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Smart hybrid coatings for corrosion protection applications
CHAPTER 10
Smart hybrid coatings for corrosion protection applications Riyas Sharafudeen Department of Chemical and Process Engineering Technology, Jubail Industrial College, Jubail Industrial City, Saudi Arabia
Contents 10.1 Introduction 10.2 Different methods of corrosion protection 10.3 Smart hybrid-coating technologies 10.4 Properties and characteristics of smart hybrid coatings 10.5 Recent developments in smart hybrid coatings 10.6 Conclusion References
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10.1 Introduction Hybrid materials are composites of two or more substances. Generally a hybrid material contains an inorganic and organic material. The first hybrid materials prepared were paints made from inorganic and organic materials. The hybrid coatings might have good biocompatibility for biomedical applications. The sol gel process developed in the 1930s was one of the major driving forces of what has become a broad field of inorganic organic hybrid materials. Hybrid materials can be classified based on the possible interactions between the inorganic and organic species in the composite, namely class I and class II. In class I hybrid materials there are weak interactions such as van der Waals and hydrogen bonding, or weak electrostatic interactions between the two species. In class II hybrid materials are those which have strong chemical interactions between the components such as covalent bonds [1]. Compared to organic coatings, organic inorganic hybrid coatings can potentially improve the anticorrosion performance. The organic phase provides excellent mechanical and Advances in Smart Coatings and Thin Films for Future Industrial and Biomedical Engineering Applications DOI: https://doi.org/10.1016/B978-0-12-849870-5.00010-0
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barrier properties, while the inorganic phase acts as an adhesion promoter and corrosion inhibitor [2]. In hybrid coatings, the organic component influences the properties of a layer as its thickness increases and improves the resistance of the layer to cracking, while it also increases layer cross-linking of chemical groups such as epoxy and acrylic or vinyl groups, during the process of addition polymerization. Doping organic substances in the inorganic matrices has a positive impact on the barrier capacity of such layers [3]. The layers of the inorganic component enhance mechanical properties such as hardness and provide higher resistance toward abrasions. These mechanical properties can be additionally enhanced by the introduction of nanoparticles such as montmorillonite [4] or ZrO2 [5] to the matrix. Nanoparticles can be introduced into a hybrid matrix in a number of ways, such as in situ, during synthesis, or in the form of a colloidal suspension, including nanometric powders [6]. Another possibility is the introduction of corrosion inhibitors to the hybrid matrix in the form of nanoparticles during the synthesis phase [7]. Organic inorganic coatings doped with corrosion inhibitors, such as cerium salts (III), borates, permanganate ions, vanadium compounds, phosphorus compounds, merkapro benzothiazol, 2-merkaprobenzimidazol, etc. [8 10], introduced in the form of nanoparticles can act as reservoirs with controlled and triggered release of the corrosion inhibitors during contact with an aggressive environment. Additionally, the chemical stability of an oxide sol gel network and the possibility of the gradual dissolution of active compounds such as corrosion inhibitors from the oxide network make this a perfect material for long-term corrosion protection. Moreover, organic inorganic hybrid coatings doped with corrosion inhibitors such as cerium salts (III) have a self-protecting or “healing” effect in that the resultant oxides and cerium hydroxides formed as a result of exposure to the corrosive agent are precipitated at the place of damage, which helps to seal the affected local area [11]. Many studies on alkoxyl silane-based hybrid coatings have been studied for their weatherability and anticorrosion performance [12]. Organic inorganic hybrid coatings based on mixed sol gel precursors have received less attention compared to alkoxyl silane-based hybrid coatings. Introducing the polysiloxane part into the polyurethane coatings enhanced the crosslinking density, glass transition temperature (Tg), mechanical properties, and general coating properties [13]. Polyurethane coatings derived from cycloaliphatic polyester showed comparable properties to those derived from the commercially viable aromatic polyester [14 16].
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10.2 Different methods of corrosion protection Corrosion is a natural phenomenon that occurs in metals under specific conditions and leads to deterioration that is reflected as potential losses in human life and assets. Corrosion prevention in a specific environment is usually performed using a corrosion-resistant material, or at least one with an exceptional lifetime. There are several methods for protection against corrosion that are based on electrochemical principles [17,18]. Corrosion of metals can be controlled by different methods such as barrier protection by coatings, electrochemical protections such as cathodic protection, impressed current protection, etc. One of the easiest and most economical ways to prevent metals from corrosion is to use barrier coatings. These coatings can be paint, plastic, or powder, including epoxy, nylon, and polyurethane. These materials are deposited or sprayed onto the metal surface as a thin film. The main drawback of barrier coatings is that these coatings must be reapplied after a period of time. Improper barrier coatings can quickly fail and lead to increased levels of corrosion. Another method of corrosion protection is by applying a coating of metal with more oxidative potential than the metal to be prevented from corrosion. For example, iron objects can be prevented from corrosion by coating them with molten zinc. This process is called galvanization. This process has been around for more than 250 years and has been used for corrosion protection of objects like artistic sculptures and playground equipment. Compared to other corrosion prevention methods, galvanization is known for lower-initial costs, sustainability, and versatility. However, galvanization cannot be done onsite, hence the objects need to be taken out of work to apply the zinc coating. The toxicity of the released zinc fumes during the galvanizing process is another disadvantage. Alloying is another option to protect metals from corrosion and offers added strength and resistance to the alloyed product. Corrosion-resistant nickel is combined with oxidation-resistant chromium to produce an alloy that can be used in oxidized and reduced chemical environments. Different alloys provide resistance to different corrosive conditions. Alloyed materials are very expensive and any cracks or scratches can result in an increase of corrosion. Cathodic protection is another common method used globally. In this method, metal is connected to another metal to form an electrochemical cell in such a way that the metal to be protected acts as cathode and the other metal acts as anode when exposed to a corrosive electrolyte. Metals used as anodes include aluminum,
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magnesium, or zinc. Cathodic protection is highly effective, but the anodes used must be checked and replaced frequently. In barrier protection, the scope for hybrid coatings is much higher than other conventional coating methods. The application of hybrid coatings provides enhanced and durable anticorrosive performance. The formation of an oxide protective layer with a covalent bond provides the basis for superior corrosion protection of hybrid coatings. The corrosion protection properties of the hybrid organoceramic coatings on aluminum substrates can be significantly improved by means of encapsulation of organic corrosion inhibitors within the coating matrix [19]. The increased adhesion between the hybrid coating and the substrate makes the hybrid coating a good candidate for anticorrosion applications. The anticorrosion performance of the cycloaliphatic-hybrid coatings was much better than that of aromatic-based hybrid coatings under outdoor weathering [20]. Acid undercutting is an issue for aliphatic-based (tetraethylorthosilicate) hybrid coating. The improved resistance to acid undercutting was observed for mixed sol gel precursor-based hybrids. Khramov et al. reported the improved corrosion protection of mercaptobenzothiazole or mercaptobenzimidazole in coatings in the presence, or absence, of β-cyclodextrin [19]. Sol gel chemistry, used in the synthesis of inorganic oxides, has gained significant interest in the coatings industry due to its unique advantages such as easy processing, high purity, and low processing temperature [21,22]. In the sol gel process, hydrolysis and condensation of metal alkoxide monomers lead to three-dimensional oxide networks [23]. Sol gel technology is based on the synthesis of a gel from an organic inorganic sol by a gelation process [24,25]. Silicon alkoxides—for example, tetraethylorthosilicate (TEOS) and tetramethylorthosilicate (TMOS)—or alkoxides of the transition elements of the periodic table (e.g., zirconium, aluminum, cerium, titanium) are used as precursors in the synthesis of an oxide network in the classical sol gel method. Chou et al. reported that silica-based hybrid coatings prepared by an acid catalyst sol gel process and applied on stainless-steel substrates demonstrated enhanced corrosion protection by forming a physical barrier that effectively separated the anode from the cathode. The atomic force microscopy (AFM) image shown in Fig. 10.1 indicates that sol gel coatings have a smooth-surface morphology regardless of the amount of organic component incorporated into the silica gel network [25]. Fig. 10.2 displays the polarization curves for bare and 10% of 3-methacryloxypropyltrimethoxysilane (MPS)
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Figure 10.1 AFM image of 10% MPS sol gel coating on a stainless-steel substrate. T.P. Chou, et al., J. Non-Cryst. Solids 290 (2001) 153 162.
