impact of some pyrrolidinium ionic liquids on copper
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Impact of some
aDepartment of Chemistry, Faculty of
El-Kharja-72511, Egypt. E-mail: emad_99
au.edu.eg; Fax: +20927925393; Tel: +20102bQuality Control Laboratory, Operations De
Cite this: RSC Adv., 2019, 9, 20760
Received 14th May 2019Accepted 27th June 2019
DOI: 10.1039/c9ra03603b
rsc.li/rsc-advances
20760 | RSC Adv., 2019, 9, 20760–207
pyrrolidinium ionic liquids oncopper dissolution behavior in acidic environment:experimental, morphological and theoreticalinsights
Emad E. El-Katori *a and Ashraf S. Abousalem b
The inhibitive and adsorption activity of some pyrrolidinium ionic liquids (ILs) for the dissolution of copper in
1 M HNO3 solution was tested using chemical methods such as weight loss and electrochemical
techniques; potentiodynamic polarization, electrochemical impedance spectroscopy (EIS) and electrical
frequency modulation (EFM) techniques. The results indicated that the ionic liquids under investigation
exhibited promising corrosion inhibitory performance in 1 M HNO3 and their efficiencies reached up to
85% at 0.05 mM. Also, an enhancement in the inhibition efficiency (% IE) accompanied the increase in
inhibitor concentration. The polarization measurements suggested that these ionic liquid inhibitors act as
mixed-type inhibitors. The adsorption of the ionic liquid inhibitors on the copper surface obey the
Langmuir adsorption isotherm. Thermodynamic parameters were calculated and discussed. The surface
morphology of the copper surface was examined using different techniques. Correlation between the
calculated % IE from experiments and some quantum chemical parameters was established.
1. Introduction
Copper is widely used in various industrial applications owingto its higher electrical and thermal conductivity, mechanicalworkability and excellent corrosion resistance. Copper alloys aremain constituents in technological device applications, such asin electronics as a conductor, and in electrical power conduits,transportation pipelines used for local and industrial waterservices such as sea water, heat exchangers, heat conductors,etc.1–4 As a result, the dissolution of copper and its inhibition invarious environments have received increasing interest inacademic research and industry.5–8 Previous reports in theliterature have been devoted to conquering the challenges ofcopper corrosion that is common in aqueous solution envi-ronments.9,10 One effective method to mitigate the degradationof copper in acid solution is the use of corrosion inhibitors,such as organic, inorganic, organic anionic and organic cationicinhibitors, which simply protect the metal surface from attackby corrosive species in the media. The safe handling and costeffectiveness, which are the important points that deserve fullconsideration when testing new corrosion inhibitors.11,12
Amongst organic heterocyclic compounds, benzotriazole (BTA)is recognized as an active inhibitor of copper dissolution in
Science, The New Valley University,
[email protected]; emad.elkatori@scinv.
3318210
partment, Jotun-11835, Egypt
77
neutral/basic media.13 It is usually approved that its inhibitionmechanism in neutral and basic media is the adsorption ofa single BTA molecule on the copper surface and the creation ofa polymeric lm of a (Cu+BTA) complex.14 But, in acidic media,BTA occurs largely as a protonated species, BTAH+. Theseprotonated species are strongly chemisorbed on the coppersurface, as the metal is positively charged in acidic media.15
This is in turn drives to a reduction in the inhibition efficiencyof BTA in acidic media. Over the last decade, extensive ongoingresearch have been devoted to the use of ILs as corrosioninhibitors because they fulll the criteria in many practicalindustrial applications.16–18 In general, ILs are organic salts inits liquid state with low melting point below 100 �C, consistingof organic cations as ammonium, imidazolium, pyridinium,piperidinium, sulfonium, phosphonium or pyrrolidinium andinorganic anions such as uoride, chloride, bromide orcyanide.19 Their intrinsic physicochemical features, like goodsolvent transport, excellent electrical conductivity, and electro-chemical activity make them potential candidates in phys-isorption and chemisorption studies.20 Some ILs have surfaceactivity, like surfactants and able to form micelles and reducingwetting interfacial tension of aggressive solutions that supportsurface wetting and adsorption.21 Similar to the action ofsurfactant, the maximum efficiency is attained near the criticalmicelle concentration (CMC) wherever the colloidal aggregatesare created. The micelle composition is varied in the existenceof ILs mainly in pyrrolidinium-based ILs. These properties havemotivated researchers to evaluate new synthesized ILs in a wide
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variety of applications, particularly such as corrosion inhibitorsin aqueous environments.
Zhou et al., evaluated the inhibition activity of 1-butyl-3-methylimidazolium tetrauoroborate ([BMIM][BF4]) ionicliquid for carbon steel dissolution in basic chloride solution viaelectrochemical measurements, the results showed that thecompound effectively inhibited cathodic and anodic processestogether and played as a mixed type inhibitor.22 Zheng et al.,represented the inhibition achievement of 1-octyl-3-methylimidazolium bromide and 1-ally-3-octylimidazoliumbromide on mild steel surface inundated in environmentinvolving sulfate ions and found that the inhibitors conrmedgood inhibitive capability (above 85%) but proceeded ascathodic inhibitors slightly.23 Also, in HCl solution, ILs areshown to be effective. For example, Zhang et al., are shown thatat 303 K, alkyl imidazolium ionic liquids: 1 butyl-3-methylimidazolium chlorides, 1-hexyl-3-methylimidazoliumchlorides and 1-octyl-3-methylimidazolium chlorides pro-hibited aluminum dissolution in a hydrochloric acid environ-ment by 82.4, 90.4, and 94.0%, respectively.24 This activity maybe attributed to the adsorption ability of the used ILs, whichmainly due to the adsorption of ILs species on the metalsurface, the compounds with large molecular size and accord-ingly, the molecular mass. So far, ILs are used as corrosioninhibitors, however, there is scarce of researches have beencarried out on the inhibition of copper dissolution in acidsolutions using ionic liquid systems.
