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Inhibiting effects of some oxadiazole derivatives on the corrosion of mild steel in perchloric acid solution
Applied Surface Science 252 (2005) 950–958 www.elsevier.com/locate/apsusc
Inhibiting effects of some oxadiazole derivatives on the corrosion of mild steel in perchloric acid solution Mounim Lebrini a, Fouad Bentiss a,b, Herve´ Vezin c, Michel Lagrene´e a,* a
Laboratoire de Cristallochimie et Physicochimie du Solide UMR 8012 ENSCL, BP. 108, F-59652 Villeneuve d’Ascq Cedex, France b Laboratoire de Chimie de Coordination et d’Analytique, Universite´ Chouaib Doukkali, Faculte´ des Sciences, B.P. 20, El Jadida, Morocco c Laboratoire de Chimie Organique et Macromole´culaire, CNRS UMR 8009, USTL Baˆt C4, F-59655 Villeneuve d’Ascq Cedex, France
Received 24 November 2004; received in revised form 23 January 2005; accepted 23 January 2005 Available online 5 March 2005
Abstract The efficiency of 3,5-bis(n-pyridyl)-1,3,4-oxadiazole (n-POX, n = 1, 2, 3), as corrosion inhibitors for mild steel in 1 M perchloric acid (HClO4) have been determined by weight loss measurements and electrochemical studies. The results show that these inhibitors revealed a good corrosion inhibition even at very low concentrations. Comparison of results among those obtained by the studied oxadiazoles shows that 3-POX was the best inhibitor. Polarisation curves indicate that n-pyridyl substituted-1,3,4-oxadiazoles are mixed type inhibitors in 1 M HClO4. The adsorption of these inhibitors follows a Langmuir isotherm model. The electronic properties of n-POX, obtained using the AM1 semi-empirical quantum chemical approach, were correlated with their experimental efficiencies using the linear resistance model (LR). # 2005 Elsevier B.V. All rights reserved. Keywords: Corrosion inhibition; Mild steel; Oxadiazole; Perchloric acid
1. Introduction The inhibition of corrosion in acid solutions can be affected by the addition of a variety of organic molecules. Compounds containing N, O, S have shown vast applications as corrosion inhibitors [1–13]. * Corresponding author. Tel.: +33 320 337 746; fax: +33 320 337 746. E-mail address: [email protected] (M. Lagrene´e).
Hackerman [14] gave the idea that higher percentage of p orbital of the free electrons on the N-atom leads to inhibitive action. N-containing compounds used as acid inhibitors include heterocyclic bases such as pyridine, quinoline, and various amines [15,16]. The influence of some Heterocyclic compounds containing more than one nitrogen atoms in their molecules, on corrosion of steel in acid solutions, was investigated by Zucchi and Trabanelli [17], with a view to establish correlation among the examined
0169-4332/$ – see front matter # 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.apsusc.2005.01.160
M. Lebrini et al. / Applied Surface Science 252 (2005) 950–958
substances between molecular structure and inhibition efficiency of the various compounds. The influence of the inhibitor upon metal corrosion is often associated with physical or chemical adsorption. However, the adsorption of inhibitors can be influenced by the type of electrolyte used. The use of organic inhibitors in acid solutions can, in some cases lead to enhancement of the metal corrosion. In general, stimulation of corrosion is not related to the type and structure of the organic molecule but is mainly dependent on the type of acid and its concentration [18–20]. In this work, some oxadiazole derivatives, namely 3,5-bis(2-pyridyl)-1,3,4-oxadiazole (2-POX), 3,5bis(3-pyridyl)-1,3,4-oxadiazole (3-POX) and 3,5bis(4-pyridyl)-1,3,4-oxadiazole (4-POX), have been studied as possible corrosion inhibitors for mild steel in molar perchloric acid (1 M HClO4). Weight loss measurements, polarisation curves and AC impedance methods have been used. The efficiency of n-POX follows the order: 3-POX > 2-POX > 4-POX. We have previously shown that these compounds are very good inhibitors and behave better in 1 M HCl than in 0.5 M H2SO4 [21]. The electronic properties of 2POX, 3-POX and 4-POX, obtained using the AM1 semi-empirical quantum chemical approach, were correlated with their experimental efficiencies using the linear resistance model (LR).
