*2.1. Effect of IL Structure on Corrosion Inhibition*

## 2.1.1. Cation Effect

Among many kinds of ionic liquid, imidazolinium and pyridinium cations based ILs have been investigated intensively. Shi et al. [37] synthesized a series of new imidazolium ionic liquids and investigated the relationship between the alkyl connecting with N(3) of imidazolium ring and corrosion inhibition performance in acidic solution. The inhibition efficiency was found to increase with increasing the carbon chain length of the alkyl connecting with N(3) of imidazolium ring. In another study, the corrosion inhibition behaviour of three synthesized imidazolium ionic liquids with similar chemical structure (namely 1-butyl-3-methylimidazolium chlorides, 1-hexyl-3-methylimidazolium chlorides and 1-octyl-3-methylimidazolium chlorides) on aluminum in hydrochloric acid has been investigated [36]. It has been reported that corrosion of aluminum in aqueous solution depends on the concentration of anions in solution. A general mechanism for the dissolution of aluminum as reported in the literature is [38]:

$$\text{Al(s)} + \text{H}\_2\text{O} \rightleftharpoons \text{AlOH}\_{\text{ads}} + \text{H}^+ + \text{e}^- \tag{1}$$

$$\rm AlOH\_{ads} + 5H\_2O + H^+ \rightleftharpoons Al^{3+} + 6H\_2O + 2e^- \tag{2}$$

$$\text{Al}^{3+} + \text{H}\_2\text{O} \rightleftharpoons \text{[AlOH]}^{2+} + \text{H}^+ \tag{3}$$

In the presence of chloride ions the reaction corresponds to:

$$\text{[AlOH]}^{2+} + \text{Cl}^- \rightarrow \text{[AlOHCl]}^+ \tag{4}$$

It is well known that imidazolium group as well as nitrogen atom in heteroaromatic ring of imidazolium compounds can be protonated in acidic solutions. Thus, the interaction of the protonated imidazolium ionic liquid molecules on the aluminum surface competes with the interaction of the ions in the solution. The adsorption of inhibitors on the aluminum surface removes or depletes some of the water molecules originally adsorbed on to the surface, which blocks the formation of AlOHads. Thus, both the oxidation reaction of AlOHads to Al3+ and the complexation reaction between the hydrated cation [AlOH]2+ species and chloride ions can be prevented. Moreover, these protonated molecules also compete with the hydrogen ions that can reduce hydrogen evolution. In this case, adsorption would have occurred through polar centers of nitrogen atom in the –C=N– group. Meanwhile, the presence of the electron donating group (Cl) on the imidazolium base IL increases the electron density on the nitrogen of the –C=N– group, resulting in high inhibition efficiency [20]. This particular effect is evidenced more with the increase in chain length of the alkyl connecting with N(3) of imidazolium cationic ring.

In another study, Likhanova et al. [29] reported the inhibition action of imidazolium and pyridinium bromide ILs on mild steel in 1 M H2SO4 at room temperature. Since these ILs affected both anodic and cathodic reactions they are classified as mixed type inhibitor. Scheme 2 represents the inhibition mechanism of the interaction between the ionic liquids and the metallic surface. The adsorption of hydronium (H3O+) ion and desorption of hydrogen gas (H2) occurs on the cathodic sites whereas the adsorption and desorption of Br<sup>−</sup> and SO2<sup>−</sup> <sup>4</sup> ions occurs on the anodic sites. The protonated imidazolium or pyridinum molecules are electrostatically adsorbed on the cathodic sites in competition with the hydronium ions available to reduce hydrogen evolution [29].

Since cations of ILs are bigger than hydrogen cations, alkyl chain and aromatic ring cover a large part of the metallic surface and lead to the water molecule displacement from surface. Both imidazolium and pyridinum based ILs show a reasonable corrosion inhibition with average corrosion efficiency within 82%–88% at 100 ppm to protect the mild steel corrosion in the aqueous

solution of sulphuric acid; their efficiencies are increased with the inhibitor concentration in the range 10–100 ppm. However, due to the larger steric effect of imidazolium ion in comparison to pyridinium, which results in a higher surface coverage area during the chemical adsorption process, imidazolium based IL provides a better inhibition effect than pyridinium.

**Scheme 2.** Corrosion inhibition mechanism of imidazolium and pyridinum molecules on steel surface in 1 M H2SO4 (Reprinted with permission from [29]. Copyright 2010 Elsevier).
