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Orange peel extracts as biodegradable corrosion inhibitor for magnesium alloy in NaCl solution: Experimental and theoretical studies
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Orange peel extracts as biodegradable corrosion inhibitor for magnesium alloy in NaCl solution: Experimental and theoretical studies Yaxu Wua,b,c, Yuanmi Zhanga,b,c, Yumiao Jianga,b,c, Yafeng Qiand, Xugeng Guoa,b,c,*, Li Wanga,b,c,*, Jinglai Zhanga,b,c,* a
Henan Engineering Research Center of Green Anticorrosion Technology for Magnesium Alloys, Henan University, Kaifeng, Henan 475004, PR China Henan Engineering Research Center of Corrosion and Protection for Magnesium Alloys, Henan University, Kaifeng, Henan 475004, PR China c College of Chemistry and Chemical Engineering, Henan University, Kaifeng, Henan 475004, PR China d National Center for Quality Supervision and Inspection of Magnesium and Magnesium Alloy Products, Hebi, Henan 458030, PR China b
A R T I C L E
I N F O
Article History: Received 27 July 2020 Revised 8 October 2020 Accepted 12 October 2020 Available online xxx Keywords: Corrosion AZ91D magnesium alloy Biodegradable inhibitor Electrochemistry Theoretical study
A B S T R A C T
Performance of orange peel extracts (OPE) to retard the magnesium alloys corrosion is thoroughly evaluated by Electrochemical impedance spectra (EIS) and potentiodynamic polarization curves. OPE would be an effective inhibitor with the efficiency of 85.7% at very low concentration of 0.030 g L 1. The inhibition effect of selected three pure components including in OPE is much inferior or comparable with that of OPE, whereas the cost of pure compounds is too expensive to be afforded. The protective self-assembly film would be formed on the magnesium surface by immersion in OPE. Structure characterization is performed by scanning electron microscopy (SEM), atomic force microscopy (AFM), and X-ray diffractometer (XRD). The adsorption of OPE on magnesium surface follows the Langmuir adsorption rule with both chemical and physical interactions. The interaction between three selected pure compounds and magnesium alloys is further calculated by density functional theory (DFT). Moreover, the possible inhibition mechanism is constructed on the basis of above results along with the FTIR measurements. The low cost and green inhibitor is explored in this work, which would replace the expensive inhibitors with the similar or better performance. © 2020 Taiwan Institute of Chemical Engineers. Published by Elsevier B.V. All rights reserved.
1. Introduction In past decades, magnesium alloys have been mainly applied in the region of aerospace and military [1,2]. Recently, their applied region is extended to automobiles, energy storage, infrastructure materials, and electronics due to the ideal weight-to-strength ratio, excellent electromagnetic shielding, and good machinability [2]. They are regarded as the promising candidate to replace heavy metal alloys in future. The predominant barrier to realize the aforementioned applications is that magnesium is vulnerable to corrosion. Numerous pathways have been developed to prevent the magnesium from corrosion [3]. Addition of inhibitors is a classical method to provide protection for metal with the advantages of easy operation, without special equipment, and economy. Organic inhibitors including N, O, S, and hetero rings in their structures and inorganic inhibitors, normally chromates and phosphates, have been applied for magnesium alloys [4-7]. Recently, ionic liquids also have been developed as the corrosion inhibitors [8]. Although all of them would suppress the corrosion with different * Corresponding author. E-mail addresses: [email protected] (X. Guo), [email protected] (L. Wang), [email protected] (J. Zhang).
degree, they have some common disadvantages, such as expensive and complicate synthesis and purification. More importantly, they are toxic and hazardous for human health and environment. Lots of efforts have been made to explore the abundant, easily available, biodegradable materials with non-environmental pollution due to more and more stringent restriction on environmental protection. Plant extracts including leaves, seeds, and stems, have been employed as the inhibitors for copper, steel, and aluminum alloy [9-11]. Ginkgo leaf, Peganum harmala seed, and Olive stem have been utilized as the inhibitors for steel in HCl solution reaching the inhibition efficiency of 92.5%, 95.0%, and 88.8%, respectively [12-14]. Similarly, Phyllanthus amarus extract has been reported as inhibitor for aluminum alloy in 2 M NaOH solution with the inhibition efficiency of 76.0% [15]. As compared with organic or inorganic inhibitors, plant extracts present better biocompatibility along with “zero” pollution. However, if they are large scale used, such as, roots, stems, and other plants, the forest and environment would be suffered resulting in other pollution problems [16]. If inhibitors would be extracted from the plant waste, they would not only be favorable to disposing them but also be beneficial to purifying the environment. Orange is popular fruit in the world market with delicious taste and high nutrition. However orange peel is discarded as waste, which is almost 50% of the total fruit mass. The thrown orange peel would
https://doi.org/10.1016/j.jtice.2020.10.010 1876-1070/© 2020 Taiwan Institute of Chemical Engineers. Published by Elsevier B.V. All rights reserved.
