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Corrosion inhibition of steel in 0.5 M H2SO4 by [(2-pyridin-4-ylethyl)thio]acetic acid
Applied Surface Science 250 (2005) 50–56 www.elsevier.com/locate/apsusc
Corrosion inhibition of steel in 0.5 M H2SO4 by [(2-pyridin-4-ylethyl)thio]acetic acid M. Bouklah a, A. Ouassini b, B. Hammouti a,*, A. El Idrissi b a
Laboratoire de Chimie des Eaux et Corrosion, Faculte´ des Sciences, Oujda, Morocco Laboratoire de Chimie Organique Physique, Faculte´ des Sciences, Oujda, Morocco
b
Received 23 November 2004; received in revised form 11 December 2004; accepted 12 December 2004 Available online 16 February 2005
Abstract The influence of [(2-pyridin-4-ylethyl)thio]acetic acid (P1) and pyridine (P2) on the corrosion inhibition of steel in sulphuric acid solution is studied using weight loss, potentiodynamic polarisation and linear polarisation resistance (Rp) and electrochemical impedance spectroscopy (EIS) measurements. Results obtained show that P1 is the best inhibitor and its inhibition efficiency (E%) increases with the increase of concentration. The highest E% of 82% is observed at 5 103 M. Potentiodynamic polarisation studies clearly reveal that P1 acts as a mixed inhibitor. The inhibitor studied reduces the corrosion rates. E% values obtained from various methods used are in good agreement. Adsorption of P1 on steel surface obeys to Langmuir adsorption isotherm. Effect of temperature indicates that E% decreases with temperature between 298 and 353 K. # 2004 Elsevier B.V. All rights reserved. Keywords: Steel; Pyridine; Inhibition; Corrosion; Acid; Langmuir
1. Introduction Inhibitors are often added to chemical cleaning and pickling processes in industrial processes to remove corrosion scales from metallic surface in concentrated acidic media at elevated temperature. This procedure is quietly required to protect metals against acid attack. Inhibitors must be stable and effective even under severe conditions in hot concentrated acid. Efforts are made to synthesis novel molecules * Corresponding author. Tel.: +212 56 500 602; fax: +212 56 500 603. E-mail address: [email protected] (B. Hammouti).
containing various structures. The most synthesised compounds called nitrogen-heterocyclic compounds are known to be excellent inhibitors for metallic against corrosion [1–7]. Electronegative functional groups and p-electron in triple or conjugated double bonds as well as heteroatoms like sulphur, phosphorus, nitrogen and oxygen in their structure are the major adsorption centres. Survey of literature reveals that pyridine [8–14] and S-containing [15–20] compounds are also effective corrosion inhibitors. The purpose of this investigation, we aim to study the effect of [(2-pyridin-4-ylethyl)thio]acetic acid (P1) and pyridine (P2) on the corrosion of steel in
0169-4332/$ – see front matter # 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.apsusc.2004.12.021
M. Bouklah et al. / Applied Surface Science 250 (2005) 50–56
0.5 M H2SO4. The behaviour of steel in 0.5 M H2SO4 with and without inhibitor is studied using gravimetric, potentiodynamic polarisation, linear polarisation resistance (Rp) and electrochemical impedance spectroscopy (EIS) measurements.
