Published: April 29, 2011 r2011 American Chemical Society 7129 dx.doi.org/10.1021/ie1024744 | Ind. Eng. Chem. Res. 2011, 50, 7129–7140 ARTICLE pubs.acs.org/IECR Synthesis of Selected Vinylimidazolium Ionic Liquids and Their Effectiveness as Corrosion Inhibitors for Carbon Steel in Aqueous Sulfuric Acid Diego Guzman-Lucero, † Octavio Olivares-Xometl,* ,‡ Rafael Martínez-Palou, † Natalya V. Likhanova, † Marco A. Domínguez-Aguilar, † and Vicente Garibay-Febles † † Programa de Investigacion y Posgrado, Instituto Mexicano del Petroleo, Eje Central Norte Lazaro Cardenas 152, Mexico 07730 D.F., Mexico ‡ Facultad de Ingeniería Química, Benemerita Universidad Autonoma de Puebla, Av. San Claudio Ciudad Universitaria, Col. San Manuel, Puebla 72570, Puebla, Mexico ABSTRACT: Five imidazolium-type ionic liquids, containing both N1 unsaturated and N3 long alkyl saturated chains as cations and bromide as anion (IL1IL5), were obtained by conventional and microwave synthesis. Compounds were tested in aqueous 1MH 2 SO 4 as corrosion inhibitors for carbon steel. Weight loss and polarization curves indicated that inhibition efficiency increased with concentration, which turns out to be dependent on alkyl chain size linked to N3 (IL4 > IL3 > IL1 > IL2 > IL5). The relatively high inhibitory properties (8895%) displayed by IL4 within 2540 °C were ascribed to a chemisorption process that involved the following: the adsorption of protonated imidazolium molecules on both the anodic and cathodic sites, the latter in competition with hydrogen ions to mitigate hydrogen evolution; and also the formation of π bond with iron by the CdN group from imidazolium ring (this way inhibitor produced more than one center of adsorption action). Surface analysis indicated a considerable reduction of corrosion products after the addition of IL4. 1. INTRODUCTION Acidic solutions are commonly used in picking operations as part of the steelmaking finishing process in which oxide and scale are removed from metallic surface. A solution of sulfuric acid is normally employed to treat carbon steel products, and inhibitor is provided to lessen acid attack. Diluted sulfuric acid can be applied to carbon steel up to 90 °C 1 in concentrations of 515 wt %. 2 Likewise, refinery operations involved the formation of sulfuric acid derived from the dissolution of sulfur compounds (e.g., hydrogen sulfide, thyosulfates, and mercaptans, etc.). It is therefore important to quantify the effect of acidic solution aggressiveness and develop corrosion inhibitors for carbon steel protection. The use of corrosion inhibitors (CIs) comprises one of the most economical ways to mitigate corrosion rate and protect metallic materials against corrosion to preserve industrial facilities, 3 especially in acidic media. 4 In this context, the treat- ment of mild steel corrosion in acidic environment through organic compounds has resulted in considerable savings for the oil industry. Several families of organic compounds, i.e., fatty amides, 5,6 pyridines, 79 imidazolines, 1012 and 1,3-azoles, 1315 have shown excellent performance as CIs. However, many of these compounds are toxic, and they do not completely fulfill the requirements imposed by the environmental protection stan- dards. This is the reason why in the past few years great efforts have been made by researchers in this area to develop new environmentally friendly CIs. 16 Ionic liquids (ILs) have attracted the attention of research- ers in the past decades due to their interesting physical and chemical properties. ILs are an excellent alternative to substitute volatile organic solvents because of their very low vapor pressures, thermal and chemical stability, no flam- mability, and their ability to act as catalyst. 17 Moreover, ILs present a wide electrochemical window so this property has been studied for electrochemical applications in batteries, 18 light emitting electrochemical cells, 19 and fuel cells. 20 There are few studies that involved ionic liquids as CIs for acid environments; these are two ILs containing 1-butyl-3-methy- limidazolium as cation, along with either chloride or hydrogen sulfate as anion, which have shown good properties as CIs for mild steel in aqueous 1.0 M HCl. 21 In this work, five imidazolium-type ionic liquids, containing both N1 unsaturated and N3 long alkyl saturated chains as cations together with bromide as anion (IL1IL5), were synthe- sized and evaluated as CIs for acid environment (Table 1). Weight loss tests and electrochemical polarization curves were applied to test the inhibitory properties of these compounds in AISI 1018 carbon steel immersed in 1.0 M H 2 SO 4 . All of the ionic liquids studied showed inhibitory properties dependent on the chain length linked to N3. The highest efficiency of IL4 was confirmed by the gravimetric and electrochemical tests, which were completed by scanning electron microscopy/energy- dispersive X-ray spectroscopy (SEM/EDX) and atomic force microscopy (AFM). Received: January 10, 2011 Accepted: April 29, 2011 Revised: April 27, 2011
