Article pubs.acs.org/IECR
Electroanalytical and Theoretical Investigations of the Corrosion Inhibition Behavior of Bis-1,2,4-Triazole Precursors EBATTand BBATT on Mild Steel in 0.1 N HNO3. Sam John* and Abraham Joseph Department of Chemistry, University of Calicut, Calicut University PO, Kerala, 673 635 India ABSTRACT: The inhibition of mild steel corrosion in 0.1 N nitric acid by the bis-1,2,4-triazole precursors EBATT and BBATT was investigated by potentiodynamic polarization (Tafel), electrochemical impedance spectroscopy (EIS), adsorption, and quantum chemical calculations at 300 K. Polarization studies showed that these molecules act as mixed-type inhibitors. As the electron density around the inhibitor molecules increases due to substitution, the inhibition efficiency also increases. The quantum chemical approach was also used to calculate electronic properties of the molecules to ascertain the correlation between inhibition effect and molecular structure. Experimental and theoretical studies agree well in this regard and confirm that BBATT is a better inhibitor than EBATT.
1. INTRODUCTION
2. EXPERIMENTAL METHODS 2.1. Inhibitors. Synthesis of 3,3′-(Ethane-1,2-diyl) Bis[4amino-1H-1,2,4-triazole 5-(4H)-thione] (EBATT). A mixture of succinic acid (0.01 mol) and thiocarbohydrazide (0.02 mol) was warmed carefully until melting occurred and then kept at 170 °C for 30 min. The reaction mixture was then cooled and mixed with water (50 mL). The precipitate was filtered off, washed with water and 95% ethanol, and finally recrystallized from dimethylformamide. Probable assignments of Fourier transform infrared (FTIR) and nuclear magnetic resonance (NMR) spectra are as follows: FTIR (KBr) υ (cm−1): 3286 (NH2), 2933 (CH2), 1562 (C N), 1284 (CN), 1072 (CS). 1H NMR (DMSO-d6; δ, ppm): 13.49 (s, 2H, 2NH) two NH protons of the amino group, 5.56(s, 4H, 2NH2), two NH2 protons, 3.06 (s, 4H, 2CH2) methylene protons of the two CH2 groups. 13C NMR (DMSO-d6; δ, ppm): 150.67 (CN), 165.91 and 162.20 (C S) end group of thione. Synthesis of 3,3′-(Butane-1,4-diyl) Bis[4-amino-1H-1,2,4triazole 5-(4H)-thione] (BBATT). A mixture of adipic acid (0.01 mol) and thiocarbohydrazide (0.02 mol) was warmed carefully until melting occurred and then kept at 170 °C for 30 min. The reaction mixture was then cooled to room temperature and mixed with water (50 mL). The precipitate was filtered off, washed with water and 95% ethanol, and finally recrystallized from dimethyl sulfoxide (DMSO). Probable assignments of FTIR and NMR spectra are as follows: FTIR (KBr) υ (cm−1): 3249 (NH2), 2948 (CH2), 1573 (CN), 1234 (CN), 1083 (CS). 1H NMR (DMSO-d6; δ, ppm): 1.72−1.56 (m, 4H, CH2), 2.7 (t, 4H, CH2), 5.52 (m, 4H, 2NH2), 13.44 (s, 2H, NH). 13C NMR (DMSO-d6; δ, ppm): 23.7 (2CH2), 24.8 (2CH2), 151.92 (CN), 165.73 (CS).
Acid solutions are generally used for the removal of rust and scale in industrial processes. Hydrochloric acid is widely used in the pickling of steel and its alloys. Inhibitors are generally used in these processes to control metal dissolution. Most known inhibitors are organic compounds containing N, S, and/or O atoms. Organic compounds containing functional electronegative groups and π electrons in triple bonds or conjugated double bonds are usually good inhibitors. The mechanism of adsorption of organic molecules on the metal surface might involve the following: (a) electrostatic interaction between negatively charged surfaces, which is created by specifically adsorbed anions on the metal and positive charge of the inhibitor; (b) interaction of an unshared electron pair in the inhibitor molecule with the metal; (c) interaction of π electrons in the inhibitor molecule with the metal; and/or (d) a combination of all of these processes. The study of corrosion processes and their inhibition by organic compounds is an active field of contemporary research.1−5 Computational quantum chemical calculations and molecular simulation studies have been used recently to explain the mechanism of corrosion inhibition.6−12 The geometry of the inhibitor molecule in its ground state and the nature of their molecular orbitals (highest occupied molecular orbital, HOMO; lowest unoccupied molecular orbital, LUMO) are directly involved in the inhibitive properties of these molecules. This article describes the corrosion inhibition behavior of the bis-1,2,4-triazole derivatives 3,3′-(ethane-1,2-diyl) bis[4-amino-1H-1,2,4-triazole 5-(4H)thione] (EBATT) and 3,3′-(butane-1,4-diyl) bis[4-amino-1H1,2,4-triazole 5-(4H)-thione] (BBATT) on mild steel in 0.1 N nitric acid solution using electroanalytical and computational methods. © 2012 American Chemical Society
Received: Revised: Accepted: Published: 16633
July 25, 2012 November 19, 2012 November 22, 2012 December 3, 2012 dx.doi.org/10.1021/ie301963a | Ind. Eng. Chem. Res. 2012, 51, 16633−16642
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Figure 1. Structures of the inhibitor molecules.
