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Studies on the effect of pyranocoumarin derivatives on the corrosion of iron in 0.5 M HCl
Corrosion Science 44 (2002) 803±813
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Studies on the eect of pyranocoumarin derivatives on the corrosion of iron in 0.5 M HCl S.A. Abd El-Maksoud 1 Scienti®c Section, Abha College of Girls, P.O. Box 142, Abha, Saudi Arabia Received 15 December 2000; accepted 10 May 2001
Abstract The eect of some new synthesized pyranocoumarins on the corrosion of iron in 0.5 M HCl was investigated. The investigation involved electrochemical polarization methods (potentiodynamic, Tafel extrapolation and the determination of the polarization resistance) as well as weight loss measurements. The results show that these compounds act as mixed type inhibitors, but the cathode is more preferentially polarized. The inhibition eciency depends on both the nature and concentrations of the investigated compounds. Compounds are found to adsorb on the iron surface according to the Langmuir adsorption isotherm. Fourier transform infrared spectrophotometry was used to obtain information on bonding mechanism between the metallic surface and the inhibitors. Ó 2002 Published by Elsevier Science Ltd. Keywords: Corrosion; Inhibitors; Iron; Pyranocoumarin; Potentiodynamic
1. Introduction An important method of protecting materials against deterioration from corrosion is by using inhibitors [1±3]. Review including extensive listing of various types of organic inhibitors has been published [4]. Acid inhibitors have many important roles in the industrial ®eld as a component in pre-treatment composition, in cleaning solutions for industrial equipment and in acidization of oil wells. Compounds with functional group containing hetero-atoms which can donate lone pair electrons are
E-mail address: [email protected] (S.A. Abd El-Maksoud). Permanent address: Department of Chemistry, Faculty of Education, El-Arish, Sinai, Egypt.
1
0010-938X/02/$ - see front matter Ó 2002 Published by Elsevier Science Ltd. PII: S 0 0 1 0 - 9 3 8 X ( 0 1 ) 0 0 0 9 0 - 7
804
S.A. Abd El-Maksoud / Corrosion Science 44 (2002) 803±813
found to be particularly useful as inhibitors for corrosion of metals. Generally the tendency to form a stronger coordination bond and, hence, to cause a higher eciency increases in the order P > S > N > O [5]. The corrosion inhibition of iron and its alloy by dierent inhibitors in acid medium has been studied by several authors [6±10]. Recently 1,5 benzodiazepin-2-one derivatives [11], pyrazole derivatives [12], 1-phenyl-5-mercepto-1,2,3,4 tetrazole [13] and 1-benzalidine-4-phenyl thiosemicarbazone derivatives [14] were used as inhibitors for iron in hydrochloric acid. 1-benzylidine-4-phenyl thiosemicarbazone derivatives on the corrosion behaviour of iron in 1 M HNO3 was investigated [15]. Polyfunctionally substituted coumarines are interesting potential biodegradable agrochemicals [16,17], eective as anticoagulants or enzyme inhibitors [18,19] and antibacterials [20,21]. The aim of the present investigation is to examine the inhibition action of the new synthesized pyranocoumarin compounds towards the corrosion of iron in acidic solutions. The choice of the inhibitors is based on the presence of an electron cloud on the aromatic rings and the presence of electron donating atoms.
2. Experimental The electrodes were made of iron specimens of the chemical composition (in percentage) C 0.016
P 0.05
S 0.019
Si 0.04
Mn 0.25
Cr 0.055
Ni 0.035
Cu 0.07
Fe Rest
For potentiodynamic measurements wires with 1 cm length / 0:53 were used whereas sheets dimensions 20 mm 20 mm 1 mm were used for weight loss measurements. The iron surface was mechanically polished with emery paper, cleaned with distilled water and degreased in ethanol, washed with distilled water and ®nally dried with ®lter paper. The polarization studies were carried out in 0.5 M hydrochloric acid solutions potentiodynamically using a Wenking Potentio-Galvano-Scan (PGS 95) and employing a three electrode cell assembly. A saturated calomel electrode (SCE) and platinum electrode were used as reference and auxiliary electrodes respectively. All solutions were prepared from Analar grad chemicals and triply distilled water. All experiments were performed at 30 0:2°C using an ultrathermostatic bath. The organic inhibitors were prepared according to literature method [22] by adding the appropriate arylidine malononitrile derivatives (0.01 mol) and three drops of piperdine to a solution of either 4-hydroxycoumarin or 7-hydroxycoumarin. The reaction mixtures were re¯uxed for three hours, then evaporated in vacuum. The remaining product is then, collected by ®ltration and crystallized from ethanol.
