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The inhibition performance of long-chain alkyl-substituted benzimidazole derivatives for corrosion of mild steel in HCl
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The inhibition performance of long-chain alkyl-substituted benzimidazole derivatives for corrosion of mild steel in HCl Dongqin Zhang a , Yongming Tang a,∗ , Sijun Qi a , Dawei Dong a , Hui Cang b , Gang Lu c a b c
School of Chemistry and Molecular Engineering, Nanjing Tech University, Nanjing 211816, PR China College of Chemical Engineering and Biological, Yancheng Institute of Technology, Yancheng 224051, PR China College of Materials Science and Engineering, Nanjing Tech University, Nanjing 210009, PR China
a r t i c l e
i n f o
Article history: Received 28 August 2015 Received in revised form 2 October 2015 Accepted 5 October 2015 Available online xxx Keywords: A. Mild steel B. Modelling studies B. Weight loss C. Acid inhibition C. Interfaces
a b s t r a c t The corrosion inhibition of a new benzimidazole derivative, 6-(dodecyloxy)-1H-benzo[d]imidazole (DBI), for mild steel in 1 M HCl was investigated in this paper. Computational chemistry was performed to explore the adsorption of DBI on metal surface. Inhibition performance of DBI is attributed to both the direct interaction of benzimidazole segment with iron surface and the barrier effect of the non-polar long chain against aggressive solution. Compared to the protonated form, the molecular form of DBI could more tightly interact with iron surface. These results show that the long-chain alkyl-substituted benzimidazole derivative is of great potential application as corrosion inhibitor. © 2015 Elsevier Ltd. All rights reserved.
1. Introduction A continuing effort has been made to develop a corrosion inhibitor that exhibits a greater inhibition effect at a low concentration in the corrosion medium as well as environment-friendly feature [1]. In recent years, benzimidazole and its derivatives with low toxicity have received considerable attention as they provide effective corrosion inhibition due to strong adsorption on metal surface [2–6]. Benzimidazole, a heterocyclic aromatic organic compound, consists of the fusion of benzene and imidazole. The nitrogen atom and the aromatic ring in molecular structure are likely the facilitator of the adsorption of benzimidazole on the metallic surface [7–11]. However, relatively high concentration of benzimidazole is essential to achieve effective inhibition for steel in acidic medium. At the concentration of 10 mM, typically, the inhibition efficiency of benzimidazole for mild steel in 1 M HCl is only around 50% [5,12]. In hope of improving the inhibition effectiveness of benzimidazole, researchers have made a lot of attempts to develop new benzimidazole derivatives. Among all the benzimidazole derivatives, 2-substituted benz-
∗ Corresponding author. Fax: +86 25 83587443. E-mail address: [email protected] (Y. Tang).
imidazole derivatives have been paid most attention to. It has been demonstrated that substitution of some groups including mercapto, alkyl, amino and aromatic groups etc. for 2-H of benzimidazole significantly enhances corrosion inhibition, as compared to benzimidazole [12–23]. Computational chemistry showed that those substituent groups, which enhance the electron-donating or electron-withdrawing properties of the active N atom on the heterocyclic ring, would strengthen or weaken the interaction with the metal surface [24–30]. In addition, several derivatives with substitution for 1-H of benzimidazole exhibit effective corrosion inhibition for mild steel in acidic medium as well [3,31]. Regardless of the mentioned derivatives, there are few reports on corrosion inhibition of the benzimidazole derivative with substituted group on benzene ring [32,33]. In the present work, we attempt to prepare a new benzimidazole derivative, 6(dodecyloxy)-1H-benzo[d]imidazole (DBI), shown in Fig. 1 and to evaluate its corrosion inhibition for mild steel in 1 M HCl. In the structure of derivative, a long-chain alkyl group is connected to benzene ring of benzimidazole via oxygen atom. Evidently, there are two distinct segments in the structure: an alkyl chain as a non-polar tail and a benzimidazole segment as a polar head. This compound is expected to act as adsorption-type inhibitor as well as straight-chain amines [34].
http://dx.doi.org/10.1016/j.corsci.2015.10.002 0010-938X/© 2015 Elsevier Ltd. All rights reserved.
Please cite this article in press as: D. Zhang, et al., The inhibition performance of long-chain alkyl-substituted benzimidazole derivatives for corrosion of mild steel in HCl, Corros. Sci. (2015), http://dx.doi.org/10.1016/j.corsci.2015.10.002
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Fig. 2. Langmuir adsorption plot of the inhibitor in 1 M HCl at 25 ◦ C.
