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Corrosion inhibition of brass in presence of terdentate ligands in chloride solution
Corrosion Science 47 (2005) 1534–1544 www.elsevier.com/locate/corsci
Corrosion inhibition of brass in presence of terdentate ligands in chloride solution A. Asan a, M. Kabasakalogˇlu b, M. Isßıklan a, Z. Kılıc¸
c,*
a
Department of Chemistry, Faculty of Science, Kırıkkale University, 71450 Kırıkkale, Turkey b Department of Chemistry, Faculty of Science, Gazi University, 06500 Ankara, Turkey Department of Chemistry, Faculty of Science, Ankara University, 06100 Tandogˇan, Ankara, Turkey
c
Received 27 February 2004; accepted 21 July 2004 Available online 23 September 2004
Abstract The inhibition of two terdentate ligands, 2-[(E)-pyridin-2-ylimino)methyl)]phenol and 2[(pyridin-2-ylamino)methyl]phenol, abbreviated L1 and L2 respectively, on the corrosion of brass in 0.10 M NaCl solution under various conditions, has been studied by means of the potentiostatic polarization and AC impedance methods. The studies show that terdentate ligands, L1 and L2 inhibit the corrosion of brass in chloride solution and that the inhibiting efficiency increases with an increase in their concentrations. Self-assembled films of these substances were also prepared on the brass surface. These films improved significantly the protecting ability of brass surface to corrosion in 0.10 M NaCl solution. When the films were modified with benzotriazole (BTA), the quality and corrosion resistance of films improved markedly. Ó 2004 Elsevier Ltd. All rights reserved. Keywords: A. Brass; B. Potentiostatic polarization; C. Neutral inhibition; D. Terdentate ligands
*
Corresponding author. Tel.: +90 312 2126720; fax: +90 312 2232395. E-mail address: [email protected] (Z. Kılıc¸).
0010-938X/$ - see front matter Ó 2004 Elsevier Ltd. All rights reserved. doi:10.1016/j.corsci.2004.07.031
A. Asan et al. / Corrosion Science 47 (2005) 1534–1544
1535
1. Introduction Due to the various industrial applications and economic importance of brass its protection against corrosion attracted much attention. However it corrodes easily in chloride containing aqueous solutions and air, which limits its use. One of the most important methods in corrosion protection is to use inhibitors. Some Schiff bases have recently been reported as effective corrosion inhibitors for mild steel, aluminium and copper [1–3]. Most organic substances employed as copper corrosion inhibitors protect the metal by forming a chelate on the metal surfaces [4]. The effectiveness of an organic substance as an inhibitor depends on the structure of the inhibitor [5] and the stability of the chelate formed on the metal surface [6]. The choice of the inhibitor is based on two considerations. First, it can be synthesized conveniently from relatively cheap raw materials. Second, the presence of an electron cloud on the aromatic ring, the electronegative sulphur, nitrogen, oxygen atoms and the relatively long chain compounds to induce greater adsorption on the metal surface promoting effective inhibition [7]. Generally, a strong coordination bond causes higher inhibition efficiency, the inhibition increases in the sequence O < N < S < P [8–17]. The inhibition of corrosion by Schiff bases can be attributed to its molecules with p-electrons of –C@N-groups and p-electrons of aromatic ring. Conjugating large p bond through which its molecules are likely to be adsorbed strongly on the metal surface [18]. The aim of this study is to investigate the corrosion of brass in 0.10 M NaCl in the presence of two terdentate ligands, L1 and L2 (Fig. 1), and to observe any relationship between the structure of these ligands and their inhibitive action under various conditions. L1 is a Schiff base and L2 is a secondary amine. The difference between them is the p-electrons between carbon and nitrogen imine bond. L1 may be a potential bidentate ligand for Cu(II) and Zn(II) cations, as to be similar Ar– CH@N-pyridine (Ar: salicyl, naphthyl, etc.) ligands [19,20]. L2 may also be a potential terdentate ligand for various transition metal cations [21]. Two electrochemical techniques potentiostatic polarization and impedance have been used to study the effect of addition of these compounds on the corrosion of brass in 0.10 M NaCl. Organic inhibitors normally form very thin and persistent chemisorbed films that lead to a remarkable decrease in the corrosion rate due to the slowing down of anodic, cathodic reaction or both. In particular, it is known that benzotriazole, BTA, its derivatives and other organic compounds offer a good protection against corrosion in different aggressive environments [22–26]. We think that the same protection against corrosion is valid for brass in the presence of L1 and L2 ligands. As it is known, in the case of L1 and L2, the presence of –OH group in the ortho positions facilitate the formation of a chelate with a six membered ring [19].