Transcript
Published: April 29, 2011
r 2011 American Chemical Society 7129 dx.doi.org/10.1021/ie1024744 | Ind. Eng. Chem. Res. 2011, 50, 7129–7140
ARTICLE
pubs.acs.org/IECR
Synthesis of Selected Vinylimidazolium Ionic Liquids andTheir Effectiveness as Corrosion Inhibitors for CarbonSteel in Aqueous Sulfuric AcidDiego Guzm�an-Lucero,† Octavio Olivares-Xometl,*,‡ Rafael Martínez-Palou,† Natalya V. Likhanova,†
Marco A. Domínguez-Aguilar,† and Vicente Garibay-Febles†
†Programa de Investigaci�on y Posgrado, Instituto Mexicano del Petr�oleo, Eje Central Norte L�azaro C�ardenas 152,M�exico 07730 D.F., M�exico‡Facultad de Ingeniería Química, Benem�erita Universidad Aut�onoma de Puebla, Av. San Claudio Ciudad Universitaria,Col. San Manuel, Puebla 72570, Puebla, M�exico
ABSTRACT: Five imidazolium-type ionic liquids, containing both N1 unsaturated and N3 long alkyl saturated chains as cationsand bromide as anion (IL1�IL5), were obtained by conventional and microwave synthesis. Compounds were tested in aqueous1MH2SO4 as corrosion inhibitors for carbon steel. Weight loss and polarization curves indicated that inhibition efficiency increasedwith concentration, which turns out to be dependent on alkyl chain size linked to N3 (IL4 > IL3 > IL1 > IL2 > IL5). The relativelyhigh inhibitory properties (88�95%) displayed by IL4within 25�40 �Cwere ascribed to a chemisorption process that involved thefollowing: the adsorption of protonated imidazoliummolecules on both the anodic and cathodic sites, the latter in competition withhydrogen ions tomitigate hydrogen evolution; and also the formation ofπ bond with iron by the CdNgroup from imidazolium ring(this way inhibitor produced more than one center of adsorption action). Surface analysis indicated a considerable reduction ofcorrosion products after the addition of IL4.
1. INTRODUCTION
Acidic solutions are commonly used in picking operations aspart of the steelmaking finishing process in which oxide and scaleare removed from metallic surface. A solution of sulfuric acid isnormally employed to treat carbon steel products, and inhibitoris provided to lessen acid attack. Diluted sulfuric acid can beapplied to carbon steel up to 90 �C 1 in concentrations of 5�15wt %.2 Likewise, refinery operations involved the formation ofsulfuric acid derived from the dissolution of sulfur compounds(e.g., hydrogen sulfide, thyosulfates, and mercaptans, etc.). It istherefore important to quantify the effect of acidic solutionaggressiveness and develop corrosion inhibitors for carbon steelprotection.
The use of corrosion inhibitors (CIs) comprises one of themost economical ways to mitigate corrosion rate and protectmetallic materials against corrosion to preserve industrialfacilities,3 especially in acidic media.4 In this context, the treat-ment of mild steel corrosion in acidic environment throughorganic compounds has resulted in considerable savings for theoil industry. Several families of organic compounds, i.e., fattyamides,5,6 pyridines,7�9 imidazolines,10�12 and 1,3-azoles,13�15
have shown excellent performance as CIs. However, many ofthese compounds are toxic, and they do not completely fulfill therequirements imposed by the environmental protection stan-dards. This is the reason why in the past few years great effortshave been made by researchers in this area to develop newenvironmentally friendly CIs.16
Ionic liquids (ILs) have attracted the attention of research-ers in the past decades due to their interesting physical andchemical properties. ILs are an excellent alternative to
substitute volatile organic solvents because of their very lowvapor pressures, thermal and chemical stability, no flam-mability, and their ability to act as catalyst.17 Moreover, ILspresent a wide electrochemical window so this property hasbeen studied for electrochemical applications in batteries,18
light emitting electrochemical cells,19 and fuel cells.20 Thereare few studies that involved ionic liquids as CIs for acidenvironments; these are two ILs containing 1-butyl-3-methy-limidazolium as cation, along with either chloride or hydrogensulfate as anion, which have shown good properties as CIs formild steel in aqueous 1.0 M HCl.21
In this work, five imidazolium-type ionic liquids, containingboth N1 unsaturated and N3 long alkyl saturated chains ascations together with bromide as anion (IL1�IL5), were synthe-sized and evaluated as CIs for acid environment (Table 1).Weight loss tests and electrochemical polarization curves wereapplied to test the inhibitory properties of these compoundsin AISI 1018 carbon steel immersed in 1.0 M H2SO4. All of theionic liquids studied showed inhibitory properties dependent onthe chain length linked to N3. The highest efficiency of IL4was confirmed by the gravimetric and electrochemical tests,which were completed by scanning electron microscopy/energy-dispersive X-ray spectroscopy (SEM/EDX) and atomic forcemicroscopy (AFM).