2. Experimental and computational details 2.1. Corrosion measurements Corrosion measurements were performed using mild steel coupons containing 0.17 wt.% C, 0.20 wt.% Si, 0.37 wt.% Mn, 0.03 wt.% S, 0.01 wt.% P and balance iron. Prior to use, the coupons with the size of 5.0 × 2.5 × 0.2 cm3 were mechanically abraded up to 1000 grit, cleaned with ethanol and deionized water, and finally dried at room temperature. Weight loss measurements were carried out in a 250 mL vessel in a thermostat water bath held at the temperature of 25 ± 1 ◦ C. The coupons were fully immersed in 1 M HCl solution containing various concentrations of the inhibitor. After 8 h of the immersion, the specimens were taken out, rinsed thoroughly with deionized water, scrubbed gently with a bristle brush to remove the corrosion product, washed successively with deionized water and ethanol, dried at room temperature and weighed. The inhibition efficiency () was calculated as follows: (%) =
v0 − v × 100 v0
(1)
where v0 and v are the corrosion rate in the absence and presence of inhibitor, respectively. Duplicate experiments were performed in each case and both mean value and deviation of corrosion rate are reported. 2.2. Computational details Density functional theory (DFT) calculations were performed to optimize geometries of the molecules without any symmetry constraint using D mol3 module of Materials Studio software 6.0. All electron calculations of the molecules were performed using the generalized gradient approximation, following the PerdewBuekeErnzerhof scheme (GGA/PBE), for the exchange-correlation functional. The “DNP” double-numeric basis set, which includes both d and p orbital polarization functions, was used in all calculations. And vibrational analysis was carried out to ensure the calculated structures reaching the minimum point on potential energy surface. In addition, the solvent (water) model was involved in all calculations.
Fig. 1. Structure of DBI.
MD simulations were performed using Discover module of Materials Studio software 6.0. To construct the simulation box, the unit cell of iron was optimized to a minimum point of energy. A Fe (1 0 0) surface was cleaved from the bulk structure and optimized, and then the resulting cell was repeated ten times in the lateral directions (10 × 10). A water slab containing the studied compounds was then added near to the upper surface of the Fe slab. The size of the resulting box with periodic boundary conditions is 28.6 × 28.6 × 45.9 Å3 . MD simulations were performed using COMPASS force field as all Fe atoms were kept “frozen” at fixed positions and both inhibitor and water molecules were allowed to freely interact with the iron surface. A simulation temperature of 298 K and NVT ensemble with a time step of 0.1 fs and simulation time of 100 ps were implemented in all MD simulations. The interaction energy (Einteract ) between Fe (1 0 0) and inhibitive molecule was calculated as follows: Einteract = Etot − (Esurf + H2 O + Einh )
(2)
where Etot is the total energy of simulation system, Esurf+H
2O
is the
energy of iron surface together with H2 O molecules, and Einh is the energy of free inhibitive molecule. 3. Results and discussion 3.1. Corrosion inhibition of DBI Table 1 depicts the corrosion rate of mild steel and inhibition efficiency from weight loss measurements in the absence and presence of DBI. It is clear that the corrosion rate of mild steel decreases significantly and the inhibition efficiency increases with increasing inhibitor concentration. Moreover, at a rather low concentration of the inhibitor, for instance, 8 × 10−6 M, the inhibition efficiency
Please cite this article in press as: D. Zhang, et al., The inhibition performance of long-chain alkyl-substituted benzimidazole derivatives for corrosion of mild steel in HCl, Corros. Sci. (2015), http://dx.doi.org/10.1016/j.corsci.2015.10.002
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Table 1 Corrosion rate and inhibition efficiency in the absence and presence of inhibitor in 1 M HCl solution.
Fig. 3. Distribution of HOMO and LUMO (left) and isosurfaces of Fukui functions (right) of DBI and DBIH.
C (M)
v (g m−2 h−1 )
(%)
0 10−6 2 × 10−6 4 × 10−6 8 × 10−6 10−5 10−4
4.80 1.24 1.06 0.81 0.39 0.35 0.24
± ± ± ± ± ± ±
– 74.2 77.9 83.1 91.9 92.7 95.0
– 0.742 0.779 0.831 0.919 0.927 0.950
0.02 0.16 0.23 0.04 0.03 0.01 0.01
already reaches more than 90%. Afterwards, the further increase in concentration of inhibitor, up to 10−4 M, merely enhances the corrosion inhibition slightly. The inhibitive effect of DBI is also confirmed by scanning electronic microscope (SEM) analysis (see Supporting information). The mild steel surface in the presence of DBI is so significantly protected from the acidic corrosion that the grinding grooves on the surface can be clearly observed after 8 h of the immersion, compared to the surface in the uninhibited solution that is seriously damaged. Also, we performed the electrochemical impedance spectroscopy (EIS) measurements to examine the modification of corrosion mechanism of mild steel by the addition of DBI (see Supporting information). The preliminary electrochemical results show that the outlines of EIS in the absence and presence of inhibitor are similar to each other, namely, only single time-constant or capacitive response corresponding to the charge transfer process of corrosion reaction is observed in both cases. This indicates that the inhibitor functions by blocking effect resulting from adsorption of the inhibitor on the surface of mild steel and does not change the corrosion mechanism of mild steel [35]. Furthermore, adsorption isotherm of DBI is fitted for the degrees of surface coverage () obtained from the weight loss mea-
Fig. 4. Equilibrium configurations of DBI and DBIH in aqueous solution. Top: topview, bottom: sideview.
