Two phenylpyrimidine derivatives as new corrosion inhibitors for cold rolled steel in hydrochloric acid solution

Accepted Manuscript Two phenylpyrimidine derivatives as new corrosion inhibitors for cold rolled steel in hydrochloric acid solution Xianghong Li, Xiaoguang Xie, Shuduan Deng, Guanben Du PII: DOI: Reference:

S0010-938X(14)00252-2 http://dx.doi.org/10.1016/j.corsci.2014.05.017 CS 5867

To appear in:

Corrosion Science

Received Date: Accepted Date:

20 December 2013 27 May 2014

Please cite this article as: X. Li, X. Xie, S. Deng, G. Du, Two phenylpyrimidine derivatives as new corrosion inhibitors for cold rolled steel in hydrochloric acid solution, Corrosion Science (2014), doi: http://dx.doi.org/ 10.1016/j.corsci.2014.05.017

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Two phenylpyrimidine derivatives as new corrosion inhibitors for cold rolled steel in hydrochloric acid solution

Xianghong Lia,b*,

a

b

Xiaoguang Xieb, Shuduan Dengc,

Guanben Duc

Faculty of Science, Southwest Forestry University, Kunming 650224, P.R. China

School of Chemical Science and Technology, Yunnan University, Kunming 650091, P.R. China

c

Yunnan key laboratory of wood adhesives and glue products, Southwest Forestry University, Kunming 650224, P.R. China

*Corresponding author. Tel.: +86 871 63861218; fax: +86 871 63863150. E-mail address: [email protected] (X.H. Li).

Abstract

The inhibition effect of two phenylpyrimidine derivatives of 4-phenylpyrimidine (4-PPM) and 5-phenylpyrimidine (5-PPM) on the corrosion of cold rolled steel (CRS) in HCl solution was studied by weight loss, polarization curves, electrochemical impedance spectroscopy (EIS) and scanning electron microscope (SEM) methods. Quantum chemical calculation and molecular dynamics (MD) were applied to theoretically determine the relationship between molecular structure and inhibition efficiency. The results show that two phenylpyrimidine derivatives are good inhibitors, and inhibition efficiency follows the order: 4-PPM > 5-PPM. The adsorption of each inhibitor on steel surface obeys Langmuir adsorption isotherm. Two phenylpyrimidine derivatives act as mixed-type inhibitors.

Keywords: A. Steel; B. Weight loss; B. Polarization; B. EIS; B. Modelling studies; C. Acid inhibition

1. Introduction

Using inhibitors is one of the most practical methods for protecting metals against corrosion,

especially in acidic media [1]. Among numerous inhibitors, N-heterocyclic compounds are considered to be the most effective corrosion inhibitors for steel in acid solution [2]. Previously, various N-heterocyclic compounds are reported as good corrosion inhibitors for steel in acidic media, such as imidazoline derivatives [3], 1,2,3-triazole derivatives [4,5], 1,2,4-triazole derivatives [6-11], benzotriazole [12,13], pyrrole [14], pyridine derivatives [15,16], pyrazole derivatives [17], bipyrazole derivatives [18] , pyrazine derivatives [19], pyridazine derivatives [20], indole derivatives [21], benzimidazole derivatives [22,23], quinoline derivatives [24], purine derivatives [25,26] and tetrazole derivatives [27-29]. They exhibit inhibition by adsorption on the metal surface, and the adsorption takes place through N, O, and S atoms, as well as those with triple or conjugated double bonds or aromatic rings in their molecular structures. Furthermore, the adsorption of inhibitor on steel/solution interface is influenced by the chemical structure of inhibitor, the nature and charged surface of metal, the distribution of charge over the whole inhibitor molecule and the type of aggressive media. If a substitution polar group (—NH2, —OH, —SH, etc.) is added to the N-heterocyclic ring, the electron density of N-heterocyclic ring is increased, and subsequently, it facilitates the adsorb ability. As an important kind of N-heterocyclic compound, pyrimidine derivatives whose molecules possess the pyrimidine ring with two N heteroatoms have favorable characteristics in inhibiting action. 2-Mercaptopyrimidine (MP) was studied as the good corrosion inhibitor for non-ferrous metals in acid solution, such as zinc in HCl [30] and H3PO4 [31], aluminium in HCl [32], copper in H2SO4 [33]. Besides non-ferrous metal, the inhibition effect of pyrimidine derivative on the steel corrosion in acid media was studied. In 1993, Zucchi et al. [34] investigated the corrosion inhibition of steel in H2SO4 by some pyrimidine derivatives. In 2001, Wang [35] reported the

corrosion inhibition by 2-mercaptopyrimidine (MP) for low carbon steel in H3PO4, and the maximum inhibition efficiency (η) at 10.0 mM is 98% in 3.0 M H3PO4. Afterwards, Awad and Abdel Gawad [36] fully studied the corrosion inhibition of pyrimidine and seven pyrimidine derivatives on iron in 2.0 M HCl, and the results showed that the inhibition efficiency (η) of these pyrimidines

follows

the

order:

2,4-diamino-6-mercaptopyrimidine

>

2,4-diamino-6-mercaptopyrimidine > 2,4,6-triaminopyrimidine > 2,4-diaminopyrimidine > 2-mercaptopyrimidine > 2-hydroxylpyrimidine > 2-aminopyrimidine > pyrimidine. According to our

recent

work

[37,38],

2-aminopyrimidne

(AP),

2-hydroxypyrimidne

(HP)

and

2-mercaptopyrimidne (MP) also act as good inhibitors on the corrosion of steel in 1.0 M HCl. However, another pyrimidine derivative of uracil (Ur) exhibits poor inhibitive ability for steel in HCl [39], H2SO4 [40] and H3PO4 [41] solutions. Through these studies, the efficiency of pyrimidine compound mainly depends on the substitution group in the pyimidine ring. Accordingly, there is a great need to obtain the correlation between the molecular structure and inhibitive performance. Quantum chemical calculation has been proven to be a very useful method in corrosion inhibitor studies [42]. The theoretical parameters of inhibitor molecule can be obtained, and then, theoretically speaking, the inhibitive mechanism can be directly accounted for the chemical reactivity of the compound under study [43]. The inhibition activity of a given inhibitor is directly correlated with the theoretical parameters including the highest occupied molecular orbital energy (EHOMO), the lowest unoccupied molecular orbital (ELUMO), dipole moment (μ), atomic charge, etc. Besides the quantum parameters of organic inhibitor, the interaction between inhibitor and metal surface should also be considered. Recently, molecular dynamics (MD) simulation could elucidate

the adsorption of inhibitor on metal surface at molecular level [44,45]. It can provide the adsorption mode of inhibitor molecule on metal surface, and obtain the adsorption energy value between the organic inhibitor and metal surface, then give insights into the difference in inhibitive performance between different inhibitors. An extensive survey literature reveals that little information is available on the corrosion inhibition of phenylpyrimidine derivatives for steel in acid solutions. Also, in order to elucidate the effect of substitution position of phenyl group on inhibition efficiency. In this paper, two phenylpyrimidine derivatives of 4-phenylpyrimidine (4-PPM) and 5-phenylpyrimidine (5-PPM) are selected based on the consideration of their molecular structures. The inhibition effect of 4-PPM and 5-PPM on the corrosion of cold rolled steel (CRS) in HCl solution was studied using weight loss, potentiodynamic polarization curves, electrochemical impedance spectroscopy (EIS) and scanning electron microscope (SEM) methods. Effects of inhibitor concentration, temperature, immersion time and acid concentration on inhibition action were fully investigated. The adsorption isotherm of inhibitor on steel surface, the standard adsorption free energy (ΔG0) and apparent activation energy (Ea) are obtained. Quantum chemical calculation of DFT including the solvent effect was applied to study the difference in molecular parameters between 4-PPM and 5-PPM. Furthermore, the adsorption of inhibitor molecule on Fe (001) surface was studied by MD simulations. It is expected to obtain general information on the adsorption and inhibition effect of 4-PPM and 5-PPM on steel in HCl solution.