electrode, and a saturated calomel electrode (SCE) as the reference electrode. The working electrode was first immersed in the test solution, and after a steady-state open-circuit potential (OCP) had been established, the electrochemical measurements were carried out with a Gill ac computercontrolled electrochemical workstation (model 1475; ACM Instruments, Grange-over-Sands, U.K.). Electrochemical impedance spectroscopy (EIS) measurements were also carried out with an ac sine wave with a root-mean-square amplitude of 10 mV and frequency range from 10 kHz to 1 Hz. Potentiodynamic polarization curves were obtained in the potential range from −250 to +250 mV at a sweep rate of 1 mV/s. 2.5. Adsorption Studies. It is generally assumed that the adsorption of inhibitor molecules on a metal surface is an essential step in the inhibition mechanism. To understand the adsorption phenomenon, various isotherms were tested, and the Langmuir isotherm was found to be the best, giving a
The structures of the inhibitor molecules are shown in Figure 1. 2.2. Medium. The medium for the study was prepared from reagent-grade HNO3 (E-Merck) using doubly distilled water. All tests were performed in aerated medium at room temperature (300 K) and atmospheric pressure. 2.3. Materials. The mild steel samples were of the following composition (wt %): C ≈ 0.2, Mn ≈ 1, P ≈ 0.03, S ≈ 0.02, Fe ≈ 98.75. For electrochemical studies, 4.8 × 1.9 cm2 coupons of mild steel were used, but only an area of 1 cm2 was exposed during measurements. The samples were polished using different grades of emery papers and then washed with ethanol, acetone, and finally distilled water before each measurement as recommended by ASTM standard G-1-72. 2.4. Electrochemical Measurements. Electrochemical tests were carried out in a conventional three-electrode electrochemical cell with the metal specimen as the working electrode, a platinum sheet (1-cm2 surface area) as the auxiliary 16634
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straight-line graph for a plot of C/θ versus C, with the surface coverage θ given by θ = Uo − Ui/Uo, where Uo is the inhibited charge-transfer resistance, Ui is the uninhibited charge-transfer resistance, and C is the concentration of the inhibitor in parts per million (ppm). 2.6. Computational Details. Complete geometry optimizations of the molecules were performed using density functional theory (DFT) with Becke’s three-parameter exchange functional along with the Lee−Yang−Parr nonlocal correlation functional (B3LYP) with the 6-31G* basis set and the Gaussian 03 program package.13−16 The energies calculated for the frontier molecular orbitals (HOMO and LUMO) can be used to interpret the adsorption characteristics of the inhibitor molecules. The calculation of the value of local softness indices σ provides some insight into the nature of adsorption of the inhibitor molecules. The local reactivity of the inhibitor molecules was analyzed through an evaluation of the Fukui indices. Fukui indices are measurements of chemical reactivity, as well as indicators of the reactive regions and the nucleophilic and electrophilic behavior of the molecule. The regions of a molecule where the Fukui function is large are chemically softer than the regions where the Fukui function is small, and by invoking the hard−soft Lewis acid−base (HSAB) principle in a local sense, one can establish the behavior of different sites with respect to hard or soft reagents. 2.7. Scanning Electron Microscopy (SEM). The surface morphology of the sample under study in the absence and presence of inhibitors was examined using a Hitachi model SU6600 scanning electron microscope at an accelerating voltage of 20.0 kV. Samples were attached on top of an aluminum stopper by means of carbon conductive adhesive tape. All micrographs of the specimens were recorded at a magnification of 500×. Figure 2. Anodic and cathodic Tafel lines for mild steel in uninhibited 1 M HCl and with different concentrations of the inhibitors (a) EBATT and (b) BBATT.
