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Evaluation of Ginkgo leaf extract as an eco-friendly corrosion inhibitor of X70 steel in HCl solution
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Evaluation of Ginkgo leaf extract as an eco-friendly corrosion inhibitor of X70 steel in HCl solution ⁎
Yujie Qianga, , Shengtao Zhanga, Bochuan Tana, Shijin Chenb a b
School of Chemistry and Chemical Engineering, Chongqing University, Chongqing 400044, China Bomin Electronics Ltd., Meizhou 514021, China
A R T I C L E I N F O
A B S T R A C T
Keywords: X70 steel EIS Corrosion inhibition AFM DFT
The corrosion inhibition of X70 steel in 1 M HCl by Ginkgo leaf extract (GLE) was investigated by conducting electrochemical measurements. The inhibition efficiency exceeded 90% in the presence of 200 mg/L GLE at all of the tested temperatures. The excellent inhibition capacity, which was attributed to the formation of inhibitor–adsorption films on the surface of the X70 steel, was confirmed by field emission scanning electron microscopy and atomic force microscopy. The adsorption of GLE on steel surface followed the Langmuir adsorption model. Potential of zero charge measurement and quantum chemical calculation were adopted to elucidate the inhibition mechanism.
1. Introduction
use of the fruit extract of Gingko as a corrosion inhibitor of J55 steel in 3.5 wt% NaCl solution saturated with CO2 [23]. To our knowledge, no work has focused on the inhibition behavior of X70 steel in HCl medium with GLE. Thus, this work aimed to investigate GLE as a corrosion inhibitor of X70 steel in 1 M HCl by using electrochemical methods (potentiodynamic polarization measurement and electrochemical impedance spectroscopy). Microscopic surface observations (field emission scanning electron microscopy (FE-SEM) and atomic force microscopy (AFM)), potential of zero charge (PZC) measurement, and density functional theory (DFT) calculations were combined to discuss the roles of the inhibitive ability and mechanism of GLE in X70 steel corrosion.
Although using organic corrosion inhibitors is the most efficient and economical approach among all anticorrosive methods, such materials cannot be used for large-scale corrosion inhibition because of the growing ecological awareness about their high hazardous environmental implications [1]. As such, non- or low-toxic alternatives must be developed to replace traditional hazardous inhibitors. Given their biodegradability and wide availability, plant extracts are natural products that have drawn considerable attention as environmentally friendly inhibitors for steel corrosion [2,3]. To date, numerous plant extracts have been employed as efficient inhibitors for steel corrosion in acid solution; these extracts include Zenthoxylum alatum [4], lupine [5], henna [6], Justicia gendarussa [7], Uncaria gambir [8], Oxandra asbeckii [9], Punica granatum [10], Artemisia pallens [11], Osmanthus fragran [12], bamboo [13,14], Salvia officinalis [15], Tagetes erecta [16], Geissospermum [17], Musa paradisiac [18], Nigella sativa [19], orange peel [20], and Thymus vulgaris [21]. These studies attributed the inhibitive ability of plant extracts to the complex constituents, including tannins, alkaloids, flavonoids, and nitrogen bases. These organic compounds are rich in heteroatoms (i.e., N, S, O), electronegative groups, and conjugated double bonds, all of which are present in good corrosion inhibitors as major adsorption centers. In recent years, Ginkgo has attracted some attention in the field of corrosion. Deng et al. investigated the inhibition effect of GLE on cold roll steel in HCl and H2SO4, and they demonstrated that GLE is more effective in 1 M HCl than in 0.5 M H2SO4 [22]. Lin et al. explored the
⁎
2. Experimental method 2.1. Preparation of GLE GLE was synthesized in a similar procedure reported by Deng et al. [22]. Fresh Ginkgo leaves were collected in Chongqing University, cleaned with distilled water, dried for 50 h at 333 K, and then ground to powder. Exactly 20 g of powder was refluxed in 80% alcohol at 353 K for 3 h. Thereafter, the refluxed solution was filtered, degreased with petroleum ether, and extracted with a separating funnel. Then, the solution was concentrated in a rotary evaporator, and then dried in a vacuum dry oven at 333 K for 24 h. Finally, a dark brown solid residue (approximately 2 g) was collected and stored in a desiccator. The solid plant extract was characterized through Fourier transform
Corresponding author. E-mail addresses: [email protected], [email protected] (Y. Qiang).
https://doi.org/10.1016/j.corsci.2018.01.008 Received 25 September 2017; Received in revised form 25 December 2017; Accepted 15 January 2018 0010-938X/ © 2018 Elsevier Ltd. All rights reserved.
Please cite this article as: Qiang, Y., Corrosion Science (2018), https://doi.org/10.1016/j.corsci.2018.01.008
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where icorr,0 and icorr denote the current densities of the unprotected and protected WE, respectively. To determine the PZC of X70 steel, the impedance of the steel electrode was measured at various potentials in 1 M HCl solution containing 200 mg/L GLE at 298 K. Each measurement was performed thrice under the same experimental conditions to ensure a satisfactory reproducibility.
infrared (FTIR) spectroscopy. The spectra were obtained by using a Thermo Scientific FTIR spectrophotometer (Nicolet iS50) in the range of 4000–400 cm−1 through KBr disk technique. 2.2. Electrode and solutions preparation The testing specimens were cut from a X70 steel sheet and contained 0.16 wt% C, 1.7 wt% Mn, 0.45 wt% Si, 0.01 wt% S, 0.02 wt% P, 0.06 wt % V, 0.05 wt% Nb, 0.35 wt% Mo, 0.06 wt% Ti, and the remainder in Fe. The steel specimens were sealed in epoxy, leaving a 1 cm2 area exposed to the aggressive solution for electrochemical tests. The steel coupons used for FE-SEM and AFM had dimensions of 0.50 cm × 0.50 cm × 0.30 cm and 1.00 cm × 1.00 cm × 0.10 cm, respectively. Prior to each measurement, the steel electrode was abraded with emery papers with 400–3000 grit, washed ultrasonically with distilled water and anhydrous alcohol, and dried under cold wind. The aggressive solution was prepared by using 1 M HCl solution without and with various concentrations (25, 50, 100, and 200 mg/L) of GLE. The 1 M HCl solution, which was treated as the blank for comparison, was diluted from AR grade 37% HCl. Besides, 1% alcohol was added to dissolve GLE absolutely. A freshly prepared solution was used for each experiment.
2.4. Surface investigation After 0.5 or 4 h immersion in 1 M HCl solution with and without 200 mg/L GLE at different temperatures, the X70 steel samples were descaled with a soft brush, thoroughly rinsed with deionized water, and dried under cold air. The morphologies of the 4 h immersion samples were observed through FE-SEM (JEOL-JSM-7800F, JEOL Ltd., Japan) under high vacuum. The morphologies of the 0.5 h immersion samples with same procedure were examined through AFM (MFP-3D-BIO, Asylum Research, America) under tapping mode. 2.5. Calculation details Quantum chemical calculation was conducted by using Gaussian 03W software to explore the relationship between the inhibition ability of GLE and the electron structure of its main constituents, which are shown in Fig. 2 [24]. Organic molecules such as isorhamnetin (IH), sciadopitysin (SD), 6-hydroxykynurenic acid (HKA), and 4-O-methylpyridoxine (MP) were in neutral form and were fully optimized by using B3LYP method at the DFT level with a 6–311++G(d, p) basis set in the gas phase. Then, several parameters, including the energy of the highest occupied molecular orbital (EHOMO), that of the lowest unoccupied molecular orbital (ELUMO), and the dipole moment (μ), were obtained. Other parameters, such as the energy gap (ΔE), electronegativity (χ), global hardness (γ), ionization potential (I), and electron affinity (A), were calculated as follows [25]:
2.3. Electrochemical tests The electrochemical measurements were performed with a CHI 760E electrochemical station equipped with a traditional three-electrode system. The X70 steel specimen was used as a working electrode (WE), a 4 cm2 platinum sheet was utilized as the counter electrode (CE), and a saturated calomel electrode served as the reference electrode (RE). In this study, all of the potentials were are in reference to the RE. The tests were conducted in a temperature-controlled water bath at a wide range of temperature (298, 308, and 318 K). First, the WE was immersed in the test solution for 1200 s to obtain a stable open circuit potential (EOCP). The corresponding OCP–time curves are depicted in Fig. 1. Then, electrochemical impedance spectroscopy (EIS) was performed on the EOCP. The disturbance signal was a 10 mV peak-to-peak sinusoidal wave in the frequency range of 100000–0.01 Hz. The EIS data were fitted and analyzed carefully by using Zsimpwin. The inhibition efficiency (η) obtained by the EIS test was calculated as follows:
η (%) =
R ct − R ct,0 × 100 R ct
i corr,0 − i corr × 100 i corr,0
(3)
I = −EHOMO
(4)
A = −EHOMO
(5)
χ = (I + A)⁄2
(6)
γ = (I − A)⁄2
(7)
The fraction of electrons transferred from the inhibitor molecules to the metal atoms (ΔN) can be calculated by [26]
(1)
ΔN =
where Rct and Rct,0 are the charge transfer resistances of the WE with and without studied organics, respectively. Finally, the potentiodynamic polarization curves were recorded at a scan rate of 1 mV s−1. The obtained values of η were deduced as follows:
η (%) =
ΔE = ELUMO−EHOMO
χFe − χInh (γFe + γInh )
(8)
where γFe and γInh are the global hardness of Fe and the inhibitor molecule; χFe and χInh are the electronegativity of Fe and the inhibitor molecule, respectively. In accordance with the literature, theoretical χFe and γFe values of 7 and 0 eV/mol were used for the bulk Fe atom [26,27]. The optimized molecular structures, HOMO, LUMO, and ESP
(2)
Fig. 1. OCP–time curves for X70 steel in 1 M HCl solution without and with different concentrations of GLE at different temperatures.
