- Email: [email protected]
Corrosion inhibition of mild steel in 1MH2SO4 by thiadiazole Schiff bases
Measurement 69 (2015) 195–201
Contents lists available at ScienceDirect
Measurement journal homepage: www.elsevier.com/locate/measurement
Corrosion inhibition of mild steel in 1MH2SO4 by thiadiazole Schiff bases P.M. Dasami ⇑, K. Parameswari, S. Chitra Department of Chemistry, PSGR Krishnammal College for Women, Peelamedu, Coimbatore 641004, Tamil Nadu, India
a r t i c l e
i n f o
Article history: Received 23 December 2014 Received in revised form 17 February 2015 Accepted 18 March 2015 Available online 26 March 2015 Keywords: Inhibitor Schiff base Corrosion Potentiodynamic polarization Langmuir adsorption isotherm Electrochemical impedance Quantum chemical study
a b s t r a c t Two novel Schiff bases derived from heterocyclic amines and aldehydes were synthesized and evaluated as corrosion inhibitors for mild steel in 1MH2SO4 by mass loss and electrochemical techniques. Inorder to understand the mechanism of inhibition, adsorption isotherms were tested. The studies showed that the inhibition efficiency depends on concentration of inhibitor and temperature of measurements. Electrochemical studies showed that the inhibitors behave as mixed type. Quantum chemical studies were used to substantiate the experimental results. Ó 2015 Elsevier Ltd. All rights reserved.
1. Introduction Mild steel is employed in the construction of tanks, heat exchangers, distillation towers, drums, pipelines, etc. [1]. In most cases, the MS containers, during their operation, come in contact with salts, mineral acids, alkalies, which cause corrosion on them. The use of inhibitors is one of the most practical methods for protection of metals against corrosion, especially in acidic media [2]. Numerous organic inhibitors have been used as inhibitors, for instance, imidazolines, amines, fatty acids, phosphate esters, etc. Generally organic compounds having higher basicity and electron density on the hetero atoms like O, N, S have tendency to resist corrosion. Nitrogen and sulphur are the active centers for the process of adsorption on the metal surface [3]. The corrosion inhibition property of these compounds is attributed to their molecular structure. The planarity and the lone pair electrones in the hetero atoms are important ⇑ Corresponding author. E-mail addresses: [email protected] (P.M. Dasami), parampps@ yahoo.co.in (K. Parameswari), [email protected] (S. Chitra). http://dx.doi.org/10.1016/j.measurement.2015.03.025 0263-2241/Ó 2015 Elsevier Ltd. All rights reserved.
features that determine the adsorption of these molecules on the metallic surface [4]. Schiff bases with the general formula -CH@N- have both the requirements in their structures. Many Schiff bases have been reported as effective corrosion inhibitors [5–8]. It has been reported that Sulphur compounds are very effective inhibitors for steel in acidic medium, because Sulphur is easily protonated in acid medium and is a stronger donor than Nitrogen [9]. Hence in the present work, an attempt has been made to synthesis of Schiff bases from Thiadiazole amine and to evaluate their inhibition performance was investigated using weight loss, electrochemical impedance spectroscopy (EIS) and potentiodynamic polarization techniques.
2. Experimental 2.1. Inhibitors The inhibitors used are 3-[(5-phenyl-1,3,4-thiadiazole-2ylimino)methyl]quinoline 2-ol(SB1) and 3-[(5-phenyl-1,3, 4-thiadiazole-2-ylimino)methyl]quinoline 2-thiol (SB 2).
196
P.M. Dasami et al. / Measurement 69 (2015) 195–201
X=OH (SB1) X=SH (SB2) 3-(((5-phenyl-1,3,4-thiadiazol-2-yl)imino)methyl)quinoline-2-ol/thiol
2.1.1. Molecular structures of the inhibitors SB 1 & SB 2 The compounds were characterized by FTIR spectra. 3054 cm 1 (OAH), 1679 cm 1 (C@N), 704 cm 1 (CAS), 2554 cm 1 (SAH) [For SB 2]. 2.1.2. IR spectrum for compound SB 1
2.2. Mild steel specimen preparation The experiments were performed with cold rolled mild steel specimen of chemical composition (C = 0.20%, Mn = 1%, Si-0.05%, S = 0.025%, P = 0.25% and Fe = 98%). For mass loss experiments, rectangular specimens of dimensions 3 cm 1 cm 0.1 cm were used. For electrochemical measurements, a cylindrical rod with exposed area 0.785 cm 1 was used. The specimens were mechanically abraded with different grades of emery papers, degreased with acetone, washed and dried. 2.3. Corrosion measurements Mass loss measurements were carried out using Weighing digital balance, Denver instrument. The
specimens were weighed before and after immersion in 1 M H2SO4 containing various concentration of inhibitors for 3 h at various temperatures. Triplicate measurements were made and average mass loss were recorded. From the loss in mass, inhibition efficiency was calculated. The electrochemical studies were carried out with IVIUM Compactstat (potentiostat/galvanostat), using the three electrode cell with MS rod as working electrode. The potential range was 200 to +200 mV with respect to the open circuit potential at a scan rate of 1 mV/s. For impedance measurements, the sweep frequency is from 10 Hz to 0.01 Hz with a signal amplitude of 10 mV. Density functional theory (DFT) has been used to analyze the characteristics of the inhibitor/surface mechanism and to describe the structural nature of the inhibitor on the corrosion process [18]. It is widely applied in the analysis of corrosion inhibition performance and the interaction
of corrosion inhibitors and interfaces. Adopting DFT/B3LYP in Gaussian09, geometry optimization and frequency analysis on the objects on the basis set of 6-31+G (d, p) were carried out [20]. 3. Results and discussion 3.1. Weight loss measurements at different time intervals (2– 6 h) The results of weight loss study showed that inhibition efficiency increases, as the concentration increases and decreases as the temperature increases. The surface coverage values calculated from weight loss increases with inhibitor concentration showing that the inhibitor was
197
P.M. Dasami et al. / Measurement 69 (2015) 195–201 Table 1 Inhibition efficiencies of the Schiff bases at various concentrations in 1 M H2SO4 at 30 ± 1 °C.