Please cite this article in press as: D. Zhang, et al., The inhibition performance of long-chain alkyl-substituted benzimidazole derivatives for corrosion of mild steel in HCl, Corros. Sci. (2015), http://dx.doi.org/10.1016/j.corsci.2015.10.002
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4 Table 2 Quantum chemical parameters of DBI and DBIH. Form
EHOMO (eV)
ELUMO (eV)
N
DBI DBIH
−5.08 −5.79
−1.27 −2.34
0.195 −0.044
surements ( = /100). The graphical tests for different isotherms show that the values can be well fitted by a Langmuir adsorption isotherm as follows: C 1 = +C Kads
(3)
where C is the concentration of DBI and Kads is the equilibrium constant of adsorption process. The fitting result is presented in Fig. 2. The Langmuir isotherm indicates that one monolayer of DBI molecule is formed on the metal surface and, furthermore, the slope of the linear plot extremely close to unity suggests that there is negligible interaction between the adsorbate species on the metal surface. Based on the Langmuir isotherm, the standard free energy 0 ) can be estimated by the following equation of adsorption (Gads [23]: Kads =
1 exp 55.5
0 −Gads
RT
(4)
0 The calculated Gads value is −47.29 ± 0.52 kJ/mol. The details of the adsorption of the inhibitor are investigated in the following computational chemistry.
3.2. Computational chemistry of DBI 3.2.1. Quantum chemical calculations Since DBI does not change the corrosion mechanism of mild steel in 1 M HCl, it is of great interest to elucidate how DBI functions on mild steel. Before further investigation, it should be mentioned that due to the weak alkaline of the derivative DBI may be protonated by Bronsted acid as follows [7,8]:
In the acidic medium, that is to say, the benzimidazole derivative may exist as protonated form (DBIH) in equilibrium with the corresponding molecular form (DBI). In the theoretical study, therefore, the two forms were taken into consideration. The structures of DBI and DBIH were first optimized using DFT calculations, taking account of solvent effect by optimizing the structures in water, and then both highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) as well as their corresponding energy values (EHOMO and ELUMO ) were analyzed. It can be seen from Fig. 3 that the distribution of HOMO and LUMO is dominantly concentrated on the benzimidazole segment of the derivative in both molecular form and protonated form, indicating that the benzimidazole segment plays the most important role in adsorption of the inhibitor because the conjugation system with -electrons can support electron donation to the vacant d-orbital of Fe atoms [36]. Furthermore, the distribution density of frontier molecular orbitals slightly decreases as DBI is protonated, suggesting that the protonation of DBI may mitigate the adsorption of inhibitor on iron surface. The inference will be further confirmed by the MD simulations. The frontier molecular orbital energies of the two forms are listed in Table 2. Generally, it is assumed that EHOMO and ELUMO are related to the susceptibility of the molecule to attack by electrophiles and nucleophiles. The lower the value of ELUMO is, the greater the ability of molecule is to accept electrons, while the
higher the value of EHOMO of inhibitor, the easier it is to donate electrons to the unoccupied D-orbital of metal surface [37]. From Table 2, the frontier molecular orbital energies (EHOMO and ELUMO ) of DBIH are lower than those of DBI. Therefore, the protonation of DBI reduces the ability of inhibitor to donate electrons to metal surface but enhances the ability to accept electrons. However, these energy parameters cannot provide the information that the electrons are transferred from inhibitor molecule to metal or otherwise. Herein, the fraction of electrons transferred (N), defined as Eq. (5), is introduced to explain the electron transfer from inhibitor to iron [36]. N =
− inh 2 (Fe + inh )
(5)
where is the work function of iron surface with the value of 3.91 eV for Fe (1 0 0) [38], inh is the absolute electronegativity of inhibitor, defined as = −(EHOMO + ELUMO)/2, and inh is the global hardness of inhibitor, defined as = (ELUMO − EHOMO )/2 [36]. In addition, Fe ≈ 0 is accepted due to the extremely small number [39]. The N value measures the electron transfer from molecule to metal if N > 0 and vice versa if N < 0 [40,41]. It is clear from Table 2 that the N value is positive for the interaction between DBI and Fe (1 0 0) but negative for the interaction between DBIH and Fe (1 0 0). It is concluded, accordingly, that DBI donates electrons to the metal surface while DBIH accepts electrons from the metal surface in the formation of adsorptive layer of the inhibitor. This result once again demonstrates that the protonation of DBI reduces the ability of inhibitor to donate electrons but enhances the ability to accept electrons. Although the electronic nature of inhibitor is clarified, it is still essential to examine the active sites responsible for donating or accepting electrons in the structures of inhibitor. This can be achieved by evaluating the Fukui functions of the two forms of inhibitor. Fukui functions are a measure of local reactivity as well as indicative of local nucleophilic or electrophilic feature in the molecules [25]. The isosurfaces of Fukui functions are graphically presented in Fig. 3. For nucleophilic attacks, the most susceptible sites of both DBI and DBIH forms are carbon and nitrogen atoms of the benzimidazole skeleton. For electrophilic attacks, however, DBI form hardly presents significant susceptible sites except for the tail carbon atom, indicating poor electrophilic capability. With the protonation of DBI, on the contrary, the benzimidazole skeleton of DBIH form as well as the oxygen atom exhibit remarkable electrophilic susceptibility. Combined with the results from the frontier orbital analysis and the fraction of electrons transferred from molecule to iron, it is reasonable to infer that the adsorption of inhibitor be mainly attributed to the interaction between the benzimidazole segment and iron surface, and further DBI form act as electron donator and DBIH form as electron acceptor. 