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Electrochemical and theoretical studies of adsorption and corrosion inhibition of aniline violet compound on carbon steel in acidic solution
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Electrochemical and theoretical studies of adsorption and corrosion inhibition of aniline violet compound on carbon steel in acidic solution Hojat Jafari a,∗, Koray Sayin b a b
Abadan Faculty of Petroleum Engineering, Petroleum University of Technology, Abadan, Iran Department of Chemistry, Institute of Science, Cumhuriyet University, 58140 Sivas, Turkey
a r t i c l e
i n f o
Article history: Received 4 February 2015 Revised 23 March 2015 Accepted 29 March 2015 Available online xxx Keywords: DFT calculations Carbon steel Electrochemical calculation AFM Acid inhibition
a b s t r a c t The inhibition ability of 4-[bis[4-(dimethylamino)phenyl]methyl]-N,N-dimethylaniline (Mv10B) on carbon steel in 1 M HCl solution was studied by electrochemical techniques. Mv10B inhibited carbon steel corrosion in 1 M HCl solution significantly and the inhibition efficiency increased with Mv10B concentration. Potentiodynamic polarization results showed that Mv10B was a mixed-type inhibitor. The adsorption of Mv10B on steel surface followed Langmuir adsorption isotherm. The inhibition performance of Mv10B was also evidenced by AFM images. Computational investigations of studied inhibitor are performed by using HF method with 6-31++G(d,p) and 6-311++G(d,p) basis sets. © 2015 Published by Elsevier B.V. on behalf of Taiwan Institute of Chemical Engineers.
1. Introduction Over the years, a number of protection measures have been suggested to delay, slow, or stop the corrosion process, the use of corrosion inhibitors is an important method to protect metallic materials against corrosion in acidic medium. The acid solutions are widely used in industry for various purposes, such as acid pickling, acidification of petroleum wells, among others [1]. Most well-known acid inhibitors are organic compounds containing nitrogen, sulfur, and oxygen atoms. The choice of the inhibitor is based on two considerations, first economic consideration and second, should contain the electron cloud on the aromatic ring or the electronegative atoms such as N, O in the relatively long chain compounds. Generally the organic compounds containing hetero atoms like O, N, S, and P are found to work as very effective corrosion inhibitors. The efficiency of these compounds depends upon electron density present around the hetero atoms, the number of adsorption active centers in the molecule and their charge density, molecular size, mode of adsorption, and formation of metallic complexes [2–5]. Adsorption of inhibitors on the metal surface involves the two types of interaction (physical adsorption and chemical adsorption). Physical adsorption is associated with the electrostatic interaction between charged molecules and the charged metal surface. The second one, chemisorption process involves charge sharing or charge
transfer from the inhibitor molecules to the metal surface to form a co-ordinate type bond and takes place in the presence of heteroatoms (P, N, S, O, etc.) with lone pairs of electrons and/or aromatic ring in the molecular structure [6–8]. Density functional theory (DFT) has grown to be a useful theoretical method to interpret experimental results, enabling one to obtain structural parameters for even huge complex molecules, and it can explains the hard and soft acid base (HSAB) behavior of organic molecules, i.e., DFT connects some traditional empirical concepts with quantum mechanical interpretations [9,10]. Therefore, DFT is a very powerful technique to probe the inhibitor/surface interaction and to analyze experimental data. These are the reasons why we selected both electrochemical and DFT methods to evaluate the efficiency of 4-[bis[4-(dimethylamino)phenyl]methyl]-N,N-dimethylaniline. In this work, we report the successful use of 4-[bis [4-(dimethylamino)phenyl]methyl]-N,N-dimethylaniline as ecofriendly corrosion inhibitor for corrosion of steel in 1 M HCl solutions by using several electrochemical techniques like: electrochemical impedance spectroscopy and potentiodynamic polarization; corrosion kinetic parameters were evaluated and surface topology was studied using AFM images. Quantum chemical calculations have been performed using DFT, and several quantum chemical indices are calculated and correlated with the inhibitive effect of inhibitor. 2. Experimental 2.1. Materials
∗
Corresponding author. Tel.: +98 9137118405. E-mail address: [email protected] (H. Jafari).
The carbon steel used had the following chemical composition (wt.%): 0.11 C, 0.56 Mn, 0.03 Si, 0.007 P, 0.005 S, 0.07 Cr,
http://dx.doi.org/10.1016/j.jtice.2015.03.030 1876-1070/© 2015 Published by Elsevier B.V. on behalf of Taiwan Institute of Chemical Engineers.
Please cite this article as: H. Jafari, K. Sayin, Electrochemical and theoretical studies of adsorption and corrosion inhibition of aniline violet compound on carbon steel in acidic solution, Journal of the Taiwan Institute of Chemical Engineers (2015), http://dx.doi.org/10.1016/j.jtice.2015.03.030
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A = −ELUMO
η=
(3)
I−A 2
(4)
1
σ=
(5)
η |EHOMO + ELUMO |
χ=
2
(6)
μ = −χ
(7)
μ2 2η
(8)
Fig. 1. Chemical structure of Mv10B.
ω=
0.03 Ni and balance Fe. The specimens of dimension 1 cm × 1 cm (exposed) × 0.5 cm (isolated with polyester resin) were used for polarization and electrochemical impedance methods. They were polished mechanically using different grade emery papers up to 1200, and washed thoroughly with triple distilled water and degreased with acetone before being immersed in the acid solution. The aggressive solution of 1 M HCl was prepared by dilution of Merck Product HCl. The concentration range of inhibitors employed was varied from 1 × 10−4 M to 5 × 10−3 M. All chemicals used in present work were of reagent-grade Merck product and used as received without further purification. Fig. 1 shows the chemical structure of Mv10B.
N=
σ + = σ xfk +
(13)
2.2. Methods
σ 0 = σ xfk 0
(14)
The apparatus for electrochemical investigations consists of computer controlled Auto Lab potentiostat/galvanostat (PGSTAT302N) corrosion measurement system at a scan rate of 1 m/Vs. The electrochemical experiments were carried out using a conventional three electrode cell assembly at 25 °C. A rectangular platinum foil was used as counter electrode and saturated calomel electrode as the reference electrode. The EIS experiments were conducted in the frequency range of 100 kHz to 0.01 Hz at open circuit potential. The AC potential amplitude was 10 mV. Time interval of 25 min was given for steady state attainment of open circuit potential. Fitting of experimental impedance spectroscopy data to the proposed equivalent circuit was done by means of home written least square software based on the Marquardt method for the optimization of functions and Macdonald weighting for the real and imaginary parts of the impedance [9]. The specimens of size 1 cm × 1 cm × 0.5 cm were abraded with emery paper (up to 1200) to give a homogeneous surface, then washed with distilled water and acetone. The specimens were immersed in 1 M HCl prepared with and without addition of 5 × 10−3 M at 25 °C for 6 h, cleaned with distilled water. The surface morphology of the electrode surface was evaluated by metallographic microscope Neophot32 and atomic force microscopy Nan Surf easyscan2. Computational progresses were done by using GaussView 5.0.8 [11], Gaussian 09 AM64L-G09RevD.01 package programme [12], ChemBioDraw Ultra Version (13.0.0.3015) [13]. HF method was selected as method with 6-31++G(d,p) and 6-311++G(d,p) basis sets for studied inhibitor. All calculations were performed in vacuo. The vibration frequency analyses were indicated that optimized structures of all inhibitors are at stationary points corresponding to local minima without imaginary frequencies. Mentioned quantum chemical descriptors were calculated by using Eqs. (1)–(17) [14–19].
