An overview of anti-corrosion properties of ionic liquids for corrosion of carbon steel in acidic media

1 Introduction

Metals and alloys are a boon to society whereas corrosion is a bane. The word corrosion is derived from the Latin word ‘corrodere’ which means to ‘gnaw to pieces’. It’s spontaneous and irreversible destruction of materials either due to chemical or electrochemical reactions at the electrode–electrolyte interface. Forms of corrosion are many and it includes pitting, differential metal, differential aeration, stress, erosion–corrosion, etc. The world has witnessed several disasters due to corrosion in the past which includes the Bhopal gas tragedy, Aloha airline disaster, a gas pipeline explosion in Mexico to name a few, which had both economic and health impacts. The rate and intensity of the corrosion depend on various factors like corrosive medium pH, temperature, potential differences between the metals in contact, mechanical factors, etc. A report by National Association for corrosion engineers (NACE) in the year 2002, states that the annual global corrosion cost exceeds 3.5% of the world’s gross domestic product. Corrosion of metals and alloys is a global issue (Ardakani et al. 2021). Corrosion cannot be prevented as metals have a natural tendency to go back to their combined form to attain stability but can only be controlled. Various methods have been employed by researchers across the world for corrosion control like electrodeposition, electroplating, coatings, alloying, inhibitors, etc. However, the use of corrosion inhibitors is believed to be the most economical method of corrosion control, as they can control corrosion to a larger extent when used in minute quantities (Verma et al. 2020). The efficiency of the inhibitors in a corrosive medium depends on a variety of factors, such as the chemical nature of the inhibitor molecule, the concentration of the inhibitor used, medium pH, temperature, immersion time, nature of metal, and its surface state, and aerated or de-aerated solution (Ardakani et al. 2021). The corrosion rate mainly depends on the change in the standard Gibb’s free energy (ΔG◦). A more negative change in the ΔG◦ value leads to a higher corrosion rate, and more positive ΔG◦ shows a lower corrosion rate of metal/alloy (Shivakumar and Mohana 2013).

Mild steel or plain carbon steel due to its relatively lower cost and good material properties are widely used in industrial applications (Saraswata et al. 2020). But mild steel is prone to corrosion due to its low corrosion resistance during processes like pickling, well-acidizing, etching, descaling, etc (Quakki et al. 2020). Organic molecules containing heteroatoms have been extensively studied as corrosion inhibitors and literature states that donor atoms in organic molecules like nitrogen, oxygen, sulfur, etc., have exhibited excellent corrosion inhibition on mild steel in acidic media (Alaoui et al. 2018; Ansari et al. 2014). However, the use of organic inhibitors at the industrial level is a challenge owing to low solubility in aqueous media, toxicity, stability, etc. In the past couple of decades, researchers have reported ionic liquids (ILs) as a green and sustainable choice of corrosion of inhibitors due to their low toxicity, high thermal and chemical stability, less volatile and noninflammable characteristics which enable its storage and use at the industrial level (Verma et al. 2017). ILs are organic salts that generally contain an organic cation and inorganic anion with a melting point below 100 °C. ILs have potential applications in the field of batteries, supercapacitors, solar cells, waste recycling, cellulose processing, tribology, etc., owing to their excellent physicochemical properties (Ardakani et al. 2021).

This review gives an overview of ionic liquids (ILs) used as corrosion inhibitors on mild steel in acidic media during the last five years, i.e., between the years 2016–2020. This review aims to give a wide-ranging review of literature and assessment of inhibition effects to enable the possible large-scale employment of ionic liquids as corrosion inhibitors on mild steel in acidic media for certain industrial operations. The operation parameters and efficiency of the inhibitor are discussed. Various approaches adopted for corrosion inhibition evaluation, mechanism, and adsorption isotherm are highlighted in this review.

2 Ionic liquids as corrosion inhibitors for mild steel in acidic media

Several ILs are reported as corrosion inhibitors for mild steel in acidic media. Most reported ILs include imidazolium, benzimidazolium, picolinium, pyrrolidinium, phosphonium cations with bromide, iodide, hexafluorophosphate, or acetate anions.

A summary of the ionic liquids that have been reported as effective corrosion inhibitors for mild steel corrosion in acidic media between the year 2016–2020 is tabulated in Table 1 along with respective efficiencies of inhibition.

