**3. Poly Ionic Liquid (PIL) Based Corrosion Inhibitor**

A special type of polyelectrolytes which carry an IL species in each of the repeating units are referred to polymerized ionic liquids or poly (ionic liquid)s (PILs), and have been proposed as alternative inhibitor materials. Thus, PILs consist of the cationic or anionic centres on their repeating units in the polymer chain (Figure 4) [29]. Although ILs are in a liquid state near room temperature, PILs are in fact solid in most cases, except a couple of exceptions [44]. Nevertheless, opposite to solid polyelectrolytes, PILs have a reportable glass transition temperature in most cases, being well below usual ionic glasses. The major advantages of using a PIL instead of an IL are the enhanced mechanical stability, improved processability, durability, and spatial controllability over the IL species. The combination of properties of ionic liquids with the flexibility and properties of macromolecular structure results in the unique properties for PILs, which can be used in various applications including solid ionic conductor, powerful dispersant and stabilizer, absorbent, premises for carbon materials, permeable polymers, etc. [45–47]. The initial research of PILs goes back to the 1970s. The major design efforts towards developing novel PILs are focused on vinylimidazolium based PILs with diverse functional substituents due to the positive charge being on an aromatic ring and adjacent to the vinyl groups [48–51]. Intensive studies on PILs significantly expanded the research scope of PILs. New structures, properties and applications have been spotted, which generate several valuable branches for researchers. Meanwhile, there are numbers of reviews, which discussed the synthesis of some PILs and introduced the application of PILs, especially imidazolium based PILs in the field of polymer science [45,52].

**Figure 4.** Basic polymerizable IL monomers. "p" represented of a polymerizing unit in an IL monomer. One polymerizable unit located on the cation (**a**) or anion (**b**). Two polymerizable units separated in the cation and anion (**c**), or located on the cations (**d**,**e**).

It should be noted that despite the very interesting properties of PILs and their wide range of applications, these eco-friendly compounds have received little interest as corrosion inhibitors. It has been reported that some ILs based on imidazolium, pyridinium and pyridazinium exhibited corrosion inhibition properties for the corrosion of various metals [21], however; there is very limited investigation for application of PILs as a corrosion inhibitors. Here, a few recent work in this area will be discussed. Olivares-Xometl and co-workers reported the poly(ionic liquid)s (PILs), derived from imidazole with different alkylic chain lengths for corrosion inhibition of aluminum alloy in diluted sulfuric acid [53]. Figure 5 shows the likely mechanism of PILs' interaction with both the metallic surface and the aggressive environment. The interaction among the hydrophobic parts of the PILs molecules could support the protective action. However, it is more likely that the main chains of the polymer form an obstacle, which may have a supportive action on inhibition, as they hinder the passage of water and aggressive ions, in agreement with their hydrophobic nature. When the alkyl side chain is composed of 12 carbons, more effective steric hindrance prevalent, as it can interact with the other lateral alkylic groups to slow molecular diffusion.

**Figure 5.** Schematic representation of corrosion and inhibition of aluminum alloy before and after PIL addition in diluted sulfuric acid [53].

However, in this study PILs displayed a short protection range for the alloy. Thus, these PILs are not suitable to be applied in acidic media, as they are not easily adsorbed due to ionic competition, which leads to the formation of a non-uniform corrosion inhibitor film on the aluminum alloy surface. In another study, by Ayman et al. [54,55] PIL based on 2-acrylamido-2 methylpropane sulfonic acid, showed an excellent corrosion inhibition performance for steel in 1 M HCl medium. The adsorption of IL on steel surface blocked the active centers, which lowered the corrosion rate of steel. It has been reported that introducing oxyethylene ammonium counter ion into the ionic liquid polymer system promotes the wetting characteristics to form anticorrosion protective layer at the solid surfaces [54]. Also, it has been indicated that PIL in this system behaved as a mixed type inhibitor and acted via adsorption on steel surface by hindering and retarding the active centers from the corrosion reaction.

#### *3.1. PIL Structure Diversity*

There have been persistent efforts devoted to the preparation of PILs in various forms and dimension scales like spherical micro-/nanoparticles, micro-/nanogels, vesicles, nanoworms, etc. [56]. Therefore PILs have a diverse chemical structure reservoir. The most recent forms of PILs, which have been mostly used in different applications, are PIL colloidal particles and PIL gels. An insight into these types of PILs and their characteristic features is presented in the following section.