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Fig. 2. Chemical constituents of GLE.
3.2. Impedance measurements
surfaces, were visualized by using Gauss View.
To clarify the surface properties of the X70 steel electrode and the kinetics of the electrode processes, EIS was conducted on X70 steel in 1 M HCl medium without and with different concentrations of GLE at 298, 308, and 318 K. The relevant Nyquist and Bode diagrams are depicted in Figs. 4 and 5, respectively. Slightly depressed capacitive semicircles, which probably correspond to the charge-transfer process of the steel dissolution [28], were observed in all curves in Fig. 4. The centers of these depressed loops were displaced below the real axis. This phenomenon may be related to the frequency dispersion of the interfacial impedance and the inhomogeneous steel surface because of the microscopic roughness and inhibitor adsorption [29]. The addition of GLE increased the size of the impedance plots, suggesting that the organic compounds in the GLE formed a protective film on the surface of the X70 steel. The capacitive loop increased with increasing inhibitor concentration, as much more inhibitor molecules adsorbed on the steel surface, indicating that the inhibitor film gradually became compact and ultimately leading to an improved protective effect. Moreover, as the temperature increased, the size of the capacitive loop decreased, implying that corrosion was accelerated by high temperature. Nevertheless, the size of the spectra was still considerably increased with the addition of GLE with respect to the blank solution, thereby demonstrating the high inhibitive ability of GLE at high temperatures. These results revealed the formation of an inhibitor-adsorption film on the steel surface and its contribution to the enhanced anti-corrosive quality. Only one time constant was observed in the Bode plots shown in Fig. 5. At each temperature, the impedance modulus over the entire frequency range increased as increasing concentrations of GLE were added. Moreover, the frequency range with the maximum phase angle became larger as the inhibitor concentration increased. These results validated the effective inhibitive ability of the investigated GLE for the corrosion of X70 steel in 1 M HCl medium. The classical equivalent circuit shown in Fig. 6 was used to model the EIS data in this study. The solution resistance (Rs), charge transfer resistance (Rct), and constant phase angle element (CPE), which is related to the double-layer capacitance (Cdl), were fitted and summarized in Table 1. The impedance function of the CPE is described as follows
3. Results and discussion 3.1. FTIR analysis The FTIR spectroscopy of GLE is shown in Fig. 3. The broad band at 3301 cm−1 is related to OeH or NeH stretching. The band located at 2923 cm−1 is attributed to CeH stretching vibration, and that at 1694 cm−1 to C]O. The band at 1647 cm−1 is due to the C]C and C]N stretching vibration. The absorption bands at 1504 and 1240 cm−1 can be assigned to the framework vibration of aromatic ring. The CeH bending in eCH3 is found to be at 1408 cm−1. There is a sharp absorption band at 1044 cm−1, which can be associated with the CeN or CeO stretching vibration. Moreover, the absorption bands below 1000 cm−1 correspond to aromatic and aliphatic CeH group. These findings demonstrate that GLE contains oxygen and nitrogen atoms in functional groups (OeH, NeH, C]O, C]C, C]N, CeN, CeO) and aromatic ring, which meets the general characteristics of conventional corrosion inhibitors.
Fig. 3. FTIR spectra of GLE.
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Fig. 5. Bode graphs of the measured impedance shown in Fig. 4. Fig. 4. Nyquist plots recorded for the X70 steel electrode in 1 M HCl solution containing various concentrations of GLE at different temperatures.
[30]:
ZCPE =
1 Y0 (jw )n
(9)
where Y0 is the magnitude of the CPE, w is the angular frequency, j is the imaginary root, and n is the deviation parameter relating to the phase shift representing the microscopic fluctuation of the metal surface. The CPE can display an inductor (n = −1), resistor (n = 0), capacitor (n = 1), or Warburg impedance (n = 0.5). The use of CPE rather than a pure capacitor resulted in the non-ideal dielectric behavior of the inhomogeneous electrode surface [31]. The Cdl value can be calculated by using Eq. (10) [32]:
C = Y0 (wmax )n − 1
Fig. 6. Corresponding equivalent circuit used to fit the EIS experimental data.
As shown in Table 1, n ranged from 0 to 1, and these values depended on the deviation of the ideal capacitance behavior. The presence of GLE increased the Rct values, and this effect was enhanced as the inhibitor concentration was increased. This phenomenon implied that an inhibitor-adsorption film was formed on the steel substrate, thereby retarding the charge transfer. Moreover, Y0 and Cdl decreased in the presence of GLE at each temperature presumably because of the gradual replacement of H2O molecules by the adsorption process of the investigated inhibitor molecules on the steel/solution interface [33]. Cdl
(10)
where wmax = 2πfmax and fmax is the frequency at the maximum value of the imaginary component of the impedance spectra. 4
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Table 1 Impedance parameters for X70 steel in 1 M HCl solution in the absence and presence of various concentrations of GLE at different temperatures. T (K)
298
308
318
a
C (mg/L)
Rs (Ω cm2)
Rct (Ω cm2)
Y0 (μ Ω−1 sn cm−2)
n
Cdl (μF cm−2)
η (%)
SDa
Blank 25 50 100 200 Blank 25 50 100 200 Blank 25 50 100 200
1.1 1.9 9.2 1.4 2.6 3.9 2.7 0.6 3.3 9.1 0.7 2.0 0.7 4.5 2.6
64.8 180.6 269.6 505.8 643.0 32.3 161.1 197.9 244.7 374.6 20.5 76.5 119.9 162.6 275.1
493.5 211.0 171.9 134.4 134.7 898.8 342.0 196.2 210.8 163.4 823.1 333.0 157.6 143.9 143.2
0.90 0.86 0.85 0.87 0.84 0.84 0.78 0.82 0.78 0.78 0.91 0.80 0.85 0.84 0.78
339.0 124.7 106.7 91.1 83.5 449.4 156.1 103.3 92.3 84.7 502.5 129.7 80.0 71.9 68.2
– 64.1 76.0 87.2 89.9 – 80.0 83.7 86.8 91.4 – 73.2 82.9 87.4 92.5
– 3.6 2.8 3.7 2.5 – 4.3 2.7 3.9 3.2 – 6.2 3.4 4.1 4.7
SD, standard deviation of 3 independent measurements.
can be interpreted by the Helmholtz model as follows [34]:
Cdl =
ε 0ε S d
values, indicating that temperature accelerated the corrosion of the steel. Fortunately, the temperature did not influence the superior inhibition performance of GLE, as the maximum η values reached up 90.0% at 298 K, 91.3% at 308 K, and 92.2% at 318 K. These results agreed with that obtained by EIS, implying that GLE can provide a favorable protection performance for steel corrosion at a wide temperature range. This improved performance may be attributed to the adsorption and formation of a protective film on the electrode surface. Remarkably, this adsorption film remained stable at high temperatures.
(11)
where ε is the permittivity of the air, ε is the local dielectric constant of the film, d is the thickness of the electric double-layer, and S is the geometrical surface area of the electrode exposed to a corrosive medium. As the inhibitor concentration increased, more inhibitor molecules adsorbed onto the surface of the X70 steel, leading to a reduced exposed electrode area, a thicker electric double-layer, and lower local dielectric constant. These factors jointly caused a decrease in Cdl [35]. Accordingly, the η values at each temperature increased with increasing inhibitor concentration, reaching 89.9% at 298 K, 91.4% at 308 K, and 92.5% at 318 K for 200 ppm GLE. These values confirmed that GLE induced an effective protective capacity against steel corrosion in 1 M HCl solution. 0
3.4. Adsorption isotherm study Adsorption isotherms are valid models for probing the adsorption mechanism of the inhibitor on the metal surface. The Langmuir isotherm was identified the most suitable model to fit the results obtained by EIS and was calculated by using Eq. (12) [38] with a linear regression coefficient (R2) of approximately 1.
