Effect of some pyrimidinic Shciff bases on the corrosion of mild steel in hydrochloric acid solution

Electrochimica Acta 50 (2005) 3446–3452

Effect of some pyrimidinic Shciff bases on the corrosion of mild steel in hydrochloric acid solution H. Ashassi-Sorkhabia,∗ , B. Shaabanib , D. Seifzadeha a

Electrochemistry Laboratory, Physical Chemistry Department, Faculty of Chemistry, Tabriz University, Tabriz, Iran b Inorganic Chemistry Department, Faculty of Chemistry, Tabriz University, Tabriz, Iran Received 30 September 2004; received in revised form 19 December 2004; accepted 19 December 2004 Available online 6 February 2005

Abstract The efficiency of benzylidene-pyrimidin-2-yl-amine (A), (4-methyl-benzylidene)-pyrimidine-2-yl-amine (B) and (4-chloro-benzylidene)pyrimidine-2-yl-amine, as corrosion inhibitors for mild steel in 1 M HCl have been determined by weight loss measurements and electrochemical polarization method. The results showed that these inhibitors revealed a good corrosion inhibition even at very low concentrations. Polarization curves indicate that all compounds are mixed type inhibitors. The effect of various parameters such as temperature and inhibitor concentration on the efficiency of the inhibitors has been studied. Activation energies of corrosion reaction in the presence and absence of inhibitors have been calculated. The adsorption of used compounds on the steel surface obeys Langmuir’s isotherm. It appears that an efficient inhibition is characterized by a relatively greater decrease in free energy of adsorption. Significant correlations are obtained between inhibition efficiency and quantum chemical parameters using quantitative structure–activity relationship (QSAR) method. © 2004 Elsevier Ltd. All rights reserved. Keywords: Corrosion inhibitors; Langmuir’s isotherm; Quantitative structure–activity relationship

1. Introduction Acid solutions are widely used in industry. The most important areas of application are acid pickling, acid cleaning, acid rescaling and oil well cleaning [1]. Corrosion inhibitors are needed to reduce the corrosion rates of metallic materials in these media. Most of the efficient inhibitors used in industry are organic compounds, which mainly contain oxygen, sulphur, nitrogen atoms, and multiple bonds in the molecule through which they are adsorbed on metal surface [2]. Some Schiff bases have been reported earlier as corrosion inhibitors for steel [3–6], copper [7], aluminum [8,9], and zinc [10]. Some research work [11,12] reveal that the inhibition efficiency of Schiff bases is much greater that of corresponding amines and aldehydes. This may be due to the presence of a C N group in the molecule. ∗

Corresponding author. Tel.: +98 4113355998; fax: +98 4113340191. E-mail addresses: habib [email protected], [email protected] (H. Ashassi-Sorkhabi). 0013-4686/$ – see front matter © 2004 Elsevier Ltd. All rights reserved. doi:10.1016/j.electacta.2004.12.019

The aim of the present investigation is to examine the inhibitive properties of three Schiff base compounds derived from 2-amino pyrimidin and corresponding aldehydes. 2. Experimental details 2.1. Materials Schiff base compounds as inhibitors, shown in Fig. 1, were synthesized from equimolar amounts of 2-amino-pyrimidin and the corresponding aldehyde through a condensation reaction in ethanol media [13,14]. The chemical composition (wt.%) of the steel specimen (determined by SPECTROLAB quantometer) is given in Table 1. The specimens were polished with emery paper no. 400–1200 grade. They were cleaned with acetone, washed with double-distilled water, and finally dried at room temperature before being immersed in the acid solution. The acid solutions (1 M) were made from analytical grade 37% HCl

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Fig. 1. Structures of studied Schiff bases.

and double-distilled water. The concentration range of inhibitor employed was 2 × 10−4 to 1 × 10−2 M in 1 M HCl.

Weight loss measurements were carried out as it was shown in literature [15]. The volume of solution was 100 ml. The steel specimens had a rectangular form (length: 2 cm, width: 1 cm, thickness: 0.08 cm). A maximum immersion time was 24 h in the case of inhibited solutions. Solutions were not stirred. At the end of the tests, the specimens were carefully washed in acetone, dried and then weighted. The temperature of solution was fixed thermostatically a desired value.

trochemical cell was used. The working electrode was prepared from a mild steel sheet, mounted in polyester such that the area exposed to solution was 1 cm2 . A saturated calomel electrode (SCE) and a platinum electrode were used as the reference and the counter electrode, respectively. All potentials are reported versus SCE. In the case of polarization measurements, the potential sweep rate was 2 mV s−1 . The immersion time before measurements was 1 h. The ac impedance measurements were carried out in the frequency range of 10 KHz to 10 mHz, at the rest potential, by applying 5 mV sine wave ac voltage. The double layer capacitance (Cdl ) and the charge transfer resistance (Rt ) were calculated from Nyquist plots as described elsewhere [15]. All experiments were performed under atmospheric conditions.