2. Experimental details [(2-Pyridin-4-ylethyl)thio]acetic acid (P1) is synthesised, purified and characterised by NMR, IR spectroscopies and element analysis before use. Pyridine (P2) is a commercial compound. The molecular structures of the studied compounds are shown in Fig. 1. Prior to all measurements, the steel samples (0.09%P, 0.38%Si, 0.01%Al, 0.05%Mn, 0.21%C, 0.05%S and the remainder iron) are abraded with a series of emery paper from 400 to 1200 grade. The specimens are washed thoroughly with bidistilled water degreased and dried with acetone. The aggressive solution (0.5 M H2SO4) is prepared by dilution of analytical grade 98% H2SO4 with double distilled water. Gravimetric measurements are carried out in a double walled glass cell equipped with a thermostated cooling condenser. The solution volume is 60 cm3. The steel specimens used have a rectangular form (2 cm 2 cm 0.05 cm). The immersion time for the weight loss is 6 h at 298 K. Electrochemical measurements are carried out in a conventional three-electrode electrolysis cylindrical Pyrex glass cell. The working electrode (WE) had the form of a disc cut form steel sheet. The exposed area to the corrosive solution is 1 cm2. A saturated calomel electrode (SCE) and a platinum electrode are used, respectively as reference and auxiliary electrodes. Running on an IBM compatible personal computer, the 352 Soft CorrTM III Software communicates with
51
EG&G Instruments potentiostat-galvanostat model 263A at a scan rate of 20 mV/min. Before recording the polarisation curves, WE is first immersed into the test solution for 30 min to attain its free corrosion potential. The steel electrode is pre-polarized at 800 mV for 10 min. The polarisation curves are obtained from 800 mV to more positive values. The test solution is de-aerated with pure nitrogen. Gas bubbling is maintained through the experiments. All potentials are given in the SCE scale. The cell is thermostated at 298 0.5 K. Electrochemical impedance spectroscopy (EIS) was carried out with a Tacussel electrochemical system at Ecorr after immersion in solution without bubbling, the circular surface of steel exposing of 1 cm2 to the solution were used as working electrode. After the determination of steady-state current at a given potential, sine wave voltage (10 mV) peak to peak, at frequencies between 100 kHz and 10 mHz were superimposed on the rest potential. Computer programs automatically controlled the measurements performed at rest potentials after 30 min of exposure. The impedance diagrams are given in the Nyquist representation. In the case of the weight loss method, the relation determines the inhibition efficiency (EW%): W corr EW % ¼ 1 0 100 Wcorr 0 are the corrosion rates of steel with and Wcorr and Wcorr without of the inhibitor, respectively. For electrochemical measurements, the inhibition efficiency is calculated as follows: I corr EI % ¼ 1 0 100 Icorr 0 are the corrosion current density values Icorr and Icorr with and without inhibitor, respectively, determined by extrapolation of cathodic Tafel lines to the corrosion potential.
3. Results and discussion 3.1. Gravimetric measurements
Fig. 1. Molecular structures of the compounds studied.
The effect of addition of the tested compound P1 and P2 at different concentrations on the corrosion of
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M. Bouklah et al. / Applied Surface Science 250 (2005) 50–56
Table 1 Steel weight loss data and corresponding inhibition efficiency of P1 and P2 at 298 K Concentration (M)
Wcorr (mmpy)
Blank
17.91
EW% –
P1 5 103 2.5 103 103 5 104 2.5 104 104
3.28 4.41 5.98 7.50 9.43 11.99
82 75 66 58 47 33
P2 5 103 2.5 103 103 5 104 2.5 104 104
14.80 15.38 15.50 16.52 16.75 17.37
17 14 13 8 6 3
steel in 0.5 M H2SO4 is studied by weight loss at 298 K after 6 h of immersion period. Table 1 regroups the corresponding values of corrosion rates of steel and inhibition efficiency. It is clear that the presence of P1 reduces the corrosion rate and consequently the attack is inhibited. By consequent one notices that P2 has a slight action in sulphuric medium. This weak inhibition is also observed recently in HCl media [8]. This phenomenon
Fig. 2. Corrosion rates in mmpy of steel according to 5 103 M of P1 and P2.
may be explained by the substitution of hydrogen atom by the alkyl chain containing S atom and the acid function. Coordinate covalent bond formation between electron pairs of S atom and metal surface can take place [21]. The inhibition efficiency increases with P1 concentration. Fig. 2 illustrates the comparison of the corrosion rate in free acid and in the presence of 5 103 M of P1 and P2. Then we may conclude that P1 is the best inhibitor and a detailed study is then conducted. 3.2. Polarisation measurements Fig. 3 shows the polarisation curves for the steel electrode in 0.5 M H2SO4 in the absence and
Fig. 3. Polarisation curves of steel in 0.5 M H2SO4 at various concentrations of P1.