Blank 0.01 0.05 0.1 0.2 0.3 0.4 0.5 0.6 0.7
Inhibition efficiency, % 2 h immersion
Inhibition efficiency, % 3 h immersion
Inhibition efficiency, % 4 h immersion
SB 2
SB 1
SB 2
SB 1
SB 2
SB 1
SB 2
SB 1
SB 2
– 27.89 45.67 80.01 83.98 87.10 94.88 96.66 96.66 96.66
– 15.99 22.73 56.71 85.67 90.30 94.73 97.55 97.58 97.58
– 28.66 50.97 84.79 86.19 87.23 94.21 97.76 97.76 97.76
– 14.47 20.08 50.08 84.43 88.08 89.03 89.98 89.98 89.98
– 26.97 49.97 82.21 85.32 87.08 89.67 90.32 90.34 90.34
– 14.03 19.18 48.87 83.45 85.67 86.98 87.77 87.78 87.78
– 25.78 45.67 80.76 84.66 86.00 88.18 88.99 88.99 88.99
– 13.72 18.88 45.89 82.28 84.67 84.28 85.35 85.35 85.35
– 24.05 44.34 78.89 83.35 85.56 86.79 87.14 87.14 87.14
3.2. Influence of temperature Inorder to study the effect of temperature on corrosion inhibition of mild steel in acid and to determine the activation energy for the corrosion process, the weight loss studies were carried out at higher temperature (303–333 K). The results are given in Table 2. It can be noted that the inhibition efficiency decreased with temperature. This may be attributed to desorption of the inhibitor molecules from metal surface of the adsorbed layer of inhibitors at higher temperatures [10].
3.3. Thermodynamic parameters Thermodynamic parameters such as activation energy Ea, free energy change DG, enthalpy change DH and change in entropy of activation DS were calculated using Arrhenius plot (Fig. 1) and Transition state plot (Fig. 2).
BLANK SB 1 SB 2 8
Temperature Weight (K) loss (g)
Inhibition Corrosion efficiency (%) rate (mpy)
Blank
303 313 323 333
0.0609 0.1152 0.1443 0.2486
– – – –
76,627 144,950 181,565 312,801
303 313 323 333
0.0038 0.0047 0.0074 0.0214
97.55 95.92 94.87 91.39
4781 5914 26,928 172,246
303 313 323 333
0.0017 0.0019 0.0032 0.0042
97.76 96.69 93.24 91.65
5284 5914 14,974 18,497
0.0031
0.0032
1/T in K
0.0033
-1
Fig. 1. Arrhenius plot for the Schiff bases.
BLANK SB 1 SB 2
0.75 0.70
Log (Corrosion rate)/T
Name of the compound
6
0.0030
Table 2 Inhibition efficiency at the maximum concentration of the inhibitors at various temperatures.
SB 2
Inhibition efficiency, % 6 h immersion
– 15.23 23.54 55.67 83.34 87.89 93.28 95.56 95.59 95.60
adsorbed on the mild steel surface forming a protective layer which prevents the metal from corrosion. Both the inhibitors showed excellent performance at 0.5 mM concentration (see Table 1). The weight loss study was also determined by changing the time of immersion. The metal was immersed in 1 M H2SO4 with and without inhibitor for different time intervals. The result show that as the time increases from 2 h to 3 h, the inhibition efficiency increases and shows a decreasing trend from 3 to 6 h. This may be explained due to increase of adsorbed of inhibitor molecules on MS surface with time. Prolonged immersion may result in desorption of the inhibitor molecules from mild steel surface [18].
SB 1
Inhibition efficiency, % 5 h immersion
SB 1
Log Corrosion rate
Concentration, mM
0.65 0.60 0.55 0.50 0.45 0.00300 0.00305 0.00310 0.00315 0.00320 0.00325 0.00330
1/T in K
-1
Fig. 2. Transition state plot for the Schiff bases.
198
P.M. Dasami et al. / Measurement 69 (2015) 195–201
Table 3 Kinetics/thermodynamic parameters for mild steel corrosion in 1 M H2SO4. Name of the inhibitor in (mM)
Activation energy, Ea (KJ/mole)
Blank SB 1 SB 2
37.3265 86.832 69.619
DG0ads (KJ/mole) 303 K
313 K
323 K
333 K
– 21.149 22.736
– 20.475 23.184
– 20.485 23.563
– 19.582 23.497
Ea values are higher in the presence of inhibitors than that of the blank acid. The increase in Ea in the presence of inhibitor is typical of physisorption [11]. Negative DG0ads values indicate spontaneity of the adsorption process and the values are less than 40 KJ/mol showing that the inhibitors are adsorbed on mild steel by physisorption [12]. The negative value of DH0 reflects the exothermic nature of dissolution process. The value of entropy change DS0 in the presence of inhibitor is negative, which implies that the activated complex represents an association rather than dissociation [13] (see Table 3).
SB 1
0.8
SB 2
0.7 0.6 0.5
C/0
DH0 (KJ/mole)
– 90.31 51.99
DS0 (KJ/mole)
– 0.99987 0.99555
3.3.1. Langmuir adsorption isotherm The plot of C/h versus C was found to be linear (Fig. 3), showing that the adsorption of the inhibitor on the mild steel surface obeys Langmuir isotherm. 3.4. Electrochemical polarization The electrochemical parameters such as corrosion current (Icorr) and corrosion potential (Ecorr) obtained from Tafel plot for SB1 (Fig. 4) is given in Table 4. Ecorr values are slightly shifted towards negative direction and Tafel constants ba and bc are changed with increase in concentration of the inhibitor, suggesting that both anodic dissolution and cathodic hydrogen evolution mechanism are affected in the presence of the inhibitor. It can be concluded that the inhibitor is mixed type [14]. Icorr values decrease with increase in concentration suggesting the effectiveness of the compounds as corrosion inhibitors. 3.5. Impedance studies
0.4 0.3 0.2 0.1 0.0 0.0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
Concentration, C Fig. 3. Langmuir Adsorption Isotherm for the inhibitors at 3 h of immersion.
10
Table 5 gives the impedance parameters viz. charge transfer resistance (Rt), double layer capacitance (Cdl) and Inhibition Efficiency (%) obtained from the Nyquist plots (Fig. 5). It was observed that there is a gradual decrease in values of Cdl with increase in concentration from 0.05 to 0.5 mM of the Schiff bases. This decrease has confirmed that the inhibitor molecules were adsorbed on the mild steel surface and decreased the roughness of the mild steel surface due to corrosion by acid [15]. The diameter of the semicircles increase with concentration, shows increase in the thickness of the protective layer and a decrease in the dielectric constant of the solution. Such behavior is characteristic for solid electrodes and often refer to a frequency dispersion, has been attributed to roughness and other in homogeneities of the solid surface [4]
Log Current, A
5 0.05mM 0.3mM 0.5mM BLANK
0
-5 -10 -15 -20
3.6. Effect of Halide ions The effect of addition of halide ions I , Br and Cl to the solutions containing 1 M H2SO4 and the inhibitors were studied by weight loss method and the data are presented in Table 6. Analysis of the data reveals that the synergistic influence of halide ions follows the order.