Received: January 10, 2011Accepted: April 29, 2011Revised: April 27, 2011
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2. EXPERIMENTAL SECTION
2.1. Materials. All reagents (Aldrich) were used withoutprevious purification. The experiments were performed onsamples of AISI 1018 carbon steel, whose chemical compositionis as follows (wt %): 0.10 C, 0.25 Si, 0.45 Mn, 0.03 P, 0.03 S, andbalanced with Fe. Specimens were abraded with wet SiC papernumber 400, 600, 1000, and 1200, degreased in hexane, andwashed in an ultrasonic bath of acetone for 5 min to removeimpurities.2.2. Measurements and Equipments. The synthesized
compounds were characterized by 1H and 13C NMR and FT-IR spectroscopies. Melting points were measured in a FisherScientific apparatus equipped with a 300 �C thermometer. 1HNMR (300 MHz) and 13C NMR (75.4 MHz) spectra wereobtained with a JEOL Eclipse-300 equipment using tetramethyl-silane (TMS) as internal standard and the solvent indicated ineach case at room temperature. Chemical shifts (δ) were reportedin parts per million. Microwave-assisted syntheses were carriedout in CEMDiscover equipment with simultaneous cooling. Themorphology on the metallic surface was observed by a SEMmicroscope model Philips XL30ESEM. Compositional resultswere obtained by the EDX module attached to the microscope.The AFM images were obtained by a digital instrument modelNanoscope V Tuna D3100 AFM. This equipment was operatedin tapping mode at ambient conditions and low operating voltagein order to reduce any damage on the sample surface. The lateralscan size was 2.5 μm with a number of views of 512, which wererecorded at a scan rate of 0.5 Hz. A Netzsch STA 409 high-resolution simultaneous thermogravimetric analysis, TGA, anddifferential scanning calorimetry, DSC, device was used for thethermal analysis at a heating rate of 5 �C/min in nitrogenatmosphere.2.3. General Procedure for IL Synthesis. 1-Vinylimidazole
(0.05 mol) was added to the corresponding alkyl bromide (0.055mol, 10% excess), mixture was then heated at 60 �C and agitatedwith a magnetic stirrer for 36 h. During this period of time, weobserved the formation of another phase or precipitate, depen-dent on the length of the alkyl chain. The ionic liquid product wasseparated as a phase under the solution or filtered. The residualstarting materials of the liquid products were removed by threesuccessive extractions with ethyl acetate (4� 20 mL). The solidproducts were purified by recrystallization from ethyl acetate,
and the ionic liquids were dried under vacuum at 60 �C for 24 h.Additionally, compounds were synthesized for the first timeunder solvent-free microwave irradiation conditions at 50 �C andwith simultaneous cooling. In this case, the products were obtainedin similar yields but in only 20 min of irradiation.1-Vinyl-3-butylimidazolium Bromide (IL1). Following the
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(500�4000 cm�1, KBr pellet, cm�1): ν 3469, 3401, 3133, 3085,2917, 2850, 1648, 1550, 1465, 1174, 719.2.4. Weight Loss Measurements. Weight loss tests were
developed on rectangular specimens with a size of 2.54 cm �1.25 cm� 0.025 cm. The immersion time was of 1, 2, 3, 4, and6 h at 25 and 40 �C in 1.0 M H2SO4 solution naturally aeratedwith and without the addition of different concentrations ofinhibitors: 10, 25, 50, and 100 ppm. The weight of eachspecimen was determined before and after testing in theaggressive medium with a digital balance model SartoriusBasic. After the exposure time of immersion, the specimenwas removed and washed with double distilled water. Thecorrosion product on the steel surface was mechanicallyremoved by rubbing it with a brush. The specimens werewashed in an ultrasonic bath with acetone, dried in a flux ofhigh-purity nitrogen, and set to dry in a stove at 100 �C for 2 h,and finally its weight loss was recorded. The experiments weredone by triplicate and the average weight loss was used todetermine corrosion rate (CR) and inhibition efficiency asfollows:
CR ¼ KWATD
ð1Þ
θ ¼ CRo �CRinho
CRoð2Þ
IE=% ¼ θ� 100 ð3Þwhere CR stands for corrosion rate (g/(m2 h)), K is a constant(1 � 104D),22 W is the weight loss (g), A is the coupon area(cm2), T is the exposure time (h), D is the steel density(g/cm3), and CRo, CRo
inh are the corrosion rates of carbonsteel in the absence and presence of inhibitor, respectively.2.5. Electrochemical Measurements. Potentiodynamic po-