2.3. Electrochemical studies

2.4. Quantum chemical study

Electrochemical experiments were carried out using an Autolab, Potentiostat-Galvanostat. A three-electrodes elec-

Quantum chemical parameters for inhibitors, obtained using the AM1 semi empirical quantum chemical approach, were correlated with their experimental inhibition efficiencies. The following quantum chemical indices were considered: the energy of the highest occupied molecular orbital (EHOMO ), the energy of the lowest unoccupied molecular orbital (ELUMO ), and the dipole moment (µ).

2.2. Weight loss measurements

Table 1 Chemical composition of used mild steel specimen (wt.%) C Si Mn P S Cr Ni Mo V Cu Nb Ti Al Co W Pb Mg Sb Sn As B Fe

0.002 1.380 0.203 0.033 0.009 0.055 0.022 0.019 0.002 0.061 0.001 0.003 0.365 0.002 0.010 0.001 0.001 0.001 0.001 0.014 0.001 97.810

3. Results and discussion 3.1. Linear polarization Figs. 2–4 present polarization curves for mild steel electrode in 1 M HCl, in the presence and absence of Schiff bases at various concentrations. Both cathodic and anodic reactions on mild steel electrode were inhibited in the presence of studied compounds. The Schiff bases affected cathodic reaction more than anodic reaction. This result suggests that the addition of inhibitors retards the hydrogen evolution reaction [1]. Electrochemical corrosion parameters, i.e., corrosion potential (Ecorr ), cathodic and anodic Tafel slopes and corrosion current (Icorr ), obtained by extrapolation of the Tafel lines, are given in Table 2.

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Fig. 2. Polarization curves for mild steel in 1 M HCl in the presence of Schiff base A at different concentrations.

Fig. 4. Polarization curves for mild steel in 1 M HCl in the presence of Schiff base C at different concentrations.

The obtained values of ηp are given in Table 3. These results show that all used Schiff base compounds act as effective inhibitors. Corrosion inhibition increases when the inhibitor concentration increases. Maximum inhibition efficiency (99.3%) was obtained at a concentration of 1 × 10−2 M in the case of C compound. 3.2. Electrochemical impedance spectroscopy (EIS) Figs. 5–7 show a typical set of Nyquist plots for mild steel in 1 M HCl in the absence and presence of Schiff bases at various concentrations. It is apparent from these plots that the impedance response of mild steel in uninhibited HCl has Fig. 3. Polarization curves for mild steel in 1 M HCl in the presence of Schiff base B at different concentrations.

Table 3 The percentage inhibition efficiency obtained from polarization studies

The inhibition efficiency, ηp , was calculated from the following equation [16]

Concentration

ηp = [( Io − I)/Io ] × 100

0.0002 0.001 0.005 0.01

(1)

where Io and I are the corrosion current densities without and with an inhibitor, respectively.

Inhibitor efficiency A

B

C

84.8 97.0 97.6 98.7

63.2 96.4 98.1 98.9

75.4 97.5 98.6 99.3

Table 2 Electrochemical corrosion parameters for mild steel in 1 M HCl in the absence and presence of Schiff bases at various concentrations ba (mV/dec)

bc (mV/dec)

RP ( cm2 )

Icorr (A cm−2 )

Inhibitor

Concentration (M)

Ecorr (mV)

Blank

–

−429

98

142

A

0.0002 0.001 0.005 0.01

−420 −419 −475 −488

55 65 205 89

109 79 158 98

31 79 158 98

5.1 × 10−4 1.0 × 10−4 8.2 × 10−5 4.4 × 10−5

B

0.0002 0.001 0.005 0.01

−441 −443 −479 −489

78 50 92 104

154 76 97 95

18 107 328 596

1.2 × 10−3 1.2 × 10−4 6.2 × 10−5 3.6 × 10−5

C

0.0002 0.001 0.005 0.01

−532 −479 −445 −437

62 77 128 270

133 81 120 196

22 204 560 2121

8.3 × 10−4 8.4 × 10−5 4.8 × 10−5 2.3 × 10−5

7.49

3.4 × 10−3

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Fig. 5. Nyquist plots for mild steel in 1 M HCl in the presence of Schiff base A at different concentrations.

