Experimental and Theoretical Study on the Corrosion Inhibition of Mild

Article pubs.acs.org/IECR

Experimental and Theoretical Study on the Corrosion Inhibition of Mild Steel by 1‑Octyl-3-methylimidazolium L‑Prolinate in Sulfuric Acid Solution Xingwen Zheng,*,†,‡,§ Shengtao Zhang,† Min Gong,§ and Wenpo Li† †

College of Chemistry and Chemical Engineering, Chongqing University, Chongqing 400044, China School of Chemical and Pharmaceutical Engineering, Sichuan University of Science & Engineering, Zigong 643000, China § Key Laboratory of Material Corrosion and Protection of Sichuan Province, Zigong 643000, China ‡

S Supporting Information *

ABSTRACT: A newly amino acid ionic liquid, 1-octyl-3-methylimidazolium L-prolinate ([Omim]Lpro), was investigated as the inhibitor for mild steel in 0.5 M H2SO4 solution using weight loss method, electrochemical measurements, scanning electron microscopy (SEM), and quantum chemical calculation. The obtained results revealed that [Omim]Lpro was a mixed-type inhibitor with a predominantly cathodic action for mild steel in 0.5 M H2SO4 solution, and inhibition efficiency reached nearly 80% at the concentration of 10 mM, in which the Omim cation played a major role in the corrosion inhibition of [Omim]Lpro. The adsorption of [Omim]Lpro on the mild steel surface was found to obey the El-Awady thermodynamic-kinetic model and Flory−Huggins isotherm equations; thus the thermodynamic and kinetic parameters governing the adsorption process were calculated and discussed. Moreover, quantum chemical calculation gave further insight into the mechanism of inhibition of [Omim]Lpro. the 1-butyl-3-methylimidazolium tetrafluoroborate,42 1-butyl-3methylimidazolium chloride,43,49 1-butyl-3-methylimidazolium hydrogen sulfate,43 1-hexyl-3-methylimidazolium chloride,49 1octyl-3-methylimidazolium chloride,49 1-butyl-3-methylimidazolium bromide,44 1-vinyl-3-butylimidazolium bromide,45 1vinyl-3-octylimidazolium bromide,45 1-vinyl-3-dodecylimidazolium bromide,45 1-vinyl-3-octadecylimidazolium bromide,45 1vinyl-3-docosylimidazolium bromide,45 1-dodecyl-4-methoxypyridinium bromide,46 tetradecylpyridinium bromide,50 1,3dioctadecylimidazolium bromide,47 n-octadecylpyridinium bromide,47 1-propyl-3-methylimidazolium bis(trifluoromethyl-sulfonyl) imide, 4 8 1-butyl-3-methylimidazolium bis(trifluoromethyl-sulfonyl) imide,48 1-hexyl-3-methylimidazolium bis(trifluoromethyl-sulfonyl) imide,48 and 1-propyl-2,3methylimidazolium bis(trifluoromethyl-sulfonyl) imide.48 Despite the facts that the imidazole-based amino acid ionic liquid is composed of imidazolium cation and amino acid anion, and the corrosion inhibition of imidazole29−32 and amino acid14−17 have been investigated by many researchers, the studies on the imidazole-based amino acid ionic liquid as an inhibitor are still scarce. Consequently, the prime aims of this Article are to evaluate the inhibition effectiveness of the inhibitor 1-octyl-3methylimidazolium L-prolinate ([Omim]Lpro) on the mild steel corrosion in 0.5 M H2SO4 solution and to investigate the synergistic effect between 1-octyl-3-methylimidazolium cation (Omim) and L-Proline (Lpro).