I > Br > Cl
-25 -0.64 -0.62 -0.60 -0.58 -0.56 -0.54 -0.52 -0.50 -0.48 -0.46 -0.44 -0.42 -0.40 -0.38
Potential, V Fig. 4. Tafel plot for mild steel in 1 M H2SO4 containing various concentrations of the inhibitor SB 1.
I has highest synergistic influence which may be explained as follows. The steel surface is originally positively charged in 1 M H2SO4. When I ion are added to the inhibiting solution, they are strongly chemisorbed by forming chemical bonds
199
P.M. Dasami et al. / Measurement 69 (2015) 195–201 Table 4 Corrosion parameters for mild steel with selected concentrations of the inhibitors in 1 M H2SO4 by potentiodynamic polarization method. Name of the compound
Concentration (mM)
Tafel slope (mV/decade)
Blank
ba
bc
Ecorr (mV)
Icorr (lF/cm2)
Inhibition efficiency (%)
69
113
492.1
287.7
SB 1
0.05 0.3 0.5
95 73 70
114 127 114
538.1 502.3 528.8
182.3 158.6 143.9
36.63 44.87 49.98
SB 2
0.05 0.3 0.5
85 77 67
119 103 96
513.3 529.1 512.3
235.8 150.5 28.2
18.03 47.68 90.19
Table 5 Impedance parameters for corrosion of mild steel for selected concentrations of the inhibitors in 1 M H2SO4. Name of the compound
Concentration Rt (mM) (X/cm2)
Blank SB 1
SB 2
Cdl (lF)/cm2
Inhibition efficiency (%)
15.08
28.2
0.05 0.3 0.5
19.25 20.25 28.90
28 26.4 21.2
21.66 25.53 47.82
0.05 0.3 0.5
18.31 29.71 46.12
29.3 21.8 16
17.64 49.24 67.30
0.05mM 0.3mM 0.5mM BLANK
-16 -14
Table 6 Effect of 1 mM KCl/1 mM KBr/1 mM KI on the inhibition efficiency of the inhibitor. Name of the inhibitor
Concentration, Inhibition efficiency (%) mM With With Without 1 mM KCl, KBr and 1 mM KBr KCl KI
With 1 mM KI
SB 1
0.02 0.04 0.06 0.08 0.1
22.50 33.43 44.01 52.44 56.42
46.59 49.38 56.77 63.15 64.94
61.18 64.51 67.43 69.27 71.37
67.52 71.54 72.63 77.14 77.66
SB 2
0.02 0.04 0.06 0.08 0.1
53.97 58.39 73.29 78.19 85.88
77.44 78.88 86.40 87.36 89.37
84.79 87.06 90.25 91.17 95.27
95.71 96.06 96.32 96.76 97.02
-12
Z'' ohm
-10 -8 -6 -4 -2 0 0
5
10
15
20
25
30
35
40
Z' ohm Fig. 5. Nyquist plot for mild steel in 1 M H2SO4 containing various concentrations of the inhibitor SB 1.
even leading to the formation of iron iodide. This strong chemisorption of I ions shift /n of the metal to more positive potential than in the case of Cl and Br and renders the surface more highly negatively charged. On the highly negatively charged metal surface, the protonated cationic inhibitor molecules are physisorbed due to electrostatic interaction. This interaction will be higher for I than for Cl or Br due to higher magnitude of negative charge on the metal surface. 3.7. Quantum chemical calculations and evaluation of inhibitors SB 1 & SB 2 The optimized geometry of the Schiff bases and their HOMO & LUMO are shown in Fig. 6. From DFT, B3LYP/
6-31+G (d, p) optimization, structural parameters were calculated and given in Table 7. According to Frontier molecular orbital theory any reaction is feasible when HOMO and LUMO of the reactants interact. HOMO acts as electron donor, and LUMO accepts electron. EHOMO and ELUMO are related to ionization potential and electron affinity respectively of the organic molecules. The difference between the two, ie, DE is an important parameter, it should be lower for a molecule to be adsorbed on the metal surface effectively. The larger the HOMO–LUMO orbital energy gap, the harder the molecule. The hardness has been associated with the stability of the chemical system [16,17]. Table 7 shows that DE is lower for SB2 compared to SB1. Hence, SB2 has higher adsorption tendency and hence has higher Inhibition Efficiency (IE) value. A quantitative explanation for the difference in IE values of pyridine derivatives was reported by Kosari et al. through the analysis of the optimized HOMO and LUMO pictures of the compound. The same observation can be extended to the present Schiff bases also. The HOMO and LUMO densities are shown in Fig. 6. It is evident that the HOMO densities are centered mainly on the quinoline ring in SB1, whereas in SB2, the electronic charge density is spread throughout the molecule including the azomethine nitrogen and the thiadiazole ring. Therefore it is possible for SB2 to be adsorbed in a planar configuration of the entire molecule, on the basis of donor-acceptor interaction. Whereas in SB1 only the quinoline ring has maximum
200
P.M. Dasami et al. / Measurement 69 (2015) 195–201
Table 7 Quantum chemical parameters of the studied inhibitors calculated at B3LYP/6-31+G (d, p) level. Name of the inhibitors SB 1 SB 2
EHOMO (eV)
ELUMO (eV)
DE (eV)
v
g
r
DN
Dipole moment
6.5950 6.5367
3.1032 3.1712
3.4917 3.3655
4.8491 4.8539
1.7459 1.6827
0.5727 0.5942
0.615986 0.637695
7.7993 7.7077
FOR SB 1: HOMO
LUMO
FOR SB 2: HOMO
LUMO
Fig. 6. HOMO & LUMO for SB 1 & SB 2.