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2. Experimental 2.1. Electrodes For polarization and electrochemical impedance studies, the thick brass sheet with a composition of Cu 70% and Zn 30% was embedded in epoxy resin, to expose a geometrical surface area of 1 cm2 to electrolyte. The electrode was polished to mirror finish, using fine grain emery paper of 1200 grade under water flow, washed with distilled water and acetone and dried prior to the experiments. Electrochemical experiments were carried out in pyrex cell with three compartments. A saturated calomel electrode, SCE, was used as the reference electrode and platinum sheet as the counter electrode. 2.2. Inhibitors The formulae of the substances tested as brass corrosion inhibitors are depicted in Fig. 1. The compounds, L1 and L2, have been synthesized according to the literature [27]. 2-[(E)-pyridin-2-ylimino)methyl)]phenol, L1, mp. 66 °C, lit. [27] mp. 65–66 °C, FTIR (KBr): 3050 cm1 (mCH arom.), 1609 cm1 (mC@N), 1588 cm1 (mC@C). 2[(pyridin-2-ylamino)methyl]phenol, L2, mp. 106 °C, lit. [27] mp. 106–108 °C, FTIR (KBr): 3329 cm1 (mNH), 3057 cm1 (mCH arom.), 1604 cm1 (mC@C). The solutions were prepared with analytical grade and chemically pure reagents using triple-distilled water. 2.3. The formation of the surface film The working electrode was mechanically polished to mirror finish, using fine grain emery paper of 1200 grade under water flow, washed with double-distilled water, and then etched in a 6.0 M HNO3 solution for 30 s, rinsed with double-distilled water and followed by rinsing with absolute ethanol. The cleaned electrode was immersed in 5.0 mM solutions of the ligands in ethanol for 30 min to form self-assembled film. After film formation on surface, electrodes were taken out, rinsed with absolute ethanol and double-distilled water, respectively. Tafel plots and Nyquistic diagram were obtained in 0.10 M NaCl solution. N
N CH2 HN
CH N OH
OH
L1
L2
Fig. 1. Molecular structures of the investigated terdentate ligands.
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Surface modified brass electrode with self-assembled film prepared by immersion into the solution of the ligands in ethanol and then immersed in 1.0 mM benzotriazole solution for 30 min. Tafel plots and AC impedance measurements were obtained in 0.10 M NaCl solutions with the surface modified brass electrode prepared as mentioned above. 2.4. AC impedance and polarization measurements The potentiostatic polarization and impedance measurements were obtained on a Volta Lab PGZ 301 system complete with Pentium II PC and Volta Lab 4.0 software. The impedance measurements were carried out in various concentrations of L1 and L2 ligands at the respective corrosion potentials with a sinusoidal potential perturbation of 5.0 mV. The preconditioning time for all the impedance measurements were 30 s and the frequency range between 100 kHz and 20 mHz. The polarization measurements were also obtained in 0.10 M NaCl + x ML1 or x ML2 solutions in the potential range between 0.2 V and +0.1 V, using sweep rate of 2.0 mV s1.
3. Results and discussion 3.1. AC impedance results Impedance spectra for brass in 0.10 M NaCl, in the absence and in the presence of various concentrations of L1 and L2 were depicted in Figs. 2 and 3, respectively. In
2.5
2 Zr (kohm.cm )
2.0
200
1.5
10
0
50
1.0
20 0.5 Bare
1
2
4
3
5
6
2
Zr (kohm.cm ) Fig. 2. Impedance plot obtained in 0.10 M NaCl in various concentrations of L1; 0, 20, 50, 100, 200 ppm.
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A. Asan et al. / Corrosion Science 47 (2005) 1534–1544 2.5
1.5
20
0
1.0
10
Zr(kohm.cm2)
2.0
Bare
1
20
0.5
0
50
2
4
3
5
6
2
Zr (kohm.cm ) Fig. 3. Impedance plot obtained in 0.10 M NaCl in various concentrations of L2; 0, 20, 50, 100, 200 ppm.
general, these circuits fall into the classical parallel capacitor–resistor combination with the series resistance is being that of the bulk solution. As can be seen from Figs. 2 and 3, the impedance diagrams show perfect semi-circles indicating a barrier layer formed on the surface and a charge transfer process mainly controlling the corrosion of brass. In fact the presence of L1 and L2 ligands enhance the value of charge transfer resistance in chloride solution. Results can be seen in Table 1. L1 and L2 with a concentration of 200 ppm have the highest protection efficiency.