1. INTRODUCTION Corrosion inhibitor used as an additive can effectively inhibit the corrosion of metals in liquid medium, and has been widely applied in many industrial fields, such as in acid pickling of steel components, cooling water recirculating systems, oil production, oil refining, and so on.1,2 However, the traditional corrosion inhibitors, especially some inorganic substances, such as chromates, nitrites, and phosphates, have been restricted to some extent in terms of potential environmental toxicity and risk.3−6 Therefore, the development of an environmentally friendly corrosion inhibitor, taking the natural plant extract7−13 and amino acid14−17 as examples, has received much attention in current research. Ionic liquid is an organic liquid substance at ambient temperature and is composed of only ions, usually a specific organic cation and an inorganic anion. Because of the unique properties of ionic liquids as well as the synthesization of many new ionic liquids, ionic liquids have been applied to many academic and industrial fields, including the electrochemical,18 synthetic,19 catalytic,20,21 material,22−24 separation,25,26 and biotechnological fields.27 Noticeably, the ionic liquids are being increasingly considered as one of the most promising alternative chemicals in the future.28 Generally, a typical ionic liquid has a bulky organic cation, such as N-alkylimidazolium, alkylammonium, alkylpyridinium, and alkylphosphonium. The imidazole,29−32 ammonium,33−35 pyridine,36−38 and phosphonium39,40 have been used as corrosion inhibitor. Because the number of ionic liquid is tremendous in theory,41 there is a great potential to explore the ionic liquid as a novel green corrosion inhibitor. Presently, ionic liquids have reportedly been used as inhibitors for carbon steel42−48 and aluminum,49,50 including © XXXX American Chemical Society

Received: June 27, 2014 Revised: September 19, 2014 Accepted: October 1, 2014

A

dx.doi.org/10.1021/ie502578q | Ind. Eng. Chem. Res. XXXX, XXX, XXX−XXX

Industrial & Engineering Chemistry Research

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2. EXPERIMENTAL SECTION 2.1. Materials and Sample Preparation. The chemical composition of mild steel used in the experiments (wt %) was C (0.16%), Si (0.18%), Mn (0.29%), P (0.014%), S (0.013%), and Fe for balance. The specimens used for weight loss test, surface analysis, and electrochemical measurements were cut into 50 mm × 25 mm × 5 mm, 15 mm × 10 mm × 5 mm, and 10 mm × 5 mm, respectively. The exposed surface area of electrochemical specimen was 0.785 cm2, while the remainder was embedded by epoxy. Prior to all measurements, the specimens were mechanically abraded with emery paper up to 1000 grit, then rinsed with distilled water, degreased in acetone, and dried at room temperature. The corrosive solution 0.5 M H2SO4 was prepared using analytical grade sulfuric acid and distilled water. The test inhibitors [Omim]Lpro and 1-octyl-3-methylimidazolium bromide ([Omim]Br) were purchased from Shanghai Cheng Jie Chemical Chemical Co. Ltd. with the purity of 99% and were not purified further before use. Lpro was purchased from Shanghai Ru Ji Biological Technology Development Co. Ltd. The molecular structure of [Omim]Lpro is given in Figure 1.

solution without and with 10 mM [Omim]Lpro for 4 h at 298 K using Tescan Vega3 SEM instrument at high vacuum. 2.5. Quantum Chemical Study. The molecular structures of the Omim cation and Lpro cation were geometrically optimized by density functional theory (DFT) using B3LYP functional with 6-311++G(d,p) basis set as implemented in the Gaussian 03W program. Quantum chemical parameters including the energy of highest occupied molecular orbital (EHOMO), the energy of lowest unoccupied molecular orbital (ELUMO), energy gap (ΔE = ELUMO − EHOMO), Mulliken charge, and dipole moment (μ) were calculated.

3. RESULTS AND DISCUSSION 3.1. Weight Loss Method. Corrosion parameters obtained from weight loss measurements for mild steel in 0.5 M H2SO4 Table 1. Corrosion Parameters Obtained from Weight Loss Measurements for Mild Steel in 0.5 M H2SO4 Solution without (Blank) and with Different Concentrations of [Omim]Lpro for 4 h at Different Temperatures T (K)

C (mM)

V− (g m−2 h−1)

298

blank 0.5 1.0 2.5 5.0 10.0 blank 0.5 1.0 2.5 5.0 10.0 blank 0.5 1.0 2.5 5.0 10.0

10.79 5.81 4.84 3.85 3.04 2.49 22.21 13.36 11.16 8.75 7.07 5.60 36.86 42.01 30.11 22.70 16.89 11.85

308

Figure 1. Molecular structure of [Omim]Lpr.

