a

ina

Journal of the Taiwan Institute of Chemical Engineers 45 (2014) 30333045

tion

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Journal of the Taiwan Instit

.e1. Introduction

Corrosion is a fundamental process playing an important role ineconomics and safety, particularly for metals and alloys. It isestimated that the annual direct cost of metallic corrosion rangesfrom 2% to 4% of the gross domestic product (GDP). Steel is widelyused in a broad spectrum of industries and machinery; however, ittends to be corroded. The corrosion of steel is a fundamentalacademic and industrial concern that has received considerableamount of attention. In order to reduce the corrosion of metals,several techniques have been applied. Using inhibitors is one of themost practical methods for protection metals against corrosion,especially in acidic media [1].

N-heterocyclic compounds are considered to be the mosteffective corrosion inhibitors for steel in acid solution [2]. Previously,many N-heterocyclic compounds are reported as good corrosion

inhibitors for steel in acid media, such as imidazoline derivatives [3],1,2,3-triazole derivatives [4], 1,2,4-triazole derivatives [5], benzo-triazole [6], pyrrole [7], pyridine derivatives [8], pyrazole derivatives[9], bipyrazole derivatives [10], pyrazine derivatives [11], pyridazinederivatives [12], indole derivatives [13], benzimidazole derivatives[14], quinoline derivatives [15], purine derivatives [16] and tetrazolederivatives [17]. They exhibit inhibition by adsorption on the steelsurface, and the adsorption takes place through N, O, and S atoms, aswell as those with triple or conjugated double bonds or aromaticrings in their molecular structures. Furthermore, the adsorption ofinhibitor on steel/solution interface is inuenced by the chemicalstructure of inhibitor, the nature and charged surface of metal andthe type of aggressive media.

As an important kind of N-heterocyclic compound, pyrimidinederivatives whose molecules possess the pyrimidine ring withtwo N heteroatoms have favorable characteristics in inhibitingaction. Some pyrimidine derivatives were reported as goodcorrosion inhibitors for steel in acid media (HCl, H2SO4 andH3PO4), such as 2-aminopyrimidne (AP) [18], 2-hydroxypyr-imidne (HP) [19] and 2-mercaptopyrimidine [1921]. However,

Received 17 February 2014

Received in revised form 15 July 2014

Accepted 16 August 2014

Available online 6 September 2014

Keywords:

Corrosion inhibitor

Sulfuric acid

Steel surface

Aminopyrimidine derivative

Adsorption

2,4-diaminopyrimdine (DAP) on steel surface in 0.5 M H2SO4 solution were studied by weight loss, open

circuit potential (OCP), potentiodynamic polarization curves, electrochemical impedance spectroscopy

(EIS) and scanning electron microscope (SEM) methods. Quantum chemical calculation of density

function theory (DFT) and molecular dynamics (MD) simulations were applied to theoretically

determine the relationship between molecular structure and inhibition efciency. The results show that

two aminopyrimidine derivatives act as good inhibitors, and inhibition efciency follows the order:

DAP > AP. The adsorption of each inhibitor on steel surface obeys Langmuir adsorption isotherm. Both

AP and DAP are arranged as mixed-type inhibitors. EIS diagram appears a large capacitive loop at high

frequencies (HF) followed by a small inductive loop at low frequencies (LF), and the addition of

aminopyrimidine inhibitor increases the impedance of electrode. 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. The donating electrons to metal follows the order: DAP > AP,

which is in completely accordance with that of inhibitive performance. MD simulations reveal both

aminopyrimidine molecules adsorb on the Fe(0 0 1) surface in the nearly at manner, and the sequence

of either adsorption energy (Eads) or binding energy (Ebin) agrees well with that of inhibition efciency.

2014 Taiwan Institute of Chemical Engineers. Published by Elsevier B.V. All rights reserved.

* Corresponding author. Tel.: +86 871 65033769; fax: +86 871 65033726.

E-mail address: ynuxiexg@163.com (X. Xie).

http://dx.doi.org/10.1016/j.jtice.2014.08.019

1876-1070/ 2014 Taiwan Institute of Chemical Engineers. Published by Elsevier B.V. All rights reserved.Adsorption and inhibition effect of twoon steel surface in H2SO4 solution

Xianghong Li a,b, Xiaoguang Xie a,*a School of Chemical Science and Technology, Yunnan University, Kunming 650091, Chb Faculty of Science, Southwest Forestry University, Kunming 650224, China

A R T I C L E I N F O

Article history:

A B S T R A C T

The adsorption and inhibi

jou r nal h o mep age: w wwminopyrimidine derivatives

effect of two aminopyrimidine derivatives of 2-aminopyrimidine (AP) and

ute of Chemical Engineers

l s evier . co m/lo c ate / j t i c e

aminopyrimidine derivatives of AP and DAP with analyticalreagent (AR) grade were obtained from Shanghai Chemical ReagentCompany of China. Their molecular structures are shown in Fig. 1.The aggressive solutions of 0.5 M H2SO4 were prepared by dilutionof AR grade 98% H2SO4 with distilled water. The concentrationrange of inhibitor is 1.010.0 mM.

2.2. Weight loss measurements

The CRS rectangular coupons of 2.5 cm 2.0 cm 0.06 cm

X. Li, X. Xie / Journal of the Taiwan Institute of Chemical Engineers 45 (2014) 303330453034another pyrimidine derivative of uracil (Ur) exhibits poor inhibitiveability for steel in HCl [22], H2SO4 [23] and H3PO4 [24] solutions.Through these studies, the efciency of pyrimidine compound mainlydepends on the substitution group in the pyimidine ring. In 2005,Awad and Abdel Gawad [25] fully studied the corrosion inhibition ofpyrimidine and seven pyrimidine derivatives on iron in 2.0 M HClusing experimental methods. The results indicate that if a polarsubstitution group (NH2, OH, SH, etc.) is added to thepyrimidine ring, the electron density of N-heterocyclic ring isincreased, and subsequently, it facilitates the adsorb ability.Accordingly, there is a great need to study the correlationbetween the molecular structure and inhibitive performance.

Quantum chemical calculation has been proven to be a veryuseful method in corrosion inhibitor studies [26]. Using quantumchemical calculation, the theoretical parameters of inhibitormolecule can be obtained, and then, theoretically, the inhibitivemechanism can be directly accounted for the chemical reactivity ofthe compound under study [27]. The inhibition activity of a giveninhibitor is directly correlated with the theoretical parametersincluding the highest occupied molecular orbital energy (EHOMO),the lowest unoccupied molecular orbital (ELUMO), dipole moment(m), atomic charge, etc. [27,28]. Besides the quantum parameters oforganic inhibitor, the interaction between inhibitor and metalsurface could also be considered. Recently, MD simulation couldelucidate the adsorption of inhibitor on metal surface at molecularlevel [29,30]. It can provide the adsorption mode of inhibitormolecule on metal surface, and obtain the value of the adsorptionenergy (Eads) between the organic inhibitor and metal surface.

