嘧啶衍生物對鋼在鹽酸溶液中的緩蝕作用

李向紅 謝小光

(1云南大學化學科學與工程學院,昆明 650091; 2西南林業(yè)大學理學院,昆明 650224）

1 Introduction

N-heterocyclic compounds are considered to be the most effective corrosion inhibitors for metals in acid media.1They exhibit inhibition by adsorption on the metal surface,and the adsorption takes place through nitrogen,oxygen,and sulfur atoms,as well as those with triple or conjugated double bonds or aromatic rings in their molecular structures.A stronger coordination bond formed between inhibitor and metal is always related to good inhibitive performance;as consequence,inhibition efficiency should increase in the order:O＜N＜S＜P.2If 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 adsorption ability.3

As an important kind of N-heterocyclic compound,pyrimidine derivatives whose molecules possess the pyrimidine ring with two N heteroatoms could also be deemed as good potential inhibitors.2-Mercaptopyrimidine(MP）was reported as a good corrosion inhibitor for non-ferrous metals in acid media,such as zinc in HCl solution,4aluminium in HCl solution,5copper in H2SO4solution.6Besides non-ferrous metals,the inhibition effects of pyrimidine derivatives on the steel corrosion in acid media were studied.In 1993,Zucchi et al.7investigated the corrosion inhibition of steel in H2SO4solution by some pyrimidine derivatives.In 2001,Wang8reported the corrosion inhibition by MP for steel in H3PO4solution,and the maximum inhibition efficiency(η）in 3.0 mol·L-1H3PO4solution is 98%at 10.0 mmol·L-1.According to our recent work,92-aminopyrimidne(AP）also acts as a good corrosion inhibitor on the corrosion of steel in 1.0 mol·L-1HCl solution,and the η is 92.4%at 20 °C when the concentration of AP is 10.0 mmol·L-1.However,another pyrimidine derivative of uracil(Ur）exhibits poor inhibitive ability for steel in H3PO410and H2SO411solutions.Through these studies,the efficiency of pyrimidine compound mainly depends on the substitution group in the pyrimidine 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.12-14Using quantum chemical calculation,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.It is found that 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,and Fukui inces.15However,many quantum parameters about organic inhibitors are calculated in gas phase.In fact,acid inhibitors are used in acidic water solutions,and they could be protonated.Thus,in the field of theoretical calculation of acid inhibitors,the solvent effect and the protonated inhibitor molecules should be taken into account.

In this paper,the inhibition effect of two pyrimidine derivatives of 2-hydroxypyrimidine(HP）and MP on the corrosion of cold rolled steel(CRS）in 1.0 mol·L-1HCl solution is studied using weight loss,potentiodynamic polarization curves,and electrochemical impedance spectroscopy(EIS）methods.The adsorption isotherm of inhibitor on steel surface and the standard adsorption free energy(ΔG0）are obtained.Quantum chemical calculation of density functional theory(DFT）including the solvent effect is applied to elucidate the relationship between the inhibitor molecular structure and inhibition efficiency.The difference in inhibition performance between neutral inhibitor molecule and protonated inhibitor molecule is further investigated.It is expected to get general information on the adsorption and inhibition effect of pyrimidine derivatives on steel in HCl solution.

2 Experimental

2.1 Materials and inhibitors

Weight loss and electrochemical tests were performed on cold rolled steel with the following composition:0.05%C,0.02%Si,0.28%Mn,0.023%P,0.019%S,and the remainder Fe.Two pyrimidine derivatives of 2-hydroxypyrimidine(HP,C4H4N2O）and 2-mercaptopyrimidine(MP,C4H4N2S）were obtained from Shanghai Chemical Reagent Company of China.Fig.1 shows the molecular structures of HP and MP,and they are of good solubility in water.The aggressive solution of 1.0 mol·L-1HCl solution was prepared by dilution of AR grade 37%HCl with distilled water.The concentration range of inhibitor is 1.0-10.0 mmol·L-1.

2.2 Weight loss measurements

The CRS rectangular coupons of 2.5 cm×2.0 cm×0.04 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 mol·L-1HCl solution without and with different concentrations of inhibitor using glass hooks and rods.The temperature was controlled at(25.0±0.1）°C using a water thermostat bath.All the aggressive acid solutions were open to air without bubbling.After immersion for 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 triplicate.The average mass loss of three parallel CRS sheets was obtained,and then the inhibition efficiency(ηw）was calculated.1

Fig.1 Chemical molecular structures of two pyrimidine derivatives

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 minimize Ohmic contribution,the Luggin capillary was placed close to the working electrode(WE）which was in the form of a square CRS embedded in polyvinyl chloride(PVC）holder using epoxy resin so that the flat surface was the 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 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 25°C using PARSTAT 2273 advanced electrochemical system(Princeton Applied Research）.