3.3. Polarization curves
θ = K ads C 1−θ
Fig. 7 shows the Tafel polarization curves of the X70 steel electrode in 1 M HCl solution with and without different concentrations of GLE at different temperatures. Table 2 lists the relevant electrochemical parameters, including corrosion current density (icorr), corrosion potential (Ecorr), anodic and cathodic Tafel slope (βa, βc), and inhibition efficiency (η), which were deduced from the polarization curves at a temperature range of 298–318 K. As shown, the addition of GLE shifted both the cathodic and anodic curves to a lower current density, causing a considerable decrease in the corrosion rate at all of the tested temperatures. Specifically, the extent of reduction of the cathodic branch was stronger than that of the anodic one. This phenomenon suggested that both anodic steel dissolution and cathodic hydrogen evolution were retarded by GLE, whereas cathodic reaction is mainly suppressed. Besides, all of the changes in the Ecorr values shown in Table 2 were lower than 85 mV. Therefore, GLE can be considered a mixed-type inhibitor with a predominant control of the cathodic action at each temperature [36,37]. The slight change of both βa and βc at each temperature revealed that the mechanism of the X70 steel corrosion was not altered with the addition of GLE [36]. Table 2 also shows that icorr was markedly decreased with the addition of GLE at all of the tested temperatures, and it continuously decreased with increasing inhibitor concentration. Consequently, the inhibitive ability, which is reflected by η, markedly improved as the inhibitor concentration increased. Temperature is an important kinetic factor that could affect the corrosion behavior of metals and change the adsorption strength inhibitors on metal surfaces. Thus, as shown in Table 2, an increase in temperature enhanced the icorr
(12)
where θ (defined as η/100) is the surface coverage, Kads is the equilibrium constant of the adsorption process, and C is the concentration of inhibitor. The plots of C versus C/θ showed straight lines with an intercept of 0 1/K, as shown in Fig. 8. The standard adsorption free energy ΔGads can be calculated by the following equation [39]:
K ads =
1 CH20
exp(
0 −ΔGads ) RT
(13)
where R is the molar gas constant, T is the absolute temperature, and the water concentration in the solution is 1000 g/L. The calculated Kads 0 and ΔGads values for GLE are provided in Fig. 8. A higher value of Kads generally implies that the inhibitor could tightly adsorb on the metal surface, which is an indicator of a good inhibitive ability [40]. The values of Kads listed in Fig. 8 followed the order of 308 > 298 > 318 K. As the temperature reached 308 K, the largest Kads value revealed that GLE possessed the strongest adsorption affinity on the steel surface and therefore exhibited the best inhibition behavior. However, when the temperature continued to increase, the Kads value decreased, implying an intensified corrosion, which hampered the adsorption of the inhibitor. Furthermore, the adsorption was con0 sidered a physisorption for ΔGads values of −20 kJ mol−1 or less ne0 gative and a chemisorption for ΔGads values of −40 kJ mol−1 or more 0 negative [41]. As shown in Fig. 6, all of the calculated ΔGads values ranged from −28 to −30 kJ mol−1, indicating that the interaction between the inhibitor and the steel surface involves both physisorption 5
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Table 2 Corrosion parameters for X70 steel in 1 M HCl solution containing different concentrations of GLE determined by the Tafel extrapolation method. T (K)
298
308
318
C (mg/L)
Ecorr (mV/ SCE)
icorr (μA cm−2)
βc (mV dec−1)
βa (mV dec−1)
η (%)
SD
Blank 25 50 100 200 Blank 25 50 100 200 Blank 25 50 100 200
−462 −461 −468 −466 −462 −450 −455 −452 −453 −454 −430 −458 −466 −469 −468
348.3 132.5 101.8 47.6 34.7 722.8 139.8 105.7 77.3 62.6 1242 362.3 183.2 160.3 96.8
−100⋅5 −91⋅6 −92.7 −86.6 −80.3 −69.8 −81⋅1 −82.7 −78.6 −79.8 −87⋅9 −85.7 −94⋅9 −104.7 −94.0
81.3 72.4 82.8 74.0 97.8 82.2 74.6 66.6 63.2 71.3 85.6 75.1 77.3 81.9 90.2
– 62.0 70.8 86.3 90.0 – 80.7 85.4 89.3 91.3 – 70.8 85.2 87.1 92.2
– 3⋅1 2⋅2 2⋅4 2⋅1 – 3⋅9 2⋅5 3⋅4 3⋅0 – 5⋅7 3⋅6 3⋅9 3⋅5
where Er is the Antropov’s “rational” potential of corrosion. If Er is negative, then the adsorption of cation is favored, whereas a positive Er value represents the preferential adsorption of anion [43]. Fig. 9 shows the dependence of Cdl on the applied potential of the steel in the 1 M HCl solution containing 200 mg/L GLE. As shown, the obtained curve displayed a parabola with a minimum at −0.501 V, which can be considered the EPZC. The EOCP (−0.471 V) of the steel under the same condition was more positive than EPZC, indicating a positively charged metal surface. This finding suggested that Cl− ions first adsorbed onto the X70 steel surface, causing the steel surface to become negatively charged. Thus, the protonated form of the main organic constitution in GLE adsorbed on the steel surface via an electrostatic interaction. These molecules formed a compact adsorption layer, which acted as a barrier against steel corrosion. In addition to physical adsorption, the neutral and cationic organic molecules may be adsorbed on the surface through the donation of lone electron pairs in heteroatoms to the vacant d orbitals of Fe; this phenomenon is a chemisorption mechanism [44]. Thus, the results confirmed that both physical and chemical adsorption simultaneously existed on the solution/metal interface obtanied from adsorption isotherm. 3.6. Surface microscopic observation Fig. 10 shows the SEM micrographs of the X70 steel surface after 4 h of immersion in 1 M HCl without and with 200 mg/L GLE at different temperatures. The uninhibited steel surface at 298 K (Fig. 10a) was highly damaged and showed obvious deterioration due to aggressive acid attack. More rough surfaces were observed as the temperature increased. In particular, the surfaces protected by GLE at all of the tested temperatures presented relatively smoother morphologies than that of the uninhibited one. These observations indicated that an inhibitor film was formed on the surface of the X70 steel. Compared with the unprotected one, the inhibited surface had a lower permeability for aggressive particles. Consequently, the X70 steel surface was effectively protected by GLE, validating the electrochemical results. AFM is a powerful tool for observing the surface appearance at the nano- to microscale levels, and it has become the preferred choice in the corrosion field [45–47]. Fig. 11 presents 3D AFM images of the corroded and inhibited steel surfaces by 200 mg/L GLE in 1 M HCl solution at 298––318 K for 0.5 h. As shown, all of the corroded steel samples cracked considerably and displayed rough structures with deep and
Fig. 7. Polarization curves for the X70 steel in 1 M HCl solution containing various concentrations of GLE at different temperatures.
and chemisorption.
3.5. Potential of zero charge (PZC) and inhibition mechanism Experiments were performed to determine the charge on the steel surface, which can be estimated by comparing the PZC (EPZC) and open circuit potential (EOCP) of the X70 steel in the solution [42] as follows: Er = EOCP − EPZC
(14)
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Fig. 8. Langmuir adsorption isotherms and relevant parameters of investigated GLE on the X70 steel surface in 1 M HCl solution at different temperatures: (a) 298 K, (b) 308 K, and (c) 318 K.
was markedly decreased at all of the tested temperatures. The relevant values of the average roughness (Ra) of the specimens are graphically presented in Fig. 12. The Ra values exhibited an incremental trend with increasing temperature and exceeded 100 nm at 318 K. With the addition of 200 mg/L GLE, Ra considerably decreased at all of the tested temperatures, demonstrating that the X70 steel specimen can be strongly protected at a wide temperature range. 3.7. Quantum chemical study Quantum chemical calculation was employed to gain insight into the inhibition mechanism of GLE by examining the structure–reactivity correlation of the compounds contained in GLE. Fig. 13 shows the optimized structures and the frontier molecular orbital (FMO) density distributions, i.e., the HOMO and the LUMO. The calculated quantum chemical properties are summarized in Table 3. As shown in Fig. 13 both the HOMO and LUMO distributions of IH, HKA, and MP were concentrated evenly over whole molecules. This system indicated that a flat-adsorption mode could be obtained to gain the largest protective area for the steel surface. By contrast, for SD, the HOMO was preferentially distributed over half of the molecule
Fig. 9. Plot of Cdl vs. applied potential in 1 M HCl containing 200 mg/L GLE.
large pits due to the aggressive attack. In the presence of 200 mg/L GLE, uniform surfaces with small pits were obtained, as shown in Fig. 11(d)–(f), indicating that the corrosion rate of the steel specimen
Fig. 10. FE-SEM images of the X70 steel specimens immersed in 1 M HCl solution (a–c) without and (d–f) with 200 mg/L GLE for 4 h at different temperatures.
