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

Corrosion: basics, adverse impacts and prevention

Corrosion is distinct as degradation of metals and their materials through the interaction of environment. Corrosion cause big safety, economic and fatality losses especially in the industrial sectors [54,55]. The economic losses are more pronounced during industry based metallurgical processes including acid cleaning, pickling, and descaling and oil-well acidization. Obviously, these industrial processes involve use aggressive solutions (15–28% acidic solution) that cause undesir- able degradation of metallic components along with the surface impuri- ties. As per the estimation of NACE, corrosion results into the global economic loss of about US $2.5 trillion that consists 3.5% of the global GDP [56]. It is estimated that this cost of corrosion can be reduced from US $375 billion (15%) to US $875 billion (35%) using previously developed methods [56,57]. Corrosion scientists and engineers developed numerous corrosion mitigation methods depending upon the nature of metal and environment. These methods include paintings, coatings, alloying and dealloying, galvanization, use of corrosion inhibi- tors and many more [58–60]. Organic inhibitors may be of inorganic or organic type. Inorganic inhibitors generally show good inhibition effectivity however they are highly toxic and their implementation is highly restricted owing to the strict environmental regulations and increasing ecological awareness. Therefore, current advancement in the field of corrosion inhibition is lying on the designing and use of en- vironmental friendly synthetic organic corrosion inhibitors.
The compounds derived through MCRs, solid state reactions (SSRs), mechanochemical mixing (MCM), microwave (MW) and ultrasound (US) irradiations and engineered materials such as ionic liquids (ILs) and polyethylene glycol (PEG) are established as most effective environ- mental sustainable corrosion inhibitors. Literature outcomes suggest that these compounds become effective by adsorbing on the interface of metal and environment [61–63]. After getting adsorbed, organic com- pounds form a barrier that avoids the contact of metal surface with ag- gressive environment. The inhibitive barrier also avoids the penetration or diffusion and reaching out the corrosive species such as O₂, H₂O and an- ions to the metal surface. It is well documented that presence of moisture and salt accelerate the corrosion rate. Corrosion inhibitors may adsorb using chemical, physical or physiochemical (mixed) mechanism. Physisorption takes place by the electrostatic potential difference among the charged inhibitor and metallic surface. Physisorption is generally con- sistent with the Gibb's free energy of adsorption (ΔG_ads) of −20 kJ mol⁻¹ or more positive. On the other hands, chemisorption occurs mainly though charge sharing and consistent with the Gibb's free energy of ad- sorption of −40 kJ mol⁻¹ or more negative. Literature study reveals that adsorption of most of the inhibitors obeys physiochemisorption mecha- nism with Gibb's free energy value in between −20 and −40 kJ mol⁻¹ [64,65]. Different adsorption isotherm model are used to describe the ad- sorption behavior of these compounds on the metal surface [66,67]. It is observed that adsorption mechanism mostly obeyed the Langmuir, Temkin and Freundlich adsorption isotherm models [66,67].

4.