- Email: [email protected]
Investigation on some Schiff bases as HCl corrosion inhibitors for carbon steel
Materials Chemistry and Physics 85 (2004) 420–426
Investigation on some Schiff bases as HCl corrosion inhibitors for carbon steel A. Yurt a , A. Balaban b , S. Ustün Kandemir a , G. Bereket a,∗ , B. Erk b a
Chemistry Department, Faculty of Arts and Science, Osmangazi University, Eskisehir 26480, Turkey b Chemistry Department, Faculty of Science, Gazi University, Ankara, Turkey
Received 10 November 2003; received in revised form 29 December 2003; accepted 29 January 2004
Abstract Potentiodynamic polarisation and ac impedance studies were carried out on the inhibition of carbon steel in 0.1 M hydrochloric acid solution by various Schiff bases containing heteroaromatic substituents. The examined Schiff bases were 2-((1E)-2-aza-2-pyrimidine-2-ylvinyl) thiophene, 2-((1Z)-1-aza-2-(2-pyridyl)vinyl)pyrimidine, 2-((1E)-2-aza-2-(1,3-thiazol-2-yl)vinyl)thiophene, 2-((1Z)-1-aza-2-(2-thienyl)vinyl)benzothiazole. Polarisation curves indicates that studied Schiff bases act essentially as anodic inhibitor. The variation in inhibitive efficiency mainly depends on the type and nature of the substituents present in the inhibitor molecule. Potentiodynamic polarisation and ac impedance measurements carried out at different concentration of studied Schiff bases reveal that these compounds are adsorbed on steel surface and the adsorption obeys Temkin’s adsorption isotherm. From the adsorption isotherms values of equilibrium constant, Kads , values of free energies of adsorption, Gads , were calculated. Activation parameters of the corrosion process such as activation energies, Ea , activation enthalpies, H∗ , and activation entropies, S∗ , were calculated from the obtained corrosion rates at different temperatures. © 2004 Elsevier B.V. All rights reserved. Keywords: Corrosion; Inhibitor; Schiff base; Carbon steel; Hydrochloric acid
1. Introduction Evaluation of corrosion inhibitors for steel in acidic media is important for some industrial facilities as well as is very interesting from theoretical aspects. Most of the effective inhibitors are organic compounds containing nitrogen, phosphorus and sulphur in their structures [1,2]. Heteroatoms such as nitrogen, oxygen and sulphur are capable of forming coordinate covalent bond with metal owing to their free electron pairs and thus acting as inhibitor. Compounds with -bonds also generally exhibit good inhibitive properties due to interaction of orbital with metal surface. Schiff bases—with RC = NR as general formula—have both the features combined with their structure which may then give rise to particularly potential inhibitors. Some Schiff bases have recently reported as effective corrosion inhibitors for mild steel [3], aluminium alloys [4,5], copper [6] in acidic media and as inhibitors for halide corrosion of copper [7,8]. The aim of this work is to investigate inhibitive effect of some Schiff bases containing nitrogen and/or sulphur het∗ Corresponding author. Tel.: +90-222-2290433; fax: +90-222-2393578. E-mail address: [email protected] (G. Bereket).
0254-0584/$ – see front matter © 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.matchemphys.2004.01.033
eroaromatic compound as substituents in their structure for the corrosion of carbon steel in 0.1 M HCl solution. Effects of concentration, temperature and substituents on the inhibition efficiencies of the selected Schiff bases have been studied systematically.
2. Experimental The working electrode was prepared from cylindrical carbon steel bar (0.4% C, 0.9% Mn, 0.04% P, 0.04% S, 98.62% Fe). The electrode was inserted in a Teflon tube and isolated with polyester so that only its cross section (0.3848 cm2 ) was allowed to contact the aggressive solutions. The electrode was polished using different grades of emery paper (600, 800 and 1200) before each experiment, and then rinsed with triply distilled water and finally degreased with acetone. The molecular structures of the examined Schiff bases are listed in Table 1. Studied Schiff bases were synthesised according to published methods [9–12]. Solutions were prepared from double distilled water. Test solutions were deaerated by bubbling ultra pure oxygen-free nitrogen gas for 30 min before use and continued during the tests. The tests were performed in a three-compartment electrochemical cell with a separate compartment for the
A. Yurt et al. / Materials Chemistry and Physics 85 (2004) 420–426
421
Table 1 List of investigated Schiff bases Molecule
Structure
Abbreviation
2-((1E)-2-aza-2-pyrimidine-2-ylvinyl)thiophene
PT
2-((1Z)-1-aza-2-(2-pyridyl)vinyl)pyrimidine
PP
2-((1E)-2-aza-2-(1,3-thiazol-2-yl)vinyl)thiophene
TT
2-((1Z)-1-aza-2-(2-thienyl)vinyl)benzothiazole
TBT
S N N N N N N N
S S N N S S N N
reference electrode connected with the main compartment via a Luggin capillary. The reference electrode was silver– silver chloride (Ag|AgCl|Cl− ) electrode and the auxiliary electrode was platinum electrode. The cell was water-jacketed and was connected to a Giant LTD6-G model thermostat. Potentiodynamic current–voltage characteristics were measured with EG & G PAR363 model potentiostat, VSG 2000 model voltage scan generator and TQ DMS2 model data management system and measured data were saved by PC386. Working electrode was first immersed into the test solution for 30 min to establish a steady state open circuit potential. After measuring the open circuit potential potentiodynamic polarisation curves were obtained with a scan rate of 0.5 mV s−1 in the potential range from −500 to +500 mV relative to the corrosion potential. Corrosion current density values were obtained by Tafel extrapolation method. Electrochemical impedance (EIS) measurements were performed at open circuit potential in the frequency range from 100 kHz to 1 Hz with a signal amplitude perturbation of 5 mV by using CHI 604 Electrochemical Analyser at 20 ◦ C. All experiments were performed using three-electrode system.
3. Results and discussion 3.1. Tafel polarisation measurements Representative Tafel polarisation curves of carbon steel in 0.1 M HCl solutions with different concentrations of PT is shown in Fig. 1. Similar polarisation curves were obtained
in 0.1 M HCl solutions with different concentrations of PP, TT and TBT. Electrochemical parameters obtained from the Tafel extrapolation method of the polarisation curves are given in Table 2. Corrosion currents obtained in 0.1 M HCl solutions with Schiff bases are lower than corrosion currents obtained in acid solution without Schiff bases. Increase in concentration of added Schiff bases causes shifting of corrosion potentials to noble direction and variation in both of Tafel slopes. But anodic Tafel slopes are more enhanced, so inhibition of corrosion of carbon steel in 0.1 M HCl solution by the studied Schiff bases is under anodic control. Increase in inhibition efficiencies with the increase of concentrations of studied Schiff bases shows that inhibition actions are due to adsorption on steel surface. Four types of adsorption may take place by organic molecules at metal/solution interface. (1) Electrostatic attraction between the charged molecules and charged metal. (2) Interaction of uncharged electron pairs in the molecule with the metal. (3) Interaction electrons with the metal. (4) Combination of (1) and (3) [13]. In hydrochloric acid solution the following mechanism is proposed for the corrosion of iron and steel [14]. According to this mechanism anodic dissolution of iron: Fe + Cl− (FeCl− )ads
(a)
(FeCl− )ads (FeCl)ads + e−
(b)
(FeCl)ads → (FeCl+ ) + e−
(c)
+
FeCl Fe
2+
+ Cl
−
(d)
The cathodic hydrogen evolution: Fe + H+ (FeH+ )ads
(e)
422
A. Yurt et al. / Materials Chemistry and Physics 85 (2004) 420–426
10
log i(m A /c m 2 )
10
10
10
10
2
1
0
-1
-2
-0.9
-0.8
-0.7
-0.6
-0.5 E (V )
-0.4
-0.3
-0.2
Fig. 1. Tafel polarisation curves for carbon steel in 0.1 M HCl (—) in the presence of 1 × 10−4 M PT (
) and 1 × 10−5 M PT (+).