The potential of potentiodynamic polarization curves was increased at 0.5 mV·s-1and started from a potential of-250 mV to+250 mV(versus OCP）.Inhibition efficiency(ηp）was calculated through the corrosion current density(icorr）values.1Electrochemical impedance spectroscopy(EIS）was carried out at OCP over a frequency range of 10 mHz-100 kHz using a 10 mV root mean square(RMS）voltage excitation.The number of points per decade is 30.Inhibition efficiency(ηR）was estimated using the charge transfer resistance(Rt）values.1

2.4 Quantum chemical calculations

Quantum chemical calculations were performed with DMol3numerical based DFT in Materials Studio 4.1 software from Accelrys Inc.16Geometrical optimizations and frequency calculations were carried out with the generalized gradient approximation(GGA）functional of Becke exchange plus Lee-Yang-Parr correlation(BLYP）17in conjunction with double numerical plus d-functions(DND）basis set.18Fine 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）19and defining water as the solvent.Through the frequency analysis,it is found that all optimized species have no imaginary frequencies.

3 Results and discussion

3.1 Effect of pyrimidine derivatives on inhibition efficiency

Fig.2 Relationship between inhibition efficiency(ηw）and concentration of inhibitor(c）in 1.0 mol·L-1 HCl solution at 25 °C weight loss method,immersion time:6 h

Fig.2 shows the relationship between inhibition efficiency(ηw）values obtained from weight loss method and the concentrations of HP and MP in 1.0 mol·L-1HCl solution at 25 °C.Apparently,ηwincreases with the increase of the inhibitor concentration.This behavior is due to the fact that the adsorption coverage of inhibitor on steel surface increases with the inhibitor concentration.For both pyrimidine compounds,when the concentration reaches about 5.0 mmol·L-1,ηwreaches certain value and changes slightly with a further increase in the inhibitor concentration.At 10.0 mmol·L-1,the maximum ηwis 87.2%for HP and 95.4%for MP,which indicates that two studied pyrimidine compounds act as good corrosion inhibitors for CRS in 1.0 mol·L-1HCl solution.Inhibition efficiency follows the order of MP＞HP.It is evident that the difference in inhibition efficiency of HP and MP was related to the presence of―OH and―SH on the pyrimidine ring.

3.2 Adsorption isotherm and adsorption free energy(△G0）

Fundamental information on the adsorption of inhibitor on metal surface can be obtained by the adsorption isotherm.Several isotherms such as Frumkin,Langmuir,Temkin,Freundlich,Bockris-Swinkels,and Flory-Huggins isotherms are employed to fit the experimental data.It is found that the adsorption of the studied inhibitors on steel surface obeys Langmuir adsorption isotherm equation:11

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

The straight lines of c/θ against c for two inhibitors are shown in Fig.3,and the corresponding linear regression parameters are listed in Table 1.Both straight lines have very good linear fit with the linear regression coefficients(r）up to 0.99 and the slopes also very close to 1.0,which suggests that the adsorption of the pyrimidine inhibitors on steel surface obeys Langmuir adsorption isotherm.Also,the adsorptive equilibrium constant(K）follows the order:MP＞HP.

Generally,large value of K means the more stability of adsorptive inhibitor on metal surface,and then the better inhibition performance of a given inhibitor.This is in good agreement with the values of ηwobtained from Fig.2.

The adsorption equilibrium constant K is related to the standard adsorption free energy ΔG0according to the following equation:20

Fig.3 Langmuir isotherm adsorption modes of HP and MP on the CRS surface in 1.0 mol·L-1 HCl solution at 25°C from weight loss measurement

Table 1 Parameters of the straight lines of c/θ-c and adsorption free energy(ΔG0）in 1.0 mol·L-1 HCl solution at 25 °C

where R is the gas constant(8.314 J·K-1·mol-1）,T is the absolute temperature(K）,and the value 55.5 is the concentration of water in solution in mol·L-1.20The ΔG0values are calculated and also given in Table 1.Generally,values of ΔG0up to-20 kJ·mol-1are consistent with the electrostatic interaction between the charged molecules and the charged metal(physisorption）while those more negative than-40 kJ·mol-1involve sharing or transfer of electrons from the inhibitor molecules to the metal surface to form a co-ordinate type of bond(chemisorption）.21In the present study,the value of ΔG0is found to be within the range from-40 to-20 kJ·mol-1;probably means that the adsorption of each pyrimidine inhibitor on steel surface contains both physical adsorption and chemical adsorption.It may be assumed that adsorption occurs firstly through the physical forces,and then the removal of water molecules from the surface is accompanied by chemical interaction between the metal surface and inhibitor.22

3.3 Effect of temperature

Temperature is an important kinetic factor that influences the corrosion rate of metal and modifies the adsorption of inhibitor on electrode surface.In order to study the effect of temperature on the corrosion inhibition,experiments were conducted at 25 to 50 °C at an interval of 5 °C.Effect of temperature on inhibition efficiency(ηw）of 10.0 mmol· L-1inhibitor is shown in Fig.4.Clearly,inhibition efficiency of either HP or MP fluctuates slightly with the experimental temperature.At 50 °C,ηwis 83.2%for HP and 95.3%for MP,which reflects that the adsorption film of pyrimidine inhibitor is more stable even at higher temperature.

