咪唑啉衍生物缓蚀剂对碳钢在CO 2盐水中的缓蚀机理

物理化学学报(Wuli Huaxue Xuebao )Acta Phys.鄄Chim.Sin .,2008,24(1)：138-142Received:July 17,2007;Revised:September 28,2007;Published on Web:November 30,2007.∗Corresponding author.Email:fuguoliu@;Tel:+86532⁃66781637.ⒸEditorial office of Acta Physico ⁃Chimica Sinica[Article]January咪唑啉衍生物缓蚀剂对碳钢在CO 2盐水中的缓蚀机理刘福国∗杜敏张静仇萌(中国海洋大学化学化工学院,海洋化学理论与工程技术教育部重点实验室,山东青岛266100)摘要：采用极化曲线和交流阻抗研究新合成咪唑啉衍生物缓蚀剂对碳钢在饱和CO 2盐水中的缓蚀性能和机理.计算了缓蚀效率和热力学参数.缓蚀效率随着缓蚀剂浓度增大而增加,但随着温度增加先增加后降低.咪唑啉衍生物在碳钢表面的吸附符合Langmuir 等温式.电化学结论由量子化学计算补充说明.关键词：交流阻抗;极化;缓蚀剂;Langmuir 等温式;量子化学中图分类号：O646Inhibition Mechanism of Imidazoline Derivative Inhibitor forQ235Steel in Saltwater Saturated with CO 2LIU Fu ⁃Guo ∗DU Min ZHANG Jing QIU Meng(Key Laboratory of Marine Chemistry Theory and Technology,Ministry of Education,College of Chemistry and ChemicalEngineering,Ocean University of China,Qingdao 266100,Shandong Province,P.R.China )Abstract ：Laboratory investigations (polarization and electrochemical impedance spectroscopy)were performed in order to assess the effectiveness and the inhibition mechanism of new synthesized imidazoline derivative inhibitor to prevent corrosion of steel in saltwater saturated with carbon dioxide.The inhibitor efficiencies and thermodynamic parameters were calculated.The inhibition efficiency increased with the increase in inhibitor concentration,but increased then decreased with temperature.It was seen that the adsorption of imidazoline derivative on steel fitted the Langmuir isotherm equation.The electrochemical results have also been supplemented by quantum chemical analysis.Key Words ：EIS;Potentiodynamic polarization;Corrosion inhibitor;Langmuir isotherm;QuantumchemistryIn remote and offshore oil sites,the product from the wells is transported as a mixture of oil,saltwater,and natural gas.Car-bon dioxide in the natural gas dissolves in saltwater and results in the formation of a weak carbonic acid often causing severe corrosion in carbon steel pipelines.This problem has caused the consideration of many corrosion control methods for oil anic substances have been used extensively as corrosion in-hibitors during the last four decades in carbon steel pipelines.Of these,heterocyclic compounds containing one or more N,O,and S atoms can affect the inhibition of corrosion,in aqueous acid solutions,of metals [1-7].Imidazoline,as a type of corrosion inhibitor,is most common-ly used for protecting oil wells,gas wells,or pipelines from CO 2corrosion due to its good adsorption character and the formationof a chemical film on the surface of iron and steel [8-10].Recent-ly,Wang et al .[11]studied the structure and inhibition efficiency of imidazoline derivatives by quantum chemical calculation,and considered that the active functional group with conjugated sys-tem to imidazoline ring can improve inhibition efficiency of imi-dazoline derivatives.Jovancicevic et al .[9]studied the effect of hydrocarbon chain length of imidazoline on the corrosion inhibi-tion of mild steel in a CO 2-containing environment,and the re-sults showed the critical importance of the chain length over a broad range (C8-C20)in the inhibition of imidazolines.In this work we studied the inhibition action of imidazoline derivative synthesized in our laboratory on carbon steel corrosion in salt-water by polarization and impedance spectroscopy techniques.Several quantum ⁃chemistry studies have been performed in138LIU Fu⁃Guo et al.