**3. Corrosion protection methods**

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 selfhealing materials, design strategies, effective parameters, and their mechanisms.