Table 2 Polarisation parameters and corresponding inhibition efficiencies for the corrosion of carbon steel in 0.1 M HCl without and with addition of various concentrations of Schiff bases at 20 ◦ C Ecorr (mV)
βa (mV)
βc (mV)
icorr (A cm−2 )
IE
θ
–
10−1
1× 1 × 10−4 5 × 10−5
−526 −440 −462
129.2 40.3 68.4
−127.54 −150.0 −141.1
450 75 100
– 83.3 77.8
– 0.833 0.778
PT
1 × 10−5 5 × 10−6 1 × 10−6
−486 −487 −491
75.0 80.5 76.9
−121.5 −125.0 −123.3
173 213 225
57.4 52.8 50.0
0.574 0.528 0.500
PP
1 5 1 5 1
× × × × ×
10−4 10−5 10−5 10−6 10−6
−487 −502 −500 −513 −510
63.8 86.7 84.8 109.8 95.9
−140.2 −118.9 −101.2 −122.5 −129.0
127 160 189 236 267
71.8 64.4 60.0 47.5 40.7
0.718 0.644 0.600 0.475 0.407
TT
1 5 1 5 1
× × × × ×
10−4 10−5 10−5 10−6 10−6
−490 −497 −503 −513 −526
67.8 80.3 97.0 108.0 116.1
−89.1 −119.4 −126.7 −114.6 −122.6
160 211 255 282 367
64.4 53.1 43.3 37.3 18.5
0.644 0.531 0.433 0.373 0.185
TBT
1 5 1 5 1
× × × × ×
10−4 10−5 10−5 10−6 10−6
−510 −513 −519 −526 −516
86.9 106.5 94.3 127.0 95.2
−125.6 −138.2 −114.1 −134.7 −105.8
227 255 289 350 367
49.5 43.4 35.8 22.2 18.4
0.495 0.434 0.358 0.222 0.184
Inhibitor
Concentration (M)
(FeH+ )ads + e− → (FeH)ads
(f)
+
(g)
−
(FeH)ads + H + e → Fe + H2
In acidic solution imine group as well as nitrogen atoms in heteroaromatic ring can be protonated. Physical adsorption may take place due to electrostatic interaction between protonated forms of Schiff bases and (FeCl− )ads species. Coordinate covalent bond formation between electron pairs of
unprotonated S atoms in heteroaromatic ring and metal surface can take place. Chemisorption of Schiff bases due to interaction of their orbitals with metal surface occurs following deprotonisation step of the physically adsorbed protonated forms of Schiff bases. Examination of experimental data shows that the inhibition efficiency values of the examined Schiff bases follows the order PT > PP > TT > TBT. The variation in inhibitive
efficiency mainly depends on the type and the nature of the substituents present in the inhibitor molecule. The difference in protection action can be attributed to the presence of different substituents to azomethine (–C=N–) group. The difference in inhibition efficiency between PT and PP arises due to presence of electron excess ring (i.e. thiophene) in PT instead of presence of electron deficient ring (i.e. pyridine) in PP as substituent. Presence of electron releasing electron excess ring in PT causes increase of electron density of –C=N– group which gives a better protective action of steel surface than PP. On the other hand difference in inhibition efficiency between PT and TT can be explained by the presence of thiazole ring in TT instead of presence of pyrimidine ring in PT. Chemisorption of studied Schiff bases arise from the interaction of (–C=N–) group, electrons of the aromatic ring and free electron pairs of heteroatoms (S or N) with steel surface. In the case of difference in inhibition efficiencies between PT and TT arise not only in the difference from the electron density of (–C=N–) group but from the difference of strength of interaction free electron pairs on N atom in pyrimidine ring and the strength of interaction free electron pairs of N in thiazole ring with metal. Free electron pairs on N atom in pyrimidine ring are more available than free electron pairs on N atom in thiazole ring, because free electron pairs on sulphur participate conjugation in thiazole ring. On the other hand the difference between inhibition efficiencies of TT and TBT arises due to presence of benzene ring in TBT as a fused to thiazole ring which turns to decrease electron density on S atom in thiazole ring as a result of elongated conjugation. Since adsorption isotherms can provide important clues to the nature of metal–inhibitor interaction it was established isotherms that describe the adsorptive behaviour of the studied corrosion inhibitor. Several adsorption isotherms were assessed and the Temkin adsorption isotherm was found to provide best description of the adsorption behaviour of the investigated Schiff bases, which has the following surface coverage, θ, and the bulk concentration, c, relationship [15] exp(fθ) = Kads c
(1)
where K is the adsorption equilibrium constant and f the molecular interaction constant. Typical plot of θ versus ln c for PT is shown in Fig. 2. From the obtained straight lines in θ–ln c graphs, equilibrium constants for adsorption process, Kads , were obtained. The equilibrium constants for adsorption process is related to the free energy of adsorption, Gads , by 1 −Gads Kads = exp (2) 55.5 RT where 55.5 is the molar concentration of water in the solution. The thermodynamic parameters for adsorption process obtained from Temkin adsorption isotherms for the studied molecules are given in Table 3. The values of free energy of adsorption, Gads , are negative which reveals the spontaneity of adsorption process and stability of the adsorbed layer
θ
A. Yurt et al. / Materials Chemistry and Physics 85 (2004) 420–426
423
0,9 0,8 0,7 0,6 0,5 0,4 0,3 0,2 0,1 0
y = 0,0788x + 1,535 R2 = 0,9038
-15
-10
-5
0
lnc
Fig. 2. Temkin adsorption isotherm of PT on carbon steel in 0.1 M HCl solutions.