Thus, the overall objective of the present work is to estimatethe corrosion inhibition activity of 1-butyl-1-methylpyrrolidinium tetrauoroborate and 1-ethyl-1-methylpyrrolidinium bis(triuoromethylsulfonyl)imide ILs forcopper dissolution in 1 M HNO3 solution via chemical, elec-trochemical experiments for corrosion monitoring. The surfacemorphology of copper specimens was analyzed by scanningelectron microscopy (SEM), energy dispersive X-rays (EDX), andatomic force microscopy (AFM). We have demonstrated thevalidity of our ndings by corroborating the experimentalresults with computed quantum chemical parameters andadsorption energies from molecular dynamics simulations.Furthermore, this research is aimed to elucidate the adsorptionof ionic liquid inhibitors on the copper surface and under-standing the role of ionic liquid constituents in the corrosioninhibition process.
2. Experimental methods2.1 Materials, inhibitors and solutions
The elemental chemical composition of Cu specimens used inour investigation was as follows in weight %: 0.0297 Fe, 0.0103Ni, 0.0053 Si, 0.023 Pb and rest are Cu (Merck Chemicals Co.).The investigated ILs were purchased from Sigma Aldrich Co.,and used without further purication, they are labeled as IL1and IL2. The molecular structure, molecular weight and phys-ical properties are given in Table 1. The corrosive solution usedwas prepared by dilution of analytical grade, 70% HNO3 (El-Gomhouria Chemicals Co.) by bi-distilled water, and the acidconcentration was standardized using NaOH (El-Gomhouria
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Chemicals Co.) before carrying out the experiments. Thetested inhibition dose of ILs ranged from 0.01 to 0.05 mM in1 M HNO3.
2.2 Weight loss method (WL)
The weight loss method (WL) was applied to the copper speci-mens with geometrical size (2 � 2 � 0.1 cm3). The coppercoupons were rstly prepared by scratching against emerypapers of different grit size from coarser to nest, then thesamples were degreased and rinsed with distilled water anddesiccated before measuring their initial weight (Wi), aer thatthe copper samples were immersed in 100 ml of corrosivesolution at controlled temperature for 3 hours immersionperiod, where the coupons were taken out from solution every30 minute and washed out with distilled water, dried and theweight aer immersion was measured (Wf). The corrosion rate(CR), the surface coverage degree (q) and the (% IE) of theinvestigated ILs were calculated from the following equations:
CR ¼ Wi �Wf
At(1)
% IE ¼ q � 100 ¼ [(Wa � Wb)/Wa] � 100 (2)
where, Wi and Wf denote to the initial and nal weight of thecopper samples in the tested solution, respectively. A, is theentire surface area of copper specimens (cm2) and t, is theimmersion time in (min).Wa andWb are the WL per unit area ofcopper specimens in the absence and presence of inhibitors,correspondingly.
2.3 Electrochemical measurements
Electrochemical experiments used the copper specimen withthe same composition were mounted in glass rod with anexposed surface area of 1.0 cm2. An epoxy resin was used to llthe gap between a copper electrode and the opening glass rod.The electrochemical measurements [potentiodynamic polari-zation (PP), electrochemical impedance spectroscopy (EIS) andelectrochemical frequency modulation (EFM)] techniques weredone at 298 K in a classic three glass cell electrode systeminvolving the copper specimen as working electrode, platinumwire as counter electrode (HANNA Instruments Co.), anda saturated calomel electrode (SCE) as reference electrode(HANNA Instruments Co.). Prior to each experiment, the spec-imens were polished with a sequence of emery papers of varioussizes up to 1200, washed out a few times with distilled water anddried before use. The copper electrode was allowed to immersein an open circuit condition for about 15 min to reach thesteady state conditions before starting the measurements (theequipment of electrochemical measurements are calibratedusing a dummy cell and measurements are repeated 3 times forreproducibility).
Potentiodynamic polarization (PP) was obtained viasweeping the electrode potential automatically from (�0.900 to+0.900) mV. (SCE) vs. open circuit potential (OCP) at a scan rateof 1.0 mV s�1.25
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Table 1 Chemical structure of the investigated ionic liquids
Inhibitorscode Structure CAS number Empirical formula
Molecular weight(g mole�1) Purity Impurities
IL1 345984-11-4 C9H20BF4N 229.07 $97.0% (T)#0.2% halogens (as chloride)#0.5% water
IL2 223436-99-5 C9H16F6N2O4S2 394.35 $98.5% (T) #0.5% water
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Electrochemical impedance spectroscopy (EIS) was appliedin a frequency range from 100 kHz to 0.5 Hz with an amplitudeof 5.0 mV peak-to-peak via AC signals at particular corrosionpotential.26
Electrochemical frequency modulation (EFM) was appliedvia two frequencies 2 and 5 Hz. The base frequency was 1.0 Hzwith 32 cycles, so the waveform repeats aer 1 s. A disturbancesignal with amplitude of 10 mV was applied for togetherdisturbance frequencies of 2 and 5 Hz. The option for thefrequencies of 2 and 5 Hz was founded on three principles.27
Electrochemical measurements were achieved with a Gamryinstrument potentiostat/Galvanostat/ZRA, this incorporates
Fig. 1 Weight loss-time plots for copper dissolution in 1 M HNO3 in the
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a Gamry framework system based on the ESA 400, Gamryapplications that impeded DC105 for DC corrosion tests, EIS300for EIS measurements and EFM140 to compute the corrosioncurrent and Tafel constants for EFM measurements. EchemAnalyst soware was used for graphing, analyzing, and ttingdata. A computer was employed for gaining the expected data.
2.4 Surface analyses (SEM, EDX and AFM)
Scanning electron microscopy (SEM) provides an insightfuldepiction of the copper surface morphology and the inhibitiveprotective layer on the surface. The surface morphology of the
absence and presence of ILs at 298 K.