S.A. Abd El-Maksoud / Corrosion Science 44 (2002) 803±813
The structure of the investigated compounds were as follows:
3. Results and discussion In this study, the corrosion rates were determined by · · · ·
potentiodynamic method; extrapolation of the anodic and cathodic Tafel lines; linear polarization method; weight loss method.
805
806
S.A. Abd El-Maksoud / Corrosion Science 44 (2002) 803±813
Fig. 1. Potentiodynamic polarization curves of iron in 0.5 M HCl in the absence and in the presence of dierent concentrations of compound 6, at the scan rate of 1 mV s 1 .
3.1. Potentiodynamic method After steady state potential was established, potentiodynamic experiment was carried out under constant potential of 850 mV vs (SCE) for 1 min, before each run, then the scan started from cathodic to the anodic direction with scan rate of 1 mV s 1 . Fig. 1 shows typical example of potentiodynamic curves for the iron electrode in 0.5 M HCl solution without and with dierent concentrations of compound 6. It is clear that the current density decreases with increasing of the concentration, this indicates that this compound is adsorbed on the metal surface and hence inhibition occurs. 3.2. Extrapolation of the anodic and cathodic Tafel lines The values of the corrosion currents and corrosion potential were estimated from the intersection of the anodic and cathodic Tafel lines. Fig. 2 shows experimental results from potentiodynamic polarization and Tafel lines intersection for iron in 0.5 M HCl without and with the addition of dierent concentration of compound 6. Tables 1 and 2 represent the electrochemical parameters for iron corrosion in 0.5 M HCl containing dierent concentrations of compound 6 and 1 10 4 M of different compounds under investigation respectively, and show the corrosion potential, Ecorr , corrosion current density, Icorr , Tafel slopes, Bc and Ba and the percentage inhibition %I. From these tables one can conclude that
S.A. Abd El-Maksoud / Corrosion Science 44 (2002) 803±813
807
Fig. 2. Intersection of the cathodic and anodic Tafel lines for iron in 0.5 M HCl in the absence and in the presence of dierent concentrations of compound 6.
Table 1 Corrosion potential Ecorr , corrosion current density Icorr , Tafel slopes Bc and Ba and inhibition eciency (%I) in 0.5 M HCl at dierent concentrations of inhibitor 6 at 303 K calculated by Tafel extrapolation Solution 0.5 0.5 0.5 0.5 0.5 0.5
M M M M M M
HCl HCl 1 10 HCl 2 10 HCl 4 10 HCl 8 10 HCl 1 10
5 5 5 5 4
M M M M M
Ecorr (mV)
Icorr (mA/cm2 )
Bc (mV/decade)
Ba (mV/decade)
%I
450 450 458 462 466 470
2.936 1.346 0.695 0.497 0.371 0.292
159 152 146 144 142 141
50 55 56 57 58 59
± 54.2 76.3 83.1 87.4 90.1
Table 2 Corrosion potential (Ecorr ), corrosion current density (Icorr ), Tafel slopes (Bc and Ba ) and inhibition eciency (%I) in 0.5 M HCl 1 10 4 M from dierent compounds at 303 K calculated by Tafel extrapolation Solution 0.5 M HCl 0.5 M HCl 1 10 compound 1 0.5 M HCl 1 10 compound 2 0.5 M HCl 1 10 compound 3 0.5 M HCl 1 10 compound 4 0.5 M HCl 1 10 compound 5 0.5 M HCl 1 10 compound 6
Ecorr (mV)
Icorr (mA/cm2 )
Bc (mV/decade)
Ba (mV/decade)
%I
4
M
450 452
2.936 1.888
159 160
50 48
± 35.7
4
M
455
1.817
156
47
38.1
4
M
457
1.679
150
52
42.8
4
M
462
1.218
149
50
58.5
4
M
466
0.534
144
48
81.8
4
M
470
0.292
141
50
90.1
808
S.A. Abd El-Maksoud / Corrosion Science 44 (2002) 803±813
1. The corrosion current densities calculated by Tafel extrapolation decrease with increasing of inhibitor concentration. 2. The corrosion potential of the inhibitor-containing solution nearly equal to that in the solution without the inhibitor, indicating that the inhibition eect is caused by geometrical blocking of the metal surface electrode by adsorbed inhibiting species [23]. 3. The percentage of inhibition increases with increasing inhibitor concentration and decreased in the order 6 > 5 > 4 > 3 > 2 > 1. 3.3. Linear polarization method For determining polarization resistance Rp , the potential of the working electrode was ramped 20 mV around open circuit potential at a scan rate of 0:1 mV s 1 . Polarization resistance was determined by the slope of the potential vs. current lines. Stearn and Geary [24] formulated the following equation for corrosion current calculation icorr B=Rp where the constant B is B
Ba XBc 2:303 Ba Bc
and Rp is the polarization resistance determined according to Mansfeld [25] from the slope of the polarization curve. Rp
S dE di
where S is the surface area of the electrode. From the polarization curves presented in Fig. 3, the polarization resistance was calculated for iron in 0.5 M HCl in the absence and in the presence of dierent concentrations of compound 6. The values of the polarization resistance, corrosion current density and the inhibition eciency are
Fig. 3. Weight loss±time curves for dierent concentrations of compound 6 in 0.5 M HCl.