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analytical reagent grade 65% HClO4 with doubly distilled water. The tested inhibitor, namely 3,5-bis(2pyridyl)-1,3,4-oxadiazole (2-POX), 3,5-bis(3-pyridyl)-1,3,4-oxadiazole (3-POX) and 3,5-bis(4-pyridyl)-1,3,4-oxadiazole (4-POX) were synthesised according to a previously described experimental procedure [22,23]. The molecular structures of 2POX, 3-POX and 4-POX are shown in Fig. 1. The concentration range of n-POX employed was 2 104 to 12 104 M. 2.2. Weight loss method Gravimetric experiments were carried out in a double glass cell equipped with a thermostated cooling condenser. The solution volume was 100 ml. The steel specimens used have a rectangular form (length = 2 cm, width = 1 cm, thickness = 0.06 cm). The maximum duration of tests was 24 h at 30 8C in non-de-aerated solutions. At the end of the tests, the specimens were carefully washed in ethanol under ultrasound and then weighed. Duplicate experiments were performed in each case and the mean value of the weight loss is reported. Weight loss allowed calculation of the mean corrosion rate in mg cm2 h1. 2.3. The electrochemical measurements
2. Experimental method 2.1. Material preparation Mild steel strips composed of (wt.%): 0.09% P, 0.38% Si, 0.01% Al, 0.05% Mn, 0.21% C, 0.05% S, and balance Fe were pre-treated prior to the experiments by grinding with emery paper SiC (grades 600 and 1200), then cleaned in ultrasonic bath with ethanol, rinsed with doubly distilled water and finally dried at room temperature. The solutions (1 M HClO4) were prepared by dilution of an
Electrochemical experiments were carried out by means of impedance equipment (Tacussel-Radiometer PGZ 3O1) and controlled with the Tacussel corrosion analysis software (Voltamaster 4). Electrochemical experiments were carried out in a standard glass threeelectrode cell with a capacity of 500 ml for polarisation curves and a polymethyl methacrylate (PMMA) cell with a capacity of 1000 ml for electrochemical spectroscopy measurements. Polarisation measurements were obtained using a conventional three-electrodes cell with a platinum counter electrode and a saturated calomel electrode
Fig. 1. Chemical formulas of the n-POX isomers.
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(SCE) as reference with Luggin capillary bridge. All tests were performed in de-aerated solutions under continuously stirred conditions at room temperature. The procedure adopted for the polarisation measurements was the same as described elsewhere [24]. The anodic and cathodic polarisation curves were recorded by a constant sweep rate of 0.5 mV s1. Inhibition efficiencies were determined from corrosion currents calculated by the Tafel extrapolation method and fitting the curve to the polarisation equation. Impedance spectra were obtained in the frequency range 100 kHz–10 mHz with 10 points per decade at the corrosion potential after 24 h of immersion. A sine wave with 10 mV amplitude was used to perturb the system. All tests were performed at 30 8C in non-deaerated solutions under unstirred conditions. Square sheets of mild steel of size (5 cm 5 cm 0.06 cm), with exposed area of 7.55 cm2, were used as the working electrode. Nyquist plot were made from these experiments. 2.4. Computational chemistry All quantum theoretical calculations were performed with SPARTAN PRO V. 1.05 software package for PC (Irvine Inc.) using ab initio Hartree-Fock 321G* basis set at RHF (Restricted Hartree Fock) level [25], starting without any geometry constraints for full geometry optimisations. 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), D = EHOMO ELUMO and the dipole moment (m).
3. Results and discussion 3.1. Weight loss measurements Gravimetric measurements of mild steel subjected to the effects of acidic media in the absence and presence of various concentrations of oxadiazole derivatives were made after 24 h of immersion at 30 8C. The inhibition efficiency E (%) was calculated as described in a previous paper [26]. Table 1 gives the corrosion rates (the overage of two measurements) and the inhibition efficiencies calculated from weight loss measurements for different concentrations of the three tested isomers of the oxadiazole in 1 M HClO4. As it can be seen in Table 1, E (%) increases with increasing inhibitor concentration. From weight loss measurements, we can conclude that the efficiency of n-POX follow the order: 3-POX > 2-POX > 4-POX. 3.2. Electrochemical impedance spectroscopy (EIS) The corrosion behaviour of mild steel in 1 M HClO4 in the absence and the presence of oxadiazole derivatives was investigated by EIS at 30 8C after immersion for 24 h. Nyquist plots of mild steel in uninhibited and inhibited acidic solutions containing
Table 1 Corrosion rates and inhibition efficiencies of oxadiazole isomers at different concentrations in 1 M HClO4 solutions Inhibitor
Concentration (104 M)
Corrosion rate (mg cm2 h1)
E (%)
Blank
–
1.98
–
2-POX
2 4 8 12
0.41 0.34 0.32 0.28
79.2 82.8 83.8 85.5
3-POX
2 4 8 12
0.33 0.25 0.21 0.16
83.0 87.0 89.0 91.6
4-POX
2 4 8 12
1.24 1.02 0.91 0.47
37.0 48.3 54.0 76.2
M. Lebrini et al. / Applied Surface Science 252 (2005) 950–958
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Fig. 2. Nyquist diagrams for mild steel in 1 M HClO4 containing different concentrations of 2-POX.
Fig. 4. Nyquist diagrams for mild steel in 1 M HClO4 containing different concentrations of 4-POX.
various concentrations of n-POX displayed one capacitive arc are shown in Figs. 2–4. The impedance parameters derived from these figures are given in Table 2. In the case of the electrochemical impedance spectroscopy, E (%) is calculated by charge transfer resistance as described elsewhere [26]. It is found (Table 2) that, as the n-POX concentration increases, the Rt values increase, but the Cdl values tend to decrease. The decrease in Cdl values is due to the adsorption of n-POX from the metal surface. Electrochemical study confirms the results of the weight loss measurements. The comparative study of 2-POX, 3-POX and 4-POX by weight loss measurements and Rt data indicated that E (%) was dependent
on the position of the nitrogen on the pyridinium substituent. The 3-POX appears to be a good inhibitor with a maximum efficiency of 93.4%.
Fig. 3. Nyquist diagrams for mild steel in 1 M HClO4 containing different concentrations of 3-POX.
Fig. 5. Polarisation curves for mild steel in 1 M HClO4 containing different concentrations of 2-POX.