electronic density and that part is adsorbed and the rest of the molecule configures perpendicular to the surface. Hence a lesser coverage by SB1 and the compound shows lower inhibition efficiency [19]. X, the electronegativity is almost same for the two compounds. g, the global hardness is a measure of stability and reactivity of a species. A hard molecule has a large energy gap and a soft molecule has a small energy gap [20]. Normally, the inhibitor with the least value of global hardness (hence the highest value of global softness, r) is expected to have the highest inhibition efficiency [21]. For the two Schiff bases both these parameters g and r agree with the experimentally determined IE order SB2 > SB1. There is no definite correlation between dipole moment and IE has been reported in the literature. 4. Conclusion The synthesized Schiff bases were found to be very good inhibitor for the corrosion of mild steel in 1 M H2SO4. The efficiency increases with increase in concentration of the inhibitors and decreases with temperature. The inhibitors act by adsorption on the mild steel surface and obey Langmuir isotherm. Electrochemical studies show that the inhibitors are mixed type suppressing both anodic metal dissolution and cathode hydrogen evolution Quantum chemical studies correlated well with the experimental data. SB 2 was found to be more effective inhibitor than SB 1 as shown by the quantum chemical studies. Addition of halide ions enhanced the efficiency in the order chloride < bromide < iodide.
References [1] I.M. Iloamaeke, T.U. Onuegbu, V.I.E. Ajiwe, U.C. Umeobika, Corrosion inhibition of mild steel by pterocarpus soyauxi leaves extract in HCl medium, Int. Instit. Sci. Technol. Edu. 2 (2012) 22–28. [2] Muktha Sharma, Jyothi Chawla, Gurmeet Singh, Cetyl trimethylammonium bromide as corrosion inhibitor for mild steel in acid medium, Ind. J. Chem. Technol. 16 (2009) 339–343. [3] R.K. Upadhyay, S.P. Mathur, Effect of Schiff’s bases as corrosion inhibitors on mild steel in sulphuric acid, E-J. Chem. 4 (2007) 408– 414. [4] V. Ramesh Saliyan, Airody Vasudeva Pisharody, Corrosion inhibition of mild steel in acid media by quinolino thiopropano hydrazone, Ind. J. Chem. Technol. 16 (2009) 162–174. [5] Ashish Kumar Singh, M.A.Q. Uraishi, Study of some bidentate Schiff bases of Isatin as corrosion inhibitors for mild steel in hydrochloric acid solution, Int. J. Electrochem. Sci. 7 (2012) 3222–3241. [6] S.D. Toliwal, Kalpesh Yadav, Tejas Pavagaddhi, Corrosion inhibition study of a new synthetic Schiff base derived from nontraditional oils on mild steel in 1N HCl solution, J. Sci. Ind. Res. 69 (2010) 43–47. [7] T. Sethi, A. Chaturvedi, R.K. Upadhyay, S.P. Mathur, Corrosion inhibitory effects of some Schiff bases on mild steel in acidic media, J. Chi. Chem. Soc. 52 (2007) 1206–1213. [8] Serpil Safak, Berrin Duran, Aysel Yurt, Gulsen Turkoglu, Schiff bases as corrosion inhibitor for aluminium in HCl solution, Corros. Sci. 54 (2014) 251–259. [9] R.T. Loto, C.A. Loto, A.P.I. Popoola, Corrosion inhibition of thiourea and thiadiazole derivatives, J. Mater. Environ. Sci. 3 (2012) 885–894. [10] S. Leelavathi, R. Rajalakshmi, Dodonaea viscosa (L.) Leaves extract as acid Corrosion inhibitor for mild Steel – A Green approach, J. Mater Environ. Sci. 4 (2013) 625–638. [11] B.A. Abd-El-Nabey, A.M. Abdel-Gaber, M.E.L. Said Ali, Essam Khamis, S. El-Housseiny, Inhibitive action of cannabis plant extract on the corrosion of copper in 0.5 M H2SO4, Int. J. Electrochem. Sci. 8 (2013) 5851–5865. [12] ’E.A. Noor, Synthesis and effect on corrosion inhibition of carbon steel in hydrochloric acid solution, Appl. Electrochem. 39 (2009) 1465–1475. [13] M.A. Hegazy, Novel cationic gemini surfactants as corrosion inhibitors for carbon steel pipelines, Corro. Sci. 51 (2009) 2610– 2618.
P.M. Dasami et al. / Measurement 69 (2015) 195–201 [14] A. Layla, Al Juhaiman, Amal Abu Mustafa, Waffak Mekhamer, Polyvinyl pyrrolidone as a green corrosion inhibitor of carbon steel in neutral solutions containing NaCl: electrochemical and thermodynamic study, Int. J. Electrochem. Sci. 7 (2012) 8578–8596. [15] F. Bentiss, M. Lebrini, M. Lagrene, Thermodynamic characterization of metal dissolution and inhibitor adsorption processes in mild steel/ 2,5-bis(n-thienyl)-1,3,4-thiadiazoles/hydrochloric acid system, Corro. Sci. 47 (2005) 2915–2931. [16] N.S. Patel, S. Jauhariand, G.N. Mehta, S.S. Al-Deyab, I. Warad, B. Hammouti, Mild steel corrosion inhibition by various plant extracts in 0.5M sulphuric acid, Int. J. Electrochem. Sci. 8 (2013) 2635–2655. [17] Singh Rajeev, D. Kumar, Singh Bhoop, V.K. Singh, Sharma Ranjana, Molecular structure, vibrational spectroscopic and HOMO, LUMO studies of S-2-picolyl-b-N-(2-acetylpyrrole) dithiocarbazate Schiff base by Quantum Chemical investigations, Res. J. Chem. Sci. 3 (2013) 79–84.
201
[18] R. Saratha, R. Meenakshi, Dimethylaminobenzylidene acetone as corrosion inhibitor for mild steel in acid medium, Rasayan J. Chem. 4 (2011) 251–263. [19] A. Kosari, M.H. Moayed, A. Davoodi, R. Parvizi, M. Momeni, H. Eshghi, H. Moradi, Electrochemical and quantum chemical assessment of two organic compounds from pyridine derivatives as corrosion inhibitors for mild steel in HCl solution under stagnant condition and hydrodynamic flow, Corro. Sci. 78 (2014) 138–150. [20] S. Martinez, Inhibitory mechanism of mimosa tannin using molecular modelling and substitutional adsorption isotherms, Mat. Chem. Phys. 77 (2002) 997–1025. [21] Jianfang Zhu, Shuangkou Chen, A theoretical investigation on the inhibition efficiencies of some schiff bases as corrosion inhibitors of steel in hydrochloric acid, Int. J. Electrochem. Sci. 7 (2012) 11884– 11894.