on the steel surface. The obtained values of the adsorption free energy, Gads , were >−49 to −58 kJ mol−1 are indicative of chemisorption [16]. In order to study the effect of temperature on the inhibition characteristic of Schiff bases polarisation experiments were conducted at 20, 30, 40 and 60 ◦ C in 0.1 M HCl in presence of Schiff bases at 10−4 M concentration. Typical polarisation curves obtained at 20, 30, 40 and 60 ◦ C in 0.1 M HCl in presence of 1 × 10−4 M PT is shown in Fig. 3. Corrosion characteristics obtained from the Tafel polarisation curves are given in Table 4. Inhibition efficiencies were found to decrease as temperatures were increased. A decrease in inhibition efficiencies with the increase in temperature might be due to weakening of physical adsorption. Physical adsorption in inhibition of corrosion of carbon steel in acidic solution is small but important because it is preceding stage of chemisorption of Schiff bases on carbon steel. In order to calculate activation parameters for the corrosion process Arrhenius equation (3) and transition state equation (4) were used Ea k = A exp (3) RT H ∗ S ∗ RT exp exp − (4) k= Nh R RT Figs. 4 and 5 show the typical plots of ln icorr versus 1/T and ln(icorr /T) versus 1/T graphs for PT, respectively. Kinetic parameters obtained from these graphs are given in Table 5. Inspection of Table 5 shows that higher values were obtained for Ea and H∗ in the presence of inhibitors indicating the higher protection efficiency which was observed for these inhibitors. There is also a parallelism between increases in Table 3 Thermodynamic parameters of adsorption for studied Schiff bases on carbon steel in 0.1 M HCl solutions at 20 ◦ C Inhibitor
Equilibrium constant of adsorption, Kads
PT PP TT TBT
2.88 4.13 8.75 1.01
× × × ×
108 108 106 107
Standard free energy of adsorption, Gads (kJ mol−1 ) −57.234 −58.112 −48.722 −49.072
A. Yurt et al. / Materials Chemistry and Physics 85 (2004) 420–426
10
2
10
1
10
0
10
-1
10
-2
10
-3
2
logi( m A /c m )
424
-1
-0.9
-0.8
-0.7
-0.6
-0.5 E (V )
-0.4
-0.3
-0.2
-0.1
0
Fig. 3. Tafel polarisation curves for carbon steel in 0.1 M HCl in the presence of 1 × 10−4 M PT at 20 ◦ C (· · · ), 30 ◦ C (
), 40 ◦ C (䊐), 60 ◦ C (+).
Table 4 Polarisation parameters and corresponding inhibition efficiencies for the corrosion of carbon steel in 0.1 M HCl without and with addition of 1 × 10−4 M concentrations of Schiff bases at different temperatures Inhibitor
Temperature (◦ C)
Concentration (M)
–
20 30 40 60
1 1 1 1
× × × ×
PT
20 30 40 60
1 1 1 1
PP
20 30 40 60
TT
TBT
Ecorr (mV)
βa (mV)
βc (mV)
icorr (A cm−2 )
10−1 10−1 10−1 10−1
−526 −525 −510 −510
129.2 134.4 118.8 151.8
−127.5 −124.2 −177.8 −188.4
450 575 1333 5833
– – – –
× × × ×
10−4 10−4 10−4 10−4
−440 −490 −507 −536
40.3 92.7 113.7 125.2
−150.0 −154.5 −201.9 −203.4
75 300 750 3500
83.3 47.8 43.7 40.0
1 1 1 1
× × × ×
10−4 10−4 10−4 10−4
−487 −484 −516 −519
63.8 67.0 111.7 118
−140.2 −121.9 −115.7 −240.4
127 244 620 4000
71.8 57.6 53.5 31.4
20 30 40 60
1 1 1 1
× × × ×
10−4 10−4 10−4 10−4
−490 −513 −513 −526
67.8 108.8 116.6 227.1
−89.1 −137.6 −188.3 −196.8
160 417 1111 5200
64.4 27.5 16.7 10.8
20 30 40 60
1 1 1 1
× × × ×
10−4 10−4 10−4 10−4
−510 −523 −532 −526
86.9 130 138.2 135
−125.6 −138.8 −168.1 −271.0
227 425 1050 4800
49.5 26.1 21.2 17.7
Table 5 Activation parameters of dissolution reaction of carbon steel in 0.1 M HCl solution containing 1 × 10−4 M concentrations of studied Schiff bases Inhibitor
Concentration (M)
– PT PP TT TBT
1 1 1 1 1
× × × × ×
10−1 10−4 10−4 10−4 10−4
Ea (kJ mol−1 )
H∗ (kJ mol−1 )
S∗ (kJ mol−1 )
54.04 76.29 70.97 70.65 62.84
51.45 73.68 68.38 68.06 60.24
−0.020 0.044 0.027 0.030 0.050
IE
inhibition efficiency and increasing of Ea and H∗ values. On the other hand S∗ values were found to be negative in acid solution without inhibitor whereas S∗ values were positive in the presence of studied inhibitors. The rate-determining step for the hydrogen evolution reaction on iron in acid solution is the recombination of adsorbed hydrogen atoms to form hydrogen molecules (step (g)). In the acid solution without inhibitor, the transition state of the rate-determining recombination step represents a more orderly arrangement relative to the initial state ((FeH)ads ),
A. Yurt et al. / Materials Chemistry and Physics 85 (2004) 420–426
425
10
Cdl
ln icorr
8 6 4 2 0 2,9
Rs
y = -9,1764x + 35,795 2 R = 0,9882
3
3,1
3,2
Rt
3,3
3,4
3,5
1/T Fig. 4. The relation between ln icorr vs. 1/T for carbon steel in 0.1 M HCl solutions containing 1 × 10−4 M PT.
3 ln (icorr /T)
2 1 0 -1 -2 2,9
y = -8,8617x + 29,043 R2 = 0,9872 3
3,1
3,2 1/T
3,3
3,4
3,5
Fig. 5. The relation between ln(icorr /T) vs. 1/T for carbon steel in 0.1 M HCl solutions containing 1 × 10−4 M PT.
and hence a negative value for the entropy of activation was obtained. As molecular hydrogen represents more orderly arrangement, entropy of activation is positive in presence of inhibitor [14].
Fig. 7. Electrochemical equivalent circuit diagram for metal–electrolyte interface.
0.1 M HCl solution with PP, TT and TBT. The equivalent circuit model for this system is shown in Fig. 7 and was identical to those reported previously [17]. Solution resistance, Rs , and charge transfer resistance, Rt , values were obtained from Nyquist plots. Inhibition efficiencies, IE, were calculated using the following formula (Eq. (5)) Rt − Rto IE (%) = × 100 (5) Rt where Rt and Rto are the values of the charge transfer resistance with and without inhibitor, respectively. The impedance parameters for the corrosion of carbon steel in 0.1 M HCl without and with addition of various concentrations of PT, PP, TT and TBT are given in Table 6. As can be seen from Table 6, Rt values and inhibition efficiencies are increased with increasing concentrations of Schiff bases. This behaviour reveals the adsorption of Schiff bases on carbon steel. The complex impedance plots have appearance of one capacitive contribution represented by one semicircle for all solutions examined. A characteristic feature of the results was the absence of a diffusive contribution to real impedance
3.2. Electrochemical impedance Corrosion inhibition behaviour of carbon steel in 0.1 M HCl solution with different concentrations of the studied Schiff bases and its blank solution after immersion for 30 min was investigated by EIS at 20 ◦ C. Fig. 6 shows the impedance diagram for carbon steel in 0.1 M HCl solution without and with 1 × 10−4 and 1 × 10−6 M concentrations of PT at 20 ◦ C. Similar Nyquist diagrams were obtained in
Fig. 6. Nyquist impedance diagrams for carbon steel in 0.1 M HCl (䊏) solutions containing 1 × 10−4 M PT ( ), 1 × 10−6 M PT (䉱).
