# Advanced Micro/Nanocapsules for Self-Healing Coatings
## 2 Self-Healing Coatings Based on Additives Including Corrosion Inhibitors  
Corrosion inhibitors are chemical compounds that slow down or prevent the material degradation of metals [12]. In coatings demonstrating self-healing properties, these inhibitors can be incorporated into the coating. When damage occurs to the coating due to  
Figure 1: The classification of self-healing coatings. its interaction with the environment, the inhibitors protect the metal through three different procedures. The first one is cathodic or anodic protection, where the metal's electrochemical potential is shifted, enabling it to be less prone to oxidation. Another procedure is the formation of a protective layer, where a passive layer is created onto the metal surface by the inhibitors, preventing further corrosion. The third process is the neutralization of the aggressive agents, such as sulfur or chloride compounds. Several chemical compounds can be used as corrosion inhibitors in self-healing coatings (Figure 2). One category is organic inhibitors, which can be adsorbed onto the metal surface, blocking active corrosion sites [13]. The effectiveness of an organic inhibitor depends on the size of the organic molecule, the number and type of groups or bonding atoms in the molecule, aromaticity, the nature and surface charge, and the type of aggressive environment. Another type is inorganic inhibitors, which precipitate as insoluble salts or can create protective layers on the metal substrates [14]. Green inhibitors are specified as both organic and inorganic, demonstrating eco-friendly properties and reducing adverse effects on the environment [15]. Another category is smart inhibitors, which are activated only when corrosion occurs [16]. Smart inhibitors can be incorporated in microcapsules or embedded in a coating matrix that responds to environmental tritillating, such as thermal or pH changes. Self-healing coatings, including corrosion inhibitors, are used in a variety of industries, such as marine, aerospace, automotive, and construction. The benefits include reduced maintenance, extended lifespan, and improved safety. [PERSON] et al. performed experiments on the synthesis of mesoporous silica nanocontainers and functionalized and loaded them with the corrosion inhibitor sodium molybdate [17]. The mesoporous silica nanocontainers were fabricated by mixing cetyltrimethylammonium bromide as a surfactant and tetraethylorthosilicate as a silica precursor. Then, the produced mesoporous silica nanocontainers were functionalized with 1-(2-aminoethyl)-3 aminopropyltrimethoxysilane, and a positive charge was obtained using anhydrous FeCl\({}_{3}\) in order to accomplish enhanced adsorption of molybdate (MoO\({}_{4}\)\({}^{2-}\)) as a corrosion inhibitor on the functionalized nanocontainers. Epoxy composite coatings incorporating loaded nanocontainers were applied onto mild steel substrates by a film applicator. Evaluation of the produced loaded mesoporous silica nanocontainers displayed the pH dependence of molybdate release from the epoxy coating. The increase in the pH from 1 to 14 resulted in a 160-fold increase in the molybdate concentration in the solution. Electrochemical impedance spectroscopy and electrochemical noise analysis were used for the assessment of the corrosion behavior of the coatings. The results revealed enhanced corrosion resistance of the coatings, including the loaded nanocontainers, which confirmed the proper corrosion-inhibition effect of the released molybdate ions. In another study, [PERSON] et al. evaluated the concentration of 2-mercaptoethanol(MBI) in graphene-based carbon hollow spheres with respect to their active/barrier corrosion protective properties after their incorporation into epoxy coatings [18]. Several  
Figure 2: Types of corrosion inhibitors. epoxy coatings, including either those loaded with inhibitor or empty hollow spheres, were applied on mild steel plates via a wet film roller applicator.