2.1.2. Anion Effect

For carbon steel and aluminium, the corrosivity of IL media strongly depends on the chemical structure of the cationic moiety and the nature of anion in the IL molecule. Specifically, the corrosion resistance of carbon steel in water-free ILs strongly depends on the IL anion. Depending on the type of IL, carbon steel may undergo severe corrosion in diluted IL media. Anions like tosylate and dimethyl phosphate generally trigger higher corrosivity specifically in water-diluted ILs. In this regard, Uerdingen et al. [39] investigated the behaviour of carbon steel, austenitic stainless steel, nickel-based alloy, copper, brass and aluminium in presence of various diluted ILs with different concentrations under flow conditions at temperatures up to 90 ◦C. The effect of the chemical structure of the IL cation and the nature of anion on the corrosivity of the metals has been studied. It is observed that diluted ILs (with water) could result in hydrolysis of IL's anions. As a result, they can produce acids (e.g., sulfuric acid, phosphoric acid) and, hence, cause a considerable increase of medium corrosivity, whereas in water-free ILs, most of the metals exhibited a high corrosion resistance.

Ashassi-Sorkhabi et al. [40] studied the effect of IL anion in corrosion inhibitor behaviour of two synthesized imidazolium ionic liquids with chlorides and hydrogen sulphate anion on mild steel in hydrochloric acid. In the structure of the imidazolium bases, the atoms of the imidazolium ring and the –C=N– group can form a big π bond. Then, in addition to the π electron of the imidazolium, bases enter unoccupied orbitals of iron. The π\* orbital can also accept the electrons of d orbitals of iron to form bonds, that produce more than one center of adsorption action. Moreover, the presence of the Cl and S on the imidazolium structure, which are electron donating groups, increases the electron density on the nitrogen of the –C=N– group, resulting in high inhibition efficiency. Therefore, in this study hydrogen sulphate counter-anion showed better inhibiting effect of mild steel in HCl compared with chloride anion.