3.3. Polarisation curves Polarisation curves of the mild steel electrode in 1 M HClO4 without and with addition of 2-POX, 3-POX and 4-POX at different concentrations are shown in Figs. 5–7. As it can be seen, the anodic reactions of mild steel electrode corrosion were inhibited with the increase of the oxadiazole derivatives concentration in 1 M HClO4. Addition of 2-POX, 3-POX and 4-POX has suppressed the cathodic reaction to greater extents than the anodic one, especially at low concentration. This result suggests that the addition of oxadiazole
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Table 2 Impedance parameters for the corrosion of mild steel in 1 M HClO4 containing different concentrations of n-POX Concentration (104 M)
Rt (V cm2)
Cdl (mF cm2)
Erest.pot (mV/SCE)
E (%)
Blank
–
6
1742
599
–
2-POX
2 4 8 12
31 42 46 52
1131 855 647 548
497 491 497 493
80.6 85.7 87.0 88.5
3-POX
2 4 8 12
56 64 70 91
119 106 99 87
519 506 505 503
89.3 90.6 91.4 93.4
4-POX
2 4 8 12
10 12 14 27
1616 1431 1239 700
521 514 519 525
40 50 57.1 77.8
Inhibitor
derivatives reduces anodic dissolution and also retards the hydrogen evolution reaction. Tafel lines of nearly equal slopes were obtained, indicating that the hydrogen evolution reaction was activation-controlled. The constancy of this cathodic slope may have indicated that the mechanism of proton discharge reaction did not change by addition of oxadiazole derivatives. Values of associated electrochemical parameters such as corrosion potential (Ecorr), corrosion current density (Icorr) obtained by extrapolation of the Tafel lines and the calculated E (%) are presented in Table 3. As it can be seen from these polarisation results, the Icorr values decrease considerably in the presence of 2-POX, 3-POX and 4-POX with increasing inhibitor concentration. The addition of oxadiazole derivatives
modifies slightly the cathodic slope. No definite trend was observed in the shift of Ecorr values, in presence of various concentrations of these inhibitors in 1 M HClO4 solutions. In anodic domain, we notice that the presence of 2-POX, 3-POX and 4-POX in 1 M HClO4 results in a reduction of anodic current density. This result indicated that these inhibitors exhibit cathodic and anodic inhibition effects. Therefore, 2-POX, 3POX and 4-POX can be classified as inhibitors of relatively mixed effect (anodic/cathodic inhibition) in 1 M HClO4. E (%) values increase with inhibitor concentrations for both inhibitors, reaching a maximum value at 12 104 M, but better performances were obtained by 3-POX. We can conclude that the ability of the
Fig. 6. Polarisation curves for mild steel in 1 M HClO4 containing different concentrations of 3-POX.
Fig. 7. Polarisation curves for mild steel in 1 M HClO4 containing different concentrations of 4-POX.
M. Lebrini et al. / Applied Surface Science 252 (2005) 950–958
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Table 3 Polarisation parameters and the corresponding inhibition efficiencies for the corrosion of mild steel in 1 M HClO4 containing different concentrations of 2-POX, 3-POX and 4-POX at 30 8C Inhibitor
Concentration (M)
Ecorr vs. SCE (mV)
Icorr (mA cm2)
E (%)
Blank
–
481
812
–
2-POX
2 4 8 12
408 429 427 432
233 193 180 165
71.3 76.2 77.8 79.6
3-POX
2 4 8 12
426 434 432 431
196 164 142 129
75.8 79.8 82.4 84.0
4-POX
2 4 8 12
468 448 454 427
567 489 423 254
30.1 39.7 47.9 68.7
molecule to chemisorb on the iron surface was dependent on the position of the nitrogen atom on the heterocyclic ring. The polarisation curves study also confirms the inhibiting character of 2-POX, 3-POX and 4-POX obtained with weight loss measurements. However, E (%) values, determined using polarisation curves were smaller than those determined by AC impedance experiments and weight loss measurements. This difference was probably caused by the shorter immersion time in the case of polarisation curve measurements. 3.4. Study of the adsorption phenomenon If we suppose that the adsorption of this inhibitor follows the langmuir adsorption, the degree of surface coverage (u) is expressed by u¼
bCinh 1 þ bCinh
plots of Cinh/u versus Cinh yield a straight line proving that the adsorption of the n-POX from 1 M HClO4 solutions on the mild steel surface obeys the Langmuir adsorption isotherm (Fig. 8). The ability of the n-POX molecules to chemisorb on the steel surface varies with the position of the nitrogen atom in the pyridyl substituent. The highest inhibition efficiency values were calculated for 3POX. In acidic medium, pyridine and related basic compounds do not remain in solution as free bases; therefore, it may be assumed that, the first contact between the metal surface and n-POX is between metal and pyridinium ions. As corrosion begins, the cation may become attached to the anodic sites, otherwise, the alkalinity produced at the cathodic sites may reform the free base, and thus, the adsorption of the oxadiazole on the metal surface can occur directly via donor-acceptor interactions between the p
(1)
where b designates the adsorption coefficient. The degree of surface coverage (u) for different concentrations of the inhibitor in acidic media have been evaluated from electrochemical measurements using the equation [27]: u¼
Cdlðu¼0Þ Cdlu Cdlðu¼0Þ Cdlðu¼1Þ
(2)
The surface coverage values (u) were tested graphically by fitting a suitable adsorption isotherm. The
Fig. 8. Langmuir adsorption plots for mild steel in 1 M HClO4 containing different concentrations of n-POX.