2. Experimental

2.1. Materials

Weight loss and electrochemical tests were performed on cold rolled steel (CRS) with the following composition (wt.%): 0.07% C, 0.3% Mn, 0.022% P, 0.010% S, 0.01% Si, 0.030% Al, and the remainder Fe. Two phenylpyrimidine derivatives of 4-PPM and 5-PPM with analytical reagent (AR) grade were obtained from Shanghai Chemical Reagent Company of China, and their molecular structures are shown in Fig. 1. The aggressive solutions of 1.0—5.0 M HCl were prepared by dilution of AR grade 37% HCl with distilled water. 4-PPM and 5-PPM have good solubility in water. Accordingly, the stock solution (10.0 mM) was prepared by directly dissolved in distilled water, and then was diluted to a certain concentration. The concentration range of inhibitor is 0.2—5.0 mM.

2.2. Weight loss measurements

The CRS rectangular coupons of 2.5 cm × 2.0 cm × 0.06 cm were abraded by a series of emery paper (grade 320-500-800) and then washed with distilled water, degreased with acetone, and finally dried at room temperature. After weighing using digital balance with sensitivity of ± 0.1 mg, the specimens were immersed in glass beakers containing 250 mL 1.0 M HCl without and with different concentrations of inhibitor using glass hooks and rods. The temperature was controlled at 20 ± 0.1 oC using a water thermostat bath. All the aggressive acid solutions were open to air without bubbling. After 6 h, the specimens were taken out, washed with bristle brush under running water to remove the corrosion product, dried with a hot air stream, and re-weighed

accurately. In order to get good reproducibility, experiments were carried out in duplicate. Then the tests were repeated at different temperatures, immersion time and HCl concentration. The average weight loss of two parallel CRS sheets was obtained, and then the inhibition efficiency (ηw) was calculated [46].

2.3. Electrochemical measurements

Electrochemical experiments were carried out in the conventional three-electrode system with a platinum counter electrode (CE) and a saturated calomel electrode (SCE) coupled to a fine Luggin capillary as the reference electrode. In order to minimise ohmic contribution, the Luggin capillary was placed close to the working electrode (WE) which was in the form of a square CRS embedded in olyvinyl chloride (PVC) holder using epoxy resin so that the flat surface was only surface in the electrode. The working surface area was 1.0 cm × 1.0 cm, and prepared as described above (Section 2.2). The electrolyte is 250 mL 1.0 M HCl without and with different concentration of 4-PPM or 5-PPM. The electrode was immersed in test solution at open circuit potential (OCP) for 2 h to be sufficient to attain a stable state before measurement. All electrochemical measurements were carried out at 20 oC using PARSTAT 2273 advanced electrochemical system (Princeton Applied Research). The potentiodynamic polarization curves were carried out by polarizing to ±250 mV with respect to open circuit potential (OCP) at a scan rate of 0.5 mV/s. Inhibition efficiency (ηp) is calculated through the corrosion current density (icorr) values [46]. Electrochemical impedance spectroscopy (EIS) was carried out at OCP over a frequency range of 100 kHz - 10 mHz using a

10 mV r.m.s. voltage excitation. The total number of points is 30. Inhibition efficiency (ηR) is estimated using the charge transfer resistance (Rt) values [46].

2.4. Scanning electron microscope (SEM)

Samples of dimension 2.5 cm × 2.0 cm × 0.06 cm were prepared as described above (Section 2.2). After immersion in 1.0 M HCl solutions without and with addition of 20 mM HMU and 20 mM BHMU at 20 oC for 6 h, the specimens were cleaned with distilled water, dried with a cold air blaster, and then examined by S-3000N scanning electron microscope (SEM) (Hitachi High-Tech Science Systems Corporation, Japan).

2.5. Quantum chemical calculations

Quantum chemical calculations were performed with DMol3 numerical based density function theory (DFT) in Materials Studio 4.1 software from Accelrys Inc [47]. Geometrical optimizations and frequency calculations were carried out with the generalized gradient approximation (GGA) functional of Becke exchange plus Lee-Yang-Parr correlation (BLYP) [48] in conjunction with double numerical plus d-functions (DND) basis set [49]. Fine convergence criteria and global orbital cutoffs were employed on basis set definitions. Considering the solvent effects, all the geometries were re-optimized at the BLYP/DND level by using COSMO (conductor-like screening model) [50] and defining water as the solvent. Through the frequency analysis, the optimized geometric structures are verified that they have no imaginary frequencies.

2.6. MD simulations

MD simulations were also performed with Discover program in Materials Studio 4.1 software from Accelrys Inc [47]. Fe (001) plane was firstly cleaved from pure Fe crystal, the surface was then optimized to the energy minimum, and then was enlarge to fabricate an appropriate supercell. After that, a vacuum slab with 1 Å thickness was built above the Fe (001) supercell with 31.53 Å × 31.53 Å × 15.30 Å of total 1331 Fe atoms. Meanwhile, the optimized inhibitor molecules in Section 2.4 were also built using the Amorphous cell program. Finally, the adsorption system was built by layer builder to place the inhibitor layer to Fe (001) supercell. All these slabs are separated by a 10 Å vacucum thickness to ensure that the interaction between the periodically repeated slabs along the normal of the surface is small enough. The adsorption system was optimized using COMPASS force field. The MD simulation was performed under 298 K, NVT ensemble, with a time step of 1.0 fs and simulation time of 1000 ps. The adsorption energy (Eads) of the inhibitor molecule on the Fe (001) surface was calculated as follows [51]:

Eads = ( Einh + Esurf ) − Etotal

(1)

where Einh and Esurf are the energies of the free inhibitor molecule and Fe (001) plane, respectively. Etotal is the total energy of Fe (001) plane together with inhibitor molecule adsorbed on the iron surface. The binding energy (Ebin) is the negative value of the adsorption energy [52]:

Ebin = − Eads

3. Results and discussion

(2)

3.1. Weight loss measurements

3.1.1. Effect of phenylpyrimidine derivatives on corrosion rate

Fig. 2 illustrates the corrosion rate (v) of CRS in 1.0 M HCl at 20 oC in the presence of different concentrations (c) of phenylpyrimidine derivatives (immersion time is 6 h). The corrosion rate reduces after addition of the studied two phenylpyrimidine derivatives, and decreases with the inhibitor concentration. This behavior is due to the fact that the adsorption coverage increases with the increase of inhibitor concentration, which shields the CRS surface efficiently from the medium. In the absence of inhibitor, the corrosion rate is as high as 4.67 g m-2 h-1. While in the presence of 5.0 mM inhibitor, the corrosion rate values are reduced to 0.37 and 0.17 g m-2 h-1 for 5-PPM and 4-PPM, respectively. At any given inhibitor concentration, the corrosion rate follows the order: v (5-PPM) < v (4-PPM), which indicates that 4-PPM exhibits better inhibitive performance between two phenylpyrimidine derivatives.

3.1.2. Effect of phenylpyrimidine derivatives on inhibition efficiency

Fig. 3 represents inhibition efficiency (ηw) values obtained from the weight loss in 1.0 M HCl solutions in the presence of various concentrations of 4-PPM and 5-PPM at 20 oC (immersion time is 6 h). Clearly, ηw increases with an increase in the inhibitor concentration. It should be noted that when the concentration of inhibitor is about 3.0 mM, ηw reaches certain value and

changes slightly with a further increase in the inhibitor concentration. At 5.0 mM, the maximum ηw is 92.2% for 5-PPM; and 96.3% for 4-PPM, which indicates two phenylpyrimidine derivatives act as good corrosion inhibitors for CRS in 1.0 M HCl. Inhibition efficiency of the examined phenylpyrimidine derivatives follows the order: 4-PPM > 5-PPM. Under the same conditions (inhibitor concentration: 5.0 mM, temperature: 20 oC), ηw of AP, HP and MP are 83.3%, 82.0% and 88.4%, respectively [10]. Thus, 4-PPM and 5-PPM exhibit better inhibitive performance comparing with AP, HP and MP [10]. It could be reasonable deduced that the substituted phenyl (—C6H6) group has higher inhibitive performance than amino (—NH2), hydroxyl (—CH3) or mercapto (—SH) group. The reason could be explained as follows: the strong conjugation between —C6H6 and pyrimidine ring significantly facilitates the adsorption of the phenyl group, and then efficiently covers more surface area than —NH2, —CH3 or —SH group.