larization curves were carried out using the conventional three-electrode cylindrical glass cell. The working electrode (WE) wasmade of AISI 1018 carbon steel with a contact area of 0.32 cm2, ahigh-purity platinum foil (99.9%) with a surface area of 4.3 cm2,which was used as the counter electrode (CE), and a saturatedcalomel electrode (SCE), via a Luggin capillary probe, wasused as the reference electrode (RE). The WE was wet-abraded with 400, 600, 100, and 1200 emery paper, degreasedin AR-grade 2-propanol and acetone, and rinsed with deion-ized water prior to each experiment. Experiments were per-formed in the absence (blank) and presence of corrosioninhibitors at 25( 1 �C. Electrochemical tests were performedin a potentiostat/galvanostat PGSTAT302N controlled by aPC through the general purpose electrochemical system(GPES). Before recording the polarization curves, the WEspecimen was immersed in the test solution for 30 min untilsteady-state open circuit potential (Eocp) was reached. Thecathodic and anodic polarization scans were performed from�250 mV to Eocp to þ250 mV at a rate of 1.6 mV s�1. Tafelslopes were constructed, and inhibition efficiencies (IE, %)were determined as follows:
IE=% ¼ jcorr � jinhcorr
jcorr
" #� 100 ð4Þ
where jcorrinh and jcorr are the corrosion current densities with
and without the addition of corrosion inhibitor, respectively.
3. RESULTS AND DISCUSSION
3.1. Synthesis of Ionic Liquids. We select ILs containingimidazolium cation with long alkyl chain because it is well-knownthat many 1,3-azole heterocycles have shown good performanceas CIs.7�17 Alkyl chains play an important role in inhibi-tion efficiency and binding energy between inhibitor and metalsurface; for example, the binding energies for imidazolinesincrease from C8 to C18 series.
23,24 Likewise, inhibition efficiencyalso depends on the presence of insaturations in molecularstructure;25�27 this is the reason why, 1-vinylimidazole wasemployed as starting material to synthesize the selected ILs forthis study.ILs containing halogens as anions present a very low solubility
in hydrocarbon environments and a relatively high water solubi-lity. When a long alkyl chain is part of a molecule, they becomeamphiphilic compounds that act as charged surfactants. ILscontaining halogens are capable of being synthesized in a one-pot and one-step synthesis reaction to be the most economical ofILs available. Bearing this in mind, five ILs were synthesizedunder conventional heating and also under microwave dielectricheating. According to NMR spectroscopy, products whosestructures are shown in Table 1 were obtained with purity greaterthan 95%. They were obtained by alkylation reactions of1-vinylimidazole with alkylbromides, in which aliphatic chainswith lengths between C4 and C22 (IL1�IL5) were developed tostudy the effect of carbon length on the inhibitory properties ofcompounds (Scheme 1). Microwave-assisted synthesis yieldedsimilar products though in times of about 20 min of dielectricheating at 50 �C. This procedure involved simultaneous coolingto avoid the spontaneous radical-free polymerization of 1-viny-limidazole and also solvent-free conditions, which implicateprocess savings and green environment protection.28,29
3.2. Weight Loss Tests of ILs as CIs. Table 2 summarizes theresults obtained after gravimetric tests of carbon steel in 1 MH2SO4 at 25 �C when added ILs developed at concentrations of10, 25, 50, and 100 ppm for different exposure times (1, 3, and6 h). The table shows that corrosion rate depends on immersiontime and also on the temperature of corrosive environment. Forall of the studied compounds, the IE (%) increased whenconcentration was increased. It was observed that the corrosionrate increased with temperature, while the IE increased with theexposure time at room temperature.It is important to note that IE follows the order IL4 > IL3 >
IL1 > IL2 > IL5 at 25 �Cwithin 1�3 h of exposure time. At 4 and6 h, the order is similar except that the IE of IL5 is higher thanthat of IL2. The structural difference among the ILs evaluated isthe N3 alkyl chain size; the compound with an alkyl chain ofC12�C18 (IL3, IL4) showed better IE than both ILs containingshort (C4 and C8) and large (C22) alkyl chains. These results arein correspondence with those obtained by several authors forimidazoline type corrosion inhibitors.24,25 It seems that C18 is theoptimum alkyl chain size to obtain a suitable binding forcebetween metal and inhibitor molecules.The results of the weight loss test at 40 �C(Table 3) show that,
in the case of IL4, 3 and 5, the IEs are higher than those at 25 �C
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Table 3. Corrosion Parameters Obtained from Weight Loss Test at 40 �C for AISI 1018 Carbon Steel in 1.0 M H2SO4 as aFunction of IL Concentration and Immersion Time
Table 2. Corrosion Parameters Obtained from Weight Loss Test at 25 �C for AISI 1018 Carbon Steel in 1.0 M H2SO4 as aFunction of IL Concentration and Immersion Time