3. RESULTS AND DISCUSSION 3.1. Potentiodynamic Polarization Studies. Polarization measurements were carried out to gather information concerning the kinetics of the anodic and cathodic reactions. The potentiodynamic polarization curves for mild steel in 0.1 N HNO3 solution in the absence and presence of various concentrations of the inhibitor molecules are shown in Figure 2. The values of electrochemical kinetic parameters, including the corrosion potential (Ecorr), corrosion current density (icorr), and Tafel slopes (βa and βc), determined from these experiments by extrapolation are listed in Table 1. The corrosion inhibition efficiency was calculated using the equation IE (%) =
Icorr * − Icorr × 100% Icorr *
the inhibitor on the metal surface. Therefore, bis-triazole can be considered as a mixed-type inhibitor. The cathodic branches of the polarization curves for the two inhibitors were aligned parallel to each other. A decrease in cathodic corrosion current density was also visible. Thus, it can be concluded that the addition of bis-triazole does not change the cathodic hydrogen evolution mechanism and that the decrease in H+ ions on the surface of mild steel occurs mainly through a charge-transfer mechanism. The suppression of the cathodic process can be attributed to the adsorption of inhibitor molecules on the cathodic sites. Thus, addition of this inhibitor not only reduces mild steel dissolution but also causes a delay in the hydrogen evolution reaction. From a thorough study of the anodic branch of the polarization curve, it is evident that the inhibitor molecules first adsorb on the mild steel surface and thereafter block the available reaction sites.17 The surface coverage was found to increase with the inhibitor concentration. The formation of a surface inhibitor film on the mild steel surface reduces the active surface area available for the attack of the corrosive medium, delays both the hydrogen evolution and iron dissolution reactions, and provides considerable protection against corrosion to mild steel.18,19 The corrosion parameters derived from these curves are listed in Table 1. It is clear that the values of corrosion current density (icorr) and corrosion rate (CR) decreased considerably with
(1)
where Icorr* and Icorr are the uninhibited and inhibited corrosion current densities, respectively, determined by extrapolation of the Tafel lines to the corrosion potential. In acidic solutions, the anodic reaction of corrosion is the passage of metal ions from the metal surface into the solution, and the cathodic reaction is the discharge of hydrogen ions to produce hydrogen gas or reduce oxygen. An inhibitor might affect either the anodic or cathodic reaction or in some cases both. Inspection of Figure 2 shows that the addition of bis-triazole has an inhibitive effect on both the anodic and cathodic parts of the polarization curves and shifts both the anodic and cathodic curves to lower current densities. This result can be ascribed to adsorption of 16635
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Table 1. Polarization Parameter and Corresponding Inhibition Efficiency for the Corrosion of Mild Steel 0.1 N HNO3 at Various Concentrations of EBATT and BBATT conc (ppm)
Ecorr (mV)
LPR (Ω cm2)
βa (mV dec−1)
−βc (mV dec−1)
icorr (mA cm−2)
CR (mm/year)
blank
−480
22.75
160
257
1.4338
33.35
−
203 189 159 149 127
0.3021 0.1953 0.1174 0.1021 0.0782
7.03 4.54 1.36 1.18 0.91
78.93 86.38 91.81 92.88 94.54
252 195 160 150 140
0.5342 0.1806 0.1157 0.0755 0.0379
6.19 2.09 1.27 0.87 0.44
62.74 87.40 91.93 94.73 97.35
IE (%)
EBATT 10 25 50 75 100
−537 −541 −541 −543 −545
34.87 35.93 42.45 45.78 54.90
95 74 65 62 62
10 25 50 75 100
−540 −530 −536 −541 −530
47.31 51.92 53.07 51.29 68.61
139 82 80 72 71
BBATT
Figure 3. Nyquist diagrams for mild steel in 0.1 N HNO3 containing different concentrations of (a) EBATT and (b) BBATT.
Figure 4. Bode diagrams for mild steel in 0.1 N HNO3 containing different concentrations of (a) EBATT and (b) BBATT.
increasing concentration of the inhibitor. This was due to the formation of a film of inhibitor molecules on the mild steel surface, which acted as a protecting barrier against corrosion. Also, the IE values increased with the inhibitor concentration, and the maximum IE value was 97.35% at 100 ppm of BBATT. 3.2. Electrochemical Impedance Spectroscopy. Impedance measurements were performed under potentiostatic conditions after 1 h of immersion. Nyquist plots of uninhibited and inhibited solutions containing different concentrations of
inhibitor molecules were obtained over the frequency range from 1 Hz to 10 kHz and are shown in Figure 3. The similarity in the shapes of these graphs throughout the experiments is due to the fact that the addition of inhibitor molecules does not change the corrosion mechanism. The corresponding Bode and impedance plots are shown in Figures 4 and 5, respectively. The capacitive loop at high frequencies represents the phenomenon associated with the electrical double layer. The Nyquist plots contain depressed semicircles centered under the 16636
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Table 2. Impedance parameter for the corrosion of mild steel in 0.1 N HNO3 with various concentrations of EBATT and BBATT conc (ppm)
Rp (Ωcm2)
Cdl (μF cm2)
blank
13.4
358.6
10 25 50 75 100
47.5 96.2 149.1 153.5 199.3
144.4 77.3 56.5 51.9 53.1
10 25 50 75 100
72.5 176.7 202.3 248.9 578.6
113.7 57.4 49.8 43.4 33.2
icorr (mAcm−2) 1.9409 EBATT 0.5492 0.2711 0.1750 0.1700 0.1309 BBATT 0.3616 0.1476 0.1257 0.1048 0.0451
CR (mm/ year)
n
22.50
0.94
−
12.77 6.31 2.03 1.97 1.52
0.84 0.86 0.89 0.87 0.88
71.78 86.07 91.01 91.27 93.27
4.91 1.71 1.09 1.22 0.53
0.84 0.87 0.88 0.88 0.92
81.52 92.42 93.38 94.62 97.68
IE (%)
Figure 5. Impedance diagrams for mild steel in 0.1 N HNO3 containing different concentrations of (a) EBATT and (b) BBATT. Figure 7. Langmuir adsorption isotherms for the inhibitors (a) EBATT and (b) BBATT.