S.A. Abd El-Maksoud / Corrosion Science 44 (2002) 803±813
809
Table 3 The inhibition eciency (%I) of dierent concentrations of compound 6 at 303 K on the basis of polarization resistance and weight loss measurements Solution
Rp (ohm)
Icorr (mA/cm2 )
%I
%P (wt. loss)
0.5 0.5 0.5 0.5 0.5 0.5
40 94 135 250 385 418
0.413 0.187 0.130 0.071 0.055 0.043
± 54.8 68.5 82.5 86.7 89.5
± 58.6 71.2 84.9 89.3 93.4
M M M M M M
HCl HCl 1 10 HCl 2 10 HCl 4 10 HCl 8 10 HCl 1 10
5 5 5 5 4
M M M M M
Table 4 The inhibition eciency (%I) of 1 10 4 from dierent compounds at 303 K on the basis of polarization resistance and weight loss measurements Solution
Rp (ohm)
0.5 M HCl 0.5 M HCl 1 10 compound l 0.5 M HCl 1 10 compound 2 0.5 M HCl 1 10 compound 3 0.5 M HCl 1 10 compound 4 0.5 M HCl 1 10 compound 5 0.5 M HCl 1 10 compound 6
Icorr (mA/cm2 )
%I
%P (wt. loss)
4
M
40 65
0.413 0.249
± 39.5
± 45.1
4
M
70
0.223
46.0
53.2
4
M
98
0.171
58.5
61.2
4
M
115
0.141
65.9
71.5
4
M
233
0.067
83.7
85.7
4
M
418
0.043
89.5
93.4
presented in Table 3 for dierent concentration of compound 6 and Table 4 for 1 10 4 M for dierent compounds under investigation. The inhibition eciency calculated is decreased in the order 6 > 5 > 4 > 3 > 2 > 1. 3.4. Weight loss method Weight loss in mg per cm2 of the surface area for iron was determined in the absence and in the presence of the additives. Fig. 4 shows the weight loss vs. time curves in the absence and in the presence of dierent concentrations, ranging from 1 10 5 to 1 10 4 M from compound 6. It is clear that, curves for additive containing systems fall below that for the inhibitor free acid, this indicates that these compounds act as inhibitors. The inhibition eciency (%P ) was calculated from the equation %P
W0
W W0
100
where W0 and W are the corrosion rate of iron without and with inhibitors, respectively. From Tables 3 and 4, it is clear that the weight loss decrease in the
810
S.A. Abd El-Maksoud / Corrosion Science 44 (2002) 803±813
Fig. 4. Linear polarization curves for iron in 0.5 M HCl in the absence and in the presence of dierent concentrations of compound 6, at scan rate of 0.1 mV s 1 .
presence of inhibitors, the inhibition eciency increases with increasing of concentration of compound 6 and the order of inhibition eciency decreases is 6 > 5 > 4 > 3 > 2 > 1, which is in a good matching with results obtained from electrochemical methods. FTIR spectral studies used to determine the manner of attachment of the active centres to the metallic surface. The spectrum of pure compound 6 was compared to the spectrum of the precipitate which formed after immersion of iron in inhibitor containing solution (Fig. 5). Dierences between the spectra were used to determine interactions between the active centres and the metallic surface. FTIR spectrum of compound 6 revealed three amino peaks at 3220, 3315 and 3396 cm 1 corresponding to the symmetrical and a symmetrical stretching band of primary amine and amidic NH2 [26]. It is also revealed an adsorption band for thion function at 1585 cm 1 and coumarin C@O at 1641. On the other hand the IR spectrum of the precipitate obtained from reaction of compound 6 with the metal surface revealed the absence of bands corresponding to primary amine, carbonyl and thion. The appearance of a broad band at 3321 cm 1 indicates the formation of a secondary amine and absence of bands at 1585 and 1641 cm 1 indicates that the bonding has occurred between the metal surface and the amine nitrogen, the carbonyl oxygen and the thion sulphur. Assuming a direct relationship between inhibition eciency and surface coverage, h, of the inhibitor. Data obtained from weight loss were used to analyse the adsorption mechanism. The plot of log h= 1 h vs. log C for compound 6 is represent in Fig. 6. The data ®t straight line, with slope nearly equal unity, indicating that these inhibitors adsorb according to the Langmuir adsorption isotherm. 4. Chemical structure and inhibition eciency Inhibition eect of dierent compounds used in this investigation was observed from dierent techniques used, and the order of inhibition eciency is
S.A. Abd El-Maksoud / Corrosion Science 44 (2002) 803±813
811
Fig. 5. FTIR spectra of compound 6 (upper spectrum) and on the iron surface (lower spectrum).