3.1.3. Adsorption isotherm and standard adsorption free energy (ΔG0)

Fundamental information on the adsorption of inhibitor on metal surface can be provided by adsorption isotherm. Several isotherms including Frumkin, Langmuir, Temkin, Freundlich, Bockris-Swinkels and Flory-Huggins isotherms are employed to fit the experimental data. It is found that the adsorption of studied phenylpyrimidine compounds on steel surface obeys Langmuir adsorption isotherm [38,46]:

c 1 = +c θ K

(3)

where c is the concentration of inhibitor, K the adsorption equilibrium constant, and θ is the surface coverage and calculated by the ration of ηw %/100.

Plots of c/θ against c yield straight lines as shown in Fig. 4, and the corresponding linear regression parameters are listed in Table 1. Both linear correlation coefficient (r) and slope are very close to 1, indicating the adsorption of two phenylpyrimidine compounds on steel surface obeys Langmuir adsorption isotherm. Also, K follows the order: 4-PPM > 5-PPM. Generally, large value of K means that the inhibitor is easily and strongly adsorbed on the metal surface, and then results in the better inhibition performance. This is in consistent with the values of ηw obtained from Fig. 3. The adsorption equilibrium constant (K) is related to the standard adsorption free energy (ΔG0) with the following equation [53]:

K=

− ΔG 0 1 exp( ) RT 55.5

(4)

where R is the gas constant (8.314 J K-1 mol-1), T the absolute temperature (K), and the value 55.5 is the concentration of water in the solution expressed in M [53]. The calculated ΔG0 values are also presented in Table 1. The value of ΔG0 for 4-PPM is lower than that for 5-PPM, further demonstrating that 4-PPM exhibits the stronger tendency to adsorb on steel surface.

3.1.4. Effect of temperature

Temperature is an important kinetic factor that modifies the adsorption of inhibitor on electrode surface. In order to study the effect of temperature on the inhibition characteristics, experiments were conducted at 20 to 50 oC at an interval of 5 oC. Fig. 5 shows the relationship between inhibition efficiency (ηw) and temperature (immersion time is 6 h). From 20 to 30 oC, inhibition efficiency changes slightly, but then decreases to more extent with the experimental

temperature to 50 oC, which can be attributed to that the higher temperatures might cause the desorption of inhibitor molecule from metal surface. When the temperature is at 50 oC, ηw is reduced to 90.0% for 4-PPM; and 85.4% for 5-PPM. According to Arrhenius equation, the natural logarithm of the corrosion rate (ln v) is a linear function with 1/T [46]:

ln v =

− Ea + ln A RT

(5)

where Ea and A represent the apparent activation energy and the pre-exponential factor, respectively. The linear regressions between ln v and 1/T were calculated, and the parameters are given in Table 2. Fig. 6 shows the Arrhenius straight lines of ln v vs. 1/T for the blank and different inhibitors. All the linear regression coefficients (r) are very close to 1, which reflects that the corrosion of steel in HCl without and with inhibitors follows the Arrhenius equation. For two inhibitors, Ea (inhibited solution) > Ea (uninhibited solution), and Ea follows the order of Ea (5-PPM) > Ea (4-PPM), which further confirm ηw decreases with increase in temperature, and the decreased degree with temperature becomes more notable for 5-PPM. Table 2 also shows that the variance in ln A is similar to that in Ea. Influenced by the cumulative effect of the magnitudes of Ea and A, the corrosion rate decreases in the presence of inhibitor.

3.1.5. Effect of immersion time

The inhibition kinetics of the inhibition efficiency functioning with the immersion time is another important role to assess the stability of inhibitive behaviour of inhibitors. In the present

study, effect of immersion time (0.5—6 h) on corrosion inhibition of 5.0 mM phenylpyrimidine compounds in 1.0 M HCl at 20 oC was studied. The relationship between inhibition efficiency (ηw) and immersion time (t) in 1.0 M HCl at 20 oC is shown in Fig. 7. For both 4-PPM and 5-PPM, inhibition efficiency is higher than 75% when the immersion time is only 1 h, which indicates the adsorption rate of phenylpyrimidine inhibitors adsorbing on the steel surface is relatively high. Also, the changed rule of ηw for two inhibitors is similar. ηw firstly increases with immersion time from 1 to 6 h, and then fluctuates slightly with prolonging time to 156 h. When the immersion time is 156 h, ηw values are 95.5% and 91.3% for 4-PPM and 5-PPM, respectively. The reasons could be attributed to the adsorptive film of inhibitor that rests upon the immersion time. The adsorptive film reaches more compact and uniform along with prolonging immersion time (1−6 h), and then the adsorptive film becomes the saturated state within 6−156 h.

3.1.6. Effect of HCl concentration on corrosion inhibition

In order to study the effect of acid concentration on the corrosion of CRS, dependence of inhibition efficiency (ηw) on the concentration of HCl (c) at 20 oC (immersion time is 6 h) is shown in Fig. 8. Clearly, increasing acid concentration resulted in decreasing ηw gradually, and the minimum ηw values in 5.0 M HCl are 70.4% and 63.0% for 4-PPM and 5-PPM, respectively. At same acid concentration solution, inhibition performance follows the order: 4-PPM > 5-PPM. To study quantificationally the effect of HCl concentration on the corrosion inhibition, assuming the corrosion rate (v) against the molar concentration of acid concentration (c) obeys the expression proposed by Mathur and Vasudevan [54]:

ln v = ln k + Bc

(6)

where k is the rate constant and B is the reaction constant. Fig. 9 is the relationship between ln v and c at different conditions. Clearly, The straight lines of ln v versus c show that the kinetic parameters could be calculated by Eq. (6), and listed in Table 3. The linear regression coefficients (r) listed in Table 3 are very close to 1, which again confirms that there is a good linear relationship between ln v and c. Eq. (6) shows that k can be regarded as a commencing rate at zero acid concentration, so k means the ability of corrosion of acid media for steel [38,40,41,46,63]. Inspection of Table 3 reveals that k decreases to more extent after adding phenylpyrimidine inhibitors in HCl solution, and follows the order: k (4-PPM) < k (5-PPM), which indicates that the corrosion of steel is drastically inhibited by these inhibitors, and the inhibition performance follows the order of 4-PPM > 5-PPM. According to Eq. (6), B is the slope of the line ln v-c, so B indicated the changed extent of corrosion rate with the acid concentration. The value of B in inhibited HCl is higher than that in uninhibited HCl, which indicates that the changed extent of v with c in inhibited HCl is bigger than that in uninhibited HCl. In addition, the value of B increases the order of B (4-PPM) > B (5-PPM), which indicates that the changed degree of corrosion rate with acid concentration for 4-PPM is greater than that for 5-PPM.

3.2. Open circuit potential (EOCP) curves

The variation of open circuit potential (EOCP) of CRS electrode with immersion time (t) in 1.0 M HCl solution in absence and presence of inhibitor at 20 oC is shown in Fig. 10. For all cases, the

initial potential is moved positive value by time and gradually remains steady value, which is similar with the previous reported EOCP-time curves of steel in 1.0 M HCl [55]. It took about 90 minutes to reach the steady state, so the steady-state was achieved after 2 h immersion for electrochemical tests. The EOCP values at 120 min in HCl without and with 5-PPM and 4-PPM are -524, -497 and -473 mV (versus SCE), respectively. The steady-state potential moves positive after adding inhibitors to 1.0 M HCl solution, which indicates that the corrosion of CRS is retarded by the phenylpyrimidine derivatives.