During the weight loss and electrochemical measurements, the temperature of the solution was controlled by a water thermostat with an accuracy of 1 K, and all experiments were open to the air and carried out under static conditions. 2.2. Weight Loss Measurement. Cleaned and weighed mild steel samples were immersed in 0.5 M H2SO4 solution in the absence and presence of different concentrations of [Omim]Lpro for 4 h at 298, 308, and 318 K. After that, the samples were taken out, scrubbed with a bristle brush, cleaned by distilled water and acetone, then dried and weighed. Triplicate experiments were performed in each test, and average values were calculated and used as results. 2.3. Electrochemical Measurement. A traditional threeelectrode cell assembly with mild steel working electrode, saturated calomel electrode (SCE), and platinum electrode was used for electrochemical measurements using the PARSTAT 2273 advanced electrochemical system. Before the experiments were started, the working electrode was immersed in test solution for 30 min at open circuit potential (OCP) to reach a steady state; correspondingly, the OCP−time curves are depicted in Figure S1 in the Supporting Information. Electrochemical impedance spectroscopy (EIS) measurements were performed in the frequency range of 10−2 to 105 Hz using a sinusoidal AC perturbation with amplitude of 5 mV peak-topeak at OCP. The EIS parameters were obtained by fitting the experimental data to an appropriate equivalent circuit using ZSimpWin software. The potentiodynamic polarization curves were obtained in the potential range of −250 to +250 mV versus OCP with a scan rate of 0.5 mV/s, and the data were collected and analyzed by electrochemical software PowerSuite ver. 2.58. 2.4. Surface Analysis. The surfaces of mild steel specimens were measured before and after immersion in 0.5 M H2SO4

318

η (%)

θ

46.2 55.1 64.3 71.9 76.9

0.462 0.551 0.643 0.719 0.769

39.9 49.7 60.6 68.2 74.8

0.399 0.497 0.606 0.682 0.748

−14.0 18.3 38.4 54.2 67.9

−0.140 0.183 0.384 0.542 0.679

solution without (blank) and with different concentrations of [Omim]Lpro for 4 h at different temperatures (298−318 K) are shown in Table 1. The corrosion rate (v), inhibition efficiency (η), and surface coverage (θ) are calculated from the following equations:

ν=

W St

η (%) =

θ=

(1)

ν0 − v × 100 v0

ν0 − v v0

(2)

(3)

where W is the weight loss of the mild steel sample, S is the total surface area of the sample, t is the immersion time, and v0 and v are the corrosion rates of the mild steel sample without and with inhibitor, respectively. In general, the results demonstrate that [Omim]Lpro inhibits the corrosion of mild steel in 0.5 M H2SO4 solution, and the corrosion rate of mild steel reduces while the inhibition efficiency of [Omim]Lpro increases with the increase of B



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Table 2. Corrosion Parameters Obtained from Weight Loss Measurements for Mild Steel in 0.5 M H2SO4 Solution with Different Inhibitors for 4 h at 298 K [Omim]Lpro C (mM) blank 0.5 1.0 2.5 5.0 10.0

−

V (g m

−2

−1

[Omim]Br −

η (%)

h )

10.79 5.81 4.84 3.85 3.04 2.49

V (g m

−2

−1

h )

10.79 9.62 8.98 6.48 3.89 0.94

46.2 55.1 64.3 71.9 76.9

Lpro η (%) 10.9 16.8 40.0 63.9 91.3

−

V (g m

−2

10.79 12.17 11.52 9.76 9.41 7.60

−1

h )

η (%)

S

−12.8 −6.7 9.6 12.8 29.6

1.87 1.98 1.52 1.12 0.27

Figure 2. (a) Polarization curves for mild steel in 0.5 mol/L H2SO4 solution without and with different concentrations of [Omim]Lpro at 298 K. (b) The typical example of Tafel extrapolation method for polarization curve of mild steel in 0.5 M H2SO4 solution (blank solution). Figure 3. Nyqusit (a) and Bode (b) plots for mild steel in 0.5 mol/L H2SO4 solution without and with different concentrations of [Omim]Lpro at 298 K.

