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

Ionic liquids as corrosion inhibitors for different metals and alloys in various electrolytes

Along with HCl and H₂SO₄ based electrolytes, NaCl solutions are also widely used by academicians and industrialists as corrosive media. Sev- eral reports dealing with the anticorrosive effect of ionic liquids in NaCl electrolytes are reported. However, these reports the quite old therefore recent studies on the anticorrosive effect of ionic liquids in NaCl media are highly required. Ionic liquids are also tested as corrosion inhibitors for aluminum [135–141], magnesium [142,143], zinc [144], copper [145–148] and other metals however these reports are moderately el- derly. Table 3 represents the collection of some major in which ionic liq- uids are evaluated as corrosion inhibitors for different metals and alloys in the media other than HCl and H₂SO₄. It is important to mention that most of the studies in which ionic liquids are evaluated as corrosion in- hibitors are quite old. In view of this, such types of works in highly recommended.

3. Summary and outlook

Ionic liquids are established as one of the most effective, economic, highly soluble and environmental friendly class of corrosion inhibitors.

Fig. 7. AFM images of API X52 iron alloys corroded in (a) 0.5 M H₂SO₄, (b) 0.5 M HCl, (c) 0.5 M H₂SO₄ + 75 ppm (EMIM)+(SCN)− and (d) 0.5 M HCl + 100 ppm (EMIM)+(SCN)−. Average surface roughness decreased in the presence of (EMIM)+(SCN)− in both electrolytes [114].

Their environmental friendly nature is attributed due to their low toxic- ity, low vapor pressure, high thermal and chemical stabilities and high solubility in the polar electrolytes including water. Several classes espe- cially imidazolium based ionic liquids are widely used corrosion


inhibitors for different metals and alloys. Most of the reported ionic liq- uids behave as interface- and mixed-type inhibitors. Their presence de- creases the values of corrosion current density and increases the values of charge transfer resistance. SEM, EDX, XPS, AFM, FT-IR, UV–vis studies

Fig. 8. Lowest energy configurations of (EMIM)+(SCN)−on Fe(110) and Fe₂O₃ (110) surface in 0.5 M HCl and 0.5 M H₂SO₄ media [114].

Fig. 9. Nyquist curves for API X52 iron alloy corrosion in 1 M H₂SO₄ in the absence and presence of [DBIM+]I− and [DPIM+]I− at different concentrations. Diameter of the Nyquist curves increases with the concentrations of [DBIM+]I− and [DPIM+]I− [115].

Table 2
Chemical structures, abbreviations, nature of metals and electrolytes of some major works reported on anticorrosive effect of ionic liquids for iron alloys in H2SO4 electrolytes.

Chemical str. of ILs	System	Ref.	Chemical str. of ILs	System	Ref.
	API 5L X52 0.5 M/H2SO4 & HCl	[114]		API 5L X52/1 M H2SO4	[115]
	Fe/0.5 M H2SO4	[116]		Fe/0.5 M H2SO4	[117]
	API-X60 Fe/1 M H2SO4	[118]		304 SS Fe/0.5 M H2SO4	[119]
	Fe/0.5 M H2SO4	[120]		Fe/0.5 M H2SO4	[121]
	API X60 Fe/1 M H2SO4	[122]		Fe/1 M H2SO4	[123]
,	Fe/2 M H2SO4	[124]		Fe/1 M H2SO4	[125]

(MTABr), (TOMABr)
Fe/1 M H₂SO₄ [126]
R = IL1:C₄H₉; IL2:C₈H₁₇; IL3:C₁₂H₂₅; IL4:C₁₈H₃₇; IL5: ^C22^H45
Fe/1 M H₂SO₄ [127]

([BsMIM]-[HSO4]), ([BsMIM][BF4])

1 M H₂SO₄ [128]
(DDI), (TMA) (TML)

(EOPC)
Fe/0.5 M H₂SO₄ [129]

Fe/1 M H₂SO₄ [130] Fe/0.5 M H₂SO₄ [131]

Table 3
Chemical structures, abbreviations, nature of metals and electrolytes of some major works reported on anticorrosive effect of ionic liquids for other alloys in different electrolytes.

Chemical str. of ILs	System	Ref.	Chemical str. of ILs	System	Ref.
	Fe/3.5% NaCl	[149]		Fe/0.01 M NaCl	[126]
	Fe/NaCl (pH 3.8 & pH 6.8)	[150]		Fe/3.5% NaCl	[151]
(DMICL)
	Fe/3.5% NaCl	[152]	, ,	Fe/CO2	[153]
Coating

Fe/production water [154]
[EMIM][BF4], [BMIM][Otf], [EMIM][Otf]

Fe/open system [155]

(TSIL)


304 SS Fe/5% sulfamic acid [156] Fe/CO₂ capture system [157]

TESFI

SS Fe/36% HCl + 5% Sb₂O

+ 4% SnCl₂


[emim]-[Otf], [emim]-[DCA], [emim][acetate], [emim][tosylate]
[158] Copper/0.1 M Na₂SO₄ [159]

showed that ionic liquids inhibit corrosion by adsorbing thereby forming surface protective film on the metal surface. Inhibition effec- tiveness of the ionic liquids increases on increasing their concentration and decreases on rise increasing the temperature. Literature study showed that most of the investigated are imidazolium based therefore use of other classes of ionic liquids including phosphonium, tetra- ammonium and pyridinium should be further should be further en- hanced. It is important to mention that anticorrosive effect of ionic liq- uids is mainly reported for iron alloys (CS and MS) therefore their implementation for other metals and alloys especially for aluminum and magnesium should be further magnified. Utilization of ionic liquids as corrosion inhibitors in other electrolytes such as NaCl, HNO₃ and H3PO₄ should be explored. Most of the ionic liquids are used as corro- sion inhibitors in aqueous phase therefore consumption of the ionic liq- uids as anticorrosive coating materials is highly recommended.