The ordinary aminopyrimidine derivatives of 2-aminopyrimidine(AP) and 2,4-diaminopyrimdine (DAP) are low cost, and have goodsolubility in water. Also, in order to elucidate whether increasing thenumber of amino group (NH2) to pyrimidine ring could improvethe inhibition efciency, AP and DAP are selected based on theconsideration of their molecular structures. In 2006, someaminopyrimidine derivatives including AP and DAP were reportedas corrosion inhibitors for carbon steel in 0.05 M HNO3 solutionusing weight loss and galvanostatic polarization measurements[31]. The maximum inhibition efciencies of AP and DAP are 36.3%and 42.6%, respectively. Afterwards, Masoud et al. [32] theoreticallyinvestigated the role of structural chemistry in the inhibitiveperformance of these aminopyrimidine compounds. However, bothAP and DAP have not yet been investigated as corrosion inhibitorsfor steel in H2SO4 solution. In addition, pyrimidine compounds couldbe protonated in the acid solution. But their protonated forms werenot theoretically studied in early work [32], and the adsorptionmode was not studied by MD simulation.

In this paper, the adsorption and inhibition effect of AP and DAPon the corrosion of steel surface in 0.5 M H2SO4 solution wasstudied rstly using weight loss, OCP, potentiodynamic polariza-tion curves, EIS and SEM methods. The adsorption isotherm ofinhibitor on steel surface is obtained. The standard adsorption freeenergy (DG0) and apparent activation energy (Ea) are calculatedand discussed. Quantum chemical calculation of DFT was appliedto study the difference in theoretical parameters between AP andDAP. Furthermore, the adsorption of inhibitor molecule onFe(0 0 1) surface was studied by MD simulations. It is expectedto obtain general information on the adsorption and inhibitioneffect of AP and DAP on steel surface in H2SO4 solution.

2. Experimental

2.1. Materials

Tests were performed on cold rolled steel (CRS) specimen withthe 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. Twowere abraded by a series of emery paper (grade 320-500-800) andthen washed with distilled water, degreased with acetone, andnally dried at room temperature. After weighing using digitalbalance with sensitivity of 0.1 mg, the specimens were immersedin glass beakers containing 250 ml 0.5 M H2SO4 without and withdifferent concentrations of inhibitor using glass hooks and rods. Thetemperature was controlled at 20 0.1 8C using a water thermostatbath. All the aggressive acid solutions were open to air withoutbubbling. After 6 h, the specimens were taken out, washed withbristle brush under running water to remove the corrosion product,dried with a hot air stream, and re-weighed accurately. In order to getgood reproducibility, experiments were carried out in duplicate. Theaverage weight loss of two parallel CRS sheets was obtained. Thecorrosion rate (v) was calculated from the following equation [33]:

v WSt

(1)

where W is the average weight loss of two parallel CRS sheets (g), Sthe total area of one CRS specimen (m2), and t is the immersiontime (6 h). With the calculated corrosion rate, the inhibitionefciency (hw) was calculated as follows:

hw % v0 vv0

100 (2)

where v0 and v are the values of corrosion rate without and withinhibitor, respectively.

2.3. Electrochemical measurements

Electrochemical experiments were carried out in the conven-tional three-electrode system with a platinum counter electrode(CE), a saturated calomel electrode (SCE) coupled to a ne Luggincapillary as the reference electrode and the working electrode(WE) which was in the form of a square CRS embedded in polyvinylchloride (PVC) holder using epoxy resin so that the at surface wasonly surface in the electrode. The WE surface area was1.0 cm 1.0 cm, and prepared as described above (Section 2.2).The electrode was immersed in test solution at open circuitpotential (OCP) for 2 h to be sufcient to attain a stable state beforemeasurement. In order to minimize ohmic contribution, the Luggincapillary was placed close to WE surface. All electrochemicalmeasurements were carried out at 20 8C using PARSTAT 2273advanced electrochemical system (Princeton Applied Research).

(a) (b)

Fig. 1. Chemical molecular structures of two aminopyrimidine derivatives: (a) 2-aminopyrimidine (AP) and (b) 2,4-diaminopyrimidine (DAP).

energy (Eads) of the inhibitor molecule on the Fe(0 0 1) surface wascalculated as follows [39]:

Eads Einh Esurf Etotal (5)

where Einh and Esurf are the energies of the free inhibitor moleculeand Fe(0 0 1) plane, respectively. Etotal is the total energy ofFe(0 0 1) plane together with inhibitor molecule adsorbed on theiron surface. The binding energy (Ebin) is the negative value of Eads[40]:

Ebin Eads (6)

3. Results and discussion

3.1. Weight loss measurements

3.1.1. Effect of aminopyrimidine derivatives on corrosion rate

Fig. 2 illustrates the corrosion rate (v) of CRS in 0.5 M H2SO4 at20 8C in the presence of different concentrations (c) of aminopyr-

X. Li, X. Xie / Journal of the Taiwan Institute of Chemical Engineers 45 (2014) 30333045 3035The potentiodynamic polarization curves were carried out bypolarizing to 250 mV with respect to open circuit potential (OCP) ata scan rate of 0.5 mV/s. Inhibition efciency (hp) is calculated throughthe corrosion current density (icorr) values:

hp % icorr0 icorrinh

icorr0 100 (3)

where icorr(0) and icorr(inh) represent corrosion current densitieswithout and with inhibitor, respectively.

EIS was carried out at OCP over a frequency range of 10 mHz100 kHz using a 10 mV root mean square (rms) voltage excitation.The total number of points is 30. Inhibition efciency (hR) isestimated using the charge transfer resistance (Rt) [33]:

hR Rtinh Rt0

Rtinh 100% (4)

where Rt(0) and Rt(inh) are charge transfer resistance values in theabsence and presence of the inhibitor, respectively.

2.4. SEM

Samples of dimension 2.5 cm 2.0 cm 0.06 cm were pre-pared as described above (Section 2.2). After immersion in 0.5 MH2SO4 solutions without and with addition of 10.0 mM AP or DAPat 20 8C for 6 h, the specimens were cleaned with distilled water,dried with a cold air blaster, and then examined by S-3000Nscanning electron microscope (Hitachi High-Tech Science SystemsCorporation, Japan).