：Inhibition Mechanism of Intidazoline Derivative Inhibitor for Q235Steel in Saltwater Saturated No.1order to relate the inhibition efficiency to the molecular proper-ties of the different types of compounds[12].1Experimental1.1MaterialsQ235steel was used for this study,its composition(w(%),mass fraction)was C0.16,Mn0.53,Si0.30,P<0.045,S<0.055,Fe bal-ance.The surfaces were polished with a series of silicon carbide papers,and then they were washed with distilled water and de-greased in acetone and dried.In the saltwater,the contents of Ca2+,Mg2+,K+,Na+,Cl-,SO2-4,and CO2-3were1.086,0.224,0.359, 9.378,17.002,0.290,and0.089g·L-1,respectively.The value of pH of the saltwater saturated with CO2was4.86.The imidazoline derivatives were synthesized as follows:All chemicals used are AR grade.1.2Potentiodynamic polarizationThe potentiodynamic polarization studies were carried out with Q235steel strips having an exposed area of1cm2at dif-ferent temperatures.The cell assembly consisted of Q235steel as working electrode,a platinum foil as counter electrode and a saturated calomel electrode(SCE)as a reference electrode with a Luggin capillary bridge.Polarization studies were carried out us-ing IM6e(ZAHNER,Germany)electrochemistry workstation.The polarization curves for Q235steel specimen in the saltwater sat-urated with CO2with or without various concentrations of the inhibitors were recorded at a sweep rate of1mV·s-1.1.3Electrochemical impedance spectrum Electrochemical impedance spectrum(EIS)was obtained using impedance spectrum analyzer(IM6e,ZAHNER,Germany).The potential amplitude was set at5mV at the corrosion potential (E corr)and the frequency range was from100kHz to3mHz.2Results and discussion2.1Potentiodynamic polarizationThe polarization curves of Q235steel in saltwater at different temperatures with varying concentrations of imidazoline deriva-tive are shown in Figs.(1,2)(curves at308K and318K are not shown).It is observed that in the presence of inhibitor,the curves were shifted to more positive potential region and the shift was found dependent on concentration of the inhibitor.The changes of E corr in the presence of5-25mg·L-1inhibitor were not remarkable or neglectable at different temperatures.And the curves display both suppression of the cathodic and anodic half reactions.Values of all kinetic parameters such as corrosion potential (E corr),cathodic and anodic Tafel slopes(b c,b a)and corrosion cur-rent density(i corr)attained by extrapolation of Tafel lines,as well as inhibitor efficiency are listed in Table1.From Table1,it is observed that imidazoline derivative shifts the polarization curves to lower corrosion current density values with affecting Tafel slopes obviously in the presence of various concentrations of inhibitor at different temperatures.The synthe-sized imidazoline derivative controlled both cathodic and anodic reactions and thus acted as a mixed鄄type inhibitor.The inhibi-tion efficiencey(η)increases with increasing concentration of in-hibitor at the same temperature,indicating that a higher surface coverage was obtained in saltwater with the optimum concentra-tion of inhibitor.Butηincreases with increasing temperature first,and thendecreases.Fig.1Polarization curves of Q235steel in saltwater with varying concentrations of inhibitor at298KFig.2Polarization curves of Q235steel in saltwater with varying concentrations of inhibitor at328K139Acta Phys.鄄Chim.Sin.,2008Vol.24According to Cao [13],the invariability of the E corr with the in-hibitor addition could be associated with a geometric blocking mechanism (Figs.(1,2)).In such a case,the inhibition efficiency equals the coverage of the adsorbed inhibitor species (θ)on the metal surface (Table 1).2.2Electrochemical impedance spectrumFig.3shows the typical Nyquist plots for Q235steel in saltwa-ter in the presence of 25μg ·L -1of imidazoline derivative at 298K.It is clear that the impedance response of Q235steel has sig-nificantly changed after the addition of inhibitor in saltwater.The distance along the real axis where the loop appears increas-es with exposure time,suggesting that the charge transfer resis-tance increases with time.The impedance spectra obtained on Q235steel in saltwater consist of one depressed capacitive loop.When Nyquist plot contains a “depressed semicircle with the center under the real axis ”,such behavior is characteristic for