According to Arrhenius equation,the natural logarithm of the corrosion rate(ln v）is a linear function with 1/T:11

Fig.4 Relationship between inhibition efficiency(ηw）and temperature in 1.0 mol·L-1 HCl solution weight loss method,immersion time:6 h

where Eaand A represent apparent activation energy and preexponential factor,respectively.The corrosion rate(v）was calculated from the following equation:

where W is the average mass loss of three parallel CRS sheets(g）,S is the total area of one CRS specimen(m2）,and t is the immersion time.

The linear regressions between ln v and 1/T were calculated,and the parameters are given in Table 2.Fig.5 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 indicates that the corrosion of steel functioning with temperature follows Arrhenius equation.

Kinetic parameter of apparent activation energy(Ea）is important to study the inhibitive mechanism.Compared with uninhibited solution,the increase of Eain inhibited solution may be interpreted as the physical adsorption.22In visa,a drop in Eawith respect to the uninhibited solution probably indicates chemisorption.23However,the criteria of Eacan not be taken as decisive due to competitive adsorption with water whose removal from the surface requires also some activation energy.24According to Solmaz et al.,25the adsorption phenomenon of an organic molecule is not considered as a mere physical or mere chemical adsorption phenomenon.In addition,Moretti et al.26proposed that the adsorption criteria of chemisorption or physisorption could be decided by other adsorption parameters.According to the adsorption parameter of ΔG0mentioned above in Section 3.2,the adsorption of oxime inhibitor would involve both physical and chemical processes.Thus,in the present study,the value of Eain the presence of each inhibitor is higher than that in the uninhibited HCl solution,which does not necessarily mean that the adsorption of inhibitor is mere the physical adsorption.27In a word,both physical adsorption and chemical adsorption would be considered simultaneously for the adsorption of either HP or MP.

Table 2 Parameters of the straight lines of ln v-1/T in 1.0 mol·L-1 HCl solution

Fig.5 Arrhenius plots related to the corrosion rate of CRS for various inhibitors in 1.0 mol·L-1 HCl solution

According to Eq.(3）,it can be seen that the lower A and the higher Ealead to the lower corrosion rate(v）and exhibit inhibition performance.After adding HP to the acid media,the value of A is higher than that in uninhibited solution.Accordingly,the decrease in corrosion rate after adding inhibitor to acid media is mostly caused by the increase of Ea.On the other hand,in the presence of MP,the decrease in corrosion rate is mostly caused by the combination of the increase of Eaand the increase of A.In a word,the effect of A on the corrosion rate needed to be considered,especially for MP inhibitor.

3.4 Effect of immersion time

The immersion time is another important parameter in assessing the stability of inhibitive behavior,so it is necessary to evaluate the inhibition efficiency for a long immersion time.In the present study,effect of immersion time(1-156 h）on corrosion inhibition of 10.0 mmol·L-1HP and 10.0 mmol·L-1MP in 1.0 mol·L-1HCl solution at 25 °C was investigated using weight loss method.Dependence inhibition efficiency(ηw）on the immersion time(t）is shown in Fig.6.For both HP and MP,inhibition efficiency is higher than 60%when the immersion time is only 1 h,which indicates that the adsorption rate of pyrimidine inhibitor adsorbing on the steel surface is relatively high.Also,the changed rule of ηwfor two inhibitors is similar.ηwfirstly increases with immersion time from 1 to 6 h,and then fluctuates slightly(＜4%）with prolonging time to 156 h.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 relative saturated state within 6-156 h.

3.5 Effect of acid concentration

Fig.6 Relationship between inhibition efficiency(ηw）and immersion time(t）in 1.0 mol·L-1 HCl solution at 25 °C

In order to study the effect of acid concentration on the corrosion of steel in the presence of 10.0 mmol·L-1inhibitor,dependence of inhibition efficiency(ηw）on the concentration of HCl solution(1.0-5.0 mol·L-1）at 25 °C is shown in Fig.7(immersion time is 6 h）.For either HP or MP inhibitor,ηwdecreases almost linearly with the HCl concentration.In 5.0 mol·L-1HCl solution,ηwvalues are reduced to 55.6%and 60.4%for HP and MP,respectively.At same acid concentration solution,inhibition performance still follows the order:MP＞HP.

It is found that the corrosion rate(v）against the molar concentration of acid(C）obeys the kinetic expression reported by Mathur and Vasudevan:29

where k is the rate constant,and B is the reaction constant.The straight lines of ln v versus C are shown in Fig.8,and the corresponding kinetic parameters are listed in Table 3.

The rate constant of k means the corrosion ability of acid for metal.29Inspection of Table 3 reveals that in the presence of pyrimidine derivatives,there is a drop of k to more extent,which indicates that the steel corrosion is retarded by the inhibitors of HP and MP.Furthermore,the value of k for MP is lower than that for HP,which confirms that the inhibition performance of MP is more superior to that of HP.According to Eq.(5）,B is the slope of the line ln v-C,thus B reflects the changed extent of v with the acid concentration.29It is observed that the value of B in the presence of inhibitor is larger than that of blank HCl solution,which suggests that the changed extent of corrosion rate with acid concentration in inhibited acid is larger than that in uninhibited acid.In addition,the value of B for MP is higher than that for HP,which indicates that the changed degree of corrosion rate with acid concentration for MP is greater than that for HP.