Bousskri et al. (2016) have studied picolinium-based ionic liquid, 1-2-(4-chlorophenyl) 2-oxoethyl) -4N–N–methyl) picolinium bromide (CPOMPB) as corrosion inhibitor for mild steel in 1 M HCl solution. They have conducted gravimetric measurements (GM) and electrochemical measurement techniques like potentiodynamic polarization measurements (PDP) and electrochemical impedance spectroscopy (EIS). The surface morphology of the corroded specimen and inhibited specimen were studied using optical microscope measurements. X-ray photoelectron spectroscopic (XPS) method enabled one to understand the relationship between the molecular structure and efficiency of CPOMPB as a corrosion inhibitor. The adsorption of CPOMPB on mild steel surface followed Langmuir adsorption isotherm. The addition of KI to the inhibitor solutions showed increased inhibition due to the synergistic effect. Efficiency up to 94% was obtained using CPOMPB and KI. The mode of adsorption was mainly physisorption and had mixed control on both anodic and cathodic reactions as per Tafel plots. Gerengi et al. (2016) have investigated 1-ethyl-1-methylpyrrolidinium (EMTFB) as a corrosion inhibitor for dissolution of low carbon steel in 0.1 M HCl medium. Surface morphology of the corroded and inhibited specimen was studied using a scanning electron microscope (SEM) and surface compositions were recorded using energy-dispersive X-ray spectroscopy (EDX) analyses. Electrochemical measurements like PDP and EIS were carried out to determine the inhibition efficiency. The synergistic effect was observed on the addition of KI into the inhibitor solution and an increase in efficiency from 75% to 98% was reported. Langmuir adsorption isotherm model was proposed for the adsorption EMTFB on the carbon steel surface. Kannan et al. (2016) have synthesized 3-(4-chlorobenzoylmethyl)-1-methylbenzimidazoliumbromide ([BMMB]⁺Br⁻) characterized using spectroscopic techniques and studied its inhibitive effect on mild steel in 1 M hydrochloric acid solution. The inhibitor showed chemisorption behavior up to 50 °C and then it decreased, suggesting a shift in the equilibrium towards the process of desorption. The adsorption followed Langmuir adsorption isotherm and the quantum chemical calculations supported the adsorption mechanism of adsorption. Tetra-n-butyl ammonium methioninate was synthesized by Kowsari et al. (2016) and was assessed as a corrosion inhibitor for the dissolution of mild steel in 1 M HCl solution. The PDP and EIS results showed improved corrosion control and had predominant control on anodic metal oxidation reaction. The adsorption followed Freundlich isotherm and was due to electrostatic attraction between the metal and the inhibitor molecule. The methionine group of the reported IL was oriented towards the anodic sites and the tetra-n-butyl ammonium group of the IL was oriented towards the cathodic site, as per the proposed mechanism and quantum chemical calculations. SEM and EDX were carried out for surface morphology and composition analyses. Studies by Tawfik (2016) has revealed that the three ionic liquid-based gemini cationic surfactants; N¹,N¹,N¹,N²,N²,N²-hexadodecylethane-1,2-diaminium bromide (G2IL), N¹,N¹, N¹,N², N²,N²-hexadodecylpropane-1,3-diaminium bromide (G3IL) and N¹,N¹,N¹,N²,N²,N²-hexadodecylhexane-1,6-diaminium bromide (G6IL) could effectively suppress the corrosion of mild steel in 1 M HCl medium. The results of weight loss (WL), EIS, and PDP indicate that the reported ILs can control the rate of corrosion in mild steel effectively with efficiency values ranging between 89-94%. The adsorption mode was mainly physisorption and followed Langmuir adsorption isotherm. Yesudass et al. (2016) have reported five alkyl imidazolium-based ionic liquids (ILs) as corrosion inhibitors for mild steel dissolution in 1 M HCl. All were the mixed type of inhibitors and followed the Langmuir adsorption isotherm model. The quantum chemical calculations revealed that the inhibitive effects of the reported ILs depended strongly on the composite descriptors that involved the molecular weight, the fraction of electrons transferred from the inhibitor to the metal as well as the dipole moment of the ILs.

