Saifi Issaadi , Tahar Douadi and Ahmed Zouaoui
Keywords: Corrosion; Inhibitor; Schiff base; mild steel; Hydrochloric acid.
Abstract:
The inhibiting effect Schiff base on the corrosion of the mild steel in 1M HCl has been studied by electrochemical impedance spectroscopy (EIS) and Tafel polarisation measurements. The Schiff base 4,4’- bis(salicylideneimino) diphenylether (S) is synthesized from salicylaldehyde and the corresponding amine. Polarization curves indicated that the studied Schiff base act as mixed type (cathodic/anodic) inhibitor and the inhibition efficiency have increased when the concentration of the Schiff base have increased. Thermodynamic adsorption parameters (Kads, ΔGads) of studied Schiff base were calculated using the Langmuir adsorption isotherm. Activation parameters of the corrosion process such as activation energies, Ea, activation enthalpies, ΔH*, and activation entropies, ΔS*, were calculated by the obtained corrosion currents at different temperatures.
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ISSN 1466-8858 Volume 12, Preprint 30 submitted 23 June 2009 The inhibitive effect of Schiff base compound on corrosion behaviour of mild steel in hydrochloric acid solution Saifi Issaadi , Tahar Douadi, Ahmed Zouaoui Issaadi2001@yahoo.fr Laboratoire d’Électrochimie des Matériaux Moléculaires et Complexes, Faculté des Sciences de l’Ingénieur, Université Ferhat Abbas, DZ-19000 Sétif (Algeria) Abstract The inhibiting effect Schiff base on the corrosion of the mild steel in 1M HCl has been studied by electrochemical impedance spectroscopy (EIS) and Tafel polarisation measurements. The Schiff base 4,4’- bis(salicylideneimino) diphenylether (S) is synthesized from salicylaldehyde and the corresponding amine. Polarization curves indicated that the studied Schiff base act as mixed type (cathodic/anodic) inhibitor and the inhibition efficiency have increased when the concentration of the Schiff base have increased. Thermodynamic adsorption parameters (Kads, ΔGads) of studied Schiff base were calculated using the Langmuir adsorption isotherm. Activation parameters of the corrosion process such as activation energies, Ea, activation enthalpies, ΔH*, and activation entropies, ΔS*, were calculated by the obtained corrosion currents at different temperatures. Keywords: Corrosion; Inhibitor; Schiff base; mild steel; Hydrochloric acid. © 2009 University of Manchester and the authors. This is a preprint of a paper that has been submitted for publication in the Journal of Corrosion Science and Engineering. It will be reviewed and, subject 1 to the reviewers’ comments, be published online at http://www.jcse.org in due course. Until such time as it has been fully published it should not normally be referenced in published work. ISSN 1466-8858 Volume 12, Preprint 30 submitted 23 June 2009 Introduction Chemical cleaning and pickling processes are widely used in industrial processes. The most important fields of application are acid pickling and industrial acid cleaning bathes. Because of their aggressiveness, the use of corrosion inhibitors reduces the rate of dissolution of metals and it is considered as the most effective method for the protection of many metals and alloys against such acid attack [1-4].The adsorption of these compounds is infuenced by the electronic structure of inhibiting molecules, steric factor, aromaticity, and electron density at donor site, presence of functional group such as -CHO, -C=N-, R-OH etc., molecular area and molecular weight of the inhibitor molecule [5-8]. Schiff bases can be synthesized easily from relatively cheap initial materials. They have been investigated for their corrosion inhibition effect on various metals and alloys in acid media. Schiff bases are the condensation product of an amine and a ketone or aldehyde. The inhibition efficiency of Schiff bases is much more than that of the corresponding amines and aldehyde due to the presence of the RC= NR group which enriches the electron cloud. In the structure of the Schiff base, the atoms of the aromatic ring and -C= N