M. Bouklah et al. / Applied Surface Science 250 (2005) 50–56
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Table 2 Electrochemical parameters of steel in 0.5 M H2SO4 at various concentrations of P1 P1 (M)
Ecorr (mV/SCE)
bc (mV/dec)
Icorr (mA cm2)
Rp (V cm2)
EI (%)
ER (%)
Blank 104 2.5 104 103 2.5 103
463 479 475 472 447
184 185 186 190 192
542 390 314 173 124
31 48 60 79 141
– 28 42 68 77
– 35 49 61 78
presence of different concentrations of P1. Values of associated electrochemical parameters and corresponding inhibition efficiencies (E%) are given in Table 2. It is clear that the addition of the molecule studied decreases the cathodic current density. Cathodic portions rise to Tafel lines indicating that the hydrogen evolution reaction is activation controlled. Parallel Tafel lines obtained suggest that the presence of compounds tested does not modify the mechanism of the hydrogen reduction. In the anodic domain, a decrease of the anodic current density is observed. The free corrosion potential determined after 30 min of immersion remains almost constant in the presence of the inhibitor. E% values increase with the P1 concentration to attain 77% at 2.5 103 M. P1 acts as a mixed inhibitor. For comparative purposes the linear polarisation resistance (RP) are calculated from the linear I–E plots at 10 mV in the vicinity of Ecorr. The corresponding RP values of steel in 0.5 M H2SO4 in the presence and absence of different concentrations of the inhibitor are
also given in Table 2. The inhibition efficiency ER% is calculated using the following equation: Rp ER ð%Þ ¼ 1 0 100 Rp R0 P and RP are the polarisation resistance with and without the inhibitor, respectively. We remark that RP increases with increasing of P1 concentration. E% increases with the concentration to reach 78% at 2.5 103 M. 3.3. EIS measurements Corrosion behaviour of steel, in acidic solution with and without pyridine compound at different concentrations after immersion for 30 min is investigated by electrochemical impedance spectroscopy (EIS) measurements at 298 K. The corresponding Nyquist diagram are shown in Fig. 4. The chargetransfer resistance (Rt) values are calculated from the difference in impedance at lower and higher frequencies, as suggested by Tsuru et al. [22]. The
Fig. 4. Nyquist diagrams of steel in acid at various concentrations of P1.
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M. Bouklah et al. / Applied Surface Science 250 (2005) 50–56
Table 3 Impedance parameters of steel in acid at various concentrations of P1 Concentration (M)
Rs (V cm2)
Rt (V cm2)
fmax (Hz)
Cdl (mF/cm2)
E%
Blank 104 2.5 104 103 2.5 103
4.86 2.92 2.74 2.97 2.94
30 44 51 58 81
51.50 44.76 35.71 35.31 22.63
103 80.80 88.09 77.70 86.89
– 32 41 48 63
double layer capacitance (Cdl) and the frequency at which the imaginary component of the impedance is maximal (Zmax) are found as represented in equation: 1 C dl ¼ wherew ¼ 2pf max vRt The impedance parameters derived from these investigations are mentioned in Table 3. The inhibition efficiency got from the charge transfer resistance is calculated by: ! Rt Eð%Þ ¼ 1 100 Rt=inh Rt/inh and Rt are the charge transfer-resistance values with and without inhibitor, respectively. As we notice in Fig. 4, the impedance diagrams show perfect semi-circles indicating a charge transfer process mainly controlling the corrosion of steel. In fact, the presence of P1 enhances the value of the transfer resistance in acidic solution. EIS study shows that the pyridine tested is an efficient inhibitor. 3.4. Effect of temperature Temperature can affect the steel corrosion in the acidic media in the presence and absence of inhibitor. To determine the action energy of the corrosion process, gravimetric measurements are taken at various temperatures (298–353 K) in the presence and absence of 103 M of P1 at 1 h of immersion. The corresponding results are given in Table 4. From these results, we can deduce that the corrosion rate increases with the rise of temperature, but in the presence of P1, the dissolution of steel is less pronounced. The inhibitive efficiency of the inhibitor decreases with the rise of temperature to fall to 25% at 353 K.
Fig. 5 shows Arrhenius plots for steel corrosion rate for both blank and inhibitor solution. The activation energies can be expressed by the Arrhenius equation: logðWÞ ¼
Ea þA RT
T is the absolute temperature, A is a constant and R is universal gas constant. Ea = 60.17 kJ/mol E0 a = 77.42 kJ/mol are the activation energies in the absence and presence of P1, respectively. It is evident that the presence of inhibitor causes increases the activation energy. E% depends on temperature. This may indicate the physical nature of adsorption mechanism [23]. 3.5. Adsorption isotherm The dependence of the ratio of the concentration and the fraction of the surface covered C/u as function of the concentration of P1, where u is the ratio E/100, is shown in Fig. 6. The obtained plot is linear with a slope equal to 1.16 near to unity showing an adsorption on the steel surface electrode according to the Langmuir isotherm: DG0ads C 1 1 ¼Cþ with K ¼ exp u K 55:55 RT
Table 4 Effect of temperature on the corrosion of steel with and without 5 103 M of P1 at 1 h Temperature (K)
0 Wcorr (mmpy)
Wcorr (mmpy)
E%
298 303 313 323 333 343 353
8.30 14.38 37.95 87.40 154.16 256.17 420.12
2.15 4.88 15.55 39.33 80.16 163.95 315.09
74 66 59 55 48 36 25
M. Bouklah et al. / Applied Surface Science 250 (2005) 50–56
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Fig. 5. Arrhenius plots of steel in acid with and without 103 M P1.