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S.A. Abd El-Maksoud / Corrosion Science 44 (2002) 803±813
Fig. 6. Relation between log h= 1
h vs. log C, for compound 6 from weight loss method.
6 > 5 > 4 > 3 > 2 > 1. Inhibition eciency of the additives depends on many factors [27] which including the number of adsorption active centres and their basicity, the mode of interaction with the metal surface and the molecular size. Compound 6 comes in the top among the investigated compounds in inhibition eciency, this may be due to, the high molecular weight, the presence of C@O of pyranone ring, the two NH2 groups and C@S which are electron donating groups, moreover the possibility of dp±dp bond formation resulting from overlap of 3d electrons from Fe atom to the 3d vacant orbital of the sulphur atom [28] which enhances the inhibition eciency of compounds 6 and 5 compared with the other investigated compounds. Compound 5 comes after compound 6 in inhibition eciency, it may be due to, the presence of aryl group between pyrane and pyranone rings in compound 5, which reduces the electron donating ability of anisole to the C@O of the pyranone ring. Compound 4 comes after compound 5 in percentage inhibition, this may be due to the lower molecular weight, as well as the absence of some active sites (C@S and NH2 ). Although compound 3 has the same active sites as compound 4, compound 4 exhibited a slightly higher eciency, this can be rationalized in terms of the high molecular weight of compound 4. Compound 2 is less eective than compound 3 due to the reduce in molecular size of compound 2, also the number of active sites is lesser than that in compound 3. Compound 2 is higher in inhibition eciency, this may be attributed to the presence of hydroxyl group attached directly to the pyranone ring, which increase the electron density on the C@O in the pyranone ring, whereas in compound 1 the hydroxyl group attached to the benzene ring, thus, compound 1 is the least eective among the investigated compounds. 5. Conclusions 1. The pyranocoumarin derivatives reduce the corrosion of iron in 0.5 M HCl. 2. The substitution plays an important role in the percentage inhibition values and the inhibition eciency observed with sulphur containing compounds were the highest.
S.A. Abd El-Maksoud / Corrosion Science 44 (2002) 803±813
813
3. As the number of active sites increase, the inhibition eciency increase too. 4. The adsorption of the investigated compounds on the iron surface obey Langmuir's adsorption isotherm. Acknowledgements The author wishes to thank Dr. I.S.A. Ha®z, for providing inhibitor compounds used in this study.
References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18] [19] [20] [21] [22] [23] [24] [25] [26] [27] [28]
J.I. Bregmann, Corrosion inhibitors, P.T. Macmillan, New York 1963. N. Hackerman, Langmuir 3 (1987) 922. C.C. Nathan, Organic inhibitors, NACE, Houston 1977. G. Trabanelli, V. Caraaiti, in: M.G. Fontana, R.W. Staehle (Eds.), Advances in Corrosion Science and Technology, vol. 1, Plenum Press, NY, 1970, p. 147. J.G.N. Thomas, Symp. Practical Aspects of Corrosion, Teddington, The Society of Chemical Industry, London, 1979. J. Vosta, S.M. Pelikanj, Werkst.Korros. 25 (1974) 750. S. Sathiyanarayanan, S.K. Dhawan, D.C. Trivedi, K. Balakrishnan, Corros. Sci. 33 (1992) 1831. R. Zvauya, J.L. Dawson, J. Appl. Electrochem. 24 (1994) 943. A.K. Mohamed, S.A. Abd El-Maksoud, A.S. Fouda, Bull. Soc. Chim. Belg. 105 (1996) 363. A.A. Aksut, A.N. Onal, Corros. Sci. 39 (1997) 761. M. Achouri, S. Elkertit, M. Salem, M.S. Hajji, G.E.M. Essassi, J. Chim. Phys. Phys-Chem Biol. 93 (1996) 2011. A. Aouniti, B. Hammouti, B. Brighle, M. Kertit, F. Behili, S. El-Kadiri, A. Ramadani, J. Chim. Phys. Phys-Chem Biol. 93 (1996) 1262. S. Kertit, B. Hammouti, Appl. Surf. Sci. 93 (1) (1996) 59. S.A. Abd El-Maksoud, A.A. El-Shafei, A.S. Fouda, Mat-wiss.U. Werkstoech 28 (1997) 439. S.A. Abd EI-Maksoud, Portugal Electrochem. Acta 16 (1998) 143. E.A.A. Hafez, M.H. Elnagdi, A.A. Elagamey, F.M.A. El-Taweel, Heterocycles 26 (1987) 903. F.M.A. El-Taweel, M.A. Sofan, M.M. Mashaly, M.A. Hanna, A.A. Elagamey, Phannazie 45 (1990) 671. T.W. Chu, Daniel, K.A. Clailorne, J.J. Clement, J.J. Plattner, Can. J. Chem. 70 (1992) 1328. J.F. Rigola, J. Pares, J. Gorbera, D. Vano, R. Merce, A. Torrens, J. Mas, E. Valenti, J. Med. Chem. 36 (1993) 801. G.B. Barlin, M.T.T. Nguyen, B. Kotecks, K.H. Rieckmann, Aust. J. Chem. 45 (1992) 1651. A.A.A. Hafez, J. Chem. Technol. Biotechnol. 