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

MðH₂OÞ_nSO₄=2Cl−ILsC]

₋

ads
þ 2IL^þ
ticorrosive property of two series of imidazolium based ionic liquids

• 
  (IL1–IL4), namely, 1-methyl imidazolium and 1,2-dimethyl
  

þ SO =2Cl →MðH₂OÞ SO₄=2Cl−ILsC]⁻ _ads þ ½2IL]^þSO²⁻=2Cl
ð8Þ

4 n 4
Above reactions can be simplified as follows:

ads
M þ X⁻↔ðMXÞ⁻
ð9Þ
imidazolium for CS/1 M HCl using experimental methods. IL1–4
inhibited corrosion by adsorbing on the metal surface that followed Temkin adsorption isotherm. IL1–4 behaved as mixed-type corrosion inhibitors. Authors observed that for both 1-methyl imidazolium (IL1 & IL2) and 1,2-dimethyl imidazolium (IL3 & IL4), increase in the size

−
ðMXÞ_ads
þ IL^þ↔ MX⁻IL^þ
ads
ð10Þ
of hydrocarbon chain from decyl (10 C-atom) to dodecyl (12 C-atom) resulted into substantial increase in the inhibition efficiency. EIS analy-

Counter ions such as chloride and sulphate ions synergistically acceler- ate the adsorption of cations at anodic sites.

2. 
   Cathodic reactions without ionic liquids (ILs) In aqueous electrolytes cathodic reactions without corrosion in- hibitors proceed as follows:
   

M þ H₃O^þ↔MðH₃OÞ^þ_ads ð11Þ
MðH₃OÞ^þ_ads ↔MðH₃OÞ_ads þ e⁻ ð12Þ
MðH₃OÞ_ads þ e⁻ þ H^þ↔M þ H₂O þ H₂ ð13Þ

3. 
   Cathodic reactions with ionic liquids (ILs)
   

M þ IL^þ↔MðILÞ^þ_ads ð14Þ
MðILÞ_a^þ_ds þ e⁻↔MðILÞ_ads ð15Þ
Generally, adsorption of ionic liquids and traditional organic inhibitors originated all the way through electrostatic force of connections be- tween charged metallic surface and inhibitor molecules. However, later than getting adsorbed, cationic species gain electrons and get trans- formed into their neutral forms.


Neutral species with free unshared paired electrons get adsorbed by transferring their non-bonding and/or π-electrons (chemisorption) into the d-orbital of the metal through coordination bonding. As metals are already electron rich, this type of electron donation causes inter- electronic hindrance that renders the metal to transfer their electrons
sis revealed that IL1-IL4 inhibit CS acidic corrosion by behaving as inter- face type of behavior. Adsorption of the IL1-IL4 in CS surface was reinforced with FT-IR technique. In another study, it was observed that imidazolium ionic liquids with identical hydrocarbon chain but dif- ferent number of methyl substituents exhibited variable protection power [72]. Authors observed that at all tested concentrations, [C₁₆M₂Im][Br] containing two methyl substituents showed higher pro- tection efficiency for MS corrosion in 1 M HCl than that of the [C₁₆MIm][Br] having only one methyl substituent. Analyses revealed that [C₁₆M₂Im][Br] and [C₁₆MIm][Br] behaved as mixed-type inhibitors with slightly anodic predominance. SEM, EDX and AFM analyses showed that [C₁₆M₂Im][Br] and [C₁₆MIm][Br] inhibited MS corrosion by adsorbing on the metallic surface and substantial improvement in the surface morphologies of SEM and AFM images was observed in the presence of [C₁₆M₂Im][Br] and [C₁₆MIm][Br]. The order of smooth- ness in the surface morphologies of metallic species derived from SEM and AFM methods was consistent with the order of weight loss and electrochemical studies i.e. [C₁₆M₁Im] [Br] < [C₁₆M₂Im] [Br]. It was pos- tulated in this study that [C₁₆M₂Im][Br] and [C₁₆MIm][Br] interact with metallic surface using donor-acceptor mechanism involving both physisorption and chemisorption mechanisms.
While studying the corrosion inhibition of three imidazolium de-
rived ionic liquids (HMIMI, BMIMI & PMIMI) for MS/1 M HCl system, Azeez et al. [73] showed that increase in the size of alkyl chain from pro- pyl to hexyl causes increase in the protection effectiveness. PDP study showed that HMIMI, BMIMI & PMIMI behave as mixed-type inhibitors and they become effective by following Langmuir adsorption isotherm. Results showed that HMIMI, BMIMI & PMIMI showed optimum protec- tiveness of 93.1%, 87.8% and 80.4% at 5 × 10⁻³ M concentration. EIS study revealed that HMIMI, BMIMI & PMIMI acted as interface-type in- hibitors. SEM, AFM and FT-IR studies were conducted to demonstrate the adsorption of HMIMI, BMIMI & PMIMI on MS surface in 1 M HCl