3.3. MD simulations Quantum chemical calculations tell us that the adsorption of the inhibitor is almost exclusively attributed to the interaction between the benzimidazole segment and iron surface. As a consequence, the benzimidazole segment, due to its rigid structure, may adopt a horizontal mode to contact with the iron surface, which can be confirmed by MD simulations. MD simulations were first performed in a vacuum slab without any solvation effect (see Supporting information). It is observed in the vacuum slab that the benzimidazole parts of both DBI and DBIH are nearly parallel to the Fe (1 0 0) plane and their long-chain alkyl groups stretch toward the vacuum slab. The calculations of interaction energy show that the Einteract value of DBIH/Fe system is more negative than that of DBI/Fe system, indicating that the protonation of DBI enhances the adsorption of the inhibitor in a vacuum slab. However, the result from
Please cite this article in press as: D. Zhang, et al., The inhibition performance of long-chain alkyl-substituted benzimidazole derivatives for corrosion of mild steel in HCl, Corros. Sci. (2015), http://dx.doi.org/10.1016/j.corsci.2015.10.002
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Fig. 5. RDF curves of N–Fe after MD simulations.
vacuum system is not fully applicable to the present case because the adsorptive performance of the inhibitor occurs at a solid/liquid interface where the solvation effect ought to be taken into consideration. Moreover, the vacuum result appears to be inconsistent with the quantum chemical results in which the decrease in frontier orbital distribution density resulting from the protonation of DBI would mitigate the interaction of the inhibitor with iron surface as the solvent effect is involved. Thus, the MD simulations in a solventcontaining system were performed in the study as shown in Fig. 4. As in the vacuum case, the benzimidazole segments remain parallel to the Fe (1 0 0) plane in the aqueous modelling system. Nonetheless, the alkyl long chains of the two forms are more conspicuously distorted in the aqueous phase than in the vacuum slab. For the inhibitor, evidently, the distortion can provide larger blocking area for iron surface against the aggressive solution. Furthermore, in the aqueous phase the Einteract value of DBI/Fe system (−464.40 kJ/mol) is more negative than that of DBIH/Fe system (−380.24 kJ/mol), indicating that DBI form could more strongly adsorb on iron surface than DBIH form. Additionally, it can also be seen from Fig. 4 that due to the stronger polarity DBIH is surrounded by a denser water layer compared to DBI, as a result, the adsorptive film of DBI could more effectively get rid of the aggressive species from the iron surface. The radial distribution functions (RDF) between N atoms and Fe atoms were analyzed after MD simulations (Fig. 5). The first peak on RDF represents the most probable distance between N atoms (and further benzimidazole segment due to the flat mode) and Fe atoms. The NFe distance values for the two forms are both around 3.2 Å, indicating the tight adsorption of the inhibitor on iron surface, and the RDF tendency of the two forms is similar in a large range from 2 to 6 Å. However, it is still observed from Fig. 5 that the NFe distance in DBI/Fe system is slightly less than that in DBIH/Fe system, indicating the stronger interaction of DBI form with iron surface than DBIH form. The above MD calculations are based on the case in which no specific adsorption occurs at the solid/liquid interface. It is well known that the steel surface bears positive charge in 1 M HCl [42], so it is difficult for the DBIH form to approach the positively charged mild steel surface due to the electrostatic repulsion.
However, the Cl− ions can first adsorbed on the steel surface creating an excess negative charge in 1 M HCl [43,44]. Accordingly, the DBIH form, which is the predominant form in 1 M HCl, may preferentially adsorb on the negatively charged surface through electrostatic interaction. In doing so, afterwards, it is possible that the protonated molecule loses its associated proton either in entering the double layer or in taking part in the reaction at the cathodic sites and adsorb by donating electrons to the metal [45,46]. The possibility is supported by the present MD simulations where the interaction of the DBI form with the mild steel surface is stronger than that of the DBIH form. Of course, we need more evidence from next experiment and theoretical calculation such as the simulation containing the specific adsorption of Cl− ions to completely clarify the adsorption mechanism of DBI. Also, it should be mentioned that the inhibition performance is not only from the direct interaction of the inhibitor with iron surface but also from the barrier effect of the non-polar long chain against aggressive solution.