The picolinium-based ILs reported by Bousskri et al. (2017) displayed 96% efficiency at 0.001 M concentration of the IL as per the electrochemical measurements. For surface morphology, optical microscope images were recorded. Two separate reports by Aoun (2017a,b) on the effect of pyridinium IL, showed mixed inhibition of corrosion reaction as per the results of electrochemical measurements. SEM images supported the formation of a thin film of IL on the surface of the metal. Deyab et al. (2017) have synthesized and characterized four derivatives of 1,2-dimethyl-1H-imidazole-3-ium tetrafluoroborate and evaluated their inhibitory performance on the corrosion of mild steel in 1 M HCl. At 200 ppm levels of the IL, 73 to 92% inhibition was observed depending on the length of the aliphatic chain present on the 1,2-dimethyl-1H-imidazole-3-ium tetrafluoroborate moiety. Guo et al. (2017) have demonstrated the corrosion inhibitory performance of 1-vinyl-3-aminopropylimidazolium hexafluorophosphate ([VAIM][PF6]) and 1-vinyl-3- aminopropylimidazolium tetrafluoroborate ([VAIM][BF4] on mild steel corrosion in 1 M HCl medium. Both showed a mixed type of corrosion control. However, the adsorption of the former IL followed Langmuir monolayer adsorption model and the latter followed EI-Awady kinetic-thermodynamic adsorption model. Feng et al. (2017) discussed the corrosion inhibition performance of 1-vinyl-3-hexylimidazolium iodide ([VHIM]I) both experimentally and theoretically on the surface of mild steel in 0.5 M H₂SO₄ by providing up to 96% efficiency at 298 K. Theoretical calculations and molecular dynamic simulation model also explained the strong adsorption of the reported IL on the metal surface. Cao et al. (2017) have successfully demonstrated the corrosion control behavior of two Bronsted acid ionic liquids (BAILs) on mild steel in 0.5 M HCl. Both BAILs suppressed the corrosion rate to a greater extent and followed the Langmuir adsorption model. UV–Vis studies indicate chemisorption phenomena apart from the physisorption mode. Srivastava et al. (2017) have reported three amino acid-based imidazolium ILs as novel and green corrosion inhibitors for mild steel in 1 M HCl electrolyte. Both experimental and theoretical findings suggested good inhibitory action of the reported ILs. Quantum chemical calculation results show that protonated inhibitors are better inhibitors than neutral inhibitors. The findings of DFT matched well with the experimental results. MD results showed that the ILs adsorbed through flat orientation and their order of effectiveness followed the experimental order of inhibition efficiency. Kumar et al. (2017) have studied a new phosphonium bromide IL as a corrosion inhibitor on mild steel in 0.5 M H₂SO₄. SEM, EDX, and AFM were used to study the surface morphology of the corroded and the inhibited specimens. The IL showed a mixed type of inhibition and obeyed the Langmuir adsorption model.

Examination of 1-methyl-3-propylimidazolium iodide (MPIMI), 1-butyl-3-methyl imidazolium iodide (BMIMI), and 1-hexyl-3-methylimidazolium iodide (HMIMI) as inhibitors on mild steel in 1 M HCl by Azeez et al. (2018) has shown effective suppression of the corrosion current and improved corrosion resistance values as indicated by the PDP and EIS data, respectively. The results of electrochemical frequency modulation (EFM) are in line with EIS and PDP data. SEM, atomic force microscope (AFM), and Fourier transform infra-red (FTIR) analyses supported the inhibition offered by the IL. El-Hajjaji et al. (2018) have investigated two new pyridazinium-based ionic liquids namely 1-(6-ethoxy-6-oxohexyl)pyridazin-1-ium bromide and 1-(2-bromoacetyl) pyridazinium bromide as corrosion inhibitor on mild steel surface in 1 M HCl from 303–333 K temperature. The highest inhibition was seen at 303 K and decreased as the temperature of the electrolyte was increased to 333 K suggesting a physisorption mode of adsorption. The theoretical studies support the displacement of water molecules by the ILs (2-amino benzyl)triphenylphosphonium bromide.