Fig. 7. Nyquist plots for the mild steel in 1 M HCl in the presence of Schiff base C at different concentrations.

significantly changed after the addition of Schiff bases in to the corrosive solutions. The results described below can be interpreted in terms of the equivalent circuit of the electrical double layer shown in Fig. 8 which has been used previously to model the iron/acid interface [17]. The capacitance (Cdl ) and the charge transfer resistance (Rt ) were calculated from Nyquist plots [15]. In the case of the electrochemical impedance spectroscopy, the inhibition efficiency is calculated using the charge transfer resistance as follow [15]: ηz (%) = [(Rt(inh) − Rt )/Rt(inh) ] × 100

(2)

where Rt and Rt(inh) are the charge transfer resistance values without and with inhibitor, respectively. The electrochemical parameters derived from the Nyquist plots and the inhibition efficiencies ηz are given in Table 4. As can be seen from Figs. 5–7, the Nyquist plots are not

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Fig. 8. Equivalent circuit of the studied system.

perfect semicircles as expected from the theory of EIS for the assumed equivalent circuit, and this difference can be explained as follows. The Nyquist plots obtained in the real system represent a general behavior where the double layer on the interface of metal/solution does not behave as a real capacitor. On the metal side, electrons control the charge distribution whereas on the solution side it is controlled by ions. Since ions are much larger than the electrons, the equivaTable 4 Electrochemical impedance parameters for mild steel in 1 M HCl in the presence and absence of Schiff bases at different concentrations, at 25 ◦ C

Fig. 6. Nyquist plots for the mild steel in 1 M HCl in the presence of Schiff base B at different concentrations.

Cdl (␮F/cm2 )

ηz (%)

349

–

18 60 159 166

157.9 72.6 38.8 34.9

54.8 79.2 88.9 90.0

1.3 1.2 1.2 1.8

46 88 164 248

70.0 68.9 53.9 25.3

79.9 80.3 84.6 92.8

1.3 1.3 1.3 2.3

34 98 204 397

97.8 48.4 31.6 14.6

71.8 86.1 90.9 95.8

Inhibitor

Cinh (M)

Rs ( cm2 )

Blank

–

1.3

6

A

0.0002 0.001 0.005 0.01

1.4 1.2 1.4 1.1

B

0.0002 0.001 0.005 0.01

C

0.0002 0.001 0.005 0.01

Rt ( cm2 )

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lent ions to the charge on the metal will occupy quite a large volume on the solution side of the double layer [18]. It can be obtained from Table 4 that, the capacitance of electrical double layer (Cdl ) decreases in the presence of inhibitors. Decrease in the Cdl , 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 act by adsorption at the metal/solution interface [12,19]. The inhibition efficiency follows the order: C>B>A As known, some atoms, such as O and N atoms of Schiff bases, which have unoccupied orbitals, so exhibit a tendency to obtain electrons. The electrons in the d orbitals can easily be offered because their applied force is small. If an inhibitor does not only offer electrons to un occupied d orbitals of metals, but it can also accept the electrons in d orbitals of metallic steel by using their antibond orbital to form stable chelate, then it may be considered an as excellent inhibitor [20]. The presence the electron donating groups on the Schiff bases structure (such as Cl and CH3 ) increases the electron density on the nitrogen of the C N group, resulting high inhibition efficiency. Among the compounds investigated in the present study, C has been found to give the best performance as corrosion inhibitor. This can be explained on the basis of the presence of chloride group on the benzene ring. 3.3. Weight loss measurements The values of inhibition efficiency and corrosion rate obtained from weight loss method at different concentrations of inhibitors at 25 ◦ C are summarized in Table 5. It has been found that all of these compounds inhibit the corrosion of mild steel in HCl solution at all concentrations used in this study. It has also been observed that the inhibition efficiency for all of these compounds increases with the

Fig. 9. Langmuir’s isotherm for adsorption of Schiff bases on the steel surface.