group can form a π bond. Then, π electrons in the Schiff base molecule not only can locate the unoccupied orbital of the transition metal, but also can accept the electrons of the d orbital of the transition metal to form feed backmetal-inhibitor bond, which is not possible with an amine. Based on the presence of nitrogen atoms and the imine functional group in its structure, Schiff base molecule may reasonably justify its use as an effective corrosion inhibitor [9,10]. The aim of this work is to investigate inhibitive effect of Schiff base compound (S) for the corrosion of mild steel in 1M HCl solution in the absence and presence of inhibitor were studied via electrochemical impedance and polarisation methods. © 2009 University of Manchester and the authors. This is a preprint of a paper that has been submitted for publication in the Journal of Corrosion Science and Engineering. It will be reviewed and, subject 2 to the reviewers’ comments, be published online at http://www.jcse.org in due course. Until such time as it has been fully published it should not normally be referenced in published work. ISSN 2.1466-8858 Experimental Volume 12, Preprint 30 submitted 23 June 2009 Compound (S) is synthesized according to published methods [11, 12]. To 1 mmol of diamine (hot EtOH, 20 mL) was added dropwise 2 mmol of salicylaldehyde (Fluka Chemical Company). m.p. = 209 0C; IR (KBr): ν(C=N) = 1620 cm-1. The 1H NMR spectrum exhibits a singulet at 13.3 ppm, corresponding to phenolic group, the aromatic protons give a multiplet centred at 6.5 ppm . A singulet at 8.5 ppm is due to the imino part. The molecule structure of commpound (S) is shown in Fig. 1. The 1H NMR spectra were recorded with using a Bruker AM-250 (France). The electrolyte solution was 1 M HCl, prepared from analytical grade 37% HCl (Merck) and double distilled water. All tests have been performed in deaerated solutions and at ambient temperature. The working electrode was prepared from a cylindrical mild steel (MS) rods having following chemical composition: C 0.7%,Mn 0.4%, Cu 0.12%, Si 0.05%, P 0.025%, S 0.025%, Sn 0.01%, Ni 0.009% and remainder iron. The electrode was inserted in Teflon tube and isolated with polyester was allowed to contact the aggressive solutions. The electrode was polished using different grades of emery paper (800 and 1200) before each experiment, rinsed with double distilled water and finally degreased with acetone. The concentration range of inhibitor employed was 1x10-4 to 5x10-3 M in 1 M HCl. Electrochemical measurements were conducted in a conventional three-electrode thermostated cell. A paltinium disk as counter electrode and standard calomel electrode (SCE) as the reference electrode have been used in the electrochemical studies. The potentiodynamic curves were recorded using a PGZ 301 voltalab 4 system connected to a personal computer. The working electrode was first immersed into the test solution for 120 min to establish a steady state open circuit potential. After measuring the open circuit potential dynamic polarisaion curves were obtained with a scan rate of 1 mV/s in the potential range from -150 to +150 mV relative to the Eocp. Corrosion current density values were obtained by Tafel extrapolation method. Electrochemical impedance (EIS) measurements were performed at © 2009 University of Manchester and the authors. This is a preprint of a paper that has been submitted for publication in the Journal of Corrosion Science and Engineering. It will be reviewed and, subject 3 to the reviewers’ comments, be published online at http://www.jcse.org in due course. Until such time as it has been fully published it should not normally be referenced in published work. ISSN 1466-8858 open circuit Preprint 30 to 80 mhz. The cyclic submitted 23 June 2009 potentiel in the frequencyVolume range 12, from 80 kHz voltammetry was carried out for bare electrode and inhibitor covered electrode in the test solution. The working electrode was scanned from negative to positive in the potential range of -0.6V to 0.0V at a scan rate of 50 mVs-1. 