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Metal corrosion is a natural and inevitable process that imposes a lot of cost on many industries and can also have irreparable consequences. Several methods, such as cathodic protection, galvanizing, painting, and coatings, are available to prevent metal corrosion. Selection of the best corrosion prevention method depends on many factors including cost, effectiveness, type of metal, and corrosive media but it can be said that coatings are probably the most convenient method to prevent corrosion of metals due to the low cost, availability of raw materials, flexibility, and simplicity. Despite having many advantages, coatings are subject to problems such as cracking and degradation. Therefore, they must be repaired or replaced. Self-healing coating has been introduced and developed during the past decades as a very effective method to overcome the problems of traditional coatings. Self-healing means healing (recover/repair) internal damages automatically and autonomously. It is an amazing property that can fill cracks and small pinholes which leads to increased service lives of coatings. This chapter presents different strategies for fabrication of self-healing materials and explains their challenges and limitations. Furthermore, the use of self-healing materials in metal corrosion through different mechanisms is discussed, and published reports in this field are reviewed.
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1. Introduction
Corrosion can be defined as an attack on materials because of an interaction of materials with the surrounding environment. Based on this definition, all materials can corrode. But in practice, the term corrosion is mainly associated with the degradation of metals [1]. Metals are widely used materials due to having remarkable properties such as high electrical and thermal conductivity, ductility, malleability, and high strength. Apart from gold, platinum, and a few others, most metals are prone to corrosion. This problem can be more important when using metal alloys as the type, speed, and rate of corrosion are different for each element. In fact, more than one type of corrosion can occur in the corrosion of alloys. Corrosion is a common and costly phenomenon that should be prevented to avoid irreparable risks such as damage to structures and human life. There are several factors that influence the rate of corrosion including diffusion, temperature, conductivity, humidity, type of ions, pH value, and electrochemical potential [2]. The corrosion rate of metals can be controlled or reduced by taking various measurements. The most common methods include cathodic protection, galvanizing, painting, and coatings. Coatings are probably the most convenient method to prevent the corrosion of metals due to the low cost, availability of raw materials, flexibility, and simplicity. There are many different types of industrial coatings, including organic (alkyd coatings, epoxy coating, polyurethane coatings, etc.) and inorganic coatings (ceramic coatings, metalized coating, etc.).
Despite having many advantages, coatings are subject to problems such as cracking and degradation. Therefore, they must be repaired or replaced. Being exposed to a corrosive environment for a long time leads to the weakening of the mechanical properties of the coating, and as a result, microcracks form and propagate. The failure of the coatings is inevitable and hence they must be repaired or replaced. In order to improve the properties of the coatings, several methods such as adding fillers in a polymer matrix have been proposed and implemented. Various fillers including zinc oxide, titanium dioxide, and silicon dioxide have been used and commercialized which enhance both mechanical properties and corrosion inhibition performance of coatings. During the past two decades, new-generation protective coatings, self-healing coatings, have been introduced and developed that can repair the damage caused by corrosive media and as a result, increase the coating lifetime. Self-healing coating has become a research hotspot in the coating field. Thus underlying their properties, mechanism, and synthesis procedures could be interesting. However, the development of these coatings is still in its early stages and it is very important to know their various aspects, including their properties, mechanism, performance, and application. This chapter provides basic information about metal corrosion and the development of methods to inhibit this phenomenon. In addition, different mechanisms of self-healing materials and published reports in this field are reviewed.