σ − = σ xfk −
(15)
EH+ = EH3 O+ − EH2 O
(16)
PA = Epro.inh. − (Enon−pro.inh. + EH+ )
(17)
EGAP = ELUMO − EHOMO
(1)
I = −EHOMO
(2)
1
ω
+
fk = Pk (N + 1) − Pk (N) −
fk = Pk (N) − Pk (N − 1) 0
fk =
Pk (N + 1) − Pk (N − 1) 2
(9) (10) (11)
(12)
where EH+ , EH3 O+ , EH2 O , Epro. inh., Enon-pro. inh. and PA are total energy of proton, total energy of hydronium, total energy of water, total energy of protonated inhibitor, total energy of non-protonated inhibitor, proton affinity, respectively. 3. Results and discussion 3.1. Electrochemical results Polarization curves were obtained for steel in 1 M HCl solution with and without inhibitors. The polarization exhibits Tafel behavior. Tafel lines which obtained in various concentrations of Mv10B in 1 M HCl solutions were shown in Fig. 2, at 25 °C respectively. The corresponding electrochemical parameters, i.e., corrosion potential (Ecorr versus SCE), corrosion current density (Icorr ), cathodic and anodic Tafel slopes (β a , β c ) and the degree of surface coverage (θ ) values were calculated from these curves and are given in Table 1. The degree of surface coverage for different concentrations of inhibitor is calculated using the following equations [20,21]:
θ=
I − I˙ I
(18)
where I and I˙ are the corrosion current densities without and with corrosion inhibitor, respectively, determined by the intersection of the extrapolated Tafel lines at the corrosion potential for steel in uninhibited and inhibited acid solution. The presence of Mv10B shifts
Please cite this article as: H. Jafari, K. Sayin, Electrochemical and theoretical studies of adsorption and corrosion inhibition of aniline violet compound on carbon steel in acidic solution, Journal of the Taiwan Institute of Chemical Engineers (2015), http://dx.doi.org/10.1016/j.jtice.2015.03.030
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Table 1 Potentiodynamic polarization parameters for the corrosion of steel in 1 M HCl solution in the absence and in the presence of different concentrations of Mv10B at 25 °C. Concentration (M)
−Ecorr (±1) (mV)
Icorr (±5) (μA/cm2 )
β a (±1) (mV/dec)
−β c (±1) (mV/dec)
Rp (±1) (Ω/cm2 )
θ (±10−2 )
Blank 1 × 10−4 5 × 10−4 1 × 10−3 5 × 10−3
470 477 484 503 516
365 158 97 56 17
68 73 69 71 66
112 133 114 115 124
206 444 781 1436 3489
– 0.56 0.74 0.85 0.95
Fig. 2. Anodic and cathodic polarization curves for steel in 1 M HCl without and with various concentration of Mv10B at 25 °C.
both anodic and cathodic branches to the lower values of current densities and thus causes a remarkable decrease in the corrosion rate. It can be clearly seen from Fig. 2 that both anodic metal dissolution of iron and cathodic hydrogen evolution reactions were inhibited after the addition of Mv10B to the aggressive solution. This result is indicative of the adsorption of inhibitor molecules on the active sites of steel surface [22]. The inhibition of both anodic and cathodic reactions is more pronounced with the increasing inhibitor concentration. However, the influence is more pronounced in the cathodic polarization plots compared to that of the anodic polarization plots. The cathodic current–potential curves (Fig. 2) giving rise to parallel lines indicate that the addition of Mv10B to the 1 M HCl solution does not modify the reduction mechanism and the reduction at steel surface takes place mainly through a charge transfer mechanism [23–25]. The slopes do not display an order with the inhibitor concentration; this feature indicates that inhibition occurred by a blocking mechanism on the available metal spaces [26-29]. The corrosion potential displayed small change in the range of −470 to −506 mV versus SCE and curves changed slightly toward the negative direction (Fig. 2). These results indicated that the presence of Mv10B compounds inhibited iron oxidation and in a lower extent hydrogen evolution, consequently these compounds can be classified as mixed corrosion inhibitors, as electrode potential displacement is lower than 85 mV in any direction [30]. The polarization resistance (Rp ) from Tafel extrapolation method was calculated using the Stern–Geary equation (Eq. (19)) [31].
Rp =
1 β a.β c × 2.303(β a + β c) Icorr
(19)
By increasing the inhibitor concentration, the polarization resistance increases in the presence of compound, indicating adsorption of the inhibitor on the metal surface to block the active sites efficiently and inhibit corrosion [32]. Fig. 3 shows the Nyquist diagrams of steel in 1 M HCl solutions containing different concentrations of Mv10B at 25 °C respectively. All the impedance spectra exhibit single depressed semicircle. The diameter of semicircle increases with the increase of Mv10B concentration. The semicircular appearance shows that the corrosion of carbon steel is controlled by the charge transfer and the presence of Mv10B does not change the mechanism of steel dissolution [33,34]. In addition, these
Fig. 3. Nyquist plots for steel in 1 M HCl without and with various concentration of Mv10B at 25 °C.
Fig. 4. Equivalent circuits compatible with the experimental impedance data in Fig. 3 for corrosion of steel electrode at different inhibitor concentrations.
Nyquist diagrams are not perfect semicircles. The deviation of semicircles from perfect circular shape is often referred to the frequency dispersion of interfacial impedance [34–37]. This behavior is usually attributed to the inhomogeneity of the metal surface arising from surface roughness or interfacial phenomena [32,33],which is typical for solid metal electrodes [38]. The equivalent circuit compatible with the Nyquist diagram recorded in the presence of inhibitor is depicted in Fig. 4. The simplest approach requires the theoretical transfer function Z (ω) to be represented by a parallel combination of a resistance Rct and a capacitance C, both in series with another resistance Rs [39].
Z (ω) = Rs +
1 1 Rct
+ iωC
(20)
where ω is the frequency in rad/s, ω = 2π f and f is frequency in Hz. To obtain a satisfactory impedance simulation of steel, it is necessary to replace the capacitor (C) with a constant phase element (CPE) Q in the equivalent circuit. The most widely accepted explanation for the presence of CPE behavior and depressed semicircles on solid electrodes is microscopic roughness, causing an inhomogeneous distribution in the solution resistance as well as in the double layer capacitance [40]. Constant phase element Qdl , Rs and Rct can be corresponded to doun solution resistance and charge ble layer capacitance, Qdl = Rn−1 Cdl transfer resistance, respectively. Computer fitting of the spectrum allows evolution of the elements of the circuit analogue. The aim of the fitting procedure is to find those values of the parameters which best describe the data, i.e., the fitting model must be consistent with the experimental data. To corroborate the equivalent circuit, the experimental data are fitted to equivalent circuit and the circuit elements are obtained. Table 2 illustrates the equivalent circuit parameters for the impedance spectra of corrosion of steel in 1 M HCl solution. The data indicate that increasing charge
Please cite this article as: H. Jafari, K. Sayin, Electrochemical and theoretical studies of adsorption and corrosion inhibition of aniline violet compound on carbon steel in acidic solution, Journal of the Taiwan Institute of Chemical Engineers (2015), http://dx.doi.org/10.1016/j.jtice.2015.03.030
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H. Jafari, K. Sayin / Journal of the Taiwan Institute of Chemical Engineers 000 (2015) 1–10 Table 2 Impedance parameters for the steel in 1 M HCl solution in the absence and presence of different concentrations of Mv10B at 25 °C. Concentration (M)
Rs (±10−1 ) (Ω/cm2 )
Rct (±1) (Ω/cm2 )
Qdl (±10−3 ) (mF/cm2 )
n (±10−2 )
Blank 1 × 10−4 5 × 10−4 1 × 10−3 5 × 10−3
1.2 1.6 1.1 1.8 1.7
209 374 546 1214 1425
0.500 0.334 0.138 0.069 0.079
0.66 0.74 0.73 0.75 0.78
transfer resistance is associated with a decrease in the double layer capacitance. It has been reported that the adsorption of organic inhibitor on the metal surface is characterized by a decrease in Cdl [40]. The decreased values of Cdl may be due to the replacement of water molecules at the electrode interface by organic inhibitor of lower dielectric constant through adsorption, suggest that inhibitor acts by adsorption at the metal–solution interface. The increase in values of Rct and the decrease in values of Cdl with increasing the concentration also indicate that Mv10B acts as primary interface inhibitor and the charge transfer controls the corrosion of steel under the open circuit conditions [31]. 3.2. Adsorption isotherm and thermodynamic parameters The adsorption process consists of the replacement of water molecules at a corroding interface according to following process [41].
Org(sol)+nH2 O(ads) → Org(ads)+nH2 O(sol)
(21)
where Org (sol) and Org (ads) are the organic molecules in the solution and adsorbed on the metal surface, respectively, and n is the number of water molecules replaced by the organic molecules. It is essential to know the mode of adsorption and the adsorption isotherm that can give important information on the interaction of inhibitor and metal surface. Ten adsorption isotherms (Langmuir, Temkin, Freundlich, Frumkin, Modified, Langmuir, Henry, Viral, Damaskin, Volmer and Flory–Huggins) [42–44] were tested for their fit to the experimental data. The linear regression coefficient values (R2 ) determined from the plotted curves. According to these results, it can be concluded that the best description of the adsorption behavior of Mv10B can be explained by Langmuir adsorption isotherm which is given by (Eq. (22)).