2.5. Quantum chemical calculations

Quantum chemical calculations were performed with DMol3

numerical based DFT in Materials Studio 4.0 software fromAccelrys Inc. [34]. Geometrical optimizations and frequencycalculations were carried out with the generalized gradientapproximation (GGA) functional of Becke exchange plus LeeYangParr correlation (BLYP) [35] in conjunction with doublenumerical plus d-functions (DND) basis set [36]. GGA/BLYP/DNDhas good precision for N-heterocyclic inhibitors [37]. Fineconvergence criteria and global orbital cutoffs were employedon basis set denitions. Considering the solvent effects, all thegeometries were re-optimized at the BLYP/DND level by usingCOSMO (conductor-like screening model) [38] and dening wateras the solvent. The optimized species were conrmed to have noimaginary frequencies.

2.6. MD simulations

MD simulations were performed with Discover program inMaterials Studio 4.0 software from Accelrys Inc. [34]. Fe(0 0 1)plane was rstly cleaved from pure Fe crystal, the surface was thenoptimized to the energy minimum, and then was enlarged tofabricate an appropriate supercell. After that, a vacuum slab with1 A thickness was built above the Fe(0 0 1) supercell with31.53 A 31.53 A 15.30 A of total 1331 Fe atoms. Meanwhile,the optimized inhibitor molecules in Section 2.5 were built usingthe Amorphous cell program. Finally, the adsorption system wasbuilt by layer builder to place the inhibitor layer to Fe(0 0 1)supercell. All these slabs are separated by a 10 A vacuum thicknessto ensure that the interaction between the periodically repeatedslabs along the normal of the surface is small enough. Theadsorption system was optimized using COMPASS force eld. TheMD simulation was performed under 298 K, NVT ensemble, with atime step of 1.0 fs and simulation time of 1000 ps. The adsorptionimidine derivatives. The corrosion rate reduces after addition oftwo studied aminopyrimidine derivatives, and decreases with theinhibitor concentration. This behavior is due to the fact that theadsorption coverage increases with the increase of inhibitorconcentration, which shields the CRS surface efciently from themedium. In the absence of inhibitor, the corrosion rate is as high as7.45 g/(m2 h). But in the presence of 10.0 mM inhibitors, thecorrosion rate values are reduced to 1.40 and 0.61 g/(m2 h) for APand DAP, respectively. At any given inhibitor concentration, thecorrosion rate follows the order: v (DAP) < v (AP), which indicatesthat DAP exhibits better inhibitive performance than AP.

3.1.2. Effect of aminopyrimidine derivatives on inhibition efciency

Fig. 3 represents inhibition efciency (hw) values obtained fromthe weight loss in 0.5 M H2SO4 solutions in the presence of variousconcentrations of AP and DAP at 20 8C (immersion time is 6 h).Clearly, hw increases with the inhibitor concentration. It should benoted that when the concentration of inhibitor is about 7.0 mM, hwchanges slightly with a further increase of the inhibitor concen-tration. At 10.0 mM, the maximum hw is 81.3% for AP; and 91.8% forDAP, which indicates both aminopyrimidine derivatives act asgood corrosion inhibitors for CRS in 0.5 M H2SO4. Inhibitionefciency of the examined aminopyrimidine derivatives followsthe order: DAP > AP. It is reasonable deduced that increasing the

111098765432100

1

2

3

4

5

6

7

8

c/

(mM

)

c (mM)

AP DA P

Fig. 2. Relationship between corrosion rate (v) and concentration of inhibitor (c) in0.5 M H2SO4 at 20 8C (weight loss method, immersion time is 6 h).

steel surface obeys Langmuir adsorption isotherm. Noticeably, theslope values are deviated from 1, which indicates that there isinteraction between the adsorbed species [41]. Also, K follows theorder: DAP > AP. Generally, large value of K means that theinhibitor is easily and strongly adsorbed on the metal surface, andthen results in the better inhibition performance. This is inconsistent with the values of hw obtained from Fig. 3.

The adsorption equilibrium constant (K) is related to thestandard adsorption free energy (DG0) through the followingequation [42]:

1 DG0 !

1110987654321020

30

40

50

60

70

80

90

100

w(%

)

c (mM)

AP DAP

Fig. 3. Relationship between inhibition efciency (hw) and concentration ofinhibitor (c) in 0.5 M H2SO4 at 20 8C (weight loss method, immersion time is 6 h).

Table 1Parameters of the straight lines of c/u c and standard adsorption free energy (DG0)in 0.5 M H2SO4 at 20 8C (weight loss method, immersion time is 6 h).

Inhibitor Linear correlation

coefcient (r)

Slope K (M1) DG0 (kJ/mol)

AP 0.9987 0.85 2.78 102 23.5DAP 0.9998 0.89 4.93 102 24.9

X. Li, X. Xie / Journal of the Taiwan Institute of Chemical Engineers 45 (2014) 303330453036number of amino group (NH2) to pyrimidine ring would improvethe inhibitive performance.

3.1.3. Adsorption isotherm and standard adsorption free energy

(DG0)Fundamental information on the adsorption of inhibitor on

metal surface can be provided by adsorption isotherm. Someisotherms including Frumkin, Langmuir, Temkin, Freundlich,BockrisSwinkels and FloryHuggins isotherms were used to tthe experimental data. It is found that the adsorption of twoaminopyrimidine compounds on steel surface obeys Langmuiradsorption isotherm [33]:

c

u 1K c (7)

where c is the concentration of inhibitor, K the adsorptionequilibrium constant, and u is the surface coverage and calculatedby the ration of hw %/100.

Plots of c/u against c yield straight lines as shown in Fig. 4, andthe corresponding linear regression parameters are listed inTable 1. The linear correlation coefcient (r) is almost to 1,indicating the adsorption of two aminopyrimidine inhibitors on111098765432102

3

4

5

6

7

8

9

10

11

12

13

AP DA P

c/

(mM

)

c (mM)

Fig. 4. Langmuir isotherm adsorption modes of AP and DAP on steel surface in 0.5 MH2SO4 at 20 8C from weight loss measurement.K 55:5

expRT

(8)

where R is the gas constant (8.314 J/(K mol)), T the absolutetemperature (K), and the value 55.5 is the concentration of water inthe solution expressed in M [39]. The calculated DG0 values are alsopresented in Table 1. Generally, values of DG0 up to 20 kJ/mol areconsistent with the electrostatic interaction between the chargedmolecules and the charged metal (physisorption), while thosemore negative than 40 kJ/mol involve sharing or transfer ofelectrons from the inhibitor molecules to the metal surface to forma co-ordinate type of bond (chemisorption) [4244]. In the presentstudy, the values of DG0 for two inhibitors are around 24 kJ/mol,which probably means that both physical adsorption and chemicaladsorption (mixed adsorption) would occur. Moreover, the value ofDG0 for DAP is lower than that for AP, further demonstrating thatDAP exhibits the stronger tendency to adsorb on metal surface.