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(Table 2) within 1�3 h of exposure time; furthermore, IL4reached top efficiency at 40 �C after 6 h. At 40 �C, IEs followedthe order IL4 > IL3 > IL5 > IL1 > IL2 for every time tested, whileat 25 �C the IL5 compound displayed a lower efficiency in thefirst 3 h of exposition. As temperature is increased, IL1 and IL2lose their efficiency in approximately 8�20%. These resultssuggest that IL1 and IL2 may be physically adsorbed on themetal surface, so when there is an increase in temperature, theyare desorbed from the surface which conveys a decrease ininhibition efficiency. In the case of IL3, -4, and -5, as temperatureincreases, physical adsorption changes to chemisorption, aprocess that is common to filming inhibitors.30 It is worthnoticing that compounds IL3�IL5 have long aliphatic chainsthat under the effect of temperature become ordered in thesolution and near the metallic surface, which may result in a moreuniform adsorption and consequently in an increase of IE.31 Inevery case, IE was enhanced when inhibitor concentration wasincreased.According to test results, IL4 reached the best performance as
CI (Figure 1) with a moderate alkyl chain size of C18. This factsuggests that the compound may present a balanced chain size tomeet the hydrophilic/lipophilic equilibrium, which favors themolecular migration from the aqueous media to the metalsurface. This behavior has been observed in ammonium surfac-tants tested as corrosion inhibitors.32 For neutral molecules,studies report that adsorption of organic inhibitors dependsmainly on some physicochemical properties of the molecule,
related to its functional groups, steric effects, and electronicdensity of donor atoms. Additionally, adsorption also depends onthe interaction of π orbitals of inhibitor with d orbitals of ironatoms, which induces great adsorption of inhibitor moleculesonto a mild steel surface, leading to the formation of a corrosionprotecting film.33�36
Little knowledge exists about how these parameters areinvolving for the case of charged organic species. ILs propertiesare highly dependent on the structure of cation and anion, andboth species can be involved in the interaction between the ILand metal surface. Shang et al.21 proposed that protonatedimidazolium molecules are also adsorbed at cathodic sites incompetition with hydrogen ions that reduce hydrogen evolutionand that the atom of imidazolium ring, the CdN group, can forma π bond. Therefore not only can the π electrons of theimidazolium basis enter unoccupied orbitals of iron, but alsotheπ* orbital can accept the electrons of d orbitals of iron to formretrodonation bonds so they produce more than one center ofadsorption action.Surfactants exert their inhibitory action by adsorption on the
metal surface in such a way that the polar or ionic group(hydrophilic part) is attached to the metal surface, while its tail(hydrophobic part) is extended to the solution. The adsorptionof surfactant on metal surface can markedly change the corro-sion-resisting property of metal.38,39 Many ionic surfactants havebeen described as CIs.40�42 In our case, the ILs containing longsaturated alkyl chains (IL3, IL4, and IL5) present an amphiphiliccharacter and behave as corrosion inhibitors.3.3. Potentiodynamic Polarization Curves. The inhibition
efficiencies of ILs were determined through electrochemicalexperiments (Table 4). The polarization curves for AISI 1018carbon steel obtained at various inhibitor concentrations of IL4and IL5 are shown in Figures 2 and 3, respectively. Similarpolarization curves were obtained for the other ILs. The values ofthe corrosion current density (jcorr) and corrosion potential(Ecorr), as well as the cathodic and anodic Tafel slopes (βc, βa),were obtained by the linear extrapolation of the Tafel slopes.These figures show the influence of inhibitor; as concentration isincreased, the corrosion current density decreased, indicatingthat the corrosion rate was mitigated. Inhibition was reached by ablocking mechanism on the anodic and cathodic sites as polar-ization curves are displaced downward on the whole potentialrange, which suggests that ILs behave as mixed type inhibitors.43
On the other hand, the presence of these compounds produced apositive shift to more noble values of Ecorr when the concentra-tion was increased, which indicates the formation of a protectivelayer on the metal surface.44,45 The presence of IL compoundsaffected the Ecorr with respect to Eocp, as it is observed by thedisplacement of the polarization curves to anodic potentials inthe presence of 100 ppm: IL1 (40 mV), IL2 (59 mV), IL3 (102mV), IL4 (28 mV), and IL5 (12 mV). These data are importantin classifying an inhibitor of anodic or cathodic type.41 If thedisplacement in potential is at least 85 mV to the right, it isanodic; if not; it is cathodic: this way IL1, IL2, IL4, and IL5behave as mixed type inhibitors, while IL3 appears to be aninhibitor of cathodic type. IL4 displayed the maximum reductionin corrosion current density, and the compounds follow theorder IL4 > IL3 > IL1 > IL2 > IL5. There is a good agreementbetween the gravimetric and electrochemical results at 25 �Cduring the first 3 h of the gravimetric tests.3.4. Adsorption Isotherms. The mode and extent of inter-
actions between inhibitor and iron surface can be studied by
Figure 1. Inhibition efficiency vs IL4 concentration at (a) 25 and(b) 40 �C as a function of immersion time.