Table 1 The impedance, Tafel polarization and inhibition efficiency values for the corrosion of brass in 0.10 M NaCl Inhibitors
C (ppm)
Rp (kohm)
g(Rp)%
Ecor
Icor (lA/cm2)
g%
Average g%
L1
0 20 50 100 200 Coated Modified
0.8 2.4 3.1 5 6.1 12.2 16
67 74 84 86 93 95
224 216 209 208 190 160 149
9.12 3.11 2.42 1.46 1.13 0.62 0.29
66 73 84 88 93 97
66.5 73.5 84 87 93 96
20 50 100 200 Coated Modified
1.9 2.7 4.6 5.2 10.5 12.6
58 70 78 85 92 94
218 215 192 195 158 150
3.53 2.65 1.75 1.30 0.75 0.47
61 71 80 86 92 95
59.5 70.5 79 85.5 92 94.5
L2
A. Asan et al. / Corrosion Science 47 (2005) 1534–1544
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10.0
6.0
Co ve
M
red
od
ifi ed
Zr(kohm.cm2)
8.0
4.0
200
2.0
8
6
4
2
10
12
14
16
2
Zr (kohm.cm ) Fig. 4. (1) Impedance curve obtained with bare electrode in 0.10 M NaCl solution containing 200 ppm of L1. (2) Impedance curve obtained in 0.10 M NaCl solution electrode surface is covered with adsorbed L1 film. (3) Impedance curve obtained in 0.10 M NaCl solution with surface modified electrode with L1 and with BTA.
8.0 6.0 4.0
ed
20 0 2
4
6
8
10
ied
r ve Co
2.0
dif Mo
2 -Zi (kohm.cm )
10.0
12
14
16
Zr (kohm.cm2) Fig. 5. (1) Impedance curve obtained with bare electrode in 0.10 M NaCl solution containing 200 ppm of L2. (2) Impedance curve obtained in 0.10 M NaCl solution electrode surface is covered with adsorbed L2 film. (3) Impedance curve obtained in 0.10 M NaCl solution with surface modified electrode with L2 and with BTA.
As can be seen from Figs. 4 and 5 the polarization resistance in 0.10 M NaCl solution increase for covered and modified brass electrodes. Modification of brass surface with L1 and L2 and then BTA enhances protection efficiency (Table 1). Same conclusion can be drawn from Tafel plots.
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The impedance, Tafel polarization and inhibition efficiency values for the corrosion of brass in 0.10 M NaCl solution were listed in Table 1. The protection efficiency was calculated using the following expression, where Rp and R0p denote polarization resistance of electrode with and without inhibitor, respectively, evaluated from AC impedance curves. g¼
Rp R0p Rp
3.2. Polarization curves Figs. 6 and 7 show the anodic and cathodic polarization curves of the brass electrode obtained with a scan rate of 2.0 mV s1 in 0.10 M NaCl solutions with and without addition of L1 and L2 at different concentrations, respectively. It is clear that
Log i, µAcm-2
3 2 1 0 -1 -2
-300
-400
-200
0
-100
Potential, mV (SCE) Fig. 6. Polarization curves for brass in 0.1 M NaCl without and in the presence of different concentrations of L1; ( ) 0.1 M NaCl + 0; ( ) 20 ppm; ( ) 50 ppm; ( ) 100 ppm; ( ) 200 ppm.
Log i, µAcm-2
3 2 1 0 -1 -2
-400
-300
-200
-100
0
Potential, mV (SCE) Fig. 7. Polarization curves for brass in 0.1 M NaCl without and in the presence of different concentrations of L2; ( ) 0.1 M NaCl + 0; ( ) 20 ppm; ( ) 50 ppm; ( ) 100 ppm; ( ) 200 ppm.
A. Asan et al. / Corrosion Science 47 (2005) 1534–1544
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corrosion rates decrease with the increasing inhibitor concentration. Both of the inhibitors increase the corrosion potential slightly, but decrease anodic and cathodic currents, markedly. The corrosion current densities were determined by extrapolation of the Tafel lines to corrosion potential. The highest protection efficiency was exhibited for 200 ppm of inhibitor for each substance. The protection efficiency was estimated using the following expression, where icorr(inh) and icorr current density with and without inhibitor, respectively. gð%Þ ¼
icorr icorr ðinh:Þ 100 icorr
After adsorption of L1 and L2 on bare brass surface from alcohol solution, the corrosion potential of brass in 0.10 M NaCl solution shifted to 160 mV and 158 mV very close to each other and corrosion currents decrease to 0.62 and 0.75 lA/cm2, respectively. BTA adsorption, after electrodes were covered with adsorbed film of L1 or L2 ligands, increases corrosion potential to 149 mV and 150 mV, respectively. On the other hand, protection efficiency also rises to 97% for L1 and 95% for L2, respectively. Figs. 7 and 8 show polarization curves of brass electrode coated with L1 and L2 adsorption film, and then their adsorption layer modified with BTA obtained in 0.10 M NaCl solution (Fig. 9). 3.3. Adsorption isotherms It is generally assumed that the adsorption of inhibitors on the metal surface is the essential step in the mechanism of inhibition. Fig. 10a and b show the relationships between surface coverage and concentration of inhibitors. The adsorption of L1 and L2 ligands in 5.0 mM solution of ethanol were found to be governed by Langmuir isotherms. C versus C/h plot gives a straight line with a slope of unity. The surface coverage values, h, were calculated from the polarization resistance values and icorr values which gave the same results.