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Table 2 Corrosion rate (CR), q and % IE data obtained from WLmeasurements for copper dissolution in 1 M HNO3 in the absence andpresence of ILs at 298 K
InhibitorsConc., mM� 10�2
Weight loss,mg cm�1 CR, mmy�1 % IE q
1 M HNO3 Blank 8.333 4.6 — —IL1 1 4.195 2.3 49.7 0.497
2 3.571 2.0 57.2 0.5723 3.099 1.7 62.8 0.6284 2.169 1.2 74.0 0.7405 1.680 0.9 79.8 0.798
IL2 1 3.515 2.0 57.8 0.5782 2.922 1.6 64.9 0.6493 2.376 1.3 71.5 0.7154 1.550 0.9 81.4 0.8145 1.285 0.7 84.6 0.846
Fig. 2 Surface tension vs. ILs total concentration in 1 M HNO3 solutionat 298 K.
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copper specimens was analyzed via (JEOL JSM-5500, Japan)model. The uninhibited and inhibited copper specimens'compositional analysis was investigated via electron dispersiveX-ray spectroscopy (EDX). Also, the conditions of copper speci-mens in the acid environments without and with the optimumconcentration of ILs was applied via atomic force microscopy(AFM) using Pico SPM 2100 with the soware version of Picoscan version 5.4.
2.5 Ionic liquid solution test (tensiometry measurements)
Tensiometry measurements were applied via Kruss (ForceTensiometer – K100) under atmospheric pressure via the ringmethod. The platinum ring was fully washed and ame-driedprior to each test. This tensiometry measurement is repeated3 times for reproducibility and the average value was deter-mined as represented in our investigation.
2.6 Computational chemical approaches
2.6.1 Quantum chemical calculations. By using quantumchemical calculations, the correlation between the properties ofstudying ILs and their electronic properties was studied. Ingeneral, the electronic properties and the chemical reactivity ofcompounds can be anticipated from the molecular structure.Such theoretical calculation tools provide a depth
Table 3 Surface tension data for the investigated ionic liquids
Inhibitors Conc., mM � 10�2 Surface tension, mN
IL1 1 602 523 444 365 30
IL2 1 552 483 384 355 30
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understanding of the chemical reactivity compounds whichapproximated quantitatively from the interpretation of somequantum chemical descriptors for example, the energies ofFrontiers molecular orbital's, for short, FMOs (HOMO andLUMO). In our investigation, the rst step in the calculation isperformed geometrical optimization on the molecular structureof the tested ILs. These computations were made with the aid ofStandard Gaussian 09 soware package (revision A02) applyingthe density functional theory (DFT) method with hybrid func-tional which is Becke three parameter-hybrid-B3 with the L–Y–P(Lee–Yang–Parr) correction, functional (B3LYP) and 6-31G(d,p)basis set in absence of symmetry limitations.28 The reason toselect this computational set up in our investigations is that thismethod is established in literature to give good approximate forthe activity of corrosion inhibitors. Also, B3LYP is suitable andyield reliable intermolecular distances and binding energies. Inaddition, since the corrosion inhibition occurs in wet (aqueous)environments, the effect of solvent (water) representing thecorrosion inhibition environment was considered in oneperspective of the computational calculations by performingthe calculation with polarized continuum model (PCM). Noimaginary frequency appeared, indicating the proper
m�1 Standard deviation Standard uncertainties
0.2494 0.300.2450 0.300.4497 0.550.0816 0.100.2494 0.300.2494 0.300.3742 0.450.3559 0.400.3399 0.400.4109 0.50
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Fig. 3 PP curves for copper dissolution in 1 M HNO3 with and without various concentrations of ILs at 298 K.
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optimization of structures. Frontiers molecular orbital's EHOMO
and ELUMO represent the energy of the highest occupiedmolecular orbital and the energy of the lowest unoccupiedmolecular orbital, respectively from which the gap energy DE ¼ELUMO � EHOMO is calculated. Other quantum chemicalparameters were calculated such as soness (s), hardness (h),and electronegativity (X) which are global chemical descriptorsreecting the chemical reactivity and molecular stability.Higher reactivity of chemical compounds is associated withincreasing soness (s) values and decreasing the hardnessvalues (h). Moreover, the more electronegative (X) of thecompound, the better the interaction with other reactingspecies.
2.6.2 Monte Carlo simulations. The adsorption of testingILs on the copper surface was investigated by Monte Carlosearch code modulated in Materials Studio version 07. Thesimulations were carried out with periodic boundary conditionsto mimic part of the corrosion inhibitor system of ILs on Cusurface.29 The constructing simulation system involves in the
Table 4 The impact of the various ILs concentration on the free corrosiobc), inhibition efficiency (% IE) and surface coverage degree (q) of coppe
Inhibitors Conc., mM � 10�2 Ecorr, mV icorr, mA cm�2
1 M HNO3 Blank 6.2 198IL1 1 28.4 116
2 24.4 81.73 15.6 48.94 22.8 45.25 36.3 41.5
IL2 1 22.4 78.22 19.6 69.83 21.4 56.44 27.6 41.75 32.8 26.2
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rst place a preparation procedure to import pure Cu crystaland cleaved its surface along the plane (111), which is the lowestenergy plane, then building appropriate supercell of appro-priate size (10 � 10) to accommodate the inhibitors, and witha vacuum thickness 30 A. Their mesh dimensions were asfollows a ¼ b ¼ 24.5 A and c ¼ 42.7 A. Furthermore, the surfaceof the Cu (111) was rst relaxed to the lowest energy state beforeinitiating the simulations. Moreover, the optimized structuresof the ILs by force-eld calculation method were introduced inthe simulations.30 The molecular dynamic simulation was per-formed using Monte Carlo search code by applying universalforce eld with a time step of 1 fs and a simulation time of 500ps done at 298 K, set NVT.
