# Special Issue: Corrosion Effects and Smart Coatings of Corrosion Protection
###### Abstract
[19] prepared intelligent bacteria-triggered multilayer films by adding pH-responsive chitosan nanocapsules to control the release of antifouling agents during bacterial growth. Chitosan and capsaicin (CAP) combine to form nanocapsules to achieve pH-responsive behavior. The electrostatic interaction between algal salt (ALG) and polydopamine (PDA) in solution leads to (PDA/Alg-CAP@CS-n). After the M membrane was scratched mechanically in artificial seawater solution, it showed self-repairing behavior due to the penetration of ions and the transfer of free chains. The design of multilayer films provides a new idea for the research of antifouling agents in the field of controlled release, and the self-healing performance can further extend its service life in the field of antifouling. Adding a color indicator or a fluorescent indicator corresponding to pH to the coating can also realize self-warning of the coating [20]. [PERSON] et al. [21] encapsulated the pH indicator phenolphthalein (PHPH) into polyurea microcapsules, which has ideal compatibility with polyurethane coatings. When the metal substrate is corroded, the oxygen molecules in the cathode area obtain electrons and react with water to generate OH\({}^{-}\), which increases the pH value of the cathode area, and phenolphthalein turns pink to indicate the occurrence of corrosion. [PERSON] et al. [22] loaded benzotriazole (BTA) and fluorescent molecular probe coumarin into pH-responsive polymer microspheres, pdvb-graft-p (DVB co AA), and then dispersed them in epoxy resin coating to prepare an intelligent coating with self-warning and self-healing functions. The intelligent coating can provide timely fluorescent self-warning at the damage site of the coating on the surface of carbon steel, copper alloy, aluminum alloy, and other metals and prevent the occurrence and spread of corrosion at the damaged part of the coating. It also has good self-warning and self-healing functions for the damage caused by fatigue fracture of the coating. In addition, smart coatings can also strategically design polymer structures and coatings that respond to internal or external stimuli. This type of intelligent coating mainly relies on external stimuli such as temperature and light to trigger a series of physical and chemical reactions to repair the coating. Temperature stimuli are the most common trigger conditions. At present, most of the temperature-responsive self-repairing is realized through the thermally reversible reaction of crosslinked linear polymers, especially the Diels-Alder (DA) reaction [23]. [PERSON] et al. [24] prepared a self-healing polyurethane (PU) network based on reversible DA reaction between anthracyl and maleimide groups. When the coating cracks, in order to consume crack energy, DA products at the crack decompose to form free anthracene and maleimide. Then, these cracks are heated. The initial stage of heating causes the material to expand, and the polymers on both sides of the cracks are in close contact with each other. Hydrogen bonds and molecules are rearranged. After heating, the DA reaction of the decomposition product occurred again, and a new DA bond was established. Therefore, the network structure of the polymer is restored and the cracks are healed. [PERSON] et al. [25] prepared a shape-memory polyurethane self-healing coating, which was mixed with microcapsules loaded with Alodine 5200 inhibitor and heating agent. An automatic induction heating mechanism is introduced in the self-healing process. When the coating is damaged, the inhibitor released by the microcapsule reacts with the aluminum alloy matrix. A passivation film is formed at the bottom of the scratch. At the same time, the released heating agent reacts with oxygen to generate a large amount of heat and promote the healing of Shape-memory polymer (SMP). The experimental results show that the introduction of automatic induction heating into shape memory polyurethane can effectively improve the self-healing performance and corrosion resistance of the coating.