Contents lists available at ScienceDirect
Measurement journal homepage: www.elsevier.com/locate/measurement
Corrosion inhibition of mild steel in 1MH2SO4 by thiadiazole Schiff bases P.M. Dasami ⇑, K. Parameswari, S. Chitra Department of Chemistry, PSGR Krishnammal College for Women, Peelamedu, Coimbatore 641004, Tamil Nadu, India
a r t i c l e
i n f o
Article history: Received 23 December 2014 Received in revised form 17 February 2015 Accepted 18 March 2015 Available online 26 March 2015 Keywords: Inhibitor Schiff base Corrosion Potentiodynamic polarization Langmuir adsorption isotherm Electrochemical impedance Quantum chemical study
a b s t r a c t Two novel Schiff bases derived from heterocyclic amines and aldehydes were synthesized and evaluated as corrosion inhibitors for mild steel in 1MH2SO4 by mass loss and electrochemical techniques. Inorder to understand the mechanism of inhibition, adsorption isotherms were tested. The studies showed that the inhibition efficiency depends on concentration of inhibitor and temperature of measurements. Electrochemical studies showed that the inhibitors behave as mixed type. Quantum chemical studies were used to substantiate the experimental results. Ó 2015 Elsevier Ltd. All rights reserved.
1. Introduction Mild steel is employed in the construction of tanks, heat exchangers, distillation towers, drums, pipelines, etc. [1]. In most cases, the MS containers, during their operation, come in contact with salts, mineral acids, alkalies, which cause corrosion on them. The use of inhibitors is one of the most practical methods for protection of metals against corrosion, especially in acidic media [2]. Numerous organic inhibitors have been used as inhibitors, for instance, imidazolines, amines, fatty acids, phosphate esters, etc. Generally organic compounds having higher basicity and electron density on the hetero atoms like O, N, S have tendency to resist corrosion. Nitrogen and sulphur are the active centers for the process of adsorption on the metal surface [3]. The corrosion inhibition property of these compounds is attributed to their molecular structure. The planarity and the lone pair electrones in the hetero atoms are important ⇑ Corresponding author. E-mail addresses: [email protected] (P.M. Dasami), parampps@ yahoo.co.in (K. Parameswari), [email protected] (S. Chitra). http://dx.doi.org/10.1016/j.measurement.2015.03.025 0263-2241/Ó 2015 Elsevier Ltd. All rights reserved.
features that determine the adsorption of these molecules on the metallic surface [4]. Schiff bases with the general formula -CH@N- have both the requirements in their structures. Many Schiff bases have been reported as effective corrosion inhibitors [5–8]. It has been reported that Sulphur compounds are very effective inhibitors for steel in acidic medium, because Sulphur is easily protonated in acid medium and is a stronger donor than Nitrogen [9]. Hence in the present work, an attempt has been made to synthesis of Schiff bases from Thiadiazole amine and to evaluate their inhibition performance was investigated using weight loss, electrochemical impedance spectroscopy (EIS) and potentiodynamic polarization techniques.
2. Experimental 2.1. Inhibitors The inhibitors used are 3-[(5-phenyl-1,3,4-thiadiazole-2ylimino)methyl]quinoline 2-ol(SB1) and 3-[(5-phenyl-1,3, 4-thiadiazole-2-ylimino)methyl]quinoline 2-thiol (SB 2).
196
P.M. Dasami et al. / Measurement 69 (2015) 195–201
X=OH (SB1) X=SH (SB2) 3-(((5-phenyl-1,3,4-thiadiazol-2-yl)imino)methyl)quinoline-2-ol/thiol
2.1.1. Molecular structures of the inhibitors SB 1 & SB 2 The compounds were characterized by FTIR spectra. 3054 cm 1 (OAH), 1679 cm 1 (C@N), 704 cm 1 (CAS), 2554 cm 1 (SAH) [For SB 2]. 2.1.2. IR spectrum for compound SB 1
2.2. Mild steel specimen preparation The experiments were performed with cold rolled mild steel specimen of chemical composition (C = 0.20%, Mn = 1%, Si-0.05%, S = 0.025%, P = 0.25% and Fe = 98%). For mass loss experiments, rectangular specimens of dimensions 3 cm 1 cm 0.1 cm were used. For electrochemical measurements, a cylindrical rod with exposed area 0.785 cm 1 was used. The specimens were mechanically abraded with different grades of emery papers, degreased with acetone, washed and dried. 2.3. Corrosion measurements Mass loss measurements were carried out using Weighing digital balance, Denver instrument. The
specimens were weighed before and after immersion in 1 M H2SO4 containing various concentration of inhibitors for 3 h at various temperatures. Triplicate measurements were made and average mass loss were recorded. From the loss in mass, inhibition efficiency was calculated. The electrochemical studies were carried out with IVIUM Compactstat (potentiostat/galvanostat), using the three electrode cell with MS rod as working electrode. The potential range was 200 to +200 mV with respect to the open circuit potential at a scan rate of 1 mV/s. For impedance measurements, the sweep frequency is from 10 Hz to 0.01 Hz with a signal amplitude of 10 mV. Density functional theory (DFT) has been used to analyze the characteristics of the inhibitor/surface mechanism and to describe the structural nature of the inhibitor on the corrosion process [18]. It is widely applied in the analysis of corrosion inhibition performance and the interaction
of corrosion inhibitors and interfaces. Adopting DFT/B3LYP in Gaussian09, geometry optimization and frequency analysis on the objects on the basis set of 6-31+G (d, p) were carried out [20]. 3. Results and discussion 3.1. Weight loss measurements at different time intervals (2– 6 h) The results of weight loss study showed that inhibition efficiency increases, as the concentration increases and decreases as the temperature increases. The surface coverage values calculated from weight loss increases with inhibitor concentration showing that the inhibitor was
197
P.M. Dasami et al. / Measurement 69 (2015) 195–201 Table 1 Inhibition efficiencies of the Schiff bases at various concentrations in 1 M H2SO4 at 30 ± 1 °C.