K is the equilibrium constant leading to DG0ads is a standard free energy of adsorption. Value of DG0ads obtained is 30.34 kJ/mol for P1. The value of DG0ads is too negative indicating that P1 is strongly adsorbed on the steel surface [24,25]. Comparison of the inhibition efficiencies obtained by the P1 reveal that we assist really to an intramolecular synergistic effect. The mode of adsorption of P1 suggested is illustrated in Fig. 7. The action inhibitory may be attributed to the presence of S and two O atoms in the linear chain which humbly reinforces the adsorption of pyridine ring containing N atom and p
electrons. These findings are confirmed by the calculation of the total partial atomic charges in P1 molecule are calculated by generating all valence bond (resonance) structures for this system and then weighting them on the basis of p-orbital electronegativities and formal considerations (PEPE: partial equalization of pelectronegativity) [26]. The negative charges of N, S, O(1) and O(2) shown in Table 5 indicate that these atoms are the centres of the adsorption phenomenon. In comparison the total partial atomic charge of the nitrogen of P2 is 0.3444(e), but the efficiency obtained at 5 103 M does not exceed more than 17%. This
Fig. 6. Langmuir isotherm adsorption model of P1 on the surface of steel in 0.5 M H2SO4.
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M. Bouklah et al. / Applied Surface Science 250 (2005) 50–56
Fig. 7. Skeletal representation of the mode of adsorption of P1. Table 5 Calculation of the total charges in P1 molecule calculated by PETRA programa
Total charges (e)
O(1)
O(2)
N
S
0.3828
0.2336
0.3445
0.1504
a
The PETRA program is available for free on-line at this address: http://www2.chemie.uni-erlangen.de/software/petra/manual/.
fact indicates that the presence of the chain containing S and carboxylic group in P1 introduces the synergistic effect and then E% jumps at 82%.
4. Conclusion [(2-Pyridin-4-ylethyl)thio]acetic acid (P1) is a good inhibitor for steel in 0.5 M H2SO4. The inhibition efficiency of P1 attains a maximum value of about 82% at 5 103 M. It acts as a mixed inhibitor without modifying the mechanism of hydrogen evolution. The inhibition efficiency of P1 decreases with the temperature. P1 adsorbs according to Langmuir adsorption isotherm. Adsorption of P1 is explained by an intramolecular synergistic effect of pyridine and S and O chain containing. References [1] A. Elouafi, B. Hammouti, H. Oudda, S. Kertit, R. Touzani, A. Ramdani, Anti-Corros. Meth. Mater. 49 (2002) 199.
[2] A. Dafali, B. Hammouti, R. Touzani, S. Kertit, A. Ramdani, K. Elkacemi, Anti-Corros. Meth. Mater. 49 (2002) 96. [3] R. Salghi, L. Bazzi, B. Hammouti, E. Zine, S. Kertit, S. El Issami, E. Ait Eddi, Bull. Electrochem. 17 (2001) 429. [4] F. Touhami, A. Aouniti, Y. Abed, B. Hammouti, S. Kertit, A. Ramdani, K. Elkacemi, Corros. Sci. 42 (2000) 929. [5] F. Touhami, A. Aouniti, Y. Abed, B. Hammouti, S. Kertit, A. Ramdani, Bull. Electrochem. 16 (2000) 245. [6] R. Salghi, L. Bazzi, B. Hammouti, S. Kertit, Bull. Electrochem. 16 (2000) 272. [7] F. Touhami, S. Kertit, B. Hammouti, A. Aouniti, Ann. Chim. Sci. Mater. 24 (1999) 581. [8] M. Bouklah, A. Attayibat, B. Hammouti, A. Ramdani, S. Radi, M. Benkaddour, Appl. Surf. Sci. 240 (2005) 341. [9] A. Aouniti, B. Hammouti, S. Kertit, Bull. Electrochem. 