Log i, µAcm-2
3 2 1 0 Modified Covered 200ppm
-1 -2 -400
-300
-200
-100
0
Potential, mV (SCE) Fig. 8. Polarization curves obtained in 0.10 M NaCl for 200 ppm L1 Schiff base, film covered brass electrode and after film modification with BTA.
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Log i, µAcm-2
3 2 1 0
200ppm Covered
-1
Modified
-2 -400
-300
0
-100
-200
Potential, mV (SCE) Fig. 9. Polarization curves obtained in 0.10 M NaCl for 200 ppm L2 ligand, film covered brass electrode and after film modification with BTA.
250
y = 1.0995x + 10.055 2 R = 0.9994
C/θ
200 150 100 50 0 0
50
250
150
200
250
y = 1.0479x + 16.825 2 R = 0.9975
200
C/θ
100
C, ppm
(a)
150 100 50 0 0
(b)
50
100
150
200
250
C, ppm
Fig. 10. Langmuir adsorption isotherms (C vs. C/h) of L1 (a) and L2 (b).
The type of adsorption, when good corrosion inhibition occurs, is found to obey the Langmuir isotherm. That is derived by assuming that the inhibitor forms a monomolecular layer on the surface. h¼
KC 1 þ KC
A. Asan et al. / Corrosion Science 47 (2005) 1534–1544
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or its arranged form C 1 ¼ þC h K where K is the equilibrium constant of adsorption. From the graphs equilibrium constants K are calculated for L1 and L2 as 0.099 and 0.059 respectively. However, there is a little difference between these values. The L1 adsorption that has –C@N-bond is more favourable than the L2 adsorption on the surface of electrode. Chemisorptions involve the share or transfer of charge from the ligands to the surface to form coordinated covalent bond. Since these terdentate ligands are very suitable to form four or six coordinated complexes with Zn(II) and Cu(II) [19,21].
4. Conclusion Two terdentate ligands, L1 and L2, are found to be effective corrosion inhibitors in 0.10 M NaCl solution for brass. The inhibition efficiencies increase with increasing concentration of the inhibitor. The highest inhibition efficiency was obtained for the same concentration (200 ppm) of the ligands. The L1 gives a little bit higher inhibition efficiency than the L2, since L2 has no –C@N-bond, which may impart better adsorptivity to the brass.These inhibitors are adsorbed on the brass surface according to Langmuir isotherm. Inhibitor molecules can form very stable chemisorbed film on brass surface, so self-assembled films can be formed. They act as a barrier film and protect brass substrate against corrosion in chloride solution. A modification with BTA improves dramatically the coverage and possibly inhibition effect of self-assembled film of these ligands by the complexation with Zn(II) and Cu(II) cations on the surface of brass.
References [1] H. Shokry, M. Yuasa, I. Sekine, R.M. Issa, H.Y. El-Baradie, G.K. Gomma, Corros. Sci. 40 (1988) 2173. [2] G.K. Gomma, M.H. Wahdan, Mater. Chem. Phys. 39 (1995) 209. [3] S. Li, S. Chen, S. Lei, H. Ma, R. Yu, D. Liu, Corros. Sci. 41 (1999) 1273. [4] M. Duprat, N. Bui, F. Dabost, Corrosion 35 (1979) 392. [5] J.M. Costa, J.M.L. Luch, Corros. Sci. 24 (1984) 929. [6] B. Dus, Z.S. Smialowska, Corrosion 28 (1972) 105. [7] M. Agaal, A.S. Mideen, M.A. Qurasihi, Corros. Sci. 36 (1994) 79. [8] F. El-Taib Heakal, S. Haruyama, Corros. Sci. 20 (1980) 887. [9] M. Metikos-Hukovic, Z. Grubac, E. Stupnisk-Lisac, Corros. Sci. 50 (1994) 2. [10] U. Upke, U. Ibok, B. Ita, O. Offiong, E. Ebenso, Mater. Chem. Phys. 40 (1995) 87. [11] A.B. Tadros, A.B. Adbennabey, J. Electroanal. Chem. 24 (1988) 433. [12] A. Aouniti, B. Hammouti, M. Brighli, S. Kertit, F. Berhili, S. El Kadiri, A. Ramdani, J. Chim. Phys. 93 (1996) 1262. [13] S. Kertit, B. Hammouti, Appl. Surf. Sci. 93 (1996) 59. [14] M. Bouayed, H. Rabaa, A. Schiri, J.Y. Saillard, A. Ben Bachir, A. Le Beuze, Corros. Sci. 41 (1999) 501.