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applying adsorption isotherms. Adsorption of inhibitor mole-cules on metal surface is a substitution process since it isaccompanied by an exchange of adsorbed water molecules withorganic molecules. The degree of surface covered (θ) on steelwas determined from the weight loss tests as a function ofinhibitor concentration at constant temperature; this way theadsorption isotherm is evaluated at equilibrium conditions.Several models of isotherms were applied to fit the surfacecoverage values at different inhibitor concentrations and tem-peratures. Experimental results of Cinh/θ vs Cinh yieldedstraight lines, as shown in Figure 4, which are in agreement
with Langmuir's isotherm:
KadsCinh ¼ θ
1� θð5Þ
where Cinh is the inhibitor concentration and Kads is theadsorption equilibrium constant. The values of the correlationcoefficients and the adsorption equilibrium constants aregiven in Table 5 for the ILs tested. The high correlationcoefficients and slopes (1.0 ( 0.1) indicated a good fitting ofexperimental data to Langmuir's isotherm. The correlations ofCinh/θ vs Cinh displayed a linear fitting along with slopes close
Table 4. Electrochemical Parameters (Ecorr, jcorr, βc, βa, and Rp) Associated with PolarizationMeasurements of AISI 1018 CarbonSteel in 1.0 M H2SO4 Solution in the Absence and Presence of Different Concentrations of Corrosion Inhibitors at 25 �C
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to unit value as indicated by eq 5. Deviations from unity wereascribed to the presence of sulfate and iron sulfate on the metalsurface, products which are less soluble in water and hencemore likely to be adhered to surface. The formation ofcorrosion products is owed to the anodic reaction, whichdestroy the film already formed on the surface, and therefore alower amount of inhibitor molecules is efficiently adsorbed.When θ tends to ∼1, a more compact film is formed, thoughthis is dependent on the molecular structure of thecompounds.46 The adsorption isotherm is useful for estimat-ing important thermodynamic parameters such as the stan-dard free energy of adsorption:
ΔGads� ¼ � RT lnð55:5KadsÞ ð6ÞΔGads� is the standard free energy of adsorption, and the valueof 55.5 is the concentration of water in solution expressedin moles.As it is known, an increase in Kads with temperature indicates
an increase in the extent of adsorption.47 The high negativevalues of ΔGads� ensure the spontaneity of the adsorptionprocess and the stability of the adsorbed layer on the metalsurface. Generally, values of ΔGads� up to �20 kJ/mol areconsistent with the electrostatic interaction between the chargedmolecules and the charged metal (physical adsorption), whilethose more negative than�40 kJ/mol involve sharing or transferof electrons from the inhibitor molecules to the metal surface toform a coordinated type of bond (chemical adsorption).36,38 Inour case, Kads increased for IL4 and IL5 when temperature wasincreased. This means that a chemical adsorption processoccurred on the metal surface; as chemisorption requires an
activation energy provided by a higher temperature to happen,whereas physisorption takes place without a change in activationenergy. Although the values of ΔGads� for IL4 and 5 are in therange of 40�43 kJ mol�1, the increase in activation energy withtemperature was higher for IL4 than for IL5, suggesting that non-Coulombic forces prevailed over electrostatic forces; besidesthat, potential and metallic charge are lower than those forcompounds IL1, 2 and 3 (ΔGads� < 40 kJ mol�1).3.5. Surface Analysis. Figure 5 shows the micrographs of the
carbon steel surface before and after carbon steel immersion in1.0 M H2SO4 with and without corrosion inhibitor. Figure 5ashows the surface after the immersion in the corrosive medium; itis observed that a uniform surface finishing is produced by themechanical grinding on the sample. Figure 5b shows the surfaceof the carbon steel specimen after immersion in 1.0 M H2SO4
solution for 6 h in the absence of inhibitor. Figure 5c shows thesurface of the carbon steel specimen after immersion in thecorrosive solution for the same period of time though in thepresence of 100 ppm IL4 inhibitor. SEM micrograph revealedthat the surfacemorphologywas strongly damaged in the absence ofthe inhibitor, but in the presence of 100 ppm inhibitor damagewas considerably diminished, which confirmed the high efficiency ofIL4 at this concentration. The elemental analysis obtained fromEDX indicated that the coupon after grinding is mainly com-posed of Fe, as is expected for carbon steel (Figure 6a), while
Figure 4. Langmuir adsorption isotherm for IL3 at 25 (a) and 40 �C (b)on the AISI 1018 carbon steel surface in 1.0 M H2SO4.