Fig.7 Relationship between inhibition efficiency(ηw）and acid concentration(C）at 25°C

Fig.8 Straight lines of ln v versus C at 25°C immersion time:6 h

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

3.6 Potentiodynamic polarization curves

Potentiodynamic polarization curves of CRS in 1.0 mol·L-1HCl solution in the presence of different concentrations of HP and MP at 25°C are shown in Fig.9.Obviously,the presence of each pyrimidine compound causes a remarkable decrease in the corrosion rate,i.e.,shifts both anodic and cathodic curves to lower current densities.In other words,both cathodic and anodic reactions of CRS electrode are drastically inhibited,which indicates that the pyrimidine compounds act as mixed-type inhibitors.

The potentiodynamic polarization parameters including corrosion current densities(icorr）,corrosion potential(Ecorr）,cathodic Tafel slope(bc）,anodic Tafel slope(ba）,and inhibition efficiency(ηp）are listed in Table 4.It can be seen from Table 4 that icorrdecreases sharply with the increase of the concentration of each pyrimidine inhibitor.In the presence of the inhibitor concentration of 10.0 mmol·L-1,the corrosion current density decreases from 319.4 μA·cm-2to 31.6 and 11.5 μA·cm-2in the case of HP and MP,respectively.Correspondingly,ηpincreases with the inhibitor concentration,due to the increase in the blocked fraction of the electrode surface by adsorption.At 10.0 mmol·L-1inhibitor concentration,inhibition efficiency(ηp）reaches up to a maximum of 90.1%for HP and 96.4%for MP,which again confirms that both pyrimidine derivatives are good inhibitors for steel in 1.0 mol·L-1HCl solution,and ηpfollows the order:MP＞HP.Compared with the corrosion potential(Ecorr）in 1.0 mol·L-1HCl solution without inhibitor,Ecorrin the presence of HP or MP does not change,which indicates that all studied pyrimidine derivatives act as mixed-type inhibitors.30Furthermore,in the presence of each inhibitor,the slight change of Tafel slopes of bcand baindicates that the mechanism of steel does not change.According to Cao,30the inhibition is caused by geometric blocking effect.Namely,the inhibition effect comes from the reduction of the reaction area on the surface of the corroding metal.

Fig.9 Potentiodynamic polarization curves for CRS in 1.0 mol·L-1 HCl solution without and with different concentrations of inhibitors at 25°C

Table 4 Potentiodynamic polarization parameters for the corrosion of CRS in 1.0 mol·L-1 HCl solution containing different concentrations of HP and MP at 25°C

3.7 Electrochemical impedance spectroscopy(EIS）

Fig.10 represents the Nyquist diagrams for CRS in 1.0 mol·L-1HCl solution in the presence of HP and MP at 25°C.As can be seen from the figures,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 behavior.31In addition,the shape is maintained throughout all tested inhibitor concentrations compared with that of blank solution,indicating that there is almost no change in the corrosion mechanism which occurs due to the inhibitor addition.32

Fig.10 Nyquist plots of the corrosion of CRS in 1.0 mol·L-1 HCl solution without and with different concentrations of inhibitors at 25 °C

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.Further inspection of Fig.10 reveals that there is a little difference between HP and MP at low inhibitor concentration(1.0 mmol·L-1）,while there is large difference at high inhibitor concentration(10.0 mmol·L-1）.Such behavior may be considered the suggestion that physisorption at lower concentration and chemisorption at higher concentration for a given inhibitor.

Noticeably,these capacitive loops are not perfect semicircles that can be attributed to the frequency dispersion effect as a result of the roughness and inhomogeneousness of the electrode surface.33Accordingly,the EIS data are simulated by the equivalent circuit shown in Fig.10B.Rsand Rtare the solution resistance and charge transfer resistance,respectively.CPE is constant phase element to replace a double layer capacitance(Cdl）for more accurate fit.The solid lines in Fig.10 correspond to the fitted plots for EIS experiment data using this electric circuit,which indicates that the experimental data can be fitted using this equivalent circuit.

The CPE is composed of a component Qdland 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,and calculated from the following equation:34

where fmaxrepresents the frequency at which the imaginary value reaches a maximum on the Nyquist plot.The electrochemical parameters of Rs,Rt,CPE,a,Cdl,and ηRare listed in Table 5.The chi-squared(χ2）is used to evaluate the precision of the fitted data.35Table 5 shows that χ2value is low,which confirms that the fitted data and the experimental data are in good agreement.It is observed that Rsis very small,which confirms that the IR drop could be neglect.Rtvalue increases prominently while Cdlreduces with the concentration of inhibitor.A large charge transfer resistance is associated with a slower corroding system.At any given inhibitor concentration,Rt(HP）＜Rt(MP）,which confirms that MP shows better inhibitive performance between two pyrimidine compounds.According to Cao,30the single capacitive loop again indicates that the adsorption mode of each pyrimidine inhibitor is geometric blocking effect.Through the light of the tendency to form a stronger coordination bond,S atom shows more stable coordination ability than O atom,which results in that it could be more easily for MP to chemisorb on steel than HP.