Commonly, metal corrosion is defined as the deterioration of metals as a result of exposure to a corrosive environment, such as the atmosphere or even more aggressive corrosive media like seawater and chemical plants [3]. Definitely, our daily life is negatively affected by metal corrosion as automobiles, buildings, infrastructure, appliances, and energy distribution systems are all susceptible to corrosion because of metal pieces. Metals are rarely used in pure form (only in special applications), and they are generally used as alloys. There are many debates about which pure metal or alloy is the best, but what is clear is that alloys can provide unique properties. For instance, stainless steel, which is an alloy of iron, is more resistant to corrosion and has a slower corrosion rate than pure iron. In addition, it is an extremely tough and highly durable material with high impact resistance in comparison with pure iron [4]. But this does not necessarily mean that alloys do not corrode. Therefore, it can be said that corrosion in metals and even alloys is inevitable and measures must be taken to prevent it. Before pointing out the methods to prevent metal corrosion, it is beneficial to understand the causes and mechanisms of metal corrosion. Although corrosion is mainly a chemical phenomenon, mechanical factors, such as tensile or compressive forces and wear, can also aggravate it in many situations called mechanically assisted corrosion. For example, in offshore environments, waves, tides, and strong winds can cause mechanically assisted corrosions such as stress-corrosion cracking, fatigue corrosion, corrosion-erosion, and fretting-corrosion [5]. Figure 1 depicts the most common types of mechanically assisted corrosion.
Figure 1.
The most probable types of mechanically assisted corrosion [5].
Generally, Corrosion consists of a series of complex chemical reactions and may initiate by several different mechanisms that depend on the surrounding environment and involves simultaneous oxidation and reduction reactions. In fact, first, an aqueous adlayer forms on the metal surface followed by electrochemical and chemical reactions. Furthermore, corrosion products may participate in these reactions. The mechanism and rate of chemical reactions that lead to corrosion depending on the corrosive factors in the surrounding environment which cause various classifications of corrosion. The most important factor contributing to metal corrosion can be classified as following [6]:
Gases (CO2, SOx, HCL, HF, H2SO4, NH3, H2S, etc.)
Moisture, dew, and condensation
Temperature
Relative humidity
Type of metal
Acidity and alkalinity (pH)
Corrosive ions (Cl−, SO42−, Mg2+, etc.)
The most common types of metal corrosion can be listed as follows:
Uniform corrosion
Uniform corrosion is one of the most typical types of corrosion and occurs when the entire surface area is exposed to a corrosive environment. Uniform corrosion is one of the most typical types of metal corrosion and occurs when the entire surface area is exposed to a corrosive environment. In this type of corrosion, the thickness of the metal decreases over time, and if it is not prevented, it continues until the metal is completely destroyed [7]. Figure 2 shows the uniform corrosion in metals.
Figure 2.
The uniform corrosion on metal surface [8].
Galvanic/bimetallic corrosion
Galvanic corrosion takes place when dissimilar metals come in contact with each other when exposed to an electrolyte. In this type of corrosion, one metal deteriorates quickly by an electrochemical reaction while the other remains unaffected. This is due to the magnitude of the potential difference between the two metals [9]. Figure 3 illustrates the galvanic corrosion in metals.
Figure 3.
The galvanic corrosion in metals [9].
Crevice corrosion
Crevice corrosion is a localized type of corrosion and is formed inside gaps or crevices on the surface of the metal. This type of corrosion can also affect the connected pieces to the metal substrate such as another welded metal or even metal attached to non-metal objects. The filling of crevices by corrosive agents, such as water, humidity, or pollutants, causes the rate of this type of corrosion to increase and eventually leads to the failure of the substrate [10]. Figure 4 depicts the crevice corrosion in metals.
Figure 4.
The crevice corrosion in metals [10].
Pitting corrosion
Pitting corrosion is a localized form of corrosion that creates holes on the surface of metals. This type of corrosion takes place when a small area of metal becomes anodic while a vast area of the substrate acts as a cathode. The shape of holes may not be similar but mostly they are hemispherical and their size can vary from shallow and wide to deep and narrow. The formation and growth of holes on the surface of metals can have irreparable consequences, especially in critical points such as joint and welded parts [11]. Figure 5 shows the pitting corrosion in metals.
Figure 5.
The pitting corrosion in metals [11].
