Article pubs.acs.org/IECR
Inhibition of Mild Steel Corrosion in Hydrochloric Acid Solution by 3-(4-((Z)-Indolin-3-ylideneamino)phenylimino)indolin-2-one Ashish Kumar Singh* Department of Chemistry, School of Mathematical and Physical Sciences, North West University (Mafikeng Campus), Private Bag X2046, Mmabatho 2735, South Africa Department of Applied Chemistry, IT, Banaras Hindu University, Varanasi, India 221 005 ABSTRACT: The inhibition effect of 3-(4-((Z)-indolin-3-ylideneamino)phenylimino)indolin-2-one Schiff base (PDBI) on mild steel corrosion in 1 M HCl solution was studied by using electrochemical techniques such as potentiodynamic polarization curves, weight loss, electrochemical impedance spectroscopy, and linear polarization resistance. PDBI has remarkable inhibition efficiency on the corrosion of mild steel in 1 M HCl solution. Polarization measurements indicated that PDBI acts as mixed type corrosion inhibitor. The adsorption of PDBI on the mild steel surface obeys Langmuir adsorption isotherm. These results were supplemented by atomic force microscopy and scanning electron microscopy.
1. INTRODUCTION The damage by corrosion generates not only high cost for inspection, repairing, and replacement, but in addition these constitute a public risk, thus the necessity of developing novel substances that behave like corrosion inhibitors especially in acid media.1 The use of organic molecules as corrosion inhibitor is one of the most practical methods for protecting against the corrosion, and it is becoming increasingly popular. The existing data show that organic inhibitors act by the adsorption on the metal surface and protective film formation.2,3 It has been shown that organic compounds containing heteroatoms with high electron density, such as phosphorus, nitrogen, sulfur, and oxygen as well as those containing multiple bonds which are considered as adsorption centers, are effective as corrosion inhibitor.4−7 The synthesized Schiff base has exhibited >90% inhibition efficiency at lower concentration than the others reported in literature.8−10 Recently, Schiff base compounds have been of interest in order to obtain efficient corrosion inhibitors since they provide much greater inhibition compared to corresponding amines and aldehydes.11−15 The presence of the -CN− group in Schiff base molecules enhances their adsorption ability and corrosion inhibition efficiency.16,17 Due to versatile importance of isatins, Ru(II) complexes of the studied Schiff base (3-(4-((Z)-indolin-3-ylideneamino)phenylimino)indolin-2-one, PDBI) have been synthesized and studied as antibacterial agents.18 The present paper reports on the anticorrosive behavior of 3-(4-((Z)-indolin-3-ylideneamino)phenylimino)indolin-2-one (a Schiff base) for mild steel in hydrochloric acid solution. For this purpose, weight loss, electrochemical impedance spectroscopy (EIS), potentiodynamic polarization, linear polarization, scanning electron microscopy (SEM), and atomic force microscopy (AFM) techniques were used.
4−5 h, given as Scheme 1, and recrystallized from ethanol.19 The schematic route of synthesis of PDBI is given as Scheme 1. The spectral data of PDBI are given in Table 1. 2.2. Solutions. The aggressive solutions were made of 37% HCl (E. Merck). The concentrations of the used inhibitors ranged from 0.68 × 10−4 to 3.42 × 10−4 M in 1 M HCl. All solutions were prepared in double distilled water. 2.3. Mild Steel Sample. The chemical composition of the working electrode, a mild steel electrode, was determined as follows (wt %): C, 0.17; Mn, 0.46; Si, 0.26; S. 0.017; P, 0.019; balance Fe. It was mechanically ground with 320, 400, 600, 800, 1000, and 1200 emery paper, washed in acetone and bidistilled water, then dried, and put into the cell. 2.4. Electrochemical Measurements. A three-electrode cell, consisting of carbon steel working electrode (WE), a platinum counter electrode (CE), and saturated calomel electrode (SCE) as a reference electrode, was used for electrochemical measurements. All experiments were performed in atmospheric condition without stirring. Prior to the electrochemical measurement, a stabilization period of 30 min was allowed, which was proved to be sufficient to attain a stable value of Ecorr; for example the OCP plot of mild steel in 1 M HCl with optimum concentration of PDBI is given as Figure 1. The EIS measurements were carried out in a frequency range from 100 kHz to 0.00001 kHz under potentiostatic conditions, with amplitude of 10 mV peak-to peak, using the AC signal at Ecorr. The potentiodynamic polarization curves were recorded in the potential range of −250 to +250 mV (SCE) with a scan rate of 1 mV s−1. All potentials were measured against SCE. The linear polarization study was carried out from the cathodic potential of −20 mV vs OCP to an anodic potential of +20 mV vs OCP with a scan rate of 0.125 mV s−1 to determine the polarization resistance (Rp).