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X.L. Cheng, H.Y. Ma, S.H. Chen, R. Ye, X. Chen, Z.M. Yao, Corros. Sci. 41 (1999) 321. S.N. Raicheva, B.V. Aleksiev, E.I. Sokolova, Corros. Sci. 34 (1993) 343. D.P. Schweinsberg, V. Ashworth, Corros. Sci. 28 (1988) 539. Y.K. Agrawal, J.D. Talati, M.D. Shah, M.N. Desai, N.K. Shah, Corros. Sci. 46 (2004) 633. T. Ho¨kelek, Z. Kılıc¸, M. Isßıklan, M. Toy, J. Mol. Struct. 523 (2000) 61. M.I. Ayad, S.A. Sallam, H.E. Mabrouk, Thermochim. Acta 189 (1991) 65. R. Srinivasan, I. Sougandi, R. Venkatesan, P. Sambasiva Rao, Proc. Indian Acad. Sci. (Chem. Sci.), 115 (2003) 91. V. Brusic, M.A. Frisch, B.N. Eldridge, F.P. Novak, F.B. Kaufman, B.M. Rusch, G.S. Frankel, J. Electrochem. Soc. 138 (1991) 2253. C. Tornkvist, D. Thierry, J. Bergman, B. Liedberg, C. Leygraf, J. Electrochem. Soc. 136 (1989) 58. J.J. Kester, T.E. Furtak, A.J. Bevolo, J. Electrochem. Soc. 129 (1982) 1716. A.M. Fenelon, C.B. Breslin, J. Appl. Electrochem. 31 (2001) 509. M. Kabasakalogˇlu, T. Kıyak, O. S ß endil, A. Asan, Appl. Surf. Sci. 193 (2002) 167. G. Pitt, Y. Bao, J. Thompson, M.C. Wani, H. Rosenkrantz, J. Metterville, J. Med. Chem. 29 (1986) 1231.
Corrosion inhibition of brass in presence of terdentate ligands in chloride solution A. Asan a, M. Kabasakalogˇlu b, M. Isßıklan a, Z. Kılıc¸
c,*
a
Department of Chemistry, Faculty of Science, Kırıkkale University, 71450 Kırıkkale, Turkey b Department of Chemistry, Faculty of Science, Gazi University, 06500 Ankara, Turkey Department of Chemistry, Faculty of Science, Ankara University, 06100 Tandogˇan, Ankara, Turkey
c
Received 27 February 2004; accepted 21 July 2004 Available online 23 September 2004
Abstract The inhibition of two terdentate ligands, 2-[(E)-pyridin-2-ylimino)methyl)]phenol and 2[(pyridin-2-ylamino)methyl]phenol, abbreviated L1 and L2 respectively, on the corrosion of brass in 0.10 M NaCl solution under various conditions, has been studied by means of the potentiostatic polarization and AC impedance methods. The studies show that terdentate ligands, L1 and L2 inhibit the corrosion of brass in chloride solution and that the inhibiting efficiency increases with an increase in their concentrations. Self-assembled films of these substances were also prepared on the brass surface. These films improved significantly the protecting ability of brass surface to corrosion in 0.10 M NaCl solution. When the films were modified with benzotriazole (BTA), the quality and corrosion resistance of films improved markedly. Ó 2004 Elsevier Ltd. All rights reserved. Keywords: A. Brass; B. Potentiostatic polarization; C. Neutral inhibition; D. Terdentate ligands
*
Corresponding author. Tel.: +90 312 2126720; fax: +90 312 2232395. E-mail address: [email protected] (Z. Kılıc¸).