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Fig. 9. Withdrawing effect of pyridinium substituent in n-POX molecules.
electrons of the heterocyclic compound and the vacant ‘‘d’’ orbitals of iron surface atoms [28]. In 2-POX and 4-POX molecules, the pyridinium substituent exerts a direct withdrawing effect on the oxadiazole heterocycle. This effect is significantly much lower in the case of 3-POX, where no direct withdrawing effect can be observed (Fig. 9) and therefore, slightly better performances are seen in this case.
The general equation describing our studied QSAR linear model can be expressed as follows: X 1=Rti ¼ ðAm j þ BEHOMO j j
ðLRÞ
1=Rt ¼ 0:00381 þ ð1:70m þ 5:03EHOMO 32:13ELUMO ÞCinh N ¼ 9;
3.5. Computational chemistry
þ CELUMO j ÞCinhi
The calculated quantum chemical indices (EHOMO, ELUMO and m) for n-POX are reported in Table 4. Experimental and calculated values of the inverse of Rt are displayed in Fig. 10. The best regression equation for this family, obtained by using the LR model was:
R ¼ 0:91;
(4) F ¼ 8:13
ðF0:99 ¼ 7:59Þ where R denotes the multiple correlation coefficient and N is the total number of experimental impedance measurements. The significance of the regression equation was obtained by calculating the Fischer’s
(3)
where Rt is the charge transfer resistance, A, B, and C are the regression coefficients of the calculated quantum chemical parameters for the molecule j and Cinh,i denotes the concentration of the inhibitor in Experiment i.
Table 4 Quantum chemical indices of 2,5-di(n-pyridyl)-1,3,4-oxadiazoles Inhibitor
HOMO (eV)
LUMO (eV)
Dipole moment (D)
2-POX 3-POX 4-POX
9.27 9.33 9.76
1.16 1.36 1.45
0.90 0.48 1.65
M. Lebrini et al. / Applied Surface Science 252 (2005) 950–958
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Fig. 10. Experimental (Exp.) and predicted (Estd.) 1/Rt values of 2,5-di(n-pyridyl)-1,3,4-oxadiazole derivatives by the LR model.
F number [29]. As shown in Fig. 6, the correlation between experimental and estimated values of Rt of the three oxadiazole derivatives (n-POX) was better using the LR model (R = 0.91). In this correlation we can observe that the inhibition corrosion efficiency is mainly supported by the coefficient of the LUMO energy of the molecule, which is connected to the electron acceptance character of the molecule. Additionally, we have previously shown [30] that the dipole moment participates in the structure-activity relationship.
4. Conclusions The 2,5-bis(n-pyridyl)-1,3,4-oxadiazole derivatives inhibit the corrosion of mild steel in 1 M HClO4 solutions, but the better performance is seen with the 3-POX. Polarisation studies showed that 3-POX, 3POX and 3-POX were mixed-type inhibitor and its E (%) increased with the inhibitor concentration. Adsorption of these inhibitors on the steel surface from acidic solutions followed the Langmuir isotherm. Using QSAR approach we have established a direct correlation, for the two isomers studied, between their molecular structure and their E (%) by using a linear resistance (LR) model joining the Rt to chemical
quantum parameters. A highly significant multiple correlation coefficient (R > 0.91) has been obtained between experimental and predicted 1/Rt using the proposed model.
References [1] W. Machu, in: Proceedings of the Third European Symposium Corrosion Inh., Ferrara, 1971, p. 107. [2] B. Sathianandan, K. Balakrishan, N. Subramanyan, Brit. Corros. J. 5 (1970) 270. [3] M.M. Osman, E. Khamis, A. Micheal, Corros. Prev. Cont. 41 (1994) 60. [4] E. Stupnisek-Lisac et, Z. Ademovic, in: Proceedings of the Eighth European Symposium Corrosion Inh., vol. 10, Ferrara, 1995, p. 257. [5] M.A. Quraishi, M.A.W. Khan, D. Jamal, M. Ajmal, S. Muralidharan et, S.V.K. Iyer, J. Appl. Electrochem. 26 (1996) 1253. [6] M.A. Quraishi, M.A.W. Khan, D. Jamal, M. Ajmal, Trans. Indian Inst. Met. 51 (1998) 431. [7] M.A. Quraishi, M.A.W. Khan, D. Jamal, M. Ajmal, S. Muralidharan et, S.V.K. Iyer, Brit. Corros. J. 32 (1997) 72. [8] B. Mernari, H.El. Attari, M. Traisnel, F. Bentiss, M. Lagrene´e, Corros. Sci. 40 (1998) 391. [9] F. Bentiss, M. Lagrene´e, M. Traisnel, J.C. Hornez, Corros. Sci. 41 (1999) 789. [10] M. El Azhar, B. Mernari, M. Traisnel, L. Gengembre, F. Bentiss, M. Lagrene´e, Corros. Sci. 43 (2001) 2229.