Erosion-corrosion
Erosion-corrosion is a form of corrosion that occurs as a result of chemical and mechanical stimuli. The mechanical effect of flow or velocity of a fluid combined with the corrosive action of the fluid causes accelerated loss of metal. This type of corrosion initiates by removing parts of the metal by erosion and is followed by contact with a corrosive medium. This usually happens when the velocity of the fluid is too high (turbulent flow) such as in tube blockages, tube inlet ends, or pump impellers. The process is a cyclic process that continues until the perforation of the metal occurs [12]. Figure 6 illustrates the erosion-corrosion in metals.
As stated before, corrosion is one of the leading problems faced by almost all industries and can lead to safety issues and ruin the integrity of equipment and supplies. Therefore, steps should be taken to prevent or minimize metal corrosion. There are various methods to protect metals from corrosion such as hot dip galvanization, cathodic protection, painting, and coating. Choosing the best method depends on cost, effectiveness, type of metal, corrosive media, etc. Cathodic protection can perform in two different ways, the impressed current cathodic protection (ICCP) system and the sacrificial anodes cathodic protection (SACP) system. In the ICCP system, the metal substrate is connected to an external DC power source and the resulting current prevents the corrosion process. The SCAP is a technique that uses a less noble metal to act as the anode. In this method, an active metal such as magnesium, aluminum, and zinc is connected to the substrate that needs to be protected and provides electrons to the structure to be protected and consumed [13]. The ICCP systems require high initial costs due to the need for an external power supply as well as skilled workers, but it is more effective than SACO systems. [14].
In the hot-dip galvanization method, fabricated steel is immersed in molten zinc which leads to forming of a series of zinc-iron alloy layers on the metal surface by a metallurgical reaction between the iron and zinc. This method is very effective for protecting steel as zinc oxide (ZnO) is formed by the reaction of zinc and oxygen which provides a robust barrier to steel corrosion [15].
Painting is another way to protect metals from corrosion. Paints are a thin passive layer that typically contains a complex mix of ingredients, each with its own purpose. These can include binders that help glue the paint together, pigments that give the paint its color and other useful properties, solvents that help the ingredients mix properly, and many others. Paints are mostly used for decorative purposes that can protect the metal to some extent from exposure to corrosive agents. However, if the paint film is damaged, corrosion will occur very quickly due to a lack of protection from the environment [16].
Coating is one of the most convenient and effective methods for the prevention of metal corrosion. Coating is a widely used and effective method due to the availability of raw materials, simplicity, low cost, flexibility, and versatility [17]. Despite having many advantages, coatings face problems like other corrosion protection methods. During the service life, the mechanical properties of the coating film change which leads to the formation of cracks. Sometimes cracks are deep within the structure where detection is difficult and repair is almost impossible. This leads to the destruction of the coating and as a result, the substrate is exposed to a corrosive environment [18]. Self-healing materials have been introduced and developed in recent years as a novel method to overcome this drawback. Self-healing materials can repair minor damage automatically and autonomously. Self-healing materials have revolutionized the coating systems which lead to a significant increase in the service life of coatings [18]. Therefore, the following section covers different aspects of these novel materials including the definition of self-healing materials, design strategies, effective parameters, and their mechanisms.
To better understand the basis of self-healing materials, it would be better to define self-healing meaning first. Self-healing materials can heal (recover/repair) internal damages automatically and autonomously. Definitely, it is a truly amazing characteristic that can fill not only the cracks but also small pinholes. The different types of materials including polymers, metals, and ceramics can have the ability to self-healing with their own self-healing mechanisms [19]. In this chapter, the focus is on self-healing polymers and their mechanisms. Different strategies can be considered to design self-healing polymers, such as the release of healing agents, reversible cross-links, and miscellaneous technologies, including electrohydrodynamics, shape memory effect, and nanoparticle migration. In this section, the different strategies are discussed.
4.1 Release of healing agents
The release of healing agents is one of the most covenant strategies for designing self-healing materials. This is accomplished by loading one or more healing agents, including monomers, dyes, catalysts, and hardeners in a container such as microcapsules, hollow fibers, and microvascular. The containers are then embedded into a polymeric matrix, which can release the healing agents by stimulation, such as physical damage or even pH alteration [20]. Figure 7 depicts a schematic of the healing process using the release of healing agents from containers.