3. Results and discussion 3.1. Tafel polarisation measurements Polarization profiles for mild steel in 1 M HCl in the absence and presence of (S) of various concentrations is shown in Fig. 2. Corrosion parameters, such as corrosion potential (Ecorr), corrosion current density (Icorr) obtained by extrapolation of the Tafel lines and the inhibition efficiency values η (%) are listed in table 1. The maximum inhibition efficiency (80 %) was obtained at 5x10-3 M. The presence of Schiff base in HCl solution resulted in shift of corrosion potential towards more negative direction in comparison with that obtained in the absence of inhibitor. These results indicate that all Schiff base act as mixed type inhibitor with predominant control of cathodic reaction and variation in both of Tafel slopes [13]. Increase in inhibition efficiencies with the increase of concentrations of studied Schiff base shows that inhibition actions are due to adsorption on mild steel surface. The inhibition efficiencies; η, were calculated using the following formula : η (%) = i0corr – icorr i0corr x 100 (1) Where i and i0 are values of the current densities wih and without inhibitor, respectively. Fig.3 shows the change of η with the inhibitor concentration. The increase in the inhibition efficiencies of mild steel, in 1M acid chlorid solution, with increasing additive concentration can be explained on the basis of additive adsorption. © 2009 University of Manchester and the authors. This is a preprint of a paper that has been submitted for publication in the Journal of Corrosion Science and Engineering. It will be reviewed and, subject 4 to the reviewers’ comments, be published online at http://www.jcse.org in due course. Until such time as it has been fully published it should not normally be referenced in published work. ISSN 1466-8858 3.2. Electrochemical impedance Volume 12, Preprint 30 submitted 23 June 2009 Corrosion inhibition behaviour of mild steel in 1M HCl solution with different concentrations of the studied Schiff base was investigated by EIS. Fig. 4 , shows the impedance diagrams for mild steel in 1M HCl solution without and with addition of compound (S) a at various concentrations. The corresponding Bode plots are shown in Fig. 5. Generally the Nyquist plots are regarded as semicircles. These types of Nyquist plots can be modeled by a simple Randel’s circuit including the charge transfer resistance (Rt) parallel with double layer capacitance (Cdl) in series with solution resistance (Rs) [14].The equivalent circuit model for this system shown in Fig. 6, was identical to those reported previously [15,16]. Solution resistance, Rs, and charge transfer resistance, Rt, values were obtained from nyquist plots. Inhibition efficiencies, ηz, were calculated using the following formula (Eq. ) η z (%) = Rt – Rt0 Rt x 100 (2) Where Rt and Rt0 are the values of the charge transfer resistance with and without inhibitor, respectively. The impedance parameters for the corrosion of mild steel in 1 HCl are given in table 2. Rt values obtained from Nyquist diagrams given in table 2, are parallel to the order of inhibition efficiencies obtained from Tafel polarisation methods. The obtained results show that the inhibition efficiency increases with inhibitor concentration . It is clear, that inhibition efficiencies of the studied Schiff base obtained from the two methods are not the same but their trends are the same. The inhibition action of (S) towards the corrosion of mild steel in 1M HCl can be explained in termes of interaction between the adsorbed species [17] which is known to depend on the chemical structure of the inhibitor [18]. Iron is well known for its coordination affinity towards nitrogen and oxygen bearing ligand. © 2009 University of Manchester and the authors. This is a preprint of a paper that has been submitted for publication in the Journal of Corrosion Science and Engineering. It will be reviewed and, subject 5 to the reviewers’ comments, be published online at http://www.jcse.org in due course. Until such time as it has been fully published it should not normally be referenced in published work. ISSN 1466-8858 3.3. Cyclic voltammetric studies Volume 12, Preprint 30 submitted 23 June 2009 Cyclic voltammograms for mild steel in 1M HCl in the absence and presence of inhbitor (S) is shown in Fig. 7. It can be seen that the presence of inhbitor (S) causes significant decrease in the current density of the anodic peaks. The results show that (S) is an effective inhibitor for miled steel . 