ZCPE = A−1(iω)−n
(2)
where A is the CPE constant (in Ω−1 Sn cm−2); i = (−1)1/2; ω is the angular frequency (ω = 2πf, where f is the angular frequency); and n is the CPE component, which gives details about the degree of surface inhomogenity.23,24 As resported in Table 2, the values of Rp and Cdl exhibit opposite trends over the entire concentration range, and this might be due to the formation of a protective layer on the surface of the electrode. The double layer between the charged metal surface and the solution was considered as an electrical double capacitor. The decrease in electrical capacity of the mild steel caused by the adsorption of inhibitor molecules on its surface was achieved by the displacement of water molecules and other ions that were originally adsorbed on the metal surface.25 The increase in the thickness of the protective layer and decrease in the value of Cdl with increasing inhibitor concentration showed the electrostatic nature of the adsorption of inhibitor molecules on the mild steel surface. This trend is in accordance with the Helmholtz model, given by the equation
Figure 6. Equivalent circuit used to fit the EIS data for mild steel and different inhibitor concentrations.
real axis. Such behavior is characteristic of solid electrodes and is often referred to frequency dispersion attributed to different physical phenomena such as roughness, inhomogeneities of the solid surface, impurities, grain boundaries, and distributions of surface active sites. Ideal capacitive behavior was not seen in this case, and hence, a constant phase element (CPE) was introduced into the circuit to provide a more accurate fit.20−22 The simplest fitting can be represented by a Randles equivalent circuit (Figure 6), which is a parallel combination of the chargetransfer resistance (Rct) and the constant phase element (CPE), both in series with the solution resistance (Rs). The impedance function of a CPE can be represented as
Cdl = 16637
ΣΣ0A d
(3)
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Table 3. Calculated Quantum Chemical Parameters for the Inhibitor Molecules inhibitor
Etotal (au)
EHOMO (eV)
ELUMO (eV)
ΔE (eV)
μ
χ
η
ΔN
EBATT BBATT
−1468 −1547
−0.2601 −0.1567
−5.8929 −5.8082
−5.6328 −5.6515
0.8808 0.9750
−3.0765 −2.9824
−2.8164 −2.8258
1.78 1.76
Table 4. Nucleophilic and Electrophilic Fukui Functions for EBATT and BBATT BBATT
EBATT
atom
f(−)
f(+)
atom
f(−)
f(+)
C1 C2 N3 C5 N6 N7 S11 C14 C16 C17 C22 C23 N24 N25 N26 S28 N29
0.0048 0.0011 0.0058 0.0003 0.0001 0.0111 0.0584 0.0008 0.0039 0.0016 0.0477 0.011 0.058 0.1115 0.0861 0.5769 0.0007
0.0002 0.0321 0.0052 0.0001 0.0104 0.0104 0.0134 0.0003 0.001 0.0004 0.006 0.4345 0.0574 0.1606 0.0714 0.1812 0.0024
C1 C2 N3 C5 C6 C7 N9 N10 N11 S14 C19 N20 N21 N22 N25 S26
0.0151 0.0081 0.0787 0.0004 0.0001 0 0.066 0.0627 0.0959 0.671 0.0001 0 0 0 0 0.0004
0.0178 0.0024 0.002 0.0308 0.0292 0.0548 0.0001 0.0036 0.0093 0.0018 0.3028 0.3345 0.0246 0.0014 0.1253 0.0226
3.3. Adsorption Studies. The selection of these inhibitor molecules was based on their mechanism of action, for example, their ability to donate electrons. The corrosion inhibition properties of several substances are directly associated with adsorption phenomena that can follow different types of adsorption isotherms, such as the Temkin, Langmuir, Freundlich, and Frumkin isotherms that have been employed to study adsorption phenomena on steel electrodes. The adsorption of an inhibitor at an electrode/electrolyte interface can take place through displacement of adsorbed water molecules at the inner Helmholtz plane of the electrode, likely in agreement with the reaction scheme Orgaq + XH2Oads ↔ Orgads+ XH2Oaq, where X, the size ratio, is the number of water molecules displaced by one molecule of organic inhibitor. Adsorption plays a significant role in the inhibition of metallic corrosion by organic molecules. Many investigators have used the Langmuir adsorption isotherm to study inhibitor characteristics, assuming that the adsorption of inhibitors on the metal surface decreases the surface area available for electrode reactions to take place.26−29 In this work, the adsorption of inhibitor molecules on the mild steel surface was found to obey the Langmuir isotherm. The straight lines obtained for Langmuir isotherms are shown in Figure 7. 