3. Results and discussion3.1 Weight loss method
The weight loss of the copper specimens in 1 M HNO3, with andwithout various concentrations of the tested ILs, was calculated
n potential (Ecorr), corrosion current density (icorr), Tafel slopes (ba andr dissolution in 1 M HNO3 at 298 K
ba, mV dec�1 ßc, mV dec�1 CR, mpy % IE q
215 141 90.0 — —288 151 52.7 41.4 0.414248 158 37.1 58.7 0.587233 228 22.2 75.3 0.753208 171 20.5 77.2 0.772232 192 18.9 79.0 0.790322 133 35.5 60.5 0.605381 195 31.7 64.7 0.647289 121 25.6 71.5 0.715252 111 18.9 78.9 0.789155 98 11.9 86.8 0.868
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Fig. 4 (a) Nyquist and (b) Bode diagrams for copper dissolution in 1 M HNO3 containing different concentrations of ILs at 298 K.
Fig. 5 Parallel circuit diagram applied for modeling the metal/solution.
Table 5 EIS parameters for Cu dissolution in 1 M HNO3 with andwithout various concentrations of ionic liquid inhibitors at 298 K
InhibitorsConc., mM� 10�2 Rct, U cm2 Cdl, mF cm�2 % IE q
IL1 Blank 108 350 — —1 225 261 52.0 0.5202 271 228 60.1 0.6013 335 215 67.8 0.6784 387 208 72.1 0.7215 448 199 76.0 0.760
IL2 Blank 108 350 — —1 291 107 62.9 0.6292 341 199 68.3 0.6833 456 175 76.3 0.7634 578 138 81.3 0.8135 735 62 85.3 0.853
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aer 3 hours of immersion at 298 K, as represented in Fig. 1.The weight loss corrosion data, including % IE are listed inTable 2. The existence of the ILs lowering the corrosion rate ofcopper specimen in 1 M HNO3. The inhibition efficiency (% IE)was noticed to increase with increasing the dose of ILs in thetested solutions.31,32 The corrosion inhibition is due to theadsorption of the applied ILs at the copper/nitric acid solutionboundary.33 This adsorption is caused by the electrostaticinteraction between ILs & the copper specimen surface. The
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inhibition efficiency raised with rising the concentration up toachieve an optimal concentration (0.05 mM). Since, the ionicliquid concentrations were increased, the corrosion rateremained constant approximately owing to the saturation of the
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Fig. 6 EFM diagrams for copper dissolution in 1 M HNO3 with and without different concentrations of IL2 at 298 K.
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IL adsorbed layer and increasing the concentration mayparticipate only in the creation of hydrogen bonding of thepyrrolidinium ring with their counter ions as well as with watermolecules in the medium.34,35 Similar results were achieved inprevious works.36,37 Moreover, the % IE of these ILs within thesame concentration follows the order IL2 > IL1, this perfor-mance may be referred to the adsorption capability of the ILsstudied, which mainly attributing to the increasing in adsorp-tion density at the metal surface with rising of the molecularsize of ILs. Hence, the variation in the anionic parts of IL1 and
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IL2 is responsible for the different size changes and differentperformance of the ILs in the inhibition efficiency. The resultslisted in Table 3 agree with the critical concentration values forthese ILs gained via the tensiometery method as represented inFig. 2.
3.2 Potentiodynamic polarization (PP)
The anodic and cathodic polarization curves of copper in 1 MHNO3 with and without various concentrations of the appliedcompounds (IL1 & IL2) are illustrated together in Fig. 3. It's
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Table 6 EFM parameters for copper dissolution in 1 M HNO3 with and without different concentrations of ILs at 298 K
InhibitorsConc., mM� 10�2, ppm icorr, mA cm�2 ba, mV dec�1 bc, mV dec�1 CF-2 CF-3 CR, mpy % IE q
IL1 Blank 255.0 85 281 1.88 3.32 115.9 — —1 113.7 146 203 1.92 2.88 51.7 55.4 0.5542 85.2 151 212 1.85 2.85 38.7 66.6 0.6663 78.6 149 200 1.91 2.95 35.7 69.2 0.6924 66.9 150 190 1.88 2.98 30.4 73.8 0.7385 55.5 152 191 1.95 2.82 25.2 78.2 0.782
IL2 1 81.4 98 55 2.15 3.11 37.0 68.1 0.6812 72.7 122 51 1.92 3.01 33.1 71.5 0.7153 61.2 111 49 1.98 2.81 27.8 76.0 0.7604 49.5 88 45 2.11 2.97 22.5 80.6 0.8065 36.3 94 39 1.88 2.89 16.5 85.8 0.858
Table 7 Langmuir adsorption parameters for the copper dissolution in1 M HNO3 involving different concentrations of ILs at 298 K
InhibitorsExperimentalmethods DG
�ads (kJ mol�1) K (M�1) Slope R2
IL1 WL �9.68 0.909 1.07 0.9439PP �9.09 0.914 0.96 0.9863EIS �9.47 0.983 1.09 0.9965EFM �10.79 1.42 1.16 0.9968
IL2 WL �10.18 1.11 1.02 0.9813PP �10.47 1.25 1.05 0.9689EIS �9.92 1.0 0.97 0.9900EFM �11.19 1.67 1.07 0.9932
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known that the polarization behavior of copper in acidic envi-ronments follows approximately identical pattern. A variety ofcorrosion kinetic parameters, for example, corrosion potential(Ecorr), corrosion current (icorr), Tafel slopes (ba and bc), surfacecoverage degree (q) and inhibition efficiency (% IE) obtainedfrom the polarization plots are given in Table 4. From the PPresults, we can say that these (ILs) function as mixed-typeinhibitors, due to the potential change in the cathodic path isequivalent to that in the anodic path. The reduction in (icorr)with rising the dose of ILs used to illustrate the efficiency of theexamined compounds as corrosion inhibitors. The free corro-sion potential in 1 M HNO3 solution is 6.2 mV vs. SCE infavorable agreement with published data.38,39 The (Ecorr) data areindicating that the inhibition takes place by blocking thesurface of the copper electrode.