sol-gel-coated stainless-steel substrates. The polarization curve of the sol gel coating was appreciably different from that of the bare stainlesssteel substrate. The scanning electron microscope (SEM) images of the bare and 10% MPS sol-gel-coated stainless-steel substrates after potentiostat polarization tests are shown in Fig. 10.3. Although corrosion is observed in both cases, the extent of corrosion was appreciably different. The corrosion pits in the bare substrate are much larger than that in sol gel coated substrates. In addition, the number of pits in the bare substrate was more than that in the coated substrate [25]. A variety of organic inorganic hybrid coatings has been developed as anticorrosive coatings for metallic surfaces, with the most important ones being epoxy silica and poly(methyl methacrylate) (PMMA) silica hybrids, which have demonstrated high-anticorrosive efficiency combined with high thermal and mechanical resistance [26,27]. In addition, corrosion inhibitors can be added while preparing the coating mixture. Cerium (IV) has been included in PMMA silica hybrids as a corrosion inhibitor [28]. Corrosion inhibitors are substances that reduce or even eliminate corrosion when present in suitable concentrations in the corrosive medium. Inhibition is accomplished by one or a combination of several mechanisms, such as adsorption, forming a ultrathin film with a thickness of only few molecular layers, in the form of visible bulky precipitates that coat the metal surface, or other common methods consisting of the combination of adsorption, conversion, and oxidation processes to form a passive layer.
Figure 10.2 Polarization curves of bare and 10% MPS sol gel coated different stainless-steel substrates. T.P. Chou, C. Chandra Shekaran, S. Limmer, C. Nguyen, G.Z. Cao, J. Mater. Sci. Lett. 21 (2002) 251 255.
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Figure 10.3 SEM images of (A) bare and (B) 10% MPS sol-gel-coated stainless-steel substrates after corrosion pitting from polarization analysis. T.P. Chou, C. Chandra Shekaran, S. Limmer, C. Nguyen, G.Z. Cao, J. Mater. Sci. Lett. 21 (2002) 251 255.
Some examples of the most commonly applied inhibitors are phosphates, chromates, silicates, hydroxides, carbonates, sulfates, aldehydes, amines, nitrogen heterocyclic compounds, urea, etc. [29].
10.3 Smart hybrid-coating technologies Hybrid coatings can be applied onto any substrate using different methods. These techniques for the deposition of smart hybrid coatings on substrates include physical vapor deposition, chemical vapor deposition, electrochemical deposition, plasma spraying, and the sol gel process. The most common techniques are spray coatings, sol gel method, and electrostatic powder coating. In spray coating, the coating mix is prepared and sprayed as thin layer over the substrate using a pressurized spray chamber using a spray gun. The narrow orifices in the spray gun distribute the hybrid material over the surface in a thin layer. The sol gel process is a chemical synthesis method where an organic inorganic network is created by progressive condensation reactions. The process was developed for synthesizing inorganic, mixed oxides. Hybrid coatings can be developed by using this technique. This method has several advantages compared to other methods. The first step to produce a hybrid coating by the sol gel method is to prepare a stable colloidal suspension, called sol, of the coating mix. This is a typical chemical transformation aiming the formation of a sol of colloidal particles or a solution of oligomers (small polymers). As sol is a fluid, it can be cast in a mold or applied onto a surface using various shaping techniques (e.g., spraying on a surface, dipping, or spinning through a set of rotating
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nozzles). As the sol is a relatively stable solution it can be stored for a certain time before further casting, or applied by any of the above methods. Spraying and electrodeposition processes emerged recently and could be the major sol gel coating application methods in the near future [24,30,31]. The sol gel method is known as a “green” technology since it uses compounds that do not introduce impurities into the coated surface and is waste-free, excludes the stage of washing, and the processing temperature is generally low (i.e., frequently close to room temperature) [32]. Additionally, materials with high-specific porosity and specific-surface area can be easily obtained by this method, which also allows the incorporation of substances, such as inhibitors. The low-synthesis temperatures minimize the thermal volatilization and degradation of the entrapped species [24,30]. A new method of forming functionalized silica nanoparticles in situ in an aqueous-based sol gel process and then cross-linking the nanoparticles to form a thin film is an excellent example of a nanoscience approach to coatings. This coating method is called the self-assembled nanophase particle (SNAP) process. The SNAP-coating process consists of three stages: (1) sol gel processing, (2) SNAP solution mixing, and (3) SNAP-coating application and curing. The sol gel processing stage involves hydrolysis and condensation reactions and is controlled by the solution pH and water content. Here the molar ratio of water to hydrolysable silane is a key factor. In the SNAP solution mixing stage, crosslinking agents and additives are added to the solution, which is then applied to a substrate by dip-coating to form the SNAP coating. SNAP films are amorphous, but exhibit a nanostructured assembly of siloxane oligomers at a separation of about 1.8 nm as well as molecular level ordering of O Si O species. The electrodeposition coating method uses electricity to deposit a smooth, thin, uniform layer of coating onto the surface of a metal substrate. The metal surface is first cleaned with a powerful combination of spray and dip alkaline cleaners in order to remove dirt, oils, and other contaminants that can interfere with the development of good adhesion or could cause surface defects. It is then rinsed with water and the surface is activated through salt immersion. Electrodeposition involves the reduction of metallic ions that are generated from the electrolyte and converted to the deposited metallic ions in a cathode surface (coating substrate). There is no need to supply reduction current from an external source; for instance, Cu can be deposited on Fe by a substitution process. The coatings are very thin and weakly adhesive, which limits their application on
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the industrial scales. Some metals, such as Ni, Cu, and Pd can be deposited through chemical reduction from an aqueous solution of their salts as the initial layer accelerates the further depositing. Hard-nickel alloys deposited through solutions containing phosphor or boron components are widely applied as reduction factors in engineering applications. Commonly the process takes place in polypropylene or stainless-steel tanks with polytetrafluoroethylene (PTFE) plating where the solution is maintained at 90°C. These tanks are equipped with some devices for precise temperature control as well as solution stirring and filtering. As with electrodeposition the substrate needs to be cleaned [33]. Electrodeposition coatings always produce the optimum result and they play a decisive role in corrosion protection and extending the service life of bodywork, wheels, and other vehicle components. The high efficiency of electrodeposition coating is the key advantage of e-coatings. During this procedure, an even film is applied to the entire surface, including areas that are contorted and therefore difficult to access. The need for reworking is largely eliminated and scrap risk is minimized. Furthermore, in comparison with spray coating, electrodeposition coating ensures a significantly lower level of material loss. The use of electrodeposition coatings cause less strain on the environment (e.g., due to solvent vapors) in comparison with other coating processes.
