocontainers, respectively). The coating with nanocontainers obviously demonstrates high anticorrosion potential and self-healing abilities since the ionic currents are significantly lower than those of the coating without nanoparticles (Fig. 11: 2-B and 1-B, respectively).
Fig. 11

(a) Time dependence of the corrosion propagation of the scratched aluminum alloy covered by a sol-gel film without (1) and with containers (2). (b) SVET ionic currents above the alloy surface coated by SiOx-ZrOx film for 54 h after starting corrosion test (Skorb et al. 2009a). Reprinted by permission of John Wiley & Sons, Inc

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.

Conclusive Remarks

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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Copyright information

© Springer International Publishing AG 2017

Authors and Affiliations

  • K. A. Yasakau
    • 1
    Email author
  • M. G. S. Ferreira
    • 1
  • M. L. Zheludkevich
    • 2
    • 3
  1. 1.Department of Materials and Ceramic EngineeringCICECO – Aveiro Institute of Materials, University of AveiroAveiroPortugal
  2. 2.Magnesium Innovation CentreMagIC at Helmholtz-Zentrum GeesthachtGeesthachtGermany
  3. 3.Institute for Materials Science, Faculty of EngineeringKiel UniversityKielGermany

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