**2. Ionic Liquid (IL) Based Corrosion Inhibitors**

Among all the different types of synthetic materials, a new class of low toxicity organic compounds known as Ionic liquids (ILs) deserves particular attention due to their rapid growth in a number of applications, they have shown effective performance as inhibitors for various metals and alloys [19,20]. In this section, an insight into ionic liquids will be discussed in detail.

ILs are the low-melting organic salts that are composed of cations and anions that melt below 100 ◦C [21,22]. The first IL was investigated in 1914 by Paul Walden with his observation on ethylammonium nitrate ([EtNH3][NO3]) with very low melting point of 13–14 ◦C [21]. Due to the unique properties such as low toxicity, negligible vapor pressure, high thermochemical and electrochemical stabilities, non-flammability, and their ability to act as catalyst, ILs have been used in a large number of applications as an eco-friendly alternative to substitute volatile organic solvents including catalysis [23], separation processes [24,25], analytics [26], lubricants [27], and electrochemical applications [28]. Common ionic liquids are formed by an organic cation (i.e., ammonium, imidazolium, pyridinium, pyrrolidinium, phosphonium, sulfonium) in combination with a complex anion (Scheme 1) [29].

**Scheme 1.** Most commonly used cations and anions in various ILs.

The common configuration of ILs consists of an amphiphilic group with a long chain, hydrophobic tail, and a hydrophilic polar head [30]. Therefore, due to their molecular configuration, they are able to form micelles and lowering interfacial tension of aggressive media, resulting in an enhancement in surface wetting and adsorption [30–32]. These properties of ILs have a useful effect on surface exposure and may be responsible for the corrosion inhibition of metals. ILs compounds are reported to show corrosion resistant behavior on copper, mild steel and aluminum. Here some of the literature examples will be discussed.

Espinosa et al. [31] studied the corrosion rate and surface interaction of oxygen-free high conductivity (OFHC) copper with two protic ammonium ionic liquids and four aprotic imidazolium species in order to investigate the best candidate for lubricant applications or as precursors of surface coatings. The protic ILs, with no heteroatoms in their composition, are the triprotic di[(2-hydroxyethyl) ammonium] succinate (MSu) and the diprotic di[bis-(2-hydroxyethyl)ammonium] adipate (DAd). The four aprotic ILs contain imidazolium cations with short or long alkyl chain substituents and reactive anions: 1-ethyl-3-methylimidazolium phosphonate ([EMIM]EtPO3H); 1-ethyl-3-methylimidazolium octylsulfate ([EMIM]C8H17SO4); 1-hexyl-3-methylimidazolium tetrafluoroborate ([HMIM]BF4) and 1-hexyl-3-methylimidazolium hexafluorophosphate ([HMIM]PF6). As it has been depicted in summary of results in Figure 2, the lowest corrosion rate is observed for the DAd, which gives low mass (Δ*m*)

and surface roughness changes (Δ*S*a) and forms adsorbed layers on copper, while MSu forms a dark blue corrosion product by reaction with copper.


**Figure 2.** (**a**) The OFHC copper change during the tests; (**b**) Contact angles, mass (Δ*m*) and surface roughness (Δ*S*a) changes of MSu and DAd after 168 h (Reprinted with permission from [31]. Copyright 2013 Elsevier).

Results show that DAd IL remains colourless during the corrosion tests (Figure 2a) and no precipitates are formed on the copper surface, while MSu forms a dark blue corrosion product that completely covers the copper surface at the end of the test. SEM observation showed more roughness in the case of use of MSu and the presence of oxygen and carbon peaks in EDX analysis. Nevertheless, EDX analysis of DAd shows only the presence of copper. They concluded that the presence of proton donor and acceptor sites in the DAd molecules can form a hydrogen bonded network which as a result will improve their lubricating performance. Moreover, all imidazolium aprotic ILs react with copper, with different results as a function of the anion.

Zhang et al. [20] reported the corrosion inhibition effect of 1-butyl-3-methylimidazolium chlorides (BMIC) and 1-butyl-3-methylimidazolium hydrogen sulfate ([BMIM]HSO4) on mild steel in 1 M HCl. As a result, it has been concluded that the inhibiting efficiencies decreased in the order of [BMIM]HSO4 > BMIC. Potentiodynamic polarization studies indicated that addition of both ILs affects both anodic metal dissolution and also cathodic hydrogen evolution reactions. Thus, those ILs could be classified as mixed type inhibitors. Also, they found that the mechanism of ILs corrosion inhibition is following the Langmiur adsorption isotherm with the high value of adsorption equilibrium constant. Since, the absolute values of standard free energy of adsorption (Δ*G*ads) in presence of the studied inhibitors were calculated to be less than 40 kJ mol<sup>−</sup>1, it has been expected the inhibitors to be physically adsorbed on the metal surface. The corrosion inhibition properties of three different imidazoline based ILs on aluminium in 1 M HCl and 0.5 M H2SO4 were investigated by Quraishi et al. [33] The weight loss study indicated that the inhibition efficiency increased with increase in the concentration of the inhibitor. Moreover, the mechanism of adsorption followed the Langmuir isotherm and behaved as mixed type inhibitors. The most extensively studied IL is based upon the imidazolium cation [31,34,35]. It was observed that the high inhibition efficiency of such inhibitors depends on the specific interaction between the functional groups of IL and the metal surface, due to the presence of the –C=N– group and electronegative nitrogen in the structure of the imidazolium coating [36]. Our indepth review of recent literatures shows that it is important to understand and establish the relation between ILs molecular structure, the counterion, the length of substituted alkyl chains and the functional groups

adsorbed on the metallic surface and corrosion inhibition. In next section, the effect of ILs' structure on the inhibition performance has been presented with literature examples.