3.4. Quantum Chemical Calculations. The frontier orbitals (HOMO and LUMO) of a chemical species are very important in defining its reactivity. Fukui30 first recognized this. A good correlation was found between the rate of corrosion and the energy of the HOMO (EHOMO), which is often associated with the electron-donating ability of the molecule. The adsorption of an inhibitor on a metal surface can occur on the basis of donor−acceptor interactions between the π electrons of the heterocyclic compound and the vacant d orbital of the surface metal atoms.31 A high value of EHOMO shows the tendency of a molecule to donate electrons to
Figure 8. Highest occupied molecular orbitals (HOMOs) and lowest unoccupied molecular orbitals (LUMOs) of the inhibitors (a) EBATT and (b) BBATT.
where d is the thickness of the protective layer, Σ is the dielectric constant of the medium, Σ0 is the vacuum permittivity, and A is the surface area of the electrode. The equation used to calculate the percentage inhibition efficiency is R p* − R p IE (%) = × 100% R p* (4) where Rp* and Rp are the values of the polarization resistance observed in the presence and absence, respectively, of inhibitor molecules. The impedance parameters are summarized in Table 2. 16638
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Figure 9. Pictorial representation of the probable modes of interaction on the mild steel surface of (a) EBATT and (b) BBATT.
= I and −ELUMO = A. Although no formal proof of this theorem exists within DFT, its validity is generally accepted. For χ and η, the operational and approximate definitions are −μ = (I + A)/2 = χ and η = (I − A)/2. When the two systems Fe and inhibitor are brought together, electrons flow from lower χ (inhibitor) to higher χ (Fe), until the chemical potentials become equal. As a first approximation, the fraction of electrons transferred,37 ΔN, is given by
appropriate acceptor molecules with low-energy empty molecular orbitals. Increasing values of EHOMO facilitate adsorption and therefore enhance the inhibition efficiency by influencing the transport process through the adsorbed layer. Similar relations were found between the rate of corrosion and ΔE = ELUMO − EHOMO.32−34 The energy of the lowest unoccupied molecular orbital indicates the ability of the molecule to accept electrons. A lower value of ELUMO indicates a greater probability of the molecule to accept electrons. Larger values of the energy difference (ΔE) provide low reactivity to a chemical species. Lower values of ΔE render good inhibition efficiency, as the energy to remove an electron from the highest occupied molecular orbital is low.35,36 In Table 3, certain quantum-chemical parameters related to the molecular electronic structure, including EHOMO, ELUMO, and ΔE, are presented. A pictorial representation of the HOMOs and LUMOs of EBATT and BBATT is presented in Figure 8a,b. A higher value of EHOMO and lower value of the gap energy, ΔE, indicate that BBATT is a better inhibitor than EBATT. The results for the calculations of the ionization potential (I) and electron affinity (A) by application of Koopmans’ theorem are also given in Table 3. According to the Hartree−Fock theorem, relationships exist between EHOMO, ELUMO, I, and A as −EHOMO
ΔN =
χFe − χinhi 2(ηFe + ηinhi)
(5)
where Fe is considered as the Lewis acid according to the HSAB concept.38 The difference in electronegativity drives the electron transfer, and the sum of the hardness parameters acts as a resistance. To calculate the fraction of electrons transferred, a theoretical value for the electronegativity of bulk iron of χFe ≈7 eV and a global hardness of ηFe ≈ 0 were used, by assuming that, for a bulk metal, I = A,39,40 because they are softer than the neutral metallic atoms. The local reactivity was analyzed by means of the condensed Fukui function. The condensed Fukui function allows one to distinguish each part of a molecule on the basis of its distinct chemical behavior due to different 16639
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Figure 10. SEM images of (a) blank mild steel, (b) in 0.1 N HNO3 without inhibitor, (c) in the presence of 100 ppm of EBATT after 48 h, (d) in the presence of 100 ppm of BBATT after 48 h.