The decrease in Cdlin comparing with that in blank solution(without inhibitor）,which can 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.36ηRincreases with the concentration of inhibitor,and follows the order: ηR(MP）＞ηR(HP）.The ηRvalues at 10.0 mmol· L-1are 88.9%and 93.2%for HP and MP,respectively.These results again confirm that all pyrimidine derivatives exhibit good inhibitive performance for CRS in HCl solution.

Table 5 EIS parameters for the corrosion of CRS in 1.0 mol·L-1 HCl solution containing HP or MP at 25 °C

Inhibition efficiencies obtained from weight loss(ηw）,potentiodynamic polarization curves(ηp）,and EIS(ηR）are in good reasonably agreement.

3.8 Quantum chemical calculations

In order to investigate the adsorption mode through light on the pyrimidine molecular structure,quantum chemical calculations were carried out.It is well known that the N-heterocyclic compound could be protonated in the acid solution.According to some quantum chemical studies about protonated N-heterocyclic inhibitor in HCl solution,37the proton affinity is clearly favored toward the hetero N atom of N-heterocyclic ring.Fig.11 shows the optimized molecular structures of two neutral and their protonated pyrimidine derivatives.Through quantum chemical calculations,the protonated affiliation energy(PA）values of HP and MP are 1088.8 and 1091.5 kJ·mol-1,respectively.This result indicates that two studied pyrimidine derivatives are easily protonated.From Fig.11,the pyrimidine ring and substitution group(―OH,―SH）are in one plane.It is well known that inhibitor can form coordination bonds between the unshared electron pair of O,N,or S atom and the unoccupied d orbit of Fe.The larger negative charge of the atom,the better is the action as an electronic donor.Mulliken charges of the atoms are listed in Table 6.By careful examination of the values of Mulliken charges,the larger negative atoms are found in N1,N3,O7,and S7,which are active adsorptive centers that could donate the lone electron pairs to the unfilled orbits of Fe.When pyrimidine compounds are protonated,the Mulliken charge of N5 becomes more negative than N3,while the Mulliken charge of O7 or S7 increases.This result implies that if the inhibitor is protonated,N5 exhibits more active than N3,while the adsorptive ability of O7 or S7 would decrease.

Fukui function is necessary in understanding the local site selectivity.The Fukui function(f(r→））is defined as:38

The nucleophic attack Fukui functionand electophilic attack Fukui functioncan be calculated as:39

where qi(N+1）,qi(N）,qi(N-1）are charge values of atom i for cation,neutral,and anion,respectively.The values ofandare also listed in Table 6.Generally,high values ofandmean the high capacity of the atom to gain and lost electron,respectively.For the nucleophic attack,the most reactive site is C2 and C6 for all neutral and protonated molecules,which can accept electrons from metal surface to form back-donating bond.The difference ofindex of reactivity between HP,and MP is small.But when compounds are protonated,values follow the order of p-HP＞HP;p-MP＞MP.This result implies that the nucleophic attack activity of protonated molecule decreases.On the other hand,the values ofindicate that it will happen on the N3 for HP,O7 for p-HP and S7 for MP and p-MP,which can denote electrons to metal surface to form coordinate bond.The atom of S7 has the largest f(r→）-value among N3,O7,and S7 atoms,which indicates that the adsorptive ability of S atom is higher than N atom.Accordingly,the better inhibition efficiency of MP is related to the higher adsorptive ability of S atom of―SH.

Fig.11 Optimized molecular structures of the neutral and protonated pyrimidine derivatives

Table 6 Quantum chemical parameters of Mulliken charge,andfor neutral and protonated pyrimidine molecules

Table 6 Quantum chemical parameters of Mulliken charge,andfor neutral and protonated pyrimidine molecules

Atom C1 C2 N3 C4 N5 C6 O7 S7 Mulliken charge HP-0.208-0.013-0.3810.556-0.396-0.014-0.606-MP-0.222-0.040-0.3440.356-0.342-0.039--0.347 p-HP-0.1730.026-0.3180.690-0.4520.077-0.543-p-MP-0.196-0.004-0.2900.479-0.4110.057--0.196 f(r→）+HP 0.0230.1740.1280.0570.1230.1740.041-MP 0.0180.1760.1200.0260.1160.175-0.103 p-HP 0.0280.1880.1020.0690.0770.1790.055-p-MP 0.0240.1860.0810.0410.0790.162-0.137 f(r→）-HP 0.0530.0650.2030.1140.1830.0620.041-MP 0.0850.0360.0670.0200.0650.036-0.476 p-HP 0.1410.0620.1060.0770.0510.1050.165-p-MP 0.0820.0350.0730.0270.0380.051-0.459