0010-938X/$ - see front matter Ó 2004 Elsevier Ltd. All rights reserved. doi:10.1016/j.corsci.2004.07.031
A. Asan et al. / Corrosion Science 47 (2005) 1534–1544
1535
1. Introduction Due to the various industrial applications and economic importance of brass its protection against corrosion attracted much attention. However it corrodes easily in chloride containing aqueous solutions and air, which limits its use. One of the most important methods in corrosion protection is to use inhibitors. Some Schiff bases have recently been reported as effective corrosion inhibitors for mild steel, aluminium and copper [1–3]. Most organic substances employed as copper corrosion inhibitors protect the metal by forming a chelate on the metal surfaces [4]. The effectiveness of an organic substance as an inhibitor depends on the structure of the inhibitor [5] and the stability of the chelate formed on the metal surface [6]. The choice of the inhibitor is based on two considerations. First, it can be synthesized conveniently from relatively cheap raw materials. Second, the presence of an electron cloud on the aromatic ring, the electronegative sulphur, nitrogen, oxygen atoms and the relatively long chain compounds to induce greater adsorption on the metal surface promoting effective inhibition [7]. Generally, a strong coordination bond causes higher inhibition efficiency, the inhibition increases in the sequence O < N < S < P [8–17]. The inhibition of corrosion by Schiff bases can be attributed to its molecules with p-electrons of –C@N-groups and p-electrons of aromatic ring. Conjugating large p bond through which its molecules are likely to be adsorbed strongly on the metal surface [18]. The aim of this study is to investigate the corrosion of brass in 0.10 M NaCl in the presence of two terdentate ligands, L1 and L2 (Fig. 1), and to observe any relationship between the structure of these ligands and their inhibitive action under various conditions. L1 is a Schiff base and L2 is a secondary amine. The difference between them is the p-electrons between carbon and nitrogen imine bond. L1 may be a potential bidentate ligand for Cu(II) and Zn(II) cations, as to be similar Ar– CH@N-pyridine (Ar: salicyl, naphthyl, etc.) ligands [19,20]. L2 may also be a potential terdentate ligand for various transition metal cations [21]. Two electrochemical techniques potentiostatic polarization and impedance have been used to study the effect of addition of these compounds on the corrosion of brass in 0.10 M NaCl. Organic inhibitors normally form very thin and persistent chemisorbed films that lead to a remarkable decrease in the corrosion rate due to the slowing down of anodic, cathodic reaction or both. In particular, it is known that benzotriazole, BTA, its derivatives and other organic compounds offer a good protection against corrosion in different aggressive environments [22–26]. We think that the same protection against corrosion is valid for brass in the presence of L1 and L2 ligands. As it is known, in the case of L1 and L2, the presence of –OH group in the ortho positions facilitate the formation of a chelate with a six membered ring [19].
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2. Experimental 2.1. Electrodes For polarization and electrochemical impedance studies, the thick brass sheet with a composition of Cu 70% and Zn 30% was embedded in epoxy resin, to expose a geometrical surface area of 1 cm2 to electrolyte. The electrode was polished to mirror finish, using fine grain emery paper of 1200 grade under water flow, washed with distilled water and acetone and dried prior to the experiments. Electrochemical experiments were carried out in pyrex cell with three compartments. A saturated calomel electrode, SCE, was used as the reference electrode and platinum sheet as the counter electrode. 2.2. Inhibitors The formulae of the substances tested as brass corrosion inhibitors are depicted in Fig. 1. The compounds, L1 and L2, have been synthesized according to the literature [27]. 2-[(E)-pyridin-2-ylimino)methyl)]phenol, L1, mp. 66 °C, lit. [27] mp. 65–66 °C, FTIR (KBr): 3050 cm1 (mCH arom.), 1609 cm1 (mC@N), 1588 cm1 (mC@C). 2[(pyridin-2-ylamino)methyl]phenol, L2, mp. 106 °C, lit. [27] mp. 106–108 °C, FTIR (KBr): 3329 cm1 (mNH), 3057 cm1 (mCH arom.), 1604 cm1 (mC@C). The solutions were prepared with analytical grade and chemically pure reagents using triple-distilled water. 2.3. The formation of the surface film The working electrode was mechanically polished to mirror finish, using fine grain emery paper of 1200 grade under water flow, washed with double-distilled water, and then etched in a 6.0 M HNO3 solution for 30 s, rinsed with double-distilled water and followed by rinsing with absolute ethanol. The cleaned electrode was immersed in 5.0 mM solutions of the ligands in ethanol for 30 min to form self-assembled film. After film formation on surface, electrodes were taken out, rinsed with absolute ethanol and double-distilled water, respectively. Tafel plots and Nyquistic diagram were obtained in 0.10 M NaCl solution. N
N CH2 HN
CH N OH
OH
L1
L2
Fig. 1. Molecular structures of the investigated terdentate ligands.
A. Asan et al. / Corrosion Science 47 (2005) 1534–1544
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Surface modified brass electrode with self-assembled film prepared by immersion into the solution of the ligands in ethanol and then immersed in 1.0 mM benzotriazole solution for 30 min. Tafel plots and AC impedance measurements were obtained in 0.10 M NaCl solutions with the surface modified brass electrode prepared as mentioned above. 2.4. AC impedance and polarization measurements The potentiostatic polarization and impedance measurements were obtained on a Volta Lab PGZ 301 system complete with Pentium II PC and Volta Lab 4.0 software. The impedance measurements were carried out in various concentrations of L1 and L2 ligands at the respective corrosion potentials with a sinusoidal potential perturbation of 5.0 mV. The preconditioning time for all the impedance measurements were 30 s and the frequency range between 100 kHz and 20 mHz. The polarization measurements were also obtained in 0.10 M NaCl + x ML1 or x ML2 solutions in the potential range between 0.2 V and +0.1 V, using sweep rate of 2.0 mV s1.