Blank 0.01 0.05 0.1 0.2 0.3 0.4 0.5 0.6 0.7
Inhibition efficiency, % 2 h immersion
Inhibition efficiency, % 3 h immersion
Inhibition efficiency, % 4 h immersion
SB 2
SB 1
SB 2
SB 1
SB 2
SB 1
SB 2
SB 1
SB 2
– 27.89 45.67 80.01 83.98 87.10 94.88 96.66 96.66 96.66
– 15.99 22.73 56.71 85.67 90.30 94.73 97.55 97.58 97.58
– 28.66 50.97 84.79 86.19 87.23 94.21 97.76 97.76 97.76
– 14.47 20.08 50.08 84.43 88.08 89.03 89.98 89.98 89.98
– 26.97 49.97 82.21 85.32 87.08 89.67 90.32 90.34 90.34
– 14.03 19.18 48.87 83.45 85.67 86.98 87.77 87.78 87.78
– 25.78 45.67 80.76 84.66 86.00 88.18 88.99 88.99 88.99
– 13.72 18.88 45.89 82.28 84.67 84.28 85.35 85.35 85.35
– 24.05 44.34 78.89 83.35 85.56 86.79 87.14 87.14 87.14
3.2. Influence of temperature Inorder to study the effect of temperature on corrosion inhibition of mild steel in acid and to determine the activation energy for the corrosion process, the weight loss studies were carried out at higher temperature (303–333 K). The results are given in Table 2. It can be noted that the inhibition efficiency decreased with temperature. This may be attributed to desorption of the inhibitor molecules from metal surface of the adsorbed layer of inhibitors at higher temperatures [10].
3.3. Thermodynamic parameters Thermodynamic parameters such as activation energy Ea, free energy change DG, enthalpy change DH and change in entropy of activation DS were calculated using Arrhenius plot (Fig. 1) and Transition state plot (Fig. 2).
BLANK SB 1 SB 2 8
Temperature Weight (K) loss (g)
Inhibition Corrosion efficiency (%) rate (mpy)
Blank
303 313 323 333
0.0609 0.1152 0.1443 0.2486
– – – –
76,627 144,950 181,565 312,801
303 313 323 333
0.0038 0.0047 0.0074 0.0214
97.55 95.92 94.87 91.39
4781 5914 26,928 172,246
303 313 323 333
0.0017 0.0019 0.0032 0.0042
97.76 96.69 93.24 91.65
5284 5914 14,974 18,497
0.0031
0.0032
1/T in K
0.0033
-1
Fig. 1. Arrhenius plot for the Schiff bases.
BLANK SB 1 SB 2
0.75 0.70
Log (Corrosion rate)/T
Name of the compound
6
0.0030
Table 2 Inhibition efficiency at the maximum concentration of the inhibitors at various temperatures.
SB 2
Inhibition efficiency, % 6 h immersion
– 15.23 23.54 55.67 83.34 87.89 93.28 95.56 95.59 95.60
adsorbed on the mild steel surface forming a protective layer which prevents the metal from corrosion. Both the inhibitors showed excellent performance at 0.5 mM concentration (see Table 1). The weight loss study was also determined by changing the time of immersion. The metal was immersed in 1 M H2SO4 with and without inhibitor for different time intervals. The result show that as the time increases from 2 h to 3 h, the inhibition efficiency increases and shows a decreasing trend from 3 to 6 h. This may be explained due to increase of adsorbed of inhibitor molecules on MS surface with time. Prolonged immersion may result in desorption of the inhibitor molecules from mild steel surface [18].
SB 1
Inhibition efficiency, % 5 h immersion
SB 1
Log Corrosion rate
Concentration, mM
0.65 0.60 0.55 0.50 0.45 0.00300 0.00305 0.00310 0.00315 0.00320 0.00325 0.00330
1/T in K
-1
Fig. 2. Transition state plot for the Schiff bases.
198
P.M. Dasami et al. / Measurement 69 (2015) 195–201
Table 3 Kinetics/thermodynamic parameters for mild steel corrosion in 1 M H2SO4. Name of the inhibitor in (mM)
Activation energy, Ea (KJ/mole)
Blank SB 1 SB 2
37.3265 86.832 69.619
DG0ads (KJ/mole) 303 K
313 K
323 K
333 K
– 21.149 22.736
– 20.475 23.184
– 20.485 23.563
– 19.582 23.497
Ea values are higher in the presence of inhibitors than that of the blank acid. The increase in Ea in the presence of inhibitor is typical of physisorption [11]. Negative DG0ads values indicate spontaneity of the adsorption process and the values are less than 40 KJ/mol showing that the inhibitors are adsorbed on mild steel by physisorption [12]. The negative value of DH0 reflects the exothermic nature of dissolution process. The value of entropy change DS0 in the presence of inhibitor is negative, which implies that the activated complex represents an association rather than dissociation [13] (see Table 3).
SB 1
0.8
SB 2
0.7 0.6 0.5
C/0
DH0 (KJ/mole)
– 90.31 51.99
DS0 (KJ/mole)
– 0.99987 0.99555
3.3.1. Langmuir adsorption isotherm The plot of C/h versus C was found to be linear (Fig. 3), showing that the adsorption of the inhibitor on the mild steel surface obeys Langmuir isotherm. 3.4. Electrochemical polarization The electrochemical parameters such as corrosion current (Icorr) and corrosion potential (Ecorr) obtained from Tafel plot for SB1 (Fig. 4) is given in Table 4. Ecorr values are slightly shifted towards negative direction and Tafel constants ba and bc are changed with increase in concentration of the inhibitor, suggesting that both anodic dissolution and cathodic hydrogen evolution mechanism are affected in the presence of the inhibitor. It can be concluded that the inhibitor is mixed type [14]. Icorr values decrease with increase in concentration suggesting the effectiveness of the compounds as corrosion inhibitors. 3.5. Impedance studies
0.4 0.3 0.2 0.1 0.0 0.0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
Concentration, C Fig. 3. Langmuir Adsorption Isotherm for the inhibitors at 3 h of immersion.
10
Table 5 gives the impedance parameters viz. charge transfer resistance (Rt), double layer capacitance (Cdl) and Inhibition Efficiency (%) obtained from the Nyquist plots (Fig. 5). It was observed that there is a gradual decrease in values of Cdl with increase in concentration from 0.05 to 0.5 mM of the Schiff bases. This decrease has confirmed that the inhibitor molecules were adsorbed on the mild steel surface and decreased the roughness of the mild steel surface due to corrosion by acid [15]. The diameter of the semicircles increase with concentration, shows increase in the thickness of the protective layer and a decrease in the dielectric constant of the solution. Such behavior is characteristic for solid electrodes and often refer to a frequency dispersion, has been attributed to roughness and other in homogeneities of the solid surface [4]
Log Current, A
5 0.05mM 0.3mM 0.5mM BLANK
0
-5 -10 -15 -20
3.6. Effect of Halide ions The effect of addition of halide ions I , Br and Cl to the solutions containing 1 M H2SO4 and the inhibitors were studied by weight loss method and the data are presented in Table 6. Analysis of the data reveals that the synergistic influence of halide ions follows the order.