10.4 Properties and characteristics of smart hybrid coatings The development of hybrid materials has been extensively investigated over the past few years. The combination of a wide variety of compositions and production processes permitted the use of these materials in different applications, including coatings for corrosion protection of metals. Hybrid coatings doped with corrosion inhibitors provide active corrosion protection. Corrosion inhibitors can be added to the coating mix before applying to the metal substrate or at the stage of film formation and crosslinking, which also enhances the corrosion protection of hybrid coatings [34]. Many kinds of inorganic inhibitors with low toxicity have been used to increase the corrosion protection of hybrid coatings. Inorganic inhibitors, such as phosphates [35], vanadates [36], and rare-earth elements [8,37], have shown positive influence on the corrosion protection of aluminum alloys. Cerium ions with relatively low cost and abundance satisfy the basics requirements for being considered as an alternative inhibitor to
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chromates. The main role of Ce31 and La31 in corrosion protection is the formation of insoluble deposits on intermetallic inclusions that prevents the local increase of pH, which is responsible for the acceleration of the intermetallic dealloying. The entrapped corrosion inhibitors become active in the corrosive electrolyte in response to pH changes caused by the corrosion process so that corrosion inhibitors slowly diffuse out of the host material. The formation of hydroxide precipitates can also act as a diffusion barrier hindering the corrosion processes in active zones. Cerium nitrate exhibits higher inhibition protection efficiency in comparison with lanthanum nitrate in hybrid coatings due to the lower solubility of cerium hydroxide [38]. Various organic corrosion inhibitors, such as triazole and thiazole derivatives [37,39] and 8-hydroxy quinolone (8HQ) [40], have also been investigated. It was reported that the organic species is adsorbed on the metal surface, avoiding the adsorption of aggressive ions such as chloride or hydroxide [40]. Different types of smart hybrid coatings are currently available. These coatings have high energy-saving potential, high flexibility, high chemical resistance, and good mechanical properties like high scratch resistance and elasticity. Hybrid powder coatings based on polyester and epoxy resins are available with excellent all-round properties and, therefore, they are the most-used powder coatings in the coatings sector. They are versatile in use and meet many requirements for indoor goods, for example, household articles like cabinet assemblies, steel furniture, shelves, fire extinguishers, colored bottles, clay pots, and others can be coated with hybrid coatings to enhance their appearance and lifespan. Hybrid coatings can be applied to almost any surface and is particularly advantageous in institutional and commercial applications as well as for cleaning rooms in the medical and food industries due to their low toxicity and corresponding ability to be applied without affecting the daily business of the organization or home. Smart hybrid coatings can be applied to industrial and commercial buildings, chemical plants, medical and pharmaceutical cleaning rooms, warehouses, floorings, military equipment, and protective coatings inside pipes. The protection of ancient stone monuments and buildings from the attack of corrosive agents from the environment can be successfully carried out using polymer-based hybrid coatings [41]. Among other advantages of hybrid coatings is their possibility to develop high-mechanical resistance upon curing and hardening under a wide range of environmental conditions, low weight due to the small thicknesses required for efficient protection, wear and abrasion resistance, and good bond with stone [42,43].
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10.5 Recent developments in smart hybrid coatings Hybrid coatings are highly functional coatings with excellent corrosion protection, good adhesion properties, and outstanding chemical resistance. The epoxy hybrid coatings are highly economical and versatile interior coatings. They can be used in automobiles as primers to increase protection against corrosion with excellent weather resistance, good flexibility, and outstanding adhesion properties. Smart hybrid coatings are employed to control temperature changes in the windows and doors in buildings due to solar radiation. These coatings block infrared radiation (IR) to 70% at high temperatures, while allowing it to go through at low temperatures. Thermochromic and vanadium dioxide (VO2)-based smart hybrid coatings are promising materials due to their reversible-phase transition and high efficiency in modulating IR transmittance. According to various researchers, other techniques based on advanced nanotechnology can be used to improve the thermochromic performance of smart hybrid coatings. These advantages make the plasmonic thermochromic coatings promising as intelligent air conditioners without energy cost [44 54]. The strength and durability of cement floors can be enhanced by smart hybrid coatings. These coatings save flooring from potential damage, but also save money as well. Hybrid-coating technologies offer superior alternative paints and coatings for not only floors but also electrical potting, which are safe and effective. This means they are free of dangerous isocyanates that can cause damage to the respiratory system and, therefore, protect those who live or work inside treated buildings by using green, and environment-friendly hybrid coating products that are strong, effective, and durable. Improvements in hybrid-coating technology are essential from the industrial point of view so that coated materials are much more capable of withstanding extreme corrosive environments, by temperature and erosion than a conventional coating. Recent research has focused on alternative technologies with significant improvements in the performance of coatings. Studies show that this can be achieved by changing the size, shape, and distribution of the coating matrix to produce ultrafine-grained coating materials [55]. Silicone is another material that can be used for making hybrid coatings. It has unique properties such as tolerance to high temperatures, inertness, and adjustable surface wettability properties. Its rubbery texture and resistance to chemicals means that it must be mechanically removed for rework. Another material which can be used in
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hybrid coatings is epoxy resins. Epoxy resin is an extremely hard material that has some unique uses. Its rigidity means that it can be used as mechanical reinforcement, but, more interestingly, it can be used as a security device. Combining epoxy with other materials (e.g., crossbars), a rigid structure can be made that actually destroys itself and adjacent items if an attempt to mechanically separate it from the printed circuit board (PCBA) is made. This security feature has a rating system federal information processing standard (FIPS) that defines the minimum levels of self-destruction allowable. Epoxy is also resistant to heat and chemical corrosion. Its hardness and set time are disadvantageous as it increases process time and makes rework virtually impossible. Hybrid coatings based on the graphene oxide zirconia dioxide/epoxy (GO ZrO2/EP) system were fabricated and applied for corrosion protection of metal substrates by Di et al. [56]. Acrylic resin is probably the most common coating material used today. It is also the least expensive of the materials in use. Its main advantages are low cost and ease of handling. Polyurethane is another common material used in hybrid coatings. Its smooth hydrophobic and oleophobic properties make it an excellent coating material. Hamidreza Asemani et al. reported that polyurethane coatings are a promising alternative for various industrial applications [57]. A