solid electrodes and of-ten referred to as frequency.Dispersion has been attributed to roughness and other inhomogeneities of the solid surface [14,15].In these cases the parallel network charge transfer resistance-dou-ble layer capacitance (R ct ⁃C dl )is usually a poor approximation es-pecially for systems where an efficient inhibitor is present.For the description of a frequency independent phase shifts between an applied AC potential and its current response,a constant phase element (CPE)is used [16].For analysis of the impedance spectra the equivalent circuit (EC)given in Fig.4was used,in which (a)and (b)are for saltwater in the absence and presence of inhibitor respectively.In this case R s refers to the solution resistance,R 1is the polarization resis-tance,and R 1+R 2presents the charge transfer resistance (R ct ).Re-sistance R 2and inductivity L may be correlated with a slow low frequency intermediate process [17].The impedance parameters derived from these figures are giv-en in Table 2.The fitting results (Table 2)show that C dl decreas-es and R ct increases with time,indicating that inhibition efficien-cy increases.This decrease in C dl results from a decrease in local dielectric constant and/or an increase in the thickness of the electrical double layer,signifying that imidazoline derivative molecule acts by adsorption at the solution/interface [18].The cha-nges in R ct ,C dl ,and R 2values were caused by the gradual replace-ment of water molecules by adsorption of the organic molecules on the metal surface,reducing the extent of dissolution [19].The factor n values also present an increasing trend with time,which corroborates the decrease of the surface inhomogeneity due to the adsorption over the sample.2.3Effect of temperatureThe logarithm of corrosion current can be represented by Ar-rhenius equation as follows:lg i corr =lg K -E a /RT (1)Effective activation energies (E a )were calculated from the plots of lg i corr versus 1/T .Data were fitted into straight lines using least square method.The values of E a were calculated from the slopes of these lines and are given in Table 3.Thermodynamic parameters,such as enthalpy and entropy of corrosion process,may be evaluated from the effect of tempera-ture.An alternative formulation of Arrhenius equation is [20]:W =RT Nhexp(ΔS 0a R )exp(-ΔH 0a RT )(2)where h is Planck ′s constant,N is the Avogadro ′s number,andΔS 0a and ΔH 0a are the entropy and enthalpy of activation,respec-tively.Straight lines are obtained with a slope of (-ΔH 0a /R )andan intercept of (ln(R /Nh )+ΔS 0a /R )from which the values of ΔH 0aT /K c (mg ·L -1)E corr /V (vs SCE)b a (mV ·dec -1)b c (mV ·dec -1)i corr (A ·mm -2)η(%)Coverage(θ/(°))2980-0.68757.6451 1.3095-0.67048.24860.23781.90.81910-0.65665.44230.16887.20.87215-0.67277.13190.11491.30.91320-0.64443.95350.09392.90.92925-0.67068.62840.07094.70.9473080-0.69563.2389 2.1575-0.67054.63650.38482.20.82210-0.65670.34040.25688.10.88115-0.67269.13510.18091.70.91720-0.64452.84370.14493.30.93325-0.67074.52620.10695.10.9513180-0.713120.3222 4.3015-0.684117.31710.69183.90.83910-0.676104.11830.48288.80.88815-0.69886.02030.33192.30.92320-0.69092.71730.25494.10.94125-0.68176.41420.19895.40.9543280-0.704157.2121 6.0215-0.681140.5154 1.02083.10.83110-0.679131.81680.80986.60.86615-0.680165.31590.66389.00.89020-0.689189.51320.54890.90.90925-0.62192.81700.43592.80.928Table 1Electrochemical data obtained from thepotentiodynamic curves carried out on Q235steel at differenttemperatures with varying concentrations of inhibitorFig.3Nyquist plots at different times for Q235steel in saltwater in the presence of 25mg ·L -1of imidazolinederivative at 298KFig.4The corresponding equivalent circuit used in modeling of the electrochemical impedance spectrum data140LIU Fu ⁃Guo et al .