Table 5. Summary of Kads and ΔGads� Data Obtained fromthe ILs Evaluated as Corrosion Inhibitors of AISI 1018Carbon Steel in 1.0 M H2SO4
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after immersion in the corrosive media (1.0 M H2SO4) arelatively high concentration of oxygen and sulfur was detectedas a result of formed oxyhydroxides and sulfides from iron indiluted sulfuric acid (Figure 6b). Finally, a considerable reduc-tion in the content of oxygen and sulfur was detected on thecoupon surface after immersion in the corrosive media contain-ing the inhibitor IL4 (Figure 6c), which showed inhibitorcapability to mitigate uniform corrosion.
3.6. Atomic Force Microscopy. AFM is one of the mostpowerful tools for observing the surface morphology as it providesa useful means of characterizing substrate microstructure.5,45
Three-dimensional images for the coupon surface morphologywere obtained with AFM (Figure 7), after immersion in thetesting solution in the absence and presence of ILs. The surfaceafter mechanical grinding shows some abrading scratches(Figure 7a), whereas the surface images of steel before and afterthe addition of IL4 show clear evidence of the good performanceof the IL4 corrosion inhibitor, as surface morphology showsmuch less damage (Figure 7c) with respect to the surface afterimmersion in uninhibited corrosive media (Figure 7b). Note thatimage resolution increased from Figure 7a to Figure 7c; thesurface roughness considerably increased from Figure 7a toFigure 7b and decreased in Figure 7c. The latter image shows asurface with a lower number of crest heights and shallower valleyson protected surface topography with IL4. This behavior was
Figure 5. SEM images (1000�) of metallic surfaces: (a) after polishing,(b) after 6 h of immersion in the corrosive media without inhibitor, and(c) after 6 h of immersion in the corrosive media with 100 ppm IL4.
Figure 6. EDX analysis of metallic surfaces: (a) after polishing, (b) after6 h of immersion in the corrosive media without inhibitor, and (c) after6 h of immersion in the corrosive media with 100 ppm IL4.
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ascribed to the presence of a film that inhibits metal dissolutionthat resulted in lower surface roughness. “Vesicle-like” parti-cles also are observed on the metal surface; these are char-acteristic of an adsorptive film which prevailed in the presenceof IL4.
3.7. Inhibition Mechanism. The corrosion of 1018 carbonsteel in 1.0 M H2SO4 was delayed by the presence of differentconcentrations of 1-alkyl-3-vinylimidazolium bromide derivatives(VImCnBr), which were evaluated in this study. The resultsindicated that the inhibition mechanism (Scheme 2) involvedboth physical and chemical adsorption of inhibitor on the metalsurface; this is influenced by the nature of the inhibitor (chemicalstructure) and the surface charge of the metal (acidic media).The physical adsorption involves electrostatic forces betweenionic charges of the adsorbed species and the electric charge atthe metal/solution interface; and the chemical adsorption in-volves charge transfer from inhibitor molecules to the metalsurface to form coordinate bonds.48 All synthesized compoundshave an anionic (Br�) and a cationic part, which contain animidazolium ring with vinyl (CH2dCH�) and alkyl groups. Someresearchers have reported49,50 that organic compounds containingnitrogen are more efficient against mild steel corrosion inhydrochloric acid than in sulfuric acid. A likely explanation isattributable to the synergistic effect between halide anions andorganic cations in the inhibitor for steel corrosion in acid media.The bromide ions in acid solution can be adsorbed on the metalsurface and form an interconnecting bridge between the metalatoms and organic cations.1 The adsorbed bromide ions facilitatethe adsorption of cations of ILs by electric charge attraction onthe metal surface;51 then the stabilization of adsorbed bromideions by means of electrostatic interaction with IL cations leads toa greater surface coverage and thereby to a greater inhibitioneffect andminor iron dissolution. This process of anodic sites canbe described by the following reactions.
