*2.2. Synergistic Corrosion Inhibition Using ILs*

As it has been discussed, one of the important advantages of ILs is the ability to select both the anion and cation to have useful properties for a particular application. Due to this feature of ILs and organic salts, they could be used as new organic inhibitors, ideally with synergistic effects. A number of publications have investigated the use of biologically safe anions and cations to produce a salt that could approach the performance of chromates, while being environmentally friendly.

Recently, Somers et al. [41] described such a family of ILs and organic salts that target dual activity by incorporating both anions and cations with proven evidence of effective inhibition. These salts were based on the imidazolinium cation with carboxylate anions. The imidazolinium has a similar structure to imidazolium with a difference in the C4−C5 double bond saturation on the core ring of the imidazolinium. Depending upon the nature of the anion in the salts, these materials were found to have interesting physical properties such as facile ion transport, as well as demonstrating synergistic corrosion inhibition on mild steel. In this study, the influence of pH on the corrosion inhibiting performance of the organic salt for mild steel in chloride environments has been investigated.

It has been shown that this environmentally friendly organic IL remains highly active at pH 2 and 8, which are common environments in which corrosion protection is required. Also at higher pH, the inhibition was controlled by the anion, and the solution showed a high level of protection. Although both the IL's components were mainly ineffective on their own at low pH, the combined salt still had an inhibition efficiency of 72%, indicating a strong synergy between the two ionic species under these conditions. Figure 3 shows an optical microscope image of mild steel samples with corrosion product intact after immersion for 24 h in salt solutions at different pH. Also, at a pH of 8, Figure 3a,b show much less corrosion product, but still show some local attack on the sample in the inhibitor containing solution. At a pH of 2 many bubbles have been observed due to the hydrogen gas evolution, where the sample at pH 2 with inhibitor does not show any bubbles, suggesting a significant reduction in the rate of reaction. The samples immersed in the neutral condition show a similar trend to those at a pH of 8, where the inhibited solution showed much less corrosion product but still with signs of localized attack.

**Figure 3.** Optical images of the surfaces of mild steel samples after immersion for 24 h in (**a**) NaCl at pH 8; (**b**) NaCl and inhibitor at pH 8; (**c**) NaCl at pH 2; (**d**) NaCl and inhibitor at pH 2; (**e**) NaCl neutral; and (**f**) NaCl and inhibitor in neutral (Reprinted with permission from [41]. Copyright 2016 American Chemical Society).

Such synergy, however, is not always achievable due to some limitation of ILs. One main disadvantages of ILs is unfavourable transport properties of these solvents, which generally present higher viscosity and surface tension than conventional organic solvents [42,43]. Also, once ILs are applied into a coating, they pose problem of miscibility with a coating formulation. For these reasons, polymerized ionic liquids or poly (ionic liquid)s (PILs) are considered more favorable than their monomers in the field of corrosion inhibitor. This is due to their low sensitivity to salts, high shear and thermal stability, high resistivity to strong acid and their efficiencies at lower concentrations. Moreover, such PIL could act as reservoir for IL with controlled release characteristics, such as the microcapsulation of inhibitor which can prevent miscibility issue with other components of coating formulation. Also, they act as controlled release type inhibitor. In the following section detail study of PILs structure and chemistry will be reviewed.