4. Conclusions DBI at a rather low concentration exhibits effective inhibition for corrosion of mild steel in acidic medium. The adsorption of DBI on mild steel surface obeys a Langmuir isotherm. Theoretical study shows that DBI adsorbs on iron surface by donating electrons to the vacant d-orbital. The adsorption is achieved by the interaction between of the benzimidazole segment of DBI and iron surface, resulting in the horizontal state of benzimidazole segment on iron surface. Meanwhile, the non-polar long chain of DBI plays an important role in the formation of barrier against the aggressive solution. In addition, compared to the protonation form of DBI (DBIH), the interaction of the molecular form of DBI with the mild steel surface is stronger based on both quantum chemical calculations and MD simulations. We hope that the present study will provide a distinctive perspective on developing new type of benzimidazole derivatives as corrosion inhibitor.
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The inhibition performance of long-chain alkyl-substituted benzimidazole derivatives for corrosion of mild steel in HCl Dongqin Zhang a , Yongming Tang a,∗ , Sijun Qi a , Dawei Dong a , Hui Cang b , Gang Lu c a b c
School of Chemistry and Molecular Engineering, Nanjing Tech University, Nanjing 211816, PR China College of Chemical Engineering and Biological, Yancheng Institute of Technology, Yancheng 224051, PR China College of Materials Science and Engineering, Nanjing Tech University, Nanjing 210009, PR China
a r t i c l e
i n f o
Article history: Received 28 August 2015 Received in revised form 2 October 2015 Accepted 5 October 2015 Available online xxx Keywords: A. Mild steel B. Modelling studies B. Weight loss C. Acid inhibition C. Interfaces
a b s t r a c t The corrosion inhibition of a new benzimidazole derivative, 6-(dodecyloxy)-1H-benzo[d]imidazole (DBI), for mild steel in 1 M HCl was investigated in this paper. Computational chemistry was performed to explore the adsorption of DBI on metal surface. Inhibition performance of DBI is attributed to both the direct interaction of benzimidazole segment with iron surface and the barrier effect of the non-polar long chain against aggressive solution. Compared to the protonated form, the molecular form of DBI could more tightly interact with iron surface. These results show that the long-chain alkyl-substituted benzimidazole derivative is of great potential application as corrosion inhibitor. © 2015 Elsevier Ltd. All rights reserved.
1. Introduction A continuing effort has been made to develop a corrosion inhibitor that exhibits a greater inhibition effect at a low concentration in the corrosion medium as well as environment-friendly feature [1]. In recent years, benzimidazole and its derivatives with low toxicity have received considerable attention as they provide effective corrosion inhibition due to strong adsorption on metal surface [2–6]. Benzimidazole, a heterocyclic aromatic organic compound, consists of the fusion of benzene and imidazole. The nitrogen atom and the aromatic ring in molecular structure are likely the facilitator of the adsorption of benzimidazole on the metallic surface [7–11]. However, relatively high concentration of benzimidazole is essential to achieve effective inhibition for steel in acidic medium. At the concentration of 10 mM, typically, the inhibition efficiency of benzimidazole for mild steel in 1 M HCl is only around 50% [5,12]. In hope of improving the inhibition effectiveness of benzimidazole, researchers have made a lot of attempts to develop new benzimidazole derivatives. Among all the benzimidazole derivatives, 2-substituted benz-
∗ Corresponding author. Fax: +86 25 83587443. E-mail address: [email protected] (Y. Tang).
imidazole derivatives have been paid most attention to. It has been demonstrated that substitution of some groups including mercapto, alkyl, amino and aromatic groups etc. for 2-H of benzimidazole significantly enhances corrosion inhibition, as compared to benzimidazole [12–23]. Computational chemistry showed that those substituent groups, which enhance the electron-donating or electron-withdrawing properties of the active N atom on the heterocyclic ring, would strengthen or weaken the interaction with the metal surface [24–30]. In addition, several derivatives with substitution for 1-H of benzimidazole exhibit effective corrosion inhibition for mild steel in acidic medium as well [3,31]. Regardless of the mentioned derivatives, there are few reports on corrosion inhibition of the benzimidazole derivative with substituted group on benzene ring [32,33]. In the present work, we attempt to prepare a new benzimidazole derivative, 6(dodecyloxy)-1H-benzo[d]imidazole (DBI), shown in Fig. 1 and to evaluate its corrosion inhibition for mild steel in 1 M HCl. In the structure of derivative, a long-chain alkyl group is connected to benzene ring of benzimidazole via oxygen atom. Evidently, there are two distinct segments in the structure: an alkyl chain as a non-polar tail and a benzimidazole segment as a polar head. This compound is expected to act as adsorption-type inhibitor as well as straight-chain amines [34].
http://dx.doi.org/10.1016/j.corsci.2015.10.002 0010-938X/© 2015 Elsevier Ltd. All rights reserved.
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Fig. 2. Langmuir adsorption plot of the inhibitor in 1 M HCl at 25 ◦ C.
