## 1 Introduction  
Metals are extensively utilized in numerous industries due to their superior physical and mechanical properties [1]. However, metallic structures are usually susceptible to corrosion, wear, and erosion causes financial damages. According to the World Corrosion Organization, the global annual cost of corrosion is approximately 1.3 trillion Euros, or 3.1% to 3.5% of a country's Gross National Product [2]. Therefore, the corrosion process of metallic structures is considered as a global economic issue. Polymeric coatings are considered as one of the most facile andeconomical strategies to protect metals from corrosion. The polymeric coatings provide barrier action as well as active corrosion inhibition due to their low permeability to corrosive chemicals. Polymeric coatings act as an obstacle between the metallic substrates and the corrosive environment around them and extend the lifetime of the metallic structures [3,4]. However, the polymeric coatings are susceptible to damages, once they encounter temperature, UV rays, and external mechanical shocks. The created damages consequently deteriorate the barrier performance of the coatings. Therefore, self-healing coatings, as smart coatings, have been developed to enhance the lifespan of the polymeric coating and consequently the metallic structures underneath. In recent years, several strategies for the development of self-healing have been employed, such as the release of healing agents, reversible bonds and also nanoparticles with the ability to migrate [5,6]. It has been proved that the release of the healing agent is the most well-studied method among the others [7,8]. When a crack initiates, the release of the healing agent protects the crack from propagation and decreases the penetration of oxygen, water, and ions into the substrate. Three strategies for storing the healing agent are as follows: the usage of micro/nanocapsules, micro/nanofibers [9], and micro/nano vascular [10] based systems. In recent years, many attempts have been made to produce an optimized self-healing coating using the capsules-based system. It was reported that the nanocapsules showed better self-healing performance in comparison with microcapsules [11,12]. Among the several employed healing agents, the alkyl resins have recently gained considerable attention. Alkyd resins are special types of polyesters with distinctive properties that are made by the polycondensation of three kinds of monomers, including polyols, polybasic acids, or anhydrides, fatty acids obtained from triglyceride oils (or vegetable oils). [13,14]. There are two types of fatty acids: drying alkyls with enough unsaturated fatty acids that cure with oxygen, and non-drying alkyls with a lower amount of unsaturated fatty acids that cannot be cured with oxygen. Non-drying alkyl can be used as a healing agent due to its crosslinking sites (the presence of carboxyl and hydroxyl groups) [15], while they are stable against oxidation reactions due to the lack of unsaturation bonds [16]. Considering some properties, including low cost, safe to work with, a renewable source, and eco-friendly, these types of oils can be extensively used in anticorrosive coatings for different industries. Figure 1 shows a plausible structure of the alkyld resin. According to Figure 1, the hydroxyl and carboxyl groups can possibly react with other functional groups, i.e., residual epoxy groups, or oxygen to form a solid binder, making it an eco-friendly healing agent for the epoxy coatings [17]. In addition, in the real condition, the reaction is catalyzed by the protons and possibly chloride ions, which are available in the environment [18]. Since the introduction of self-healing coatings, several methods, i.e., in-situ and interfacial polymerization, multi-stage emulsion polymerization, and solvent evaporation, have been developed to encapsulate healing agents within micro/nanocapsules [19,20]. These methods are expensive, time consuming, and need purification.