Please cite this article as: Y. Wu et al., Orange peel extracts as biodegradable corrosion inhibitor for magnesium alloy in NaCl solution: Experimental and theoretical studies, Journal of the Taiwan Institute of Chemical Engineers (2020), https://doi.org/10.1016/j.jtice.2020.10.010
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result in serious environmental problem because of the rotting. Orange peel extracts (OPE) have presented inhibition behavior for carbon steel in acidic medium [17]. Three main components, ascorbic acid, naringin, and neohesperidin, (See Fig. S1) are determined by Nouha M’hiri et al., which is helpful to elucidate their role in the corrosion inhibition. Unfortunately, it is not discussed in previous literature. Their inhibition effect on the carbon steel is simply attributed to the antioxidant activities. There are O atoms, conjugated rings, and hydroxyl groups in OPE, which is similar with the reported organic inhibitors. The inhibitor would adsorb on the metal surface by donated electrons of O atoms and unoccupied orbitals of metal. In addition, the hydroxyl groups would react with magnesium on the surface, which would retard the corrosion of internal magnesium. Therefore, their inhibition behavior is deserved to be further determined. Moreover, the application scope of OPE should be further enlarged to other metals rather than only carbon steel. In this contribution, the inhibition behavior of OPE on AZ91D magnesium alloys has been investigated by electrochemical test along with scanning electron microscope (SEM), atomic force microscope (AFM), fourier transform infrared spectrometer (FTIR), and X-ray diffraction (XRD). The inhibition performance of OPE is compared with three major pure components included in OPE by both experimental and theoretical calculations. When magnesium alloys are immersed in OPE, the protective self-assembly film would be formed on the surface. Its inhibition performance is also investigated as compared to samples with addition of inhibitor directly. Finally, the possible inhibition mechanism is conjectured on the basis of the experimental and theoretical studies. It is expected that a novel green inhibitor for magnesium alloys would be confirmed from the waste, which possesses lots of incomparable advantages. 2. Experimental and theoretical details 2.1. Materials The employed working electrodes were prepared by AZ91D magnesium alloy with major composition of Mg as well as rest: Al, 8.7, Zn, 0.64, Mn, 0.023, Si, 0.014, Fe, 0.002, and Cu, 0.002 (wt.%). The electrodes were sealed in epoxy, leaving around 1 cm2 area exposed to the aggressive solution for electrochemical tests. Prior to each measurement, the samples (1.0 cm £ 1.0 cm £ 0.5 cm) were abraded by a series of metallographic abrasive paper with 100, 800, 1200, and 3000 grades to remove the oxides from the surface. After rinsed with distilled water, the electrodes were washed ultrasonically for 3 mins with anhydrous ethanol. Finally, the clean electrodes were dried in the air quickly. Raw orange (orange color) were bought from the local market in Henan province, China. The extract is obtained by refluxing. Then, OPE was dissolved in 0.05 wt.% NaCl solution and prepared for electrochemical and surface analysis tests. The extract was characterized by FT-IR (VERTEX 70, Bruker, USA) in the range of 4000500 cm1 through KBr disk technique. The 0.05 wt.% NaCl solution is employed to be corrosive environment, which is utilized to simulate the marine atmospheric environment. 2.2. Electrochemical tests The electrochemical measurements including EIS measurements and potentiodynamic polarization tests were performed with RST5000F electrochemical station. The traditional three electrode cell was employed including saturated calomel electrode (SCE) as reference, Platinum electrode as counter electrode (CE), and magnesium alloys as working electrode (WE). Before each electrochemical test, the electrode was allowed to be corroded freely and its open circuit potential (OCP) was recorded as a function of time up to 1000s to stabilize the system. The frequency range of 10 kHz to 1 Hz was applied
for EIS measurements with zero amplitude sinusoidal voltage of 7 mV at OCP. The potentiodynamic polarization was performed with a scan rate of 10 mV s 1 from 400 mV to 100 mV versus OCP. Each experiment was repeated at least three times to ensure the reproducibility. 2.3. Surface studies The structure characteristics of AZ91D Mg alloy samples were observed by SEM (SU3500, HITACHI, Japan) and AFM (Dimension ICON, Bruker, USA) techniques. The corrosion products on the surface was analyzed by XRD (SmartLab SE, Rigaku, Japan). During XRD analysis, the step size is 0.01° and the scan range is 580° along with a scanning rate of 10° min1. With exception of fresh sample, other samples were washed by chromic acid solution (200 g L1 CrO3 + 10 g L 1 AgNO3) in an ultrasonic bath for 1520 mins to remove the corrosion products. Following it, the sample are ultrasonically washed by three times in absolute ethanol for 5 mins/each time to remove chromic acid. Finally, they were cleaned with distilled water and dried in the air for test. To further confirm the possible groups existed on the surface, the corrosive products were also determined by FTIR. Before test, the Mg alloy species was immersed in the NaCl solution including 0.030 g L 1 OPE for ten days to get the enough corrosive products. 