Fig. 3. Schematic presentation of inhibition of Tafel (anodic and cathodic) reactions for mid steel corrosion in 1 M HCl using choline derived ionic liquids [69].

and analyses showed that they become effective by adsorbing on the surface. In the presence of HMIMI, BMIMI & PMIMI, SEM and AFM mi- crographs are significantly improved by the formation of inhibitive films. DFT study showed that HMIMI, BMIMI & PMIMI interacted through donor-acceptor interactions and their FMOs (Frontier Molecu- lar Orbitals) were mainly localized over the imidazolium ring indicating that the imidazolium moiety directly interacted through metallic sur- face and rest parts of the molecules enhance corrosion inhibition effect by enhancing the hydrophobicity. As the substituent size of hydrocar- bon chain increases from propyl to hexyl, the hydrophobicity also in- creases in the same sequence. The FMOs (optimized, HOMO & LUMO) of HMIMI, BMIMI & PMIMI are presented in Fig. 4.
Increase in the inhibition efficiency on increasing the size of alkyl chain has also been reported elsewhere [74]. In this study authors de- scribed the anticorrosion power of three imidazolium based ionic liq- uids designated as [(CH₂)₃COOHMIm][HSO₄], [(CH₂)₂COOHMIm] [HSO₄] and [(CH₂)₃COOHMIm][HSO₄] for CS/0.5 M HCl system and ob- served that inhibition efficiencies of evaluated ionic liquids increase
on increasing the number of methylene (>CH₂) substituent. Tafel polar- ization studies suggested that presence of [(CH₂)₃COOHMIm][HSO₄], [(CH)₂COOHMIm][HSO₄] and [(CH₂)₃COOHMIm][HSO₄] causes cause
significant change in the shape of polarization anodic and cathodic curves.
On the basis of this observation authors concluded that [(CH₂)
₃COOHMIm][HSO₄], [(CH₂)₂COOHMIm][HSO₄] and [(CH₂)₃COOHMIm]
[HSO₄] inhibit CS corrosion in 0.5 M HCl by blocking the anodic oxida- tion and cathodic reduction reactions. It was also observed that corro- sion current densities significantly decreased in the presence of [(CH₂) ₃COOHMIm][HSO₄], [(CH₂)₂COOHMIm][HSO₄] and [(CH₂)₃COOHMIm]
[HSO₄]. This observation suggested that investigated ionic liquids be- come effective by blocking the active sites responsible for the aggressive corrosion. The Tafel polarization curves for CS/0.5 M HCl system with and without [(CH₂)₃COOHMIm][HSO₄], [(CH₂)₂COOHMIm][HSO₄] and
[(CH₂)₃COOHMIm][HSO₄] are presented in Fig. 5. Adsorption of the [(CH₂)₃COOHMIm][HSO₄], [(CH₂)₂COOHMIm][HSO₄] and [(CH₂)
₃COOHMIm][HSO₄] on CS surface in 0.5 M HCl was supported by SEM

Fig. 4. Optimized, HOMO and LUMO Frontier Molecular Orbital pictures of the HMIMI, BMIMI and PMIMI derived through Gaussian software package. HOMOs and LUMOs are mainly appeared at imidazolium moiety which suggests that imidazolium species is directly participated in the charge sharing [73].

and AFM analyses. It was observed that significant smoothness in the metallic surfaces were observed in the presence of [(CH₂)₃COOHMIm] [HSO₄], [(CH₂)₂COOHMIm][HSO₄] and [(CH₂)₃COOHMIm][HSO₄] and
the smoothness in the surface morphologies were consistent with the order of their practical inhibition efficiency. DFT analyses showed that investigated ionic liquids interacted through donor acceptor

Fig. 5. Tafel (anodic and cathodic) polarization curves for CS corrosion in 0.5 M HCl with and without (A) [(CH₂)₃COOHMIm][HSO₄], (B) [(CH₂)₂COOHMIm][HSO₄] and

(C) [(_CH2)₃COOHMIm][HSO₄] at different concentrations [74].

interactions in which imidazolium moiety acted as adsorption centers. FMOs of [(CH₂)₃COOHMIm][HSO₄], [(CH₂)₂COOHMIm][HSO₄] and
[(CH₂)₃COOHMIm][HSO₄] are presented in Fig. 6.
The information on chemical structures, abbreviations and nature of electrolytes and iron alloys of some major work reported in acidic HCl solutions are presented in Table 1 [71–74]. It can be seen that most of the investigated ionic liquids are imidazolium based. Mostly these ionic liquids adsorb on metallic surface using Langmuir adsorption iso- therm model. These ionic liquids mainly acted as mixed- and interface- type corrosion inhibitors. DFT analyses revealed that mostly these ionic liquids interact using donor-acceptor interactions in which electron rich imidazolium moiety behaves as adsorption or active center. Molecular dynamic (MD) and Monte Carlo (MC) simulations showed that imidazolium based ionic liquids mostly gained the flat or horizontal ori- entation and behaved as effective anticorrosive materials.