Table 3. Electrochemical Parameters for Mild Steel in 0.5 mol/L H2SO4 Solution without and with Different Concentrations of [Omim]Lpro at 298 K C (mM)

Ecorr (V)

Icorr (mA cm−2)

βa (mV dec−1)

βc (mV dec−1)

η (%)

blank 0.5 1.0 2.5 5.0 10.0

−0.4528 −0.4617 −0.4572 −0.4618 −0.4709 −0.4714

0.6789 0.4191 0.3290 0.2489 0.1982 0.1828

46.4 55.8 51.1 51.8 54.6 54.3

−113.6 −121.6 −132.4 −130.5 −140.2 −151.6

38.3 51.5 63.3 70.8 73.1

Figure 4. Equivalent circuit model used to fit the EIS for mild steel in test solution.

[Omim]Lpro concentration. However, due to the intensified molecular thermal motion under increased temperature, the corrosion rate of mild steel is improved and the inhibition efficiency of [Omim]Lpro is decreased with rising temperature, indicating that the corrosion inhibition of [Omim]Lpro is

strongly influenced by temperature. Consequently, at 318 K, the corrosion rate of mild steel in solution containing 0.5 mM [Omim]Lpro 0.5 M H2SO4 is higher than that in blank C



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Table 4. EIS Parameters for Mild Steel in 0.5 mol/L H2SO4 Solution without and with Different Concentrations of [Omim]Lpro at 298 K C (mM)

Rs (Ω cm2)

Y0 × 10−6 (S sn cm−2)

n

Rct (Ω cm2)

Cdl (μF cm−2)

L (Ω cm2)

RL (Ω cm2)

Rp (Ω cm2)

η (%)

blank 0.5 1.0 2.5 5.0 10.0

1.1 1.1 1.2 1.6 1.2 1.5

183.2 153.4 144.0 153.5 154.1 133.3

0.89 0.88 0.87 0.86 0.84 0.82

38.0 65.7 79.7 97.7 141.6 213.3

98.7 81.9 75.3 74.1 72.5 63.5

119 183 244 276 425 1034

267 196 268 306 368 533

33.3 49.2 61.4 74.1 102.3 152.3

42.2 52.3 61.1 73.2 82.2

Figure 5. El-Awady (a) and Flory−Huggins (b) isotherm equations for mild steel in 0.5 M H2SO4 solution containing different concentrations of [Omim]Lpro at different temperatures.

solution. This may be related to the surface activity of [Omim]Lpro. Because of the weak inhibition effect of [Omim]Lpro at low concentration, a large amount of hydrogen can be quickly generated, which stir the solution and form bubbles on the mild steel surface. Also, then the bubbles burst, generating the impact force to promote the corrosion of mild steel. Ionic liquid [Omim]Lpro completely ionizes in aqueous solution to give Omim cation and Lpro anion. However, because of the high concentration of hydrogen ions in acid solution, Lpro anion and Lpro both will be protonated forming Lpro cation. Hence, to investigate the contribution of Omim cation and Lpro anion for [Omim]Lpro as inhibitor of mild steel in 0.5 M H2SO4 solution, the corrosion inhibitions of [Omim]Br and Lpro have also been examined by weight loss method; the results are given in Table 2. As can be seen from Table 2, the inhibitive efficiency of Lpro is lower than that of [Omim]Br at the same concentration, which suggests that Omim cation plays a major role in the corrosion inhibition of [Omim]Lpro. The results also illustrate that [Omim]Lpro has a higher inhibitive efficiency than [Omim]Br at low concentration, implying a synergistic effect between Omim cation and Lpro anion. Thus, the synergies coefficient (S) is calculated and given in Table 2 using the following equation:51

Figure 6. Arrhenius plots (a) and transition state plots (b) for mild steel in 0.5 M H2SO4 solution without and with different concentrations of [Omim]Lpro.