3.3. Potentiodynamic polarization curves

Potentiodynamic polarization curves of CRS in 1.0 M HCl containing 5-PPM and 4-PPM at 20 oC are shown in Figs. 11(A) and (B), respectively. The addition of each compound causes a remarkable decrease in the corrosion rate i.e., shifts the both anodic and cathodic curves to lower current densities. Namely, both cathodic and anodic reactions of CRS electrode are drastically inhibited by the phenylpyrimidine compounds. The cathodic polarization curves in Fig. 11 indicate that the hydrogen evolution reaction is activation controlled [13] and the presence of the inhibitor does not affect the mechanism of hydrogen-reduction of H+ to H2 through charge transfer [56]. For anodic polarization curves as shown in Fig. 11, it is worth noting that three portions (I, II, and III) are observed, which represent the inhibited region, flat region and uninhibited region, respectively. That is, with increase of anodic potentials, the anodic currents increase at a slope of ba1 in portion I. After passing a certain potential, the desorption potential (Edes), the anodic currents increase steeply and dissolve at a

small slope of ba2 in portion II, a relative flat is observed at this stage. This flat can be attributed to the equilibrium of adsorption and desorption of inhibitor molecules on steel surface [57]. The potential value related to the flat is the desorption potential (Edes vs. SCE), which means the commencement of desorption of the absorbed species on the electrode surface, above which the coverage of inhibitor decreases rapidly. For portion III, the anodic current densities change drastically, resulting in a sharp increase in Tafel slope of ba3 in the high polarization potential region. The steep increase in current is related to marked desorption of adsorbed inhibitor [58]. According to Solmaz [59,60], this observation on anodic curve means that the inhibitor film behaves as a protective barrier at the steel surface and the dissolution of the metal takes place under this film (or un-covered surface), and increasing current sharply in the region of III may be the deformation of the inhibitor film from the metal surface. In a word, the cathodic curves display the Tafel behaviour. On the other hand, the anodic polarization curve does not display an extensive Tafel region, which may be due to concentration effects, as well as roughening of the surface. Thus, due to absence of linearity in anodic branch, accurate evaluation of the anodic Tafel slope by Tafel extrapolation of the anodic branch is rather difficult [61,62]. Accordingly, the electrochemical parameters of corrosion processes could be determined by Tafel extrapolation of the cathodic curve to the cathodic linear region back to the corrosion potential [63,64]. But it has been shown that in the Tafel extrapolation method, the use of both the anodic and cathodic Tafel regions is undoubtedly preferred over just only one Tafel region used [65]. Recently, Amin et al. [66,67] proposed that the corrosion parameters could be obtained by firstly extending the cathodic polarization curve, and then fitting the anodic region of about 80 to 150 mV (vs. Ecorr). Namely, it is possible to calculate the anodic Tafel line from the

experimental data. This method could be acceptable owing to good consistent with other methods of corrosion rate determination [66,67]. The values of corrosion current densities (icorr), corrosion potential (Ecorr), cathodic Tafel slope (bc), anodic Tafel slope (ba), inhibition efficiency (ηp) are summarized in Table 4. It is apparent that icorr decreases considerably in the presence of each inhibitor, and decreases with increasing the inhibitor concentration. Correspondingly, ηp increases with the inhibitor concentration, due to the increase in the blocked fraction of the electrode surface by adsorption. ηp of 5.0 mM inhibitor reaches up to a maximum of 89.6% for 5-PPM; and 91.7% for 4-PPM, which again confirms that two phenylpyrimidine derivatives are good inhibitors in 1.0 M HCl, and ηp follows the same order: 4-PPM > 5-PPM. The presence of each inhibitor does not prominently shift the corrosion potential, which indicates two studied phenylpyrimidine derivatives act as mixed-type inhibitors [68]. Tafel slopes of bc and ba change upon addition of these inhibitors, which means that the inhibitor molecules are adsorbed on both the anodic and cathodic sites resulting in the changing rules of the potential with current densities.

3.4. Electrochemical impedance spectroscopy

Figs. 12(A) and (B) represent the Nyquist diagrams for CRS in 1.0 M HCl in the presence of 5-PPM and 4-PPM at 20 oC, respectively. All the impedance spectra exhibit one single capacitive loop, which indicates that the corrosion of steel is mainly controlled by the charge transfer process, and usually related to the charge transfer of the corrosion process and double layer behaviour [69,70]. In addition, the shape is maintained throughout all tested inhibitor concentrations

compared with that of blank solution. Thus, there is almost no change in the corrosion mechanism whether the inhibitor is added [71]. The diameter of the capacitive loop in the presence of inhibitor is larger than that in the absence of inhibitor (blank solution), and increases with the inhibitor concentration. This suggests that the impedance of inhibited substrate increases with the inhibitor concentration. Noticeably, these capacitive loops are not perfect semicircles, which can be attributed to the frequency dispersion effect as a result of the roughness and inhomogeneousness of the electrode surface [72]. Accordingly, the EIS data are simulated by the equivalent circuit shown in Fig. 13. Rs and Rt are the solution resistance and charge transfer resistance, respectively. CPE is constant phase element to replace a double layer capacitance (Cdl) for more accurate fit [73]. The solid lines in Fig. 14 correspond to the fitted plots for EIS experiment data using this electric circuit of Fig. 13, which confirms this equivalent circuit could be used to fit the experimental data. In addition, the chi-squared (χ2) is used to evaluate the precision of the fitted data [74]. Table 5 reveals that χ2 values are low, which indicates that the fitted data have good agreement with the experimental data. The CPE is composed of a component Qdl and a coefficient a which quantifies different physical phenomena like surface inhomogeneousness resulting from surface roughness, inhibitor adsorption, porous layer formation, etc. The double layer capacitance (Cdl) can be simulated via CPE from the following equation [20]:

Cdl = Qdl × (2π f max ) a −1

(7)

where fmax represents the frequency at which the imaginary value reaches a maximum on the Nyquist plot. The electrochemical parameters of Rs, Rt, CPE, a, Cdl, fmax and ηR are also

summarized in Table 5. Rs is very small, which confirms that the IR drop could be negligible. Rt increases prominently while Cdl reduces with the concentration of inhibitor. A large charge transfer resistance (Rt) is associated with a slower corroding system. At any given inhibitor concentration, Rt (5-PPM) < Rt (4-PPM), which again indicates that 4-PPM exhibits better inhibitive performance between two phenylpyrimidine derivatives. The decrease in Cdl in comparing with that in blank solution (without inhibitor), which could result from a decrease in local dielectric constant and/or an increase in the thickness of the electrical double layer, suggests that the inhibitor molecules function by adsorption at the metal/solution interface [12]. ηR increases with the concentration of inhibitor, and follows the order:ηR (4-PPM) > ηR (5-PPM). The ηR values at 5.0 mM are 89.9% and 90.6% for 5-PPM and 4-PPM, respectively. Thus, two phenylpyrimidine derivatives show good inhibitive performance for CRS in HCl solution. Inhibition efficiencies obtained from weight loss (ηw), potentiodynamic polarization curves (ηp) and EIS (ηR) are in good reasonably agreement.

3.5. SEM surface examination

The SEM images of CRS surfaces are shown in Fig. 14. It can be seen from Fig. 14(a) that the CRS surface before immersion appears more uniform and some abrading scratches. However, it is not absolute smooth and uniform, and appears small black holes and covered with grains, which may be attributed to the defect of steel. Inspection of Fig. 14(b) reveals that the CRS surface after immersion in uninhibited 1.0 M HCl for 6 h exhibits an aggressive attack of the

corroding medium on the steel surface. The corrosion products appear too uneven, and are arranged layer upon layer. As for Fig. 14(c), corrosion degree of the steel surface remarkably decreases in the presence of 5.0 mM 5-PPM, and the surface seems smoother. Fig. 14(d) clearly shows that the steel surface in the presence of 5.0 mM 4-PPM is drastically inhibited owing to that the more uniform film could be formed on steel surface.