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[11] R.S. Chaudhary, A. Singh, J. Electrochem. Soc. India 46 (1997) 119. [12] F. Bentiss, M. Traisnel, M. Lagrene´e, J. Appl. Electrochem. 31 (2001) 41. [13] L. Wang, G.-J. Yin, G. Yin, Corros. Sci. 43 (2001) 1197. [14] N. Harckerman et, R.M. Hurd, in: Proceedings of the First International Congress on Mettalic Corrosion, Butterworths, London, 1962, p. 166. [15] B. Sathianandan, K. Balakrishan, N. Subramanyan, Brit. Corros. J. 5 (1970) 270. [16] P.N.G. Shankar, K.I. Vasu, J. Electrochem. Soc. India 32 (1983) 47. [17] F. Zucchi, G. Trabanelli, in: Proceedings of the Seventh European Symposium Corrosion Inh., Ferrara, 1990, p. 330. [18] G. Schmitt, Br. Corrosion. J. 19 (1984) 165. [19] S. Rengamani, S. Muralidharan, M. Anbu Kulandainathan, S. Venkatakrishna Iyer, J. Appl. Electrochem. 24 (1994) 355. [20] M. El Azhar, M. Traisnel, B. Mernari, L. Gengembre, F. Bentiss, M. Lagrene´e, Appl. Surf. Sci. 185 (2002) 197.
[21] F. Bentiss, M. Traisnel, M. Lagrene´e, Corros. Sci. 42 (2000) 127. [22] B.L. Sharma, S.K. Tandon, Pharmazie 39 (H-12) (1984) 858. [23] F. Bentiss, M. Lagrene´e, D. Barby, Synth. Commun. 31 (6) (2001) 935. [24] F. Bentiss, M. Lagrene´e, M. Traisnel, J.C. Hornez, Corrosion 55 (1999) 968. [25] M.J.S. Dewar, E.G. Zoebisch, E.H. Healy, J.P. Stewart, J. Am. Chem. Soc. 107 (1985) 3902. [26] F. Bentiss, M. Traisnel, L. Gengenbre, M. Lagrene´e, Appl. Surf. Sci. 152 (1999) 237. [27] D. Landolt, Corrosion et chimie de surface des metaux, 1st ed. Alden, Oxford, 1999, p. 495. [28] N. Hackerman, A.C. Makrids, J. Phys. Chem. 59 (1955) 707. [29] G.W. Snedecor, W.G. Cochran, Statistical Methods, Iowa State University Press, Ames, IA, 1972, p. 117. [30] M. Lagrene´e, B. Mernari, N. Chaibi, M. Traisnel, H. Vezin, F. Bentiss, Corros. Sci. 43 (2001) 951.
Inhibiting effects of some oxadiazole derivatives on the corrosion of mild steel in perchloric acid solution Mounim Lebrini a, Fouad Bentiss a,b, Herve´ Vezin c, Michel Lagrene´e a,* a
Laboratoire de Cristallochimie et Physicochimie du Solide UMR 8012 ENSCL, BP. 108, F-59652 Villeneuve d’Ascq Cedex, France b Laboratoire de Chimie de Coordination et d’Analytique, Universite´ Chouaib Doukkali, Faculte´ des Sciences, B.P. 20, El Jadida, Morocco c Laboratoire de Chimie Organique et Macromole´culaire, CNRS UMR 8009, USTL Baˆt C4, F-59655 Villeneuve d’Ascq Cedex, France
Received 24 November 2004; received in revised form 23 January 2005; accepted 23 January 2005 Available online 5 March 2005
Abstract The efficiency of 3,5-bis(n-pyridyl)-1,3,4-oxadiazole (n-POX, n = 1, 2, 3), as corrosion inhibitors for mild steel in 1 M perchloric acid (HClO4) have been determined by weight loss measurements and electrochemical studies. The results show that these inhibitors revealed a good corrosion inhibition even at very low concentrations. Comparison of results among those obtained by the studied oxadiazoles shows that 3-POX was the best inhibitor. Polarisation curves indicate that n-pyridyl substituted-1,3,4-oxadiazoles are mixed type inhibitors in 1 M HClO4. The adsorption of these inhibitors follows a Langmuir isotherm model. The electronic properties of n-POX, obtained using the AM1 semi-empirical quantum chemical approach, were correlated with their experimental efficiencies using the linear resistance model (LR). # 2005 Elsevier B.V. All rights reserved. Keywords: Corrosion inhibition; Mild steel; Oxadiazole; Perchloric acid
1. Introduction The inhibition of corrosion in acid solutions can be affected by the addition of a variety of organic molecules. Compounds containing N, O, S have shown vast applications as corrosion inhibitors [1–13]. * Corresponding author. Tel.: +33 320 337 746; fax: +33 320 337 746. E-mail address: [email protected] (M. Lagrene´e).