(ABTPPB) was evaluated as a corrosion inhibitor on the surface of the mild steel dipped in 0.5 M H₂SO₄ by Ghoyal et al. (2018). The studies suggest an enlarged capacitive loop in the Nyquist plot, which indicates the improved corrosion resistance and results of PDP and quantum chemical calculations, also directs the same. Corrosion inhibition of mild steel in 1 M HCl was achieved by using1-alkyl-3-methylimidazolium bromides as reported by Langova et al. (2018), the highest being 94% by the IL 1-octyl-3-methylimidazolium bromide. The results fit well with Langmuir isotherm, Flory–Huggins isotherm, and El-Awady thermodynamic-kinetic model for adsorption. The adsorption ability of choline-based ILs on mild steel in 1 M HCl was demonstrated by Verma et al. (2018) using theoretical as well as experimental methods. These choline-based ILs were efficient up to 92–96% as per the results of weight loss methods and electrochemical analyses and obeyed Temkin adsorption isotherm. The theoretical calculations showed that the adsorption of IL on mild steel occurred spontaneously, and it was mainly through donor-acceptor interactions between the ILs and the metal surface. Two newly synthesized fluoro-substituted imidazolium-based ILs two imidazolium-based ionic liquids i.e., 1-(2-aminoethyl)-1-dodecyl-2-methyl-4,5-dihydro-1H-imidazole-1-ium chloride ([ADMDI]Cl) and 1-(2-aminoethyl)-1-dodecyl-2-(trifluoromethyl)-4,5-dihydro-1H-imidazole-1-ium chloride ([ADTDI]Cl) were investigated as the corrosion inhibitor on mild steel in 0.5 M HCl solution at 368 K by Zhang et al. (2018). The PDP measurements show that the IL effectively controlled the cathodic hydrogen evolution reaction, thereby decreasing the overall rate of corrosion. Increasing the temperature of the electrolyte increased the efficiency suggesting the chemisorption mode of inhibition, following the Langmuir adsorption isotherm model. The theoretical calculations also showed that the trifluoromethyl group in [ADTDI]Cl boosts the transfer of more free electrons as compared to the methyl group in [ADMDI]Cl, suggesting the better inhibition ability of the former, especially at a higher temperature.

Ahmed et al. (2019) have reported five ILs with bromide and iodide counter ions as corrosion inhibitors on mild steel in 0.5 M H₂SO₄. The electrochemical measurements included PDP and cyclic voltammetry (CV). It was inferred that the ILs with bromide counterion were more efficient than the iodide counter ion; the mode of adsorption was both by physisorption and chemisorption process. The effect of the length of the alkyl chain, cation, and the anion of an IL on the corrosion inhibition property of the ILs on mild steel in 1 M HCl was investigated by Al-Rashed and Nazeer (2019) by EIS and PDP techniques. SEM and FTIR were used for surface analyses of the corroded and inhibited specimen. They concluded that the longer the length of the alkyl chain, the better is its inhibition ability, and the efficiency of the IL with N(CN)₂ counter ion gave better protection for mild steel than the SCN and PF₆ ions. Bhaskaran and Singh (2019) have reported 3-(4-fluorobenzyl)-1-methyl-1H-imidazole-3-ium bromide [FBMIm]Br as an effective green corrosion inhibitor for the dissolution of mild steel in 0.5 M H₂SO₄ electrolyte. An excellent efficiency up to 98 to 99% at 298 K was reported as per EIS and PDP results. Two ILs namely 1-(4-sulfonic acid) butyl-3-ethyl imidazolium hydrogen sulfate and 1-(4-sulfonic acid) butyl-3-decyl imidazolium hydrogen sulfate were found to suppress the mild steel dissolution in 0.5 M HCl due to both physical and chemical adsorption as per the reports by Cao et al. (2019a) The UV-visible and X-ray photoelectron spectroscopy (XPS) analysis results supported the interaction between ILs and mild steel surfaces. The longer alkyl chains in ILs gave better corrosion protection. The synergistic effect of KI along with the IL 1,1′ (1,4 phenylene bis(methylene))bis(3 (carboxymethyl) 1H- imidazole -3- ium) chloride on corrosion inhibition effect on mild steel in 0.5 M HCl was reported by Cao et al. (2019b). The electrochemical, surface, and theoretical approaches, all of them proved that the IL served as an efficient inhibitor and the presence of KI in 1:1 ratio to IL increased the maximum efficiency further by 2%. 3-((4-amino-2-methylpyrimidin-5-yl)methyl)-5-(2-hydroxyethyl)-4-methylthiazol-3-ium chloride (AMPMHMC) in 1 M HCl solution largely controlled the rate of corrosion of mild steel as per the investigations by Farag et al. (2019). The adsorption of the IL on a mild steel surface followed Langmuir adsorption isotherm and showed 91.4% inhibition at 40 ppm of the IL. Hajjaji et al. (2019) have synthesized two pyridazine derivatives namely, 1-decylpyridazin-1-ium iodide (DPI) and 1-tetradecylpyridazin-1-ium iodide (TPI), and studied them as corrosion inhibitors for corrosion of mild steel in 1 M HCl solution in the temperature range of 303–333 K. Both DPI and TPI served as good corrosion suppressors by exhibiting efficiencies of 86.7% and 88.6%, respectively at 303 K. At an immersion period of 12 h the efficiency reached up to 97.6%. The DFT studies were following the experimental results. A dicationic IL 1,4- (Divinyl-imidazolium bromide) butane was synthesized as a green corrosion inhibitor for mild steel corrosion in 1 M sulfuric acid solution by Jannat et al. (2019). The EIS and PDP measurements were carried out to measure the resistance and corrosion current values and in the presence of the IL, the resistance values increased with a decrease in the corrosion current, suggesting good corrosion control. The quantum chemical calculations were also supporting the experimental results. Parveen et al. (2019) have reported 1-methyl-3-propylimidazolium iodide (MPII) as a corrosion inhibitor on mild steel in 1 M sulfuric acid solution. They have used the weight loss methods, PDP, and EIS methods for efficiency analyses. The IL displayed a mixed type of inhibition with both anodic and cathodic reactions control and followed the Langmuir adsorption isotherm model. Three 1-butyl-3-methyl-imidazolium based ionic liquids are reported as corrosion inhibitors on mild steel dissolution in 1 M HCl by Verma et al. (2019) and the efficiencies ranged between 93–97% as per the results of electrochemical and computational experiments.