Table 4 represents the change in the inhibition efficiency (%IE) of ILs at different concentrations calculated from the
Fig. 7 Langmuir adsorption isotherm for copper dissolution in 1 M HNO
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polarization plots at 298 K. The order of the inhibition efficiencyof these compounds is: IL2 > IL1 and is in excellent agreementwith that gained from weight loss method.
3 involving different concentrations of ILs at 298 K.
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Fig. 8 SEMmicrographs for (A) copper free in polished state (B) exposed to 1 M HNO3 solution, (C) exposed to 1 M HNO3 solution with 0.05 mMIL1 and (D) exposed to 1 M HNO3 solution with 0.05 mM IL2 at 298 K.
Table 8 Surface composition (mass%) of copper after 12 h of inun-dation in 1 M HNO3 + 0.05 mM ILs (IL1 & IL2)
Mass% C O Cu
Pure — — 100Blank 13.1 4.5 82.4IL1 21.5 6.7 71.8IL2 32.9 4.3 62.8
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3.3 Electrochemical impedance spectroscopy (EIS)
Fig. 4 illustrates Nyquist & Bode diagrams for the Cu electrode atcompetent corrosion potentials aer 30 min inundation in 1 MHNO3 with and without various concentrations of the appli-cable inhibitors at 298 K. As the ILs concentrations increase, thesemi-circle width increases. Generally, the variation from idealsemicircle was referred to the frequency spreading as well as thesurface heterogeneity.9,40,41 EIS test of the investigated ILs was
20768 | RSC Adv., 2019, 9, 20760–20777
evaluated via the equivalent circuit, Fig. 5, which shows a singlecharge transfer reaction and approximated well with ourexperimental data. The constant phase element, CPE, is inser-ted in the circuit instead of a pure double layer capacitor toprovide a more perfect t. The double layer capacitance, Cdl, fora circuit involving a CPE parameter (Y0 and n) are computedfrom eqn (3):
Cdl ¼ Y0un�1/sin[n(p/2)] (3)
where, Y0, is the extent of the CPE, u ¼ 2pfmax, fmax is thefrequency at which the imaginary impedance module ismaximal and the factor, n, is a exible parameter that alwayslies between 0.5 and 1.0. The semi-circle width characterizes thecharge transfer resistance, Rct, corresponding to the polariza-tion resistance, Rp, and inversely proportional to the icorr value.The rising in Rct reects the increase in the double layerthickness that adsorbed via ILs.42,43 As well, the data of the
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Fig. 9 EDX micrographs for (A) copper free in polished state (B) exposed to 1 M HNO3 solution, (C) exposed to 1 M HNO3 solution with 0.05 mMIL1 and (D) exposed to 1 M HNO3 solution with 0.05 mM IL2 at 298 K.
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double-layer capacitance (Cdl) reduce by inserting ionic liquidinhibitors into the applicable corrosive medium. Furthermore,double layer capacitance is computed from the following eqn(4):
Cdl ¼ 33o(A/d) (4)
where, 3, is the double-layer dielectric constant, 3o the vacuumelectrical permittivity, d the double-layer thickness, and A is thesurface area. Generally, the reduction in Cdl data is assigned tothe substitution of the adsorbed water molecules at the coppersurface by the ILs inhibitor molecules which having lowerdielectric constant.44 Moreover, the reduction in the surfacearea which proceeds as sites for charging may be believed asa different reason for the decrease in Cdl. These points recom-mend that the role of inhibitory molecules is advanced by itsadsorption at the metal–solution interface. The impedancecorrosion parameters are reported in Table 5. The % IE wascomputed from the charge transfer resistance data from the eqn(5).
%IE ¼ q� 100 ¼ �1� �
R�ct
�Rct
��� 100 (5)
where, Rct and R�ct are the charge-transfer resistance values with
and without inhibitors, respectively.
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The obtained results show that the value of Rct raises with therising of the inhibitor concentration, which leads to an improvein % IE and double layer capacitance (Cdl) data reduce in theexistence of the inhibitors. The lower Cdl value results froma decrease in local dielectric constant and/or an increase in thedouble layer thickness. Thus, it is reasonable to presume thatinhibitor compound function by adsorption at the metal–acidinterface. It is clear that there is a perfect agreement betweenthe two dissimilar electrochemical techniques owing to thesame trend of inhibition of ILs inhibitors.
3.4 Electrochemical frequency modulation (EFM)
Recently, Bosch et al. suggested electrochemical frequencymodulation (EFM) as a novel electrochemical method forcorrosion monitoring.27 EFM is a non-destructive and a quickcorrosion rate measurement method that directly provide thecorrosion current data without previous knowledge of Tafelconstants.
It is known that the corrosion process is non-linear innature, a potential distortion by one or more sine waves willproduce responses at higher frequencies than the inputfrequencies of the applied signal. Practically, no notice isprovided to the inter-modulation or electrochemical frequency
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Fig. 10 3D AFM images for (A) copper free in polished state (B) exposed to 1 M HNO3 solution, (C) exposed to 1 M HNO3 solution containing0.05 mM IL1 and (D) exposed to 1 M HNO3 solution containing 0.05 mM IL2 after 12 h at 298 K.
Table 9 The surface roughness average (Sa) of copper after 12 h ofimmersion in 1 M HNO3 containing 0.05 mM ILs at 298 K
Status Sa (nm)
Pure metal 76.5Pure metal + 1.0 M HNO3 180.2Pure metal + 1.0 M HNO3 + IL1 (0.05 mM) 118.4Pure metal + 1.0 M HNO3 + IL2 (0.05 mM) 82.8
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modulation. However, EFM illustrated that this non-linearresponse includes enough information about the corrodingsystem so that the corrosion current can be calculated directly.The great strength of the EFM is the causality factors which act
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as an internal test of the EFM measurement validity, with thecausality factors the experimental EFM results can beconrmed.45,46
Fig. 6 shows the current response involving not only theinput frequencies, but also the other frequencies which are thesum, difference, and multiples of the two input frequencies.Inter-modulation spectra driven from EFM measurements ofcopper with and without various concentrations of the testedILs in 1 M HNO3. The EFM inter-modulation spectra wereconstructed of copper in nitric acid medium with and withoutvarious concentrations of the investigated inhibitors at 298 Kare also studied.