Figure 7.
A schematic of healing process using release of healing agents from containers [21].
4.1.1 Microencapsulation
In the encapsulation process, self-healing microcapsules are synthesized by loading a self-healing agent (solids, liquid, or gases) in an inert shell which in turn isolates and protects them from the external environments. Therefore, self-healing microcapsules are made of two parts, the core (healing agent) and the shell, which may vary in shape (spherical or irregular shape) and size (from nano to micro). Figure 8 shows a schematic of a microcapsule structure. Generally, microcapsules have a diameter of 3–800 μm, and consist of 10–90 wt% core materials [23]. Different types of materials can be used as the healing agent and shell. Core materials or healing agents are selected based on the application of healing material such as stabilization against environmental degradation, improvement of longtime efficiency, maintenance of non-toxicity of degradation products, and easy handling through solidification of liquid core [24]. Some of the most common materials used in self-healing coatings are cerium nitrate, dodecylamine, polyethyleneimine, linseed oil, and sodium alginate. The shell material can be various from traditional organic polymers to novel inorganic materials. Usually, core materials are loaded in poly(Urea formaldehyde) (PUF), poly(melamine formaldehyde), cellulose nanofibers (CNF), halloysite nanotubes, etc. Table 1 listed some conventional materials used in self-healing coating for different substrates.
Some conventional materials used in self-healing coating for different substrates.
Interfacial polymerization and in-situ polymerization are the most common methods for the synthesis of microcapsules, which are defined as follows [34]:
Interfacial polymerization: interfacial polymerization is an encapsulation procedure that mainly developed in the late 1960s [35]. This procedure consists of four main steps and two sets of monomers. One monomer is soluble in the oil phase and the other one is soluble in the water phase. At first monomer A (soluble in the water phase) is dissolved in the water phase. Secondly, monomer B (soluble in the oil phase) is dissolved in the oil phase. Then the oil phase is introduced into the water phase and emulsification is carried out under constant stirring. Finally, the polymerization reaction takes place through a chemical reaction between the monomers A and B which is initiated by changes in pH (acids or bases) and can be accelerated by the use of catalysts. The polymerization reaction leads to the form of a polymer film at the interface of monomers and the polymer formed is deposited around the drops which leads to encapsulation [35]. The encapsulation efficiency is enhanced when a low-molecular-weight shell material is used, due to the higher mobility of the small molecules. However, this reduces the shell’s strength [36]. In addition, stirring speed is manipulated to control the size of the microcapsules. The higher the speed of the stirrer, the smaller the microcapsules are formed.
In-situ polymerization: encapsulation via in-situ polymerization technique is very similar to interfacial polymerization. The difference is that there are no reactants in the core material in in situ polymerization. In fact, in this process, polymerization takes place in the continuous phase, rather than in the interface between the continuous phase and the core material. In-situ polymerization also includes four steps. First, the core material is dispersed in the water phase. In the next step, the shell material is introduced into the water phase. Typically, this includes monomers that react in continuous phase and form a polymer. Then the pH is reduced by the addition of acid to initiate the polymerization reaction. Finally, the polymer film formed covers droplets. The size of the microcapsules is controlled by the agitator speed, so that the higher the speed of the stirrer, the smaller the microcapsules are achieved [37].