：Inhibition Mechanism of Intidazoline Derivative Inhibitor for Q235Steel in Saltwater Saturated No.1and ΔS 0a are calculated (Table 3).In aqueous solution,the adsorption of organic molecules is generally accompanied by the desorption of water molecules.The adsorption of an organic adsorbate at the metal/solution in-terface is considered a “substitutional adsorption ”phenomenon.Therefore,the positive values of ΔH 0a and ΔS 0a related to“substi-tutional adsorption ”can be attributed to the increase in the sol-vent entropy and to a more positive water desorption enthalpy [21].2.4Adsorption isothermSeveral adsorption isotherms were studied and the modified form of Langmuir adsorption isotherm known as thermodynam-ic/kinetic model was found to be closest to the description of the adsorption behavior of the studied inhibitor:lg(θ/(1-θ))=lg K ′+y lg c (3)where c is inhibitor concentration and K ′is a constant related to the equilibrium constant of adsorption (K )by the following rela-tionship:K =K ′/y (4)where y is the number of inhibitor molecules occupying one ac-tive site and K is related to standard free energy of adsorption ΔG ads by the equation:K =(1/55.5)exp(-ΔG 0ads /RT )(5)From the plots of lg(θ(1-θ))versus lg c ,the values of lg K ,y and ΔG ads are calculated and given in Table 4.The values of ΔG ads obtained indicate that the inhibitor studied is strongly adsorbed on the mild steel surface at all the tempera-ture ranges studied but the adsorption increases with the increase in temperature.The effective activation energies show higher values in the presence of inhibitor than in the absence of it.Therefore,the inhibitor retards the corrosion process at lower temperature but this inhibition action is reduced at higher tem-perature.The calculated effective activation energies show that the imidazoline derivative inhibits corrosion more effectively at higher concentrations.The values of 1/y show that at lower temperature onemolecule occupies more active sites than at higher temperature.There may be changed in orientation of imidazole rings at higher temperature causing the molecule to occupy less active sites.2.5Interfacial capacitanceIt is obvious that only in the case of the geometric blocking effect does the inhibition efficiency (η)equal the coverage of the adsorbed inhibitive species (θ)on the metal surface.To verify this point,one can study the relationship between ηand relative coverage (θ).The latter is estimated by interfacial capacitance measurements according to the following equation:y =1-C c /C 0=(1-C s /C 0)θ(6)Here C c and C 0are the interfacial capacitance of the metal elec-trode in solutions with or without the inhibitor,and C s is the in-terfacial capacitance of the electrode when θequals 1.If ηequals θ,the plot of y vs θmust be a straight line passing through the origin.Fig.5shows the straight line through the ori-gin almost.From the slope of the line one can estimate the value of C s that cannot be measured directly,and the value of C s is 38.4μF ·cm -2.2.6Quantum chemical analysisFig.6gives the optimized geometry of the inhititor ion and the various optimized parameters are given in Table 5.The energies of HOMO and LUMO of iron taken from the literature [19]equal to -7.81and -0.25eV,respectively.Fig.6suggests that the in-hibitor is non-planar molecule with no symmetry elements.Therefore,the coverage of the surface is not as uniform as ob-served for planar molecules.Therefore,adsorption is assisted by the already adsorbed anions like Cl -,giving synergistic effects.Value of dipole moment suggests that it is a polar compound and can easily donate π⁃electrons forming strong d π⁃p πbond-ing [22].Charge on phosphorous atom is found to be 1.8C and there is increased negative charge on the N attached to the central PTable 2The simulative electrochemical parameters for Q235steel in saltwater in the absence and presence of inhibitorc-1t /h R s /ΩR ct (k Ω·cm )R 2(Ω·cm )L (kH ·cm η(%)0- 1.1030.831---2511.2494.0177.4200.184.4240.9945.26110.1275.286.048 