I > Br > Cl
-25 -0.64 -0.62 -0.60 -0.58 -0.56 -0.54 -0.52 -0.50 -0.48 -0.46 -0.44 -0.42 -0.40 -0.38
Potential, V Fig. 4. Tafel plot for mild steel in 1 M H2SO4 containing various concentrations of the inhibitor SB 1.
I has highest synergistic influence which may be explained as follows. The steel surface is originally positively charged in 1 M H2SO4. When I ion are added to the inhibiting solution, they are strongly chemisorbed by forming chemical bonds
199
P.M. Dasami et al. / Measurement 69 (2015) 195–201 Table 4 Corrosion parameters for mild steel with selected concentrations of the inhibitors in 1 M H2SO4 by potentiodynamic polarization method. Name of the compound
Concentration (mM)
Tafel slope (mV/decade)
Blank
ba
bc
Ecorr (mV)
Icorr (lF/cm2)
Inhibition efficiency (%)
69
113
492.1
287.7
SB 1
0.05 0.3 0.5
95 73 70
114 127 114
538.1 502.3 528.8
182.3 158.6 143.9
36.63 44.87 49.98
SB 2
0.05 0.3 0.5
85 77 67
119 103 96
513.3 529.1 512.3
235.8 150.5 28.2
18.03 47.68 90.19
Table 5 Impedance parameters for corrosion of mild steel for selected concentrations of the inhibitors in 1 M H2SO4. Name of the compound
Concentration Rt (mM) (X/cm2)
Blank SB 1
SB 2
Cdl (lF)/cm2
Inhibition efficiency (%)
15.08
28.2
0.05 0.3 0.5
19.25 20.25 28.90
28 26.4 21.2
21.66 25.53 47.82
0.05 0.3 0.5
18.31 29.71 46.12
29.3 21.8 16
17.64 49.24 67.30
0.05mM 0.3mM 0.5mM BLANK
-16 -14
Table 6 Effect of 1 mM KCl/1 mM KBr/1 mM KI on the inhibition efficiency of the inhibitor. Name of the inhibitor
Concentration, Inhibition efficiency (%) mM With With Without 1 mM KCl, KBr and 1 mM KBr KCl KI
With 1 mM KI
SB 1
0.02 0.04 0.06 0.08 0.1
22.50 33.43 44.01 52.44 56.42
46.59 49.38 56.77 63.15 64.94
61.18 64.51 67.43 69.27 71.37
67.52 71.54 72.63 77.14 77.66
SB 2
0.02 0.04 0.06 0.08 0.1
53.97 58.39 73.29 78.19 85.88
77.44 78.88 86.40 87.36 89.37
84.79 87.06 90.25 91.17 95.27
95.71 96.06 96.32 96.76 97.02
-12
Z'' ohm
-10 -8 -6 -4 -2 0 0
5
10
15
20
25
30
35
40
Z' ohm Fig. 5. Nyquist plot for mild steel in 1 M H2SO4 containing various concentrations of the inhibitor SB 1.
even leading to the formation of iron iodide. This strong chemisorption of I ions shift /n of the metal to more positive potential than in the case of Cl and Br and renders the surface more highly negatively charged. On the highly negatively charged metal surface, the protonated cationic inhibitor molecules are physisorbed due to electrostatic interaction. This interaction will be higher for I than for Cl or Br due to higher magnitude of negative charge on the metal surface. 3.7. Quantum chemical calculations and evaluation of inhibitors SB 1 & SB 2 The optimized geometry of the Schiff bases and their HOMO & LUMO are shown in Fig. 6. From DFT, B3LYP/
6-31+G (d, p) optimization, structural parameters were calculated and given in Table 7. According to Frontier molecular orbital theory any reaction is feasible when HOMO and LUMO of the reactants interact. HOMO acts as electron donor, and LUMO accepts electron. EHOMO and ELUMO are related to ionization potential and electron affinity respectively of the organic molecules. The difference between the two, ie, DE is an important parameter, it should be lower for a molecule to be adsorbed on the metal surface effectively. The larger the HOMO–LUMO orbital energy gap, the harder the molecule. The hardness has been associated with the stability of the chemical system [16,17]. Table 7 shows that DE is lower for SB2 compared to SB1. Hence, SB2 has higher adsorption tendency and hence has higher Inhibition Efficiency (IE) value. A quantitative explanation for the difference in IE values of pyridine derivatives was reported by Kosari et al. through the analysis of the optimized HOMO and LUMO pictures of the compound. The same observation can be extended to the present Schiff bases also. The HOMO and LUMO densities are shown in Fig. 6. It is evident that the HOMO densities are centered mainly on the quinoline ring in SB1, whereas in SB2, the electronic charge density is spread throughout the molecule including the azomethine nitrogen and the thiadiazole ring. Therefore it is possible for SB2 to be adsorbed in a planar configuration of the entire molecule, on the basis of donor-acceptor interaction. Whereas in SB1 only the quinoline ring has maximum
200
P.M. Dasami et al. / Measurement 69 (2015) 195–201
Table 7 Quantum chemical parameters of the studied inhibitors calculated at B3LYP/6-31+G (d, p) level. Name of the inhibitors SB 1 SB 2
EHOMO (eV)
ELUMO (eV)
DE (eV)
v
g
r
DN
Dipole moment
6.5950 6.5367
3.1032 3.1712
3.4917 3.3655
4.8491 4.8539
1.7459 1.6827
0.5727 0.5942
0.615986 0.637695
7.7993 7.7077
FOR SB 1: HOMO
LUMO
FOR SB 2: HOMO
LUMO
Fig. 6. HOMO & LUMO for SB 1 & SB 2.
electronic density and that part is adsorbed and the rest of the molecule configures perpendicular to the surface. Hence a lesser coverage by SB1 and the compound shows lower inhibition efficiency [19]. X, the electronegativity is almost same for the two compounds. g, the global hardness is a measure of stability and reactivity of a species. A hard molecule has a large energy gap and a soft molecule has a small energy gap [20]. Normally, the inhibitor with the least value of global hardness (hence the highest value of global softness, r) is expected to have the highest inhibition efficiency [21]. For the two Schiff bases both these parameters g and r agree with the experimentally determined IE order SB2 > SB1. There is no definite correlation between dipole moment and IE has been reported in the literature. 4. Conclusion The synthesized Schiff bases were found to be very good inhibitor for the corrosion of mild steel in 1 M H2SO4. The efficiency increases with increase in concentration of the inhibitors and decreases with temperature. The inhibitors act by adsorption on the mild steel surface and obey Langmuir isotherm. Electrochemical studies show that the inhibitors are mixed type suppressing both anodic metal dissolution and cathode hydrogen evolution Quantum chemical studies correlated well with the experimental data. SB 2 was found to be more effective inhibitor than SB 1 as shown by the quantum chemical studies. Addition of halide ions enhanced the efficiency in the order chloride < bromide < iodide.