increase in concentration. This fact could be expressed that these compounds acts as adsorption inhibitors. Plotting suitable adsorption isotherm could prove it. In fact, log θ/(1 − θ) against log C of Schiff bases gives straight lines with unit slope (Fig. 9) indicates that adsorption of Schiff bases on the steel surface follows the Langmuir’s adsorption isotherm. From these results it could be concluded that there is no interaction between the inhibitor molecules adsorbed at the metal surface [21]. The values of corrosion rate and inhibition efficiency of Schiff bases at 0.1 M concentration at different temperatures are given in Table 6. It can be seen that the inhibition efficiency for all of the compounds slightly decreases with increasing temperature from 25 to 43 ◦ C. It may be explained by desorption of adsorbed inhibitor from the steel surface. The values of activation energy (Ea ) were calculated using Arrhenius equation [22,23]. The free energy of inhibitor adsorption ( Gads ) at 25 ◦ C was calculated from the following equation [21]: Gads = −RT ln(55.5K)

Table 5 Corrosion parameters for mild steel in 1 M HCl in the presence and absence of Schiff bases at different concentrations, obtained from weight loss measurements at 25 ◦ C Inhibitor

Concentration (M)

Surface coverage (θ)

Inhibition efficiency, ηw (%)

Blank

–

–

–

A

0.0002 0.001 0.005 0.01

0.582 0.902 0.961 0.990

58.2 90.2 96.1 99.0

0.0002 0.001 0.005 0.01

0.879 0.967 0.985 0.989

87.9 96.7 98.5 98.9

0.0002 0.001 0.005 0.01

0.679 0.962 0.987 0.994

67.9 96.2 98.7 99.4

B

C

(3)

Table 6 Corrosion parameters of mild steel in 1 M HCl in the presence and absence of 0.01 M of Schiff bases at different temperature, obtained from weight loss measurements Inhibitor Temperature Corrosion Surface Inhibition (◦ C) rate (mg/cm2 h) coverage (θ) efficiency, ηw (%) Blank

25 34 43

0.549 0.982 1.220

– – –

– – –

A

25 34 43

0.005 0.021 0.049

0.990 0.978 0.960

99.0 97.8 96.0

B

25 34 43

0.006 0.014 0.045

0.989 0.986 0.963

98.9 98.6 96.3

C

25 34 43

0.003 0.016 0.029

0.995 0.984 0.976

99.5 98.4 97.6

H. Ashassi-Sorkhabi et al. / Electrochimica Acta 50 (2005) 3446–3452 Table 7 Activation energy (Ea ) of corrosion and free Gibbs energy ( Gads ) of Schiff bases adsorption obtained from weight loss measurements

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Table 8 Quantum chemical parameters for used Schiff bases obtained from AM1 method

Inhibitor

Temperature (◦ C)

Ea (kJ/mol)

Gads (kJ/mol)

Inhibitor

EHOMO (eV)

ELUMO (eV)

E (eV)

µ (D)

Blank

–

34.92

–

A

25 34 43

79.56

−31.53 −33.03 −34.00

A B C

−9.3487 −9.1665 −9.3598

−0.9333 −0.9132 −1.0908

−10.2820 −10.0797 −10.4506

0.580 1.835 2.797

B

25 34 43

88.97

−31.93 −32.56 −33.52

C

25 34 43

99.36

−32.99 −33.98 −34.97

where K is given by: K = θ/C(1 − θ)

(4)

where θ is the degree of coverage on the metal surface, C the concentration of inhibitor in mol l−1 and K the equilibrium constant. The values of the free energy and the activation energy are given in Table 7. It was found that, Ea values for inhibited systems are higher than Ea for uninhibited system. This results that physical adsorption occurred in the first stage, which explains the nature of organic molecules–metal interactions. On the other hand, physical adsorption is related to lower values of the activation energy (30–50 kJ mol−1 ). So these criteria cannot be taken as decisive to competitive adsorption with water whose removal from the surface requires also some activation energy [24]. The low and negative value of Gads indicated the spontaneous adsorption of inhibitor on the surface of mild steel [25,26]. 3.4. Quantum chemical study Since the electronic properties can change by an addition of a functional group to the molecule, the inhibitor efficiency can be treated as a controlled property. The synthesis of better corrosion inhibitors then can be achieved by controlling electronic properties of a selected group of Schiff base molecules. In addition, the correlations between the inhibition efficiency and molecular parameters can be used for pre-selection of new inhibitors, which are, at the moment, taken essentially from empirical knowledge’s [19]. Quantum structure–activity relationships (QSAR) has been used to study the effect of molecular structure on the inhibition efficiency of the used Shciff base compounds. Satisfactory correlation has been recorded between the inhibition efficiency of some inhibitors and some quantum chemical parameters by other investigators [12,27–29]. The calculated quantum chemical indices (EHOMO = energy of the highest occupied molecular orbital, ELUMO = energy of the lowest unoccupied molecular orbital and dipole moment µ) of Schiff bases obtained using AM1