Ardakani et al. (2020) have synthesized a polymeric IL derived from imidazolium IL and studied its corrosion inhibition properties on mild steel corrosion in 1 M HCl by EIS and PDP methods. The adsorption of the inhibitor followed the Frumkin isotherm model, and it was a mixed inhibitor with predominant control on the cathodic hydrogen evolution reaction. Aslam et al. (2020) have demonstrated three amino ester salts based ILs as corrosion inhibitors on mild steel in 1 M HCl solution. As per the PDP results the ILs effectively controlled the anodic metal dissolution as well as the cathodic hydrogen evolution reactions. The quantum chemical calculations supported the results that were experimentally determined. Cui et al. (2020) demonstrated the control on the corrosion rate of mild steel in H₂S and HCl solutions by using five different imidazolium-based ILs. The ILs inhibited the corrosion by mere electrostatic interactions between the ILs and the metal surface and followed the Langmuir adsorption model. Isopentyltriphenylphosphonium bromide ionic liquid was reported as an effective inhibitor of corrosion of mild steel in 0.5 M H₂SO₄ by Goyal et al. (2020) Inhibition as high as 99% was achieved with a 0.01 M concentration of the IL. The IL formed a monolayer on the surface of mild steel and followed the Langmuir adsorption isotherm model. The inhibition was due to the physical adsorption of ILs on the mild steel surface. El-Hajjaji et al. (2020) have demonstrated pyridinium-derived ionic liquids as corrosion inhibitors for mild steel in 1 M HCl. These ILs behaved as an anodic inhibitor as per the result of PDP analysis. Both physisorption and chemisorption mode of adsorption was observed. The radial distribution function indicated the adsorption of IL on the metal surface via chemisorption mode. Subasree and Subasree and Selvi (2020) have synthesized and evaluated three imidazolium-based ionic liquids as corrosion inhibitors for mild steel dissolution in a 1 M HCl medium. An increase in the alkyl substituents in the ILs gave better inhibition performance. The formation of a complex between the iron and the inhibitor IL was confirmed by UV–Vis results. Tan et al. (2020) have reported 1-hexadecyl-3-methylimidazolium bromide (HMIBr) as a corrosion inhibitor for mild steel corrosion in 1 M HCl solution. Up to 97% efficiency was obtained at 0.001 M concentration of the IL. N-methyl-2-hydroxyethyl ammonium oleate ([m-2HEA] [Ol]) has been investigated as a corrosion inhibitor on mild steel in 0.1 M HCl medium. The results of PDP indicated a mixed inhibition type. Briefly, the inhibition was also measured in sulfuric acid medium, and it provided up to 54% efficiency by Schmitzhaus et al. (2020). As per the reports by Zafari et al. (2020), 2-benzyl-1-butyl-3-(3-(triethoxysilyl)propyl)-1H-benzo[d]imidazolium chloride (BTOSPB) provided up to 99.5% corrosion control in 1 M HCl on mild steel surface. The XPS analysis confirmed the film formation on the metal surface by the IL. The maximum efficiency was obtained at higher medium concentrations and temperatures.