14 (1998) 193. [10] K.M. El-Sobki, H. Abbas, React. Solids 5 (1988) 191. [11] G. Subramaniam, K. Balasubramanian, P. Sridhar, Corros. Sci. 30 (1990) 1019. [12] Y. Xiao-Ci, Z. Hong, L. Ming-Dao, R. Hong-Xuan, Y. Lu-An, Corros. Sci. 42 (2000) 645. [13] M. Lashkari, M.R. Arshadi, Chem. Phys. 299 (2004) 131. [14] H. Ashassi-Sorkhabi, B. Shaabani, D. Seifzadeh, Appl. Surf. Sci. 239 (2005) 154. [15] B. Aida, A. Tadros, B.A. Abd-el-Nabey, J. Electroanal. Chem. 246 (1988) 433. [16] B.A. Abd El-Nabey, E. Khamis, G.E. Thompson, J.L. Dawson, Surf. Coat. Tech. 30 (1987) 199. [17] M.Th. Makhlouf, S.A. El-Shatory, A. El-Said, Mater. Chem. Phys. 46 (1996) 76. [18] S. Chander, G. Kar, D.W. Fuerstenau, Corros. Sci. 19 (1979) 405. [19] M. Bouklah, B. Hammouti, A. Aouniti, T. Benhadda, Prog. Org. Coat. 49 (2004) 225. [20] M. Bouklah, N. Benchat, A. Aouniti, B. Hammouti, M. benkaddour, M. Lagrene´ e, H. Vezin, F. Bentiss, Prog. Org. Coat. 51 (2004) 119. [21] A. Yurt, A. Balaban, S. Ustun Kandemir, G. Bereket, B. Erk, Mater. Chem. Phys. 85 (2004) 420. [22] T. Tsuru, S. Haruyama, B. Gijutsu, J. Jpn. Soc. Corros. Eng. 27 (1978) 573. [23] A. Popova, E. Sokolova, S. Raicheva, M. Chritov, Corros. Sci 45 (2003) 33. [24] J.D. Talati, D.K. Gandhi, Corros. Sci. 23 (1983) 1315. [25] M.M. Osman, M.N. Shalaby, Mater. Chem. Phys. 77 (2002) 261. [26] W.-D. Ihlenfeldt, J. Gasteiger, J. Comput. Chem. 8 (1994) 793.
Corrosion inhibition of steel in 0.5 M H2SO4 by [(2-pyridin-4-ylethyl)thio]acetic acid M. Bouklah a, A. Ouassini b, B. Hammouti a,*, A. El Idrissi b a
Laboratoire de Chimie des Eaux et Corrosion, Faculte´ des Sciences, Oujda, Morocco Laboratoire de Chimie Organique Physique, Faculte´ des Sciences, Oujda, Morocco
b
Received 23 November 2004; received in revised form 11 December 2004; accepted 12 December 2004 Available online 16 February 2005
Abstract The influence of [(2-pyridin-4-ylethyl)thio]acetic acid (P1) and pyridine (P2) on the corrosion inhibition of steel in sulphuric acid solution is studied using weight loss, potentiodynamic polarisation and linear polarisation resistance (Rp) and electrochemical impedance spectroscopy (EIS) measurements. Results obtained show that P1 is the best inhibitor and its inhibition efficiency (E%) increases with the increase of concentration. The highest E% of 82% is observed at 5 103 M. Potentiodynamic polarisation studies clearly reveal that P1 acts as a mixed inhibitor. The inhibitor studied reduces the corrosion rates. E% values obtained from various methods used are in good agreement. Adsorption of P1 on steel surface obeys to Langmuir adsorption isotherm. Effect of temperature indicates that E% decreases with temperature between 298 and 353 K. # 2004 Elsevier B.V. All rights reserved. Keywords: Steel; Pyridine; Inhibition; Corrosion; Acid; Langmuir
1. Introduction Inhibitors are often added to chemical cleaning and pickling processes in industrial processes to remove corrosion scales from metallic surface in concentrated acidic media at elevated temperature. This procedure is quietly required to protect metals against acid attack. Inhibitors must be stable and effective even under severe conditions in hot concentrated acid. Efforts are made to synthesis novel molecules * Corresponding author. Tel.: +212 56 500 602; fax: +212 56 500 603. E-mail address: [email protected] (B. Hammouti).
containing various structures. The most synthesised compounds called nitrogen-heterocyclic compounds are known to be excellent inhibitors for metallic against corrosion [1–7]. Electronegative functional groups and p-electron in triple or conjugated double bonds as well as heteroatoms like sulphur, phosphorus, nitrogen and oxygen in their structure are the major adsorption centres. Survey of literature reveals that pyridine [8–14] and S-containing [15–20] compounds are also effective corrosion inhibitors. The purpose of this investigation, we aim to study the effect of [(2-pyridin-4-ylethyl)thio]acetic acid (P1) and pyridine (P2) on the corrosion of steel in
0169-4332/$ – see front matter # 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.apsusc.2004.12.021
M. Bouklah et al. / Applied Surface Science 250 (2005) 50–56
0.5 M H2SO4. The behaviour of steel in 0.5 M H2SO4 with and without inhibitor is studied using gravimetric, potentiodynamic polarisation, linear polarisation resistance (Rp) and electrochemical impedance spectroscopy (EIS) measurements.