S=

1 − θ1 − θ2 + θ1θ 2 1 − θ1 + 2

(4)

where θ1, θ2, and θ1+2 are the surface coverages of [Omim]Br, Lpro, and [Omim]Lpro, respectively. It can be seen that the values of S are greater than 1 at low concentration (0.5−5 mM), indicating that the interaction between Omim and Lpro is a synergistic effect. However, when the concentration reaches 10 mM, the value of S becomes less than 1. This means that an antagonistic effect occurs between Omim and Lpro, which may be related to the competitive adsorption of Omim and Lpro cation on the mild steel surface.51 3.2. Potentiodynamic Polarization Curves. Figure 2a gives the potentiodynamic polarization curves for mild steel in 0.5 M H2SO4 solution containing different concentrations of [Omim]Lpro at 298 K. It is obvious that the mild steel electrode immersed in test solution shows a cathodic region of Tafel behavior; however, the anodic polarization curve does not D



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cathodic sites; however, it is obvious that the inhibitory effect of [Omim]Lpro on the cathode reaction is significantly greater than that on the anode reaction. Additionally, the data in Table 3 clearly show the displacement in Ecorr is less than 20 mV, and the Ecorr shifts toward the negative side when increasing the inhibitor concentration, which means [Omim]Lpro is a mixedtype inhibitor with a predominantly cathodic action.9,48 Table 3 also illustrates both βa and βc values change after adding inhibitor; this indicates that [Omim]Lpro inhibits both the anodic and the cathodic reactions, and the concentration of inhibitor has a slight impact on the values of βa but a more significant influence on the values of βc. The shift in βc indicates the influence of the inhibitor molecules on the kinetics of hydrogen evolution.53,54 The values of βa that changed in inhibited solution may be due to the adsorption of sulfate ion or inhibitor molecules on the mild steel surface. 55,56 Furthermore, there is a good correlation between the inhibition efficiencies obtained from potentiodynamic polarization and weight loss experiments. 3.3. EIS Measurements. The Nyquist and Bode plots of mild steel obtained in 0.5 M H2SO4 solution with various concentrations of [Omim]Lpro at 298 K are shown in Figure 3. It is shown that the Nyquist plot for mild steel in the test solution consists of a depressed capacitive loop at high frequencies (HF) and an inductive loop at low frequencies (LF). The capacitive loop attributed to the time constant of charge transfer and double layer capacitance (Cdl) is not a perfect semicircle in acid solution,12,57 which often refers to frequency dispersion as a result of the roughness and inhomogeneity of electrode surface.58−60 The LF inductive loop may be attributed to the relaxation process obtained by adsorption species as (SO42−)ads and (H+)ads on the electrode surface, exhibiting negative change in the surface coverage with potential on the surface of the mild steel.12,57 An electrical circuit given in Figure 4 is employed to fit the obtained EIS data.12,57 In the circuit, Rs is the solution resistance, Rct is the charge transfer resistance, L and RL represent the inductive elements, and CPE is the constant phase element and can be described as follows:58,60,61 ZCPE =

1 Y0(jω)n

(5)

where Y0 is the CPE constant, n is the phase shift, which can be explained as a degree of surface inhomogeneity,61,62 j is the imaginary unit, and ω is the angular frequency. Thus, the value of Cdl is calculated according to the following equation:63,64

Figure 7. SEM images of mild steel surface: (a) not immersed, (b) immersed in 0.5 M H2SO4 solution, and (c) immersed in 0.5 M H2SO4 solution with 10.0 Mm [Omim]Lpro at 298 K for 4 h.