From the thorough review of literature, it may be inferred that the ILs serve as excellent candidates for corrosion control application. Almost all ILs reported in the literature mentioned above give excellent inhibition efficiency ranging from 80 percent to 90% except for 5-methoxy-1,2,3,3-tetramethyl-3H-indolium iodide (IBIL-I), 1-(2-carboxyethyl)-2,3,3-trimethyl-3H-indolium iodide (IBIL-II), 2,3,3-trimethyl-1-(pyren-2-ylmthyl)-3H-indolium iodide (IBIL-III) where the authors attribute it to the role of bromide counter ion being more efficient inhibitor than the iodide counter ion inhibitor (Ahmed et al. 2019) (see Table 1). Almost all the reported ILs were of mixed types of inhibitors which has control on both the anodic metal dissolution and cathodic reduction reaction.

The adsorption isotherm studies reveal that almost all reported ILs as corrosion inhibitors for mild steel in acid media obey and fit well to Langmuir adsorption isotherm barring a few which follow Temkin and Frumkin adsorption isotherms (Ayawei et al. 2017). Langmuir adsorption isotherm indicates a dynamic equilibrium phenomenon where the surface coverage by the inhibitor is due to balanced relative rates of adsorption of ILs and desorption of the solvent molecules. Adsorption of the IL onto the surface of the mild steel is proportional to the fraction of the surface of the mild steel that is open while desorption of the solvent molecules is proportional to the fraction of the mild steel surface that is covered. In the case of the Temkin adsorption isotherm model, the effect of adsorbate interactions in the process of adsorption is considered. It assumes that due to the increase in surface coverage by the ILs on the mild steel surface there is a linear decrease in the heat of adsorption of all molecules in the layer. However, this isotherm is valid only for a specific range of IL concentrations. The Frumkin adsorption isotherm model gives an important parameter termed “a”, this refers to the peripheral interaction between IL inhibitor molecules into the adsorbent layer and the surface non-uniformity. When a < 0, it indicates a repulsive interaction, while a > 0 indicates peripheral adsorption amongst the adsorbent IL molecules (Ardakani et al. 2020).

3 Common methods adopted for the evaluation of corrosion rate and inhibition efficiency

Most used electrochemical and non-electrochemical methods of corrosion rate analysis in the absence and presence of inhibitors are discussed in the following sections. Of which, weight loss measurement technique and density functional theory calculations are non-electrochemical and EIS and PDP measurements are electrochemical methods. Both the electrochemical and non-electrochemical techniques have their pros and cons which are discussed below.

The weight loss technique is a simple-to-do method and applies to solid, particulate, gaseous, or aqueous environments. The corrosion products or deposits may be visually inspected. The rate of corrosion in the absence and presence of the inhibitors can be assessed using a high accuracy weighing balance without using many sophisticated tools of analysis which is relatively inexpensive. However, this technique is time-consuming and gives only average readings, and doesn’t account for non-uniform forms of corrosion, and specimens undergoing a lower degree of corrosion cannot be determined by this technique (Speight 2014).

On the other hand, the EIS analysis, an AC technique overcomes most of the downsides of weight loss methods by enabling the determination of low corrosion rates in a short time and high accuracy measurements by application of the small excitation amplitude with minimum perturbation to the corrosion potential. The results of EIS are reliable and repeatable. The EIS data gives enormous information about the various electrochemical reactions and reaction kinetics. The fast processes like electrical double layer formation, ohmic resistance, resistance associated with charge transfer due to electrochemical reactions, and slow processes like the formation of layers due to adsorbed reaction intermediates are all accounted for in EIS data. These data are beneficial in the computation of corrosion rates in the presence and absence of inhibitors. It provides material regarding electrode–electrolyte double layer formation and formation of surface oxide films and their dielectric properties. However, the primary disadvantage of EIS measurements is that conversion of polarization resistance into a corrosion rate without prior knowledge of the Tafel slopes values and the Stern–Geary coefficient is not possible. Apart from these, certain other limitations like lack of determination of relative anodic and cathodic reaction kinetics due to different alloying elements, secondary phases, solutions, etc. exist. Also, single EIS data may fit into different equivalent circuits all of which give low error figures which results in an improper calculation of resistance and capacitance data thereby assessing the corrosion rate. These kinds of errors may be eliminated by choosing a proper equivalent circuit model based on a deep understanding of the mechanism of corrosion reactions, assisted by surface characterization, physicochemical analysis, or from previously published reports (Feliu 2020).