Hackerman [14] gave the idea that higher percentage of p orbital of the free electrons on the N-atom leads to inhibitive action. N-containing compounds used as acid inhibitors include heterocyclic bases such as pyridine, quinoline, and various amines [15,16]. The influence of some Heterocyclic compounds containing more than one nitrogen atoms in their molecules, on corrosion of steel in acid solutions, was investigated by Zucchi and Trabanelli [17], with a view to establish correlation among the examined
0169-4332/$ – see front matter # 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.apsusc.2005.01.160
M. Lebrini et al. / Applied Surface Science 252 (2005) 950–958
substances between molecular structure and inhibition efficiency of the various compounds. The influence of the inhibitor upon metal corrosion is often associated with physical or chemical adsorption. However, the adsorption of inhibitors can be influenced by the type of electrolyte used. The use of organic inhibitors in acid solutions can, in some cases lead to enhancement of the metal corrosion. In general, stimulation of corrosion is not related to the type and structure of the organic molecule but is mainly dependent on the type of acid and its concentration [18–20]. In this work, some oxadiazole derivatives, namely 3,5-bis(2-pyridyl)-1,3,4-oxadiazole (2-POX), 3,5bis(3-pyridyl)-1,3,4-oxadiazole (3-POX) and 3,5bis(4-pyridyl)-1,3,4-oxadiazole (4-POX), have been studied as possible corrosion inhibitors for mild steel in molar perchloric acid (1 M HClO4). Weight loss measurements, polarisation curves and AC impedance methods have been used. The efficiency of n-POX follows the order: 3-POX > 2-POX > 4-POX. We have previously shown that these compounds are very good inhibitors and behave better in 1 M HCl than in 0.5 M H2SO4 [21]. The electronic properties of 2POX, 3-POX and 4-POX, obtained using the AM1 semi-empirical quantum chemical approach, were correlated with their experimental efficiencies using the linear resistance model (LR).
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analytical reagent grade 65% HClO4 with doubly distilled water. The tested inhibitor, namely 3,5-bis(2pyridyl)-1,3,4-oxadiazole (2-POX), 3,5-bis(3-pyridyl)-1,3,4-oxadiazole (3-POX) and 3,5-bis(4-pyridyl)-1,3,4-oxadiazole (4-POX) were synthesised according to a previously described experimental procedure [22,23]. The molecular structures of 2POX, 3-POX and 4-POX are shown in Fig. 1. The concentration range of n-POX employed was 2 104 to 12 104 M. 2.2. Weight loss method Gravimetric experiments were carried out in a double glass cell equipped with a thermostated cooling condenser. The solution volume was 100 ml. The steel specimens used have a rectangular form (length = 2 cm, width = 1 cm, thickness = 0.06 cm). The maximum duration of tests was 24 h at 30 8C in non-de-aerated solutions. At the end of the tests, the specimens were carefully washed in ethanol under ultrasound and then weighed. Duplicate experiments were performed in each case and the mean value of the weight loss is reported. Weight loss allowed calculation of the mean corrosion rate in mg cm2 h1. 2.3. The electrochemical measurements
2. Experimental method 2.1. Material preparation Mild steel strips composed of (wt.%): 0.09% P, 0.38% Si, 0.01% Al, 0.05% Mn, 0.21% C, 0.05% S, and balance Fe were pre-treated prior to the experiments by grinding with emery paper SiC (grades 600 and 1200), then cleaned in ultrasonic bath with ethanol, rinsed with doubly distilled water and finally dried at room temperature. The solutions (1 M HClO4) were prepared by dilution of an
Electrochemical experiments were carried out by means of impedance equipment (Tacussel-Radiometer PGZ 3O1) and controlled with the Tacussel corrosion analysis software (Voltamaster 4). Electrochemical experiments were carried out in a standard glass threeelectrode cell with a capacity of 500 ml for polarisation curves and a polymethyl methacrylate (PMMA) cell with a capacity of 1000 ml for electrochemical spectroscopy measurements. Polarisation measurements were obtained using a conventional three-electrodes cell with a platinum counter electrode and a saturated calomel electrode
Fig. 1. Chemical formulas of the n-POX isomers.
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(SCE) as reference with Luggin capillary bridge. All tests were performed in de-aerated solutions under continuously stirred conditions at room temperature. The procedure adopted for the polarisation measurements was the same as described elsewhere [24]. The anodic and cathodic polarisation curves were recorded by a constant sweep rate of 0.5 mV s1. Inhibition efficiencies were determined from corrosion currents calculated by the Tafel extrapolation method and fitting the curve to the polarisation equation. Impedance spectra were obtained in the frequency range 100 kHz–10 mHz with 10 points per decade at the corrosion potential after 24 h of immersion. A sine wave with 10 mV amplitude was used to perturb the system. All tests were performed at 30 8C in non-deaerated solutions under unstirred conditions. Square sheets of mild steel of size (5 cm 5 cm 0.06 cm), with exposed area of 7.55 cm2, were used as the working electrode. Nyquist plot were made from these experiments. 2.4. Computational chemistry All quantum theoretical calculations were performed with SPARTAN PRO V. 1.05 software package for PC (Irvine Inc.) using ab initio Hartree-Fock 321G* basis set at RHF (Restricted Hartree Fock) level [25], starting without any geometry constraints for full geometry optimisations. 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), D = EHOMO ELUMO and the dipole moment (m).