2. Experimental details [(2-Pyridin-4-ylethyl)thio]acetic acid (P1) is synthesised, purified and characterised by NMR, IR spectroscopies and element analysis before use. Pyridine (P2) is a commercial compound. The molecular structures of the studied compounds are shown in Fig. 1. Prior to all measurements, the steel samples (0.09%P, 0.38%Si, 0.01%Al, 0.05%Mn, 0.21%C, 0.05%S and the remainder iron) are abraded with a series of emery paper from 400 to 1200 grade. The specimens are washed thoroughly with bidistilled water degreased and dried with acetone. The aggressive solution (0.5 M H2SO4) is prepared by dilution of analytical grade 98% H2SO4 with double distilled water. Gravimetric measurements are carried out in a double walled glass cell equipped with a thermostated cooling condenser. The solution volume is 60 cm3. The steel specimens used have a rectangular form (2 cm 2 cm 0.05 cm). The immersion time for the weight loss is 6 h at 298 K. Electrochemical measurements are carried out in a conventional three-electrode electrolysis cylindrical Pyrex glass cell. The working electrode (WE) had the form of a disc cut form steel sheet. The exposed area to the corrosive solution is 1 cm2. A saturated calomel electrode (SCE) and a platinum electrode are used, respectively as reference and auxiliary electrodes. Running on an IBM compatible personal computer, the 352 Soft CorrTM III Software communicates with
51
EG&G Instruments potentiostat-galvanostat model 263A at a scan rate of 20 mV/min. Before recording the polarisation curves, WE is first immersed into the test solution for 30 min to attain its free corrosion potential. The steel electrode is pre-polarized at 800 mV for 10 min. The polarisation curves are obtained from 800 mV to more positive values. The test solution is de-aerated with pure nitrogen. Gas bubbling is maintained through the experiments. All potentials are given in the SCE scale. The cell is thermostated at 298 0.5 K. Electrochemical impedance spectroscopy (EIS) was carried out with a Tacussel electrochemical system at Ecorr after immersion in solution without bubbling, the circular surface of steel exposing of 1 cm2 to the solution were used as working electrode. After the determination of steady-state current at a given potential, sine wave voltage (10 mV) peak to peak, at frequencies between 100 kHz and 10 mHz were superimposed on the rest potential. Computer programs automatically controlled the measurements performed at rest potentials after 30 min of exposure. The impedance diagrams are given in the Nyquist representation. In the case of the weight loss method, the relation determines the inhibition efficiency (EW%): W corr EW % ¼ 1 0 100 Wcorr 0 are the corrosion rates of steel with and Wcorr and Wcorr without of the inhibitor, respectively. For electrochemical measurements, the inhibition efficiency is calculated as follows: I corr EI % ¼ 1 0 100 Icorr 0 are the corrosion current density values Icorr and Icorr with and without inhibitor, respectively, determined by extrapolation of cathodic Tafel lines to the corrosion potential.
3. Results and discussion 3.1. Gravimetric measurements
Fig. 1. Molecular structures of the compounds studied.
The effect of addition of the tested compound P1 and P2 at different concentrations on the corrosion of
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M. Bouklah et al. / Applied Surface Science 250 (2005) 50–56
Table 1 Steel weight loss data and corresponding inhibition efficiency of P1 and P2 at 298 K Concentration (M)
Wcorr (mmpy)
Blank
17.91
EW% –
P1 5 103 2.5 103 103 5 104 2.5 104 104
3.28 4.41 5.98 7.50 9.43 11.99
82 75 66 58 47 33
P2 5 103 2.5 103 103 5 104 2.5 104 104
14.80 15.38 15.50 16.52 16.75 17.37
17 14 13 8 6 3
steel in 0.5 M H2SO4 is studied by weight loss at 298 K after 6 h of immersion period. Table 1 regroups the corresponding values of corrosion rates of steel and inhibition efficiency. It is clear that the presence of P1 reduces the corrosion rate and consequently the attack is inhibited. By consequent one notices that P2 has a slight action in sulphuric medium. This weak inhibition is also observed recently in HCl media [8]. This phenomenon
Fig. 2. Corrosion rates in mmpy of steel according to 5 103 M of P1 and P2.