3.4. Adsorption isotherm and thermodynamics calculations The interactions between the inhibitor and the mild steel surface can be examined by the adsorption isotherm. The degree of surface coverage values (θ) for various concentrations of the inhibitor in acidic media have been evaluated from the polarisation measurements Table 2. Suitable adsorption isotherm was obtained, using these calculated values. The linear relationships of C/θ versus C, depicted in Fig. 8, suggest that the adsorption of compound (S) on the mild steel obeyed the Langmuir adsorption isotherm [19]. This isotherm can be represented as C q 1 = k +C (3) The degree of surface coverage of each inhibitor at a given concentration can be calculated using the equation: q= i0 – i i0 (4) The strong correlation (R2 = 0.997) of the Langmuir adsorption isotherm for (S) was observed. Langmuir adsorption isotherm assumes that the adsorption of organic molecule on the adsorbent is monolayer.Values of adsorption equlibrium constant (Kads) calculated from the Langmuir adsorption isotherm is 1.1×104 . The relatively high values of the adsorption equilibrium constant reflect the high adsorption ability of these molecules on mild steel © 2009 University of Manchester and the authors. This is a preprint of a paper that has been submitted for publication in the Journal of Corrosion Science and Engineering. It will be reviewed and, subject 6 to the reviewers’ comments, be published online at http://www.jcse.org in due course. Until such time as it has been fully published it should not normally be referenced in published work. ISSN 1466-8858 surface. The Volume 30 23 June free energy of adsorption ΔGads12,ofPreprint the inhibitors on mild steelsubmitted surface can be 2009 determined using the following relation: ΔGads = -RT lnKads (5) The negative values of ΔGads indicating the spontaneously adsorption of these molecules on the metal surface is found to be -22.76 kJ mol-1 . Values of ΔGads around -20 kJ mol-1 or lower are consistent with the electrostatic interaction between charged organic molecules and the charged metal surface (physisorption); those around -40 kJ mol-1 or higher involve charge sharing or transfer from the organic molecule to the metal surface to form a co-ordinate type of bond (chemisorption). The values of ΔGads for both compounds being less than -40 kJ mol-1 indicates physical adsorption. In addition to electrostatic interaction, there may be some other interactions. The effect of temperature on the corrosion parameters of mild steel in free and inhibited solutions of 1 M HCl was studied using polarization technique in temperature range of 20-40 0 C. The acid solutions were inhibited by addition of 5.10-3 M of inhibitor (S). The obtained corrosion parameters are given in Table 3 and show when temperature increases, in the absence and presence of inhibitor, the Icorr increases. From Table 3 can find that the investigated Schiff base hase inhibiting properties at all the studied temperatures and when temperature increases the values of inhibition efficiencies decrease. A decrease in inhibition efficiencies with the increase in temperature might be due to weakening of physical adsorption. The activation energy of corrosion process with and without the inhibitor could be calculated using the equation k=A exp ( -Ea ) RT (6) © 2009 University of Manchester and the authors. This is a preprint of a paper that has been submitted for publication in the Journal of Corrosion Science and Engineering. It will be reviewed and, subject 7 to the reviewers’ comments, be published online at http://www.jcse.org in due