The higher peaks are applied to calculate the corrosioncurrent density (icorr), the Tafel slopes (ba and bc) and the
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Table 10 Different quantum chemistry descriptors of the investigated ILs; global hardness (h), global softness s (S), electronegativity (X), dipolemoment (m)
Quantum parameters Total energy (Ha) EHOMO, eV ELUMO, eV DE, eV h s (S) X m, Debye
IL1/water �834.245796 �8.762 �1.342 7.420 3.710 0.270 5.052 18.15IL2/water �2158.29015 �7.659 0.832 8.491 4.245 0.236 3.414 24.59
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causality factors (CF-2 and CF-3). These electrochemical corro-sion parameters at various concentrations of the investigatedinhibitors for copper dissolution in 1 M HNO3 at 298 K aredetermined at once and reported in Table 6.
IEEFM the inhibition efficiency (%), calculated from the eqn(6), increases by the abundance of the investigated inhibitors intest solution.
IEEFMð%Þ ¼ 1� i�corr � icorr
i�corr
� 100 (6)
where, icorr and i�corr are corrosion current densities with and
without inhibitors, correspondingly. The causality factors CF-2and CF-3 summarized in Table 6 are very close to their theo-retical values of 2.0 and 3.0, respectively. This suggests that thecalculated EFM parameters are valid and of excellent quality.Hence, the results from (EFM) method demonstrated that allthe investigated inhibitors have good inhibiting power forcopper corrosion in nitric acid. Derived from these data, theelectrochemical frequency modulation test appears able toexamine the corrosion inhibition of copper in nitric acid byvarious concentrations of ILs compounds.
3.5 Adsorption isotherm
It is generally accepted that the inhibitors mitigate the metalcorrosion by adsorbing on the copper surface and this process
Fig. 11 Optimized molecular structures with electronic densities of HO
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involves replacing the water molecules on the metal surface byionic liquid inhibitor molecules.20 Therefore, the positiveinformation on the copper corrosion inhibition via ILs can beobtained by applying different adsorption isotherms. Finally,experimental data fromWL, PP, EIS and EFMmeasurements forthe corrosion inhibition of copper in 1 M HNO3 medium by ILsare tted to different adsorption isotherms such as Langmuir,Temkin, Frumkin, Freundlich and El-Awady thermodynamicadsorption isotherms. It was found that the Langmuir adsorp-tion isotherm gives the best t obtained. Langmuir adsorptionisotherm is identied by the subsequent equation:47
C
q¼ 1
Kads
þ C (7)
where, q is the surface coverage degree, C is the concentration,Kads is the equilibrium constant of adsorption–desorptionprocess. The plot of C/q as a function of C for the inhibition ofcopper dissolution in 1 M HNO3 is illustrated in Fig. 7 and theadsorption parameters produced from the diagrams are re-ported in Table 7. The strong correlations (0.9968) and thelinearity of the diagrams are veried the power of this approach.In Table 5, Kads values are indicated that the adsorptionstrength or desorption between adsorbate and adsorbent. It isalso detected from Table 7 that the slopes deviated from theunity value as predicted from an ideal Langmuir adsorption
MO and LUMO of ILs study.
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Fig. 12 Mulliken population analysis with atomic charge ranges from�ve charged with red color and +ve charged with green color in the testedILs.
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equation. This variable may be assigned to interaction betweenadsorbed ILs species on the copper surface. It has beensupposed from the indication of the Langmuir isotherm equa-tion that adsorbed species don't cooperate with one another;this hypothesis isn't true for organic molecules at low concen-tration which can be absorbed at the anodic and cathodic sitestogether on a metal surface. From PP plots shown in Fig. 3, itcan be seen that ILs suppress largely the anodic processes dueto the adsorption of ILs molecules on the cathodic site which isthe reason for the observed variation from unit gradient.
The values of the free energy of adsorption also reported inTable 7 which are computed via the subsequent equation:48
DG�ads ¼ �RT lnð55:5 KÞ (8)
where, R is the molar gas constant, T is the absolute tempera-ture, and the constant value of 55.55 is the water molarconcentration. As reported in Table 7, the DG
�ads values are
negative, and this indicates the spontaneity of the adsorptionprocess and stability of the adsorbed ILs lms on the coppersurface.
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3.6 Surface analysis
3.6.1 SEM and EDX. SEM and EDX tests are used to conrmthe presence of the ILs on the copper surface or just peeled offthe copper surface. SEM micrographs are signal of the changesthat join with the corrosion and the protection of the coppersurface together as shown in (Fig. 8A–D). Fig. 8A illustrates thefree metal. Fig. 8B illustrates the loss caused to the surface bynitric acid. Fig. 8C illustrates SEM micrographs of the coppersurface aer treating with 1 M HNO3 involving 0.05 mM of ionicliquids. From these micrographs, it's clear that the coppersurface appears to be unaffected by corrosion nearly. This is dueto adsorption of ILs creating a thin protecting lm of the ILs onthe copper surface. This lm is greatly responsible for theeffective inhibition by these inhibitors. The matching EDXanalyses are reported in Table 8 and Fig. 9A–D for ILs with andwithout 0.05 mM ionic liquids. In addition, it's essential todeduce the presence of C, O and Cu peaks in the EDX graphs ofthe copper surface matching to the sample inundated for 12 h.In solutions involving the favorable concentration of the ionic
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Fig. 13 Side and top side snapshots of the most stable orientation of ILs simulated as whole part in vacuum condition.