4.1.2 Hollow fiber embedment
The use of hollow fiber as a container for healing agents is another interesting method as agents can restore up to 97% of their initial flexural strength. In this method, the healing mechanism is similar to that of microcapsule-based methods with the difference that the healing agent is stored in hollow tubes or fibers until they are ruptured by damage [38]. Figure 9 depicts a schematic of the self-healing concept using hollow fibers. Recently, fibers such as hollow glass fibers, hollow site nanotubes, titanium dioxide nanotubes, and polymeric fibers have been used as a container for self-healing applications [38]. In the hollow fibers approach, a healing agent and a curing agent (i.e., epoxy resin and its hardener) are loaded in separate hollow fibers that react together when both fibers are broken due to external damage. One of the most remarkable advantages of this self-healing method is releasing a large amount of healing agent because of the large size of containers that can be suitable for filling cracks or large holes. However, this can also be a limitation. Because the rupture of hollow fibers depletes the healing agent contained within it which leads to a limitation on the number of times that a damaged region can be healed. On the other hand, this self-healing system faces a significant drawback which is the development of a practical technique for filling the hollow fibers with the liquid healing agent [39].
Figure 9.
A schematic of self-healing concept using hollow fibers [34].
4.1.3 Microvascular system
To overcome the limitation of hollow fiber containers, microvascular networks have been introduced and developed by White et al. [40, 41] which are similar to the biological vascular system. In this method, the microvascular networks can repair the damaged area autonomously. Thanks to the small diameter of the microvascular, only the vascular in the damaged area will discharge the healing agent when damage occurs and the rest of the containers will remain intact. Figure 10 shows a schematic of a microvascular network for self-healing applications. Although this method can increase the number of healing cycles, the fabrication process of such a microvascular network is complex and it is hard to achieve synthetic materials with such properties for practical applications.
Figure 10.
A schematic of a microvascular network for self-healing application [41].
4.2 Reversible cross-links
In chemistry, cross-linking is the linking of one polymer chain to another one. The cross-linking is an irreversible process that can change the polymers’ physical properties such as elasticity, mechanical behavior, and surface characteristics [42]. But highly cross-linked materials have the drawback of brittleness and tend to crack. Some approaches, including Diels–Alder (DA) and Retro-DA reactions as well as the use of ionomers and supramolecular polymers, are available to bring reversibility in cross-linked polymeric. Reversibly cross-linked polymers exhibit self-healing properties but they need an external trigger such as thermal or chemical activation. Thus, these systems show a non-autonomous healing phenomenon [34].
4.3 Miscellaneous technologies
Some other emerging technologies are available in the literature which are discussed in brief in this section. These methods have very limited applications and their explanation is beyond the goals of this chapter thus they are only mentioned. These methods include electro hydrodynamics, conductivity, shape memory effect, nanoparticle migrations, and co-deposition [34].
As stated before, self-healing materials offer vast scientific attention due to the wide range of applications. In recent years, these new-generation protective coatings have been developed and commercialized for metal corrosion. In this regard, self-healable polymers find potential candidates for coatings due to their suitable thermo-mechanical behavior, lightweight, excellent adhesion, corrosion resistance, and good chemical resistance [43]. The major components of all coatings are the binders. So, it is obvious that the selection of the binder will affect the general performance of the coating. The most common binders are epoxy resin and polyurethane, however many other polymers, such as acrylic resins and polyesters, are used in many applications [44]. One of the most convenient methods proposed for self-healing coatings is adding containers loaded with a self-healing agent to a polymeric matrix (binders). Self-healing coatings can remarkably enhance the anti-corrosion performance and service life of metals. These protective coatings can restore the physical coating barrier, seal or close defects, or even inhibit the corrosion reactions at the coating defects. This is mostly accomplished by adding microcapsules loaded by a polymerizable healing agent or corrosion inhibitors to routine binders. The other self-healing design strategies are less commonly reported and therefore they are not discussed in this chapter. It is also worth noting that the coatings described here are mainly epoxy- or polyurethane-based organic systems. Two main classifications can consider for self-healing coating, autonomous and non-autonomous. In the following section, these two mechanisms are discussed in detail.
5.1 Autonomous healing mechanisms
Autonomous self-healing coating can repair their bulk integrity or functional properties without any external physical intervention. The Autonomous healing ability is usually achieved by embedding extrinsic polymerizable healing agents in the coating. In this system healing agents are mostly stored in microcapsules and released when the coating is damaged, which then polymerizes to fill the damage or form a protective film that inhibits the electrochemical reactions occurring at the exposed metal substrate [18].