3.6. Quantum chemical calculations

Fig. 15 shows the optimized molecular structures of 4-PPM and 5-PPM. The pyrimidine and benzene rings are not in one plane for both 4-PPM and 5-PPM. Organic inhibitor could form coordination bonds between the unshared electron pairs of N atom and the empty d-orbital of Fe atom. The larger negative charge of the atom, the better is the action as an electronic donor. Each phenylpyrimidine molecule contains two N atoms in the pyrimidine ring. Mulliken charges of the atoms are listed in Table 6. The larger negative atoms are found in N3 and N5 atoms for both inhibitors; which are active adsorptive atom. Furthermore, the Mulliken charges of two N atoms in 4-PPM are negative than those in 5-PPM. This result implies that N3 and N5 atoms of 4-PPM exhibits more active than those of 5-PPM. Fukui function is necessary in understanding the local site selectivity. The Fukui function



( f (r ) ) is defined as [75]:

  ⎛ ∂ρ (r ) ⎞ f (r ) = ⎜ ⎟ ⎝ ∂N ⎠V ( r )



+

(8)



−

The nucleophic attack Fukui function ( f (r ) ) and electrophilic attack Fukui function ( f (r ) ) can be calculated as [76]:

 f i (r ) + = qi ( N + 1) − qi ( N )

(9)

 f i ( r ) − = qi ( N ) − qi ( N − 1)

(10)

Where qi(N + 1), qi(N), qi(N - 1) are charge values of atom i for cation, neutral and anion,



respectively. The values of f (r )



of f (r )

+



and f (r )

−

+



and f (r )

−

are also given in Table 6. Generally, high values

mean the high capacity of the atom to gain and lost electron, respectively.

For the nucleophic attack, the most reactive sites are N3 and C6 for 4-PPM; and C4 and C1 for 5-PPM, which can accept electrons from metal surface to form back-donating bond. On the other



hand, the values of f (r )

−

indicate that they will happen on the N3 and N5 for two inhibitors;

which can denote electrons to metal surface to form coordinate bond. The dipole moment (μ) is widely used the polarity of the molecule, and also related to the inhibitive ability [77]. The large value of dipole moment probably increases the inhibitor adsorption through electronic force [78]. In Table 7, μ (4-PPM) > μ (5-PPM) and η (4-PPM) > η (5-PPM), which indicates that the better inhibitive performance of 4-PPM would be arisen from intermolecular electrostatic force to some extent. Besides the above mentioned quantum chemical parameters, the global reactivity of a molecule depends on the distributions of frontier molecular orbitals. HOMO (the highest occupied molecular orbital) is often associated with the capacity of a molecule to donate electrons, whereas LUMO (the lowest unoccupied molecular orbital) represents the ability of the molecule to accept electrons. The electric/orbital density distributions of HOMO and LUMO for two phenylpyrimidine molecules are shown in Fig. 16. The electron density of both HOMO and LUMO are localized principally on the pyrimidine ring. That is, there is electron transferring in the interaction between the pyrimidine and metal surface. For 4-PPM, the densities of both

HOMO and LUMO are on the substituted benzene ring, which indicates the benzene ring of 4-PPM could be both the acceptor of the electron and the donor of the electron. For 5-PPM, HOMO is located on the benzene ring, but LUMO is absent on the benzene ring, which reflects the benzene ring of 5-PPM could be the donor of electron, not acceptor of the electron. Thereby, comparing with 5-PPM, it is reasonable to assume that the benzene ring of 4-PPM has additional ability to accept electrons from d-orbitals of metal to form back-donating bond. This assumption would also be the main reason of η (4-PPM) > η (5-PPM). The values of energy of highest occupied molecular orbital (EHOMO), energy of lowest unoccupied molecular orbital (ELUMO) and the separation energy (ELUMO - EHOMO, ΔE) are also presented in Table 7. High value of EHOMO indicates a tendency of the molecule to donate electrons to empty molecular orbitals of acceptor molecules [79]. Conversely, the ELUMO represents the ability of the molecule to accept electrons, and the lower value of ELUMO suggests the molecule accepts electrons more probable [79]. From Table 7, EHOMO(4-PPM) < EHOMO(5-PPM), which is not agreement with the inhibition efficiency order of 4-PPM > 5-PPM, thus the better inhibition efficiency of 4-PPM molecule is not due to increasing energy of the HOMO. On the other hand, ELUMO obeys the order: 4-PPM < 5-PPM, which is completely agreement with the inhibition efficiency order. The separation energy (ΔE) is an important parameter as a function of reactivity of the inhibitor molecule towards the adsorption on metallic surface. As ΔE decreases, the reactivity of the molecule increases in visa, which facilitates adsorption and enhances the efficiency of inhibitor [80]. Inspection of the data in Table 7 revealsΔE obeys the order of 4-PPM < 5-PPM, which is in completely accordance with the order of inhibition efficiency of 4-PPM > 5-PPM. Thus, there is a good correlation between ΔE and

inhibition efficiency. Additionally, the energies of HOMO and LUMO orbital of the inhibitor molecule are related to the ionization potential (I) and the electron affinity (Y), respectively, by the following relations [81]:

I = − EHOMO

(11)

Y = − ELUMO

(12)

Then absolute electronegativity (β), global hardness (γ) and global softness (s) of the inhibitor molecule are approximated as follows [82]:

I +Y 2 I −Y γ= 2

β=

s=

1

γ

(13) (14)

(15)

The number of electrons transferred from the inhibitor to metallic surface (ΔN) is calculated depending on the quantum chemical method [83]:

ΔN =

β Fe − β inh 2(γ Fe + γ inh )

(16)

For Fe, the theoretical values of βFe and γFe are 7 eV and 0 eV, respectively [84]. The values of β, γ, s and ΔN are also listed in Table 7. According to some studies [84,85], the parameter of β is related to the chemical potential, and higher value of β means better inhibitive performance. On the other hand, γ is equal to ΔE/2, and the lower γ implies more polarizability and higher inhibition efficiency. The parameter of s is reciprocal to γ, thus high value of s is related to more efficiency. Values ofΔN exhibit inhibitive performance resulted from electrons donations. IfΔN < 3.6, the inhibition efficiency increases with the increase in electron-donation

ability to the metal surface [85]. In the present study, inhibition efficiency follows the order: 4-PPM > 5-PPM. Thus, there is a good correlation between inhibition efficiency and the parameters of β, γ and s.

3.8. Molecular dynamics (MD) simulations

MD simulations have been done to further study the adsorption behavior of the two inhibitor molecules of 4-PPM and 5-PPM on the Fe (001) surface. Through the analysis of the temperature and energy, it takes about 200 ps for the adsorption system containing both Fe (001) surface and the studied inhibitor molecule reaches equilibrium. The adsorption system is at steady state and fluctuates slightly from 500 ps to 1000 ps. Fig. 17 shows the adsorption configurations on Fe (001) surface for inhibitor molecules, and the corresponding Eads and Ebin values are listed in Table 8. As can be seen from Fig. 17, two inhibitor molecules of 4-PPM and 5-PPM are adsorbed on Fe (001) surface with a nearby flat orientation. The formation of the flat orientation on steel surface can be attributed to the relatively equal distribution of HOMO and LUMO densities on the whole molecule. In other words, the whole inhibitor molecule can adsorb on steel surface through pyrimidine ring and substituted benzene ring simultaneously. Moreover, the values of the adsorption energy (Eads) in Table 8 reveal the sequence of 4-PPM > 5-PPM, which indicates that 4-PPM shows a stronger tendency to adsorb on steel surface than 5-PPM. On the other hand, magnitude of Ebin is indicative of stability of adsorptive system. More negative value of Ebin of 4-PPM suggests a more stable adsorption system and leads to the higher inhibitive action than 5-PPM. Accordingly, inhibition efficiency for two studied inhibitors is ranked as 4-PPM > 5-PPM

based on the parameters of Eads and Ebin. Thus, the theoretical inference is in good agreement with experimental data.