55 (1992) 95. I.S.A. Ha®z, F.M. Farouk, A.M. Hussien, Z. Nautarforch. 14b (1999) 273. C. Cao, Corros. Sci. 29 (1996) 2073. M. Stern, M. Geary, J. Electrochem. Soc. 104 (1956) 56. F. Mansfeld, Corrosion 29 (1973) 397. Z. Rappoport, CRC Handbook of Tables for Organic Compounds Identi®cation, third ed., CRC Press, Boca Raton, FL, 1983, pp. 442±443. A.K. Mohamed, T.H. Raka, M.N.H. Moussa, Bull. Soc. Chim. Fr. 127 (1990) 375. B. Bonnelly, T.C. Downie, R. Grzekowiak, H.R. Hamburg, D. Short, Corros. Sci. 18 (1978) 109.
www.elsevier.com/locate/corsci
Studies on the eect of pyranocoumarin derivatives on the corrosion of iron in 0.5 M HCl S.A. Abd El-Maksoud 1 Scienti®c Section, Abha College of Girls, P.O. Box 142, Abha, Saudi Arabia Received 15 December 2000; accepted 10 May 2001
Abstract The eect of some new synthesized pyranocoumarins on the corrosion of iron in 0.5 M HCl was investigated. The investigation involved electrochemical polarization methods (potentiodynamic, Tafel extrapolation and the determination of the polarization resistance) as well as weight loss measurements. The results show that these compounds act as mixed type inhibitors, but the cathode is more preferentially polarized. The inhibition eciency depends on both the nature and concentrations of the investigated compounds. Compounds are found to adsorb on the iron surface according to the Langmuir adsorption isotherm. Fourier transform infrared spectrophotometry was used to obtain information on bonding mechanism between the metallic surface and the inhibitors. Ó 2002 Published by Elsevier Science Ltd. Keywords: Corrosion; Inhibitors; Iron; Pyranocoumarin; Potentiodynamic
1. Introduction An important method of protecting materials against deterioration from corrosion is by using inhibitors [1±3]. Review including extensive listing of various types of organic inhibitors has been published [4]. Acid inhibitors have many important roles in the industrial ®eld as a component in pre-treatment composition, in cleaning solutions for industrial equipment and in acidization of oil wells. Compounds with functional group containing hetero-atoms which can donate lone pair electrons are
E-mail address: [email protected] (S.A. Abd El-Maksoud). Permanent address: Department of Chemistry, Faculty of Education, El-Arish, Sinai, Egypt.
1
0010-938X/02/$ - see front matter Ó 2002 Published by Elsevier Science Ltd. PII: S 0 0 1 0 - 9 3 8 X ( 0 1 ) 0 0 0 9 0 - 7
804
S.A. Abd El-Maksoud / Corrosion Science 44 (2002) 803±813
found to be particularly useful as inhibitors for corrosion of metals. Generally the tendency to form a stronger coordination bond and, hence, to cause a higher eciency increases in the order P > S > N > O [5]. The corrosion inhibition of iron and its alloy by dierent inhibitors in acid medium has been studied by several authors [6±10]. Recently 1,5 benzodiazepin-2-one derivatives [11], pyrazole derivatives [12], 1-phenyl-5-mercepto-1,2,3,4 tetrazole [13] and 1-benzalidine-4-phenyl thiosemicarbazone derivatives [14] were used as inhibitors for iron in hydrochloric acid. 1-benzylidine-4-phenyl thiosemicarbazone derivatives on the corrosion behaviour of iron in 1 M HNO3 was investigated [15]. Polyfunctionally substituted coumarines are interesting potential biodegradable agrochemicals [16,17], eective as anticoagulants or enzyme inhibitors [18,19] and antibacterials [20,21]. The aim of the present investigation is to examine the inhibition action of the new synthesized pyranocoumarin compounds towards the corrosion of iron in acidic solutions. The choice of the inhibitors is based on the presence of an electron cloud on the aromatic rings and the presence of electron donating atoms.