3. Results and discussion 3.1. AC impedance results Impedance spectra for brass in 0.10 M NaCl, in the absence and in the presence of various concentrations of L1 and L2 were depicted in Figs. 2 and 3, respectively. In
2.5
2 Zr (kohm.cm )
2.0
200
1.5
10
0
50
1.0
20 0.5 Bare
1
2
4
3
5
6
2
Zr (kohm.cm ) Fig. 2. Impedance plot obtained in 0.10 M NaCl in various concentrations of L1; 0, 20, 50, 100, 200 ppm.
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A. Asan et al. / Corrosion Science 47 (2005) 1534–1544 2.5
1.5
20
0
1.0
10
Zr(kohm.cm2)
2.0
Bare
1
20
0.5
0
50
2
4
3
5
6
2
Zr (kohm.cm ) Fig. 3. Impedance plot obtained in 0.10 M NaCl in various concentrations of L2; 0, 20, 50, 100, 200 ppm.
general, these circuits fall into the classical parallel capacitor–resistor combination with the series resistance is being that of the bulk solution. As can be seen from Figs. 2 and 3, the impedance diagrams show perfect semi-circles indicating a barrier layer formed on the surface and a charge transfer process mainly controlling the corrosion of brass. In fact the presence of L1 and L2 ligands enhance the value of charge transfer resistance in chloride solution. Results can be seen in Table 1. L1 and L2 with a concentration of 200 ppm have the highest protection efficiency.
Table 1 The impedance, Tafel polarization and inhibition efficiency values for the corrosion of brass in 0.10 M NaCl Inhibitors
C (ppm)
Rp (kohm)
g(Rp)%
Ecor
Icor (lA/cm2)
g%
Average g%
L1
0 20 50 100 200 Coated Modified
0.8 2.4 3.1 5 6.1 12.2 16
67 74 84 86 93 95
224 216 209 208 190 160 149
9.12 3.11 2.42 1.46 1.13 0.62 0.29
66 73 84 88 93 97
66.5 73.5 84 87 93 96
20 50 100 200 Coated Modified
1.9 2.7 4.6 5.2 10.5 12.6
58 70 78 85 92 94
218 215 192 195 158 150
3.53 2.65 1.75 1.30 0.75 0.47
61 71 80 86 92 95
59.5 70.5 79 85.5 92 94.5
L2
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10.0
6.0
Co ve
M
red
od
ifi ed
Zr(kohm.cm2)
8.0
4.0
200
2.0
8
6
4
2
10
12
14
16
2
Zr (kohm.cm ) Fig. 4. (1) Impedance curve obtained with bare electrode in 0.10 M NaCl solution containing 200 ppm of L1. (2) Impedance curve obtained in 0.10 M NaCl solution electrode surface is covered with adsorbed L1 film. (3) Impedance curve obtained in 0.10 M NaCl solution with surface modified electrode with L1 and with BTA.
8.0 6.0 4.0
ed
20 0 2
4
6
8
10
ied
r ve Co
2.0
dif Mo
2 -Zi (kohm.cm )
10.0
12
14
16
Zr (kohm.cm2) Fig. 5. (1) Impedance curve obtained with bare electrode in 0.10 M NaCl solution containing 200 ppm of L2. (2) Impedance curve obtained in 0.10 M NaCl solution electrode surface is covered with adsorbed L2 film. (3) Impedance curve obtained in 0.10 M NaCl solution with surface modified electrode with L2 and with BTA.
As can be seen from Figs. 4 and 5 the polarization resistance in 0.10 M NaCl solution increase for covered and modified brass electrodes. Modification of brass surface with L1 and L2 and then BTA enhances protection efficiency (Table 1). Same conclusion can be drawn from Tafel plots.
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The impedance, Tafel polarization and inhibition efficiency values for the corrosion of brass in 0.10 M NaCl solution were listed in Table 1. The protection efficiency was calculated using the following expression, where Rp and R0p denote polarization resistance of electrode with and without inhibitor, respectively, evaluated from AC impedance curves. g¼
Rp R0p Rp
3.2. Polarization curves Figs. 6 and 7 show the anodic and cathodic polarization curves of the brass electrode obtained with a scan rate of 2.0 mV s1 in 0.10 M NaCl solutions with and without addition of L1 and L2 at different concentrations, respectively. It is clear that
Log i, µAcm-2
3 2 1 0 -1 -2
-300
-400
-200
0
-100
Potential, mV (SCE) Fig. 6. Polarization curves for brass in 0.1 M NaCl without and in the presence of different concentrations of L1; ( ) 0.1 M NaCl + 0; ( ) 20 ppm; ( ) 50 ppm; ( ) 100 ppm; ( ) 200 ppm.