3.7. Explanation for adsorption and inhibition

The adsorption and inhibition effect of 4-PPM and 5-PPM in HCl solution can be explained as follows: both 4-PPM and 5-PPM might be protonated in the acid solution as following:

4-PPM + xH + ↔ [4-PPMH x ]x +

(17)

5-PPM + xH + ↔ [5-PPMH x ]x +

(18)

Thus, in aqueous acidic solutions, the phenylpyrimidine compounds exists either as neutral molecules (4-PPM, 5-PPM) or cations ([4-PPMHx]x+, [5-PPMHx]x+). Generally, two modes of adsorption could be considered. According to the quantum chemical calculations results of Section 3.6, the neutral 4-PPM and 5-PPM may be adsorbed on the metal surface via the chemisorption mechanism, involving the displacement of water molecules from the metal surface and the sharing electrons between the N atoms and iron. The inhibitor molecules can be also adsorbed on the metal surface on the basis of donor-acceptor interactions between π-electrons of two rings and vacant d-orbitals of iron. In another hand, it is well known that the steel surface charges positive charge in acid solution [86], so it is difficult for the protonated [4-PPMHx]x+ and [5-PPMHx]x+ to approach the positively charged steel surface (H3O+/metal interface) due to the electrostatic repulsion. Since chloride ions (Cl-) have a smaller degree of hydration, being specifically adsorbed, they create an excess negative charge towards the solution and favor more adsorption of the cations, [4-PPMHx]x+ and [5-PPMHx]x+ may adsorb through electrostatic interactions between the

positively charged molecules and the negatively charged metal surface, i.e. there may be a synergism between Cl- and protonated [4-PPMHx]x+ or [5-PPMHx]x+. It should be noted that the molecular structure of [4-PPMHx]x+ or [5-PPMHx]x+ remains unchanged with respect to its neutral form, the N atoms on the ring remaining strongly blocked, so when protonated [4-PPMHx]x+ or [5-PPMHx]x+ adsorbed on metal surface, coordinate bond may be formed by partial transference of electrons from the polar atom (N atom) to the metal surface.

4. Conclusion

(1) Two phenylpyrimidine derivatives of 4-PPM and 5-PPM are good inhibitors for the corrosion of steel in 1.0 M HCl solution. Inhibition efficiency (ηw) increases with the inhibitor concentration, and the maximum ηw at 5.0 mM is 96.3% for 4-PPM; and 92.2% for 5-PPM at 20 oC. Inhibition efficiency (ηw) follows the order: 4-PPM > 5-PPM. (2) The adsorption of 4-PPM or 5-PPM obeys Langmuir adsorption isotherm, and the adsorption equilibrium constant follows the order: 4-PPM > 5-PPM. Inhibition efficiency (ηw) decreases with the temperature. The values of both apparent activation energy (Ea) and pre-exponential factor (A) in the presence of inhibitors are higher than those in the absence of inhibitor. Prolonging immersion time results in increasing inhibition efficiency (ηw) from 1 to 6 h, and then almost remains constant until 156 h. (5) The natural logarithm of the corrosion rate (ln v) is a linear function with the molar concentration of HCl (c). The reaction constant (k) decreases significantly after adding 4-PPM and 5-PPM compounds in 1.0—5.0 M HCl, while the reaction constant (B) increases.

(6) Both 4-PPM and 5-PPM act as mixed-type inhibitors. EIS spectra exhibit individual capacitive loop. The presence of inhibitor in 1.0 M HCl solutions increases Rt while reduces Cdl. (7) The larger negative Mulliken charges are found in N3 and N5 atoms for both 4-PPM and 5-PPM; which are adsorptive centers. The nucleophic attack active atoms are C2 and O6 atoms for N3 and C6 for 4-PPM; and C4 and C1 for 5-PPM. The electrophilic attack active atoms are N3 and N5 for both 4-PPM and 5-PPM. The electron densities of both HOMO and LUMO are localized principally on the pyrimidine ring, which could be both the acceptor of the electron and the donor of the electron. Comparing with 5-PPM, the substituted benzene ring of 4-PPM has additional ability to accept electrons from d-orbitals of metal to form back-donating bond. There is a good correlation between inhibition efficiency and the quantum parameters of ELUMO,ΔE, β, γ and s. (8) MD simulations reveal that 4-PPM and 5-PPM molecules adsorb on the Fe (001) surface in the nearly flat manner, and the sequence of either Eads or Ebin is in accordance with that of inhibition efficiency.

Acknowledgements

This work was carried out in the frame of research projects funded by the National Natural Science Foundation of China (51361027). The electrochemical measurements were carried out PARSTAT 2273 advanced electrochemical system (Princeton Applied Research) provided by Advanced Science Instrument Sharing Center of Southwest Forestry University.

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[71] N. Labjar, M. Lebrini, F. Bentiss, N.E. Chihib, S. El Hajjaji, C. Jama, Corrosion inhibition of carbon steel and antibacterial properties of aminotris-(methylnephosnic) acid, Mater. Chem. Phys. 119 (2010) 330-336. [72] M. Lebrini, M. Lagrenée, H. Vezin, M. Traisnel, F. Bentiss, Experimental and theoretical study for corrosion inhibition of mild steel in normal hydrochloric acid solution by some new macrocyclic polyether compounds, Corros. Sci. 49 (2007) 2254-2269. [73] A.R.S. Priya, V.S. Muralidharam, A. Subramania, Development of novel acidizing inhibitors for carbon steel corrosion in 15% boiling hydrochloric acid, Corrosion 64 (2008) 541-552. [74] W.R. Osório, L.C. Peixoto, P.R. Goulart, A. Garcia, Electrochemical corrosion parameters of as-cast Al–Fe alloys in a NaCl solution, Corros. Sci. 52 (2010) 2979-2993. [75] R.G. Parr, W. Yang, Density function approach to the frontier-electron theory of chemical reactivity, J. Am. Chem. Soc. 106 (1984) 4049-4050. [76] S.Q. Hu, J.Q. Hu, C.C. Fan, X.L. Jia, J. Zhang, W.Y. Guo, Theoretical and experimental study of corrosion inhibition performance of new imidazoline corrosion inhibitors, Acta Chim. Sinica 68 (2010) 2051-2058. [77] R.M. Issa, M.K. Awad, F.M. Atlam, Quantum chemical studies on the inhibition of corrosion of copper surface by substituted uracils, Appl. Surf. Sci. 255 (2008) 2433-2441. [78] M. Lashkari, M.R. Arshadi, DFT studies of pyridine corrosion inhibitors in electrical double layer: solvent, substrate, and electric field effects, Chem. Phys. 299 (2004) 131 - 137. [79] T. Ghailane, R.A. Balkhmima, R. Ghailane, A. Souizi, R. Touir, M. Ebn Touhami, K. Marakchi, N. Komiha Experimental and theoretical studies for mild steel corrosion inhibition in 1 M HCl by two new benzothiazine derivatives, Corros. Sci. 76 (2013) 317-324.