3. Results and discussion 3.1. Weight loss measurements Gravimetric measurements of mild steel subjected to the effects of acidic media in the absence and presence of various concentrations of oxadiazole derivatives were made after 24 h of immersion at 30 8C. The inhibition efficiency E (%) was calculated as described in a previous paper [26]. Table 1 gives the corrosion rates (the overage of two measurements) and the inhibition efficiencies calculated from weight loss measurements for different concentrations of the three tested isomers of the oxadiazole in 1 M HClO4. As it can be seen in Table 1, E (%) increases with increasing inhibitor concentration. From weight loss measurements, we can conclude that the efficiency of n-POX follow the order: 3-POX > 2-POX > 4-POX. 3.2. Electrochemical impedance spectroscopy (EIS) The corrosion behaviour of mild steel in 1 M HClO4 in the absence and the presence of oxadiazole derivatives was investigated by EIS at 30 8C after immersion for 24 h. Nyquist plots of mild steel in uninhibited and inhibited acidic solutions containing
Table 1 Corrosion rates and inhibition efficiencies of oxadiazole isomers at different concentrations in 1 M HClO4 solutions Inhibitor
Concentration (104 M)
Corrosion rate (mg cm2 h1)
E (%)
Blank
–
1.98
–
2-POX
2 4 8 12
0.41 0.34 0.32 0.28
79.2 82.8 83.8 85.5
3-POX
2 4 8 12
0.33 0.25 0.21 0.16
83.0 87.0 89.0 91.6
4-POX
2 4 8 12
1.24 1.02 0.91 0.47
37.0 48.3 54.0 76.2
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Fig. 2. Nyquist diagrams for mild steel in 1 M HClO4 containing different concentrations of 2-POX.
Fig. 4. Nyquist diagrams for mild steel in 1 M HClO4 containing different concentrations of 4-POX.
various concentrations of n-POX displayed one capacitive arc are shown in Figs. 2–4. The impedance parameters derived from these figures are given in Table 2. In the case of the electrochemical impedance spectroscopy, E (%) is calculated by charge transfer resistance as described elsewhere [26]. It is found (Table 2) that, as the n-POX concentration increases, the Rt values increase, but the Cdl values tend to decrease. The decrease in Cdl values is due to the adsorption of n-POX from the metal surface. Electrochemical study confirms the results of the weight loss measurements. The comparative study of 2-POX, 3-POX and 4-POX by weight loss measurements and Rt data indicated that E (%) was dependent
on the position of the nitrogen on the pyridinium substituent. The 3-POX appears to be a good inhibitor with a maximum efficiency of 93.4%.
Fig. 3. Nyquist diagrams for mild steel in 1 M HClO4 containing different concentrations of 3-POX.
Fig. 5. Polarisation curves for mild steel in 1 M HClO4 containing different concentrations of 2-POX.
3.3. Polarisation curves Polarisation curves of the mild steel electrode in 1 M HClO4 without and with addition of 2-POX, 3-POX and 4-POX at different concentrations are shown in Figs. 5–7. As it can be seen, the anodic reactions of mild steel electrode corrosion were inhibited with the increase of the oxadiazole derivatives concentration in 1 M HClO4. Addition of 2-POX, 3-POX and 4-POX has suppressed the cathodic reaction to greater extents than the anodic one, especially at low concentration. This result suggests that the addition of oxadiazole
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Table 2 Impedance parameters for the corrosion of mild steel in 1 M HClO4 containing different concentrations of n-POX Concentration (104 M)
Rt (V cm2)
Cdl (mF cm2)
Erest.pot (mV/SCE)
E (%)
Blank
–
6
1742
599
–
2-POX
2 4 8 12
31 42 46 52
1131 855 647 548
497 491 497 493
80.6 85.7 87.0 88.5
3-POX
2 4 8 12
56 64 70 91
119 106 99 87
519 506 505 503
89.3 90.6 91.4 93.4
4-POX
2 4 8 12
10 12 14 27
1616 1431 1239 700
521 514 519 525
40 50 57.1 77.8
Inhibitor
derivatives reduces anodic dissolution and also retards the hydrogen evolution reaction. Tafel lines of nearly equal slopes were obtained, indicating that the hydrogen evolution reaction was activation-controlled. The constancy of this cathodic slope may have indicated that the mechanism of proton discharge reaction did not change by addition of oxadiazole derivatives. Values of associated electrochemical parameters such as corrosion potential (Ecorr), corrosion current density (Icorr) obtained by extrapolation of the Tafel lines and the calculated E (%) are presented in Table 3. As it can be seen from these polarisation results, the Icorr values decrease considerably in the presence of 2-POX, 3-POX and 4-POX with increasing inhibitor concentration. The addition of oxadiazole derivatives
modifies slightly the cathodic slope. No definite trend was observed in the shift of Ecorr values, in presence of various concentrations of these inhibitors in 1 M HClO4 solutions. In anodic domain, we notice that the presence of 2-POX, 3-POX and 4-POX in 1 M HClO4 results in a reduction of anodic current density. This result indicated that these inhibitors exhibit cathodic and anodic inhibition effects. Therefore, 2-POX, 3POX and 4-POX can be classified as inhibitors of relatively mixed effect (anodic/cathodic inhibition) in 1 M HClO4. E (%) values increase with inhibitor concentrations for both inhibitors, reaching a maximum value at 12 104 M, but better performances were obtained by 3-POX. We can conclude that the ability of the
Fig. 6. Polarisation curves for mild steel in 1 M HClO4 containing different concentrations of 3-POX.
Fig. 7. Polarisation curves for mild steel in 1 M HClO4 containing different concentrations of 4-POX.