3.1.4. Effect of temperature

Temperature is an important factor that modies the adsorp-tion of inhibitor on electrode surface. In order to study the effect oftemperature on the inhibition characteristics, experiments wereconducted at 2050 8C at an interval of 5 8C. Fig. 5 shows therelationship between inhibition efciency (hw) of 10.0 mM and

80

85

90

95

w (%

)

AP DAP55504540353025201570

75

temperature (oC)

Fig. 5. Relationship between inhibition efciency (hw) and temperature in 0.5 MH2SO4 (weight loss method, immersion time is 6 h).

temperature. From 20 to 30 8C, inhibition efciency changesslightly, but then sharply decreases with the experimentaltemperature, which can be attributed to that the high temperaturemay cause the desorption of inhibitor molecule from metal surface.When the temperature is at 50 8C, hw is reduced to 75.6% for AP;and 81.6% for DAP.

According to Arrhenius equation, the natural logarithm ofcorrosion rate (ln v) is a linear function with 1/T [33]:

ln v EaRT

ln A (9)

where Ea and A represent apparent activation energy and pre-

the presence of each aminopyrimidine inhibitor is higher than thatin the uninhibited H2SO4 solution, which indicates that theadsorption of inhibitor is mainly the physical adsorption, while thechemical bonding between inhibitor molecule and steel surfacewould also occur [50].

Also, Eq. (9) shows that Ea/R is the slope of the straight lineln v 1/T, so Ea could elucidate the effect of temperature oncorrosion inhibition. The relationships between the temperaturedependence of percentage hw of an inhibitor and the Ea can beclassied into three groups according to temperature effects [51].

(i) hw decreases with increase in temperature, Ea (inhibitedsolution) > Ea (uninhibited solution);

(ii) hw increases with increase in temperature, Ea (inhibitedsolution) < Ea (uninhibited solution);

(iii) hw does not change with temperature, Ea (inhibited solutio-n) = Ea (uninhibited solution).

For two inhibitors, Ea (inhibited solution) > Ea (uninhibited

Table 2Parameters of the straight lines of ln v 1/T in 0.5 M H2SO4.

Inhibitor Linear correlation coefcient (r) Ea (kJ/mol) A (g/(m2h))

0.9980 51.71 1.20 1010AP 0.9965 60.16 6.75 1010DAP 0.9952 78.24 4.67 1013

X. Li, X. Xie / Journal of the Taiwan Institute of Chemical Engineers 45 (2014) 30333045 3037exponential 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 Arrheniusstraight lines of ln v vs. 1/T for the blank and different inhibitors. Allthe linear regression coefcients (r) are very close to 1, whichreects that the corrosion of steel in H2SO4 without and withinhibitors follows Arrhenius equation.

Kinetic parameter of Ea is important to study the inhibitivemechanism. Compared with uninhibited solution, the increase ofEa in inhibited solution may be interpreted as the physicaladsorption [45]. Conversely, a drop in Ea with respect to theuninhibited solution probably indicates chemisorption [46].However, the criteria of Ea cannot be taken as decisive due tocompetitive adsorption with water whose removal from thesurface requires additional activation energy [47]. According toSolmaz et al. [48], the adsorption phenomenon of an organicmolecule is not considered as a mere physical or mere chemicaladsorption phenomenon. Additionally, Moretti et al. [49] proposedthe adsorption criteria of chemisorption or physisorption could dedecided by other adsorption parameters. According to anotheradsorption parameter of DG0 in Section 3.1.3, the adsorption ofaminopyrimidine inhibitor would involve both physical andchemical processes. Thus, in the present study, the value of Ea in

1

2

3

4

5 blan k 10.0 mM AP 10.0 mM DA P

ln v

(g m-

2 h-

1 )0.00340.00330.00320.0031-1

0

1/T (K-1)

Fig. 6. Arrhenius plots related to the corrosion rate of CRS for various inhibitors in0.5 M H2SO4.solution), and Ea follows the order of Ea (DAP) > Ea (AP), whichfurther conrm hw decreases with increase in temperature, and thedecreased degree with temperature becomes more notable for DAP.

Table 2 also shows that the variance in A is similar to that in Ea.Inuenced by the comprehensive effect of the magnitudes of Ea andA, the corrosion rate decreases in the presence of inhibitor.

3.2. OCP curves

Fig. 7 shows the variation of open circuit potential value (EOCP)of CRS electrode with immersion time (t) in 0.5 M H2SO4 solution inthe absence and presence of 10.0 mM AP, 10.0 mM DAP at 20 8C. Inthe absence of inhibitor (blank solution), the curve shows a slightincrease of EOCP toward positive direction (020 min) followed byshifting in lower values of potential, and then gradually reaches thesteady state. It took about 80 min to reach the steady state. In thepresence of inhibitors of AP and DAP, the initial potential is movedpositive value by time and gradually remains constant. It tookabout 60 min to reach the steady state, so the steady-state wasachieved after 2 h immersion for electrochemical tests. The EOCP at120 min in H2SO4 without and with 10.0 mM AP and 10.0 mM DAPare 488, 484 and 493 mV (vs. SCE), respectively. The change ofsteady-state potential could be negligible after adding inhibitors ofAP and DAP to 0.5 M H2SO4 solution, which indicates that both APand DAP can be arranged as mixed-type inhibitors.

200180160140120100806040200-0.53

-0.52

-0.51

-0.50

-0.49

-0.48

-0.47

-0.46

c

b

E OCP

(V

v

s. S

CE)

t (min )

a

a --- 0.5 M H2SO4b --- 0.5 M H2SO4 + 10.0 mM APc --- 0.5 M H2SO4 + 10.0 mM DAP

Fig. 7. EOCPt curves for CRS in 0.5 M H2SO4 solution at 20 8C.

-1.0-1.5-2.0-2.5-3.0-3.5-4.0-4.5-5.0-5.5-6.0-0.8

-0.7

-0.6

-0.5

-0.4

-0.3

-0.2

E (V

vs.

SCE

)

log i (A cm-2)

blan k 2.0 mM AP 5.0 mM AP 10.0 mM AP

Fig. 8. Potentiodynamic polarization curves for CRS in 0.5 M H2SO4 without andwith different concentrations of AP at 20 8C (immersion time is 2 h).