C
θ
=
1 +C Kads
(22)
where θ is the surface coverage degree, C is the concentration of inhibitor and Kads is the adsorptive equilibrium constant. Suggest that the adsorption of Mv10B on the steel surface obeyed the Langmuir adsorption isotherm (Fig. 5). This isotherm postulates that there is no interaction between the adsorbed molecules, and the energy of adsorption is independent on the surface coverage (θ ). Langmuir’s isotherm assumes that the solid surface contains a fixed number of adsorption sites and each holds one adsorbed species [42]. The slopes of the straight lines obtained from of the Langmuir isotherm plots are near unity. So, it could be concluded that each Mv10B unit occupies more than one adsorption site on the steel surface. A modified Langmuir adsorption isotherm [42] could be applied to this phenomenon, which is given by the corrected equation:
C
θ
=
m + mC Kads
(23)
This is an indicative from some divergence from pure monolayer adsorption and can be attributed to interactions between adsorbate species on the metal surface as well as changes in the adsorption heat with increasing surface coverage, factors which were not taken
Fig. 5. Langmuir adsorption plot for steel electrode in 1 M HCl at 25 °C.
into consideration in derivation of the isotherm. Eq. (23) illustrates that only mθ can show the real coverage of the adsorption material on the metallic surface. The correction factor m can be deemed as the correlation coefficient between the surface coverage and the corrosion current [42]. The value of m shows the degree of fitting Langmuir adsorption isothermal equation for the system. The value of m approaches 1, the adsorption conforms to Langmuir adsorption isothermal equation. This supposition is more reasonable. The standard free energy of adsorption of inhibitor (࢞Gads ) on steel surface can be evaluated with the following equation:
Gads = −RT ln(55.5Kads )
(24)
The value 55.5 in the above equation is the concentration of water in solution in mol/l [45]. The negative values of ࢞Gads suggest that the adsorption of Mv10B on the steel surface is spontaneous. Generally, the values of ࢞Gads around or less than −30 kJ/mol are associated with the electrostatic interaction between charged molecules and the charged metal surface (physisorption); while those around or higher than −40 kJ/mol mean charge sharing or transfer from the inhibitor molecules to the metal surface to form a coordinate type of metal bond (chemisorption). The values of Kads and ࢞Gads are 3749 l/mol and −30.35 kJ/mol. The ࢞Gads values are around −30 kJ/mol, which means that the absorption of Mv10B on the steel surface belongs to the physisorption and the adsorptive film has an electrostatic character [46]. 3.3. Surface photographs The surface photographs of steel, which were immersed in blank and containing Mv10B after 6 h, were presented in Fig. 6. It can be seen clearly from Fig. 6B, there is a good surface coverage on the steel surface in the presence of inhibitor which provides a good corrosion inhibition efficiency. This is due to the involvement of inhibitor molecules in the interaction with the reaction sites of steel surface, resulting in a decrease in the contact between iron and the aggressive medium and sequentially exhibited excellent inhibition effect. It was observed that the sample in contact solution gave high corrosion attack (Fig. 6A), on the other hand, in the presence of Mv10B smooth surface obtained, and the degree of attack decreased (Fig. 6B). This is in good agreement with the result obtained from the EIS tests that with the increasing of Mv10B concentration in solution increase the exponent n of the double layer capacitance. The results are further proved by AFM photographs of blank steel surface and steel surface exposed to uninhibited and inhibited acid solution (Fig. 7). The average roughness of blank steel surface (Fig. 7A) is 1.33 μm which was found to be 200 nm in the presence of inhibited 1 M HCl solution (Fig. 7B). A smoother layer with clearly different morphology is as a result of the formation of a protective layer by the adsorbed inhibitor.
Please cite this article as: H. Jafari, K. Sayin, Electrochemical and theoretical studies of adsorption and corrosion inhibition of aniline violet compound on carbon steel in acidic solution, Journal of the Taiwan Institute of Chemical Engineers (2015), http://dx.doi.org/10.1016/j.jtice.2015.03.030
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Fig. 6. Optical micrographs of steel electrode exposed to (A) 1 M HCl solution. (B)In the presence of 5 × 10− 3 M of Mv10B.
Fig. 7. 2D and 3D of AFM images of carbon steel exposed to 1 M HCl solution. (A) In the absence of 5 × 10− 3 M Mv10B. (B) In the presence of 5 × 10− 3 M Mv10B (scan size:49.5 μm × 50 μm).
Please cite this article as: H. Jafari, K. Sayin, Electrochemical and theoretical studies of adsorption and corrosion inhibition of aniline violet compound on carbon steel in acidic solution, Journal of the Taiwan Institute of Chemical Engineers (2015), http://dx.doi.org/10.1016/j.jtice.2015.03.030
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H. Jafari, K. Sayin / Journal of the Taiwan Institute of Chemical Engineers 000 (2015) 1–10 Table 3 Quantum chemical descriptors for non-protonated inhibitors at HF/ 6-31++G(d,p) and HF/6311++G(d,p) levels in vacuo. ELUMO a
EGAP a
ηa
σb
χa
ωa
Nb
HF/6-31++G(d,p) Inhibitor −10.079
−2.299
7.780
3.890
0.257
6.189
4.923
0.203
HF/6-311++G(d,p) Inhibitor −10.104
−2.303
7.801
3.900
0.256
6.203
4.933
0.203
EHOMO a
a b
In eV. In per eV.
Fig. 8. The change of hydrogen gas evolution on the carbon steel electrode.
The inhibitor layer is very compact and as such provide absolute coverage, with some metal sites still exposed to acid attack. 3.4. Hydrogen gas evolution measurements The change in the volume of H2 gas evolved from the corrosion reaction as a function of reaction time is represented graphically in Fig. 8. It is clear from Fig. 8 that, in the solution without inhibitor, the rate of hydrogen evolution is small at the beginning of the reaction, and then it increases markedly due to increase in surface area as steel dissolves. The addition of Mv10B to the corrosive media reduces considerably the rate of H2 evolution. After 120 h of exposure, in 1 M HCl solution 151 ml/cm2 H2 gas evolved whereas in 5 × 10−3 M Mv10B containing HCl solution only 4.6 ml/cm hydrogen gas evolved per unit area of steel indicate that, inhibitor molecules adsorb strictly onto the metal surface and blocking the electrochemical reaction efficiently through decreasing the available surface area. 3.5. Molecular structure and quantum chemical calculation 3.5.1. Non-protonated inhibitors Optimized structure of studied inhibitor is obtained at each level and represented in Fig. 9 at HF/6-31++G(d,p) level in vacuo. For nonprotonated inhibitors, quantum chemical descriptors are calculated by using HF/6-31++G(d,p) and HF/6-311++G(d,p) levels in vacuo. Mentioned parameters for each level are listed in Table 3. In many computational corrosion studies, some quantum chemical descriptors have been calculated from HOMO and LUMO energies. In calculations, HF method is selected for our inhibitor. Since, HF method gives more reliable molecular orbital energies than other methods. In Table 3, numerical values of quantum chemical descriptors are given at HF/6-31++G(d,p) and HF/6-311++G(d,p) level in vacuo for mentioned inhibitor. Calculated results for each descriptor are too close to each other. There are not significant differences between each level. Some selected structural parameters are given in Table 2 at HF/6-31++G(d,p) level in vacuo. According to Table 4, optimized structural parameters of relevant inhibitor are identical to each other which are around the same central atom.