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Table 3 Polarisation parameters and the corresponding inhibition efficiencies for the corrosion of mild steel in 1 M HClO4 containing different concentrations of 2-POX, 3-POX and 4-POX at 30 8C Inhibitor
Concentration (M)
Ecorr vs. SCE (mV)
Icorr (mA cm2)
E (%)
Blank
–
481
812
–
2-POX
2 4 8 12
408 429 427 432
233 193 180 165
71.3 76.2 77.8 79.6
3-POX
2 4 8 12
426 434 432 431
196 164 142 129
75.8 79.8 82.4 84.0
4-POX
2 4 8 12
468 448 454 427
567 489 423 254
30.1 39.7 47.9 68.7
molecule to chemisorb on the iron surface was dependent on the position of the nitrogen atom on the heterocyclic ring. The polarisation curves study also confirms the inhibiting character of 2-POX, 3-POX and 4-POX obtained with weight loss measurements. However, E (%) values, determined using polarisation curves were smaller than those determined by AC impedance experiments and weight loss measurements. This difference was probably caused by the shorter immersion time in the case of polarisation curve measurements. 3.4. Study of the adsorption phenomenon If we suppose that the adsorption of this inhibitor follows the langmuir adsorption, the degree of surface coverage (u) is expressed by u¼
bCinh 1 þ bCinh
plots of Cinh/u versus Cinh yield a straight line proving that the adsorption of the n-POX from 1 M HClO4 solutions on the mild steel surface obeys the Langmuir adsorption isotherm (Fig. 8). The ability of the n-POX molecules to chemisorb on the steel surface varies with the position of the nitrogen atom in the pyridyl substituent. The highest inhibition efficiency values were calculated for 3POX. In acidic medium, pyridine and related basic compounds do not remain in solution as free bases; therefore, it may be assumed that, the first contact between the metal surface and n-POX is between metal and pyridinium ions. As corrosion begins, the cation may become attached to the anodic sites, otherwise, the alkalinity produced at the cathodic sites may reform the free base, and thus, the adsorption of the oxadiazole on the metal surface can occur directly via donor-acceptor interactions between the p
(1)
where b designates the adsorption coefficient. The degree of surface coverage (u) for different concentrations of the inhibitor in acidic media have been evaluated from electrochemical measurements using the equation [27]: u¼
Cdlðu¼0Þ Cdlu Cdlðu¼0Þ Cdlðu¼1Þ
(2)
The surface coverage values (u) were tested graphically by fitting a suitable adsorption isotherm. The
Fig. 8. Langmuir adsorption plots for mild steel in 1 M HClO4 containing different concentrations of n-POX.
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Fig. 9. Withdrawing effect of pyridinium substituent in n-POX molecules.
electrons of the heterocyclic compound and the vacant ‘‘d’’ orbitals of iron surface atoms [28]. In 2-POX and 4-POX molecules, the pyridinium substituent exerts a direct withdrawing effect on the oxadiazole heterocycle. This effect is significantly much lower in the case of 3-POX, where no direct withdrawing effect can be observed (Fig. 9) and therefore, slightly better performances are seen in this case.
The general equation describing our studied QSAR linear model can be expressed as follows: X 1=Rti ¼ ðAm j þ BEHOMO j j
ðLRÞ
1=Rt ¼ 0:00381 þ ð1:70m þ 5:03EHOMO 32:13ELUMO ÞCinh N ¼ 9;
3.5. Computational chemistry
þ CELUMO j ÞCinhi
The calculated quantum chemical indices (EHOMO, ELUMO and m) for n-POX are reported in Table 4. Experimental and calculated values of the inverse of Rt are displayed in Fig. 10. The best regression equation for this family, obtained by using the LR model was:
R ¼ 0:91;
(4) F ¼ 8:13
ðF0:99 ¼ 7:59Þ where R denotes the multiple correlation coefficient and N is the total number of experimental impedance measurements. The significance of the regression equation was obtained by calculating the Fischer’s
(3)
where Rt is the charge transfer resistance, A, B, and C are the regression coefficients of the calculated quantum chemical parameters for the molecule j and Cinh,i denotes the concentration of the inhibitor in Experiment i.
Table 4 Quantum chemical indices of 2,5-di(n-pyridyl)-1,3,4-oxadiazoles Inhibitor
HOMO (eV)
LUMO (eV)
Dipole moment (D)
2-POX 3-POX 4-POX
9.27 9.33 9.76
1.16 1.36 1.45
0.90 0.48 1.65
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Fig. 10. Experimental (Exp.) and predicted (Estd.) 1/Rt values of 2,5-di(n-pyridyl)-1,3,4-oxadiazole derivatives by the LR model.
F number [29]. As shown in Fig. 6, the correlation between experimental and estimated values of Rt of the three oxadiazole derivatives (n-POX) was better using the LR model (R = 0.91). In this correlation we can observe that the inhibition corrosion efficiency is mainly supported by the coefficient of the LUMO energy of the molecule, which is connected to the electron acceptance character of the molecule. Additionally, we have previously shown [30] that the dipole moment participates in the structure-activity relationship.
4. Conclusions The 2,5-bis(n-pyridyl)-1,3,4-oxadiazole derivatives inhibit the corrosion of mild steel in 1 M HClO4 solutions, but the better performance is seen with the 3-POX. Polarisation studies showed that 3-POX, 3POX and 3-POX were mixed-type inhibitor and its E (%) increased with the inhibitor concentration. Adsorption of these inhibitors on the steel surface from acidic solutions followed the Langmuir isotherm. Using QSAR approach we have established a direct correlation, for the two isomers studied, between their molecular structure and their E (%) by using a linear resistance (LR) model joining the Rt to chemical
quantum parameters. A highly significant multiple correlation coefficient (R > 0.91) has been obtained between experimental and predicted 1/Rt using the proposed model.
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