The dipole moment(μ）is widely used to represent the polarity of the molecule,and related to the inhibitive ability.The large value of dipole moment probably increases the inhibitor adsorption through electronic force.40In Table 7,the μ of HP is larger than that of MP.On the contrary,the inhibition efficiency follows the order:MP＞HP.These results indicate that the better inhibitive performance of MP might not be arisen form intermolecular electrostatic force.Table 7 shows that the protonated molecules of p-HP and p-MP have larger dipole moment values than corresponding neutral molecules of HP and MP.Thus,p-HP and p-MP could be easily adsorbed via physical adsorption compared with HP and MP.It is well known that the charge of the metal surface can be determined from the value of Ecorr-Eq=0(Ecorr:corrosion potential,Eq=0:zero charge potential）.41The Eq=0of iron is-530 mV(vs SCE）in HCl solution,42and the value of Ecorrobtained in 1.0 mol·L-1HCl solution is-440 mV(vs SCE）.Thus,the steel surface charges positive charge in HCl solution because of Ecorr-Eq=0＞0.Since the acid anion of Cl-could be specifically adsorbed,it creates excess negatives charge towards the solution and favors more adsorption of the cations,p-HP or p-MP may adsorb on the negatively charged metal surface.In other words,there may be a synergism between anion(Cl-）and protonated inhibitor.

Besides the above mentioned quantum chemical parameters,the global reactivity of a molecule depends on molecular distributions.HOMO is often associated with the capacity of a molecule to donate electrons,whereas LUMO represents the ability of the molecule to accept electrons.The electric/orbital density distributions of HOMO and LUMO for the studied inhibitors(neutral and protonated molecules）are shown in Fig.12,respectively.It is found that that the electron density of the frontier orbital is well proportioned.For four molecules,the electron densities of both HOMO and LUMO are localized principally on the pyrimidine ring,which indicates that the pyrimidine ring could be both the acceptor of the electron and the donor of the electron.That is,there is electron transferring in the interaction between the pyrimidine ring and metal surface.The adsorption of inhibitor on steel may be in a manner in which the plane of the pyrimidine ring is parallel to the metal surface.It should be noted that HOMO density is absent on O7 atom for HP,whereas present on S7 for MP.The result indicates that thesubstituted S7 atom on pyrimidine ring of MP is an additional adsorption centre in comparison with the substituted O7 atom of HP.Accordingly,the efficiency of MP is higher than that of HP.The LUMO density is absent on either O7 of HP or S7 of MP;however,LUMO density is located on either O7 of p-HP or S7 of p-MP.Thus,it is reasonable to deduce that the protonated molecule exhibits better adsorptive ability than neutral molecule.

Table 7 Quantum chemical parameters of μ,E HOMO,E LUMO,ΔE,and ΔN for neutral and protonated pyrimidine molecules

Fig.12 Frontier molecule orbital density distributions of pyrimidine compounds

The values of energy of the highest occupied molecular orbital(EHOMO）,energy of the lowest unoccupied molecular orbital(ELUMO）,and the separation energy(ELUMO-EHOMO,ΔE）are also presented in Table 7.High value of EHOMOindicates a tendency of the molecule to donate electrons to act with acceptor molecules with low-energy,empty molecular orbital.3Similarly,the ELUMOrepresents the ability of the molecule to accept electrons,and the lower value of ELUMOsuggests the molecule accepts electrons more probable.3From Table 7,EHOMOobeys the order:MP＞HP,while ELUMOobeys the order:MP＜HP.When compound is protonated,the order is still same.Obviously,the two sequences are in completely accordance with the order of inhibition efficiency.This may explain that the better inhibition efficiency of MP molecule than HP is due to both lower ELUMOand higher EHOMO.Both EHOMOand ELUMOvalues of protonated molecules decrease compared with those of neutral molecules,Therefore,it could be deduced that the protonated molecule strengthens accepting electrons from metal surface while weakens donating electrons to Fe atom.

The separation energy(energy gap）ΔE(ELUMO-EHOMO）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.3Inspection of the data in Table 7 shows that the ΔE value follows the order of MP＜HP and p-MP＜p-HP,which confirms that MP(or p-MP）facilitates its adsorption on the metal surface so that it has higher inhibition efficiency.Furthermore,ΔE value of protonated molecule is lower than neutral molecule,which again confirms that the protonated molecule exhibits better inhibitive performance than neutral molecule.

The energies of HOMO and LUMO of the inhibitor molecule are related to the ionization potential(I）and the electron affinity(Y）,respectively,by the following relations:45

Then absolute electronegativity(β）and global hardness(γ）of the inhibitor molecule are approximated as follows:44

The fraction of electrons transferred from the inhibitor to metallic surface(ΔN）is calculated depending on the quantum chemical method:45

For Fe,the theoretical values of βFeand γFeare 7 and 0 eV,respectively.46The values of ΔN are also listed in Table 7.According to some studies,46values of ΔN exhibit inhibitive performance resulted from electrons donations.If ΔN＜3.6,the inhibition efficiency increases with the increase in electron-donation ability to the metal surface.47It can be seen from Table 7 that ΔN follows the order of MP＞HP and p-MP＞p-HP,which is in good agreement with the order of inhibition efficiency of these inhibitors.When the compounds are protonated,ΔN follows the order of p-HP＞HP and p-MP＜MP,which again implies that the ability of donating electrons decreases for protonated molecule.