Fig. 9. Optimized structures of investigated inhibitor at HF/6-31++G(d,p) level in vacuo. Table 4 Optimized structural parameters of studied inhibitor at HF/6-31++G(d,p) level in vacuo. ˚ Bond lengths (A) 1C−2C 1C−3C 1C−4C 7C−22C 12C−21C 17C−20C
1.442 1.442 1.442 1.347 1.347 1.347
20C−23C 20C−24C 21C−25C 21C−26C 22C−27C 22C−28C
1.455 1.455 1.455 1.455 1.455 1.455
Bond angle (°) 2C−1C−3C 2C−1C−4C 3C−−1C−4C 7C−22C−27C 7C−22C−28C 27C−22C−28C
120.0 120.0 120.0 120.6 120.6 118.9
12C−21C−25C 12C−21C−26C 25C−21C−26C 17C−20C−23C 17C−20C−24C 23C−20C−24C
120.6 120.6 118.9 120.6 120.6 118.9
3.5.2. Determination of active site Determination of active site in inhibitor is important to mechanism of corrosion. Active sites of inhibitors can be determined with computational chemistry methods by using contour diagram of some occupied molecular orbitals, molecular electrostatic potential (MEP)
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Fig. 10. Contour diagrams of HOMO, HOMO-1 and HOMO-2 of investigated inhibitors at HF method with 6-31G basis set in water. (For interpretation of the references to colour in the text, the reader is referred to the web version of this article.)
maps, MEP contours, condensed Fukui functions and local softness of heteroatoms in investigated inhibitor. 3.5.2.1. Condensed Fukui functions and local softness. The active site in inhibitor can be analyzed by using condensed Fukui functions. The higher value of Fukui functions means that the highest active and Fukui functions provide important information which are the active sites for nucleophilic attacks, electrophilic attack and radicalic attack in inhibitor and they are related to highest ƒk + value, highest ƒk − value and highest ƒk 0 value, respectively. Additionally, tendencies of electron transferring of inhibitors toward metal can be discussed with hard–soft–acid–base (HSAB) approximation and soft species are to be most effective for metallic bulks. Since, metallic bulk is softer than metal atoms. Beyond them, local softness values can be calculated for electrophilic attack (σ − ), radicalic attack (σ 0 ) and nucleophilic attack (σ + ). Knowing the local softness value for heteroatoms would
be more useful. Condensed Fukui functions of heteroatoms and local softness are calculated HF/6-31++G(d,p) and HF/6-311++G(d,p) levels in vacuo with Eqs. (10)–(15) and listed in Table 5. According to the Fukui functions and local softness values, nitrogen atoms are identical to each other for electrophilic, nucleophilic and radicalic attacks. Any nitrogen atom therefore can be protonated. 3.5.2.2. Contour diagram of some molecular orbitals. Frontier molecular orbitals are very important to explain corrosion mechanisms. In addition to HOMO and LUMO properties, properties of HOMO-1, HOMO-2, HOMO-3 and HOMO-4 are investigated. In corrosion mechanism, electron can be transferred from metal to inhibitor or inhibitor to metal. If LUMO energy of inhibitor is the lowest, electron transfers from metal to inhibitor and if energy of HOMO is the highest, electrons transfer from inhibitor to metal surface. Additionally, electrons transfer from HOMO-1 and HOMO-2 to metal
Please cite this article as: H. Jafari, K. Sayin, Electrochemical and theoretical studies of adsorption and corrosion inhibition of aniline violet compound on carbon steel in acidic solution, Journal of the Taiwan Institute of Chemical Engineers (2015), http://dx.doi.org/10.1016/j.jtice.2015.03.030
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H. Jafari, K. Sayin / Journal of the Taiwan Institute of Chemical Engineers 000 (2015) 1–10 Table 5 Fukui functions and local softness of investigated inhibitor. Pk (N)
Pk (N + 1)
ƒk −
ƒk 0
ƒk +
σ−
σ0
σ+
HF/6-31++G(d,p) in vacuo 20N 7.16447 21N 7.16447 22N 7.16447
7.54332 7.54332 7.54332
7.57825 7.57825 7.57825
0.37885 0.37885 0.37885
0.20689 0.20689 0.20689
0.03493 0.03493 0.03493
0.09736 0.09736 0.09736
0.05317 0.05317 0.05317
0.00897 0.00897 0.00897
HF/6-311++G(d,p) in vacuo 20N 7.2108 21N 7.2108 22N 7.2108
7.54292 7.54292 7.54292
7.57535 7.57535 7.57535
0.33212 0.33212 0.33212
0.182275 0.182275 0.182275
0.03243 0.03243 0.03243
0.08502 0.08502 0.08502
0.04666 0.04666 0.04666
0.00830 0.00830 0.00830
Heteroatomsa
a
Pk (N − 1)
Heteroatoms are represented in Fig. 1.
Fig. 11. MEP maps of investigated inhibitors at HF method with 6-31G basis set in water. (For interpretation of the references to colour in the text, the reader is referred to the web version of this article.)
surface, but it depends on energy differences between HOMO and relevant molecular orbitals. Energy values and contour diagrams of mentioned molecular orbitals are calculated at HF/6-31++G(d,p) and HF/6-311++G(d,p) levels in vacuo and represented in Fig. 10. In contour diagrams of selected molecular orbitals, there are red and green lobes in different sizes. The bigger lobes mean that the highest contribution to molecular orbitals. According to the Fig. 10, there are not energy differences between HOMO and HOMO-1. Therefore, electrons must transfer from HOMO or HOMO-1 to metal surface. According to the contour diagram of HOMO and HOMO-1, any nitrogen atom can be protonated. Additionally, there are contributions of pi electrons in benzene ring to contour diagram of HOMO and HOMO-1. 3.5.2.3. Molecular electrostatic potential (MEP) maps and contours. Different values of electrostatic potential at MEP map are represented by different colors which are red, yellow, green, light blue and blue. The red and yellow regions in MEP map are related to electrophilic active region and the light blue and blue regions in MEP map are related to nucleophilic active region. In MEP contours, there are two colors which are yellow and red and these color lines related to positively charged and negatively charged regions, respectively. Steric
effect can be easily seen from MEP contour. MEP maps and contours are represented in Fig. 11. According to MEP maps of studied inhibitors, electrophilic active regions are mainly observed over the benzene ring and nitrogen atom and partially over the nitrogen atoms. In MEP contours, there are not red lines around the nitrogen atoms and these results mean that inhibitor cannot adsorb to metal surface on the board plane. MEP contour and MEP maps imply that inhibitors can adsorb to metal surface as parallel. 3.5.3. Protonated inhibitor In Section 3.6.2, nitrogen atoms are determined as identical to each other and any nitrogen atom can be used for protonation. There are similarities between the results in HF/6-31++G(d,p) and HF/6311++G(d,p) level. Therefore, HF/6-31++G(d,p) level is taken into account to protonated inhibitor. In this section, 21N is represented in Fig. 9 is protonated and optimized structure of protonated inhibitor is represented in Fig. 12. Some bond lengths are calculated as 1.007, 1.501, 1.501 and 1.470 A˚ for H−21N, 12C−21N, 21N−25N and 21N−26N bonds, respectively. According to the bond lengths, bond lengths which are around the 21N atom in protonated inhibitor are
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inhibitive effect of Mv10B. Both experimental and theoretical calculations are in agreement. References
Fig. 12. Optimized structures of protonated inhibitor at HF/6-31++G(d,p) level in vacuo.