1.073 6.10918.5320.488.372 2.3747.55626.6389.789.596 1.6518.90538.7419.590.41201.59010.15449.6478.191.01441.71310.81562.4550.290.5C dl (μF ·cm )229.9(0.882)120.1(0.726)93.8(0.819)79.1(0.837)60.4(0.859)47.1(0.884)37.9(0.905)35.1(0.922)(n )c /(mg ·L -1)E a /(kJ ·mol -1)ΔH 0a /(kJ·mol -1)ΔS 0a /(J·mol -1·K -1)034.88632.28273.09540.23238.63289.311048.57945.19298.531555.14955.69307.122059.71759.09327.752564.48264.81337.48Table 3Calculated values of E a for the corrosion of Q235steel in saltwater in the presence of inhibitorT /K ln K y ΔG ads /(kJ ·mol -1)R 229821.50.84-38.50.97930823.90.87-39.90.98431825.50.86-41.20.98232826.90.96-43.10.980Table 4ln K and free Gibbs energy (ΔG ads )of inhibitoradsorption obtained from potentiodynamic polarizationFig.5Relationship between y and 兹in saltwater with varying concentrations of inhibitor141Acta Phys.鄄Chim.Sin.,2008Vol.24atom and O compared to other atoms in the molecule.This sug-gests that these could be possible centers of adsorption.To de-termine the type of interaction between iron and the inhibitor by molecular orbital approach,the energies of frontier orbitals are considered.Since E HOMO (Fe)-E LUM O (In)(-7.16647eV)is less com-pared to E HOM O (In)-E LUMO (Fe)(-9.63292eV),there is a strong possi-bility of electrons from Fe to be given to the inhibitor further strengthening the adsorption.The introduction of the ρE term shows that the higher the elec-trophilic frontier electron density of atom,the higher the activity,which shows that the ability of the atom to donate electrons has an important effect on the herbicidal activity.And the ρN E shows that the greater the ability of the carbon atom to accept electrons,the higher the activity.The results indicated that the imidazole ring was an electron-donating site binding to the electropositiveregion of receptor and the P atom was an electron-accepting site binding to the electronegative region of receptor (Table 6).3Conclusions(1)From the corrosion point view,imidazoline derivative acts as good-mixed inhibitor system.(2)The inhibitor is found to affect both the anodic and cathod-ic processes and its inhibition efficiency increases with the in-hibitor concentration,but increases then decreases with tempera-ture.(3)The modified form of Langmuir adsorption isotherm known as thermodynamic/kinetic model was found to be closest to the description of the adsorption behavior of the studied inhibitor.(4)Molecule as a whole is involved in the adsorption process through the π鄄electron density and N atoms which are well spread over the molecule.(5)Quantum chemical calculations show that the inhibitor molecules also act as electron acceptor when they interact with mild steel surface.References1Zhang,J.P.;Zhang,Q.Y.;Ren,H.;Zhao,W.;Zhang,H.P.Appl.Surf.Sci.,2007,253:74162Quraishi,M.A.;Khan,M.A.W.;Jamal,D.;Ajmal,M.;Muralidharan,S.;Iyer,S.V.K.J.Appl.Electrochem.,1996,26:12533Rodrrguez ⁃Valdez,L.M.;Villamisar,W.;Casales,M.Corros.Sci.,2006,48:40534Mernari,B.;Attari,H.E.;Traisnel,M.;Bentiss,F.;Lagrenree,M.Corros.Sci.,1998,40:3915El 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Corros.J.,1997,32:4922Bhrara,K.;Singh,G.Appl.Surf.Sci.,2006,253:846Fig.6Optimized geometry of the inhibitor ion Parameter Value total anergy (eV)-5014.11622energy of HOMO (eV)-9.88292energy of LUMO (eV)-0.64353dipole (10-30C ·m)9.316molecular point group C 1electronic energy (eV)-44792.19827total charge on N3(C)-0.2588total charge on N4(C)-0.0658total charge on N8(C)-0.5111total charge on O12(C)-0.6615total charge on P13(C) 1.79total charge on O14(C)-0.9584total charge on O15(C)-0.9960total charge on O16(C)-0.6943Table 5Optimized PM6parameters forthe inhibitor using MOPAC 7.0Atom ρEρNN30.407751480.00037307N40.006799000.00084800N80.000026950.00516491P130.000006310.00101376Table 6The values of ρN(nucleophilic frontier electron density)and ρE (electrophilic frontierelectron density)142。