2. Experimental and computational details 2.1. Corrosion measurements Corrosion measurements were performed using mild steel coupons containing 0.17 wt.% C, 0.20 wt.% Si, 0.37 wt.% Mn, 0.03 wt.% S, 0.01 wt.% P and balance iron. Prior to use, the coupons with the size of 5.0 × 2.5 × 0.2 cm3 were mechanically abraded up to 1000 grit, cleaned with ethanol and deionized water, and finally dried at room temperature. Weight loss measurements were carried out in a 250 mL vessel in a thermostat water bath held at the temperature of 25 ± 1 ◦ C. The coupons were fully immersed in 1 M HCl solution containing various concentrations of the inhibitor. After 8 h of the immersion, the specimens were taken out, rinsed thoroughly with deionized water, scrubbed gently with a bristle brush to remove the corrosion product, washed successively with deionized water and ethanol, dried at room temperature and weighed. The inhibition efficiency () was calculated as follows: (%) =
v0 − v × 100 v0
(1)
where v0 and v are the corrosion rate in the absence and presence of inhibitor, respectively. Duplicate experiments were performed in each case and both mean value and deviation of corrosion rate are reported. 2.2. Computational details Density functional theory (DFT) calculations were performed to optimize geometries of the molecules without any symmetry constraint using D mol3 module of Materials Studio software 6.0. All electron calculations of the molecules were performed using the generalized gradient approximation, following the PerdewBuekeErnzerhof scheme (GGA/PBE), for the exchange-correlation functional. The “DNP” double-numeric basis set, which includes both d and p orbital polarization functions, was used in all calculations. And vibrational analysis was carried out to ensure the calculated structures reaching the minimum point on potential energy surface. In addition, the solvent (water) model was involved in all calculations.
Fig. 1. Structure of DBI.
MD simulations were performed using Discover module of Materials Studio software 6.0. To construct the simulation box, the unit cell of iron was optimized to a minimum point of energy. A Fe (1 0 0) surface was cleaved from the bulk structure and optimized, and then the resulting cell was repeated ten times in the lateral directions (10 × 10). A water slab containing the studied compounds was then added near to the upper surface of the Fe slab. The size of the resulting box with periodic boundary conditions is 28.6 × 28.6 × 45.9 Å3 . MD simulations were performed using COMPASS force field as all Fe atoms were kept “frozen” at fixed positions and both inhibitor and water molecules were allowed to freely interact with the iron surface. A simulation temperature of 298 K and NVT ensemble with a time step of 0.1 fs and simulation time of 100 ps were implemented in all MD simulations. The interaction energy (Einteract ) between Fe (1 0 0) and inhibitive molecule was calculated as follows: Einteract = Etot − (Esurf + H2 O + Einh )
(2)
where Etot is the total energy of simulation system, Esurf+H
2O
is the
energy of iron surface together with H2 O molecules, and Einh is the energy of free inhibitive molecule. 3. Results and discussion 3.1. Corrosion inhibition of DBI Table 1 depicts the corrosion rate of mild steel and inhibition efficiency from weight loss measurements in the absence and presence of DBI. It is clear that the corrosion rate of mild steel decreases significantly and the inhibition efficiency increases with increasing inhibitor concentration. Moreover, at a rather low concentration of the inhibitor, for instance, 8 × 10−6 M, the inhibition efficiency
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Table 1 Corrosion rate and inhibition efficiency in the absence and presence of inhibitor in 1 M HCl solution.
Fig. 3. Distribution of HOMO and LUMO (left) and isosurfaces of Fukui functions (right) of DBI and DBIH.
C (M)
v (g m−2 h−1 )
(%)
0 10−6 2 × 10−6 4 × 10−6 8 × 10−6 10−5 10−4
4.80 1.24 1.06 0.81 0.39 0.35 0.24
± ± ± ± ± ± ±
– 74.2 77.9 83.1 91.9 92.7 95.0
– 0.742 0.779 0.831 0.919 0.927 0.950
0.02 0.16 0.23 0.04 0.03 0.01 0.01
already reaches more than 90%. Afterwards, the further increase in concentration of inhibitor, up to 10−4 M, merely enhances the corrosion inhibition slightly. The inhibitive effect of DBI is also confirmed by scanning electronic microscope (SEM) analysis (see Supporting information). The mild steel surface in the presence of DBI is so significantly protected from the acidic corrosion that the grinding grooves on the surface can be clearly observed after 8 h of the immersion, compared to the surface in the uninhibited solution that is seriously damaged. Also, we performed the electrochemical impedance spectroscopy (EIS) measurements to examine the modification of corrosion mechanism of mild steel by the addition of DBI (see Supporting information). The preliminary electrochemical results show that the outlines of EIS in the absence and presence of inhibitor are similar to each other, namely, only single time-constant or capacitive response corresponding to the charge transfer process of corrosion reaction is observed in both cases. This indicates that the inhibitor functions by blocking effect resulting from adsorption of the inhibitor on the surface of mild steel and does not change the corrosion mechanism of mild steel [35]. Furthermore, adsorption isotherm of DBI is fitted for the degrees of surface coverage () obtained from the weight loss mea-
Fig. 4. Equilibrium configurations of DBI and DBIH in aqueous solution. Top: topview, bottom: sideview.