[80] Sudheer, M.A. Quraishi, Electrochemical and theoretical investigation of triazole derivatives on corrosion inhibition behavior of copper in hydrochloric acid medium, Corros. Sci. 70 (2013) 161-169. [81] H. Tian, W. Li, K. Cao, B. Hou, Potent inhibition of copper corrosion in neutral chloride media by novel thiadiazole derivatives, Corros. Sci. 73 (2013) 281-291. [82] M.A. Hegazy, A.M. Badawi, S.S. Abd El Rehim, W.M. Kamel, Corrosion inhibition of carbon steel using novel N-(2-(2-mercapto acetoxy)ethyl)-N,N-dimethyl) dodecan-1-aminium bromide during acid pickling, Corros. Sci. 69 (2013) 110-122. [83] N. Kovačević, A. Kokalj, The relation between adsorption bonding and corrosion inhibition of azole molecules on copper, Corros. Sci. 73 (2013) 7-17. [84] Z. El Adnani, M. Mcharfi, M. Sfaira, M. Benzakour, A.T. Benjelloun, M. Ebn Touhami, DFT theoretical study of 7-R-3methylquinoxalin-2(1H)-thiones (R=H; CH3; Cl) as corrosion inhibitors in hydrochloric acid, Corros. Sci. 68 (2013) 223-230. [85] H. Ju, Z.P. Kai, Y. Li, Aminic nitrogen-bearing polydentate Schiff base compounds as corrosion inhibitors for iron in acidic media: A quantum chemical calculation, Corros. Sci. 50 (2008) 865-871. [86] M. Behpour, S.M. Ghoreishi, N. Soltani, M. Salavati-Niasari, The inhibitive effect of some bis-N,S-bidentate Schiff bases on corrosion behaviour of 304 stainless steel in hydrochloric acid solution, Corros. Sci. 51 (2009) 1073 – 1082.

Figure Captions

Fig.

1.

Chemical

molecular

structures

of

two

phenylpyrimidine

derivatives:

(a)

4-phenylpyrimidine (4-PPM); (b) 5-phenylpyrimidine (5-PPM). Fig. 2. Relationship between corrosion rate (v) and concentration of inhibitor (c) in 1.0 M HCl at 20 oC (weight loss method, immersion time is 6 h). Fig. 3. Relationship between inhibition efficiency (ηw) and concentration of inhibitor (c) in 1.0 M HCl at 20 oC (weight loss method, immersion time is 6 h). Fig. 4.

Langmuir isotherm adsorption modes of 4-PPM and 5-PPM on the CRS surface in 1.0 M HCl at 20 oC from weight loss measurement.

Fig. 5. Relationship between inhibition efficiency (ηw) and temperature in 1.0 M HCl (Weight loss method, immersion time is 6 h). Fig. 6. Arrhenius plots related to the corrosion rate of CRS for various inhibitors in 1.0 M HCl. Fig. 7. Relationship between inhibition efficiency (ηw) and immersion time (t) in 1.0 M HCl at 20°C (Weight loss method). Fig. 8. Relationship between inhibition efficiency (ηw) and HCl concentration (c) at 20°C (weight loss method, immersion time is 6 h). Fig. 9. The straight lines of ln v versus c at 20 oC (Weight loss method, immersion time is 6 h). Fig. 10. EOCP – time curves for CRS in 1.0 M HCl solutions at 20 oC: (a) 1.0 M HCl; (b) 1.0 M HCl + 5.0 mM 5-PPM; (c) 1.0 M HCl + 5.0 mM 4-PPM. Fig. 11. Potentiodynamic polarization curves for CRS in 1.0 M HCl without and with different concentrations of inhibitors at 20 oC (immersion time is 2 h): (A) 5-PPM; (B) 4-PPM. Fig. 12. Nyquist plots of the corrosion of CRS in 1.0 M HCl at 20 oC without and with different

concentrations of inhibitors (2 hours immersion at the open circuit potential prior to measurement): (A) 5-PPM; (B) 4-PPM. Fig. 13. The equivalent circuit model of EIS. Fig. 14. SEM micrographs of CRS surface: (a) before immersion; (b) after 6 h of immersion at 20 o

C in 1.0 M HCl; (c) after 6 h of immersion at 20 oC in 5.0 mM 5-PPM + 1.0 M HCl; (d)

after 6 h of immersion at 20 oC in 5.0 mM 4-PPM + 1.0 M HCl. Fig.

15.

Optimized

molecular

structures

of

two

phenylpyrimidine

derivatives:

(a)

4-phenylpyrimidine (4-PPM); (b) 5-phenylpyrimidine (5-PPM). Fig. 16. The frontier molecule orbital density distributions of (a) 4-phenylpyrimidine (4-PPM) and (b) 5-phenylpyrimidine (5-PPM): HOMO (left); LUMO (right). Fig. 17. Equilibrium adsorption configuration of inhibitors on Fe (001) planes obtained by MD simulations: (a) 4-phenylpyrimidine (4-PPM); (b) 5-phenylpyrimidine (5-PPM).

Tables

Table 1 Parameters of the straight lines of c/θ - c and adsorption free energy (ΔG0) in 1.0 M HCl at 20 oC (Weight loss method, immersion time is 6 h) Inhibitor 5-PPM 4-PPM

Linear correlation coefficient (r) 0.9999 0.9998

ΔG0 (kJ mol-1)

Slope

K (M-1)

0.99

2.14 × 103

-28.5

0.97

2.88 × 103

-29.2

Table 2 Parameters of the straight lines of ln v – 1/T in 1.0 M HCl Inhibitor blank

Linear correlation coefficient (r) 0.9998

Ea (kJ mol-1)

A (g m-2 h-1)

63.6

9.94 × 1011

5-PPM

0.9940

78.7

3.54 × 1013

4-PPM

0.9913

73.4

6.13 × 1012

Table 3 Parameters of the linear regression between ln v and C for the corrosion of steel in HCl Inhibitor —

Linear correlation coefficient (r) 0.9974

k (g m-2 h-1) 3.09

B (g m-2 h-1 M -1) 0.39

5-PPM

0.9959

0.19

0.76

4-PPM

0.9938

0.083

0.88

Table 4 Potentiodynamic polarization parameters for the corrosion of CRS in 1.0 M HCl solution containing different concentrations of 4-PPM and 5-PPM at 20 oC (immersion time is 2 h) Inhibitor

c (mM)

— 5-PPM

4-PPM

Ecorr (mV vs. SCE)

icorr

-bc

(μA cm-2) (mV dec –1)

ba (mV dec –1)

ηp (%)

0

-465.2

219.5

103

53

—

1.0

-464.0

78.7

117

60

64.1

3.0

-471.4

36.9

112

59

83.2

5.0

-487.1

22.8

102

95

89.6

1.0

-466.3

117

123

75.0

3.0

-462.6

27.2

114

77

87.6

5.0

-440.5

18.3

107

101

91.7

54.9

Table 5 EIS parameters for the corrosion of CRS in 1.0 M HCl containing 4-PPM and 5-PPM at 20oC Inhibitor

c

Rs

Rt

(mM) (Ω cm2) —

0

5-PPM 1.0

4-PPM

0.84 2.09

CPE

a

(Ω cm2) (μΩ-1 sa cm-2)

fmax (Hz)

χ2

Cdl (μF cm-2)

ηR (%)

337.8

0.8981

13.74

214

9.39 × 10-3

—

116.8

268.5

0.9009

13.74

173

9.17 × 10-3

67.6

0.8859

7.88

112

2.71 × 10-3 1.16 × 10-3

37.88

3.0

1.09

212.9

174.1

5.0

0.91

376.4

101.9

0.8675

7.88

61

1.0

0.77

173.5

222.7

0.8745

7.88

136

7.88

71

1.39 × 10-2 87.4

53

1.84 × 10-3

3.0

1.39

300.1

113.2

5.0

2.18

400.9

80.2

0.8798 0.8921

7.88

1.36 × 10-2

82.2 89.9 78.2

90.6

Table 6





+

−

Quantum chemical parameters of Mulliken charge, f (r ) and f (r ) for 4-PPM and 5-PPM molecules Atom

C1

Mulliken charge

f (r) +

f (r)-

4-PPM

5-PPM

4-PPM

5-PPM

-0.251

0.098

0.035

0.083

4-PPM

5-PPM

0.051

0.020

C2

-0.020

-0.077

0.087

0.061

0.044

0.054

N3

-0.348

-0.327

0.100

0.051

0.121

0.081

C4

0.057

0.062

0.027

0.146

0.071

0.066

N5

-0.362

-0.327

0.073

0.051

0.109

0.081

C6

0.242

-0.077

0.105

0.061

0.040

0.054

C7

0.105

0.108

0.014

0.019

0.039

0.051

C8

-0.213

—

0.051

—

0.042

—

C9

-0.170

—

0.016

—

0.019

—

C10

-0.177

-0.234

0.064

0.043

0.055

0.046

C11

-0.174

-0.172

0.018

0.018

0.037

0.032

C12

-0.236

-0.182

0.047

0.062

0.027

0.075

C13

—

-0.172

—

0.018

—

0.032

C14

—

-0.234

—

0.043

—

0.046

Table 7 Quantum chemical parameters for inhibitor molecules at GGA/BLYP/DND/ COSMO level Molecule