pigment particle can also be incorporated into hybrid coatings. Pigment particles in hybrid coatings can be classified into two groups, namely, particles forming in sols and traditionally added pigments. The particles forming in sols are different from traditional pigments. There are strong chemical interactions between sol particles and organic components when hybrid coatings are dried. When the concentration of traditional pigments in the coating exceeds their critical pigment volume concentration (CPVC), the apertures in the films accrue sharply, which degrades the physical barrier properties of coatings. The CPVC of traditional pigments lies in the 50% 60% range. However, there is no CPVC for the sol particles in hybrid coatings, and hybrid films can be formed at random ratios of inorganic organic components. When the average diameter of sol particles exceeds 100 nm the transparency of hybrid coatings decreases abruptly to make the films opaque, but sol particles do not have the color aptitude and conditioning of traditional pigments [34]. When the concentration of traditional pigments is below CPVC, the permeability of coatings monotonically decreases with increases of the added pigment loading [58]. The pigments in flakes, such as in montmorillonite clay or aluminum flakes, should reduce coating permeability remarkably. Nanoscale
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montmorillonite clay is dispersed in the precursor solution of the hybrid coatings to form coatings for alloys. Formulation of the coating with a diaminosilane curing agent and clay nanofillers significantly improves the coating resistance and reduces the water uptake to about 3% 5% after 8 weeks of immersion in a 3.5% NaCl solution [59]. The list goes on and new ideas are constantly being tried to solve the problems of specific applications. Further recent improvements in coating methods are summarized next. In the electrostatic spray (ES) or powder coating (ES) technique an electrically charged coating material is sprayed onto earthed-metal items. The metal items are then placed in an industrial oven and heated until the powder melts. The item is cooled to leave a high-quality plastic coating. The fluidized bed coating technique uses fluidized bed powders that behave like a liquid when pressurized air is allowed to bubble up through it. A metal item, heated to the correct temperature, is then completely immersed into this fluidized bed. The powder melts on contact with the metal and the item is then lifted out of the fluidized bed. The item is cooled to leave a high-quality plastic coating. The flock spraying method uses uncharged powder that is blown onto a hot metal item where it melts to form a coating. The item is then left to cool to leave a good-quality plastic coating. In the case of flame spraying, uncharged powder is blown inside a flame onto a preheated metal item. The powder melts on transit from the application gun to the metal surface and forms a solid coating as the metal item cools down. This is suitable for onsite coating or repairs. For example, in order to protect wires as the wires are extruded from the extruder they are passed through a molten plastic. The wire is then allowed to cool to leave a continuous, protective coating. In the spin-coating method the object that requires coating, typically the inside of a bottle or cylinder, is heated up to the desired temperature. Fluidized bed powder is then injected into the object. The object is then immediately spun and tumbled to give a complete and consistent coating inside the bottle. Any unused powder is then tipped out of the object. Corrosion protection of steel by hybrid sol gel coating was reported by Avci and Abanoz [60]. Coatings prepared by spraying coating mix with different epoxy resins and polyurethane hybrid coatings heat treating at 110 150°C showed better mechanical strength, flexibility, and corrosion resistance than those with coatings of tetraethylorthosilicate and 3-glycidoxypropyltrimetoxysilane prepared by the sol gel method.
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Figure 10.4 Schematic illustration of a three-layered system based on hybrid sol gel coatings and how the preemptive healing system works. R.B. Figueira, I.R. Fontinha, C.J.R. Silva, E.V. Pereira, Coatings 6 (1) (2016) 12.
It is undeniable that future applications and research will be focused primarily on the investigation of more environment-friendly precursors/ reagents or industrial colloid particles to replace the traditional precursors in the formation of sol gel protective coatings without changing their general properties. Combined systems will also be developed in order to achieve the best protection possible against corrosion, such as metal-rich sol gel coatings containing zinc or magnesium particles, or other inhibitor particles, which combine the barrier properties of the sol gel layer and cathodic protection of sacrificial particles. Multilayer systems, such as the one proposed in Fig. 10.4 in which a preemptive behavior, are also being considered. Water and electrolyte diffusion through the sol gel top coat can generate corrosion reactions at the metal oxide interface. Preemptive healing could occur through the trapping of water and corrosives and/or the release of inhibitors from the hybrid sol gel that targets the corrosion reactions and shuts them down. Self-healing agents also heal the top coat from the effects of weathering or other damage. This system combines different sol gel layers with different functions, which is a very promising area to explore [18]. Intrinsically conductive polymers (ICPs), which can undergo redox reactions with the surrounding environment, represent a potential class of organic coatings for corrosion protection applications. Polyaniline (PANI)
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Figure 10.5 Generic scheme showing the corrosion-healing mechanism of a hybrid sol gel coating containing inhibitor-loaded nanoparticles. R.B. Figueira, I.R. Fontinha, C.J.R. Silva, E.V. Pereira, Coatings 6 (1) (2016) 12.
with adjustable electronic and protonic conductivities can be utilized as ICPs in different forms for corrosion protection of various ferrous and nonferrous metals [61]. Nanoscale coatings have recently been proposed for anticorrosion selfhealing, antiicing, self-cleaning, etc. Fig. 10.5 represents a generic scheme showing the corrosion-healing mechanism of a hybrid sol gel coating containing inhibitor-loaded nanoparticles [18]. Different types of nanomaterials have been incorporated in anticorrosion coatings by adopting various approaches. The basic approach utilizes the incorporation of inorganic nanomaterials with traditional organic coatings to enhance certain functionalities of the formulated nanocomposite coating. One of the recent trends in nanotechnology is to design nanomaterial-based coatings with multifunctionality [62 64].
10.6 Conclusion The corrosion of metals and their objects exposed to harsh environmental conditions can be successfully prevented using smart hybrid coatings. The development of hybrid materials has been extensively studied in recent years. A wide variety of organic and inorganic materials in different compositions can be used to make hybrid coatings, which can be applied as coatings for corrosion protection of metals. Electrochemical analyses proved that relatively dense hybrid coatings provided excellent corrosion protection by forming a physical barrier, which effectively separates the
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anodic part from the cathodic part. Some preliminary biocompatibility tests conducted on hybrid coatings recommends that the hybrid coatings can be applied for biomedical applications due to their good biocompatibility. The composition of the coating mix and types of epoxy material selected for making the hybrid coating determine the efficiency of corrosion protection. Corrosion control by hybrid coatings is an environmentfriendly and promising technology for surface protection and corrosion prevention.