5.1.1 Self-healing based on defect-filling effect
In this type of healing mechanism, the stored polymerizable healing agent releases due to generating a defect caused by mechanical damage. These materials can polymerize into a film by reacting with hardener or even with moisture or oxygen in the environment and can fill the defect. One of the classic examples of this type of healing mechanism was reported by White and Sottos [32] in 2001. In their proposed system, healing was accomplished by incorporating microencapsulated dicyclopentadiene in the PUF shell within an epoxy matrix. The average healing efficiency was reported 60% of the original fracture load. This method was further developed in the following years to be used for various metal substrates. Table 2 listed several autonomous self-healing coatings based on defect-filling effects for different applications.
Several autonomous self-healing coatings based on defect-filling effects for different applications.
5.1.2 Self-healing based on corrosion inhibitors
The embedding of corrosion inhibitors in coatings is another mechanism of autonomous self-healing systems. Some of the most widely used corrosion inhibitors in autonomous self-healing coating are nitrites, phosphates, vanadates, molybdates, tungstates, borates, mercaptobenzothiazole, benzotriazole, imidazoline, 8-hydroxyquinoline, and aliphatic amines [18]. In this type of self-healing coatings, the healing process is accomplished by anodic dissolution and cathodic reactions which leads to corrosion inhibition. The conceptual design of inhibitor-based coatings is simple and this can be achieved by adding corrosion inhibitors directly to a polymeric matrix. In the early studies, the addition of doping nitrates and phosphates of cerium into organic coatings was proposed to prevent the corrosion of zinc, galvanized steel, and aluminum alloy [53, 54, 55, 56]. This method is subject to problems such as poor compatibility between the organic coating resins and particle agglomeration. However, recently, the approach of encapsulating inhibitor agents has received more attention because in this way inhibitors can be released in a stable and controlled manner [57]. Table 3 listed a number of self-healing based on corrosion inhibitors for different applications.
A number of self-healing based on corrosion inhibitors for different application.
5.2 Non-autonomous healing mechanisms
In non-autonomous systems, healing effects accomplish by an external stimuli, such as heat and light, which trigger the chemical reactions or physical transitions necessary for bond formation or molecular chain movement. In fact, this type of coating are healed by recovering the intrinsic chemical bonds and/or physical configurations of the polymer networks in the coating matrices. The external stimulus provides the activation energy required for bond breakage/reformation. For example a heat stimulus can enhance the reactions of the broken bonds by bringing them closer together. The most common light stimulus are sunlight, near infrared (NIR) light, and UV light. But heat sources can be artificially applied (e.g., by a heat gun) or generated from the service environments (e.g., sunlight, abrasion) [18]. Non-autonomous healing polymers have vast applications in healthcare, aerospace, construction and electronics industries. Thus, a comprehensive review of these materials beyond the scope of this chapter and only some systems that are more closely related to protective coatings are discussed in the following section.
5.2.1 Non-autonomous self-healing based on dynamic bonds
The dynamic bonds refer to reversible break and reform bonds which allow a continuous modification of the constitution by reorganization and exchange of building blocks [64]. For instance, at a certain temperature thermally reversible bonds can decompose, which allows the polymer chains to flow to the defect and re-crosslink to repair the defect [65]. Diels-Alder (DA) reaction is one of the most prominent examples of this healing system. A recent work by Chuo et al. [66] demonstrated the possibility of a tetra-functional furan-capped aniline trimer, a trifunctional maleimide, and a trifunctional to prevent metal corrosion. The corrosion current density in the polarization curve was used to evaluate the corrosion protection efficiency of the scratched coating. The results of a cycle test showed that the proposed system could recover protection efficiency from 79.8% to 99.2%. Some other researchers reported self-healing effects by using light stimuli. For example, UV-sensitive self-healing polymers have been developed based on reversible photo-crosslinking reactions [67], or near-infrared light-triggered self-healing ability was used for biocomposites [68]. Light-responsive self-healing polymers have several major advantages over thermally induced self-healing systems, e.g., the healing can be triggered instantaneously, remotely, and on demand. In fact, unlike heat-stimuli systems where all surfaces are exposed, in light-responsive systems, the light stimulus can be applied exactly to the damaged area, which leads to a reduction in side reactions and degradation in the intact coating during the healing process [18].