2. Ionic liquids as corrosion inhibitors for iron alloys in H₂SO₄

Similar to HCl, H₂SO₄ based electrolytes are widely used as electro- lytes by academicians as well as industrialists. Lower concentration is widely used for academicians whereas highly concentrated H₂SO₄ is mainly employed by industrialist. Imidazolium based ionic liquids are extensively as inhibitors in H₂SO₄ based electrolytes. Substituents play a significant role while determining the effectiveness of ionic liquids. Generally, imidazolium ILs with hydrophobic hydrocarbon chain

(s) exhibit excellent corrosion inhibition activity as they form highly protective micelles at the interface of the metal and environment (e.g. HCl & H₂SO₄ solution). Recently, Corrales-Luna et al. [114] studied the corrosion inhibition effect of an ionic liquid namely, 1-ethyl 3- methylimidazolium thiocyanate [(EMIM)+(SCN)−] for API X52 iron al- loys in 0.5 M HCl and 0.5 M H₂SO₄ media by computational and exper- imental methods. PDP study revealed that (EMIM)+(SCN)− behaves as mixed type corrosion inhibitor. (EMIM)+(SCN)− showed highest pro- tection effectiveness of 82.9% (75 ppm) and 74.4% (100 ppm) in 0.5 M H₂SO₄ and 0.5 M HCl solutions, respectively. Protection power of (EMIM)+(SCN)− increases with its concentration and diminished with rise in temperature. (EMIM)+(SCN)− inhibits metal corrosion by adsorption mechanism that obeyed Langmuir isotherm model. Ad- sorption of the (EMIM)+(SCN)− of metals surface was reinforced with SEM, EDX, AFM and XPS methods.
SEM studies coupled with EDX revealed that significant improve- ment in the surface morphology of the protected species was observed and this improvement was consistent with the order of inhibition effec- tiveness. AFM analyses showed that surface of the uninhibited metallic specimens were highly rough with the average roughness of 405.1 nm and 367.7 nm in 0.5 M H₂SO₄ and 0.5 M HCl solutions, respectively. However, in the presence of (EMIM)+(SCN)−, average roughness im- proved to 56.9 nm and 43.2 nm in 0.5 M H₂SO₄ and 0.5 M HCl solutions, respectively. The improvement in the surface morphology is attributed due to the adsorption of ionic liquid at the interface of the metal and electrolytes (0.5 M H₂SO₄ and 0.5 M HCl). Inhibited and uninhibited AFM images of metal surface corroded in 0.5 M H₂SO₄ and 0.5 M HCl are presented in Fig. 7. Adsorption of the (EMIM)+(SCN)− on metal surface was also supported by XPS analyses. Interactions between (EMIM)+(SCN)− and metal surface was studied using MD simulations and it was observed that tested ionic liquid spontaneously adsorb on the metallic surface using flat or nearly flat orientation. For MD simulations, Fe (110) and Fe₂O₃ (110) surface was chosen and it was observed that on both surface adsorption energy of (EMIM)+(SCN)− in H₂SO₄ are relatively higher than that of in HCl medium. Negative values of adsorp- tion energies in all circumstances indicated that interaction and adsorp- tion of (EMIM)+(SCN)− on metal surface is a spontaneous process. Based on the experimental and MD simulations studies, authors pro- posed a mechanism for the adsorption of (EMIM)+(SCN)− on metal surface. Lowest energy configurations of (EMIM)+(SCN)− on Fe
(110) and Fe₂O₃ (110) surfaces are given in Fig. 8.
Arellanes-Lozada et al. [115] while studying the inhibition effect of two imidazolium ionic liquids observed that increase in alkyl chain length from propyl to butyl causes significant improved in their inhibi- tion property for API X52 iron alloy in 1 M H₂SO₄. Polarization study suggested that studied iodide based ionic liquids acted as good corro- sion inhibitors and their presence affect the nature of anodic and ca- thodic Tafel polarization curves. This observed revealed that [DPIM+]I

• 
  and [DBIM+]I− affect both anodic oxidation and cathodic reduction
  

reactions. Decrease in the values of current densities in the presence of ILs indicated that they block the active sites present over the metal sur- face. Adsorption of [DPIM+]I− and [DBIM+]I− was also supported by EIS studies. Diameter of the Nyquist curves increase in the presence of [DPIM+]I− and [DBIM+]I− and this increase in the diameter of Nyquist curves is consistent with the concentrations of [DPIM+]I−

Fig. 6. Frontier (optimized, HOMO and LUMO) molecular orbital pictures of [(CH₂)₃COOHMIm][HSO₄], [(CH₂)₂COOHMIm][HSO₄] and [(CH₂)₃COOHMIm][HSO₄] derived through Gaussian software package [74].