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Fig. 11. 3D AFM images of the X70 steel specimens immersed in 1 M HCl solution without GLE for 0.5 h at (a) 298 K, (b) 308 K, and (c) 318 K; 3D AFM images of the X70 steel specimens immersed in 1 M HCl solution with 200 mg/L GLE for 0.5 h at (a) 298 K, (b) 308 K, and (c) 318 K.
tends to accept electrons, whereas a high EHOMO reflects a strong electron-donating tendency to a suitable acceptor with an empty molecular orbital and low ELUMO [49]. Thus, an energy gap indicates the chemical stability of an inhibitor, and a lower ΔE value typically leads to greater adsorption on the metal surface, resulting in higher inhibition efficiencies [34]. The calculated ΔE values of the compounds followed the order of HKA < SD < IH < MP, indicating that HKA probably played the most prominent role in retarding the corrosion process. The electronegativity χ is an indicator of the electron-attraction ability of a molecule. A higher χ corresponds to a lower chance of electron donation from the molecule, and vice versa [45]. Moreover, a higher value of global hardness γ indicates a higher resistance of a molecule toward charge transfer [31,41]. The dipole moment μ is also a crucial indicator, although the correlation between μ and the inhibitive capacity is yet to be verified [28,36,50]. The lowest μ value of HKA favors the accumulation of the inhibitor on the metal surface, thereby increasing the inhibition efficiency, which agrees well with the result obtained from the ΔE values. The ΔN value of HKA was the maximum, suggesting that the reaction activity of the HKA molecule was the highest, and the number of electrons from HKA to Fe atom was the greatest [48]. Thus, HKA may demonstrate the highest inhibition efficiency among the four compounds, which agrees with the conclusion obtained from ΔE. In summary, the inhibition capacity of GLE could be ascribed to the adsorption of its numerous organic constituents on the surface of the X70 steel. The main components of GLE are HKA, SD, IH, and MP, all of which contain conjugated structures, such as benzene ring, O, N atoms in O-herterocyclic ring and functional groups (NeH, OeH, C]O, CeO), thus meeting the general characteristics of traditional organic inhibitors.
Fig. 12. Average roughness (Ra) of X70 steel immersed in 1 M HCl solution without and with 200 mg/L GLE for 0.5 h at different temperatures.
containing two OH groups, whereas the LUMO was spread on the other half presumably because of its large twisted structure. As such, the SD molecule had a greater directionality during bonding with steel surface [48]. Half of the SD molecules preferentially bonded with positively polarized anodic reaction sites, thereby leading to a decreased rate of anodic steel dissolution reaction. The other half of the molecule preferentially attracted electrons from the cathodic sites on the steel surface, thus reducing the rate of the cathodic hydrogen evolution reaction. Moreover, the red (negative) and blue (positive) regions of the ESP map were related to nucleophilic and electrophilic activities, respectively [25]. As evidenced in Fig. 14 that, all of the red regions were distributed in the electronegative groups with heteroatoms, such as O, N atoms or O-herterocyclic rings, which readily formed covalent bonds with electrophilic agents, such as Fe atoms. ELUMO and EHOMO represented the electron-receiving and -donating ability of a molecule. In general, a low ELUMO implies that an inhibitor
4. Conclusions On the basis of the systematic experimental and theoretical results in this work, the following conclusions can be drawn: (1) Electrochemical results indicated that GLE can be classified as an effective mixed-type inhibitor for the corrosion of X70 steel in 1 M HCl. Moreover, the η values obtained from the polarization test reached up to 90.0% at 298 K, 91.3% at 308 K, and 92.2% at 318 K 8
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Fig. 13. Optimized structures and frontier orbital density distributions of IH, SD, HKA, and MP.
(3) The adsorption of GLE on the X70 steel surface obeyed the Langmuir adsorption model and involved both physisorption and chemisorption. (4) The inhibition capacity of GLE could be ascribed to the adsorption of its numerous organic constituents, such as HKA, SD, IH, and MP, on the surface of the X70 steel.
for 200 mg/L GLE, which agrees well with those gained by EIS. In particular, the inhibitive ability remained favorable at high temperatures. (2) FE-SEM and AFM observations verified the inhibition performance of GLE obtained electrochemically, indicating that a dense and ordered protective film was formed on the steel surface.
Table 3 Calculated quantum chemical indices of isorhamnetin (IH), sciadopitysin (SD), 6-hydroxykynurenic acid (HKA), and 4-O-methylpyridoxine (MP). Substance
EHOMO (eV)
ELUMO (eV)
ΔE (eV)
I (eV)
A (eV)
χ (eV)
γ (eV)
μ (Debye)
ΔN
IH SD HKA MP
−5.96 −6.22 −5.32 −6.14
−1.73 −2.29 −1.74 −0.85
4.23 3.93 3.58 5.29
5.96 6.22 5.32 6.14
1.73 2.29 1.74 0.85
3.85 4.26 3.53 3.50
2.12 1.97 1.79 2.65
7.38 11.89 4.13 6.70
0.74 0.70 0.97 0.66
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granatum peel and main constituents, Mater. Chem. Phys. 131 (2012) 621–633. [11] S. Garai, S. Garai, P. Jaisankar, J.K. Singh, A. Elango, A comprehensive study on crude methanolic extract of Artemisia pallens (Asteraceae) and its active component as effective corrosion inhibitors of mild steel in acid solution, Corros. Sci. 60 (2012) 193–204. [12] L. Li, X. Zhang, J. Lei, J. He, S. Zhang, F. Pan, Adsorption and corrosion inhibition of Osmanthus fragran leaves extract on carbon steel, Corros. Sci. 63 (2012) 82–90. [13] X. Li, S. Deng, H. Fu, Inhibition of the corrosion of steel in HCl, H2SO4 solutions by bamboo leaf extract, Corros. Sci. 62 (2012) 163–175. [14] X. Li, S. Deng, H. Fu, X. Xie, Synergistic inhibition effects of bamboo leaf extract/ major components and iodide ion on the corrosion of steel in H3PO4 solution, Corros. Sci. 78 (2014) 29–42. [15] N. Soltani, N. Tavakkoli, M. Khayatkashani, M.R. Jalali, A. Mosavizade, Green approach to corrosion inhibition of 304 stainless steel in hydrochloric acid solution by the extract of Salvia officinalis leaves, Corros. Sci. 62 (2012) 122–135. [16] P. Mourya, S. Banerjee, M.M. Singh, Corrosion inhibition of mild steel in acidic solution by Tagetes erecta (Marigold flower) extract as a green inhibitor, Corros. Sci. 85 (2014) 352–363. [17] M. Faustin, A. Maciuk, P. Salvin, C. Roos, M. Lebrini, Corrosion inhibition of C38 steel by alkaloids extract of Geissospermum laeve in 1 M hydrochloric acid: electrochemical and phytochemical studies, Corros. Sci. 92 (2015) 287–300. [18] G. Ji, S. Anjum, S. Sundaram, R. Prakash, Musa paradisica peel extract as green corrosion inhibitor for mild steel in HCl solution, Corros. Sci. 90 (2015) 107–117. [19] M. Chellouli, D. Chebabe, A. Dermaj, H. Erramli, N. Bettach, N. Hajjaji, M.P. Casaletto, C. Cirrincione, A. Privitera, A. Srhiri, Corrosion inhibition of iron in acidic solution by a green formulation derived from Nigella sativa L, Electrochim. Acta 204 (2016) 50–59. [20] N. M’hiri, D. Veys-Renaux, E. Rocca, I. Ioannou, N.M. Boudhrioua, M. Ghoul, Corrosion inhibition of carbon steel in acidic medium by orange peel extract and its main antioxidant compounds, Corros. Sci. 102 (2016) 55–62. [21] A. Ehsani, M.G. Mahjani, M. Hosseini, R. Safari, R. Moshrefi, H. Mohammad Shiri, Evaluation of Thymus vulgaris plant extract as an eco-friendly corrosion inhibitor for stainless steel 304 in acidic solution by means of electrochemical impedance spectroscopy, electrochemical noise analysis and density functional theory, J. Colloid Interf. Sci. 490 (2017) 444–451. [22] S. Deng, X. Li, Inhibition by Ginkgo leaves extract of the corrosion of steel in HCl and H2SO4 solutions, Corros. Sci. 55 (2012) 407–415. [23] A. Singh, Y. Lin, E.E. Ebenso, W. Liu, J. Pan, B. 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Meghdadi, Enhanced corrosion resistance of mild steel in 1 M HCl solution by trace amount of 2-phenylbenzothiazole derivatives: experimental, quantum chemical calculations and molecular dynamics (MD) simulation studies, Corros. Sci. 114 (2017) 133–145. [28] Y. Qiang, L. Guo, S. Zhang, W. Li, S. Yu, J. Tan, Synergistic effect of tartaric acid with 2,6-diaminopyridine on the corrosion inhibition of mild steel in 0.5 M HCl, Sci. Rep. 6 (2016) 33305. [29] Z. Tao, W. He, S. Wang, G. Zhou, Electrochemical study of cyproconazole as a novel corrosion inhibitor for copper in acidic solution, Ind. Eng. Chem. Res. 52 (2013) 17891–17899. [30] G. Sığırcık, D. Yildirim, T. Tüken, Synthesis and inhibitory effect of N,N'-bis(1phenylethanol)ethylenediamine against steel corrosion in HCl Media, Corros. Sci. 120 (2017) 184–193. [31] N. Yilmaz, A. Fitoz, ÿ. Ergun, K.C. 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Fig. 14. (a) Contours and (b) surface maps of the electrostatic potential (ESP) of IH, SD, HKA, and MP.