course. Until such time as it has been fully published it should not normally be referenced in published work. ISSN 1466-8858 where Ea is 12, Preprint 30 T the absolute temperature, submitted 23 the activation energy, A Volume the frequency factor, RJune the 2009 gas constant and k is the rate of corrosion reaction, that is directly proportional to the corrosion current density. The apparent activation energies (Ea) and pre-exponential factors (A) at 5.10-3 M of inhibitors are calculated by linear regression between ln (Icorr) and 1/T Fig 9, and also the results shown in Table 4. All the linear regression coefficients are close to 1, indicating that the mild steel corrosion in hydrochloric acid can be elucidated using the kinetic model. Enthalpy and entropy of activation (ΔH*, ΔS*) were calculated from the equation: K= RT exp ( ΔS*)exp (- ΔH*) Nh R RT (7) where h is the Plank constant, N is the Avogadros number. A plot of ln(Icorr/T) versus 1/T equa 7, gave straight lines as shown in Fig 10, for mild steel dissolution in 1 HCl in the absence and presence of 5.10-3 M of inhibitor (S ). Straight lines are obtained with a slope of ΔH*/R and an intercept of lnR/Nh + ΔS*/R from which the values of ΔH* and ΔS* are calculated and listed in table 4. Inspection of these data reveals the positive sign for both Ea and ΔH*, reflecting the endothermic nature of corrosion process. It is obviously seen that the activation energy strongly increases in the presence of inhibitor, this result due to that the inhibitor species are physically adsorbed on the metal surface [20]. The negative values of ΔS* pointed to a greater order produced during the process of activation. This can be achieved by the formation of activated complex represents association or fixation with consequent loss in the degrees of freedom of the system during the process [21]. © 2009 University of Manchester and the authors. This is a preprint of a paper that has been submitted for publication in the Journal of Corrosion Science and Engineering. It will be reviewed and, subject 8 to the reviewers’ comments, be published online at http://www.jcse.org in due course. Until such time as it has been fully published it should not normally be referenced in published work. ISSN 4.1466-8858 Conclusion Volume 12, Preprint 30 submitted 23 June 2009 The corrosion behaviour of mild steel was investigated in 1 M HCl with and without addition of various concentrations of Schiff base at different concentrations, using potentiodynamic and electrochemical impedance techniques. -Polarization curves indicated that all studied Schiff base act as mixed type (cathodic/anodic) inhibitor. -The Schiff bases adsorbed on mild steel surface followed Langmuir adsorption isotherm. The calculated values of free energy adsorption of the studied Schiff base indicated that their adsorption on mild steel in 1M HCl is physical in nature. References [1] H. Ashassi-Sorkhabia, M.R. Majidi, K. Seyyedi, Appl. Surf. Sci 225, 176-185 (2004). [2] K.C. Emregül, A.A. Akay, O. Atakol, Mater. Chem. Phys 93, 325-329 (2005). [3] J. Cruz, R. Martinez, J. Genesca, E.G. Ochoa, J. Electroanal. Chem 566, 111-121(2004). [4] H. Ashassi-Sorkhabia, B. Shaabani, D. Seifzadeh, Appl. Surf. Sci 239, 154-164 (2005). [5] F. Bentiss, M. Lagrenee, M. Traisnel, J.C. Hornez. Corros Sci 41, 789-803 (1999) [6] F.B. Growcock, W.W. Frenier, P.A. Andreozzi. Corrosion 45, 1007-15 (1989). [7] I. Lukovits, E. Kalman, G. Palinkas. Corrosion 51, 201-5 (1995). [8] R.C. Ayers Jr, N. Hackerman. J Electrochem Soc 110, 507-13 (1963). [9] J.D. Talati, M.N. Desai, N.K. Shah, Mater. Chem. Phys 93; 54-64(2005). [10] A. Aytac¸ , Ü. Özmen, M. Kabasakalo¢glu, Mater. Chem. Phys 89, 176-181(2005). [11] Y. S. Sharma, H. N. Pandey, Mathur. Polyhedron 13, 3111-3117 (1994) [12] P. Gili, M. G.M. Reyes, P. M. Zarza, I.L. F. Machado, M. F. C. Guedes da Silva, A A Lemos, A. J. L. Pombiero. Inorg. Chim. Acta 244, 25-36 (1996) [13] K. F. Khaled, N. Hackenman. Electrochimica Acta 49, 485-495 (2004) © 2009 University of Manchester and the authors. This is a preprint of a paper