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Smart hybrid coatings for corrosion protection applications Riyas Sharafudeen Department of Chemical and Process Engineering Technology, Jubail Industrial College, Jubail Industrial City, Saudi Arabia
Contents 10.1 Introduction 10.2 Different methods of corrosion protection 10.3 Smart hybrid-coating technologies 10.4 Properties and characteristics of smart hybrid coatings 10.5 Recent developments in smart hybrid coatings 10.6 Conclusion References
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10.1 Introduction Hybrid materials are composites of two or more substances. Generally a hybrid material contains an inorganic and organic material. The first hybrid materials prepared were paints made from inorganic and organic materials. The hybrid coatings might have good biocompatibility for biomedical applications. The sol gel process developed in the 1930s was one of the major driving forces of what has become a broad field of inorganic organic hybrid materials. Hybrid materials can be classified based on the possible interactions between the inorganic and organic species in the composite, namely class I and class II. In class I hybrid materials there are weak interactions such as van der Waals and hydrogen bonding, or weak electrostatic interactions between the two species. In class II hybrid materials are those which have strong chemical interactions between the components such as covalent bonds [1]. Compared to organic coatings, organic inorganic hybrid coatings can potentially improve the anticorrosion performance. The organic phase provides excellent mechanical and Advances in Smart Coatings and Thin Films for Future Industrial and Biomedical Engineering Applications DOI: https://doi.org/10.1016/B978-0-12-849870-5.00010-0
© 2020 Elsevier Inc. All rights reserved.
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barrier properties, while the inorganic phase acts as an adhesion promoter and corrosion inhibitor [2]. In hybrid coatings, the organic component influences the properties of a layer as its thickness increases and improves the resistance of the layer to cracking, while it also increases layer cross-linking of chemical groups such as epoxy and acrylic or vinyl groups, during the process of addition polymerization. Doping organic substances in the inorganic matrices has a positive impact on the barrier capacity of such layers [3]. The layers of the inorganic component enhance mechanical properties such as hardness and provide higher resistance toward abrasions. These mechanical properties can be additionally enhanced by the introduction of nanoparticles such as montmorillonite [4] or ZrO2 [5] to the matrix. Nanoparticles can be introduced into a hybrid matrix in a number of ways, such as in situ, during synthesis, or in the form of a colloidal suspension, including nanometric powders [6]. Another possibility is the introduction of corrosion inhibitors to the hybrid matrix in the form of nanoparticles during the synthesis phase [7]. Organic inorganic coatings doped with corrosion inhibitors, such as cerium salts (III), borates, permanganate ions, vanadium compounds, phosphorus compounds, merkapro benzothiazol, 2-merkaprobenzimidazol, etc. [8 10], introduced in the form of nanoparticles can act as reservoirs with controlled and triggered release of the corrosion inhibitors during contact with an aggressive environment. Additionally, the chemical stability of an oxide sol gel network and the possibility of the gradual dissolution of active compounds such as corrosion inhibitors from the oxide network make this a perfect material for long-term corrosion protection. Moreover, organic inorganic hybrid coatings doped with corrosion inhibitors such as cerium salts (III) have a self-protecting or “healing” effect in that the resultant oxides and cerium hydroxides formed as a result of exposure to the corrosive agent are precipitated at the place of damage, which helps to seal the affected local area [11]. Many studies on alkoxyl silane-based hybrid coatings have been studied for their weatherability and anticorrosion performance [12]. Organic inorganic hybrid coatings based on mixed sol gel precursors have received less attention compared to alkoxyl silane-based hybrid coatings. Introducing the polysiloxane part into the polyurethane coatings enhanced the crosslinking density, glass transition temperature (Tg), mechanical properties, and general coating properties [13]. Polyurethane coatings derived from cycloaliphatic polyester showed comparable properties to those derived from the commercially viable aromatic polyester [14 16].
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10.2 Different methods of corrosion protection Corrosion is a natural phenomenon that occurs in metals under specific conditions and leads to deterioration that is reflected as potential losses in human life and assets. Corrosion prevention in a specific environment is usually performed using a corrosion-resistant material, or at least one with an exceptional lifetime. There are several methods for protection against corrosion that are based on electrochemical principles [17,18]. Corrosion of metals can be controlled by different methods such as barrier protection by coatings, electrochemical protections such as cathodic protection, impressed current protection, etc. One of the easiest and most economical ways to prevent metals from corrosion is to use barrier coatings. These coatings can be paint, plastic, or powder, including epoxy, nylon, and polyurethane. These materials are deposited or sprayed onto the metal surface as a thin film. The main drawback of barrier coatings is that these coatings must be reapplied after a period of time. Improper barrier coatings can quickly fail and lead to increased levels of corrosion. Another method of corrosion protection is by applying a coating of metal with more oxidative potential than the metal to be prevented from corrosion. For example, iron objects can be prevented from corrosion by coating them with molten zinc. This process is called galvanization. This process has been around for more than 250 years and has been used for corrosion protection of objects like artistic sculptures and playground equipment. Compared to other corrosion prevention methods, galvanization is known for lower-initial costs, sustainability, and versatility. However, galvanization cannot be done onsite, hence the objects need to be taken out of work to apply the zinc coating. The toxicity of the released zinc fumes during the galvanizing process is another disadvantage. Alloying is another option to protect metals from corrosion and offers added strength and resistance to the alloyed product. Corrosion-resistant nickel is combined with oxidation-resistant chromium to produce an alloy that can be used in oxidized and reduced chemical environments. Different alloys provide resistance to different corrosive conditions. Alloyed materials are very expensive and any cracks or scratches can result in an increase of corrosion. Cathodic protection is another common method used globally. In this method, metal is connected to another metal to form an electrochemical cell in such a way that the metal to be protected acts as cathode and the other metal acts as anode when exposed to a corrosive electrolyte. Metals used as anodes include aluminum,
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magnesium, or zinc. Cathodic protection is highly effective, but the anodes used must be checked and replaced frequently. In barrier protection, the scope for hybrid coatings is much higher than other conventional coating methods. The application of hybrid coatings provides enhanced and durable anticorrosive performance. The formation of an oxide protective layer with a covalent bond provides the basis for superior corrosion protection of hybrid coatings. The corrosion protection properties of the hybrid organoceramic coatings on aluminum substrates can be significantly improved by means of encapsulation of organic corrosion inhibitors within the coating matrix [19]. The increased adhesion between the hybrid coating and the substrate makes the hybrid coating a good candidate for anticorrosion applications. The anticorrosion performance of the cycloaliphatic-hybrid coatings was much better than that of aromatic-based hybrid coatings under outdoor weathering [20]. Acid undercutting is an issue for aliphatic-based (tetraethylorthosilicate) hybrid coating. The improved resistance to acid undercutting was observed for mixed sol gel precursor-based hybrids. Khramov et al. reported the improved corrosion protection of mercaptobenzothiazole or mercaptobenzimidazole in coatings in the presence, or absence, of β-cyclodextrin [19]. Sol gel chemistry, used in the synthesis of inorganic oxides, has gained significant interest in the coatings industry due to its unique advantages such as easy processing, high purity, and low processing temperature [21,22]. In the sol gel process, hydrolysis and condensation of metal alkoxide monomers lead to three-dimensional oxide networks [23]. Sol gel technology is based on the synthesis of a gel from an organic inorganic sol by a gelation process [24,25]. Silicon alkoxides—for example, tetraethylorthosilicate (TEOS) and tetramethylorthosilicate (TMOS)—or alkoxides of the transition elements of the periodic table (e.g., zirconium, aluminum, cerium, titanium) are used as precursors in the synthesis of an oxide network in the classical sol gel method. Chou et al. reported that silica-based hybrid coatings prepared by an acid catalyst sol gel process and applied on stainless-steel substrates demonstrated enhanced corrosion protection by forming a physical barrier that effectively separated the anode from the cathode. The atomic force microscopy (AFM) image shown in Fig. 10.1 indicates that sol gel coatings have a smooth-surface morphology regardless of the amount of organic component incorporated into the silica gel network [25]. Fig. 10.2 displays the polarization curves for bare and 10% of 3-methacryloxypropyltrimethoxysilane (MPS)
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Figure 10.1 AFM image of 10% MPS sol gel coating on a stainless-steel substrate. T.P. Chou, et al., J. Non-Cryst. Solids 290 (2001) 153 162.