2. EXPERIMENTAL SECTION 2.1. Synthesis of Corrosion Inhibitor. The synthesis of the studied compound was carried out through a condensation reaction between isatin and o-phenylenediamine in ethanol for
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© 2012 American Chemical Society
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September 9, 2011 January 3, 2012 January 26, 2012 January 26, 2012 dx.doi.org/10.1021/ie2020476 | Ind. Eng. Chem. Res. 2012, 51, 3215−3223
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Article
Scheme 1. Structure and Synthetic Route of PDBI
in 1 M HCl without and with addition of 2.73 × 10−3 M of the prepared inhibitor at 308 K for 3 h, steel specimens were cleaned with distilled water and dried using a cold air blaster and then the surface was investigated by a Jeol JSM-5400 scanning electron microscope. 2.7. Atomic Force Microscopy. The surface morphology of a mild steel specimen was investigated by using atomic force microscope. Atomic force microscopy was performed using a NT-MDT multimode atomic force microscope, Moscow, Russia, controlled by Solver scanning probe microscope controller. Semicontact mode was used with the tip mounted on a 100 μm long, single-beam cantilever with resonant frequency in the range of (2.4−2.5) × 105 Hz, and the corresponding spring constant of 11.5 N m−1 with NOVA program used for image rendering.21 The mild steel strips of 1.0 cm × 1.0 cm × 0.025 cm sizes were prepared as described in section 2.6. After immersion in 1 M HCl with and without addition of 2.73 × 10−3 M of PDBI at 308 K for 3 h, the specimens were cleaned with distilled water, dried, and then used for AFM.
Table 1. Spectral Data of PDBI δ, ppm ν, cm−1 IR (KBr)
1
H NMR (DMSO-d6)
ν(NH), 3280 m
11.96 (s, 2× >NH, D2O exchangeable) 7.59−6.76 (m, 12H, ArH)
ν(CO), 1725 s
13
C NMR (DMSO-d6)
115.88−151.79 (aromatic carbons) 159.31 (CN); 166.38 (CO)
ν(CN), 1614 s
3. RESULTS AND DISCUSSION 3.1. Electrochemical Impedance Spectroscopy. Impedance method provides information about the kinetics of the electrode processes and simultaneously about the surface properties of the investigated systems. The shape of impedance gives mechanistic information. The method is widely used for investigation of the corrosion inhibition processes.22 Nyquist and Bode plots of mild steel in 1 M HCl solution in the absence and presence of different concentrations of PDBI are presented in Figure 2a,b. It follows from Figure 2a that a high-frequency (HF) depressed charge-transfer semicircle was observed (one time constant in Bode plot) followed by a well-defined inductive loop in the low-frequency (LF) regions. The HF semicircle is attributed to the time constant of charge transfer and double-layer capacitance.23,24 The LF inductive loop may be attributed to the relaxation process obtained by adsorption − + species as Clads and Hads on the electrode surface.25 The relaxation time of a surface state, which means the time required for returning of the charge distribution to equilibrium after an electric perturbation, is defined as26
Figure 1. OCP plots for mild steel in 1 M HCl in the absence and presence of different concentrations of PDBI.
2.5. Weight Loss Measurements. Weight loss experiments were done according to the method described previously.20 Weight loss measurements were performed at 308 K (except for temperature effect) for 3 h (except for immersion time effect) by immersing the mild steel coupons into acid solution (100 mL) without and with various amounts of inhibitors. After the elapsed time, the specimens were taken out, washed, dried, and weighed accurately. The inhibition efficiency (μWL, %) and surface coverage (θ) were determined by using the following equations: μWL /% = θ=
w0 − wi × 100 w0
w0 − wi w0
(1)
(2)
τ = CR
where w0 and wi are the weight loss value in the absence and presence of inhibitor. 2.6. Scanning Electron Microscopy. The carbon steel specimens (size, 1.0 cm × 1 cm × 0.025 cm) were abraded with emery paper (grade 320−400−600−800−1000−1200) and then were washed with distilled water and acetone. After immersion
(3)
where C is capacitance and R is charge-transfer resistance. The inhibitor adsorption needs some time to reach equilibrium. Since this time is short, given in Table 2 (very small time constant), only the time constant is expressed. The phase angle at high frequencies provided a general idea of anticorrosion performance. The more negative the phase 3216
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Figure 2. (a) Nyquist plot, (b) Bode-magnitude plot, and (c) phase angle plots obtained for the mild steel in 1 M HCl in the absence and presence of different concentrations of PDBI.