Cdl = Y0(ω)n − 1 = Y0(2πfZ

im‐Max

)n − 1

(6)

where ω is the angular frequency at the maximum value of the imaginary part (Zim‑Max) of the impedance spectrum. The impedance parameters including Rs, Rct, Y0, n, Cdl, L, RL, polarization resistance (Rp), and inhibition efficiency (η) obtained from EIS are reported in Table 4. The polarization resistance65 and inhibition efficiency are defined as follows:

display an extensive Tafel region, which may be due to passivation and pitting52 or the deposition of corrosion products on the mild steel surface.53 Accordingly, the electrochemical parameters, including corrosion potential (Ecorr), corrosion current density (Icorr), anodic Tafel slope (βa), and cathodic Tafel slope (βc), are determined by Tafel extrapolation method, and a representative example is shown in Figure 2b. The electrochemical parameters and inhibition efficiency (η) are listed in Table 3. As it is shown in Figure 2a, in the presence of inhibitor, both the anodic and the cathodic reactions are suppressed, suggesting that [Omim]Lpro reduces the mild steel anodic dissolution and also retards the hydrogen ions reduction on the

Rp = η=

R ct × RL R ct + RL

R ct − R ct,0 R ct

(7)

× 100%

(8)

where Rct,0 and Rct are the charge transfer resistances without and with inhibitor, respectively. E



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Figure 8. Optimized geometric structures and the distributions of HOMO and LUMO for Omim cation (a) and Lpro cation (b).

molecules act by adsorption on the metal/solution interface.66,67 The n values in the range of 0.89−0.82 are close to unity, suggesting that the CPE is related to the capacitance58,63 and the dissolution mechanism of mild steel in test solution is controlled by the charge transfer process.9,54 However, the values of n decrease with the increase of inhibitor concentration, which may be due to the adsorption of the inhibitor. Inhibition efficiencies obtained from EIS are in good correspondence with the data obtained by weight loss and potentiodynamic polarization tests. 3.4. Adsorption Isotherm and Thermodynamic Analysis. The corrosion inhibition of organic molecules mainly depends on their adsorption capacity on the metal surface. The adsorption isotherm can provide basic information about the

Table 5. Quantum Chemical Parameters for the Omim and Lpro Cations molecules

EHOMO (eV)

ELUMO (eV)

ΔE (eV)

μ (D)

Omim cation Lpro cation

−10.619 −12.871

−5.039 −5.046

5.580 7.825

15.189 3.579

It is apparent from Table 4 that the values of Rct/Rp increase and the Cdl values decrease with the increase of [Omim]Lpro concentration; this may be due to the increase in the surface coverage on the electrode surface by the adsorption of inhibitor, which also leads to the increase of inhibition efficiencies. The decreased value of Cdl indicates a decrease in local dielectric constant and/or an increase in the thickness of the electrical double layer, suggesting that the inhibitor F



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interaction between the inhibitor and the metal surface. The surface coverage (θ), which is often used to characterize the adsorption of inhibitor, has been obtained from weight loss measurements and is reported in Table 1. Hence, to obtain more information about the mode of adsorption of [Omim]Lpro on mild steel surface, various adsorption isotherms are employed to fit the experimental data. Notably, the El-Awady thermodynamic-kinetic model68,69 and the Flory−Huggins68,70 adsorption isotherm are found to be the most suitable isotherms to explain the adsorption behavior of the investigated inhibitor; the relationships between the surface coverage (θ) and the concentration of the inhibitor (C) can be determined by the following equation, respectively: ln

θ = y ln C + ln K ′ 1−θ

ln

θ = x ln(1 − θ ) + ln(xK ads) C

ln ν = −

ln

⎛ ΔGads ⎞ 1 ⎟ exp⎜ − ⎝ 55.5 RT ⎠

ΔSa ΔHa ν R = ln + − T Nh R RT

(12)