longer than in non-protonated inhibitor. As for the bond angles, some bond angles are calculated as 106.5, 113.1 and 112.7° for H−21N−12C, 12C−21N−25N and 12C−21N−26N, respectively. According to the bond angles, 12C−21N−25N and 12C−21N−26N angles in protonated inhibitor are narrower than in non-protonated inhibitor. The energies of non-protonated and protonated inhibitor are calculated as −1127.8742 and −1127.3808 a.u., respectively. This result implies that protonated inhibitor is more stable than non-protonated inhibitor and proton affinity for this protonation is calculated by using Eqs. (16) and (17) and its value is equal to 564.4 kJ/mol. 4. Conclusions Mv10B was synthesized and investigated as corrosion inhibitor for carbon steel in 1 M HCl solution with different concentrations using a series of techniques. The following points can be emphasized: 1. Mv10B has an excellent inhibition effect for the corrosion of carbon steel in 1 M HCl solution especially in high concentration. Its inhibition efficiency is both concentration and temperature dependent. The high inhibition efficiency of Mv10B was attributed to the formation of a film on the steel surface. 2. Corrosion current density is increased by increasing the temperature, but, the rate of its increase is lower in the presence of Mv10B compound. 3. Polarization measurements demonstrate that Mv10B behaved as mixed type corrosion inhibitor by inhibiting both anodic metal dissolution and cathodic hydrogen evolution reactions. 4. Impedance measurements indicate that with increasing inhibitor concentration, the polarization resistance (Rct ) increased, while the double-layer capacitance (Cdl ) decreased. 5. The adsorption of Mv10B molecules on carbon steel surface has been described Langmuir adsorption isotherm. The values of ࢞Gads and Kads indicate the spontaneous interaction with surface and high adsorption ability of studied inhibitor. 6. The high resolution AFM micrographs were showed that the corrosion of carbon steel in 1 M HCl solution was described by corrosion attack and the addition of inhibitor to the aggressive solutions diminished the corrosion of carbon steel. 7. Data obtained from quantum chemical calculations using DFT at the B3LYP/6-31G(d,p) level of theory were correlated to the
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Contents lists available at ScienceDirect
Journal of the Taiwan Institute of Chemical Engineers journal homepage: www.elsevier.com/locate/jtice
Electrochemical and theoretical studies of adsorption and corrosion inhibition of aniline violet compound on carbon steel in acidic solution Hojat Jafari a,∗, Koray Sayin b a b
Abadan Faculty of Petroleum Engineering, Petroleum University of Technology, Abadan, Iran Department of Chemistry, Institute of Science, Cumhuriyet University, 58140 Sivas, Turkey
a r t i c l e
i n f o
Article history: Received 4 February 2015 Revised 23 March 2015 Accepted 29 March 2015 Available online xxx Keywords: DFT calculations Carbon steel Electrochemical calculation AFM Acid inhibition
a b s t r a c t The inhibition ability of 4-[bis[4-(dimethylamino)phenyl]methyl]-N,N-dimethylaniline (Mv10B) on carbon steel in 1 M HCl solution was studied by electrochemical techniques. Mv10B inhibited carbon steel corrosion in 1 M HCl solution significantly and the inhibition efficiency increased with Mv10B concentration. Potentiodynamic polarization results showed that Mv10B was a mixed-type inhibitor. The adsorption of Mv10B on steel surface followed Langmuir adsorption isotherm. The inhibition performance of Mv10B was also evidenced by AFM images. Computational investigations of studied inhibitor are performed by using HF method with 6-31++G(d,p) and 6-311++G(d,p) basis sets. © 2015 Published by Elsevier B.V. on behalf of Taiwan Institute of Chemical Engineers.
1. Introduction Over the years, a number of protection measures have been suggested to delay, slow, or stop the corrosion process, the use of corrosion inhibitors is an important method to protect metallic materials against corrosion in acidic medium. The acid solutions are widely used in industry for various purposes, such as acid pickling, acidification of petroleum wells, among others [1]. Most well-known acid inhibitors are organic compounds containing nitrogen, sulfur, and oxygen atoms. The choice of the inhibitor is based on two considerations, first economic consideration and second, should contain the electron cloud on the aromatic ring or the electronegative atoms such as N, O in the relatively long chain compounds. Generally the organic compounds containing hetero atoms like O, N, S, and P are found to work as very effective corrosion inhibitors. The efficiency of these compounds depends upon electron density present around the hetero atoms, the number of adsorption active centers in the molecule and their charge density, molecular size, mode of adsorption, and formation of metallic complexes [2–5]. Adsorption of inhibitors on the metal surface involves the two types of interaction (physical adsorption and chemical adsorption). Physical adsorption is associated with the electrostatic interaction between charged molecules and the charged metal surface. The second one, chemisorption process involves charge sharing or charge
transfer from the inhibitor molecules to the metal surface to form a co-ordinate type bond and takes place in the presence of heteroatoms (P, N, S, O, etc.) with lone pairs of electrons and/or aromatic ring in the molecular structure [6–8]. Density functional theory (DFT) has grown to be a useful theoretical method to interpret experimental results, enabling one to obtain structural parameters for even huge complex molecules, and it can explains the hard and soft acid base (HSAB) behavior of organic molecules, i.e., DFT connects some traditional empirical concepts with quantum mechanical interpretations [9,10]. Therefore, DFT is a very powerful technique to probe the inhibitor/surface interaction and to analyze experimental data. These are the reasons why we selected both electrochemical and DFT methods to evaluate the efficiency of 4-[bis[4-(dimethylamino)phenyl]methyl]-N,N-dimethylaniline. In this work, we report the successful use of 4-[bis [4-(dimethylamino)phenyl]methyl]-N,N-dimethylaniline as ecofriendly corrosion inhibitor for corrosion of steel in 1 M HCl solutions by using several electrochemical techniques like: electrochemical impedance spectroscopy and potentiodynamic polarization; corrosion kinetic parameters were evaluated and surface topology was studied using AFM images. Quantum chemical calculations have been performed using DFT, and several quantum chemical indices are calculated and correlated with the inhibitive effect of inhibitor. 2. Experimental 2.1. Materials
∗
Corresponding author. Tel.: +98 9137118405. E-mail address: [email protected] (H. Jafari).
The carbon steel used had the following chemical composition (wt.%): 0.11 C, 0.56 Mn, 0.03 Si, 0.007 P, 0.005 S, 0.07 Cr,
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A = −ELUMO
η=
(3)
I−A 2
(4)
1
σ=
(5)
η |EHOMO + ELUMO |
χ=
2
(6)
μ = −χ
(7)
μ2 2η
(8)
Fig. 1. Chemical structure of Mv10B.
ω=
0.03 Ni and balance Fe. The specimens of dimension 1 cm × 1 cm (exposed) × 0.5 cm (isolated with polyester resin) were used for polarization and electrochemical impedance methods. They were polished mechanically using different grade emery papers up to 1200, and washed thoroughly with triple distilled water and degreased with acetone before being immersed in the acid solution. The aggressive solution of 1 M HCl was prepared by dilution of Merck Product HCl. The concentration range of inhibitors employed was varied from 1 × 10−4 M to 5 × 10−3 M. All chemicals used in present work were of reagent-grade Merck product and used as received without further purification. Fig. 1 shows the chemical structure of Mv10B.
N=
σ + = σ xfk +
(13)
2.2. Methods
σ 0 = σ xfk 0
(14)
The apparatus for electrochemical investigations consists of computer controlled Auto Lab potentiostat/galvanostat (PGSTAT302N) corrosion measurement system at a scan rate of 1 m/Vs. The electrochemical experiments were carried out using a conventional three electrode cell assembly at 25 °C. A rectangular platinum foil was used as counter electrode and saturated calomel electrode as the reference electrode. The EIS experiments were conducted in the frequency range of 100 kHz to 0.01 Hz at open circuit potential. The AC potential amplitude was 10 mV. Time interval of 25 min was given for steady state attainment of open circuit potential. Fitting of experimental impedance spectroscopy data to the proposed equivalent circuit was done by means of home written least square software based on the Marquardt method for the optimization of functions and Macdonald weighting for the real and imaginary parts of the impedance [9]. The specimens of size 1 cm × 1 cm × 0.5 cm were abraded with emery paper (up to 1200) to give a homogeneous surface, then washed with distilled water and acetone. The specimens were immersed in 1 M HCl prepared with and without addition of 5 × 10−3 M at 25 °C for 6 h, cleaned with distilled water. The surface morphology of the electrode surface was evaluated by metallographic microscope Neophot32 and atomic force microscopy Nan Surf easyscan2. Computational progresses were done by using GaussView 5.0.8 [11], Gaussian 09 AM64L-G09RevD.01 package programme [12], ChemBioDraw Ultra Version (13.0.0.3015) [13]. HF method was selected as method with 6-31++G(d,p) and 6-311++G(d,p) basis sets for studied inhibitor. All calculations were performed in vacuo. The vibration frequency analyses were indicated that optimized structures of all inhibitors are at stationary points corresponding to local minima without imaginary frequencies. Mentioned quantum chemical descriptors were calculated by using Eqs. (1)–(17) [14–19].