The PDP method is a widely used DC electrochemical tool for corrosion rate analysis using the Tafel plot. The method is built on the electrochemical theory of corrosion developed by Wagner and Traud. The polarization curves may be recorded both under steady-state or non-steady-state potentiostat/galvanostatic conditions using polarization signal amplitudes. However, due to the application of large polarization signal amplitudes, there is a possibility of irreversible changes at the corroding electrode surface which restricts the usage of this technique in corrosion monitoring applications only (Lorenz and Mansfeld 1981).

The density functional theory (DFT) calculations are widely accepted green corrosion inhibition techniques because of a theoretical approach. It is a simple method to study the structure of molecule and corrosion inhibitor behavior. The main disadvantage of the DFT method is the challenge in defining the most appropriate method for a particular application. Before choosing a DFT method, one must consult the literature to determine the suitability of that choice for that particular application (Verma 2018).

A brief description of each of the measurement techniques and analysis mentioned in the above section is given in the following sections.

3.2 Potentiodynamic polarization measurements (Tafel plot)

This is a destructive method for measuring the corrosion rate of a specimen, where the metal is drifted away from its rest potential and allowed to undergo redox reactions. In this method, the mild steel is immersed in a known concentration of an acidic medium with and without inhibitor and is allowed to attain the open circuit potential (OCP). After attaining OCP, the Tafel curves were recorded at a scan rate of 1 mV s⁻¹ from −250 mV to +250 mV of OCP, in most of the reported literature (Fu et al. 2011, Kumari et al. 2017, Saranya et al., 2016). The linear portions of the anode and cathode curve are then extrapolated and several corrosion parameters like the anodic slope (β_a), cathodic slope (−β_c), corrosion current density (i_corr) can be obtained by the Tafel extrapolation method and the inhibition efficiency (η%) can be calculated using the Eq. (3) (Tang et al. 2013).

(3)η%=icorr−icorr(inh)icorr×100

where, i_corr and i_corr(inh) are the corrosion current densities expressed in μA cm⁻² in the absence and presence of inhibitor, respectively.

Apart from the above-mentioned corrosion parameters, the Tafel curves give information about the type of corrosion control the inhibitor has on the metal dissolution. The inhibitor may act either on anode sites or cathode sites. Glancing at the cathode and anode curves in the absence and presence of the inhibitor and comparing the two curves one can easily infer of the inhibitor slows down the anode reaction or the cathode reaction Figure 1. Also, if the corrosion potential (E_corr) shifts towards more positive values and the shift is beyond 85 mV then the inhibitor acts as an anodic inhibitor, where anodic sites are blocked by the inhibitor thereby slowing down the metal dissolution rate. Similarly, if the corrosion potential shifts towards more negative values and if the shift is beyond −85 mV, then the inhibitor acts as a cathodic inhibitor, where cathodic sites are blocked, and the overall corrosion rate is reduced. However, in a case where the shift in E_corr is within ±85 mV, then the inhibitor is called a mixed type of inhibitor which has control on both cathodic and anodic reactions (Behpour et al. 2010).

Figure 1:

Representative Tafel curves in the presence and absence of the inhibitor.

3.3 Electrochemical impedance spectroscopy (EIS) measurements

The EIS corrosion measurement technique is a powerful and nondestructive measurement tool that gives in-depth information about the electrode–electrolyte interfacial corrosion reactions. Here, a small amplitude of AC signal is applied on the electrode to assess the capacitive and resistive behavior. The EIS measurements yield data in the form of Nyquist plots, Bode phase angle plots, and Bode magnitude plots. In Nyquist plots, the real impedance is plotted against imaginary impedance (Quaraishi and Sardar 2003). For mild steel, usually, the Nyquist plots have a single capacitive loop, which is simulated into an equivalent circuit model. Normally, the best fit was obtained for an R_s(QR_ct) circuit (Doharea et al. 2017). The capacitor C is replaced with Q called a constant phase element which represents nonideal capacitive behavior and gives a better fit in the simulation model. R_s represents the solution resistance and R_ct represents the charge transfer resistance. In Bode phase angle plots, frequency is plotted against the phase angle theta and in Bode magnitude plots, frequency is plotted against the impedance modulus (Z_mod). The Bode phase angle and Bode magnitude plots are useful in assessing the frequency-specific impedance behavior which cannot be obtained by Nyquist plots. In the Nyquist plots, the value of R_ct increases and Q decreases in the presence of inhibitor which indicates improved corrosion rate (Figure 2). The value of inhibition efficiency is obtained by Eq. (4) (Saha et al. 2016).