2. Experimental The electrodes were made of iron specimens of the chemical composition (in percentage) C 0.016
P 0.05
S 0.019
Si 0.04
Mn 0.25
Cr 0.055
Ni 0.035
Cu 0.07
Fe Rest
For potentiodynamic measurements wires with 1 cm length / 0:53 were used whereas sheets dimensions 20 mm 20 mm 1 mm were used for weight loss measurements. The iron surface was mechanically polished with emery paper, cleaned with distilled water and degreased in ethanol, washed with distilled water and ®nally dried with ®lter paper. The polarization studies were carried out in 0.5 M hydrochloric acid solutions potentiodynamically using a Wenking Potentio-Galvano-Scan (PGS 95) and employing a three electrode cell assembly. A saturated calomel electrode (SCE) and platinum electrode were used as reference and auxiliary electrodes respectively. All solutions were prepared from Analar grad chemicals and triply distilled water. All experiments were performed at 30 0:2°C using an ultrathermostatic bath. The organic inhibitors were prepared according to literature method [22] by adding the appropriate arylidine malononitrile derivatives (0.01 mol) and three drops of piperdine to a solution of either 4-hydroxycoumarin or 7-hydroxycoumarin. The reaction mixtures were re¯uxed for three hours, then evaporated in vacuum. The remaining product is then, collected by ®ltration and crystallized from ethanol.
S.A. Abd El-Maksoud / Corrosion Science 44 (2002) 803±813
The structure of the investigated compounds were as follows:
3. Results and discussion In this study, the corrosion rates were determined by · · · ·
potentiodynamic method; extrapolation of the anodic and cathodic Tafel lines; linear polarization method; weight loss method.
805
806
S.A. Abd El-Maksoud / Corrosion Science 44 (2002) 803±813
Fig. 1. Potentiodynamic polarization curves of iron in 0.5 M HCl in the absence and in the presence of dierent concentrations of compound 6, at the scan rate of 1 mV s 1 .
3.1. Potentiodynamic method After steady state potential was established, potentiodynamic experiment was carried out under constant potential of 850 mV vs (SCE) for 1 min, before each run, then the scan started from cathodic to the anodic direction with scan rate of 1 mV s 1 . Fig. 1 shows typical example of potentiodynamic curves for the iron electrode in 0.5 M HCl solution without and with dierent concentrations of compound 6. It is clear that the current density decreases with increasing of the concentration, this indicates that this compound is adsorbed on the metal surface and hence inhibition occurs. 3.2. Extrapolation of the anodic and cathodic Tafel lines The values of the corrosion currents and corrosion potential were estimated from the intersection of the anodic and cathodic Tafel lines. Fig. 2 shows experimental results from potentiodynamic polarization and Tafel lines intersection for iron in 0.5 M HCl without and with the addition of dierent concentration of compound 6. Tables 1 and 2 represent the electrochemical parameters for iron corrosion in 0.5 M HCl containing dierent concentrations of compound 6 and 1 10 4 M of different compounds under investigation respectively, and show the corrosion potential, Ecorr , corrosion current density, Icorr , Tafel slopes, Bc and Ba and the percentage inhibition %I. From these tables one can conclude that
S.A. Abd El-Maksoud / Corrosion Science 44 (2002) 803±813
807
Fig. 2. Intersection of the cathodic and anodic Tafel lines for iron in 0.5 M HCl in the absence and in the presence of dierent concentrations of compound 6.