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liquids, the creation of a thin protective lm of ILs is matchingwith the SEM micrographs.49
3.6.2 Atomic force microscopy (AFM). Further morpholog-ical evaluation of the copper surface was applied via AFM in thepresence and absence of the ILs with concentrations (0.05 mM)aer inundation 12 h. The AFM micrographs were shown inFig. 10A–D. The average roughness parameter is listed in Table9. It is evidently observed from the micrographs that the coppersample show a coarse surface caused by acid corrosion. But, thecopper specimens get smoothened in the presence of 0.05 mM
Fig. 14 Side and top view snapshots of the most stable orientation of IL
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ILs (IL1 & IL2) solutions in 1 M HNO3 which inhibited corrosionon the surface, as shown in Fig. 10C–D. The possibly the crea-tion of adsorbed thin protective lm of inhibitors on the coppersurface was the reason behind the reduction in the copperspecimens' roughness as reported in Table 9.
3.7 Quantum chemical calculations
3.7.1 DFT computations. The active sites in the tested ILsare the key enabling regions on the compounds that adsorb on
s simulated as whole part in water condition.
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Table 11 Adsorption energies of the tested ILs on Cu (111), simulatedas whole and separated constituents in vacuum and water
System Adsorption energy, kcal mol�1
IL1/vacuum �2.742IL1
+/vacuum �16.652IL1
�/vacuum �0.722IL2/vacuum �3.245IL2
+/vacuum �16.00IL2
�/vacuum �1.811IL1/aqueous �17.526IL1
+/aqueous �29.832IL1
�/aqueous �5.911IL2/aqueous �21.622IL2
+/aqueous �26.613IL2
�/aqueous �11.655
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metal substrate. This type of chemical reaction can be identiedby interpretation of the parameters from quantum chemicalcalculations. Such method provides a plethora of informationon the reactivity of chemical species and an atomic-level insighton the effect of molecular structure. In our investigation, wecarried out the calculations at the (DFT) level to obtain therelevant quantum chemical parameters.50 Some quantumchemical data from DFT calculations is given in Table 10. Theoptimized geometric structures and density distribution ofelectrons of EHOMO and ELUMO are presented in Fig. 11.
For IL1, the electron density of (HOMO) is localized mainlyover the anion part with a partial spread on the neighboringcarbon atoms of the cationic part. On the other side, the LUMOcan be seen spread over the cationic part of IL1 with greatestlocalization on the aromatic rings, especially N atoms. For IL2, itis obvious that the electron densities of HOMO and LUMOoccur in the anionic part of the compound.51–54 This indicatesthe big role that the anionic part contributes in the corrosioninhibition process. Looking at Table 10, it can infer that thetrend in inhibition of compounds relates to the decreasing inthe ELUMO, also lower total energy of IL2 reects the higherstability of compounds on metal surface. This decreasing trendin ELUMO is accompanying with increasing the electron affinityof compounds, meaning that the capacity of compounds to
Fig. 15 Comparison of the adsorption energies in kcal mol�1 for the diff
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accept free electrons from corroding metal surface increases.Another observation is that IL2 possess larger magnitude of thedipole moment, this could correlate to higher dipole–dipoleinteraction of ILs ionic species which indicates higher adsorp-tion and strengthen the effect of blocking the active sites oncorroding metal surface. The gap energy DE is important toexplore the static molecular reactivity. It is noted that the activecenters of the ILs compounds are mainly distributed around thepolar atoms, meaning that those polar atoms donate electronsto Fe atoms with empty d-orbitals.
The ionization potential of the compound (�EHOMO) showshigher tendency of the studied ILs to donate electrons.55 It isgenerally established that the lower ionization potential [higherthe value of (EHOMO) values are indicative of higher capacity ofcompounds to donate electrons to an appropriate electronacceptor such as chemical species having unoccupied molec-ular orbitals at low energy level, which is in our case the Cusurface atoms]. It also can be noticed that the total energy ofcompound IL2 is lower than IL1, meaning that the IL2 is morestable than IL1. Likewise, the dipole moment of IL2 is greaterthan IL1, indicating the higher polarizability of IL2 compoundthat facilitate the adsorption of compound on metal surface.
The distribution of the Mulliken atomic charges on theatoms of the ILs is shown in Fig. 12. We have observed that theoxygen atoms with other carbon atoms is the structure of ILsstudied possesses negative charges, these atoms are the activesites and responsible for nucleophilic attack with the Cu surfacethrough electron/charge transfer from negatively charges sitesto the vacant-d orbital of Cu surface atoms. The region withpositive charges are mainly appearing on the anionic part of ILs,these regions are also active centers of electrophilic attack thatoccurs between the Cu surface and inhibitors through theformation of back-bond when the electrons from the Cu surfaceare freely transferred to the anti-bonding orbitals of thesepositively charged atoms.
3.7.2 Molecular dynamics simulations. The equilibriumcongurations of the ILs on the iron surface has been searchedusing a molecular dynamic simulation method which providesdeep understanding of the interactions of the inhibitors withCu surface. Molecular dynamics simulation can reasonably
erent ILs in vacuum and water simulations.
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predict the more favorable conguration of the metal inhibitorinterface.56,57 As a result, the side and top views of the adsorp-tion orientation of the two ILs inhibitors on Cu (111) surface invacuum and aqueous solution are displayed in Fig. 13 and 14.As obviously shown from the gures the two inhibitors havea planar orientation of Cu (111) surface, this position offersstrong adsorption, where the formation of coordination bond ismore likely through the sharing of p-electrons in the aromaticring with copper, and the transfer of electrons from d-orbital ofCu to anti-bonding orbital's in the tested ILs.
Overall, from Table 11 the adsorption energy is higher inaqueous solution than in a vacuum, indicating the facilitatingrole of solvent in the adsorption process of compounds on Cusurface. Furthermore, the high adsorption energy in aqueoussolutions (�21.622 kcal mol�1) was recorded for IL2 compound,indicating the spontaneous and strong adsorption of thiscompound on Cu surface. Also, the adsorption energy of IL2 ishigher than IL1 in all simulations, which means that IL2exhibited better corrosion inhibition performance compared toIL1 as shown in Fig. 15.