4 Conclusions

(1）Two pyrimidine derivatives of HP and MP act as good inhibitors for the corrosion of CRS in 1.0 mol·L-1HCl solution.Inhibition efficiency(ηw）increases with the inhibitor concentration,and the maximum ηwis 83.6%for HP and 95.4%for MP at 10.0 mmol·L-1.Inhibition efficiency follows the order:MP＞HP.

(2）The adsorption of either MP or HP obeys Langmuir adsorption isotherm.The parameter of adsorption free energy(ΔG0）indicates that the adsorption involves both physical adsorption and chemical adsorption.

(3）Both HP and MP are arranged as mixed-type inhibitors in 1.0 mol·L-1HCl solution.EIS spectra exhibit one capacitive loop,and the presence of each inhibitor enhances Rtwhile reduces Cdl.

(4）The large protonated affiliation energy(PA）values confirm that two studied pyrimidine derivatives are easily protonated.The chemical molecules of HP,MP,p-HP,and p-MP are in one plane.The larger negative Mulliken charges are found in N1,N3,O7,and S7,which are adsorptive centers.The nucleophic attack active atoms are C2 and C6 atoms.The electrophilic attack active atoms are N3 for HP,O7 for p-HP,S7 for MP and p-MP.

(5）The electron densities of both HOMO and LUMO are localized principally on the pyrimidine ring.The pyrimidine ring could be both the acceptor of the electron and the donor of the electron.The better inhibition efficiency of MP than HP could be explained with the quantum parameters of ELUMO,EHOMO,ΔE,and ΔN.

(6）The protonated molecules of p-HP and p-MP could be easily adsorbed via physical adsorption;there may be a synergism between anion(Cl-）and protonated inhibitor.Comparing with neutral molecule,the protonated molecule exhibits better inhibitive performance.The protonated molecule strengthens accepting electrons while weakens donating electrons.

(1）Li,X.H.;Deng,S.D.;Fu,H.Acta Phys.-Ch im.Sin.2011,27,2841.[李向紅,鄧書端,付 惠.物理化學學報,2011,27,2841.]doi:10.3866/PKU.WHXB20112841

(2）Bentiss,F.;Traisnel,M.;Gengembre,L.;Lagrenée,M.Appl.Surf.Sci.1999,152,237.doi:10.1016/S0169-4332(99）00322-0

(3）Yan,Y.;Li,W.H.;Cai,L.K.;Hou,B.R.Electrochim.A cta 2008,53,5953.doi:10.1016/j.electacta.2008.03.065

(4）Wang,L.J.Yunnan Univ.2001,23,203.[王 林.云南大學學報,2001,23,203.]

(5）Monticell,C.;Brunoro,G.;Frignani,A.J.Electroch em.S oc.1992,139,706.

(6）Lin,Z.H.;Wang,F.C.;Tian,Z.Q.A cta P hys.-Chim.Sin.1992,8,87. [林仲華,王逢春,田中群.物理化學學報,1992,8,87.]doi:10.3866/PKU.WHXB19920116

(7）Zucchi,F.;Trabanelli,G.;Brunoro,G.Mater.Corros.1993,44,264.

(8）Wang,L.Corrosion Sci.2001,41,1637.

(9）Li,X.H.;Deng,S.D.;Fu,H.Chin.J.Ap pl.Chem.2012,29,209.[李向紅,鄧書端,付 惠.應(yīng)用化學,2012,29,209.]

(10）Makhlouf,M.T.;El-Shahawy,A.S.;El-Shatory,S.A.Mater.Chem.P hys.1996,43,153.doi:10.1016/0254-0584(95）01611-W

(11）Li,X.H.;Deng,S.D.;Fu,H.Corrosion Sci.2009,51,1344.doi:10.1016/j.corsci.2009.03.023

(12）Kokalj,A.;Peljhan,S.;Fin?gar,M.;Milo?ev,I.J.Am.Ch em.S oc.2010,132,16657.doi:10.1021/ja107704y

(13）Feng,L.J.;Yang,H.Y.;Wang,F.H.Electrochim.Acta 2011,58,427.doi:10.1016/j.electacta.2011.09.063

(14）El Adnani,Z.;Mcharfi,M.;Sfaira,M.;Benzakour,M.;Benjelloun,A.T.;Ebn Touhami,M.Corrosion Sci.2013,68,223.doi:10.1016/j.corsci.2012.11.020

(15）Zhang,J.;Zhao,W.M.;Guo,W.Y.;Wang,Y.;Li,Z.P.Acta Phys.-Chim.Sin.2008,24,1239.[張 軍,趙衛(wèi)民,郭文躍,王 勇,李中譜.物理化學學報,2008,24,1239.]doi:10.3866/PKU.WHXB20080720

(16）Materials Studio 4.1;Accelrys Inc.:San Diego,CA,2006.

(18）Lee,C.;Yang,W.;Parr,R.G.P hys.R ev.B 1988,37,785.doi:10.1103/PhysRevB.37.785

(19）Klamt,A.;Schüürmann,G.J.Chem.Soc.Perkin.Trans.1993,2,799.