sol-gel-coated stainless-steel substrates. The polarization curve of the sol gel coating was appreciably different from that of the bare stainlesssteel substrate. The scanning electron microscope (SEM) images of the bare and 10% MPS sol-gel-coated stainless-steel substrates after potentiostat polarization tests are shown in Fig. 10.3. Although corrosion is observed in both cases, the extent of corrosion was appreciably different. The corrosion pits in the bare substrate are much larger than that in sol gel coated substrates. In addition, the number of pits in the bare substrate was more than that in the coated substrate [25]. A variety of organic inorganic hybrid coatings has been developed as anticorrosive coatings for metallic surfaces, with the most important ones being epoxy silica and poly(methyl methacrylate) (PMMA) silica hybrids, which have demonstrated high-anticorrosive efficiency combined with high thermal and mechanical resistance [26,27]. In addition, corrosion inhibitors can be added while preparing the coating mixture. Cerium (IV) has been included in PMMA silica hybrids as a corrosion inhibitor [28]. Corrosion inhibitors are substances that reduce or even eliminate corrosion when present in suitable concentrations in the corrosive medium. Inhibition is accomplished by one or a combination of several mechanisms, such as adsorption, forming a ultrathin film with a thickness of only few molecular layers, in the form of visible bulky precipitates that coat the metal surface, or other common methods consisting of the combination of adsorption, conversion, and oxidation processes to form a passive layer.
Figure 10.2 Polarization curves of bare and 10% MPS sol gel coated different stainless-steel substrates. T.P. Chou, C. Chandra Shekaran, S. Limmer, C. Nguyen, G.Z. Cao, J. Mater. Sci. Lett. 21 (2002) 251 255.
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Figure 10.3 SEM images of (A) bare and (B) 10% MPS sol-gel-coated stainless-steel substrates after corrosion pitting from polarization analysis. T.P. Chou, C. Chandra Shekaran, S. Limmer, C. Nguyen, G.Z. Cao, J. Mater. Sci. Lett. 21 (2002) 251 255.
Some examples of the most commonly applied inhibitors are phosphates, chromates, silicates, hydroxides, carbonates, sulfates, aldehydes, amines, nitrogen heterocyclic compounds, urea, etc. [29].
10.3 Smart hybrid-coating technologies Hybrid coatings can be applied onto any substrate using different methods. These techniques for the deposition of smart hybrid coatings on substrates include physical vapor deposition, chemical vapor deposition, electrochemical deposition, plasma spraying, and the sol gel process. The most common techniques are spray coatings, sol gel method, and electrostatic powder coating. In spray coating, the coating mix is prepared and sprayed as thin layer over the substrate using a pressurized spray chamber using a spray gun. The narrow orifices in the spray gun distribute the hybrid material over the surface in a thin layer. The sol gel process is a chemical synthesis method where an organic inorganic network is created by progressive condensation reactions. The process was developed for synthesizing inorganic, mixed oxides. Hybrid coatings can be developed by using this technique. This method has several advantages compared to other methods. The first step to produce a hybrid coating by the sol gel method is to prepare a stable colloidal suspension, called sol, of the coating mix. This is a typical chemical transformation aiming the formation of a sol of colloidal particles or a solution of oligomers (small polymers). As sol is a fluid, it can be cast in a mold or applied onto a surface using various shaping techniques (e.g., spraying on a surface, dipping, or spinning through a set of rotating
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nozzles). As the sol is a relatively stable solution it can be stored for a certain time before further casting, or applied by any of the above methods. Spraying and electrodeposition processes emerged recently and could be the major sol gel coating application methods in the near future [24,30,31]. The sol gel method is known as a “green” technology since it uses compounds that do not introduce impurities into the coated surface and is waste-free, excludes the stage of washing, and the processing temperature is generally low (i.e., frequently close to room temperature) [32]. Additionally, materials with high-specific porosity and specific-surface area can be easily obtained by this method, which also allows the incorporation of substances, such as inhibitors. The low-synthesis temperatures minimize the thermal volatilization and degradation of the entrapped species [24,30]. A new method of forming functionalized silica nanoparticles in situ in an aqueous-based sol gel process and then cross-linking the nanoparticles to form a thin film is an excellent example of a nanoscience approach to coatings. This coating method is called the self-assembled nanophase particle (SNAP) process. The SNAP-coating process consists of three stages: (1) sol gel processing, (2) SNAP solution mixing, and (3) SNAP-coating application and curing. The sol gel processing stage involves hydrolysis and condensation reactions and is controlled by the solution pH and water content. Here the molar ratio of water to hydrolysable silane is a key factor. In the SNAP solution mixing stage, crosslinking agents and additives are added to the solution, which is then applied to a substrate by dip-coating to form the SNAP coating. SNAP films are amorphous, but exhibit a nanostructured assembly of siloxane oligomers at a separation of about 1.8 nm as well as molecular level ordering of O Si O species. The electrodeposition coating method uses electricity to deposit a smooth, thin, uniform layer of coating onto the surface of a metal substrate. The metal surface is first cleaned with a powerful combination of spray and dip alkaline cleaners in order to remove dirt, oils, and other contaminants that can interfere with the development of good adhesion or could cause surface defects. It is then rinsed with water and the surface is activated through salt immersion. Electrodeposition involves the reduction of metallic ions that are generated from the electrolyte and converted to the deposited metallic ions in a cathode surface (coating substrate). There is no need to supply reduction current from an external source; for instance, Cu can be deposited on Fe by a substitution process. The coatings are very thin and weakly adhesive, which limits their application on
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the industrial scales. Some metals, such as Ni, Cu, and Pd can be deposited through chemical reduction from an aqueous solution of their salts as the initial layer accelerates the further depositing. Hard-nickel alloys deposited through solutions containing phosphor or boron components are widely applied as reduction factors in engineering applications. Commonly the process takes place in polypropylene or stainless-steel tanks with polytetrafluoroethylene (PTFE) plating where the solution is maintained at 90°C. These tanks are equipped with some devices for precise temperature control as well as solution stirring and filtering. As with electrodeposition the substrate needs to be cleaned [33]. Electrodeposition coatings always produce the optimum result and they play a decisive role in corrosion protection and extending the service life of bodywork, wheels, and other vehicle components. The high efficiency of electrodeposition coating is the key advantage of e-coatings. During this procedure, an even film is applied to the entire surface, including areas that are contorted and therefore difficult to access. The need for reworking is largely eliminated and scrap risk is minimized. Furthermore, in comparison with spray coating, electrodeposition coating ensures a significantly lower level of material loss. The use of electrodeposition coatings cause less strain on the environment (e.g., due to solvent vapors) in comparison with other coating processes.