Acknowledgments This research was supported by National Natural Science Foundation of China (No. 21676035), Sail Plan of Guangdong, China (No. 2015YT02D025), Graduate Research and Innovation Foundation of Chongqing, China (No. CYB17022). References [1] E. Kowsari, S.Y. Arman, M.H. Shahini, H. Zandi, A. Ehsani, R. Naderi, A. PourghasemiHanza, M. Mehdipour, In situ synthesis, electrochemical and quantum chemical analysis of an amino acid-derived ionic liquid inhibitor for corrosion protection of mild steel in 1 M HCl solution, Corros. Sci. 112 (2016) 73–85. [2] A.Y. El-Etre, M. Abdallah, Z.E. El-Tantawy, Corrosion inhibition of some metals using lawsonia extract, Corros. Sci. 47 (2005) 385–395. [3] M. Mehdipour, B. Ramezanzadeh, S.Y. Arman, Electrochemical noise investigation of Aloe plant extract as green inhibitor on the corrosion of stainless steel in 1 M H2SO4, J. Ind. Eng. Chem. 21 (2015) 318–327. [4] L.R. Chauhan, G. Gunasekaran, Corrosion inhibition of mild steel by plant extract in dilute HCl medium, Corros. Sci. 49 (2007) 1143–1161. [5] A.M. Abdel-Gaber, B.A. Abd-El-Nabey, M. Saadawy, The role of acid anion on the inhibition of the acidic corrosion of steel by lupine extract, Corros. Sci. 51 (2009) 1038–1042. [6] A. Ostovari, S.M. Hoseinieh, M. Peikari, S.R. Shadizadeh, S.J. Hashemi, Corrosion inhibition of mild steel in 1 M HCl solution by henna extract: a comparative study of the inhibition by henna and its constituents (Lawsone, Gallic acid, α-d-Glucose and Tannic acid), Corros. Sci. 51 (2009) 1935–1949. [7] A.K. Satapathy, G. Gunasekaran, S.C. Sahoo, K. Amit, P.V. Rodrigues, Corrosion inhibition by Justicia gendarussa plant extract in hydrochloric acid solution, Corros. Sci. 51 (2009) 2848–2856. [8] M.H. Hussin, M.J. Kassim, The corrosion inhibition and adsorption behavior of Uncaria gambir extract on mild steel in 1 M HCl, Mater. Chem. Phys. 125 (2011) 461–468. [9] M. Lebrini, F. Robert, A. Lecante, C. Roos, Corrosion inhibition of C38 steel in 1 M hydrochloric acid medium by alkaloids extract from Oxandra asbeckii plant, Corros. Sci. 53 (2011) 687–695. [10] M. Behpour, S.M. Ghoreishi, M. Khayatkashani, N. Soltani, Green approach to corrosion inhibition of mild steel in two acidic solutions by the extract of Punica
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theoretical, and surface morphological studies of corrosion inhibition effect of green naphthyridine derivatives on mild steel in hydrochloric acid, J. Phys. Chem. C 120 (2016) 3408–3419. P. Dohare, K.R. Ansari, M.A. Quraishi, I.B. Obot, Pyranpyrazole derivatives as novel corrosion inhibitors for mild steel useful for industrial pickling process: experimental and Quantum Chemical study, J. Ind. Eng. Chem. 52 (2017) 197–210. Y. Qiang, S. Zhang, L. Guo, X. Zheng, B. Xiang, S. Chen, Experimental and theoretical studies of four allyl imidazolium-based ionic liquids as green inhibitors for copper corrosion in sulfuric acid, Corros. Sci. 119 (2017) 68–78. A. Dutta, S.K. Saha, U. Adhikari, P. Banerjee, D. Sukul, Effect of substitution on corrosion inhibition properties of 2-(substituted phenyl) benzimidazole derivatives on mild steel in 1 M HCl solution: a combined experimental and theoretical approach, Corros. Sci. 123 (2017) 256–266. D. Zhang, Y. Tang, S. Qi, D. Dong, H. Cang, G. Lu, The inhibition performance of long-chain alkyl-substituted benzimidazole derivatives for corrosion of mild steel in HCl, Corros. Sci. 102 (2016) 517–522. X. Zheng, S. Zhang, W. Li, M. Gong, L. Yin, Experimental and theoretical studies of two imidazolium-based ionic liquids as inhibitors for mild steel in sulfuric acid solution, Corros. Sci. 95 (2015) 168–179.
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Evaluation of Ginkgo leaf extract as an eco-friendly corrosion inhibitor of X70 steel in HCl solution ⁎
Yujie Qianga, , Shengtao Zhanga, Bochuan Tana, Shijin Chenb a b
School of Chemistry and Chemical Engineering, Chongqing University, Chongqing 400044, China Bomin Electronics Ltd., Meizhou 514021, China
A R T I C L E I N F O
A B S T R A C T
Keywords: X70 steel EIS Corrosion inhibition AFM DFT
The corrosion inhibition of X70 steel in 1 M HCl by Ginkgo leaf extract (GLE) was investigated by conducting electrochemical measurements. The inhibition efficiency exceeded 90% in the presence of 200 mg/L GLE at all of the tested temperatures. The excellent inhibition capacity, which was attributed to the formation of inhibitor–adsorption films on the surface of the X70 steel, was confirmed by field emission scanning electron microscopy and atomic force microscopy. The adsorption of GLE on steel surface followed the Langmuir adsorption model. Potential of zero charge measurement and quantum chemical calculation were adopted to elucidate the inhibition mechanism.
1. Introduction
use of the fruit extract of Gingko as a corrosion inhibitor of J55 steel in 3.5 wt% NaCl solution saturated with CO2 [23]. To our knowledge, no work has focused on the inhibition behavior of X70 steel in HCl medium with GLE. Thus, this work aimed to investigate GLE as a corrosion inhibitor of X70 steel in 1 M HCl by using electrochemical methods (potentiodynamic polarization measurement and electrochemical impedance spectroscopy). Microscopic surface observations (field emission scanning electron microscopy (FE-SEM) and atomic force microscopy (AFM)), potential of zero charge (PZC) measurement, and density functional theory (DFT) calculations were combined to discuss the roles of the inhibitive ability and mechanism of GLE in X70 steel corrosion.
Although using organic corrosion inhibitors is the most efficient and economical approach among all anticorrosive methods, such materials cannot be used for large-scale corrosion inhibition because of the growing ecological awareness about their high hazardous environmental implications [1]. As such, non- or low-toxic alternatives must be developed to replace traditional hazardous inhibitors. Given their biodegradability and wide availability, plant extracts are natural products that have drawn considerable attention as environmentally friendly inhibitors for steel corrosion [2,3]. To date, numerous plant extracts have been employed as efficient inhibitors for steel corrosion in acid solution; these extracts include Zenthoxylum alatum [4], lupine [5], henna [6], Justicia gendarussa [7], Uncaria gambir [8], Oxandra asbeckii [9], Punica granatum [10], Artemisia pallens [11], Osmanthus fragran [12], bamboo [13,14], Salvia officinalis [15], Tagetes erecta [16], Geissospermum [17], Musa paradisiac [18], Nigella sativa [19], orange peel [20], and Thymus vulgaris [21]. These studies attributed the inhibitive ability of plant extracts to the complex constituents, including tannins, alkaloids, flavonoids, and nitrogen bases. These organic compounds are rich in heteroatoms (i.e., N, S, O), electronegative groups, and conjugated double bonds, all of which are present in good corrosion inhibitors as major adsorption centers. In recent years, Ginkgo has attracted some attention in the field of corrosion. Deng et al. investigated the inhibition effect of GLE on cold roll steel in HCl and H2SO4, and they demonstrated that GLE is more effective in 1 M HCl than in 0.5 M H2SO4 [22]. Lin et al. explored the
⁎
2. Experimental method 2.1. Preparation of GLE GLE was synthesized in a similar procedure reported by Deng et al. [22]. Fresh Ginkgo leaves were collected in Chongqing University, cleaned with distilled water, dried for 50 h at 333 K, and then ground to powder. Exactly 20 g of powder was refluxed in 80% alcohol at 353 K for 3 h. Thereafter, the refluxed solution was filtered, degreased with petroleum ether, and extracted with a separating funnel. Then, the solution was concentrated in a rotary evaporator, and then dried in a vacuum dry oven at 333 K for 24 h. Finally, a dark brown solid residue (approximately 2 g) was collected and stored in a desiccator. The solid plant extract was characterized through Fourier transform
Corresponding author. E-mail addresses: [email protected], [email protected] (Y. Qiang).
https://doi.org/10.1016/j.corsci.2018.01.008 Received 25 September 2017; Received in revised form 25 December 2017; Accepted 15 January 2018 0010-938X/ © 2018 Elsevier Ltd. All rights reserved.