in the absence of ILs :
Feþ xH2O T ½FeðH2OÞx�ads ð7Þ
½FeðH2OÞx�ads þ SO42� T Fe½ðH2OÞxSO4
2��ads ð8Þ
Fe½ðH2OÞxSO42��ads f ½FeðH2OÞxSO4�ads þ 2e� ð9Þ
½FeðH2OÞxSO4�ads f Fe2þ þOH� þ SO42� þHþ ð10Þ
Fe2þ þOH� þ 1=2O2 f FeOOHþ e� ð11Þwhereas in the presence of ILs :
Feþ xH2O T ½FeðH2OÞx�ads ð12Þ
½FeðH2OÞx�ads þ SO42� T Fe½ðH2OÞxSO4
2��ads ð13Þ
Fe½ðH2OÞxSO42��ads þ VImCn
þ f Fe½ðH2OÞxSO42�VImCn
þ�adsf ½FeðH2OÞxSO4
��adsVImCnþ þ e�
f ½ð½FeðH2OÞxSO4�VImCnÞ��ads ð14Þ
½ð½FeðH2OÞxSO4�VImCnÞ��ads þ VImCnþ þ SO4
2�
f fð½FeðH2OÞxSO4�adsVImCnÞ�VImCnþSO4
2�=VImCnþgadsð15Þ
Feþ Br� T ðFeBr�Þads ð16Þ
ðFeBr�Þads þ VImC þn f ðFeBr�VImCn
þÞads ð17Þ
ðFeVImCnÞads f ðFeVImCnþÞads þ e� ð18Þ
Figure 7. Three-dimensional AFM images of carbon steel: (a) aftermechanical grinding (data scale 50 nm), (b) after 6 h of immersion in thecorrosive media without inhibitor (700 nm), and (c) after 6 h ofimmersion in the corrosive media containing 100 ppm of IL4 (1.0 μm).
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In addition, the IL cations compete with hydronium (H3Oþ) for
available electrons on themetallic surface, where the inhibitor cationsize is much larger than the hydrogen molecule due to the aliphaticflexible chain. After electron assimilation, the inhibitor cation evolvedto its neutral form with a CHdN group of the imidazolium ringand the vinyl group having a free electron pair that promoted thechemical adsorption on the metallic surface to be protected fromcorrosion. Additionally, the vinyl group takes place in the hydro-genation reaction by using hydrogen from the ethylic group(eq 24). This process can be described by the following reactions:
in the absence of ILs :
FeþH3Oþ T FeðH3O
þÞads ð19Þ
FeðH3OþÞads þ e� f ½FeðH3OÞ�ads ð20Þ
ðFeH3OÞads þHþ þ e- f FeþH2 þH2O ð21Þ
whereas in the presence of ILs :
Feþ VImCnþ T FeðVImCn
þÞads ð22Þ
½FeðVImCnþÞ�ads þ e� f ðFeVImCnÞads ð23Þ
½FeðVImCnþÞ�ads þ 2H f ðFeImEtCnÞads ð24Þ
3.8. Thermal Behavior. ILs employed in this study seem to beenvironmentally benign corrosion inhibitors as they present avery low vapor pressure.52 Furthermore, ILs showed a highthermal stability as 1% of weight loss matter does not occurbelow 200 �C, and they either are in liquid phase at roomtemperature or present low melting points (Table 6). On the
other hand, the best performance obtained at 40 �C, whencompared to 25 �C, suggests that these compounds could beconsidered for further study as a promising corrosion inhibitor.Investigations about the inhibitory properties for these com-pounds at moderate temperatures are now in course.
4. CONCLUSIONS
The five imidazolium-type ionic liquids (IL1�IL5) contain-ingN1 unsatured andN3 long alkyl saturated chains (cation) andbromide (anion) showed corrosion inhibition properties for theprotection of carbon steel in aqueous 1.0MH2SO4, as confirmed byweight loss test and polarization curves. Inhibition efficiencyincreased with concentration (10�100 ppm), and it was dependenton the alkyl chain size linked to N3 (IL4 > IL3 > IL1 > IL2 > IL5).
The inhibition mechanism of ILs was attributed to the strongadsorption ability of these surfactants to form a protective layerthat isolates the mild steel surface from an aggressive environ-ment. ILs behaved as mixed type corrosion inhibitors. Theirmoderate thermal stability suggests that these compounds maybe considered for further study to determine the upper level ofapplication as corrosion inhibitors. The relatively high inhibitoryproperties (88�95%) displayed by IL4 within 25�40 �C maysupport this study.
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