that has been submitted for publication in the Journal of Corrosion Science and Engineering. It will be reviewed and, subject 9 to the reviewers’ comments, be published online at http://www.jcse.org in due course. Until such time as it has been fully published it should not normally be referenced in published work. ISSN 1466-8858 [14] H. Ashassi-Sorkhabi, Volume 12, Preprint 30 Surf. Sci 239,154-164 submitted 23 June 2009 B. Shaabani, D. Seifzadeh. Appl. (2005) [15] M. Bouklah, M. hammoutin. T. Benkaddourn. Benhadda. Journal of applied Electrochemistry 35, 1095-1101(2005) [16] F. Mansfeld, M.W. Kending, W.J. Lorenz. J. Electrochem. Soc; 132 ,290-310 (1985). [17] H.L. Wang, H.B. Fan, J.S. Zheng. Mater. Chem. Phys 77,655-661 (2002) [18] X L. Cheng, H. Y. Ma, S. H. Chen, R. Yu, X. Chen, Z .M. Yao. Sci 41,321-333(1999) [19] C. Kaan. Emregul, Orhan Atakol. Materials Chemistry and Physics 83, 373-379 (2004) [20] G. Moretti, F .Guidi, G. Grion. Corr. Sci 46 (387–403)2004 [21] M. Benabdellah, A. Aouniti, A. Dafali, B. Hammouti, M. Benkaddour, A. Yahyi, A. Ettouhami. Appl. Surf. Sci 252, 8341-8355 (2006). © 2009 University of Manchester and the authors. This is a preprint of a paper that has been submitted for publication in the Journal of Corrosion Science and Engineering. It will be reviewed and, subject 10to the reviewers’ comments, be published online at http://www.jcse.org in due course. Until such time as it has been fully published it should not normally be referenced in published work. ISSN 1466-8858 Volume 12, Preprint 30 submitted 23 June 2009 Table 1 Polarisation parameters and corresponding inhibition efficiency for the mild steel in 1 M HCl with and without addition of various concentrations of Schiff base S. Inhibitor Cncentration -Ecorr (M) HCl S (mV) Icorr ba -bc ηp (µAcm-2) (mV/dec) (mV/dec) (%) - q 1 467 521 166 68 1x10-4 451 215 69 177 58.7 58 5x10-4 474 172 111 131 66.9 66 -3 1x10 480 124 125 175 76.9 76 5x10-3 478 100 76 71.7 80.8 80 Table 2 Electrochimical impedance parameters for mild steel in 1 M HCl without and with addition of various concentrations of studied Schiff base S. Inhibitor C Rs (M) (Ω cm2) HCl S Rt Cdl ηz (Ω cm2 ) (µF cm-2 ) (%) 1 0.7 44 215 - 1x10-4 0.9 129 101 65.8 5x10-4 0.97 140 98 68.5 1x10-3 1.02 155 83 71.6 5x10-3 1.44 191 81 76.9 © 2009 University of Manchester and the authors. This is a preprint of a paper that has been submitted for publication in the Journal of Corrosion Science and Engineering. It will be reviewed and, subject 11to the reviewers’ comments, be published online at http://www.jcse.org in due course. Until such time as it has been fully published it should not normally be referenced in published work. ISSN 1466-8858 Table 3 Volume 12, Preprint 30 submitted 23 June 2009 Polarization parameters for the corrosion of the mild steel in 1 HCl without and with addition of 5.10-3 M of schiff base at different temperatures. Inhibitor Temperature (0C) -Ecorr (mV) Icorr(µAcm-2) ba(mV) -bc(mV) ηt (%) HCl S 20 467 521 68.8 156 - 30 500 591 128 103 - 40 507 626 115 126 - 20 478 100 76 71.7 80.8 30 512 165 107 116 72.0 40 507 233 98 129 62.7 Table 4 Activation parameters of dissolution reaction of mild steel in 1 HCl solution containing 5.10-3 M concentrations of studied Schiff base Inhibitor Ea (kJ mol-1) ΔH* (kJ mol-1) HCl 6.73 6.4 S 32.11 29.45 ΔS* (J mol-1 K-1) - 171 -104.49 © 2009 University of Manchester and the authors. This is a preprint of a paper that has been submitted for publication in the Journal of Corrosion Science and Engineering. It will be reviewed and, subject 12to the reviewers’ comments, be published online at http://www.jcse.org in due course. Until such time as it has been fully published it should not normally be referenced in published work. ISSN 1466-8858 Volume 12, Preprint 30 submitted 23 June 2009 O CH N OH N CH HO 4,4’- bis(salicylideneimino) diphenylether Fig. 1. Structure of studied Schiff base (S) © 2009 University of Manchester and the authors. This is a preprint of a paper that has been submitted for publication in the Journal of Corrosion