Please cite this article as: Qiang, Y., Corrosion Science (2018), https://doi.org/10.1016/j.corsci.2018.01.008
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where icorr,0 and icorr denote the current densities of the unprotected and protected WE, respectively. To determine the PZC of X70 steel, the impedance of the steel electrode was measured at various potentials in 1 M HCl solution containing 200 mg/L GLE at 298 K. Each measurement was performed thrice under the same experimental conditions to ensure a satisfactory reproducibility.
infrared (FTIR) spectroscopy. The spectra were obtained by using a Thermo Scientific FTIR spectrophotometer (Nicolet iS50) in the range of 4000–400 cm−1 through KBr disk technique. 2.2. Electrode and solutions preparation The testing specimens were cut from a X70 steel sheet and contained 0.16 wt% C, 1.7 wt% Mn, 0.45 wt% Si, 0.01 wt% S, 0.02 wt% P, 0.06 wt % V, 0.05 wt% Nb, 0.35 wt% Mo, 0.06 wt% Ti, and the remainder in Fe. The steel specimens were sealed in epoxy, leaving a 1 cm2 area exposed to the aggressive solution for electrochemical tests. The steel coupons used for FE-SEM and AFM had dimensions of 0.50 cm × 0.50 cm × 0.30 cm and 1.00 cm × 1.00 cm × 0.10 cm, respectively. Prior to each measurement, the steel electrode was abraded with emery papers with 400–3000 grit, washed ultrasonically with distilled water and anhydrous alcohol, and dried under cold wind. The aggressive solution was prepared by using 1 M HCl solution without and with various concentrations (25, 50, 100, and 200 mg/L) of GLE. The 1 M HCl solution, which was treated as the blank for comparison, was diluted from AR grade 37% HCl. Besides, 1% alcohol was added to dissolve GLE absolutely. A freshly prepared solution was used for each experiment.
2.4. Surface investigation After 0.5 or 4 h immersion in 1 M HCl solution with and without 200 mg/L GLE at different temperatures, the X70 steel samples were descaled with a soft brush, thoroughly rinsed with deionized water, and dried under cold air. The morphologies of the 4 h immersion samples were observed through FE-SEM (JEOL-JSM-7800F, JEOL Ltd., Japan) under high vacuum. The morphologies of the 0.5 h immersion samples with same procedure were examined through AFM (MFP-3D-BIO, Asylum Research, America) under tapping mode. 2.5. Calculation details Quantum chemical calculation was conducted by using Gaussian 03W software to explore the relationship between the inhibition ability of GLE and the electron structure of its main constituents, which are shown in Fig. 2 [24]. Organic molecules such as isorhamnetin (IH), sciadopitysin (SD), 6-hydroxykynurenic acid (HKA), and 4-O-methylpyridoxine (MP) were in neutral form and were fully optimized by using B3LYP method at the DFT level with a 6–311++G(d, p) basis set in the gas phase. Then, several parameters, including the energy of the highest occupied molecular orbital (EHOMO), that of the lowest unoccupied molecular orbital (ELUMO), and the dipole moment (μ), were obtained. Other parameters, such as the energy gap (ΔE), electronegativity (χ), global hardness (γ), ionization potential (I), and electron affinity (A), were calculated as follows [25]:
2.3. Electrochemical tests The electrochemical measurements were performed with a CHI 760E electrochemical station equipped with a traditional three-electrode system. The X70 steel specimen was used as a working electrode (WE), a 4 cm2 platinum sheet was utilized as the counter electrode (CE), and a saturated calomel electrode served as the reference electrode (RE). In this study, all of the potentials were are in reference to the RE. The tests were conducted in a temperature-controlled water bath at a wide range of temperature (298, 308, and 318 K). First, the WE was immersed in the test solution for 1200 s to obtain a stable open circuit potential (EOCP). The corresponding OCP–time curves are depicted in Fig. 1. Then, electrochemical impedance spectroscopy (EIS) was performed on the EOCP. The disturbance signal was a 10 mV peak-to-peak sinusoidal wave in the frequency range of 100000–0.01 Hz. The EIS data were fitted and analyzed carefully by using Zsimpwin. The inhibition efficiency (η) obtained by the EIS test was calculated as follows:
η (%) =
R ct − R ct,0 × 100 R ct
i corr,0 − i corr × 100 i corr,0
(3)
I = −EHOMO
(4)
A = −EHOMO
(5)
χ = (I + A)⁄2
(6)
γ = (I − A)⁄2
(7)
The fraction of electrons transferred from the inhibitor molecules to the metal atoms (ΔN) can be calculated by [26]
(1)
ΔN =
where Rct and Rct,0 are the charge transfer resistances of the WE with and without studied organics, respectively. Finally, the potentiodynamic polarization curves were recorded at a scan rate of 1 mV s−1. The obtained values of η were deduced as follows:
η (%) =
ΔE = ELUMO−EHOMO
χFe − χInh (γFe + γInh )
(8)
where γFe and γInh are the global hardness of Fe and the inhibitor molecule; χFe and χInh are the electronegativity of Fe and the inhibitor molecule, respectively. In accordance with the literature, theoretical χFe and γFe values of 7 and 0 eV/mol were used for the bulk Fe atom [26,27]. The optimized molecular structures, HOMO, LUMO, and ESP
(2)
Fig. 1. OCP–time curves for X70 steel in 1 M HCl solution without and with different concentrations of GLE at different temperatures.
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Fig. 2. Chemical constituents of GLE.
3.2. Impedance measurements
surfaces, were visualized by using Gauss View.
To clarify the surface properties of the X70 steel electrode and the kinetics of the electrode processes, EIS was conducted on X70 steel in 1 M HCl medium without and with different concentrations of GLE at 298, 308, and 318 K. The relevant Nyquist and Bode diagrams are depicted in Figs. 4 and 5, respectively. Slightly depressed capacitive semicircles, which probably correspond to the charge-transfer process of the steel dissolution [28], were observed in all curves in Fig. 4. The centers of these depressed loops were displaced below the real axis. This phenomenon may be related to the frequency dispersion of the interfacial impedance and the inhomogeneous steel surface because of the microscopic roughness and inhibitor adsorption [29]. The addition of GLE increased the size of the impedance plots, suggesting that the organic compounds in the GLE formed a protective film on the surface of the X70 steel. The capacitive loop increased with increasing inhibitor concentration, as much more inhibitor molecules adsorbed on the steel surface, indicating that the inhibitor film gradually became compact and ultimately leading to an improved protective effect. Moreover, as the temperature increased, the size of the capacitive loop decreased, implying that corrosion was accelerated by high temperature. Nevertheless, the size of the spectra was still considerably increased with the addition of GLE with respect to the blank solution, thereby demonstrating the high inhibitive ability of GLE at high temperatures. These results revealed the formation of an inhibitor-adsorption film on the steel surface and its contribution to the enhanced anti-corrosive quality. Only one time constant was observed in the Bode plots shown in Fig. 5. At each temperature, the impedance modulus over the entire frequency range increased as increasing concentrations of GLE were added. Moreover, the frequency range with the maximum phase angle became larger as the inhibitor concentration increased. These results validated the effective inhibitive ability of the investigated GLE for the corrosion of X70 steel in 1 M HCl medium. The classical equivalent circuit shown in Fig. 6 was used to model the EIS data in this study. The solution resistance (Rs), charge transfer resistance (Rct), and constant phase angle element (CPE), which is related to the double-layer capacitance (Cdl), were fitted and summarized in Table 1. The impedance function of the CPE is described as follows
3. Results and discussion 3.1. FTIR analysis The FTIR spectroscopy of GLE is shown in Fig. 3. The broad band at 3301 cm−1 is related to OeH or NeH stretching. The band located at 2923 cm−1 is attributed to CeH stretching vibration, and that at 1694 cm−1 to C]O. The band at 1647 cm−1 is due to the C]C and C]N stretching vibration. The absorption bands at 1504 and 1240 cm−1 can be assigned to the framework vibration of aromatic ring. The CeH bending in eCH3 is found to be at 1408 cm−1. There is a sharp absorption band at 1044 cm−1, which can be associated with the CeN or CeO stretching vibration. Moreover, the absorption bands below 1000 cm−1 correspond to aromatic and aliphatic CeH group. These findings demonstrate that GLE contains oxygen and nitrogen atoms in functional groups (OeH, NeH, C]O, C]C, C]N, CeN, CeO) and aromatic ring, which meets the general characteristics of conventional corrosion inhibitors.
Fig. 3. FTIR spectra of GLE.
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Fig. 5. Bode graphs of the measured impedance shown in Fig. 4. Fig. 4. Nyquist plots recorded for the X70 steel electrode in 1 M HCl solution containing various concentrations of GLE at different temperatures.
[30]:
ZCPE =
1 Y0 (jw )n
(9)
where Y0 is the magnitude of the CPE, w is the angular frequency, j is the imaginary root, and n is the deviation parameter relating to the phase shift representing the microscopic fluctuation of the metal surface. The CPE can display an inductor (n = −1), resistor (n = 0), capacitor (n = 1), or Warburg impedance (n = 0.5). The use of CPE rather than a pure capacitor resulted in the non-ideal dielectric behavior of the inhomogeneous electrode surface [31]. The Cdl value can be calculated by using Eq. (10) [32]:
C = Y0 (wmax )n − 1
Fig. 6. Corresponding equivalent circuit used to fit the EIS experimental data.