X. Li, X. Xie / Journal of the Taiwan Institute of Chemical Engineers 45 (2014) 3033304530383.3. Potentiodynamic polarization curves

Potentiodynamic polarization curves of CRS in 0.5 M H2SO4 inthe presence of different concentrations of AP and DAP at 20 8C areshown in Figs. 8 and 9, respectively. Obviously, the presence ofeach inhibitor causes a remarkable decrease in the corrosion ratei.e., shifts both anodic and cathodic curves to lower currentdensities. Both anodic and cathodic reactions of CRS electrode aredrastically retarded, which suggests that the both AP and DAP actas mixed-type inhibitors. This result clearly shows that thecorrosion inhibition of steel is controlled under cathodic andanodic reactions.

As can be seen from Figs. 8 and 9, the steel electrode immersedin 0.5 M H2SO4 solution displays a cathodic region of Tafelbehavior. However, the anodic polarization curve does not displayan extensive Tafel region, may be due to concentration effects orroughening of the surface. Therefore, due to absence of linearity inanodic branch, accurate evaluation of the anodic Tafel slope by Tafelextrapolation of the anodic branch is impossible [5254]. Accord-

ingly, the electrochemical parameters of corrosion processes could

-1.0-1.5-2.0-2.5-3.0-3.5-4.0-4.5-5.0-5.5-6.0-0.8

-0.7

-0.6

-0.5

-0.4

-0.3

-0.2

E (V

vs. SC

E)

log i (A cm-2)

blank 2.0 mM DA P 5.0 mM DA P 10.0 mM DAP

Fig. 9. Potentiodynamic polarization curves for CRS in 0.5 M H2SO4 without andwith different concentrations of DAP at 20 8C (immersion time is 2 h).be determined by Tafel extrapolation of the cathodic curve to thecathodic linear region back to the corrosion potential [55,56].But it has been shown that in the Tafel extrapolation method,use of both the anodic and cathodic Tafel regions is undoubtedlypreferred over the use of only one Tafel region [57]. Recently,Amin et al. [58,59] proposed that the corrosion parameters couldbe obtained by rstly extending the cathodic polarizationcurve, and then tting the anodic region of about 10150 mV(vs. Ecorr). Namely, it is possible to calculate the anodic Tafel linefrom the experimental data. This method is acceptable owingto good consistent with other methods of corrosion ratedetermination [58,59].

The values of corrosion current densities (icorr), corrosionpotential (Ecorr), cathodic Tafel slope (bc), anodic Tafel slope (ba)and inhibition efciency (hp) are summarized in Table 3. It can beseen from Table 3 that icorr decreases signicantly with theincrease of the concentration of each aminopyrimidine deriva-tive. When the inhibitor concentration is 10.0 mM, icorr decreasesfrom 595.6 mA/cm2 to 107.5 and 61.7 mA/cm2 in the case of APand DAP, respectively. Correspondingly, hp increases with theinhibitor concentration, due to the increase in the blockedfraction of the electrode surface by adsorption. Inhibitionefciency (hp) at 10.0 mM reaches up to a maximum of 82.0%for AP; and 89.6% for DAP, which again conrms that bothaminopyrimidine derivatives are good inhibitors for steel in0.5 M H2SO4, and hp follows the order: DAP > AP. Compared withEcorr in 0.5 M H2SO4 solution without inhibitor, Ecorr in thepresence of either AP or DAP does not change, which indicatestwo studied aminopyrimidine derivatives act as mixed-typeinhibitors [60,61]. Furthermore, Tafel slopes of bc and ba do notchange markedly in the presence of each inhibitor, whichsuggests that the mechanism of steel in H2SO4 media does notalter after adding inhibitors [6062].

3.4. Electrochemical impedance spectroscopy (EIS)

Figs. 10 and 11 represent the Nyquist diagrams for CRS in 0.5 MH2SO4 in the presence of AP and DAP at 20 8C, respectively. Clearly,the impedance spectra exhibit a large capacitive loop at highfrequencies followed by a small inductive loop at low frequencyvalues. In the presence of each aminopyrimidine derivative,comparing with blank solution, the shape is maintained through-out all tested concentrations, indicating that almost no change inthe corrosion mechanism occurs due to the inhibitor addition [63].

The capacitive loop indicates that the corrosion of steel ismainly controlled by a charge transfer process, and usually relatedto the charge transfer of the corrosion process and double layerbehavior. On the other hand, the inductive loop may be attributedto the relaxation process obtained by adsorption species like FeSO4[64] or inhibitor species [65] on the electrode surface.

The diameter of the capacitive loop in the presence of inhibitoris bigger than that in the absence of inhibitor (blank solution) andincreases with the inhibitor concentration. This indicates that theimpedance of inhibited substrate increases with the inhibitorconcentration. Noticeably, these capacitive loops are not perfectsemicircles which can be attributed to the frequency dispersioneffect as a result of the roughness and inhomogeneousness of theelectrode surface [66]. Accordingly, the capacitive loops at highfrequencies are simulated by the equivalent circuit shown inFig. 12 [65]. The circuit employed allows the identication of bothsolution resistance (Rs) and charge transfer resistance (Rt). It isworth mentioning that the double layer capacitance (Cdl) valueis affected by imperfections of the surface, and that this effect issimulated via a constant phase element (CPE) [66]. The CPE iscomposed of a component Qdl and a coefcient a that quantiesdifferent physical phenomena like surface inhomogeneousness

resulting from surface roughness, inhibitor adsorption, porouslayer formation, etc. The capacitance can be calculated from thefollowing equation [6,53]:

Cdl Qdl 2p fmaxa1 (10)

where fmax represents the frequency at which the imaginary valuereaches a maximum on the Nyquist plot. The electrochemical

more uniform and some abrading scratches. However, it is notabsolute smooth and uniform, and covered with small grains,which may be attributed to the defect of steel, and probably anoxide inclusion or carbides [67]. Fig. 13(b), the CRS surface afterimmersion in uninhibited 0.5 M H2SO4 for 6 h shows an aggressiveattack of the corroding medium on the steel surface. The corrosionproducts appear too uneven, and the surface is rather rough.Fig. 13(c) and (d) shows that corrosion degree of the steel surface

Table 3Potentiodynamic polarization parameters for the corrosion of CRS in 0.5 M H2SO4 solution containing different concentrations of AP and DAP at 20 8C (immersion time is 2 h).