Table 1 Corrosion potential Ecorr , corrosion current density Icorr , Tafel slopes Bc and Ba and inhibition eciency (%I) in 0.5 M HCl at dierent concentrations of inhibitor 6 at 303 K calculated by Tafel extrapolation Solution 0.5 0.5 0.5 0.5 0.5 0.5
M M M M M M
HCl HCl 1 10 HCl 2 10 HCl 4 10 HCl 8 10 HCl 1 10
5 5 5 5 4
M M M M M
Ecorr (mV)
Icorr (mA/cm2 )
Bc (mV/decade)
Ba (mV/decade)
%I
450 450 458 462 466 470
2.936 1.346 0.695 0.497 0.371 0.292
159 152 146 144 142 141
50 55 56 57 58 59
± 54.2 76.3 83.1 87.4 90.1
Table 2 Corrosion potential (Ecorr ), corrosion current density (Icorr ), Tafel slopes (Bc and Ba ) and inhibition eciency (%I) in 0.5 M HCl 1 10 4 M from dierent compounds at 303 K calculated by Tafel extrapolation Solution 0.5 M HCl 0.5 M HCl 1 10 compound 1 0.5 M HCl 1 10 compound 2 0.5 M HCl 1 10 compound 3 0.5 M HCl 1 10 compound 4 0.5 M HCl 1 10 compound 5 0.5 M HCl 1 10 compound 6
Ecorr (mV)
Icorr (mA/cm2 )
Bc (mV/decade)
Ba (mV/decade)
%I
4
M
450 452
2.936 1.888
159 160
50 48
± 35.7
4
M
455
1.817
156
47
38.1
4
M
457
1.679
150
52
42.8
4
M
462
1.218
149
50
58.5
4
M
466
0.534
144
48
81.8
4
M
470
0.292
141
50
90.1
808
S.A. Abd El-Maksoud / Corrosion Science 44 (2002) 803±813
1. The corrosion current densities calculated by Tafel extrapolation decrease with increasing of inhibitor concentration. 2. The corrosion potential of the inhibitor-containing solution nearly equal to that in the solution without the inhibitor, indicating that the inhibition eect is caused by geometrical blocking of the metal surface electrode by adsorbed inhibiting species [23]. 3. The percentage of inhibition increases with increasing inhibitor concentration and decreased in the order 6 > 5 > 4 > 3 > 2 > 1. 3.3. Linear polarization method For determining polarization resistance Rp , the potential of the working electrode was ramped 20 mV around open circuit potential at a scan rate of 0:1 mV s 1 . Polarization resistance was determined by the slope of the potential vs. current lines. Stearn and Geary [24] formulated the following equation for corrosion current calculation icorr B=Rp where the constant B is B
Ba XBc 2:303 Ba Bc
and Rp is the polarization resistance determined according to Mansfeld [25] from the slope of the polarization curve. Rp
S dE di
where S is the surface area of the electrode. From the polarization curves presented in Fig. 3, the polarization resistance was calculated for iron in 0.5 M HCl in the absence and in the presence of dierent concentrations of compound 6. The values of the polarization resistance, corrosion current density and the inhibition eciency are
Fig. 3. Weight loss±time curves for dierent concentrations of compound 6 in 0.5 M HCl.
S.A. Abd El-Maksoud / Corrosion Science 44 (2002) 803±813
809
Table 3 The inhibition eciency (%I) of dierent concentrations of compound 6 at 303 K on the basis of polarization resistance and weight loss measurements Solution
Rp (ohm)
Icorr (mA/cm2 )
%I
%P (wt. loss)
0.5 0.5 0.5 0.5 0.5 0.5
40 94 135 250 385 418
0.413 0.187 0.130 0.071 0.055 0.043
± 54.8 68.5 82.5 86.7 89.5
± 58.6 71.2 84.9 89.3 93.4
M M M M M M
HCl HCl 1 10 HCl 2 10 HCl 4 10 HCl 8 10 HCl 1 10
5 5 5 5 4
M M M M M
Table 4 The inhibition eciency (%I) of 1 10 4 from dierent compounds at 303 K on the basis of polarization resistance and weight loss measurements Solution
Rp (ohm)
0.5 M HCl 0.5 M HCl 1 10 compound l 0.5 M HCl 1 10 compound 2 0.5 M HCl 1 10 compound 3 0.5 M HCl 1 10 compound 4 0.5 M HCl 1 10 compound 5 0.5 M HCl 1 10 compound 6
Icorr (mA/cm2 )
%I
%P (wt. loss)
4
M
40 65
0.413 0.249
± 39.5
± 45.1
4
M
70
0.223
46.0
53.2
4
M
98
0.171
58.5
61.2
4
M
115
0.141
65.9
71.5
4
M
233
0.067
83.7
85.7
4
M
418
0.043
89.5
93.4
presented in Table 3 for dierent concentration of compound 6 and Table 4 for 1 10 4 M for dierent compounds under investigation. The inhibition eciency calculated is decreased in the order 6 > 5 > 4 > 3 > 2 > 1. 3.4. Weight loss method Weight loss in mg per cm2 of the surface area for iron was determined in the absence and in the presence of the additives. Fig. 4 shows the weight loss vs. time curves in the absence and in the presence of dierent concentrations, ranging from 1 10 5 to 1 10 4 M from compound 6. It is clear that, curves for additive containing systems fall below that for the inhibitor free acid, this indicates that these compounds act as inhibitors. The inhibition eciency (%P ) was calculated from the equation %P
W0
W W0
100
where W0 and W are the corrosion rate of iron without and with inhibitors, respectively. From Tables 3 and 4, it is clear that the weight loss decrease in the
810
S.A. Abd El-Maksoud / Corrosion Science 44 (2002) 803±813
Fig. 4. Linear polarization curves for iron in 0.5 M HCl in the absence and in the presence of dierent concentrations of compound 6, at scan rate of 0.1 mV s 1 .