2.4. Theoretical details To explore the interfacial properties between inhibitor and the metal surface atom. The structures of inhibitor (three major components in OPE), Mg ð1010Þsurface, and inhibitor@Mg ð1010Þsurface were optimized by generalized gradient approximation (GGA) along with Perdew-Burke-Ernzerhof (PBE) method [18,19]. Then, the Grimme’s D3 dispersion correction was employed to further refine the energy [20,21]. All calculations were implemented using Vienna ab-initio software package (VASP) with energy cut-off of 500 eV and the force on each atom of 0.1 eV A1. Projected density of states (PDOS) were calculated at the same level based on the optimized geometries [22-25]. 3. Results and discussion 3.1. Characterize of orange peel extracts Three components, ascorbic acid, naringin, and neohesperidin, have been testified to be included in OPE in previous literature [17], which plays a critical role in inhibition. By the mass spectrum (MS), they are confirmed to be included in our OPE. Further, they are further measured by FTIR. The FTIR spectroscopy of OPE is shown in Fig. 1a. The strong and broad peak at 3400 cm1 is attributed to the OH stretching vibration. The -OH group is plentiful in three components, which is consistent with the strongest strength. The second strong peak at 1065 cm1 is associated with CO stretching vibration. The small sharp peaks at 2929 and 1642 cm1 correspond to CH and C=O stretching vibration. The vibration of aromatic ring is related with the small peaks around 1515 and 1270 cm1, while the vibration of aromatic CH group is related with peaks below 1000 cm1. The appearance of O atoms, conjugated rings, and -OH groups ensures the basic requirement for corrosion inhibitors. 3.2. Corrosion inhibition 3.2.1. Electrochemical impedance spectroscopy (EIS) The EIS spectra with and without different concentrations of extracts obey the one-time constant, which is fitted by the equivalent electronic circuit described in Fig. S2. The Cdl is replaced by CPE in equivalent electronic circuit because of the inhomogeneous metal
Please cite this article as: Y. Wu et al., Orange peel extracts as biodegradable corrosion inhibitor for magnesium alloy in NaCl solution: Experimental and theoretical studies, Journal of the Taiwan Institute of Chemical Engineers (2020), https://doi.org/10.1016/j.jtice.2020.10.010
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Fig. 1. The FT-IR spectra of OPE (a) and the corrosive products on the metal surface immersed in 0.05 wt.% NaCl solution with 0.030 g L 1 OPE after ten days (b).
surface. The detailed definition of CPE and Cdl please refer to refs [26-28] a ZCPE ¼ Y1 0 ðjvÞ
ð1Þ
where Y0 is the CPE magnitude constant, a is the CPE exponent, j is an imaginary number, and v is the angular frequency. Cd1 ¼ Y0 ð2pf max Þa1
ð2Þ
where 2pfmax equals v, fmax is the maximum frequency. As shown in Fig. 2a, 0.030 g L 1 OPE is the threshold. Below it, the diameter of capacitive loops is increased with the enhancement of concentration indicating the larger inhibition performance. After that, the inhibition performance is decreased when the concentration is further increased. The similar conclusion could be deduced from Bode plots (Fig. S3a). The appearance of depressed semicircle indicates that capacitive layer is formed on the metal surface due to the adsorption of extract along with the roughness of metal surface. The detailed data fitted by EIS results are tabulated in Table 1. The variation of solution resistance (Rs) is slight and irregular for all concentrations indicating that the negligible effect of electrolyte on the corrosion. Polarization resistance (Rp) presents the similar variation rule with the change of semicircle diameter since the corrosion process is predominantly controlled by charge transfer process. The CPE exponent (a) is around 0.8 with slight differentiation, which is attributed to the similar inhibition mechanism with and without OPE. The highest inhibition efficiency is 85.7% with addition of 0.030 g L 1 OPE in 0.05 wt.% NaCl solution, which is defined by the following expression [29]: nE % ¼
RpðinhÞ Rp 100 RpðinhÞ
ð3Þ
where Rp(inh) and Rp are polarization resistance with and without OPE. The more extracts would be adsorbed on the metal surface with the increasing concentrations leading to the larger surface coverage (u ). However, the further more extracts would not be adsorbed on the metal surface due to the competition among them. Alternatively, some of them would react with the corrosive product resulting in some soluble products. As a result, the coating surface is reduced in contrast. The adsorption is a key item to suppress the corrosion.