Table 1
Chemical structures, abbreviations, nature of metals and electrolytes of some major works reported on anticorrosive effect of ionic liquids for iron alloys in HCl electrolytes.

Chemical str. of ILs	System	Ref.	Chemical str. of ILs	System	Ref.
	Fe/1 M HCl	[71]		Fe/1 M HCl	[72]

Fe/1 M HCl [73] Fe/0.5 M HCl [74]

Fe/1 M HCl [75] Fe/1 M HCl [76]

(CPEPB)



(DBImL), (DBImA)
Fe/1 M HCl & H₂SO₄
[77]

(G2IL): n = 2; (G3IL): n = 3; (G6IL): n = 6

(CTAB), (SDS)
Fe/2 M HCl [78]

Fe/1 M HCl [79] Fe/1 M HCl [80]

([C4C1im][FeCl4])

Fe/1 M HCl [22] Fe/1 M HCl [81,82]

[EMIM]+[EtSO4]−, [EMIM]+[Ac]−, [BMIM]+[SCN]−, [BMIM]

+[Ac]−, [BMIM]+[DCA]−

Fe/1 M HCl [83]

[EMIM]+[BF4]−, [BDMIM]+[BF4]−, [C10MIM]
+[BF4]−

Fe/2 M HCl [20]

Fe/1 M HCl [84]
(EMIm Cl), (BMIm Cl), (BMIm PF6), (BMIm BF4), (BMIm Br), (HMIm Cl)
Fe/1 M HCl [85]

		[HMIM][TfO] [HMIM][BF4], [HMIM][PF6] & [HMIM][I]
Fe/1 M HCl	[86]		Fe/1 M HCl	[87]
Fe/1 M HCl	[88]		Fe/1 M HCl & H2SO4	[89]
Fe/1 M HCl	[90]		Fe/1 M HCl	[91]
Fe/1 M HCl	[69]		Fe/1 M HCl	[92]
Fe/0.5 M HCl	[93]		X70 steel/0.5 M HCl	[94]
Fe/1 M HCl	[95]		Fe/1 N HCl	[96]
Fe/1 M HCl	[97]		Q235 steel/1 M HCl	[98]

Table 1 (continued)
Chemical str. of ILs	System	Ref.	Chemical str. of ILs	System	Ref.
	Fe/1 M HCl	[99]		Fe/1 M HCl	[100]
	API-X52	[101]		Fe, Ni, and 304	[102]
	Fe/1 M HCl			SS/water
	Fe/0.5 M HCl	[103]		Fe/2 N HCl	[104]
	Fe/1 M HCl	[92]		Fe/0.5 M HCl	[106]
	Fe/1 M HCl	[107]		Fe/1 M HCl	[108]
	316L SS	[109]		Fe/1 M HCl	[69]
	Fe/1 M HCl
	Fe/0.5 M HCl	[110]		Fe/1 M HCl	[111]
	Fe/H2S and HCl	[112]		Fe/1 N HCl	[113]

and [DBIM+]I−. This finding suggested that both [DPIM+]I− and [DBIM+]I− adsorb at the interface of metal and 1 M H₂SO₄ solution and form inhibitive barrier. Fig. 9 represent the Nyquist curves for API X52 iron alloy corrosion in 1 M H₂SO₄ in the absence and presence of [DPIM+]I− and [DBIM+]I−.
There is numerous other ionic liquids are evaluated as corrosion in- hibitors for iron alloys in sulphuric acid media. Summary of chemical structure, abbreviation, nature of metal and electrolyte are presented in Table 2. Extensively survey shows that most of the reported ionic liq- uids behave as mixed type inhibitors and their presence adversely affect the nature of both anodic and cathodic Tafel curves. Their presence de- creases the values of corrosion current densities. Their adsorption on the metallic surface obeyed mostly Langmuir adsorption isotherm model. Obviously, higher the hydrophobicity higher is the corrosion in- hibition efficiency. EIS studies showed that most of the ILs tested as cor- rosion inhibitors behave as interface-type inhibitors and their presence increases the values of charge transfer resistance. Outcomes of the AFM, EDX, SEM and SPX studies suggested that ILs become effective by adsorbing on the metallic surface. Computational studies showed that ionic liquids interact with the metal surface using donor-acceptor mechanism and they gain flat or nearly flat orientations on metal surface.

3.