may be explained by the substitution of hydrogen atom by the alkyl chain containing S atom and the acid function. Coordinate covalent bond formation between electron pairs of S atom and metal surface can take place [21]. The inhibition efficiency increases with P1 concentration. Fig. 2 illustrates the comparison of the corrosion rate in free acid and in the presence of 5 103 M of P1 and P2. Then we may conclude that P1 is the best inhibitor and a detailed study is then conducted. 3.2. Polarisation measurements Fig. 3 shows the polarisation curves for the steel electrode in 0.5 M H2SO4 in the absence and
Fig. 3. Polarisation curves of steel in 0.5 M H2SO4 at various concentrations of P1.
M. Bouklah et al. / Applied Surface Science 250 (2005) 50–56
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Table 2 Electrochemical parameters of steel in 0.5 M H2SO4 at various concentrations of P1 P1 (M)
Ecorr (mV/SCE)
bc (mV/dec)
Icorr (mA cm2)
Rp (V cm2)
EI (%)
ER (%)
Blank 104 2.5 104 103 2.5 103
463 479 475 472 447
184 185 186 190 192
542 390 314 173 124
31 48 60 79 141
– 28 42 68 77
– 35 49 61 78
presence of different concentrations of P1. Values of associated electrochemical parameters and corresponding inhibition efficiencies (E%) are given in Table 2. It is clear that the addition of the molecule studied decreases the cathodic current density. Cathodic portions rise to Tafel lines indicating that the hydrogen evolution reaction is activation controlled. Parallel Tafel lines obtained suggest that the presence of compounds tested does not modify the mechanism of the hydrogen reduction. In the anodic domain, a decrease of the anodic current density is observed. The free corrosion potential determined after 30 min of immersion remains almost constant in the presence of the inhibitor. E% values increase with the P1 concentration to attain 77% at 2.5 103 M. P1 acts as a mixed inhibitor. For comparative purposes the linear polarisation resistance (RP) are calculated from the linear I–E plots at 10 mV in the vicinity of Ecorr. The corresponding RP values of steel in 0.5 M H2SO4 in the presence and absence of different concentrations of the inhibitor are
also given in Table 2. The inhibition efficiency ER% is calculated using the following equation: Rp ER ð%Þ ¼ 1 0 100 Rp R0 P and RP are the polarisation resistance with and without the inhibitor, respectively. We remark that RP increases with increasing of P1 concentration. E% increases with the concentration to reach 78% at 2.5 103 M. 3.3. EIS measurements Corrosion behaviour of steel, in acidic solution with and without pyridine compound at different concentrations after immersion for 30 min is investigated by electrochemical impedance spectroscopy (EIS) measurements at 298 K. The corresponding Nyquist diagram are shown in Fig. 4. The chargetransfer resistance (Rt) values are calculated from the difference in impedance at lower and higher frequencies, as suggested by Tsuru et al. [22]. The
Fig. 4. Nyquist diagrams of steel in acid at various concentrations of P1.
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Table 3 Impedance parameters of steel in acid at various concentrations of P1 Concentration (M)
Rs (V cm2)
Rt (V cm2)
fmax (Hz)
Cdl (mF/cm2)
E%
Blank 104 2.5 104 103 2.5 103
4.86 2.92 2.74 2.97 2.94
30 44 51 58 81
51.50 44.76 35.71 35.31 22.63
103 80.80 88.09 77.70 86.89
– 32 41 48 63
double layer capacitance (Cdl) and the frequency at which the imaginary component of the impedance is maximal (Zmax) are found as represented in equation: 1 C dl ¼ wherew ¼ 2pf max vRt The impedance parameters derived from these investigations are mentioned in Table 3. The inhibition efficiency got from the charge transfer resistance is calculated by: ! Rt Eð%Þ ¼ 1 100 Rt=inh Rt/inh and Rt are the charge transfer-resistance values with and without inhibitor, respectively. As we notice in Fig. 4, the impedance diagrams show perfect semi-circles indicating a charge transfer process mainly controlling the corrosion of steel. In fact, the presence of P1 enhances the value of the transfer resistance in acidic solution. EIS study shows that the pyridine tested is an efficient inhibitor. 3.4. Effect of temperature Temperature can affect the steel corrosion in the acidic media in the presence and absence of inhibitor. To determine the action energy of the corrosion process, gravimetric measurements are taken at various temperatures (298–353 K) in the presence and absence of 103 M of P1 at 1 h of immersion. The corresponding results are given in Table 4. From these results, we can deduce that the corrosion rate increases with the rise of temperature, but in the presence of P1, the dissolution of steel is less pronounced. The inhibitive efficiency of the inhibitor decreases with the rise of temperature to fall to 25% at 353 K.