Table 6 Electrochemical parameters of impedance for carbon steel in 0.1 M HCl without and with addition of various concentrations of studied Schiff bases Inhibitor
Concentration (M)
Rs ()
Rt ()
–
1 × 10−1
10.02
114.62
–
9.47 10.10 4.29 11.81
274.74 257.67 173.23 140.71 146.76
58.3 55.5 33.8 18.5 21.8
IE (%)
PT
1 5 1 5 1
× × × × ×
10−4
PP
1 5 1 5 1
× × × × ×
10−4 10−5 10−5 10−6 10−6
15.43 7.84 7.64 8.16 9.76
252.57 194.86 172.84 140.17 127.38
54.6 41.2 33.7 18.2 10.0
TT
1 5 1 5 1
× × × × ×
10−4 10−5 10−5 10−6 10−6
5.73 10.01 10.01 10.21 8.76
230.27 174.75 161.42 138.96 135.41
50.2 34.4 28.9 17.5 20.2
TBT
1 5 1 5 1
× × × × ×
10−4 10−5 10−5 10−6 10−6
6.98 5.97 6.64 4.84 3.73
173.02 169.27 166.69 140.16 136.27
33.7 32.2 31.2 18.2 15.8
10−5 10−5 10−6 10−6
426
A. Yurt et al. / Materials Chemistry and Physics 85 (2004) 420–426
Fig. 8. Nyquist impedance diagram of carbon steel in 0.1 M HCl solutions containing 1 × 10−4 M PT (䊏), 1 × 10−4 M PP ( ), 1 × 10−4 M TT (䊉), 1 × 10−4 M TBT (䉱).
(Z ) at low frequencies. This result indicated that the dissolution mechanism of carbon steel was being controlled by the charge transfer rate across the phase boundary in the presence and absence of the inhibitor. Moreover, the presence of the inhibitor did not alter the mechanism of the dissolution of the carbon steel. The impedance loops measured are often depressed semicircles with their centre below the real axis. This kind of phenomenon is known as dispersing effect. Similar results have been reported in literature for the corrosion of copper in 5% Cl in the presence of Schiff bases [6]. The dispersion of the obtained experimental data was attributed to the dispersion in the capacitive loop and the inhomogeneties of the electrode surface. As mentioned in Tafel polarisation results the effectiveness of studied Schiff bases as corrosion inhibitors depends on the type and nature of the substituents present in the inhibitor molecule. Fig. 8 shows the Nyquist diagram for carbon steel in 0.1 M HCl solution with 1 × 10−4 M PT, PP, TT and TBT. Rt values obtained from Nyquist diagrams given in Fig. 8 are parallel to the order of inhibition efficiencies obtained from Tafel polarisation methods. Moreover as can be seen from impedance data given in Table 6, inhibition efficiencies of the studied Schiff bases obtained from EIS methods are not same as obtained from Tafel extrapolation method given in Table 2, but the trends of their inhibition efficiencies are the same.
4. Conclusions The corrosion behaviour of carbon steel was investigated in 0.1 M HCl with and without addition of various concentrations of Schiff bases at different temperatures, using potentiodynamic and EIS techniques:
• Polarisation measurements showed that all studied Schiff bases causes a decrease in the corrosion current and shifting of corrosion potentials to noble direction. So inhibition of corrosion of carbon steel in 0.1 M HCl by the Schiff bases are under anodic control. • Inhibition efficiencies of Schiff bases depends on the substituent present in inhibitor molecule and follows the order PT > PP > TT > TBT. • The inhibition efficiency of Schiff bases decrease with temperature and its decrease leads to increase activation energy of the corrosion process. • Impedance measurements at Ecorr showed a capacitive loop related to dielectric properties of the surface film. Increasing the charge transfer resistance, Rt , values of inhibitor with increase in the concentration of Schiff bases shows that inhibitive abilities of inhibitors depends on the adsorption of molecule on metal surface. • The adsorption of Schiff bases on carbon steel in 0.1 M HCl solution obeys Temkin’s adsorption isotherm. References [1] B.G. Clubby, Chemical Inhibitors for Corrosion Control, Royal Soc. Chem., Cambridge, 1990 p. 141. [2] E. Sputnik, Z. Ademovic, Lisac, Proceedings of the Eighth European Symposium on Corrosion Inhibitors (8SEIC), Sez V, Suppl., Ann. Univ. Ferrara, NS, 1995, p. 257. [3] C.D. Bain, E.B. Throughton, Y.T. Tao, J. Evall, G.M. Whiteside, J.G. Nuzzo, J. Am. Soc. 111 (1989) 321. [4] R.G. Nuzzo, E.M. Korenic, L.H. Dubois, J. Chem. Phys. 93 (1980) 767. [5] G. Hahner, Ch. Woll, M. Buck, M. Grunze, Langmuir 9 (1) (1993) 955. [6] S.L. Li, S. Chen, S.B. Lei, H. Ma, R. Yu, D. Liu, Corros. Sci. 41 (1999) 1273. [7] S.L. Li, Y.G. Wang, S.H. Chen, R. Yu, S.B. Lei, Corros. Sci. 41 (1999) 1769. [8] Z. Quan, S.H. Chen, Y. Li, X. Cui, Corros. Sci. 44 (2002) 703. [9] B. Erk, A. Dilmac, Y. Baran, A. Balaban, Synth. React. Inorg. Met. Org. Chem. 10 (2000) 30. [10] N.I. Dodoff, Ü. Özdemir, N. Karacan, M.C. Georgieva, S.M. Konstantinov, M.E. Stefanova, Z. Naturforch 54B (1999) 1553. [11] F.E. Anderson, C.J. Duca, J.V. Scudi, J. Am. Chem. Soc. 73 (1951) 4967. [12] N.K. Jha, D.M. Joshi, Synth. React. Inorg. Met. Org. Chem. 14 (1984) 455. [13] H. Shorky, M. Yuasa, I. Sekine, R.M. Issa, H.Y. El-Baradie, G.K. Gomma, Corros. Sci. 40 (1998) 2173. [14] M. Morad, J. Morvan, J. Pagetti, Proceedings of the Eighth European Symposium on Corrosion Inhibitors (8SEIC), Sez V, Suppl. N. 10, Ann. Univ. Ferrara, NS, 1995, p. 159. [15] M. Hosseini, S.F.L. Mertens, M. Ghorbani, M.R. Arshadi, Mater. Chem. Phys. 78 (2003) 800. [16] Z. Szlarska-Smialowska, Corros. Sci. 18 (1978) 557. [17] F. Mansfeld, M.W. Kending, W.J. Lorenz, J. Electrochem. Soc. 132 (1985) 290.