Inhibitor c (mM) Ecorr (mV vs. SCE) icorr (mA/cm2) bc (mV dec1) ba (mV dec1) hp (%)

0 485 595.6 132 37

AP 2.0 463 369.5 156 21 38.05.0 469 190.0 140 29 68.1

10.0 454 107.5 127 18 82.0

DAP 2.0 461 278.6 135 22 53.25.0 461 145.7 136 23 75.5

10.0 463 61.7 135 21 89.6

X. Li, X. Xie / Journal of the Taiwan Institute of Chemical Engineers 45 (2014) 30333045 3039parameters of Rs a, Rt, Cdl and hR are presented in Table 4. Clearly, Rtvalue increases prominently while Cdl reduces with the concen-tration of inhibitor. A large charge transfer resistance is associatedwith a slower corroding system. At any given inhibitor concentra-tion, Rt (AP) < Rt (DAP), which conrms that DAP exhibits thebetter inhibitive performance between two aminopyrimidinecompounds.

The decrease of Cdl in inhibited solution comparing with that inblank solution (without inhibitor), which can result from adecrease in local dielectric constant and/or an increase in thethickness of the electrical double layer, suggests that the inhibitormolecules function by adsorption at the metal/solution interface[64]. hR increases with the concentration of inhibitor, and followsthe order: hR (DAP) > hR (AP). The maximum hR values are 82.6%and 89.8% for AP and DAP, respectively. These results again conrmthat two aminopyrimidine derivatives exhibit good inhibitiveperformance for CRS in H2SO4 solution.

Inhibition efciencies obtained from weight loss (hw), poten-tiodynamic polarization curves (hp) and EIS (hR) are in goodreasonably agreement.

3.5. SEM surface examination

The SEM images of CRS surfaces are shown in Fig. 13. It can beseen from Fig. 13(a) that the CRS surface before immersion appears

60

90

120

150

cm2 )

blan k 2.0 mM AP 5.0 mM AP 10.0 mM AP2101801501209060300-30

0

30Zim

(

Zre ( cm2)

Fig. 10. Nyquist plots of the corrosion of CRS in 0.5 M H2SO4 without and withdifferent concentrations of AP at 20 8C (immersion time is 2 h).decreases in the presence of 10.0 mM AP or DAP, and the surfaceseems smooth. It should be noted that Fig. 13(d) clearly shows thatthe some compact spherical or bread-like particles distribute onthe steel surface in the presence of 10.0 mM DAP, which do notexist in Fig. 13(a) and (b). Thus, it might be concluded that theseparticles are the adsorption lm of the inhibitor, which efcientlyretard the corrosion of CRS.

3.6. Quantum chemical calculations

Fig. 14(a) and (b) shows the optimized neutral molecularstructures of P and AP under the level of GGA/BLYP/DND/COSMO,respectively. Noticeably, both AP and DAP could be protonated inthe acid solution. Thus, in aqueous acidic solutions, both neutraland protonated molecules should be considered. According tosome quantum chemical studies about protonated N-heterocyclicinhibitor in HCl solution [68], the proton afnity is clearly favoredtoward the hetero N atom of N-heterocyclic ring. To further judgewhich N atom is protonated, the protonated afliation energy (PA)is dened as:

PA Eeinh ZPVEinh Eep-inh ZPVEp-inh (11)

where Ee(inh) and Ee(p-inh) are total electron energies for inhibitorand protonated inhibitor, respectively. Ee(ZPVE) and Ee(ZPVE) are

80

120

160

200

cm2 )

blank 2.0 mM DA P 5.0 mM DA P 10 .0 mM DA P400350300250200150100500-40

0

40Zim

(

Zre ( cm2)

Fig. 11. Nyquist plots of the corrosion of CRS in 0.5 M H2SO4 without and withdifferent concentrations of DAP at 20 8C (immersion time is 2 h).

Zero-point vibrational energies for inhibitor and protonatedinhibitor, respectively. The atom with large PA means it is easilyprotonated by H+. For AP, the protonation on N6 and N8 have thesame PA value of 1123.3 kJ/mol. But for DAP, PA of the protonationon N6 (1052.4 kJ/mol) is lower than that on N8 (1123.3 kJ/mol), soN8 in DAP is easily protonated. Accordingly, the molecularstructures of protonated 2-aminopyrimidine (p-AP) and protonat-

atom. The larger negative charge of the atom, the better is theaction as an electronic donor. Each pyrimidine molecule containsmore N atoms in the pyrimidine ring and amino group. Mullikencharges of the atoms are listed in Table 5. The larger negativeatoms are found in N1 for AP and p-AP, N12 for DAP and p-DAP;which are active adsorptive atom. When the N atom of pyrimidinering is protonated, the Mulliken charge of protonated N atom (N6in p-AP, N8 in p-DAP) becomes more negative than anotherunprotonated N atom of ring (N8 in p-AP, N6 in p-DAP), whichimplies that if the inhibitor is protonated, the adsorptive ability ofprotonated N atom would increases. In addition, the Mullikencharge follows the order: N12 (p-DAP) > N1 (p-AP) and N12(DAP) > N1 (AP). This result implies that whether the inhibitor isprotonated, the N atom in DAP exhibits more denoting ability thanthat in AP, which is in accordance with that of inhibitiveperformance.

Fukui function is necessary in understanding the local siteselectivity. The Fukui function f ~r is dened as [69]:

f ~r @r~r@N

V~r

(12)

The nucleophic attack Fukui function f ~r and electrophilicattack Fukui function f ~r can be respective calculated as [70]:

f i~r qiN 1 qiN (13)

f i~r qiN qiN 1 (14)

where qi(N + 1), qi(N), qi(N 1) are charge values of atom i forcation, neutral and anion, respectively. The values of f ~r and

CPE

Rt

Rs

Fig. 12. The equivalent circuit model of the capacitive loop in EIS.

Table 4EIS parameters for the corrosion of CRS in 0.5 M H2SO4 containing AP and DAP at

20 8C.

Inhibitor c (mM) Rs (V cm2) Rt (V cm

2) a Cdl (mF/cm2) hR (%)

0 1.10 34.9 0.9459 107

AP 2.0 0.89 55.1 0.9426 93 36.7

5.0 1.63 100.3 0.9418 81 65.2

10.0 2.25 201.1 0.9362 65 82.6

DAP 2.0 1.21 74.7 0.9425 84 53.3

5.0 1.46 123.4 0.9343 76 71.7

10.0 2.05 342.6 0.9442 52 89.8

X. Li, X. Xie / Journal of the Taiwan Institute of Chemical Engineers 45 (2014) 303330453040ed 2,4-diaminopyrimdine (p-DAP) are shown in Fig. 14(c) and (d),respectively. For the pyrimidine ring, it contains two N atoms, sothe larger negative charge of N atom would be easily protonated.