Fig. 5 shows Arrhenius plots for steel corrosion rate for both blank and inhibitor solution. The activation energies can be expressed by the Arrhenius equation: logðWÞ ¼
Ea þA RT
T is the absolute temperature, A is a constant and R is universal gas constant. Ea = 60.17 kJ/mol E0 a = 77.42 kJ/mol are the activation energies in the absence and presence of P1, respectively. It is evident that the presence of inhibitor causes increases the activation energy. E% depends on temperature. This may indicate the physical nature of adsorption mechanism [23]. 3.5. Adsorption isotherm The dependence of the ratio of the concentration and the fraction of the surface covered C/u as function of the concentration of P1, where u is the ratio E/100, is shown in Fig. 6. The obtained plot is linear with a slope equal to 1.16 near to unity showing an adsorption on the steel surface electrode according to the Langmuir isotherm: DG0ads C 1 1 ¼Cþ with K ¼ exp u K 55:55 RT
Table 4 Effect of temperature on the corrosion of steel with and without 5 103 M of P1 at 1 h Temperature (K)
0 Wcorr (mmpy)
Wcorr (mmpy)
E%
298 303 313 323 333 343 353
8.30 14.38 37.95 87.40 154.16 256.17 420.12
2.15 4.88 15.55 39.33 80.16 163.95 315.09
74 66 59 55 48 36 25
M. Bouklah et al. / Applied Surface Science 250 (2005) 50–56
55
Fig. 5. Arrhenius plots of steel in acid with and without 103 M P1.
K is the equilibrium constant leading to DG0ads is a standard free energy of adsorption. Value of DG0ads obtained is 30.34 kJ/mol for P1. The value of DG0ads is too negative indicating that P1 is strongly adsorbed on the steel surface [24,25]. Comparison of the inhibition efficiencies obtained by the P1 reveal that we assist really to an intramolecular synergistic effect. The mode of adsorption of P1 suggested is illustrated in Fig. 7. The action inhibitory may be attributed to the presence of S and two O atoms in the linear chain which humbly reinforces the adsorption of pyridine ring containing N atom and p
electrons. These findings are confirmed by the calculation of the total partial atomic charges in P1 molecule are calculated by generating all valence bond (resonance) structures for this system and then weighting them on the basis of p-orbital electronegativities and formal considerations (PEPE: partial equalization of pelectronegativity) [26]. The negative charges of N, S, O(1) and O(2) shown in Table 5 indicate that these atoms are the centres of the adsorption phenomenon. In comparison the total partial atomic charge of the nitrogen of P2 is 0.3444(e), but the efficiency obtained at 5 103 M does not exceed more than 17%. This
Fig. 6. Langmuir isotherm adsorption model of P1 on the surface of steel in 0.5 M H2SO4.
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M. Bouklah et al. / Applied Surface Science 250 (2005) 50–56
Fig. 7. Skeletal representation of the mode of adsorption of P1. Table 5 Calculation of the total charges in P1 molecule calculated by PETRA programa
Total charges (e)
O(1)
O(2)
N
S
0.3828
0.2336
0.3445
0.1504
a
The PETRA program is available for free on-line at this address: http://www2.chemie.uni-erlangen.de/software/petra/manual/.
fact indicates that the presence of the chain containing S and carboxylic group in P1 introduces the synergistic effect and then E% jumps at 82%.
4. Conclusion [(2-Pyridin-4-ylethyl)thio]acetic acid (P1) is a good inhibitor for steel in 0.5 M H2SO4. The inhibition efficiency of P1 attains a maximum value of about 82% at 5 103 M. It acts as a mixed inhibitor without modifying the mechanism of hydrogen evolution. The inhibition efficiency of P1 decreases with the temperature. P1 adsorbs according to Langmuir adsorption isotherm. Adsorption of P1 is explained by an intramolecular synergistic effect of pyridine and S and O chain containing. References [1] A. Elouafi, B. Hammouti, H. Oudda, S. Kertit, R. Touzani, A. Ramdani, Anti-Corros. Meth. Mater. 49 (2002) 199.
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