functional groups or substituents. Thus, the site for nucleophilic attack is the place where the value of f +k is a maximum, and the site for electrophilic attack is the place where the value of f −k is a maximum. The values of the Fukui functions for nucleophilic and electrophilic attack are given for the two inhibitors in Table 4 (only for the nitrogen, oxygen and carbon atoms). Inspection of the values of Fukui functions presented in Table 4 shows that BBATT has more propitious zones for nucleophilic attack (28S, 25N, 24N, 11S, 22C, and 26N) than EBATT (14S, 11N, 3N, and 9N). The data in Table 4 show that BBATT has more susceptible sites for adsorption on the iron surface, which reflects its higher inhibition. The location of the HOMO in each system and the higher values of the Fukui indices of the corresponding atoms are in perfect agreement with each other. Both characteristics helped in identifying the zones by which the molecule would be adsorbed on the mild steel surface. Schematic representations of the probable modes of adsorption of the inhibitor molecules on the metal surface are given in Figure 9a,b. 3.5. Scanning Electron Microscopy (SEM). Surface examination using SEM was carried out to understand the effect of inhibitor molecules on the surface morphology of mild steel. Figure 10a shows an SEM image of a polished mild steel surface. Figure 10b shows an SEM image of the surface of mild steel after immersion in acid without inhibitor molecules for 48 h. This micrograph shows the effect of acid on surface damage. Figure 10c shows an SEM image of the surface of mild steel immersed in acid solution containing 100 ppm EBATT, and Figure 10d shows the surface of mild steel immersed in acid solution containing 100 ppm BBATT. The faceting observed in Figure 10b is absent in Figure 10c,d. Also, the surface is free from pits and visibly smooth. Therefore, it can be concluded that corrosion is much less extensive in the presence of
inhibitors and a more polished surface was obtained with BBATT than EBATT, which, in turn, confirms its higher inhibition efficiency. 3.6. Mechanism of Adsorption. The anodic dissolution of iron in acidic solutions has been reported to proceed as Fe + OH ↔ FeOHads + H + e−
(6)
FeOHads → FeOH+ + e−
(7)
(rate‐determining step)
FeOH+ + H+ ↔ Fe 2 + + H 2O
(8)
The accompanying cathodic hydrogen evolution reaction follows the steps Fe + H+ ↔ FeHads+
(9)
FeHads+ + e− ↔ FeHads
(10)
FeHads + H+ + e− → Fe + H 2
(11)
As a consequence of this reaction, including the high solubility of the corrosion products, the metal loses weight in solution. Corrosion inhibition is thought to be initiated by the inhibitor species, leading to the specific adsorption of the inhibitor on the metal surface. The inhibition efficiency increased with increasing concentration of the inhibitors. This suggests that more inhibited molecules are adsorbed on the metal surface at higher concentration, leading to greater surface coverage. Adsorption takes place through hetero atoms such as N, S, O, and P; double or triple bonds; or aromatic rings. The inhibition efficiency should increase in the order P > S > N > O. 16640
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(7) Malyszko, J.; Scendo, M. Electrode kinetics of Cu(II)/Cu(I) systems platinum in water + dimethylsulphoxide (DMSO) mixtures. J. Electroanal. Chem. 1998, 250, 61. (8) Wang, H. L.; Fan, H. B.; Zheng, J. S. Corrosion inhibition of mild steel in hydrochloric acid solution by a mercapto-triazole compound. Mater. Chem. Phys. 2002, 77, 655. (9) Babic-Samardzija, K.; Lupu, C.; Hackerman, N.; Barron, A. R. Inhibitive properties, adsorption and surface study of butyn-1-ol and pentyn-1-ol alcohols as corrosion inhibitors for iron in HCl. J.Mater. Chem. 2005, 15, 1908. (10) Khaled, K. F.; Amin, M. A. Corrosion monitoring of mild steel in sulphuric acid solutions in presence of some thiazole derivatives Molecular dynamics, chemical and electrochemical studies. Corros. Sci. 2009, 51, 1964. (11) Xia, S.; Qiu, M.; Yu, L.; Liu, F.; Zhao, H. Molecular dynamics and density functional theory study on relationship between structure of imidazoline derivatives and inhibition performance. Corros. Sci. 2008, 50, 