5.2.2 Non-autonomous self-healing based on shape memory polymers (SMPs)
Shape memory materials are a kind of smart materials that can recover their original shape from a deformed state by applying external stimuli such as heat or light. Both polymers and alloys can perform shape memory behaviors with different mechanisms. In shape memory alloys (SMAs), such as NiTi-based, Cu-based (CuAlNi and CuZnAl), and Fe-based alloys, the shape memory effects are governed by the phase transformation among twinned martensite, detwinned martensite, and austenite [69]. In shape memory polymers (SMPs), the shape memory effects are usually due to the viscoelastic transformation of polymer chains when cycled through a thermal transition temperature, such as a glass transition temperature (Tg) or a melting temperature (Tm) [70]. From the engineering point of view, tailoring the properties is much easier for polymers than metals/alloys. Polymers have traditionally lower material prices and processing costs [71]. Furthermore, SMPs have lower weight, easy fabrication methods, and higher elasticity. Hence, they have been proposed for biomedical [72], aerospace [73], and many other applications.
This chapter provides brief information on metal corrosion and corrosion prevention methods. Some of the most common metal corrosion mechanisms, such as uniform, galvanic, crevice, pitting, and erosion-corrosion, as well as mechanically assisted corrosion, are introduced. In addition, various methods of preventing metal corrosion, such as cathodic protection, galvanizing, painting, and coatings, are discussed and their advantages and disadvantages are addressed. A summary of the information mentioned in this chapter is summarized here.
Investigations show that corrosion is an inevitable and costly phenomenon, and its prevention is of great importance. The Cathodic protection as a classic method is efficient, but it is expensive and requires skilled workers and manpower as well as periodic inspections. Galvanizing is also an effective method but has only limited applications for steel. Furthermore, the galvanized coating can easily chip off and create an uneven surface that leads to corrosion and possible loss of life. Painting is also mainly decorative and does not help to prevent corrosion effectively. Paints can easily deteriorate due to environmental factors or mechanical damage and expose the surface to corrosive factors. It can be said that coatings are the most efficient method to prevent corrosion as they are cost-effective and provide a significant barrier to corrosive factors. Definitely, coatings prolong the service life of metals, but they face the problem of cracking defects and mechanical damage. However, a new generation of coatings called self-healing coatings has been introduced and developed in recent years, which has solved the mentioned problems.
Self-healing materials can heal (recover/repair) internal damages automatically and autonomously. Different design strategies such as the release of healing agents, reversible cross-links, and miscellaneous technologies including electrohydrodynamics, and shape memory effect are available for self-healing fabrication. Release of healing agents is probably the most convenient method to fabricate self-healing coatings in which healing agents are loaded in a container and added to a polymer matrix, and when damage occurs, it releases and repairs the damaged area. This method is widely used and effective because the healing agent can be released in a stable and controlled manner. Two main classifications can consider for self-healing coating, autonomous and non-autonomous. Autonomous self-healing coating can repair their bulk integrity or functional properties without any external physical intervention. But In non-autonomous systems, healing effects accomplish by external stimuli such as heat and light. Both mechanisms are very important and have their own applications despite their few limitations.
Considering the above mentioned properties, self-healing coating are a promising method for prevention of metal corrosion and they have revolutionized the coating systems. Although they are still in the early stages of their development, they have shown a great potential for wide applications and seems to completely replace traditional coatings in the future.
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Written By
Shahin Kharaji
Submitted: 12 December 2022Reviewed: 13 December 2022Published: 06 January 2023
Open access peer-reviewed chapter