(13)

where v is the corrosion rate, R is the universal gas constant, T is the thermodynamic temperature, λ is the pre-exponential factor, h is Planck’s constant, N is Avogadro’s number, Ea is the apparent activation energy, ΔSa is the apparent entropy of activation, and ΔHa is the apparent enthalpy of activation. The activation parameters, including Ea, λ, ΔHa, and ΔSa, at different concentrations of the inhibitors are calculated by Arrhenius and transition state equations using weight loss data from Table 1. The results give straight lines as shown in Figure 6, and the calculated values are presented in Table S2 (Supporting Information). It is evident from Supporting Information Table S2 that the values of Ea and λ follow the same trend in inhibited solutions and are higher than those in uninhibited solution. Usually, an increase in Ea after addition of inhibitor indicates that a physical (electrostatic) adsorption occurred in the first stage.53,74,75 However, the values of Ea gradually decrease when increasing the inhibitor concentration, which can be due to the chemisorption of the inhibitor.53,74,75 This result corresponds to the thermodynamic analysis as discussed before in section 3.4. Additionally, the positive values of ΔHa reveal the endothermic nature of the dissolution process for the steel in H2SO4 solution53,72,73 and indicate that the dissolution of steel is difficult.53,75 Moreover, the values of Ea and ΔHa change in the same manner, which can be qualified by the known thermodynamic equation ΔHa = Ea − RT.74 Supporting Information Table S2 also shows that the values of ΔSa of mild steel dissolution reaction in 0.5 M H2SO4 solution in the presence of [Omim]Lpro are higher than those in the blank solution, which means an increase in disorder takes place during the course of the transition from reactant to the activated complex during the corrosion process.53,73 However, the values of ΔSa decrease with the increase of inhibitor concentration, meaning a decrease in disorder of inhibitor molecules on the surface attributed to the formation of stable adsorption film on the mild steel surface.75,76 3.6. Surface Morphology. The SEM images of mild steel specimens before and after being immersed in 0.5 M H2SO4 solution without and with 10 mM [Omim]Lpro for 4 h at 298 K are shown in Figure 7. The results show that the surface was badly damaged due to the attack of the corrosive solution in the absence of inhibitor. Comparatively, the steel surface is much less damaged in the presence of 10.0 mM [Omim]Lpro, which reflects the corrosion inhibition of [Omim]Lpro. 3.7. Quantum Chemical Calculations. To further understand the inhibitive mechanism of [Omim]Lpro, quantum chemical calculations have been performed. Specifically, the optimized geometric structures and the distributions of HOMO and LUMO for Omim and Lpro cations are given in Figure 8, whereas the calculated quantum chemical parameters including EHOMO, ELUMO, ΔE, and μ are reported in Table 5. According to the frontier molecular orbital theory, EHOMO is often related to the ability of the molecule to donate electrons, and a high EHOMO value means a strong electron-donating ability.53,56,77 ELUMO is related to the ability of the molecule to accept electrons; the lower is the value of ELUMO, the much easier could it be for the molecule to accept electrons.53,56,77 It

(9)

(10)

where x is the number of inhibitor molecules occupying one active site or the number of water molecules replaced by one molecule of adsorbate, x = 1/y, Kads is the adsorptive equilibrium constant, and Kads = K′(1/y).68−70 The adsorptive equilibrium constant Kads is related to the standard free energy of adsorption ΔGads by the following equation:70 K ads =

Ea + ln λ RT

(11)

where the value of 55.5 is the molar concentration of water in the solution expressed in mol/L, R is the universal gas constant, and T is the thermodynamic temperature. As expected, the fitting curves give straight lines as shown in Figure 5. The calculated values of x, y, 1/y, Kads, and ΔGads are listed in Table S1 (Supporting Information). The results show that the two isotherms of the El-Awady and Flory−Huggins models yield similar results at the same temperature, indicating that the used isotherm models are appropriate to fit the experimental data. According to Supporting Information Table S1, the values of 1/y or x are greater than 1 at 298 and 308 K, which means one inhibitor molecule replaces more than one water molecule, and there is no significant difference of the values at 298 and 308 K. However, the value 1/y or x is close to 1 at 318 K, indicating that one inhibitor molecule replaces one water molecule, and the values of 1/y or x are significantly different at 308 and 318 K. The difference may reflect the change in adsorption mode for inhibitor on the mild steel at different temperatures. The values of Kads decrease with the increase of temperature, which explains the decrease in the inhibition efficiency with increasing temperature. Moreover, the values of ΔGads between −20 and −40 kJ/mol suggest a strong interaction between the inhibitor and the surface of mild steel, and the interaction involves both physisorption and chemisorption.70,71 3.5. Activation Parameters. Temperature has a significant impact on the corrosion rate of metal and the performance of inhibitor. Consequently, to further understand the inhibitive mechanism of [Omim]Lpro, the influence of temperature on the corrosion rate of mild steel in 0.5 M H2SO4 solution containing different concentrations of [Omim]Lpro has been investigated using Arrhenius and transition state equations,53,72−76 respectively: G