σ − = σ xfk −
(15)
EH+ = EH3 O+ − EH2 O
(16)
PA = Epro.inh. − (Enon−pro.inh. + EH+ )
(17)
EGAP = ELUMO − EHOMO
(1)
I = −EHOMO
(2)
1
ω
+
fk = Pk (N + 1) − Pk (N) −
fk = Pk (N) − Pk (N − 1) 0
fk =
Pk (N + 1) − Pk (N − 1) 2
(9) (10) (11)
(12)
where EH+ , EH3 O+ , EH2 O , Epro. inh., Enon-pro. inh. and PA are total energy of proton, total energy of hydronium, total energy of water, total energy of protonated inhibitor, total energy of non-protonated inhibitor, proton affinity, respectively. 3. Results and discussion 3.1. Electrochemical results Polarization curves were obtained for steel in 1 M HCl solution with and without inhibitors. The polarization exhibits Tafel behavior. Tafel lines which obtained in various concentrations of Mv10B in 1 M HCl solutions were shown in Fig. 2, at 25 °C respectively. The corresponding electrochemical parameters, i.e., corrosion potential (Ecorr versus SCE), corrosion current density (Icorr ), cathodic and anodic Tafel slopes (β a , β c ) and the degree of surface coverage (θ ) values were calculated from these curves and are given in Table 1. The degree of surface coverage for different concentrations of inhibitor is calculated using the following equations [20,21]:
θ=
I − I˙ I
(18)
where I and I˙ are the corrosion current densities without and with corrosion inhibitor, respectively, determined by the intersection of the extrapolated Tafel lines at the corrosion potential for steel in uninhibited and inhibited acid solution. The presence of Mv10B shifts
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Table 1 Potentiodynamic polarization parameters for the corrosion of steel in 1 M HCl solution in the absence and in the presence of different concentrations of Mv10B at 25 °C. Concentration (M)
−Ecorr (±1) (mV)
Icorr (±5) (μA/cm2 )
β a (±1) (mV/dec)
−β c (±1) (mV/dec)
Rp (±1) (Ω/cm2 )
θ (±10−2 )
Blank 1 × 10−4 5 × 10−4 1 × 10−3 5 × 10−3
470 477 484 503 516
365 158 97 56 17
68 73 69 71 66
112 133 114 115 124
206 444 781 1436 3489
– 0.56 0.74 0.85 0.95
Fig. 2. Anodic and cathodic polarization curves for steel in 1 M HCl without and with various concentration of Mv10B at 25 °C.
both anodic and cathodic branches to the lower values of current densities and thus causes a remarkable decrease in the corrosion rate. It can be clearly seen from Fig. 2 that both anodic metal dissolution of iron and cathodic hydrogen evolution reactions were inhibited after the addition of Mv10B to the aggressive solution. This result is indicative of the adsorption of inhibitor molecules on the active sites of steel surface [22]. The inhibition of both anodic and cathodic reactions is more pronounced with the increasing inhibitor concentration. However, the influence is more pronounced in the cathodic polarization plots compared to that of the anodic polarization plots. The cathodic current–potential curves (Fig. 2) giving rise to parallel lines indicate that the addition of Mv10B to the 1 M HCl solution does not modify the reduction mechanism and the reduction at steel surface takes place mainly through a charge transfer mechanism [23–25]. The slopes do not display an order with the inhibitor concentration; this feature indicates that inhibition occurred by a blocking mechanism on the available metal spaces [26-29]. The corrosion potential displayed small change in the range of −470 to −506 mV versus SCE and curves changed slightly toward the negative direction (Fig. 2). These results indicated that the presence of Mv10B compounds inhibited iron oxidation and in a lower extent hydrogen evolution, consequently these compounds can be classified as mixed corrosion inhibitors, as electrode potential displacement is lower than 85 mV in any direction [30]. The polarization resistance (Rp ) from Tafel extrapolation method was calculated using the Stern–Geary equation (Eq. (19)) [31].
Rp =
1 β a.β c × 2.303(β a + β c) Icorr
(19)
By increasing the inhibitor concentration, the polarization resistance increases in the presence of compound, indicating adsorption of the inhibitor on the metal surface to block the active sites efficiently and inhibit corrosion [32]. Fig. 3 shows the Nyquist diagrams of steel in 1 M HCl solutions containing different concentrations of Mv10B at 25 °C respectively. All the impedance spectra exhibit single depressed semicircle. The diameter of semicircle increases with the increase of Mv10B concentration. The semicircular appearance shows that the corrosion of carbon steel is controlled by the charge transfer and the presence of Mv10B does not change the mechanism of steel dissolution [33,34]. In addition, these
Fig. 3. Nyquist plots for steel in 1 M HCl without and with various concentration of Mv10B at 25 °C.
Fig. 4. Equivalent circuits compatible with the experimental impedance data in Fig. 3 for corrosion of steel electrode at different inhibitor concentrations.
Nyquist diagrams are not perfect semicircles. The deviation of semicircles from perfect circular shape is often referred to the frequency dispersion of interfacial impedance [34–37]. This behavior is usually attributed to the inhomogeneity of the metal surface arising from surface roughness or interfacial phenomena [32,33],which is typical for solid metal electrodes [38]. The equivalent circuit compatible with the Nyquist diagram recorded in the presence of inhibitor is depicted in Fig. 4. The simplest approach requires the theoretical transfer function Z (ω) to be represented by a parallel combination of a resistance Rct and a capacitance C, both in series with another resistance Rs [39].
Z (ω) = Rs +
1 1 Rct
+ iωC
(20)
where ω is the frequency in rad/s, ω = 2π f and f is frequency in Hz. To obtain a satisfactory impedance simulation of steel, it is necessary to replace the capacitor (C) with a constant phase element (CPE) Q in the equivalent circuit. The most widely accepted explanation for the presence of CPE behavior and depressed semicircles on solid electrodes is microscopic roughness, causing an inhomogeneous distribution in the solution resistance as well as in the double layer capacitance [40]. Constant phase element Qdl , Rs and Rct can be corresponded to doun solution resistance and charge ble layer capacitance, Qdl = Rn−1 Cdl transfer resistance, respectively. Computer fitting of the spectrum allows evolution of the elements of the circuit analogue. The aim of the fitting procedure is to find those values of the parameters which best describe the data, i.e., the fitting model must be consistent with the experimental data. To corroborate the equivalent circuit, the experimental data are fitted to equivalent circuit and the circuit elements are obtained. Table 2 illustrates the equivalent circuit parameters for the impedance spectra of corrosion of steel in 1 M HCl solution. The data indicate that increasing charge
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H. Jafari, K. Sayin / Journal of the Taiwan Institute of Chemical Engineers 000 (2015) 1–10 Table 2 Impedance parameters for the steel in 1 M HCl solution in the absence and presence of different concentrations of Mv10B at 25 °C. Concentration (M)
Rs (±10−1 ) (Ω/cm2 )
Rct (±1) (Ω/cm2 )
Qdl (±10−3 ) (mF/cm2 )
n (±10−2 )
Blank 1 × 10−4 5 × 10−4 1 × 10−3 5 × 10−3
1.2 1.6 1.1 1.8 1.7
209 374 546 1214 1425
0.500 0.334 0.138 0.069 0.079
0.66 0.74 0.73 0.75 0.78
transfer resistance is associated with a decrease in the double layer capacitance. It has been reported that the adsorption of organic inhibitor on the metal surface is characterized by a decrease in Cdl [40]. The decreased values of Cdl may be due to the replacement of water molecules at the electrode interface by organic inhibitor of lower dielectric constant through adsorption, suggest that inhibitor acts by adsorption at the metal–solution interface. The increase in values of Rct and the decrease in values of Cdl with increasing the concentration also indicate that Mv10B acts as primary interface inhibitor and the charge transfer controls the corrosion of steel under the open circuit conditions [31]. 3.2. Adsorption isotherm and thermodynamic parameters The adsorption process consists of the replacement of water molecules at a corroding interface according to following process [41].
Org(sol)+nH2 O(ads) → Org(ads)+nH2 O(sol)
(21)
where Org (sol) and Org (ads) are the organic molecules in the solution and adsorbed on the metal surface, respectively, and n is the number of water molecules replaced by the organic molecules. It is essential to know the mode of adsorption and the adsorption isotherm that can give important information on the interaction of inhibitor and metal surface. Ten adsorption isotherms (Langmuir, Temkin, Freundlich, Frumkin, Modified, Langmuir, Henry, Viral, Damaskin, Volmer and Flory–Huggins) [42–44] were tested for their fit to the experimental data. The linear regression coefficient values (R2 ) determined from the plotted curves. According to these results, it can be concluded that the best description of the adsorption behavior of Mv10B can be explained by Langmuir adsorption isotherm which is given by (Eq. (22)).