2021. (12) Zhao, P.; Liang, Q.; Li, Y. Electrochemical, SEM/EDS and quantum chemical study of phthalocyanines as corrosion inhibitors for mild steel in 1 mol/L HCl. Appl. Surf. Sci. 2005, 252, 1596. (13) Sastri, V. S.; Perumareddi, J. R. Molecular orbital theoretical studies of some organic corrosion inhibitors. Corrosion 1997, 53, 617. (14) McCafferty, E.; Pravadic, V.; Zettlemoyer, A. C. Dielectric behavior of adsorbed water films on the α-Fe2O3 surface. Trans. Faraday Soc. 1970, 66, 1720. (15) Ailor, W. H. Handbook of Corrosion Testing and Evaluation; John Wiley & Sons: New York, 1971. (16) Talati, J. D.; Modi., R. M. Dihydroxy-benzenes as corrosion inhibitors for aluminium−copper alloy in sodium hydroxide. Br. Corros. J. 1975, 10, 103. (17) Ebenso, E. E.; Arslan, T.; Kandemirli, F.; Caner, N.; Love, I. Quantum chemical studies of some rhodanine azosulpha drugs as corrosion inhibitors for mild steel in acidic medium. Int. J. Quantum Chem. 2010, 110, 1003. (18) Arslan, T.; Kandemirli, F.; Ebenso, E. E.; Love, I.; Alemu, H. Quantum chemical studies on the corrosion inhibition of some sulphonamides on mild steel in acidic medium. Corros. Sci. 2009, 51, 35. (19) Gece, G. The use of quantum chemical methods in corrosion inhibitor studies. Corros. Sci. 2008, 50, 2981. (20) Solmaz, R.; Kardas, G.; Culha, M.; Yazici, B.; Erbil, M. Investigation of adsorption and inhibitive effect of 2-mercaptothiazoline on corrosion of mild steel in hydrochloric acid media. Electrochim. Acta 2008, 53, 5941. (21) Fuchs-Godec, R. The adsorption CMC determination and corrosion inhibition of some N-alkyl quaternary ammonium salts on carbon steel surface in 2 M H2SO4. Colloids Surf. A: Physiochem. Eng. Aspects 2006, 280, 130. (22) Chetouani, A.; Hammouti, B.; Benhadda, T.; Daoudi, M. Inhibitive action of bipyrazolic type organic compounds towards corrosion of pure iron in acidic media. Appl. Surf. Sci. 2005, 249, 375. (23) Labjar, N.; Lebrini, M.; Bentiss, F.; Chihib, N. E.; Hajjaji, S.; Jama, C. Corrosion inhibition of carbon steel and antibacterial properties of aminotris-(methylenephosphonic) acid. Mater. Chem. Phys. 2010, 119, 330. (24) Amin, M. A.; Abed El-Rehim, S. S.; El-Sherbini, E. E. F.; Bayyomi, R. S. The inhibition of low carbon steel corrosion in hydrochloric acid solutions by succinic acid: Part I. Weight loss, polarization, EIS, PZC, EDX and SEM studies. Electrochim. Acta 2007, 52, 3588. (25) Veloz, M. A.; Gonzalez, I. Electrochemical study of carbon steel corrosion in buffered acetic acid solutions with chlorides and H2S. Electrochim. Acta 2002, 48, 135. (26) Sherif, E. M.; Park, S. M. Effects of 1,4-naphthoquinone on aluminum corrosion in 0.50 M sodium chloride solutions. Electrochim. Acta 2006, 51, 1313. (27) Goncalves, R. S.; Azambuja, D. S.; Serpa Lucho, A. M. Electrochemical studies of propargyl alcohol as corrosion inhibitor for
4. CONCLUSIONS (1) The 3,3′-(ethane-1,2-diyl) bis[4-amino-1H-1,2,4-triazole 5-(4H)-thione] (EBATT) and 3,3′-(butane-1,4-diyl) bis[4-amino-1H-1,2,4-triazole 5-(4H)-thione] (BBATT) molecules act as inhibitors for mild steel in aerated 0.1 N HNO3. The inhibition efficiency was fairly high, as evidenced by electrochemical studies. (2) The percentage inhibition efficiency increased with increasing concentration of EBATT and BBATT and decreased with longer exposure periods at 300 K. The surface area available for the attack of the corrosive species decreased with increasing inhibitor concentration. (3) Results of polarization studies suggest that EBATT and BBATT act as mixed-type inhibitors. (4) The inhibitor molecules adsorbed on the metal surface and blocked the reaction sites. Higher surface coverage on the metal surface was obtained with higher inhibitor concentrations. Scanning electron microscopy results confirmed the adsorption of inhibitor molecules on the metal surface. (5) The relationship between the efficiency of inhibition of mild steel corrosion in 0.1 N HNO3 by EBATT and BBATT, and the EHOMO, ELUMO, ELUMO − EHOMO, and ΔN values were calculated by DFT. The local reactivity analyzed by the condensed Fukui function also reveals that BBATT is a better inhibitor than EBATT. The results of quantum-chemical calculations and the electroanalytical results were in conformity with each other.