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4 Table 2 Quantum chemical parameters of DBI and DBIH. Form
EHOMO (eV)
ELUMO (eV)
N
DBI DBIH
−5.08 −5.79
−1.27 −2.34
0.195 −0.044
surements ( = /100). The graphical tests for different isotherms show that the values can be well fitted by a Langmuir adsorption isotherm as follows: C 1 = +C Kads
(3)
where C is the concentration of DBI and Kads is the equilibrium constant of adsorption process. The fitting result is presented in Fig. 2. The Langmuir isotherm indicates that one monolayer of DBI molecule is formed on the metal surface and, furthermore, the slope of the linear plot extremely close to unity suggests that there is negligible interaction between the adsorbate species on the metal surface. Based on the Langmuir isotherm, the standard free energy 0 ) can be estimated by the following equation of adsorption (Gads [23]: Kads =
1 exp 55.5
0 −Gads
RT
(4)
0 The calculated Gads value is −47.29 ± 0.52 kJ/mol. The details of the adsorption of the inhibitor are investigated in the following computational chemistry.
3.2. Computational chemistry of DBI 3.2.1. Quantum chemical calculations Since DBI does not change the corrosion mechanism of mild steel in 1 M HCl, it is of great interest to elucidate how DBI functions on mild steel. Before further investigation, it should be mentioned that due to the weak alkaline of the derivative DBI may be protonated by Bronsted acid as follows [7,8]:
In the acidic medium, that is to say, the benzimidazole derivative may exist as protonated form (DBIH) in equilibrium with the corresponding molecular form (DBI). In the theoretical study, therefore, the two forms were taken into consideration. The structures of DBI and DBIH were first optimized using DFT calculations, taking account of solvent effect by optimizing the structures in water, and then both highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) as well as their corresponding energy values (EHOMO and ELUMO ) were analyzed. It can be seen from Fig. 3 that the distribution of HOMO and LUMO is dominantly concentrated on the benzimidazole segment of the derivative in both molecular form and protonated form, indicating that the benzimidazole segment plays the most important role in adsorption of the inhibitor because the conjugation system with -electrons can support electron donation to the vacant d-orbital of Fe atoms [36]. Furthermore, the distribution density of frontier molecular orbitals slightly decreases as DBI is protonated, suggesting that the protonation of DBI may mitigate the adsorption of inhibitor on iron surface. The inference will be further confirmed by the MD simulations. The frontier molecular orbital energies of the two forms are listed in Table 2. Generally, it is assumed that EHOMO and ELUMO are related to the susceptibility of the molecule to attack by electrophiles and nucleophiles. The lower the value of ELUMO is, the greater the ability of molecule is to accept electrons, while the
higher the value of EHOMO of inhibitor, the easier it is to donate electrons to the unoccupied D-orbital of metal surface [37]. From Table 2, the frontier molecular orbital energies (EHOMO and ELUMO ) of DBIH are lower than those of DBI. Therefore, the protonation of DBI reduces the ability of inhibitor to donate electrons to metal surface but enhances the ability to accept electrons. However, these energy parameters cannot provide the information that the electrons are transferred from inhibitor molecule to metal or otherwise. Herein, the fraction of electrons transferred (N), defined as Eq. (5), is introduced to explain the electron transfer from inhibitor to iron [36]. N =
− inh 2 (Fe + inh )
(5)
where is the work function of iron surface with the value of 3.91 eV for Fe (1 0 0) [38], inh is the absolute electronegativity of inhibitor, defined as = −(EHOMO + ELUMO)/2, and inh is the global hardness of inhibitor, defined as = (ELUMO − EHOMO )/2 [36]. In addition, Fe ≈ 0 is accepted due to the extremely small number [39]. The N value measures the electron transfer from molecule to metal if N > 0 and vice versa if N < 0 [40,41]. It is clear from Table 2 that the N value is positive for the interaction between DBI and Fe (1 0 0) but negative for the interaction between DBIH and Fe (1 0 0). It is concluded, accordingly, that DBI donates electrons to the metal surface while DBIH accepts electrons from the metal surface in the formation of adsorptive layer of the inhibitor. This result once again demonstrates that the protonation of DBI reduces the ability of inhibitor to donate electrons but enhances the ability to accept electrons. Although the electronic nature of inhibitor is clarified, it is still essential to examine the active sites responsible for donating or accepting electrons in the structures of inhibitor. This can be achieved by evaluating the Fukui functions of the two forms of inhibitor. Fukui functions are a measure of local reactivity as well as indicative of local nucleophilic or electrophilic feature in the molecules [25]. The isosurfaces of Fukui functions are graphically presented in Fig. 3. For nucleophilic attacks, the most susceptible sites of both DBI and DBIH forms are carbon and nitrogen atoms of the benzimidazole skeleton. For electrophilic attacks, however, DBI form hardly presents significant susceptible sites except for the tail carbon atom, indicating poor electrophilic capability. With the protonation of DBI, on the contrary, the benzimidazole skeleton of DBIH form as well as the oxygen atom exhibit remarkable electrophilic susceptibility. Combined with the results from the frontier orbital analysis and the fraction of electrons transferred from molecule to iron, it is reasonable to infer that the adsorption of inhibitor be mainly attributed to the interaction between the benzimidazole segment and iron surface, and further DBI form act as electron donator and DBIH form as electron acceptor. 3.3. MD simulations Quantum chemical calculations tell us that the adsorption of the inhibitor is almost exclusively attributed to the interaction between the benzimidazole segment and iron surface. As a consequence, the benzimidazole segment, due to its rigid structure, may adopt a horizontal mode to contact with the iron surface, which can be confirmed by MD simulations. MD simulations were first performed in a vacuum slab without any solvation effect (see Supporting information). It is observed in the vacuum slab that the benzimidazole parts of both DBI and DBIH are nearly parallel to the Fe (1 0 0) plane and their long-chain alkyl groups stretch toward the vacuum slab. The calculations of interaction energy show that the Einteract value of DBIH/Fe system is more negative than that of DBI/Fe system, indicating that the protonation of DBI enhances the adsorption of the inhibitor in a vacuum slab. However, the result from
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Fig. 5. RDF curves of N–Fe after MD simulations.