C
θ
=
1 +C Kads
(22)
where θ is the surface coverage degree, C is the concentration of inhibitor and Kads is the adsorptive equilibrium constant. Suggest that the adsorption of Mv10B on the steel surface obeyed the Langmuir adsorption isotherm (Fig. 5). This isotherm postulates that there is no interaction between the adsorbed molecules, and the energy of adsorption is independent on the surface coverage (θ ). Langmuir’s isotherm assumes that the solid surface contains a fixed number of adsorption sites and each holds one adsorbed species [42]. The slopes of the straight lines obtained from of the Langmuir isotherm plots are near unity. So, it could be concluded that each Mv10B unit occupies more than one adsorption site on the steel surface. A modified Langmuir adsorption isotherm [42] could be applied to this phenomenon, which is given by the corrected equation:
C
θ
=
m + mC Kads
(23)
This is an indicative from some divergence from pure monolayer adsorption and can be attributed to interactions between adsorbate species on the metal surface as well as changes in the adsorption heat with increasing surface coverage, factors which were not taken
Fig. 5. Langmuir adsorption plot for steel electrode in 1 M HCl at 25 °C.
into consideration in derivation of the isotherm. Eq. (23) illustrates that only mθ can show the real coverage of the adsorption material on the metallic surface. The correction factor m can be deemed as the correlation coefficient between the surface coverage and the corrosion current [42]. The value of m shows the degree of fitting Langmuir adsorption isothermal equation for the system. The value of m approaches 1, the adsorption conforms to Langmuir adsorption isothermal equation. This supposition is more reasonable. The standard free energy of adsorption of inhibitor (࢞Gads ) on steel surface can be evaluated with the following equation:
Gads = −RT ln(55.5Kads )
(24)
The value 55.5 in the above equation is the concentration of water in solution in mol/l [45]. The negative values of ࢞Gads suggest that the adsorption of Mv10B on the steel surface is spontaneous. Generally, the values of ࢞Gads around or less than −30 kJ/mol are associated with the electrostatic interaction between charged molecules and the charged metal surface (physisorption); while those around or higher than −40 kJ/mol mean charge sharing or transfer from the inhibitor molecules to the metal surface to form a coordinate type of metal bond (chemisorption). The values of Kads and ࢞Gads are 3749 l/mol and −30.35 kJ/mol. The ࢞Gads values are around −30 kJ/mol, which means that the absorption of Mv10B on the steel surface belongs to the physisorption and the adsorptive film has an electrostatic character [46]. 3.3. Surface photographs The surface photographs of steel, which were immersed in blank and containing Mv10B after 6 h, were presented in Fig. 6. It can be seen clearly from Fig. 6B, there is a good surface coverage on the steel surface in the presence of inhibitor which provides a good corrosion inhibition efficiency. This is due to the involvement of inhibitor molecules in the interaction with the reaction sites of steel surface, resulting in a decrease in the contact between iron and the aggressive medium and sequentially exhibited excellent inhibition effect. It was observed that the sample in contact solution gave high corrosion attack (Fig. 6A), on the other hand, in the presence of Mv10B smooth surface obtained, and the degree of attack decreased (Fig. 6B). This is in good agreement with the result obtained from the EIS tests that with the increasing of Mv10B concentration in solution increase the exponent n of the double layer capacitance. The results are further proved by AFM photographs of blank steel surface and steel surface exposed to uninhibited and inhibited acid solution (Fig. 7). The average roughness of blank steel surface (Fig. 7A) is 1.33 μm which was found to be 200 nm in the presence of inhibited 1 M HCl solution (Fig. 7B). A smoother layer with clearly different morphology is as a result of the formation of a protective layer by the adsorbed inhibitor.
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Fig. 6. Optical micrographs of steel electrode exposed to (A) 1 M HCl solution. (B)In the presence of 5 × 10− 3 M of Mv10B.
Fig. 7. 2D and 3D of AFM images of carbon steel exposed to 1 M HCl solution. (A) In the absence of 5 × 10− 3 M Mv10B. (B) In the presence of 5 × 10− 3 M Mv10B (scan size:49.5 μm × 50 μm).
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H. Jafari, K. Sayin / Journal of the Taiwan Institute of Chemical Engineers 000 (2015) 1–10 Table 3 Quantum chemical descriptors for non-protonated inhibitors at HF/ 6-31++G(d,p) and HF/6311++G(d,p) levels in vacuo. ELUMO a
EGAP a
ηa
σb
χa
ωa
Nb
HF/6-31++G(d,p) Inhibitor −10.079
−2.299
7.780
3.890
0.257
6.189
4.923
0.203
HF/6-311++G(d,p) Inhibitor −10.104
−2.303
7.801
3.900
0.256
6.203
4.933
0.203
EHOMO a
a b
In eV. In per eV.
Fig. 8. The change of hydrogen gas evolution on the carbon steel electrode.
The inhibitor layer is very compact and as such provide absolute coverage, with some metal sites still exposed to acid attack. 3.4. Hydrogen gas evolution measurements The change in the volume of H2 gas evolved from the corrosion reaction as a function of reaction time is represented graphically in Fig. 8. It is clear from Fig. 8 that, in the solution without inhibitor, the rate of hydrogen evolution is small at the beginning of the reaction, and then it increases markedly due to increase in surface area as steel dissolves. The addition of Mv10B to the corrosive media reduces considerably the rate of H2 evolution. After 120 h of exposure, in 1 M HCl solution 151 ml/cm2 H2 gas evolved whereas in 5 × 10−3 M Mv10B containing HCl solution only 4.6 ml/cm hydrogen gas evolved per unit area of steel indicate that, inhibitor molecules adsorb strictly onto the metal surface and blocking the electrochemical reaction efficiently through decreasing the available surface area. 3.5. Molecular structure and quantum chemical calculation 3.5.1. Non-protonated inhibitors Optimized structure of studied inhibitor is obtained at each level and represented in Fig. 9 at HF/6-31++G(d,p) level in vacuo. For nonprotonated inhibitors, quantum chemical descriptors are calculated by using HF/6-31++G(d,p) and HF/6-311++G(d,p) levels in vacuo. Mentioned parameters for each level are listed in Table 3. In many computational corrosion studies, some quantum chemical descriptors have been calculated from HOMO and LUMO energies. In calculations, HF method is selected for our inhibitor. Since, HF method gives more reliable molecular orbital energies than other methods. In Table 3, numerical values of quantum chemical descriptors are given at HF/6-31++G(d,p) and HF/6-311++G(d,p) level in vacuo for mentioned inhibitor. Calculated results for each descriptor are too close to each other. There are not significant differences between each level. Some selected structural parameters are given in Table 2 at HF/6-31++G(d,p) level in vacuo. According to Table 4, optimized structural parameters of relevant inhibitor are identical to each other which are around the same central atom.
Fig. 9. Optimized structures of investigated inhibitor at HF/6-31++G(d,p) level in vacuo. Table 4 Optimized structural parameters of studied inhibitor at HF/6-31++G(d,p) level in vacuo. ˚ Bond lengths (A) 1C−2C 1C−3C 1C−4C 7C−22C 12C−21C 17C−20C
1.442 1.442 1.442 1.347 1.347 1.347
20C−23C 20C−24C 21C−25C 21C−26C 22C−27C 22C−28C
1.455 1.455 1.455 1.455 1.455 1.455
Bond angle (°) 2C−1C−3C 2C−1C−4C 3C−−1C−4C 7C−22C−27C 7C−22C−28C 27C−22C−28C
120.0 120.0 120.0 120.6 120.6 118.9
12C−21C−25C 12C−21C−26C 25C−21C−26C 17C−20C−23C 17C−20C−24C 23C−20C−24C
120.6 120.6 118.9 120.6 120.6 118.9
3.5.2. Determination of active site Determination of active site in inhibitor is important to mechanism of corrosion. Active sites of inhibitors can be determined with computational chemistry methods by using contour diagram of some occupied molecular orbitals, molecular electrostatic potential (MEP)
Please cite this article as: H. Jafari, K. Sayin, Electrochemical and theoretical studies of adsorption and corrosion inhibition of aniline violet compound on carbon steel in acidic solution, Journal of the Taiwan Institute of Chemical Engineers (2015), http://dx.doi.org/10.1016/j.jtice.2015.03.030
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Fig. 10. Contour diagrams of HOMO, HOMO-1 and HOMO-2 of investigated inhibitors at HF method with 6-31G basis set in water. (For interpretation of the references to colour in the text, the reader is referred to the web version of this article.)