(20）Cano,E.;Polo,J.L.;Iglesia,A.L.;Bastidas,J.M.A dsorption 2004,10,219.doi:10.1023/B:ADSO.0000046358.35572.4c

(21）Zhang,F.;Tang,Y.M.;Cao,Z.Y.;Jing,W.H.;Wu,Z.L.;Chen,Y.Z.Corrosion S ci.2012,61,1.doi:10.1016/j.corsci.2012.03.045

(22）Behpour,M.;Ghoreishi,S.M.;Soltani,N.;Salavati-Niasari,M.Corrosion Sci.2009,51,1073.doi:10.1016/j.corsci.2009.02.011

(23）Bentiss,F.;Lebrini,M.;Lagrenée,M.Corrosion Sci.2005,47,2915.doi:10.1016/j.corsci.2005.05.034

(24）Vra?ar,L.M.;Dra??,D.M.Corrosion Sci.2002,44,1669.doi:10.1016/S0010-938X(01）00166-4

(25）Solmaz,R.;Kardas,G.;Yazici,B.;Erbil,M.Colloids Surf.A 2008,312,7.doi:10.1016/j.colsurfa.2007.06.035

(26）Moretti,G.;Guidi,F.;Grion,G.Corrosion Sci.2004,46,387.doi:10.1016/S0010-938X(03）00150-1

(27）Li,X.H.;Deng,S.D.;Mu,G.N.;Fu,H.;Yang,F.Z.Corrosion S ci.2008,50,420.doi:10.1016/j.corsci.2007.08.014

(28）Granese,S.L.;Rosales,B.M.;Oviede,C.;Zerbino,J.O.Corrosion Sci.1992,33,1439.doi:10.1016/0010-938X(92）90182-3

(29）Mathur,P.B.;Vasudevan,T.Corrosion 1982,38,171.doi:10.5006/1.3579270

(30）Cao,C.Corrosion Sci.1996,38,2073.doi:10.1016/S0010-938X(96）00034-0

(31）Behpour,M.;Ghoreishi,S.M.;Mohammadi,N.;Soltani,N.;Salavati-Niasari,M.Corrosion Sci.2010,52,4046.doi:10.1016/j.corsci.2010.08.020

(32）Labjar,N.;Lebrini,M.;Bentiss,F.;Chihib,N.E.;El Hajjaji,S.;Jama,C.Mater.Chem.P hys.2010,119,330.doi:10.1016/j.matchemphys.2009.09.006

(33）Lebrini,M.;Lagrenée,M.;Vezin,H.;Traisnel,M.;Bentiss,F.Corrosion Sci.2007,49,2254.doi:10.1016/j.corsci.2006.10.029

(34）Priya,A.R.S.;Muralidharam,V.S.;Subramania,A.Corrosion 2008,64,541.doi:10.5006/1.3278490

(35）Osório,W.R.;Moutinho,D.J.;Peixoto,L.C.;Ferreira,I.L.;Garcia,A.E lectrochim.A cta 2011,56,8412.doi:10.1016/j.electacta.2011.07.028

(36）Lagrenée,M.;Mernari,B.;Bouanis,M.;Traisnel,M.;Bentiss,F.Corrosion Sci.2002,44,573.doi:10.1016/S0010-938X(01）00075-0

(37）Li,D.Z.;Zhang,S.G.;Bian,H.;Zhang,L.C.Comput.A ppl.Chem.2009,26,324.[李大枝,張士國,卞 賀,張立超.計算機與應(yīng)用化學,2009,26,324.]

(38）Parr,R.G.;Yang,W.J.Am.Ch em.S oc.1984,106,4049.doi:10.1021/ja00326a036

(39）Gómez,B.;Likhanova,N.V.;Aguilar,M.A.D.;Olivares,O.;Hallen,J.M.;Martínez-Magadán,J.M.J.Ph ys.Ch em.A 2005,109,8950.doi:10.1021/jp052188k

(40）Lashkari,M.;Arshadi,M.R.Chem.Phys.2004,299,131.doi:10.1016/j.chemphys.2003.12.019

(41）Schweinsberg,D.P.;Ashworth,V.Corrosion Sci.1988,28,539.doi:10.1016/0010-938X(88）90022-4

(42）Banerjee,G.;Malhotra,S.N.Corrosion 1992,48,10.doi:10.5006/1.3315912

(43）Feng,Y.;Chen,S.;Zhang,H.;Li,P.;Wu,L.;Guo,W.Appl.Surf.Sci.2006,253,2812.doi:10.1016/j.apsusc.2006.05.061

(44）Ju,H.;Kai,Z.P.;Li,Y.Corrosion Sci.2008,50,865.doi:10.1016/j.corsci.2007.10.009

(45）Lukovits,I.;Kálmán,E.;Zucchi,F.Corrosion 2001,57,3.doi:10.5006/1.3290328

(46）Sastri,V.S.;Perumareddi,J.R.Corrosion 1997,53,617.doi:10.5006/1.3290294

(47）Awad,M.K.;Mustafa,M.R.;Elnga,M.M.A.J.Mol.Struct.-Theochem 2010,959,66.doi:10.1016/j.theochem.2010.08.008