As shown in Table 1, n ranged from 0 to 1, and these values depended on the deviation of the ideal capacitance behavior. The presence of GLE increased the Rct values, and this effect was enhanced as the inhibitor concentration was increased. This phenomenon implied that an inhibitor-adsorption film was formed on the steel substrate, thereby retarding the charge transfer. Moreover, Y0 and Cdl decreased in the presence of GLE at each temperature presumably because of the gradual replacement of H2O molecules by the adsorption process of the investigated inhibitor molecules on the steel/solution interface [33]. Cdl
(10)
where wmax = 2πfmax and fmax is the frequency at the maximum value of the imaginary component of the impedance spectra. 4
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Table 1 Impedance parameters for X70 steel in 1 M HCl solution in the absence and presence of various concentrations of GLE at different temperatures. T (K)
298
308
318
a
C (mg/L)
Rs (Ω cm2)
Rct (Ω cm2)
Y0 (μ Ω−1 sn cm−2)
n
Cdl (μF cm−2)
η (%)
SDa
Blank 25 50 100 200 Blank 25 50 100 200 Blank 25 50 100 200
1.1 1.9 9.2 1.4 2.6 3.9 2.7 0.6 3.3 9.1 0.7 2.0 0.7 4.5 2.6
64.8 180.6 269.6 505.8 643.0 32.3 161.1 197.9 244.7 374.6 20.5 76.5 119.9 162.6 275.1
493.5 211.0 171.9 134.4 134.7 898.8 342.0 196.2 210.8 163.4 823.1 333.0 157.6 143.9 143.2
0.90 0.86 0.85 0.87 0.84 0.84 0.78 0.82 0.78 0.78 0.91 0.80 0.85 0.84 0.78
339.0 124.7 106.7 91.1 83.5 449.4 156.1 103.3 92.3 84.7 502.5 129.7 80.0 71.9 68.2
– 64.1 76.0 87.2 89.9 – 80.0 83.7 86.8 91.4 – 73.2 82.9 87.4 92.5
– 3.6 2.8 3.7 2.5 – 4.3 2.7 3.9 3.2 – 6.2 3.4 4.1 4.7
SD, standard deviation of 3 independent measurements.
can be interpreted by the Helmholtz model as follows [34]:
Cdl =
ε 0ε S d
values, indicating that temperature accelerated the corrosion of the steel. Fortunately, the temperature did not influence the superior inhibition performance of GLE, as the maximum η values reached up 90.0% at 298 K, 91.3% at 308 K, and 92.2% at 318 K. These results agreed with that obtained by EIS, implying that GLE can provide a favorable protection performance for steel corrosion at a wide temperature range. This improved performance may be attributed to the adsorption and formation of a protective film on the electrode surface. Remarkably, this adsorption film remained stable at high temperatures.
(11)
where ε is the permittivity of the air, ε is the local dielectric constant of the film, d is the thickness of the electric double-layer, and S is the geometrical surface area of the electrode exposed to a corrosive medium. As the inhibitor concentration increased, more inhibitor molecules adsorbed onto the surface of the X70 steel, leading to a reduced exposed electrode area, a thicker electric double-layer, and lower local dielectric constant. These factors jointly caused a decrease in Cdl [35]. Accordingly, the η values at each temperature increased with increasing inhibitor concentration, reaching 89.9% at 298 K, 91.4% at 308 K, and 92.5% at 318 K for 200 ppm GLE. These values confirmed that GLE induced an effective protective capacity against steel corrosion in 1 M HCl solution. 0
3.4. Adsorption isotherm study Adsorption isotherms are valid models for probing the adsorption mechanism of the inhibitor on the metal surface. The Langmuir isotherm was identified the most suitable model to fit the results obtained by EIS and was calculated by using Eq. (12) [38] with a linear regression coefficient (R2) of approximately 1.
3.3. Polarization curves
θ = K ads C 1−θ
Fig. 7 shows the Tafel polarization curves of the X70 steel electrode in 1 M HCl solution with and without different concentrations of GLE at different temperatures. Table 2 lists the relevant electrochemical parameters, including corrosion current density (icorr), corrosion potential (Ecorr), anodic and cathodic Tafel slope (βa, βc), and inhibition efficiency (η), which were deduced from the polarization curves at a temperature range of 298–318 K. As shown, the addition of GLE shifted both the cathodic and anodic curves to a lower current density, causing a considerable decrease in the corrosion rate at all of the tested temperatures. Specifically, the extent of reduction of the cathodic branch was stronger than that of the anodic one. This phenomenon suggested that both anodic steel dissolution and cathodic hydrogen evolution were retarded by GLE, whereas cathodic reaction is mainly suppressed. Besides, all of the changes in the Ecorr values shown in Table 2 were lower than 85 mV. Therefore, GLE can be considered a mixed-type inhibitor with a predominant control of the cathodic action at each temperature [36,37]. The slight change of both βa and βc at each temperature revealed that the mechanism of the X70 steel corrosion was not altered with the addition of GLE [36]. Table 2 also shows that icorr was markedly decreased with the addition of GLE at all of the tested temperatures, and it continuously decreased with increasing inhibitor concentration. Consequently, the inhibitive ability, which is reflected by η, markedly improved as the inhibitor concentration increased. Temperature is an important kinetic factor that could affect the corrosion behavior of metals and change the adsorption strength inhibitors on metal surfaces. Thus, as shown in Table 2, an increase in temperature enhanced the icorr
(12)
where θ (defined as η/100) is the surface coverage, Kads is the equilibrium constant of the adsorption process, and C is the concentration of inhibitor. The plots of C versus C/θ showed straight lines with an intercept of 0 1/K, as shown in Fig. 8. The standard adsorption free energy ΔGads can be calculated by the following equation [39]:
K ads =
1 CH20
exp(
0 −ΔGads ) RT
(13)
where R is the molar gas constant, T is the absolute temperature, and the water concentration in the solution is 1000 g/L. The calculated Kads 0 and ΔGads values for GLE are provided in Fig. 8. A higher value of Kads generally implies that the inhibitor could tightly adsorb on the metal surface, which is an indicator of a good inhibitive ability [40]. The values of Kads listed in Fig. 8 followed the order of 308 > 298 > 318 K. As the temperature reached 308 K, the largest Kads value revealed that GLE possessed the strongest adsorption affinity on the steel surface and therefore exhibited the best inhibition behavior. However, when the temperature continued to increase, the Kads value decreased, implying an intensified corrosion, which hampered the adsorption of the inhibitor. Furthermore, the adsorption was con0 sidered a physisorption for ΔGads values of −20 kJ mol−1 or less ne0 gative and a chemisorption for ΔGads values of −40 kJ mol−1 or more 0 negative [41]. As shown in Fig. 6, all of the calculated ΔGads values ranged from −28 to −30 kJ mol−1, indicating that the interaction between the inhibitor and the steel surface involves both physisorption 5
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Table 2 Corrosion parameters for X70 steel in 1 M HCl solution containing different concentrations of GLE determined by the Tafel extrapolation method. T (K)
298
308
318
C (mg/L)
Ecorr (mV/ SCE)
icorr (μA cm−2)
βc (mV dec−1)
βa (mV dec−1)
η (%)
SD
Blank 25 50 100 200 Blank 25 50 100 200 Blank 25 50 100 200
−462 −461 −468 −466 −462 −450 −455 −452 −453 −454 −430 −458 −466 −469 −468
348.3 132.5 101.8 47.6 34.7 722.8 139.8 105.7 77.3 62.6 1242 362.3 183.2 160.3 96.8
−100⋅5 −91⋅6 −92.7 −86.6 −80.3 −69.8 −81⋅1 −82.7 −78.6 −79.8 −87⋅9 −85.7 −94⋅9 −104.7 −94.0
81.3 72.4 82.8 74.0 97.8 82.2 74.6 66.6 63.2 71.3 85.6 75.1 77.3 81.9 90.2
– 62.0 70.8 86.3 90.0 – 80.7 85.4 89.3 91.3 – 70.8 85.2 87.1 92.2
– 3⋅1 2⋅2 2⋅4 2⋅1 – 3⋅9 2⋅5 3⋅4 3⋅0 – 5⋅7 3⋅6 3⋅9 3⋅5
where Er is the Antropov’s “rational” potential of corrosion. If Er is negative, then the adsorption of cation is favored, whereas a positive Er value represents the preferential adsorption of anion [43]. Fig. 9 shows the dependence of Cdl on the applied potential of the steel in the 1 M HCl solution containing 200 mg/L GLE. As shown, the obtained curve displayed a parabola with a minimum at −0.501 V, which can be considered the EPZC. The EOCP (−0.471 V) of the steel under the same condition was more positive than EPZC, indicating a positively charged metal surface. This finding suggested that Cl− ions first adsorbed onto the X70 steel surface, causing the steel surface to become negatively charged. Thus, the protonated form of the main organic constitution in GLE adsorbed on the steel surface via an electrostatic interaction. These molecules formed a compact adsorption layer, which acted as a barrier against steel corrosion. In addition to physical adsorption, the neutral and cationic organic molecules may be adsorbed on the surface through the donation of lone electron pairs in heteroatoms to the vacant d orbitals of Fe; this phenomenon is a chemisorption mechanism [44]. Thus, the results confirmed that both physical and chemical adsorption simultaneously existed on the solution/metal interface obtanied from adsorption isotherm. 3.6. Surface microscopic observation Fig. 10 shows the SEM micrographs of the X70 steel surface after 4 h of immersion in 1 M HCl without and with 200 mg/L GLE at different temperatures. The uninhibited steel surface at 298 K (Fig. 10a) was highly damaged and showed obvious deterioration due to aggressive acid attack. More rough surfaces were observed as the temperature increased. In particular, the surfaces protected by GLE at all of the tested temperatures presented relatively smoother morphologies than that of the uninhibited one. These observations indicated that an inhibitor film was formed on the surface of the X70 steel. Compared with the unprotected one, the inhibited surface had a lower permeability for aggressive particles. Consequently, the X70 steel surface was effectively protected by GLE, validating the electrochemical results. AFM is a powerful tool for observing the surface appearance at the nano- to microscale levels, and it has become the preferred choice in the corrosion field [45–47]. Fig. 11 presents 3D AFM images of the corroded and inhibited steel surfaces by 200 mg/L GLE in 1 M HCl solution at 298––318 K for 0.5 h. As shown, all of the corroded steel samples cracked considerably and displayed rough structures with deep and
Fig. 7. Polarization curves for the X70 steel in 1 M HCl solution containing various concentrations of GLE at different temperatures.