References [1] I.M. Iloamaeke, T.U. Onuegbu, V.I.E. Ajiwe, U.C. Umeobika, Corrosion inhibition of mild steel by pterocarpus soyauxi leaves extract in HCl medium, Int. Instit. Sci. Technol. Edu. 2 (2012) 22–28. [2] Muktha Sharma, Jyothi Chawla, Gurmeet Singh, Cetyl trimethylammonium bromide as corrosion inhibitor for mild steel in acid medium, Ind. J. Chem. Technol. 16 (2009) 339–343. [3] R.K. Upadhyay, S.P. Mathur, Effect of Schiff’s bases as corrosion inhibitors on mild steel in sulphuric acid, E-J. Chem. 4 (2007) 408– 414. [4] V. Ramesh Saliyan, Airody Vasudeva Pisharody, Corrosion inhibition of mild steel in acid media by quinolino thiopropano hydrazone, Ind. J. Chem. Technol. 16 (2009) 162–174. [5] Ashish Kumar Singh, M.A.Q. Uraishi, Study of some bidentate Schiff bases of Isatin as corrosion inhibitors for mild steel in hydrochloric acid solution, Int. J. Electrochem. Sci. 7 (2012) 3222–3241. [6] S.D. Toliwal, Kalpesh Yadav, Tejas Pavagaddhi, Corrosion inhibition study of a new synthetic Schiff base derived from nontraditional oils on mild steel in 1N HCl solution, J. Sci. Ind. Res. 69 (2010) 43–47. [7] T. Sethi, A. Chaturvedi, R.K. Upadhyay, S.P. Mathur, Corrosion inhibitory effects of some Schiff bases on mild steel in acidic media, J. Chi. Chem. Soc. 52 (2007) 1206–1213. [8] Serpil Safak, Berrin Duran, Aysel Yurt, Gulsen Turkoglu, Schiff bases as corrosion inhibitor for aluminium in HCl solution, Corros. Sci. 54 (2014) 251–259. [9] R.T. Loto, C.A. Loto, A.P.I. Popoola, Corrosion inhibition of thiourea and thiadiazole derivatives, J. Mater. Environ. Sci. 3 (2012) 885–894. [10] S. Leelavathi, R. Rajalakshmi, Dodonaea viscosa (L.) Leaves extract as acid Corrosion inhibitor for mild Steel – A Green approach, J. Mater Environ. Sci. 4 (2013) 625–638. [11] B.A. Abd-El-Nabey, A.M. Abdel-Gaber, M.E.L. Said Ali, Essam Khamis, S. El-Housseiny, Inhibitive action of cannabis plant extract on the corrosion of copper in 0.5 M H2SO4, Int. J. Electrochem. Sci. 8 (2013) 5851–5865. [12] ’E.A. Noor, Synthesis and effect on corrosion inhibition of carbon steel in hydrochloric acid solution, Appl. Electrochem. 39 (2009) 1465–1475. [13] M.A. Hegazy, Novel cationic gemini surfactants as corrosion inhibitors for carbon steel pipelines, Corro. Sci. 51 (2009) 2610– 2618.
P.M. Dasami et al. / Measurement 69 (2015) 195–201 [14] A. Layla, Al Juhaiman, Amal Abu Mustafa, Waffak Mekhamer, Polyvinyl pyrrolidone as a green corrosion inhibitor of carbon steel in neutral solutions containing NaCl: electrochemical and thermodynamic study, Int. J. Electrochem. Sci. 7 (2012) 8578–8596. [15] F. Bentiss, M. Lebrini, M. Lagrene, Thermodynamic characterization of metal dissolution and inhibitor adsorption processes in mild steel/ 2,5-bis(n-thienyl)-1,3,4-thiadiazoles/hydrochloric acid system, Corro. Sci. 47 (2005) 2915–2931. [16] N.S. Patel, S. Jauhariand, G.N. Mehta, S.S. Al-Deyab, I. Warad, B. Hammouti, Mild steel corrosion inhibition by various plant extracts in 0.5M sulphuric acid, Int. J. Electrochem. Sci. 8 (2013) 2635–2655. [17] Singh Rajeev, D. Kumar, Singh Bhoop, V.K. Singh, Sharma Ranjana, Molecular structure, vibrational spectroscopic and HOMO, LUMO studies of S-2-picolyl-b-N-(2-acetylpyrrole) dithiocarbazate Schiff base by Quantum Chemical investigations, Res. J. Chem. Sci. 3 (2013) 79–84.
201
[18] R. Saratha, R. Meenakshi, Dimethylaminobenzylidene acetone as corrosion inhibitor for mild steel in acid medium, Rasayan J. Chem. 4 (2011) 251–263. [19] A. Kosari, M.H. Moayed, A. Davoodi, R. Parvizi, M. Momeni, H. Eshghi, H. Moradi, Electrochemical and quantum chemical assessment of two organic compounds from pyridine derivatives as corrosion inhibitors for mild steel in HCl solution under stagnant condition and hydrodynamic flow, Corro. Sci. 78 (2014) 138–150. [20] S. Martinez, Inhibitory mechanism of mimosa tannin using molecular modelling and substitutional adsorption isotherms, Mat. Chem. Phys. 77 (2002) 997–1025. [21] Jianfang Zhu, Shuangkou Chen, A theoretical investigation on the inhibition efficiencies of some schiff bases as corrosion inhibitors of steel in hydrochloric acid, Int. J. Electrochem. Sci. 7 (2012) 11884– 11894.