10.4 Properties and characteristics of smart hybrid coatings The development of hybrid materials has been extensively investigated over the past few years. The combination of a wide variety of compositions and production processes permitted the use of these materials in different applications, including coatings for corrosion protection of metals. Hybrid coatings doped with corrosion inhibitors provide active corrosion protection. Corrosion inhibitors can be added to the coating mix before applying to the metal substrate or at the stage of film formation and crosslinking, which also enhances the corrosion protection of hybrid coatings [34]. Many kinds of inorganic inhibitors with low toxicity have been used to increase the corrosion protection of hybrid coatings. Inorganic inhibitors, such as phosphates [35], vanadates [36], and rare-earth elements [8,37], have shown positive influence on the corrosion protection of aluminum alloys. Cerium ions with relatively low cost and abundance satisfy the basics requirements for being considered as an alternative inhibitor to
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chromates. The main role of Ce31 and La31 in corrosion protection is the formation of insoluble deposits on intermetallic inclusions that prevents the local increase of pH, which is responsible for the acceleration of the intermetallic dealloying. The entrapped corrosion inhibitors become active in the corrosive electrolyte in response to pH changes caused by the corrosion process so that corrosion inhibitors slowly diffuse out of the host material. The formation of hydroxide precipitates can also act as a diffusion barrier hindering the corrosion processes in active zones. Cerium nitrate exhibits higher inhibition protection efficiency in comparison with lanthanum nitrate in hybrid coatings due to the lower solubility of cerium hydroxide [38]. Various organic corrosion inhibitors, such as triazole and thiazole derivatives [37,39] and 8-hydroxy quinolone (8HQ) [40], have also been investigated. It was reported that the organic species is adsorbed on the metal surface, avoiding the adsorption of aggressive ions such as chloride or hydroxide [40]. Different types of smart hybrid coatings are currently available. These coatings have high energy-saving potential, high flexibility, high chemical resistance, and good mechanical properties like high scratch resistance and elasticity. Hybrid powder coatings based on polyester and epoxy resins are available with excellent all-round properties and, therefore, they are the most-used powder coatings in the coatings sector. They are versatile in use and meet many requirements for indoor goods, for example, household articles like cabinet assemblies, steel furniture, shelves, fire extinguishers, colored bottles, clay pots, and others can be coated with hybrid coatings to enhance their appearance and lifespan. Hybrid coatings can be applied to almost any surface and is particularly advantageous in institutional and commercial applications as well as for cleaning rooms in the medical and food industries due to their low toxicity and corresponding ability to be applied without affecting the daily business of the organization or home. Smart hybrid coatings can be applied to industrial and commercial buildings, chemical plants, medical and pharmaceutical cleaning rooms, warehouses, floorings, military equipment, and protective coatings inside pipes. The protection of ancient stone monuments and buildings from the attack of corrosive agents from the environment can be successfully carried out using polymer-based hybrid coatings [41]. Among other advantages of hybrid coatings is their possibility to develop high-mechanical resistance upon curing and hardening under a wide range of environmental conditions, low weight due to the small thicknesses required for efficient protection, wear and abrasion resistance, and good bond with stone [42,43].
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10.5 Recent developments in smart hybrid coatings Hybrid coatings are highly functional coatings with excellent corrosion protection, good adhesion properties, and outstanding chemical resistance. The epoxy hybrid coatings are highly economical and versatile interior coatings. They can be used in automobiles as primers to increase protection against corrosion with excellent weather resistance, good flexibility, and outstanding adhesion properties. Smart hybrid coatings are employed to control temperature changes in the windows and doors in buildings due to solar radiation. These coatings block infrared radiation (IR) to 70% at high temperatures, while allowing it to go through at low temperatures. Thermochromic and vanadium dioxide (VO2)-based smart hybrid coatings are promising materials due to their reversible-phase transition and high efficiency in modulating IR transmittance. According to various researchers, other techniques based on advanced nanotechnology can be used to improve the thermochromic performance of smart hybrid coatings. These advantages make the plasmonic thermochromic coatings promising as intelligent air conditioners without energy cost [44 54]. The strength and durability of cement floors can be enhanced by smart hybrid coatings. These coatings save flooring from potential damage, but also save money as well. Hybrid-coating technologies offer superior alternative paints and coatings for not only floors but also electrical potting, which are safe and effective. This means they are free of dangerous isocyanates that can cause damage to the respiratory system and, therefore, protect those who live or work inside treated buildings by using green, and environment-friendly hybrid coating products that are strong, effective, and durable. Improvements in hybrid-coating technology are essential from the industrial point of view so that coated materials are much more capable of withstanding extreme corrosive environments, by temperature and erosion than a conventional coating. Recent research has focused on alternative technologies with significant improvements in the performance of coatings. Studies show that this can be achieved by changing the size, shape, and distribution of the coating matrix to produce ultrafine-grained coating materials [55]. Silicone is another material that can be used for making hybrid coatings. It has unique properties such as tolerance to high temperatures, inertness, and adjustable surface wettability properties. Its rubbery texture and resistance to chemicals means that it must be mechanically removed for rework. Another material which can be used in
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hybrid coatings is epoxy resins. Epoxy resin is an extremely hard material that has some unique uses. Its rigidity means that it can be used as mechanical reinforcement, but, more interestingly, it can be used as a security device. Combining epoxy with other materials (e.g., crossbars), a rigid structure can be made that actually destroys itself and adjacent items if an attempt to mechanically separate it from the printed circuit board (PCBA) is made. This security feature has a rating system federal information processing standard (FIPS) that defines the minimum levels of self-destruction allowable. Epoxy is also resistant to heat and chemical corrosion. Its hardness and set time are disadvantageous as it increases process time and makes rework virtually impossible. Hybrid coatings based on the graphene oxide zirconia dioxide/epoxy (GO ZrO2/EP) system were fabricated and applied for corrosion protection of metal substrates by Di et al. [56]. Acrylic resin is probably the most common coating material used today. It is also the least expensive of the materials in use. Its main advantages are low cost and ease of handling. Polyurethane is another common material used in hybrid coatings. Its smooth hydrophobic and oleophobic properties make it an excellent coating material. Hamidreza Asemani et al. reported that polyurethane coatings are a promising alternative for various industrial applications [57]. A pigment particle can