The molecular dynamic simulations were carried out on theoptimized ILs structure, and as separate ionic parts (cationicand anionic) species that comprise the studied ILs. All theadsorption energies give negative values, this means that theadsorption is a spontaneous and favorable process on Cusurface. Intriguingly, the cationic part has higher adsorptionenergy than its associated anionic part, this reects the keyobvious contribution of the cationic part in the inhibitionprocess. It can also be seen that the cationic part of the inves-tigated ILs moved closer to the Cu surface when water is presentin the simulation and the adsorption energy observed withhigher negative values. This led us to conclude that the cationicpart of the ionic liquid has a signicant effect on corrosioninhibition potential, meaning that the corrosion inhibition isgreatly affected by the cationic part. In conducting the simula-tion for the intimate ionic liquid where the cationic and anionicpart are not separated, it was found that IL2 gives higheradsorption energy, in either vacuum or water, compared to IL1,this is in good agreement with the results from experimentalstudies. While, this observation was not the case when per-forming the simulation of ILs as separated constituents, as wenoticed that the effect of cationic part outweighs the anionicpart. This nding is of interest as it corroborates the conclusionfrom quantum chemical studies and maximizes the effect ofanion species of ILs on the corrosion inhibition. This means thephysicochemical properties of ILs are due to the intrinsic elec-tronic properties of associated ionic species composing the ILswhere the effect of any part should not be overlooked. It isgenerally recognized that the adsorption of ILs molecules andits size & shape of the metal surface are controlled by thehydrophobic tail length and the charged head group interac-tions.38 As indicated in previous studies that the higher chainlength of ILs with the same head group results in higher % IE.The chain length compatibility changes lm properties such asaggregate lifetime, surface tension and etc.39,40 In our systems,we studied two ILs containing different anionic parts varying inhydrocarbon chain lengths, the impact of anionic part
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compatibility was found intriguing since the inhibition effi-ciency of ILs for copper is also estimated by the impact of theanionic parts along with the electrostatic interaction of the headgroups. The variation in the corrosion rate of copper in theexistence of IL1 and IL2 at the same inhibition dose is conrmedby the top explanation.
3.8 Mechanism of corrosion
In HNO3 solution, copper surface is positively charged and theanodic corrosion reactions in the absence of inhibitors followthe below equation steps:
Cu + NO3� / (CuNO3
�)ads (9)
(CuNO3�)ads / (CuNO3)ads + e� (10)
(CuNO3)ads / CuNO3+ + e� (11)
CuNO3+ / Cu2+ + NO3
� (12)
The adsorption of nitrate polyatomic ion (NO3�) as in eqn (9)
recharges themetal surface, making it negatively charged again.While, in acidic solutions, the ILs exists in their protonated([Inh]+) forms and this protonated species are easily adsorbedon corroding copper surface via coulombic attraction accordingto eqn (13):
(CuNO3�)ads + [Inh]+ / (CuNO3
�[Inh]+)ads (13)
Simultaneously, the cathodic corrosion reactions involve thefollowing steps:
Cu + H+ / (CuH+)ads (14)
(CuH+)ads + e� / (CuH)ads (15)
(CuH)ads + H+ + e� / Cu + H2 (16)
In our study, the corrosion inhibition action ofpyrrolidinium-based ILs on copper surface in HNO3 solutionsaffects the discharging of H+ through competitive adsorption onthe cathodic positions which results in H2 evolution, so the H2
evolution process occurs according to the subsequent steps ineqn (17) and (18):
Cu + [Inh]+ / Cu[Inh]+ads (17)
Cu[Inh]+ads + e� / Cu[Inh]ads (18)
In general, organic compounds adsorb on the copper metalsurface is associated with the substitution of water via theinhibitor molecules in the aqueous solution (Orgsol) replacingthe water molecules (H2Oads) at the copper/solution interface:9
Orgsol + xH2O / Orgads + H2Osol (19)
where, x is the number of H2Omolecules substituted. Thus, it isassumed that the metal surface charge is governed by thecorrosion potential of surface (Ecorr) described by the potential
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of zero charge which affect the adsorption potential ofcompounds on copper surface (PZC) Eq ¼ 0, hence affecting theoverall inhibition process.48 In general, when the differencebetween Ecorr and Eq ¼ 0 is negative, the metal surface acquiresa net negative charge which in turn supports the adsorption ofpositively charged species. On the other hand, the adsorption ofnegatively charged species is favorable when Ecorr � Eq ¼ 0 ispositive. The protonated [ILs]+ species are directly adsorbedonto the copper metal surface as a result will be unfavorablethat ILs molecules are adsorbed onto the copper surface in1.0 M HNO3 medium according to eqn (9). But, it appears thatthe adsorbed NO3
� ions didn't perfectly shielded the repulsiveimpact of the +ve charged copper surface on [ILs]+ as obviousfrom the inhibiting activity hypothesis of the ILs inhibitors.
4. Conclusions
The % IE depends on the dose of the ILs compounds studied.The PP measurement results conrmed that both ILs followingthe same postulated corrosion inhibition mechanism and actmainly as mixed type inhibitors. The adsorption of the ionicliquid inhibitors on the copper surface obey Langmuiradsorption isotherm. The adsorption of the investigated ionicliquids on the copper surface is spontaneous. The reduction ofCdl data when raising the dose of inhibitors is due to theadsorption of ILs molecules in the metal-corrosive solutionsurface. The % IE of the investigated ionic liquids correlate wellwith some quantum chemical descriptors. Monte Carlo simu-lations provide the most stable adsorption pattern for bothcompounds was on the copper surface resulting in increasingthe surface coverage and showed the effect of anionic constit-uent on the overall corrosion inhibition action of ILs.
Conflicts of interest
There are no conicts to declare.
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