Organic inhibitor could form coordination bonds between theunshared electron pairs of N atom and the empty d-orbital of FeFig. 13. SEM micrographs of CRS surface: (a) before immersion; (b) after 6 h of immersioH2SO4 and (d) after 6 h of immersion at 20 8C in 10.0 mM DAP + 0.5 M H2SO4.f ~r are also given in Table 5. Generally, high values of f ~r andf ~r mean the high capacity of the atom to gain and lost electron,respectively. For either neutral or protonated inhibitor molecule,the most reactive sites of the nucleophic attack are C5 and C9 for APand p-AP; C 5 for DAP and p-DAP. On the other hand, the values of

n at 20 8C in 0.5 M H2SO4; (c) after 6 h of immersion at 20 8C in 10.0 mM AP + 0.5 M

X. Li, X. Xie / Journal of the Taiwan Institute of Chemical Engineers 45 (2014) 30333045 3041f ~r indicate that they will happen on N1 for AP and p-AP, C4 andN6 for DAP and p-DAP; which can denote electrons to metalsurface to form coordinate bond.

The dipole moment (m) is widely used the polarity of themolecule, and also related to the inhibitive ability [71]. The large mprobably increases the inhibitor adsorption through electronicforce [72]. In Table 6, m(DAP) > m(AP) and m(p-DAP) > m(p-AP),which indicates that the sequence of inhibitive performance couldbe arisen form intermolecular electrostatic force. Further inspec-tion of Table 6 shows that the protonated molecules of p-AP and p-DAP have larger dipole moment values than corresponding neutralmolecules of AP and DAP. Thus, p-AP and p-DAP could easily adsorbon metal surface via physical adsorption compared with AP andDAP. The charge of the metal surface can be determined from thevalue of Ecorr Eq=0 (zero charge potential) [73]. The Eq=0 of iron is550 mV vs. SCE in H2SO4 [74]. In the present system, the value ofEcorr obtained in 0.5 M H2SO4 is 485 mV vs. SCE. The steel surface

Fig. 14. Optimized molecular structures of four pyrimidin

Table 5Quantum chemical parameters of Mulliken charge, f ~r and f ~r for AP, p-AP, DAP

Atom Mulliken charge f ~r

AP p-AP DAP p-DAP AP p-A

N1 0.724 0.690 0.736 0.709 0.034 0.0C4 0.226 0.198 0.317 0.297 0.019 0.0C5 0.021 0.069 0.035 0.013 0.170 0.1N6 0.409 0.472 0.452 0.411 0.125 0.0C7 0.515 0.674 0.500 0.655 0.045 0.0

N8 0.409 0.356 0.480 0.582 0.125 0.1C9 0.021 0.011 0.458 0.602 0.170 0.1N12 0.744 0.715 charges positive charge in H2SO4 solution because ofEcorr Eq=0 > 0. Since SO42 could adsorb on the metal surface[2], they create an excess negative charge toward the solution andfavor more adsorption of the cations. Both p-AP and p-DAP mayadsorb on the negatively charged metal surface. In other words,there may be a synergism between adsorbed SO4

2 and protonatedmolecule [11,16].

Besides the above-mentioned quantum chemical parametersfor local reactivity, the global reactivity of a molecule depends onthe distributions of frontier molecular orbitals. The highestoccupied molecular orbital (HOMO) is often associated with thecapacity of a molecule to donate electrons, whereas the lowestunoccupied molecular orbital (LUMO) represents the ability of themolecule to accept electrons. The electric/orbital density distribu-tions of HOMO and LUMO for four aminopyrimidine molecules areshown in Fig. 15. The electron densities of both HOMO and LUMOare localized principally on the pyrimidine ring. That is, there is

e molecules: (a) P, (b) p-AP, (c) DAP, and (d) p-DAP.

and p-DAP molecules.

f ~r

P DAP p-DAP AP p-AP DAP p-DAP

50 0.033 0.054 0.206 0.198 0.124 0.124

23 0.049 0.046 0.124 0.122 0.148 0.148

81 0.189 0.178 0.048 0.076 0.048 0.050

68 0.085 0.067 0.097 0.060 0.126 0.126

51 0.052 0.069 0.055 0.052 0.047 0.050

09 0.127 0.067 0.097 0.102 0.059 0.023

82 0.100 0.111 0.048 0.046 0.034 0.041

0.057 0.079 0.094 0.105

electron transferring in the interaction between the pyrimidineand metal surface. For AP, p-AP, DAP and p-DAP, HOMO is locatedon all substituted amino groups, but LUMO is absent on 2-positionof amino group, which reects the all substituted amino groups

could be the donor of electron. The amino group(s) except for 2-position of amino group could be also the acceptor of the electron.

The values of energy of EHOMO, ELUMO and the separation energy(ELUMO EHOMO, DE) are also presented in Table 6. High value of

Table 6Quantum chemical parameters for inhibitor molecules at GGA/BLYP/DND/COSMO level.

Molecule m (D) EHOMO (eV) ELUMO (eV) DE (eV) b (eV) g (eV) s (eV1) DN

AP 1.3019 5.645 1.975 3.670 3.810 1.835 0.545 0.869p-AP 4.6395 6.709 3.264 3.445 4.986 1.722 0.580 0.585DAP 4.0020 5.337 1.418 3.919 3.378 1.960 0.510 0.924p-DAP 6.3738 6.048 2.511 3.537 4.280 1.768 0.565 0.769

X. Li, X. Xie / Journal of the Taiwan Institute of Chemical Engineers 45 (2014) 303330453042Fig. 15. The frontier molecule orbital density distributions of (a) AP, (b) p-AP, (c) DAP and (d) p-DAP: HOMO (left); LUMO (right).

EHOMO indicates a tendency of the molecule to donate electrons toempty molecular orbitals of acceptor molecules [75]. Conversely,the ELUMO represents the ability of the molecule to accept electrons,and the lower value of ELUMO suggests the molecule acceptselectrons more probable [75]. DE is an important parameter as afunction of reactivity of the inhibitor molecule toward theadsorption on metallic surface. As DE decreases, the reactivity ofthe molecule increases in visa, which facilitates adsorption andenhances the efciency of inhibitor [76]. From Table 6, EHOMOobeys the order: DAP > AP, and when compound is protonated, theorder is still same of p-DAP > p-AP. Obviously, these twosequences are in completely accordance with the order ofinhibition efciency. This may explain the better inhibitionefciency of DAP molecule than AP is due to the higher EHOMO.In addition, EHOMO of protonated molecule decreases comparedwith that of neutral molecule. Therefore, it could be deduced thatthe protonated molecule weakens donating electrons to metalsurface. On the other hand, ELUMO obeys the order: DAP > AP and p-DAP > p-AP, which is on the contrary with the inhibition efciencyorder. Inspection of the data in Table 6 reveals DE obeys the orderof AP < DAP and p-AP < p-DAP, which is also not correlated withthe order of inhibition efciency.