Investigation on some Schiff bases as HCl corrosion inhibitors for carbon steel A. Yurt a , A. Balaban b , S. Ustün Kandemir a , G. Bereket a,∗ , B. Erk b a
Chemistry Department, Faculty of Arts and Science, Osmangazi University, Eskisehir 26480, Turkey b Chemistry Department, Faculty of Science, Gazi University, Ankara, Turkey
Received 10 November 2003; received in revised form 29 December 2003; accepted 29 January 2004
Abstract Potentiodynamic polarisation and ac impedance studies were carried out on the inhibition of carbon steel in 0.1 M hydrochloric acid solution by various Schiff bases containing heteroaromatic substituents. The examined Schiff bases were 2-((1E)-2-aza-2-pyrimidine-2-ylvinyl) thiophene, 2-((1Z)-1-aza-2-(2-pyridyl)vinyl)pyrimidine, 2-((1E)-2-aza-2-(1,3-thiazol-2-yl)vinyl)thiophene, 2-((1Z)-1-aza-2-(2-thienyl)vinyl)benzothiazole. Polarisation curves indicates that studied Schiff bases act essentially as anodic inhibitor. The variation in inhibitive efficiency mainly depends on the type and nature of the substituents present in the inhibitor molecule. Potentiodynamic polarisation and ac impedance measurements carried out at different concentration of studied Schiff bases reveal that these compounds are adsorbed on steel surface and the adsorption obeys Temkin’s adsorption isotherm. From the adsorption isotherms values of equilibrium constant, Kads , values of free energies of adsorption, Gads , were calculated. Activation parameters of the corrosion process such as activation energies, Ea , activation enthalpies, H∗ , and activation entropies, S∗ , were calculated from the obtained corrosion rates at different temperatures. © 2004 Elsevier B.V. All rights reserved. Keywords: Corrosion; Inhibitor; Schiff base; Carbon steel; Hydrochloric acid
1. Introduction Evaluation of corrosion inhibitors for steel in acidic media is important for some industrial facilities as well as is very interesting from theoretical aspects. Most of the effective inhibitors are organic compounds containing nitrogen, phosphorus and sulphur in their structures [1,2]. Heteroatoms such as nitrogen, oxygen and sulphur are capable of forming coordinate covalent bond with metal owing to their free electron pairs and thus acting as inhibitor. Compounds with -bonds also generally exhibit good inhibitive properties due to interaction of orbital with metal surface. Schiff bases—with RC = NR as general formula—have both the features combined with their structure which may then give rise to particularly potential inhibitors. Some Schiff bases have recently reported as effective corrosion inhibitors for mild steel [3], aluminium alloys [4,5], copper [6] in acidic media and as inhibitors for halide corrosion of copper [7,8]. The aim of this work is to investigate inhibitive effect of some Schiff bases containing nitrogen and/or sulphur het∗ Corresponding author. Tel.: +90-222-2290433; fax: +90-222-2393578. E-mail address: [email protected] (G. Bereket).
0254-0584/$ – see front matter © 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.matchemphys.2004.01.033
eroaromatic compound as substituents in their structure for the corrosion of carbon steel in 0.1 M HCl solution. Effects of concentration, temperature and substituents on the inhibition efficiencies of the selected Schiff bases have been studied systematically.
2. Experimental The working electrode was prepared from cylindrical carbon steel bar (0.4% C, 0.9% Mn, 0.04% P, 0.04% S, 98.62% Fe). The electrode was inserted in a Teflon tube and isolated with polyester so that only its cross section (0.3848 cm2 ) was allowed to contact the aggressive solutions. The electrode was polished using different grades of emery paper (600, 800 and 1200) before each experiment, and then rinsed with triply distilled water and finally degreased with acetone. The molecular structures of the examined Schiff bases are listed in Table 1. Studied Schiff bases were synthesised according to published methods [9–12]. Solutions were prepared from double distilled water. Test solutions were deaerated by bubbling ultra pure oxygen-free nitrogen gas for 30 min before use and continued during the tests. The tests were performed in a three-compartment electrochemical cell with a separate compartment for the
A. Yurt et al. / Materials Chemistry and Physics 85 (2004) 420–426
421
Table 1 List of investigated Schiff bases Molecule
Structure
Abbreviation
2-((1E)-2-aza-2-pyrimidine-2-ylvinyl)thiophene
PT
2-((1Z)-1-aza-2-(2-pyridyl)vinyl)pyrimidine
PP
2-((1E)-2-aza-2-(1,3-thiazol-2-yl)vinyl)thiophene
TT
2-((1Z)-1-aza-2-(2-thienyl)vinyl)benzothiazole
TBT
S N N N N N N N
S S N N S S N N
reference electrode connected with the main compartment via a Luggin capillary. The reference electrode was silver– silver chloride (Ag|AgCl|Cl− ) electrode and the auxiliary electrode was platinum electrode. The cell was water-jacketed and was connected to a Giant LTD6-G model thermostat. Potentiodynamic current–voltage characteristics were measured with EG & G PAR363 model potentiostat, VSG 2000 model voltage scan generator and TQ DMS2 model data management system and measured data were saved by PC386. Working electrode was first immersed into the test solution for 30 min to establish a steady state open circuit potential. After measuring the open circuit potential potentiodynamic polarisation curves were obtained with a scan rate of 0.5 mV s−1 in the potential range from −500 to +500 mV relative to the corrosion potential. Corrosion current density values were obtained by Tafel extrapolation method. Electrochemical impedance (EIS) measurements were performed at open circuit potential in the frequency range from 100 kHz to 1 Hz with a signal amplitude perturbation of 5 mV by using CHI 604 Electrochemical Analyser at 20 ◦ C. All experiments were performed using three-electrode system.
3. Results and discussion 3.1. Tafel polarisation measurements Representative Tafel polarisation curves of carbon steel in 0.1 M HCl solutions with different concentrations of PT is shown in Fig. 1. Similar polarisation curves were obtained
in 0.1 M HCl solutions with different concentrations of PP, TT and TBT. Electrochemical parameters obtained from the Tafel extrapolation method of the polarisation curves are given in Table 2. Corrosion currents obtained in 0.1 M HCl solutions with Schiff bases are lower than corrosion currents obtained in acid solution without Schiff bases. Increase in concentration of added Schiff bases causes shifting of corrosion potentials to noble direction and variation in both of Tafel slopes. But anodic Tafel slopes are more enhanced, so inhibition of corrosion of carbon steel in 0.1 M HCl solution by the studied Schiff bases is under anodic control. Increase in inhibition efficiencies with the increase of concentrations of studied Schiff bases shows that inhibition actions are due to adsorption on steel surface. Four types of adsorption may take place by organic molecules at metal/solution interface. (1) Electrostatic attraction between the charged molecules and charged metal. (2) Interaction of uncharged electron pairs in the molecule with the metal. (3) Interaction electrons with the metal. (4) Combination of (1) and (3) [13]. In hydrochloric acid solution the following mechanism is proposed for the corrosion of iron and steel [14]. According to this mechanism anodic dissolution of iron: Fe + Cl− (FeCl− )ads
(a)
(FeCl− )ads (FeCl)ads + e−
(b)
(FeCl)ads → (FeCl+ ) + e−
(c)
+
FeCl Fe
2+
+ Cl
−
(d)
The cathodic hydrogen evolution: Fe + H+ (FeH+ )ads
(e)
422
A. Yurt et al. / Materials Chemistry and Physics 85 (2004) 420–426
10
log i(m A /c m 2 )
10
10
10
10
2
1
0
-1
-2
-0.9
-0.8
-0.7
-0.6
-0.5 E (V )
-0.4
-0.3
-0.2
Fig. 1. Tafel polarisation curves for carbon steel in 0.1 M HCl (—) in the presence of 1 × 10−4 M PT (
) and 1 × 10−5 M PT (+).