Table 2. Impedance Parameters for Mild Steel in 1 M HCl in the Absence and Presence of Different Concentrations of PDBI inhibitor
concn of inhibitor (10−4 M)
Rs (Ω cm2)
Q (10−6 Ω−1 sn cm−2)
α
L (H)
Rct (Ω cm2)
RL (μF cm−2)
Cdl (Ω cm2)
τ (s)
μEIS (%)
PDBI
0.68 2.04 2.73
1.3 1.1 1.0 1.2
165.5 110.0 68.5 45.5
0.821 0.845 0.897 0.904
11 10.2 3.90 1.90
16.2 149.6 284.8 370.5
1.3 13.6 32.8 59.8
455.1 51.7 43.6 29.5
0.0073 0.0077 0.0124 0.0109
89 94 96
angle, the more capacitive the electrochemical behavior.27 Charge-transfer resistance increment could raise the current tendency to pass through the capacitor in the circuit. Also, depression of the phase angle at relaxation frequency with decreasing PDBI concentration (Figure 2c) indicated the decrease of capacitive response with the decrease of inhibitor concentration. Such a phenomenon could be attributed to higher corrosion inhibition activity at low concentrations of inhibitor. To get a more accurate fit of these experimental data, the measured impedance data were analyzed by fitting in to an equivalent circuit given in Figure 3. Excellent fit with this model
One constant phase element (CPE) is substituted for the capacitive element to give a more accurate fit, as the obtained capacitive loop is a depressed semicircle. The depression in Nyquist semicircles is a feature for solid electrodes and often referred to as frequency dispersion and attributed to the roughness and other inhomogeneities of the solid electrode.29 The CPE is a special element whose admittance value is a function of the angular frequency (ω) and whose phase is independent of the frequency. The admittance and impedance of CPE is given by
YCPE = Y0(i ω)n
(4)
where Y0 is the magnitude of CPE, i is an imaginary number (i2 = −1), and n = α/(π/2) in which α is the phase angle of CPE. The point of intersection between the inductive loop and the real axis represents (Rs + Rct). The electrochemical parameters, including Rs, Rct, RL, L, Y0 and n, obtained from fitting the recorded EIS using the electrochemical circuit of Figure 3, are listed in Table 2. Cdl values derived from CPE parameters according to eq 4 are listed in Table 2.
Figure 3. Electrochemical equivalent circuit used to fit the impedance measurements.
Cdl = Y0(ωmax )n − 1
was obtained for all experimental data. The equivalent circuit consists of the double-layer capacitance (Cdl) in parallel to the charge-transfer resistance (Rct), which is in series to the parallel of inductive elements (L) and RL. The presence of L in the impedance spectra in the presence of the inhibitor indicates that mild steel is still dissolved by the direct charge transfer at the PDBI-adsorbed mild steel surface.28
(5)
where ωmax is angular frequency (ωmax = 2πf max) at which the imaginary part of impedance (−Zi) is maximal and f max is the AC frequency at maximum. 3.2. Potentiodynamic Polarization Measurements. Figure 4 showed potentiodynamic polarization curves for the mild steel electrode in 1 M HCl solution with and without different concentrations of PDBI. It is clear that the current 3217
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efficiencies and polarization resistance parameters are presented in Table 3. The results obtained from Tafel polarization and EIS showed good agreement with the results obtained from linear polarization resistance. 3.4. Weight Loss Measurements. The effect of inhibitor concentration on inhibition efficiency of steel in 1 M HCl was first examined. Figure 5 represents such behavior in the
Figure 4. Typical polarization curves for corrosion of mild steel in 1 M HCl in the absence and presence of different concentrations of PDBI.
density decreases with the presence of Schiff bases; this indicated that PDBI adsorbed on the metal surface, and hence inhibition occurs. Values of corrosion potential (Ecorr) and corrosion current density (Icorr), obtained by extrapolation of the Tafel lines, cathodic, and anodic Tafel slope (bc, ba), and μP (%) for different concentrations of PDBI in 1 M HCl, are given in Table 3. The potentiodynamic curves show that there is a clear reduction of both the anodic and cathodic currents in the presence of PDBI compared with those for the blank solution. It is clear that the cathodic reaction (hydrogen evolution) and the anodic reaction (dissolution metal) were inhibited. The values of cathodic Tafel slope (bc) for PDBI are found to increase in the presence of inhibitor. The Tafel slope variations suggest that PDBI influence the kinetics of the hydrogen evolution reaction.30 This indicates an increase in the energy barrier for proton discharge, leading to less gas evolution.31 The approximately constant values of anodic Tafel slope (ba) for PDBI indicate that PDBI was first adsorbed onto the metal surface and impeded by merely blocking the reaction sites of the metal surface without affecting the anodic reaction mechanism.32 There is no definite trend observed in the Ecorr values in the presence of PDBI. In literature,33 it has been reported that (i) if the displacement in Ecorr is >85 mV with respect to Ecorr, the inhibitor can be seen as a cathodic or anodic type and (ii) if displacement in Ecorr is