Log i, µAcm-2
3 2 1 0 -1 -2
-400
-300
-200
-100
0
Potential, mV (SCE) Fig. 7. Polarization curves for brass in 0.1 M NaCl without and in the presence of different concentrations of L2; ( ) 0.1 M NaCl + 0; ( ) 20 ppm; ( ) 50 ppm; ( ) 100 ppm; ( ) 200 ppm.
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corrosion rates decrease with the increasing inhibitor concentration. Both of the inhibitors increase the corrosion potential slightly, but decrease anodic and cathodic currents, markedly. The corrosion current densities were determined by extrapolation of the Tafel lines to corrosion potential. The highest protection efficiency was exhibited for 200 ppm of inhibitor for each substance. The protection efficiency was estimated using the following expression, where icorr(inh) and icorr current density with and without inhibitor, respectively. gð%Þ ¼
icorr icorr ðinh:Þ 100 icorr
After adsorption of L1 and L2 on bare brass surface from alcohol solution, the corrosion potential of brass in 0.10 M NaCl solution shifted to 160 mV and 158 mV very close to each other and corrosion currents decrease to 0.62 and 0.75 lA/cm2, respectively. BTA adsorption, after electrodes were covered with adsorbed film of L1 or L2 ligands, increases corrosion potential to 149 mV and 150 mV, respectively. On the other hand, protection efficiency also rises to 97% for L1 and 95% for L2, respectively. Figs. 7 and 8 show polarization curves of brass electrode coated with L1 and L2 adsorption film, and then their adsorption layer modified with BTA obtained in 0.10 M NaCl solution (Fig. 9). 3.3. Adsorption isotherms It is generally assumed that the adsorption of inhibitors on the metal surface is the essential step in the mechanism of inhibition. Fig. 10a and b show the relationships between surface coverage and concentration of inhibitors. The adsorption of L1 and L2 ligands in 5.0 mM solution of ethanol were found to be governed by Langmuir isotherms. C versus C/h plot gives a straight line with a slope of unity. The surface coverage values, h, were calculated from the polarization resistance values and icorr values which gave the same results.
Log i, µAcm-2
3 2 1 0 Modified Covered 200ppm
-1 -2 -400
-300
-200
-100
0
Potential, mV (SCE) Fig. 8. Polarization curves obtained in 0.10 M NaCl for 200 ppm L1 Schiff base, film covered brass electrode and after film modification with BTA.
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Log i, µAcm-2
3 2 1 0
200ppm Covered
-1
Modified
-2 -400
-300
0
-100
-200
Potential, mV (SCE) Fig. 9. Polarization curves obtained in 0.10 M NaCl for 200 ppm L2 ligand, film covered brass electrode and after film modification with BTA.
250
y = 1.0995x + 10.055 2 R = 0.9994
C/θ
200 150 100 50 0 0
50
250
150
200
250
y = 1.0479x + 16.825 2 R = 0.9975
200
C/θ
100
C, ppm
(a)
150 100 50 0 0
(b)
50
100
150
200
250
C, ppm
Fig. 10. Langmuir adsorption isotherms (C vs. C/h) of L1 (a) and L2 (b).
The type of adsorption, when good corrosion inhibition occurs, is found to obey the Langmuir isotherm. That is derived by assuming that the inhibitor forms a monomolecular layer on the surface. h¼
KC 1 þ KC
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or its arranged form C 1 ¼ þC h K where K is the equilibrium constant of adsorption. From the graphs equilibrium constants K are calculated for L1 and L2 as 0.099 and 0.059 respectively. However, there is a little difference between these values. The L1 adsorption that has –C@N-bond is more favourable than the L2 adsorption on the surface of electrode. Chemisorptions involve the share or transfer of charge from the ligands to the surface to form coordinated covalent bond. Since these terdentate ligands are very suitable to form four or six coordinated complexes with Zn(II) and Cu(II) [19,21].
4. Conclusion Two terdentate ligands, L1 and L2, are found to be effective corrosion inhibitors in 0.10 M NaCl solution for brass. The inhibition efficiencies increase with increasing concentration of the inhibitor. The highest inhibition efficiency was obtained for the same concentration (200 ppm) of the ligands. The L1 gives a little bit higher inhibition efficiency than the L2, since L2 has no –C@N-bond, which may impart better adsorptivity to the brass.These inhibitors are adsorbed on the brass surface according to Langmuir isotherm. Inhibitor molecules can form very stable chemisorbed film on brass surface, so self-assembled films can be formed. They act as a barrier film and protect brass substrate against corrosion in chloride solution. A modification with BTA improves dramatically the coverage and possibly inhibition effect of self-assembled film of these ligands by the complexation with Zn(II) and Cu(II) cations on the surface of brass.
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