and chemisorption.
3.5. Potential of zero charge (PZC) and inhibition mechanism Experiments were performed to determine the charge on the steel surface, which can be estimated by comparing the PZC (EPZC) and open circuit potential (EOCP) of the X70 steel in the solution [42] as follows: Er = EOCP − EPZC
(14)
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Fig. 8. Langmuir adsorption isotherms and relevant parameters of investigated GLE on the X70 steel surface in 1 M HCl solution at different temperatures: (a) 298 K, (b) 308 K, and (c) 318 K.
was markedly decreased at all of the tested temperatures. The relevant values of the average roughness (Ra) of the specimens are graphically presented in Fig. 12. The Ra values exhibited an incremental trend with increasing temperature and exceeded 100 nm at 318 K. With the addition of 200 mg/L GLE, Ra considerably decreased at all of the tested temperatures, demonstrating that the X70 steel specimen can be strongly protected at a wide temperature range. 3.7. Quantum chemical study Quantum chemical calculation was employed to gain insight into the inhibition mechanism of GLE by examining the structure–reactivity correlation of the compounds contained in GLE. Fig. 13 shows the optimized structures and the frontier molecular orbital (FMO) density distributions, i.e., the HOMO and the LUMO. The calculated quantum chemical properties are summarized in Table 3. As shown in Fig. 13 both the HOMO and LUMO distributions of IH, HKA, and MP were concentrated evenly over whole molecules. This system indicated that a flat-adsorption mode could be obtained to gain the largest protective area for the steel surface. By contrast, for SD, the HOMO was preferentially distributed over half of the molecule
Fig. 9. Plot of Cdl vs. applied potential in 1 M HCl containing 200 mg/L GLE.
large pits due to the aggressive attack. In the presence of 200 mg/L GLE, uniform surfaces with small pits were obtained, as shown in Fig. 11(d)–(f), indicating that the corrosion rate of the steel specimen
Fig. 10. FE-SEM images of the X70 steel specimens immersed in 1 M HCl solution (a–c) without and (d–f) with 200 mg/L GLE for 4 h at different temperatures.
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Fig. 11. 3D AFM images of the X70 steel specimens immersed in 1 M HCl solution without GLE for 0.5 h at (a) 298 K, (b) 308 K, and (c) 318 K; 3D AFM images of the X70 steel specimens immersed in 1 M HCl solution with 200 mg/L GLE for 0.5 h at (a) 298 K, (b) 308 K, and (c) 318 K.
tends to accept electrons, whereas a high EHOMO reflects a strong electron-donating tendency to a suitable acceptor with an empty molecular orbital and low ELUMO [49]. Thus, an energy gap indicates the chemical stability of an inhibitor, and a lower ΔE value typically leads to greater adsorption on the metal surface, resulting in higher inhibition efficiencies [34]. The calculated ΔE values of the compounds followed the order of HKA < SD < IH < MP, indicating that HKA probably played the most prominent role in retarding the corrosion process. The electronegativity χ is an indicator of the electron-attraction ability of a molecule. A higher χ corresponds to a lower chance of electron donation from the molecule, and vice versa [45]. Moreover, a higher value of global hardness γ indicates a higher resistance of a molecule toward charge transfer [31,41]. The dipole moment μ is also a crucial indicator, although the correlation between μ and the inhibitive capacity is yet to be verified [28,36,50]. The lowest μ value of HKA favors the accumulation of the inhibitor on the metal surface, thereby increasing the inhibition efficiency, which agrees well with the result obtained from the ΔE values. The ΔN value of HKA was the maximum, suggesting that the reaction activity of the HKA molecule was the highest, and the number of electrons from HKA to Fe atom was the greatest [48]. Thus, HKA may demonstrate the highest inhibition efficiency among the four compounds, which agrees with the conclusion obtained from ΔE. In summary, the inhibition capacity of GLE could be ascribed to the adsorption of its numerous organic constituents on the surface of the X70 steel. The main components of GLE are HKA, SD, IH, and MP, all of which contain conjugated structures, such as benzene ring, O, N atoms in O-herterocyclic ring and functional groups (NeH, OeH, C]O, CeO), thus meeting the general characteristics of traditional organic inhibitors.
Fig. 12. Average roughness (Ra) of X70 steel immersed in 1 M HCl solution without and with 200 mg/L GLE for 0.5 h at different temperatures.
containing two OH groups, whereas the LUMO was spread on the other half presumably because of its large twisted structure. As such, the SD molecule had a greater directionality during bonding with steel surface [48]. Half of the SD molecules preferentially bonded with positively polarized anodic reaction sites, thereby leading to a decreased rate of anodic steel dissolution reaction. The other half of the molecule preferentially attracted electrons from the cathodic sites on the steel surface, thus reducing the rate of the cathodic hydrogen evolution reaction. Moreover, the red (negative) and blue (positive) regions of the ESP map were related to nucleophilic and electrophilic activities, respectively [25]. As evidenced in Fig. 14 that, all of the red regions were distributed in the electronegative groups with heteroatoms, such as O, N atoms or O-herterocyclic rings, which readily formed covalent bonds with electrophilic agents, such as Fe atoms. ELUMO and EHOMO represented the electron-receiving and -donating ability of a molecule. In general, a low ELUMO implies that an inhibitor
4. Conclusions On the basis of the systematic experimental and theoretical results in this work, the following conclusions can be drawn: (1) Electrochemical results indicated that GLE can be classified as an effective mixed-type inhibitor for the corrosion of X70 steel in 1 M HCl. Moreover, the η values obtained from the polarization test reached up to 90.0% at 298 K, 91.3% at 308 K, and 92.2% at 318 K 8
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Fig. 13. Optimized structures and frontier orbital density distributions of IH, SD, HKA, and MP.
(3) The adsorption of GLE on the X70 steel surface obeyed the Langmuir adsorption model and involved both physisorption and chemisorption. (4) The inhibition capacity of GLE could be ascribed to the adsorption of its numerous organic constituents, such as HKA, SD, IH, and MP, on the surface of the X70 steel.
for 200 mg/L GLE, which agrees well with those gained by EIS. In particular, the inhibitive ability remained favorable at high temperatures. (2) FE-SEM and AFM observations verified the inhibition performance of GLE obtained electrochemically, indicating that a dense and ordered protective film was formed on the steel surface.
Table 3 Calculated quantum chemical indices of isorhamnetin (IH), sciadopitysin (SD), 6-hydroxykynurenic acid (HKA), and 4-O-methylpyridoxine (MP). Substance
EHOMO (eV)
ELUMO (eV)
ΔE (eV)
I (eV)
A (eV)
χ (eV)
γ (eV)
μ (Debye)
ΔN
IH SD HKA MP
−5.96 −6.22 −5.32 −6.14
−1.73 −2.29 −1.74 −0.85
4.23 3.93 3.58 5.29
5.96 6.22 5.32 6.14
1.73 2.29 1.74 0.85
3.85 4.26 3.53 3.50
2.12 1.97 1.79 2.65
7.38 11.89 4.13 6.70
0.74 0.70 0.97 0.66
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Fig. 14. (a) Contours and (b) surface maps of the electrostatic potential (ESP) of IH, SD, HKA, and MP.
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