Table 2 Polarisation parameters and corresponding inhibition efficiencies for the corrosion of carbon steel in 0.1 M HCl without and with addition of various concentrations of Schiff bases at 20 ◦ C Ecorr (mV)
βa (mV)
βc (mV)
icorr (A cm−2 )
IE
θ
–
10−1
1× 1 × 10−4 5 × 10−5
−526 −440 −462
129.2 40.3 68.4
−127.54 −150.0 −141.1
450 75 100
– 83.3 77.8
– 0.833 0.778
PT
1 × 10−5 5 × 10−6 1 × 10−6
−486 −487 −491
75.0 80.5 76.9
−121.5 −125.0 −123.3
173 213 225
57.4 52.8 50.0
0.574 0.528 0.500
PP
1 5 1 5 1
× × × × ×
10−4 10−5 10−5 10−6 10−6
−487 −502 −500 −513 −510
63.8 86.7 84.8 109.8 95.9
−140.2 −118.9 −101.2 −122.5 −129.0
127 160 189 236 267
71.8 64.4 60.0 47.5 40.7
0.718 0.644 0.600 0.475 0.407
TT
1 5 1 5 1
× × × × ×
10−4 10−5 10−5 10−6 10−6
−490 −497 −503 −513 −526
67.8 80.3 97.0 108.0 116.1
−89.1 −119.4 −126.7 −114.6 −122.6
160 211 255 282 367
64.4 53.1 43.3 37.3 18.5
0.644 0.531 0.433 0.373 0.185
TBT
1 5 1 5 1
× × × × ×
10−4 10−5 10−5 10−6 10−6
−510 −513 −519 −526 −516
86.9 106.5 94.3 127.0 95.2
−125.6 −138.2 −114.1 −134.7 −105.8
227 255 289 350 367
49.5 43.4 35.8 22.2 18.4
0.495 0.434 0.358 0.222 0.184
Inhibitor
Concentration (M)
(FeH+ )ads + e− → (FeH)ads
(f)
+
(g)
−
(FeH)ads + H + e → Fe + H2
In acidic solution imine group as well as nitrogen atoms in heteroaromatic ring can be protonated. Physical adsorption may take place due to electrostatic interaction between protonated forms of Schiff bases and (FeCl− )ads species. Coordinate covalent bond formation between electron pairs of
unprotonated S atoms in heteroaromatic ring and metal surface can take place. Chemisorption of Schiff bases due to interaction of their orbitals with metal surface occurs following deprotonisation step of the physically adsorbed protonated forms of Schiff bases. Examination of experimental data shows that the inhibition efficiency values of the examined Schiff bases follows the order PT > PP > TT > TBT. The variation in inhibitive
efficiency mainly depends on the type and the nature of the substituents present in the inhibitor molecule. The difference in protection action can be attributed to the presence of different substituents to azomethine (–C=N–) group. The difference in inhibition efficiency between PT and PP arises due to presence of electron excess ring (i.e. thiophene) in PT instead of presence of electron deficient ring (i.e. pyridine) in PP as substituent. Presence of electron releasing electron excess ring in PT causes increase of electron density of –C=N– group which gives a better protective action of steel surface than PP. On the other hand difference in inhibition efficiency between PT and TT can be explained by the presence of thiazole ring in TT instead of presence of pyrimidine ring in PT. Chemisorption of studied Schiff bases arise from the interaction of (–C=N–) group, electrons of the aromatic ring and free electron pairs of heteroatoms (S or N) with steel surface. In the case of difference in inhibition efficiencies between PT and TT arise not only in the difference from the electron density of (–C=N–) group but from the difference of strength of interaction free electron pairs on N atom in pyrimidine ring and the strength of interaction free electron pairs of N in thiazole ring with metal. Free electron pairs on N atom in pyrimidine ring are more available than free electron pairs on N atom in thiazole ring, because free electron pairs on sulphur participate conjugation in thiazole ring. On the other hand the difference between inhibition efficiencies of TT and TBT arises due to presence of benzene ring in TBT as a fused to thiazole ring which turns to decrease electron density on S atom in thiazole ring as a result of elongated conjugation. Since adsorption isotherms can provide important clues to the nature of metal–inhibitor interaction it was established isotherms that describe the adsorptive behaviour of the studied corrosion inhibitor. Several adsorption isotherms were assessed and the Temkin adsorption isotherm was found to provide best description of the adsorption behaviour of the investigated Schiff bases, which has the following surface coverage, θ, and the bulk concentration, c, relationship [15] exp(fθ) = Kads c
(1)
where K is the adsorption equilibrium constant and f the molecular interaction constant. Typical plot of θ versus ln c for PT is shown in Fig. 2. From the obtained straight lines in θ–ln c graphs, equilibrium constants for adsorption process, Kads , were obtained. The equilibrium constants for adsorption process is related to the free energy of adsorption, Gads , by 1 −Gads Kads = exp (2) 55.5 RT where 55.5 is the molar concentration of water in the solution. The thermodynamic parameters for adsorption process obtained from Temkin adsorption isotherms for the studied molecules are given in Table 3. The values of free energy of adsorption, Gads , are negative which reveals the spontaneity of adsorption process and stability of the adsorbed layer
θ
A. Yurt et al. / Materials Chemistry and Physics 85 (2004) 420–426
423
0,9 0,8 0,7 0,6 0,5 0,4 0,3 0,2 0,1 0
y = 0,0788x + 1,535 R2 = 0,9038
-15
-10
-5
0
lnc
Fig. 2. Temkin adsorption isotherm of PT on carbon steel in 0.1 M HCl solutions.