Science and Engineering. It will be reviewed and, subject 13to the reviewers’ comments, be published online at http://www.jcse.org in due course. Until such time as it has been fully published it should not normally be referenced in published work. ISSN 1466-8858 Volume 12, Preprint 30 submitted 23 June 2009 1,5 1,0 1 0,5 0,0 43 5 2 2 i(mA/cm ) -0,5 -1,0 -1,5 1: HCl 1M -4 2: 1x10 M -4 3: 5x10 M -3 4: 1x10 M -3 5: 5x10 M -2,0 -2,5 -3,0 -3,5 -650 -600 -550 -500 -450 -400 -350 -300 E(mVvs SCE) Fig. 2. Tafel polarisation curves for mild steel in 1M HCl in the presence and absence of different concentrations of compound S 80 75 h% 70 65 60 55 0,000 0,001 0,002 0,003 0,004 0,005 C(M) Fig. 3 . Effect of inhibitor concentration on the efficiencies of mild steel in 1M HCl for compound S. © 2009 University of Manchester and the authors. This is a preprint of a paper that has been submitted for publication in the Journal of Corrosion Science and Engineering. It will be reviewed and, subject 14to the reviewers’ comments, be published online at http://www.jcse.org in due course. Until such time as it has been fully published it should not normally be referenced in published work. ISSN 1466-8858 Volume 12, Preprint 30 HCl1M -4 10 -4 5.10 -3 10 -3 5.10 60 50 40 -2 -Zi(ohm.cm ) submitted 23 June 2009 30 20 10 0 0 20 40 60 80 100 120 140 160 180 200 220 2 Zr(ohm.cm ) Fig. 4. Nyquist impedance diagrams for mild steel in 1M HCl solution in the presence and absence of different concentrations of compound S 2,4 2,0 1,8 2 1: HCl 1M -4 2: 1.10 M -4 2: 5.10 M -3 2: 1.10 M -3 2: 5.10 M 1 1,6 LogZ(ohm.cm ) 5 4 3 2 2,2 1,4 1,2 1,0 0,8 0,6 0,4 0,2 0,0 -0,2 -2 -1 0 1 2 3 4 5 Logf(Hz) Fig. 5. Bods plots for mild steel in 1 M HCl in the absence and presence of S at different concentrations. © 2009 University of Manchester and the authors. This is a preprint of a paper that has been submitted for publication in the Journal of Corrosion Science and Engineering. It will be reviewed and, subject 15to the reviewers’ comments, be published online at http://www.jcse.org in due course. Until such time as it has been fully published it should not normally be referenced in published work. ISSN 1466-8858 Volume 12, Preprint 30 submitted 23 June 2009 CdI Rs Rt Fig.6. Electrochemical equivalent circuit diagram for metal- electrolyte interface obtained for compound S. 1-H C l 1M 3 2- S : 5.10- M 300 1 250 2 I(m A/Cm) 200 150 2 100 50 0 -50 -600 -500 -400 -300 -200 -100 0 E (m V vsS C E ) Fig. 7. Cyclic voltammograms for mild steel in 1M HCl in the absence and presence of inhbitors S1 and S2 at 5.10-3 M 0,007 2 R = 0.9999 0,006 0,005 C /q 0,004 0,003 0,002 0,001 0,000 0,000 0,001 0,002 0,003 0,004 0,005 C (M ) Fig.8. Langmuir adsorption isotherm for compound S. © 2009 University of Manchester and the authors. This is a preprint of a paper that has been submitted for publication in the Journal of Corrosion Science and Engineering. It will be reviewed and, subject 16to the reviewers’ comments, be published online at http://www.jcse.org in due course. Until such time as it has been fully published it should not normally be referenced in published work. ISSN 1466-8858 Volume 12, Preprint 30 submitted 23 June 2009 6,4 6,2 ln Icorr (m A cm-2) 6,0 5,8 5,6 S HCl 5,4 5,2 5,0 4,8 4,6 3,15 3,20 3,25 3,30 3,35 3,40 3,45 1000/T (K-1) Fig. 9. Plotting ln (Icorr) vs. 1/T to calculate the activation energy of corrosion process in the presence of inhibitor S 0,8 0,6 0,2 -2 -1 ln Icorr/T m( A cm K ) 0,4 0,0 -0,2 -0,4 S HCL -0,6 -0,8 -1,0 -1,2 3,15 3,20 3,25 3,30 3,35 3,40 3,45 -1 1000/T (K ) Fig. 10. Arrhenius plots of ln (Icorr/T) versus 1/T in the absence and presence of 5.10-3 M Schiff base S. © 2009 University of Manchester and the authors. This is a preprint of a paper that has been submitted for publication in the Journal of Corrosion Science and Engineering. It will be reviewed and, subject 17to the reviewers’ comments, be published online at http://www.jcse.org in due course. Until such time as it has been fully published it should not normally be referenced in published work.