Additionally, the energies of HOMO and LUMO orbital ofthe inhibitor molecule are related to the ionization potential (I)and the electron afnity (Y) by the following relations,respectively [77]:

I EHOMO (15)

Y ELUMO (16)

Then absolute electronegativity (b), global hardness (g) andglobal softness (s) of the inhibitor molecule are approximated asfollows [78]:

b I Y2

(17)

g I Y2

(18)

s 1g

(19)

The number of electrons transferred from the inhibitor tometallic surface (DN) is calculated depending on the quantumchemical method [79]:

DN bFe binh2gFe g inh

(20)

For Fe, the theoretical values of bFe and gFe are 7 eV and 0 eV,respectively [79]. The values of b, g, s and DN are also listed inTable 6. According to some studies [80,81], the parameter of b isrelated to the chemical potential, and higher value of b meansbetter inhibitive performance. On the other hand, g is equal toDE/2, and the lower g implies more polarizability and higherinhibition efciency. The parameter of s is reciprocal to g, thus highvalue of s is related to more efciency. Accordingly, there is nocorrelation between inhibition efciency and the parameters of b,g, s. Values of DN exhibit inhibitive performance resulted fromelectrons donations. If DN < 3.6, the inhibition efciency increaseswith the increase in electron-donation ability to the metal surface

1) p

X. Li, X. Xie / Journal of the Taiwan Institute of Chemical Engineers 45 (2014) 30333045 3043Fig. 16. Equilibrium adsorption conguration of inhibitor molecules on Fe(0 0 lanes obtained by MD simulations: (a) AP, (b) p-AP, (c) DAP, and (d) p-DAP.

X. Li, X. Xie / Journal of the Taiwan Institute of Chemical Engineers 45 (2014) 303330453044[81]. From Table 6, there is a good correlation between inhibitionefciency and the parameter of DN.

3.7. MD simulations

MD simulations had been done to further study the adsorptionbehavior of neutral molecules as well as protonated molecules on theFe(0 0 1) surface. Through the analysis of the temperature and energy,it takes about 200 ps for the adsorption system containing bothFe(0 0 1) surface and the studied inhibitor molecule reaches equilibri-um. The adsorption system is at steady state and uctuates slightlyfrom 500 ps to 1000 ps. Fig. 16 shows the adsorption congurations onFe(0 0 1) surface for neural molecules and protonated molecules. Thecorresponding Eads and Ebin values are listed in Table 7.

As can be seen from Fig. 16, two neutral molecules (AP and DAP)and two protonated molecules (p-AP and p-DAP) are adsorbed onFe(0 0 1) surface with a nearby at orientation. The formation ofthe at orientation on steel surface can be attributed to therelatively equal distribution of HOMO and LUMO densities on thewhole molecule. In other words, the whole inhibitor moleculecould adsorb on steel surface through pyrimidine ring andsubstituted amino group(s) simultaneously.

The values of Eads in Table 7 reveal the sequence of DAP >AP andp-DAP > p-AP, which indicates that increasing the number ofamino group (NH2) to pyrimidine ring exhibit a stronger tendencyto adsorb on steel surface, and then leads to the higher inhibitiveperformance. On the other hand, magnitude of Ebin is indicative ofstability of adsorptive system. Ebin follows the order: DAP < AP andp-DAP < p-AP. More negative value of Ebin suggests a more stableadsorption system and leads to the higher inhibitive action. Basedon the parameters of Eads and Ebin, inhibition efciency for twostudied inhibitors is ranked as DAP > AP. Thus, the theoreticalinference is in good agreement with experimental data. Further-more, Eads value of protonated molecule is lower than neutralmolecule, which indicates that the adsorptive ability of protonatedmolecule decreases comparing with its neutral molecule.

4. Conclusion

(1) Two aminopyrimidine derivatives of AP and DAP are goodinhibitors for the corrosion of CRS in 0.5 M H2SO4 solution. hwincreases with the inhibitor concentration, and the maximumhw at 10.0 mM is 81.3% for AP; and 91.8% for DAP at 20 8C.Inhibition efciency (hw) follows the order: DAP > AP.

(2) The adsorption of AP or DAP obeys Langmuir adsorption

Table 7Values of adsorption energy (Eads) and binding energy (Ebin) between the molecules

and Fe(0 0 1) plane.

Molecule Eads (kJ/mol) Ebin (kJ/mol)

AP 536.04 536.04p-AP 219.96 219.96DAP 788.12 788.12p-DAP 557.01 557.01isotherm, and the adsorption equilibrium constant follows theorder: DAP > AP. hw decreases with the temperature. Thevalues of both Ea and A in the presence of inhibitor are higherthan those in the absence of inhibitor.

(3) Both AP and DAP act as mixed-type inhibitors. EIS spectra exhibita large capacitive loop at high frequencies followed by a smallinductive loop at low frequency values. The presence of inhibitorin 0.5 M H2SO4 solutions increases Rt while reduces Cdl.

(4) Either neutral or protonated aminopyrimidine molecules, thepyrimidine ring and substituted N atom(s) are in one plane. Thelarger negative Mulliken charges are found in N atoms for bothneutral and protonated molecules, which are adsorptivecenters. The electron densities of both HOMO and LUMO arewell localized principally on the whole molecule, which couldbe both the acceptor and the donor of the electrons. There is agood correlation between inhibition efciency and thequantum parameters of m, EHOMO and DN.

(5) MD simulations reveal that neutral and protonated moleculesadsorb on the Fe(0 0 1) surface in the nearly at manner, andthe sequence of either Eads or Ebin is in accordance with that ofinhibition efciency. The adsorptive ability of protonatedmolecule decreases comparing with its neutral molecule.

Acknowledgement

This work was carried out in the frame of research projectfunded by the National Natural Science Foundation of China(51361027).

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Adsorption and inhibition effect of two aminopyrimidine derivatives on steel surface in H2SO4 solutionIntroductionExperimentalMaterialsWeight loss measurementsElectrochemical measurementsSEMQuantum chemical calculationsMD simulations

Results and discussionWeight loss measurementsEffect of aminopyrimidine derivatives on corrosion rateEffect of aminopyrimidine derivatives on inhibition efficiencyAdsorption isotherm and standard adsorption free energy (G0)Effect of temperature

OCP curvesPotentiodynamic polarization curvesElectrochemical impedance spectroscopy (EIS)SEM surface examinationQuantum chemical calculationsMD simulations

ConclusionAcknowledgementReferences