(4)η%=Rct(inh)−Rct(b)Rct(inh)×100

where, R_ct(inh) and R_ct(b) are the charge transfer resistances of inhibited and blank specimens, respectively.

Figure 2:

Representative Nyquist plot displaying increased resistance in the presence of inhibitor than in the absence of the inhibitor.

In Bode phase angle plots, a shift in mid-frequency phase maximum (θ_max) towards lower frequency, along with an increase in θ_max as compared to the value of a blank solution, suggests the improved corrosion resistance. From the Bode magnitude plots, the value of low-frequency impedance modulus (Z_mod) increases, indicating better corrosion protection in the presence of inhibitor as compared to its value in the absence of the inhibitor (Shaban et al. 2016).

4 Common methods adopted for the surface analyses of the corroded and inhibited specimens and metal-inhibitor interactions

5 General mechanism of inhibition reported in the literature

Generally, in acidic media, the corrosion inhibitors get adsorbed onto the surface of mild steel either by the process of physical adsorption or by the chemical adsorption phenomenon. Most of the reported literature shows the mixed type of corrosion control by the ILs on mild steel dissolution in HCl and H₂SO₄ media. The ILs generally had control on both the anodic metal dissolution reaction as well as the cathodic hydrogen evolution reaction. Some ILs showed inhibition by mere physical adsorption whereas some ILs were chemically adsorbed on the surface of the mild steel. This was inferred based on the results of temperature studies. If the efficiency of the inhibitor reduces as the temperature of the electrolyte was increased, then it was said to be physically adsorbed on the mild steel surface and the ILs desorb when the temperature is increased as the ILs gain kinetic energy. However, if the efficiency values increase with an increase in the temperature, then the ILs are believed to be chemically adsorbed on the mild steel surface. Under acidic conditions, the charge on the mild steel surface is positive since the corrosion potential E_corr is negative. The chloride or sulfate ions of the acid are then attracted towards the positively charged mild steel surface. In the presence of ILs, which have both positively charged organic moiety and the negatively charged counter ion, the adsorbed chloride or sulfate ions attract the positively charged organic moiety of the ILs. This kind of interaction is purely electrostatic and results in the physisorption of the ILs onto the surface of the mild steel thereby blocking the active metal sites from a further attack of the corrosive ions. Contrarily, the ILs in acidic media largely exist in their protonated form, these cations adsorb on the cathodic sites of the metal surface thereby controlling the cathodic hydrogen evolution reaction. The ILs mostly contain heteroatoms like N, S, and O which have free lone pairs of electrons, these lone pairs are donated to orbitals of the oxidized metal and form a co-ordinate bond (chemisorption). This prevents further dissolution of the mild steel in an acidic medium (Kowsari et al. 2014, Subasree and Selvi 2020, Zheng et al. 2015).

6 Conclusions

1. The ILs serve as excellent green corrosion inhibitors for mild steel dissolution in acidic media. Most of the reported inhibition efficiency values ranged between 80 to 90% at minute concentrations.

2. The most used electrochemical tools for corrosion rate analyses are weight loss, EIS, and PDP techniques. Here, only EIS is a nondestructive method of corrosion analysis whereas weight loss and PDP methods are destructive methods of evaluation although they are accurate. Quantum chemical studies and molecular dynamic models are used to support the experimental findings.

3. Generally, SEM, AFM, EDX, techniques are used to study the surface morphology and interactions of the ILs with a mild steel surface. A few researchers have reported FTIR, UV–Vis, and contact angle measurements for the same.

4. Largely, the ILs adsorb on the mild steel surface both by physisorption and chemisorption mode and has control on both anodic metal dissolution and cathodic hydrogen evolution reaction.

7 Scope for further expansions

1. The non-destructive corrosion monitoring technique i.e., electrochemical frequency modulation (EFM) may be explored in the presence of ILs as corrosion inhibitors.

2. The effectiveness of ILs as corrosion inhibitors for mild steel corrosion in industrial applications may be explored. Also, the reports of IL inhibitor efficiencies in a neutral and alkaline medium are few and the same may be studied.

3. Mechanism of inhibition reported in the literature are general mechanisms or theoretical and enough experimental evidence is currently scarce in the literature, the same may be worked upon.