presence of inhibitors, the inhibition eciency increases with increasing of concentration of compound 6 and the order of inhibition eciency decreases is 6 > 5 > 4 > 3 > 2 > 1, which is in a good matching with results obtained from electrochemical methods. FTIR spectral studies used to determine the manner of attachment of the active centres to the metallic surface. The spectrum of pure compound 6 was compared to the spectrum of the precipitate which formed after immersion of iron in inhibitor containing solution (Fig. 5). Dierences between the spectra were used to determine interactions between the active centres and the metallic surface. FTIR spectrum of compound 6 revealed three amino peaks at 3220, 3315 and 3396 cm 1 corresponding to the symmetrical and a symmetrical stretching band of primary amine and amidic NH2 [26]. It is also revealed an adsorption band for thion function at 1585 cm 1 and coumarin C@O at 1641. On the other hand the IR spectrum of the precipitate obtained from reaction of compound 6 with the metal surface revealed the absence of bands corresponding to primary amine, carbonyl and thion. The appearance of a broad band at 3321 cm 1 indicates the formation of a secondary amine and absence of bands at 1585 and 1641 cm 1 indicates that the bonding has occurred between the metal surface and the amine nitrogen, the carbonyl oxygen and the thion sulphur. Assuming a direct relationship between inhibition eciency and surface coverage, h, of the inhibitor. Data obtained from weight loss were used to analyse the adsorption mechanism. The plot of log h= 1 h vs. log C for compound 6 is represent in Fig. 6. The data ®t straight line, with slope nearly equal unity, indicating that these inhibitors adsorb according to the Langmuir adsorption isotherm. 4. Chemical structure and inhibition eciency Inhibition eect of dierent compounds used in this investigation was observed from dierent techniques used, and the order of inhibition eciency is
S.A. Abd El-Maksoud / Corrosion Science 44 (2002) 803±813
811
Fig. 5. FTIR spectra of compound 6 (upper spectrum) and on the iron surface (lower spectrum).
812
S.A. Abd El-Maksoud / Corrosion Science 44 (2002) 803±813
Fig. 6. Relation between log h= 1
h vs. log C, for compound 6 from weight loss method.
6 > 5 > 4 > 3 > 2 > 1. Inhibition eciency of the additives depends on many factors [27] which including the number of adsorption active centres and their basicity, the mode of interaction with the metal surface and the molecular size. Compound 6 comes in the top among the investigated compounds in inhibition eciency, this may be due to, the high molecular weight, the presence of C@O of pyranone ring, the two NH2 groups and C@S which are electron donating groups, moreover the possibility of dp±dp bond formation resulting from overlap of 3d electrons from Fe atom to the 3d vacant orbital of the sulphur atom [28] which enhances the inhibition eciency of compounds 6 and 5 compared with the other investigated compounds. Compound 5 comes after compound 6 in inhibition eciency, it may be due to, the presence of aryl group between pyrane and pyranone rings in compound 5, which reduces the electron donating ability of anisole to the C@O of the pyranone ring. Compound 4 comes after compound 5 in percentage inhibition, this may be due to the lower molecular weight, as well as the absence of some active sites (C@S and NH2 ). Although compound 3 has the same active sites as compound 4, compound 4 exhibited a slightly higher eciency, this can be rationalized in terms of the high molecular weight of compound 4. Compound 2 is less eective than compound 3 due to the reduce in molecular size of compound 2, also the number of active sites is lesser than that in compound 3. Compound 2 is higher in inhibition eciency, this may be attributed to the presence of hydroxyl group attached directly to the pyranone ring, which increase the electron density on the C@O in the pyranone ring, whereas in compound 1 the hydroxyl group attached to the benzene ring, thus, compound 1 is the least eective among the investigated compounds. 5. Conclusions 1. The pyranocoumarin derivatives reduce the corrosion of iron in 0.5 M HCl. 2. The substitution plays an important role in the percentage inhibition values and the inhibition eciency observed with sulphur containing compounds were the highest.
S.A. Abd El-Maksoud / Corrosion Science 44 (2002) 803±813
813
3. As the number of active sites increase, the inhibition eciency increase too. 4. The adsorption of the investigated compounds on the iron surface obey Langmuir's adsorption isotherm. Acknowledgements The author wishes to thank Dr. I.S.A. Ha®z, for providing inhibitor compounds used in this study.
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