on the steel surface. The obtained values of the adsorption free energy, Gads , were >−49 to −58 kJ mol−1 are indicative of chemisorption [16]. In order to study the effect of temperature on the inhibition characteristic of Schiff bases polarisation experiments were conducted at 20, 30, 40 and 60 ◦ C in 0.1 M HCl in presence of Schiff bases at 10−4 M concentration. Typical polarisation curves obtained at 20, 30, 40 and 60 ◦ C in 0.1 M HCl in presence of 1 × 10−4 M PT is shown in Fig. 3. Corrosion characteristics obtained from the Tafel polarisation curves are given in Table 4. Inhibition efficiencies were found to decrease as temperatures were increased. A decrease in inhibition efficiencies with the increase in temperature might be due to weakening of physical adsorption. Physical adsorption in inhibition of corrosion of carbon steel in acidic solution is small but important because it is preceding stage of chemisorption of Schiff bases on carbon steel. In order to calculate activation parameters for the corrosion process Arrhenius equation (3) and transition state equation (4) were used Ea k = A exp (3) RT H ∗ S ∗ RT exp exp − (4) k= Nh R RT Figs. 4 and 5 show the typical plots of ln icorr versus 1/T and ln(icorr /T) versus 1/T graphs for PT, respectively. Kinetic parameters obtained from these graphs are given in Table 5. Inspection of Table 5 shows that higher values were obtained for Ea and H∗ in the presence of inhibitors indicating the higher protection efficiency which was observed for these inhibitors. There is also a parallelism between increases in Table 3 Thermodynamic parameters of adsorption for studied Schiff bases on carbon steel in 0.1 M HCl solutions at 20 ◦ C Inhibitor
Equilibrium constant of adsorption, Kads
PT PP TT TBT
2.88 4.13 8.75 1.01
× × × ×
108 108 106 107
Standard free energy of adsorption, Gads (kJ mol−1 ) −57.234 −58.112 −48.722 −49.072
A. Yurt et al. / Materials Chemistry and Physics 85 (2004) 420–426
10
2
10
1
10
0
10
-1
10
-2
10
-3
2
logi( m A /c m )
424
-1
-0.9
-0.8
-0.7
-0.6
-0.5 E (V )
-0.4
-0.3
-0.2
-0.1
0
Fig. 3. Tafel polarisation curves for carbon steel in 0.1 M HCl in the presence of 1 × 10−4 M PT at 20 ◦ C (· · · ), 30 ◦ C (
), 40 ◦ C (䊐), 60 ◦ C (+).
Table 4 Polarisation parameters and corresponding inhibition efficiencies for the corrosion of carbon steel in 0.1 M HCl without and with addition of 1 × 10−4 M concentrations of Schiff bases at different temperatures Inhibitor
Temperature (◦ C)
Concentration (M)
–
20 30 40 60
1 1 1 1
× × × ×
PT
20 30 40 60
1 1 1 1
PP
20 30 40 60
TT
TBT
Ecorr (mV)
βa (mV)
βc (mV)
icorr (A cm−2 )
10−1 10−1 10−1 10−1
−526 −525 −510 −510
129.2 134.4 118.8 151.8
−127.5 −124.2 −177.8 −188.4
450 575 1333 5833
– – – –
× × × ×
10−4 10−4 10−4 10−4
−440 −490 −507 −536
40.3 92.7 113.7 125.2
−150.0 −154.5 −201.9 −203.4
75 300 750 3500
83.3 47.8 43.7 40.0
1 1 1 1
× × × ×
10−4 10−4 10−4 10−4
−487 −484 −516 −519
63.8 67.0 111.7 118
−140.2 −121.9 −115.7 −240.4
127 244 620 4000
71.8 57.6 53.5 31.4
20 30 40 60
1 1 1 1
× × × ×
10−4 10−4 10−4 10−4
−490 −513 −513 −526
67.8 108.8 116.6 227.1
−89.1 −137.6 −188.3 −196.8
160 417 1111 5200
64.4 27.5 16.7 10.8
20 30 40 60
1 1 1 1
× × × ×
10−4 10−4 10−4 10−4
−510 −523 −532 −526
86.9 130 138.2 135
−125.6 −138.8 −168.1 −271.0
227 425 1050 4800
49.5 26.1 21.2 17.7
Table 5 Activation parameters of dissolution reaction of carbon steel in 0.1 M HCl solution containing 1 × 10−4 M concentrations of studied Schiff bases Inhibitor
Concentration (M)
– PT PP TT TBT
1 1 1 1 1
× × × × ×
10−1 10−4 10−4 10−4 10−4
Ea (kJ mol−1 )
H∗ (kJ mol−1 )
S∗ (kJ mol−1 )
54.04 76.29 70.97 70.65 62.84
51.45 73.68 68.38 68.06 60.24
−0.020 0.044 0.027 0.030 0.050
IE
inhibition efficiency and increasing of Ea and H∗ values. On the other hand S∗ values were found to be negative in acid solution without inhibitor whereas S∗ values were positive in the presence of studied inhibitors. The rate-determining step for the hydrogen evolution reaction on iron in acid solution is the recombination of adsorbed hydrogen atoms to form hydrogen molecules (step (g)). In the acid solution without inhibitor, the transition state of the rate-determining recombination step represents a more orderly arrangement relative to the initial state ((FeH)ads ),
A. Yurt et al. / Materials Chemistry and Physics 85 (2004) 420–426
425
10
Cdl
ln icorr
8 6 4 2 0 2,9
Rs
y = -9,1764x + 35,795 2 R = 0,9882
3
3,1
3,2
Rt
3,3
3,4
3,5
1/T Fig. 4. The relation between ln icorr vs. 1/T for carbon steel in 0.1 M HCl solutions containing 1 × 10−4 M PT.
3 ln (icorr /T)
2 1 0 -1 -2 2,9
y = -8,8617x + 29,043 R2 = 0,9872 3
3,1
3,2 1/T
3,3
3,4
3,5
Fig. 5. The relation between ln(icorr /T) vs. 1/T for carbon steel in 0.1 M HCl solutions containing 1 × 10−4 M PT.
and hence a negative value for the entropy of activation was obtained. As molecular hydrogen represents more orderly arrangement, entropy of activation is positive in presence of inhibitor [14].
Fig. 7. Electrochemical equivalent circuit diagram for metal–electrolyte interface.
0.1 M HCl solution with PP, TT and TBT. The equivalent circuit model for this system is shown in Fig. 7 and was identical to those reported previously [17]. Solution resistance, Rs , and charge transfer resistance, Rt , values were obtained from Nyquist plots. Inhibition efficiencies, IE, were calculated using the following formula (Eq. (5)) Rt − Rto IE (%) = × 100 (5) Rt where Rt and Rto are the values of the charge transfer resistance with and without inhibitor, respectively. The impedance parameters for the corrosion of carbon steel in 0.1 M HCl without and with addition of various concentrations of PT, PP, TT and TBT are given in Table 6. As can be seen from Table 6, Rt values and inhibition efficiencies are increased with increasing concentrations of Schiff bases. This behaviour reveals the adsorption of Schiff bases on carbon steel. The complex impedance plots have appearance of one capacitive contribution represented by one semicircle for all solutions examined. A characteristic feature of the results was the absence of a diffusive contribution to real impedance
3.2. Electrochemical impedance Corrosion inhibition behaviour of carbon steel in 0.1 M HCl solution with different concentrations of the studied Schiff bases and its blank solution after immersion for 30 min was investigated by EIS at 20 ◦ C. Fig. 6 shows the impedance diagram for carbon steel in 0.1 M HCl solution without and with 1 × 10−4 and 1 × 10−6 M concentrations of PT at 20 ◦ C. Similar Nyquist diagrams were obtained in
Fig. 6. Nyquist impedance diagrams for carbon steel in 0.1 M HCl (䊏) solutions containing 1 × 10−4 M PT ( ), 1 × 10−6 M PT (䉱).