semi empirical quantum chemical approach are shown in Table 8. The average values of inhibition efficiency (ηm ), obtained using three experimental methods (EIS, linear polarization and weight loss) were used for QSAR modeling (Table 9). Linear and non-linear QSAR models have been used in this case. Not only the ␲ electron of the Schiff bases enter unoccupied orbitals of iron, but the ␲* orbital can also accept the electrons of d orbitals of metallic iron to form feed back bonds [12]. In order to prove the forming of feed back bonds, a linear regression analysis was performed on the average inhibition efficiency versus the HOMO energy (EH ) and LUMO energy (EL ) of the Schiff bases A, B, C, and the following equation was obtained. η (%) = 3.362 EHOMO − 7.094ELUMO + 122.761

(5)

Inhibition efficiency increases with increasing values of EHOMO . The results seem to indicate that charge transfer from the inhibitor takes place during the adsorption on the metal surface. Increasing values of the EHOMO may facilitate adsorption (and therefore inhibition) by influencing the transport process through the adsorbed layer. EHOMO is often associated with the electron donating ability of a molecule. Therefore, the energy of the lowest unoccupied molecular orbital (ELUMO ) indicates the ability of the molecules to accept electrons. The lower value of ELUMO , the more probable, that the molecule would accept electrons [12,18]. The coefficients of EH (positive) and EL (negative) prove that the main aspect of the forming of feed back bonds is the ability of the inhibitors to offer electrons, so the inhibition Table 9 Inhibition efficiency obtained from different methods and their average Inhibitor

Cinh

Hw (%)

ηp (%)

ηz (%)

ηm (%)

A

0.0002 0.001 0.005 0.01

58.2 90.2 96.1 99.0

84.8 97.0 97.6 98.7

54.8 79.2 88.9 90.0

65.9 88.8 94.2 95.9

B

0.0002 0.001 0.005 0.01

87.9 96.7 98.5 98.9

63.2 96.4 98.1 98.9

79.9 80.3 84.6 92.8

77.1 91.1 93.7 96.9

C

0.0002 0.001 0.005 0.01

67.9 96.2 98.7 99.4

75.4 97.5 98.6 99.3

71.8 86.1 90.9 95.8

77.8 93.3 96.1 98.2

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Fig. 10. Correlation between the experimental inhibition efficiency (obtained from LP, EIS and weight loss methods) and the calculated inhibition efficiency (obtained from QSAR).

efficiency values of the three Schiff bases follow the order: C>B>A The non-linear equation was used to correlate all quantum chemical parameters (EH , EL , µ) and inhibitor concentration (Cinh ) with the experimental inhibition efficiencies. The non-linear model proposed by Lukovits et al. [30] for the interaction of corrosion inhibitors with metal surface in acidic solutions has been used in this part of the study. The following proposed relation between inhibition efficiency, and quantum chemical index was used: η (%) = (Axj + B)Ci /(1 + (Axj + B)Ci )

(6)

where A and B are the regression coefficients to be determined by regression analysis; xj a quantum chemical index characteristic of molecule j; Ci denotes the concentration in an experiment i. In this case, xj is constructed, as a composite index of quantum chemical parameters; EHOMO , ELUMO and µ. Following equation is obtained for the three compounds. η (%) =

(AEHOMO + BELUMO + Cµ − D)Cinh 1 + (AEHOMO + BELUMO + Cµ − D)Cinh

(7)

where A, B, C, D are −526.325, 977.031, 107.937 and 2150.147, respectively. The plot of the experimental and the calculated inhibition efficiency of compound A, B, and C is presented in Fig. 10. Highly significant multiple correlation coefficient (R2 ) between experimental and calculated efficiencies was obtained.

4. Conclusion 1. All examined Schiff base compounds act as good corrosion inhibitor in HCl media. 2. These compounds inhibit both anodic and cathodic reaction by adsorption on the steel surface.

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