also be incorporated into hybrid coatings. Pigment particles in hybrid coatings can be classified into two groups, namely, particles forming in sols and traditionally added pigments. The particles forming in sols are different from traditional pigments. There are strong chemical interactions between sol particles and organic components when hybrid coatings are dried. When the concentration of traditional pigments in the coating exceeds their critical pigment volume concentration (CPVC), the apertures in the films accrue sharply, which degrades the physical barrier properties of coatings. The CPVC of traditional pigments lies in the 50% 60% range. However, there is no CPVC for the sol particles in hybrid coatings, and hybrid films can be formed at random ratios of inorganic organic components. When the average diameter of sol particles exceeds 100 nm the transparency of hybrid coatings decreases abruptly to make the films opaque, but sol particles do not have the color aptitude and conditioning of traditional pigments [34]. When the concentration of traditional pigments is below CPVC, the permeability of coatings monotonically decreases with increases of the added pigment loading [58]. The pigments in flakes, such as in montmorillonite clay or aluminum flakes, should reduce coating permeability remarkably. Nanoscale
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montmorillonite clay is dispersed in the precursor solution of the hybrid coatings to form coatings for alloys. Formulation of the coating with a diaminosilane curing agent and clay nanofillers significantly improves the coating resistance and reduces the water uptake to about 3% 5% after 8 weeks of immersion in a 3.5% NaCl solution [59]. The list goes on and new ideas are constantly being tried to solve the problems of specific applications. Further recent improvements in coating methods are summarized next. In the electrostatic spray (ES) or powder coating (ES) technique an electrically charged coating material is sprayed onto earthed-metal items. The metal items are then placed in an industrial oven and heated until the powder melts. The item is cooled to leave a high-quality plastic coating. The fluidized bed coating technique uses fluidized bed powders that behave like a liquid when pressurized air is allowed to bubble up through it. A metal item, heated to the correct temperature, is then completely immersed into this fluidized bed. The powder melts on contact with the metal and the item is then lifted out of the fluidized bed. The item is cooled to leave a high-quality plastic coating. The flock spraying method uses uncharged powder that is blown onto a hot metal item where it melts to form a coating. The item is then left to cool to leave a good-quality plastic coating. In the case of flame spraying, uncharged powder is blown inside a flame onto a preheated metal item. The powder melts on transit from the application gun to the metal surface and forms a solid coating as the metal item cools down. This is suitable for onsite coating or repairs. For example, in order to protect wires as the wires are extruded from the extruder they are passed through a molten plastic. The wire is then allowed to cool to leave a continuous, protective coating. In the spin-coating method the object that requires coating, typically the inside of a bottle or cylinder, is heated up to the desired temperature. Fluidized bed powder is then injected into the object. The object is then immediately spun and tumbled to give a complete and consistent coating inside the bottle. Any unused powder is then tipped out of the object. Corrosion protection of steel by hybrid sol gel coating was reported by Avci and Abanoz [60]. Coatings prepared by spraying coating mix with different epoxy resins and polyurethane hybrid coatings heat treating at 110 150°C showed better mechanical strength, flexibility, and corrosion resistance than those with coatings of tetraethylorthosilicate and 3-glycidoxypropyltrimetoxysilane prepared by the sol gel method.
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Figure 10.4 Schematic illustration of a three-layered system based on hybrid sol gel coatings and how the preemptive healing system works. R.B. Figueira, I.R. Fontinha, C.J.R. Silva, E.V. Pereira, Coatings 6 (1) (2016) 12.
It is undeniable that future applications and research will be focused primarily on the investigation of more environment-friendly precursors/ reagents or industrial colloid particles to replace the traditional precursors in the formation of sol gel protective coatings without changing their general properties. Combined systems will also be developed in order to achieve the best protection possible against corrosion, such as metal-rich sol gel coatings containing zinc or magnesium particles, or other inhibitor particles, which combine the barrier properties of the sol gel layer and cathodic protection of sacrificial particles. Multilayer systems, such as the one proposed in Fig. 10.4 in which a preemptive behavior, are also being considered. Water and electrolyte diffusion through the sol gel top coat can generate corrosion reactions at the metal oxide interface. Preemptive healing could occur through the trapping of water and corrosives and/or the release of inhibitors from the hybrid sol gel that targets the corrosion reactions and shuts them down. Self-healing agents also heal the top coat from the effects of weathering or other damage. This system combines different sol gel layers with different functions, which is a very promising area to explore [18]. Intrinsically conductive polymers (ICPs), which can undergo redox reactions with the surrounding environment, represent a potential class of organic coatings for corrosion protection applications. Polyaniline (PANI)
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Figure 10.5 Generic scheme showing the corrosion-healing mechanism of a hybrid sol gel coating containing inhibitor-loaded nanoparticles. R.B. Figueira, I.R. Fontinha, C.J.R. Silva, E.V. Pereira, Coatings 6 (1) (2016) 12.
with adjustable electronic and protonic conductivities can be utilized as ICPs in different forms for corrosion protection of various ferrous and nonferrous metals [61]. Nanoscale coatings have recently been proposed for anticorrosion selfhealing, antiicing, self-cleaning, etc. Fig. 10.5 represents a generic scheme showing the corrosion-healing mechanism of a hybrid sol gel coating containing inhibitor-loaded nanoparticles [18]. Different types of nanomaterials have been incorporated in anticorrosion coatings by adopting various approaches. The basic approach utilizes the incorporation of inorganic nanomaterials with traditional organic coatings to enhance certain functionalities of the formulated nanocomposite coating. One of the recent trends in nanotechnology is to design nanomaterial-based coatings with multifunctionality [62 64].
10.6 Conclusion The corrosion of metals and their objects exposed to harsh environmental conditions can be successfully prevented using smart hybrid coatings. The development of hybrid materials has been extensively studied in recent years. A wide variety of organic and inorganic materials in different compositions can be used to make hybrid coatings, which can be applied as coatings for corrosion protection of metals. Electrochemical analyses proved that relatively dense hybrid coatings provided excellent corrosion protection by forming a physical barrier, which effectively separates the
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anodic part from the cathodic part. Some preliminary biocompatibility tests conducted on hybrid coatings recommends that the hybrid coatings can be applied for biomedical applications due to their good biocompatibility. The composition of the coating mix and types of epoxy material selected for making the hybrid coating determine the efficiency of corrosion protection. Corrosion control by hybrid coatings is an environmentfriendly and promising technology for surface protection and corrosion prevention.
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