vacuum system is not fully applicable to the present case because the adsorptive performance of the inhibitor occurs at a solid/liquid interface where the solvation effect ought to be taken into consideration. Moreover, the vacuum result appears to be inconsistent with the quantum chemical results in which the decrease in frontier orbital distribution density resulting from the protonation of DBI would mitigate the interaction of the inhibitor with iron surface as the solvent effect is involved. Thus, the MD simulations in a solventcontaining system were performed in the study as shown in Fig. 4. As in the vacuum case, the benzimidazole segments remain parallel to the Fe (1 0 0) plane in the aqueous modelling system. Nonetheless, the alkyl long chains of the two forms are more conspicuously distorted in the aqueous phase than in the vacuum slab. For the inhibitor, evidently, the distortion can provide larger blocking area for iron surface against the aggressive solution. Furthermore, in the aqueous phase the Einteract value of DBI/Fe system (−464.40 kJ/mol) is more negative than that of DBIH/Fe system (−380.24 kJ/mol), indicating that DBI form could more strongly adsorb on iron surface than DBIH form. Additionally, it can also be seen from Fig. 4 that due to the stronger polarity DBIH is surrounded by a denser water layer compared to DBI, as a result, the adsorptive film of DBI could more effectively get rid of the aggressive species from the iron surface. The radial distribution functions (RDF) between N atoms and Fe atoms were analyzed after MD simulations (Fig. 5). The first peak on RDF represents the most probable distance between N atoms (and further benzimidazole segment due to the flat mode) and Fe atoms. The NFe distance values for the two forms are both around 3.2 Å, indicating the tight adsorption of the inhibitor on iron surface, and the RDF tendency of the two forms is similar in a large range from 2 to 6 Å. However, it is still observed from Fig. 5 that the NFe distance in DBI/Fe system is slightly less than that in DBIH/Fe system, indicating the stronger interaction of DBI form with iron surface than DBIH form. The above MD calculations are based on the case in which no specific adsorption occurs at the solid/liquid interface. It is well known that the steel surface bears positive charge in 1 M HCl [42], so it is difficult for the DBIH form to approach the positively charged mild steel surface due to the electrostatic repulsion.
However, the Cl− ions can first adsorbed on the steel surface creating an excess negative charge in 1 M HCl [43,44]. Accordingly, the DBIH form, which is the predominant form in 1 M HCl, may preferentially adsorb on the negatively charged surface through electrostatic interaction. In doing so, afterwards, it is possible that the protonated molecule loses its associated proton either in entering the double layer or in taking part in the reaction at the cathodic sites and adsorb by donating electrons to the metal [45,46]. The possibility is supported by the present MD simulations where the interaction of the DBI form with the mild steel surface is stronger than that of the DBIH form. Of course, we need more evidence from next experiment and theoretical calculation such as the simulation containing the specific adsorption of Cl− ions to completely clarify the adsorption mechanism of DBI. Also, it should be mentioned that the inhibition performance is not only from the direct interaction of the inhibitor with iron surface but also from the barrier effect of the non-polar long chain against aggressive solution.
4. Conclusions DBI at a rather low concentration exhibits effective inhibition for corrosion of mild steel in acidic medium. The adsorption of DBI on mild steel surface obeys a Langmuir isotherm. Theoretical study shows that DBI adsorbs on iron surface by donating electrons to the vacant d-orbital. The adsorption is achieved by the interaction between of the benzimidazole segment of DBI and iron surface, resulting in the horizontal state of benzimidazole segment on iron surface. Meanwhile, the non-polar long chain of DBI plays an important role in the formation of barrier against the aggressive solution. In addition, compared to the protonation form of DBI (DBIH), the interaction of the molecular form of DBI with the mild steel surface is stronger based on both quantum chemical calculations and MD simulations. We hope that the present study will provide a distinctive perspective on developing new type of benzimidazole derivatives as corrosion inhibitor.
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Please cite this article in press as: D. Zhang, et al., The inhibition performance of long-chain alkyl-substituted benzimidazole derivatives for corrosion of mild steel in HCl, Corros. Sci. (2015), http://dx.doi.org/10.1016/j.corsci.2015.10.002