Recent developments in sustainable corrosion inhibition using ionic liquids: a review

Ionic liquids: property and application

Ionic liquids (ILs) that are also pronounced as ionic melts, ionic electrolytes, ionic glasses, liquid and fused salts and ionic fluids are salts in liquid state [1]. Generally, ILs exist in liquid state at below 100 °C while some the ILs are liquid below or at ambient temperature, for exam- ple, 1-ethyl-3-methylimidazolium dicyanamide ([EMIM] [N (CN)₂]⁻) acquires the melting point of −6 °F (−21 °C) [2–4]. Ethanolammonium nitrate was established as first IL that acquires the melting point of 52–55 °C however ethylammonium nitrate (EAN; m. p. 12 °C) reported (in 1914) by Paul Walden was first ionic liquid that exists in liquid state below room temperature [5–7]. After the discovery of ammonium based ionic liquid, several classes of ionic liquids including imidazolium, pyridinium, phosphonium, tetra-ammonium etc. are developed extensively during 1970s–1980s [8–10]. Although, ionic liquids are composed of cations and anions, they are poor conductors of electricity. Ionic liquids acquire high viscosity, non-ionizing ability, low combustibil- ity, and favorable salvation ability, and high liquid array, ability to dissolve polar and non-polar compounds, high thermal and chemical stabilities and low vapor pressure [11,12]. Ionic liquids can be classified in different classes depending upon their nature. Fig. 1 represents the brief classification of ILs. Acidic ionic liquids are those that able to donate proton where as basic ILs are characterized by their ability to gain the proton.
Because of their ionic nature, ionic liquids can be easily hydrolyzed. Ionic liquids are widely used as solvents, catalysts and reagents for syn- thesis of several biologically and industrially useful chemicals. Ionic liq- uids themselves acquire several useful applications [6,13–15] (Fig. 2). During last three decades, utilization of ILs as environmental friendly al- ternatives for traditional corrosion inhibitors is attain particular atten- tion because of their numerous beneficial properties together with low melting point, low vapor pressure, low toxicity, high polarity and extremely high stability to thermal and chemical treatments [16–23].
The protection efficiency of the ILs greatly depends upon the nature of cations and anions. The chemical and physical properties together with corrosion inhibition effectiveness can be suitably tailored by proper selection of cations and anions [24,25]. Because of this ability ILs are called as designer chemicals [26]. Unlike to traditional corrosion inhibitors, ILs are environment friendly in nature and this attributed due to non-inflammability negligible volatility, and non-toxic properties. Literature study shows that several review articles are published in
high quality journals dealing with numerous aspects of ILs [3,27–30]. Recently, numerous review articles are published on the corrosion inhi- bition effect of ionic liquids and their number is increasing day-by-day. Present review article is designed to describe the recent (2015–2020) advancement in the anticorrosive effect of ionic liquids for different metals and alloys in various common electrolytes. Several classes of ionic liquids are widely used as corrosion inhibitors therefore their re- cent collection is highly recommended. In view of this, present study aims to describe the collection of recent advancements in sustainable corrosion inhibitors using ionic liquids.

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