maps, MEP contours, condensed Fukui functions and local softness of heteroatoms in investigated inhibitor. 3.5.2.1. Condensed Fukui functions and local softness. The active site in inhibitor can be analyzed by using condensed Fukui functions. The higher value of Fukui functions means that the highest active and Fukui functions provide important information which are the active sites for nucleophilic attacks, electrophilic attack and radicalic attack in inhibitor and they are related to highest ƒk + value, highest ƒk − value and highest ƒk 0 value, respectively. Additionally, tendencies of electron transferring of inhibitors toward metal can be discussed with hard–soft–acid–base (HSAB) approximation and soft species are to be most effective for metallic bulks. Since, metallic bulk is softer than metal atoms. Beyond them, local softness values can be calculated for electrophilic attack (σ − ), radicalic attack (σ 0 ) and nucleophilic attack (σ + ). Knowing the local softness value for heteroatoms would
be more useful. Condensed Fukui functions of heteroatoms and local softness are calculated HF/6-31++G(d,p) and HF/6-311++G(d,p) levels in vacuo with Eqs. (10)–(15) and listed in Table 5. According to the Fukui functions and local softness values, nitrogen atoms are identical to each other for electrophilic, nucleophilic and radicalic attacks. Any nitrogen atom therefore can be protonated. 3.5.2.2. Contour diagram of some molecular orbitals. Frontier molecular orbitals are very important to explain corrosion mechanisms. In addition to HOMO and LUMO properties, properties of HOMO-1, HOMO-2, HOMO-3 and HOMO-4 are investigated. In corrosion mechanism, electron can be transferred from metal to inhibitor or inhibitor to metal. If LUMO energy of inhibitor is the lowest, electron transfers from metal to inhibitor and if energy of HOMO is the highest, electrons transfer from inhibitor to metal surface. Additionally, electrons transfer from HOMO-1 and HOMO-2 to metal
Please cite this article as: H. Jafari, K. Sayin, Electrochemical and theoretical studies of adsorption and corrosion inhibition of aniline violet compound on carbon steel in acidic solution, Journal of the Taiwan Institute of Chemical Engineers (2015), http://dx.doi.org/10.1016/j.jtice.2015.03.030
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H. Jafari, K. Sayin / Journal of the Taiwan Institute of Chemical Engineers 000 (2015) 1–10 Table 5 Fukui functions and local softness of investigated inhibitor. Pk (N)
Pk (N + 1)
ƒk −
ƒk 0
ƒk +
σ−
σ0
σ+
HF/6-31++G(d,p) in vacuo 20N 7.16447 21N 7.16447 22N 7.16447
7.54332 7.54332 7.54332
7.57825 7.57825 7.57825
0.37885 0.37885 0.37885
0.20689 0.20689 0.20689
0.03493 0.03493 0.03493
0.09736 0.09736 0.09736
0.05317 0.05317 0.05317
0.00897 0.00897 0.00897
HF/6-311++G(d,p) in vacuo 20N 7.2108 21N 7.2108 22N 7.2108
7.54292 7.54292 7.54292
7.57535 7.57535 7.57535
0.33212 0.33212 0.33212
0.182275 0.182275 0.182275
0.03243 0.03243 0.03243
0.08502 0.08502 0.08502
0.04666 0.04666 0.04666
0.00830 0.00830 0.00830
Heteroatomsa
a
Pk (N − 1)
Heteroatoms are represented in Fig. 1.
Fig. 11. MEP maps of investigated inhibitors at HF method with 6-31G basis set in water. (For interpretation of the references to colour in the text, the reader is referred to the web version of this article.)
surface, but it depends on energy differences between HOMO and relevant molecular orbitals. Energy values and contour diagrams of mentioned molecular orbitals are calculated at HF/6-31++G(d,p) and HF/6-311++G(d,p) levels in vacuo and represented in Fig. 10. In contour diagrams of selected molecular orbitals, there are red and green lobes in different sizes. The bigger lobes mean that the highest contribution to molecular orbitals. According to the Fig. 10, there are not energy differences between HOMO and HOMO-1. Therefore, electrons must transfer from HOMO or HOMO-1 to metal surface. According to the contour diagram of HOMO and HOMO-1, any nitrogen atom can be protonated. Additionally, there are contributions of pi electrons in benzene ring to contour diagram of HOMO and HOMO-1. 3.5.2.3. Molecular electrostatic potential (MEP) maps and contours. Different values of electrostatic potential at MEP map are represented by different colors which are red, yellow, green, light blue and blue. The red and yellow regions in MEP map are related to electrophilic active region and the light blue and blue regions in MEP map are related to nucleophilic active region. In MEP contours, there are two colors which are yellow and red and these color lines related to positively charged and negatively charged regions, respectively. Steric
effect can be easily seen from MEP contour. MEP maps and contours are represented in Fig. 11. According to MEP maps of studied inhibitors, electrophilic active regions are mainly observed over the benzene ring and nitrogen atom and partially over the nitrogen atoms. In MEP contours, there are not red lines around the nitrogen atoms and these results mean that inhibitor cannot adsorb to metal surface on the board plane. MEP contour and MEP maps imply that inhibitors can adsorb to metal surface as parallel. 3.5.3. Protonated inhibitor In Section 3.6.2, nitrogen atoms are determined as identical to each other and any nitrogen atom can be used for protonation. There are similarities between the results in HF/6-31++G(d,p) and HF/6311++G(d,p) level. Therefore, HF/6-31++G(d,p) level is taken into account to protonated inhibitor. In this section, 21N is represented in Fig. 9 is protonated and optimized structure of protonated inhibitor is represented in Fig. 12. Some bond lengths are calculated as 1.007, 1.501, 1.501 and 1.470 A˚ for H−21N, 12C−21N, 21N−25N and 21N−26N bonds, respectively. According to the bond lengths, bond lengths which are around the 21N atom in protonated inhibitor are
Please cite this article as: H. Jafari, K. Sayin, Electrochemical and theoretical studies of adsorption and corrosion inhibition of aniline violet compound on carbon steel in acidic solution, Journal of the Taiwan Institute of Chemical Engineers (2015), http://dx.doi.org/10.1016/j.jtice.2015.03.030
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inhibitive effect of Mv10B. Both experimental and theoretical calculations are in agreement. References
Fig. 12. Optimized structures of protonated inhibitor at HF/6-31++G(d,p) level in vacuo.
longer than in non-protonated inhibitor. As for the bond angles, some bond angles are calculated as 106.5, 113.1 and 112.7° for H−21N−12C, 12C−21N−25N and 12C−21N−26N, respectively. According to the bond angles, 12C−21N−25N and 12C−21N−26N angles in protonated inhibitor are narrower than in non-protonated inhibitor. The energies of non-protonated and protonated inhibitor are calculated as −1127.8742 and −1127.3808 a.u., respectively. This result implies that protonated inhibitor is more stable than non-protonated inhibitor and proton affinity for this protonation is calculated by using Eqs. (16) and (17) and its value is equal to 564.4 kJ/mol. 4. Conclusions Mv10B was synthesized and investigated as corrosion inhibitor for carbon steel in 1 M HCl solution with different concentrations using a series of techniques. The following points can be emphasized: 1. Mv10B has an excellent inhibition effect for the corrosion of carbon steel in 1 M HCl solution especially in high concentration. Its inhibition efficiency is both concentration and temperature dependent. The high inhibition efficiency of Mv10B was attributed to the formation of a film on the steel surface. 2. Corrosion current density is increased by increasing the temperature, but, the rate of its increase is lower in the presence of Mv10B compound. 3. Polarization measurements demonstrate that Mv10B behaved as mixed type corrosion inhibitor by inhibiting both anodic metal dissolution and cathodic hydrogen evolution reactions. 4. Impedance measurements indicate that with increasing inhibitor concentration, the polarization resistance (Rct ) increased, while the double-layer capacitance (Cdl ) decreased. 5. The adsorption of Mv10B molecules on carbon steel surface has been described Langmuir adsorption isotherm. The values of ࢞Gads and Kads indicate the spontaneous interaction with surface and high adsorption ability of studied inhibitor. 6. The high resolution AFM micrographs were showed that the corrosion of carbon steel in 1 M HCl solution was described by corrosion attack and the addition of inhibitor to the aggressive solutions diminished the corrosion of carbon steel. 7. Data obtained from quantum chemical calculations using DFT at the B3LYP/6-31G(d,p) level of theory were correlated to the
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Please cite this article as: H. Jafari, K. Sayin, Electrochemical and theoretical studies of adsorption and corrosion inhibition of aniline violet compound on carbon steel in acidic solution, Journal of the Taiwan Institute of Chemical Engineers (2015), http://dx.doi.org/10.1016/j.jtice.2015.03.030