*5.1. Self-Healing Coating*

Coating with self-healing properties is an advanced application of emerging corrosion inhibitors. The concept of self-healing which is initiated in the nineties by Dry [103] and Sottos [104] is the known phenomena seen in the nature and refers to self-repair. Self-healing coatings can be classified into two main classes' namely (1) extrinsic and (2) intrinsic self-healing systems. In extrinsic self-healing systems such as capsule-based and vascular systems, the healing agents are added as a separate phase into the matrix, while intrinsic systems such as ionomers, hydrogen-bonded systems, etc., are those which are free from healing agent and do not require any external energy to trigger the response [105,106]. They can repair the mechanical damage spontaneously due to the architecture of the molecules themselves and avoiding rupture and corrosion of underlying substrate. Extrinsic technique possesses several advantages over intrinsic, which will be discussed in this section.

In contrast to conventional anticorrosion coating, emerging corrosion inhibitor embedded in self-healing coating can act in response to corrosion attack, decrease the corrosion rate thereby enabling less maintenance and durability of the coating. For achieving this goal, the coatings have to provide release of the active and repairing material rapidly after integrity changes in coating. The main idea is to load active agents (e.g., corrosion inhibitors) into nanocontainers surrounded by a shell which controlled the permeability and then to introduce them into the coating matrix. Consequently, nanocontainers are keeping corrosion inhibitor in a "trapped" state and distributing uniformly in the passive matrix. Thus, the undesirable interaction between the corrosion inhibitor (active material) and the passive matrix which leads to spontaneous leakage could be prevented. When the local environment undergoes changes or if the active surface is affected by the outer impact, the nanocontainers respond to that signal and release encapsulated inhibitor. Various methods to add the self-healing properties to coatings have been investigated including encapsulation, reversible chemistry, microvascular networks, nanoparticle phase separation, polyionomers, hollow fibres, and monomer phase separation [107]. Microvascular is a strategy in which material with interconnected series of network channels has been designed. In this approach, circulatory system continuously

transports the necessary chemicals and building blocks of healing to the site of damage. Therefore, coating on a substrate containing a micro channel network is healed. This is the most biomimetic approach and it is difficult to achieve practically and at large scales in synthetic materials. Nanotubes are another approach that may be able to deliver larger amounts of liquid healing agent to the crack plane. Halloysites (aluminosilicate nanotubes) which are one of the most abundant natural nanotubes have recently been applied as containers in the automotive and maritime industries for corrosion protection. They have been developed as an entrapment system for loading, storage, and controlled release of corrosion inhibitor in coatings [108]. The drawbacks of using nanotubes lie in its poorly defined composition and its narrow particle size [107]. So far the most successful approach in self-healing the polymeric component of organic coatings is microencapsulation. This approach has significant advantages including protection of reactive materials (inhibitors) from corroding environment, controlled evaporation of inhibitors, safe handling of toxic inhibitors and controlled release of the inhibitors for delayed release or long acting release [109].