Table 6 Electrochemical parameters of impedance for carbon steel in 0.1 M HCl without and with addition of various concentrations of studied Schiff bases Inhibitor
Concentration (M)
Rs ()
Rt ()
–
1 × 10−1
10.02
114.62
–
9.47 10.10 4.29 11.81
274.74 257.67 173.23 140.71 146.76
58.3 55.5 33.8 18.5 21.8
IE (%)
PT
1 5 1 5 1
× × × × ×
10−4
PP
1 5 1 5 1
× × × × ×
10−4 10−5 10−5 10−6 10−6
15.43 7.84 7.64 8.16 9.76
252.57 194.86 172.84 140.17 127.38
54.6 41.2 33.7 18.2 10.0
TT
1 5 1 5 1
× × × × ×
10−4 10−5 10−5 10−6 10−6
5.73 10.01 10.01 10.21 8.76
230.27 174.75 161.42 138.96 135.41
50.2 34.4 28.9 17.5 20.2
TBT
1 5 1 5 1
× × × × ×
10−4 10−5 10−5 10−6 10−6
6.98 5.97 6.64 4.84 3.73
173.02 169.27 166.69 140.16 136.27
33.7 32.2 31.2 18.2 15.8
10−5 10−5 10−6 10−6
426
A. Yurt et al. / Materials Chemistry and Physics 85 (2004) 420–426
Fig. 8. Nyquist impedance diagram of carbon steel in 0.1 M HCl solutions containing 1 × 10−4 M PT (䊏), 1 × 10−4 M PP ( ), 1 × 10−4 M TT (䊉), 1 × 10−4 M TBT (䉱).
(Z ) at low frequencies. This result indicated that the dissolution mechanism of carbon steel was being controlled by the charge transfer rate across the phase boundary in the presence and absence of the inhibitor. Moreover, the presence of the inhibitor did not alter the mechanism of the dissolution of the carbon steel. The impedance loops measured are often depressed semicircles with their centre below the real axis. This kind of phenomenon is known as dispersing effect. Similar results have been reported in literature for the corrosion of copper in 5% Cl in the presence of Schiff bases [6]. The dispersion of the obtained experimental data was attributed to the dispersion in the capacitive loop and the inhomogeneties of the electrode surface. As mentioned in Tafel polarisation results the effectiveness of studied Schiff bases as corrosion inhibitors depends on the type and nature of the substituents present in the inhibitor molecule. Fig. 8 shows the Nyquist diagram for carbon steel in 0.1 M HCl solution with 1 × 10−4 M PT, PP, TT and TBT. Rt values obtained from Nyquist diagrams given in Fig. 8 are parallel to the order of inhibition efficiencies obtained from Tafel polarisation methods. Moreover as can be seen from impedance data given in Table 6, inhibition efficiencies of the studied Schiff bases obtained from EIS methods are not same as obtained from Tafel extrapolation method given in Table 2, but the trends of their inhibition efficiencies are the same.
4. Conclusions The corrosion behaviour of carbon steel was investigated in 0.1 M HCl with and without addition of various concentrations of Schiff bases at different temperatures, using potentiodynamic and EIS techniques:
• Polarisation measurements showed that all studied Schiff bases causes a decrease in the corrosion current and shifting of corrosion potentials to noble direction. So inhibition of corrosion of carbon steel in 0.1 M HCl by the Schiff bases are under anodic control. • Inhibition efficiencies of Schiff bases depends on the substituent present in inhibitor molecule and follows the order PT > PP > TT > TBT. • The inhibition efficiency of Schiff bases decrease with temperature and its decrease leads to increase activation energy of the corrosion process. • Impedance measurements at Ecorr showed a capacitive loop related to dielectric properties of the surface film. Increasing the charge transfer resistance, Rt , values of inhibitor with increase in the concentration of Schiff bases shows that inhibitive abilities of inhibitors depends on the adsorption of molecule on metal surface. • The adsorption of Schiff bases on carbon steel in 0.1 M HCl solution obeys Temkin’s adsorption isotherm. References [1] B.G. Clubby, Chemical Inhibitors for Corrosion Control, Royal Soc. Chem., Cambridge, 1990 p. 141. [2] E. Sputnik, Z. Ademovic, Lisac, Proceedings of the Eighth European Symposium on Corrosion Inhibitors (8SEIC), Sez V, Suppl., Ann. Univ. Ferrara, NS, 1995, p. 257. [3] C.D. Bain, E.B. Throughton, Y.T. Tao, J. Evall, G.M. Whiteside, J.G. Nuzzo, J. Am. Soc. 111 (1989) 321. [4] R.G. Nuzzo, E.M. Korenic, L.H. Dubois, J. Chem. Phys. 93 (1980) 767. [5] G. Hahner, Ch. Woll, M. Buck, M. Grunze, Langmuir 9 (1) (1993) 955. [6] S.L. Li, S. Chen, S.B. Lei, H. Ma, R. Yu, D. Liu, Corros. Sci. 41 (1999) 1273. [7] S.L. Li, Y.G. Wang, S.H. Chen, R. Yu, S.B. Lei, Corros. Sci. 41 (1999) 1769. [8] Z. Quan, S.H. Chen, Y. Li, X. Cui, Corros. Sci. 44 (2002) 703. [9] B. Erk, A. Dilmac, Y. Baran, A. Balaban, Synth. React. Inorg. Met. Org. Chem. 10 (2000) 30. [10] N.I. Dodoff, Ü. Özdemir, N. Karacan, M.C. Georgieva, S.M. Konstantinov, M.E. Stefanova, Z. Naturforch 54B (1999) 1553. [11] F.E. Anderson, C.J. Duca, J.V. Scudi, J. Am. Chem. Soc. 73 (1951) 4967. [12] N.K. Jha, D.M. Joshi, Synth. React. Inorg. Met. Org. Chem. 14 (1984) 455. [13] H. Shorky, M. Yuasa, I. Sekine, R.M. Issa, H.Y. El-Baradie, G.K. Gomma, Corros. Sci. 40 (1998) 2173. [14] M. Morad, J. Morvan, J. Pagetti, Proceedings of the Eighth European Symposium on Corrosion Inhibitors (8SEIC), Sez V, Suppl. N. 10, Ann. Univ. Ferrara, NS, 1995, p. 159. [15] M. Hosseini, S.F.L. Mertens, M. Ghorbani, M.R. Arshadi, Mater. Chem. Phys. 78 (2003) 800. [16] Z. Szlarska-Smialowska, Corros. Sci. 18 (1978) 557. [17] F. Mansfeld, M.W. Kending, W.J. Lorenz, J. Electrochem. Soc. 132 (1985) 290.