μ(D)

EHOMO (eV)

ELUMO (eV)

ΔE (eV)

β (eV) γ (eV)

s (eV-1)

ΔN

4-PPM

4.1562

-6.039

-2.454

3.585

4.246

1.792

0.558

0.768

5-PPM

4.1241

-5.898

-2.227

3.671

4.062

1.834

0.545

0.800

Table 8 Values of adsorption energy (Eads) and binding energy (Ebin) between the molecules and Fe (001) plane Molecule

Eads (kJ mol-1)

Ebin (kJ mol-1)

4-PPM

403.95

-403.95

5-PPM

361.50

-361.50

Figures

N

N

(a) Fig.

N

N

(b)

1. Chemical molecular structures of two phenylpyrimidine 4-phenylpyrimidine (4-PPM); (b) 5-phenylpyrimidine (5-PPM).

derivatives:

(a)

5

5-PPM 4-PPM

3

-2

-1

v (g m h )

4

2

1

0

0

1

2

3

4

5

6

c (mM)

Fig. 2. Relationship between corrosion rate (v) and concentration of inhibitor (c) in 1.0 M HCl at 20 oC (weight loss method, immersion time is 6 h).

100 90 80

η w (%)

70 60 50

4-PPM 5-PPM

40 30 20

0

1

2

3

4

5

6

c (mM)

Fig. 3. Relationship between inhibition efficiency (ηw) and concentration of inhibitor (c) in 1.0 M HCl at 20 oC (weight loss method, immersion time is 6 h).

6 5

c/θ (mM)

4 3 2 4-PPM 5-PPM

1 0

0

1

2

3

4

5

6

c (mM)

Fig. 4.

Langmuir isotherm adsorption modes of 4-PPM and 5-PPM on the CRS surface in 1.0 M HCl at 20 oC from weight loss measurement.

100 4-PPM 5-PPM

η w (%)

95

90

85

80 15

20

25

30

35

40

45

50

55

o

temperature ( C)

Fig. 5. Relationship between inhibition efficiency (ηw) and temperature in 1.0 M HCl (Weight loss method, immersion time is 6 h).

5 4

2

-2

-1

ln (v, g m h )

3

1 0 -1 -2 -3

blank 5.0 mM 5-PPM 5.0 mM 4-PPM 0.0031

0.0032

0.0033

0.0034

-1

1/T (K )

Fig. 6. Arrhenius plots related to the corrosion rate of CRS for various inhibitors in 1.0 M HCl.

100 95

η w (%)

90 85 80 4-PPM 5-PPM

75 70

0

12 24 36 48 60 72 84 96 108 120 132 144 156 168 t (h)

Fig. 7. Relationship between inhibition efficiency (ηw) and immersion time (t) in 1.0 M HCl at 20°C (Weight loss method).

100

90

η w (%)

80

70

5.0 mM 4-PPM 5.0 mM 5-PPM

60

50

0

1

2

3

4

5

6

c (M)

Fig. 8. Relationship between inhibition efficiency (ηw) and HCl concentration (c) at 20°C (weight loss method, immersion time is 6 h).

3

-2

-1

ln (v, g m h )

2

1

0 blank 5.0 mM 5-PPM 5.0 mM 4-PPM

-1

-2

0

1

2

3

4

5

6

c (M)

Fig. 9. The straight lines of ln v versus c at 20 oC (Weight loss method, immersion time is 6 h).

-0.47

c -0.48

EOCP (V vs. SCE)

-0.49

b

-0.50 -0.51 -0.52

a

-0.53 -0.54 -0.55

0

30

60

90

120

150

180

210

time (minute)

Fig. 10. EOCP – time curves for CRS in 1.0 M HCl solutions at 20 oC: (a) 1.0 M HCl; (b) 1.0 M HCl + 5.0 mM 5-PPM; (c) 1.0 M HCl + 5.0 mM 4-PPM.

-0.2

d

a --- blank b --- 1.0 mM c --- 3.0 mM d --- 5.0 mM

-0.3

II

c a

-0.4 E (V vs. SCE)

b

III

I -0.5 -0.6

A -0.7

d c -0.8

-7

-6

-5

-4

-3

b

a

-2

-1

-2

log (i, A cm )

-0.2 a --- blank b --- 1.0 mM c --- 3.0 mM d --- 5.0 mM

-0.3

III II I

E (V vs. SCE)

-0.4 -0.5 -0.6

B -0.7 -0.8

d

c

b a

-7

-6

-5

-4

-3

-2

-1

-2

log (i, A cm )

Fig. 11. Potentiodynamic polarization curves for CRS in 1.0 M HCl without and with different concentrations of inhibitors at 20 oC (immersion time is 2 h): (A) 5-PPM; (B) 4-PPM.

250

200

4.52 Hz

7.88 Hz

150 Zim (Ω cm2)

blank 5.0 mM 10.0 mM 20.0 mM ____ Fitting Curves

A

13.74 Hz 1.49 Hz 100

7.88 Hz 2.59 Hz

13.74 Hz

50

2.59 Hz

0 0

50

100

150

200

250

300

350

400

2

Zre (Ω cm )

250

B

200

7.88 Hz

1.0 mM 3.0 mM 5.0 mM ___ Fittting Curves

13.74 Hz Zim (Ω cm2)

150 7.88 Hz

2.59 Hz 4.52 Hz

100

1.49 Hz

7.88 Hz 2.59 Hz 50

0.85 Hz

0 0

50

100

150

200

250

300

350

400

450

2

Zre (Ω cm )

Fig. 12. Nyquist plots of the corrosion of CRS in 1.0 M HCl at 20 oC without and with different concentrations of inhibitors (2 hours immersion at the open circuit potential prior to measurement): (A) 5-PPM; (B) 4-PPM.

CPE Rs Rt Fig. 13. The equivalent circuit model of EIS.

a

b

5 µm

c

5 µm

d

5 µm

5 µm

Fig. 14. SEM micrographs of CRS surface: (a) before immersion; (b) after 6 h of immersion at 20 o C in 1.0 M HCl; (c) after 6 h of immersion at 20 oC in 5.0 mM 5-PPM + 1.0 M HCl; (d) after 6 h of immersion at 20 oC in 5.0 mM 4-PPM + 1.0 M HCl.

(a) 4-PPM

(b) 5-PPM Fig. 15. Optimized molecular structures of two phenylpyrimidine 4-phenylpyrimidine (4-PPM); (b) 5-phenylpyrimidine (5-PPM).

derivatives:

(a)

(a) 4-PPM

(b) 5-PPM Fig. 16. The frontier molecule orbital density distributions of (a) 4-phenylpyrimidine (4-PPM) and (b) 5-phenylpyrimidine (5-PPM): HOMO (left); LUMO (right).

64

(a)

(b) Fig. 17. Equilibrium adsorption configuration of inhibitors on Fe (001) planes obtained by MD simulations: (a) 4-phenylpyrimidine (4-PPM); (b) 5-phenylpyrimidine (5-PPM).

65

Research highlights

► 4-PPM and 5-PPM are new good inhibitors for steel in HCl solution. ► Inhibition efficiency follows the order: 4-PPM > 5-PPM. ► The adsorption of phenylpyrimidine inhibitor obeys Langmuir adsorption isotherm. ► There is a correlation between quantum chemical parameters and inhibition action. ► 4-PPM and 5-PPM molecules adsorb on Fe (001) surface in the nearly flat manner.

66