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AUTHOR INFORMATION
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[email protected]. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS One of the authors (S.J.) is grateful to CSIR, New Delhi, India, for providing a senior research fellowship. REFERENCES
(1) John, S.; Joseph, B.; Aravindakshan, K. K.; Joseph, A. Inhibition of mild steel corrosion in 1 M hydrochloric acid by 4-(N,Ndimethylaminobenzilidine)-3-mercapto-6-methyl-1,2,4-triazin(4H)-5one. Mater. Chem. Phys. 2010, 22, 374. (2) John, S.; Joseph, B.; Balakrishnan, K. V.; Aravindakshan, K. K.; Joseph, A. Electrochemical and quantum chemical study of 4-[(E)[(2,4-dihydroxy phenyl) methylidine] amino]-6-methyl-3-sulphanylidine-2,3,4,5-tetra hydro-1,2,4-triazin-5-one [DMSTT]. Mater. Chem. Phys. 2010, 123, 218. (3) Fiala, A.; Chibani, A.; Darchen, A.; Boulkamh, A.; Djebbar, K. Investigations of the inhibition of copper corrosion in nitric acid solutions by ketene dithioacetal derivatives. Appl. Surf. Sci. 2007, 253, 9347. (4) Zucchi, F.; Trabanelli, G.; Monticelli, C. The inhibition of copper corrosion in 0.1 M NaCl under heat exchange conditions. Corros. Sci. 1996, 38, 147. (5) Singh, A. K.; Quraishi, M. A. Inhibiting effects of 5-substituted isatin-based Mannich bases on the corrosion of mild steel in hydrochloric acid solution. J. Appl. Electrochem. 2010, 40, 1293. (6) Ramesh, S. V.; Adhikari, A. V. N′-[4-(diethylamino)benzylidine]3-{[8-(trifluoromethyl) quinolin-4 yl]thio}propano hydrazide) as an effective inhibitor of mild steel corrosion in acid media. Mater. Chem. Phys. 2009, 115, 618. 16641
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nickel, copper, and copper/nickel (55/45) alloy. Corros. Sci. 2002, 44, 467. (28) Mansfeld, F.; Kendig, M. W. Determination of the polarization resistance from impedance measurements. Mater. Corros. 1983, 34, 397. (29) Macdonald, R. Franceshetti, D. R. Impedance Spectroscopy; Wiley: New York, 1987. (30) Outirite, M.; Lagrenee, M.; Traisnel, M.; Jama, C.; Vezin, H.; Bentiss, F. ac impedance, X-ray photoelectron spectroscopy and density functional theory studies of 3,5-bis(n-pyridyl)-1,2,4-oxadiazoles as efficient corrosion inhibitors for carbon steel surface in hydrochloric acid solution. Electrochim. Acta 2010, 55, 1670. (31) Lopez, D. A.; Simison, S. N.; de Sanchez, S. R. The influence of steel microstructure on CO2 corrosion. EIS studies on the inhibition efficiency of benzimidazole. Electrochim. Acta 2003, 48, 845. (32) Khaled, K. F.; Al-Qahtani, M. M. The inhibitive effect of some tetrazole derivatives towards Al corrosion in acid solution: Chemical, electrochemical and theoretical studies. Mater. Chem. Phys. 2009, 113, 150. (33) Saleh, M. R.; Shams El Din, A. M. Efficiency of organic acids and their anions in retarding the dissolution of aluminium. Corros. Sci. 1981, 12, 688. (34) Fukui, K.; Yonezawa, T.; Shingu, H. A. Molecular Orbital Theory of Reactivity in Aromatic Hydrocarbons. J. Chem. Phys. 1952, 20, 722. (35) Fang, J.; Li, J. Quantum chemistry study on the relationship between molecular structure and corrosion inhibition efficiency of amides. J. Mol. Struct. (THEOCHEM) 2002, 593, 179. (36) Awad, M. K.; Issa, R. M.; Atlam, F. M. Theoretical investigation of the inhibition of corrosion by some triazole Schiff bases. Mater. Corros. 2009, 60, 813. (37) Li, W.; Zhao, X.; Liu, F.; Deng, J.; Hou, B. Investigation on the corrosion inhibitive effect of 2H-pyrazole triazole derivatives in acidic solution. Mater. Corros. 2009, 60, 287. (38) Hasanov, R.; Sadikoglu, M.; Bilgic, S. Electrochemical and quantum chemical studies of some Schiff bases on the corrosion of steel in H2SO4 solution. Appl. Surf. Sci. 2007, 253, 3913. (39) Amin, M. A.; Khaled, K. F.; Fadl-Allah, S. A. Testing validity of the Tafel extrapolation method for monitoring corrosion of cold rolled steel in HCl solutionsExperimental and theoretical studies. Corros. Sci. 2010, 52, 140. (40) Wang, H.; Wang, X.; Wang, H.; Wang, L.; Liu, A. DFT study of new bipyrazole derivatives and their potential activity as corrosion inhibitors. J. Mol. Model. 2007, 1, 147.
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