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was reported previously that the low value of ΔE and high value of μ are responsible for enhancing the inhibition effciency.53,66,72 Remarkably, the ELUMO values of Omim and Lpro cations are very close, but the values of EHOMO and μ are higher and the ΔE value is lower for Omim cation than those for Lpro cation. All of these mean Omim cation has stronger corrosion inhibition than Lpro cation, which is consistent with the result in Table 2 obtained from weight loss tests. 3.8. Mechanism of Inhibition. Generally, the corrosion inhibition mechanism in acid medium is the adsorption of inhibitor onto the metal surface. According to the results obtained from the experiments and theoretical analysis, [Omim]Lpro is a mixed-type inhibitor with a predominantly cathodic action for mild steel in 0.5 M H2SO4 solution, which mainly physically adsorbs on the mild steel surface, as well as to some extent existing as chemical adsorption. In acidic solutions, [Omim]Lpro exists predominantly as protonated species (Omim cation and Lpro cation); these cations may directly adsorb on the cathodic sites of the mild steel surface and decrease the evolution of hydrogen. On the other hand, the adsorption on anodic sites probably occurs through the π electrons of the imidazole ring and the free lone-pair electrons of heteroatoms (nitrogen and oxygen), which decreases the anodic dissolution of mild steel.2,8,13 However, due to the weak electron-donating ability of cations, the tendency of chemical adsorption is very little. In addition, it can be presumed that when the concentration of [Omim]Lpro is lower, due to the sufficient active adsorption sites in the cathode region, there is no competition between Omim cation and Lpro cation, showing a synergistic effect, but when the concentration of [Omim]Lpro reaches a critical value, Omim cation and Lpro cation will adsorb on the cathodic active sites through the competition, displaying an antagonistic effect.

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This project is supported financially by the Program of Education Department of Sichuan Province (no. 11ZB245), and the Opening Project of Key Laboratory of Material Corrosion and Protection of Sichuan Province (no. 2012CL05).

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4. CONCLUSIONS [Omim]Lpro is a mixed-type inhibitor with a predominantly cathodic action for mild steel in 0.5 M H2SO4 solution, and the Omim cation plays a major role in the corrrosion inhibition of [Omim]Lpro. The experimental results demonstrated that a synergistic effect between the Omim cation and Lpro anion occurs at low concentration, while an antagonistic effect happens at high concentration. The inhibition efficiencies could be increased with the increase of [Omim]Lpro concentration, but also be decreased by rising temperature. The adsorption of [Omim]Lpro on the mild steel surface is found to obey the El-Awady thermodynamic-kinetic model and Flory−Huggins isotherm equation.

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ASSOCIATED CONTENT

* Supporting Information S

Figure S1 illustrates the OCP−time curves for mild steel in 0.5 M H2SO4 solution containing different concentrations of [Omim]Lpro at 298 K. Tables S1 and S2 list the results of adsorption parameters and activation parameters for mild steel in 0.5 M H2SO4 solution containing different concentrations of [Omim]Lpro at various temperatures. This material is available free of charge via the Internet at http://pubs.acs.org.

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REFERENCES

AUTHOR INFORMATION

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*Tel.: +86-13568337780. E-mail: [email protected]. H



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Experimental and Theoretical Study on the Corrosion Inhibition of Mild

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