3.2.2. Tafel polarization Polarization curves obtained for AZ91D Mg alloys measured in the absence and presence of OPE in 0.05 wt.% NaCl solution are presented Fig. 3a along with the fitting data in Table 2. With the addition of OPE, the corrosion current density (icorr) is greatly reduced in different extent. The corrosion potential (Ecorr) also varies with concentration of OPE, however, no clear trend is followed. The maximum shift of Ecorr happens for 0.030 g L 1 OPE and the minimum shift of Ecorr appears for 0.005 g L 1 OPE. All of Ecorr(s) present the mixed inhibition behavior with the shift less than 85 mV [30], however, the shift direction is not the totally same with both negative and positive directions. There is only slight change for both anodic and cathodic Tafel slopes (ba and bc) indicating that anodic and cathodic reactions are suppressed with the similar mechanism. Following the relationship of expression: np % ¼
icorr icorrðinhÞ 100 icorr
ð4Þ
where icorr and icorr(inh) are the corrosion current density in the absence and presence of inhibitors, respectively. The best inhibition efficiency of 84.0% is obtained for 0.030 g L 1 OPE in 0.05 wt.% NaCl solution. There is a good coherence between EIS and potentiodynamic polarization test for both the inhibition efficiency and inhibition behavior. 3.3. Immersion time According to the EIS and potentiodynamic polarization test, the involvement of 0.030 g L 1 OPE in 0.05 wt.% NaCl solution would get the best inhibition efficiency. For magnesium alloys, the inhibitor used in the solution is not our ultimate goal. The ideal inhibitor would form the film on the metal surface to provide more durable resistance. The fresh Mg alloys are immersed in 0.030 g L 1 OPE for 12 h, 24 h, 36 h, 48 h, and 72 h, respectively. Then, they are put in 0.05 wt.% NaCl solution without OPE to do the EIS and potentiodynamic polarization test along with the date presented in Fig. 2b, Fig. 3b, Fig. S3b, and Tables 3-4. The immersion samples present the similar results for both EIS and potentiodynamic polarization curves which is not stated out in detail. The inhibition performance is improved with the elongation of immersed time reaching the maximum value of 82.7% for 36 h. The inhibition efficiency is similar with that measured by addition of OPE indicating the formation of
Please cite this article as: Y. Wu et al., Orange peel extracts as biodegradable corrosion inhibitor for magnesium alloy in NaCl solution: Experimental and theoretical studies, Journal of the Taiwan Institute of Chemical Engineers (2020), https://doi.org/10.1016/j.jtice.2020.10.010
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Fig. 2. Nyquist plot for AZ91D Mg alloy electrode in 0.05 wt.% NaCl solution (a) without and with different concentrations of orange peel extracts. (b) after different immersion times in 0.030 g L 1 orange peel extract-ethanol solution. (c) in the absence and presence of differently mixed inhibitors with the same concentrations of 0.030 g L 1. (A is ascorbic acid, B is neohesperidin and C is naringin).
Table 1 Electrochemical parameters obtained from Nyquist and Bode plots for AZ91D Mg alloy electrode in 0.05 wt.% NaCl without and with different concentrations of orange peel extracts. C (g L 1)
Blank 0.005 0.010 0.020 0.030 0.050 0.100 0.200
Rs (V cm2) 514.9 604.3 520.0 477.4 524.9 556.0 516.3 469.1
Rp (V cm2)
CPEdl (mV1 Sn cm2)
a
682.4 1705.0 4060.0 4391.0 4786.0 3693.0 2382.0 1455.0
27.9 24.2 21.7 22.5 18.5 19.4 19.2 20.3
0.84 0.88 0.87 0.86 0.87 0.90 0.89 0.88
protective film on the magnesium surface. After that, the inhibition efficiency is almost kept with slight fluctuation even if the immersion time is further elongated. Initially, the OPE does not have enough time to reach the Mg surface resulting in the less inhibition efficiency within 24 h. Therefore, the inhibition efficiency would be improved with the elongated immersion time. When the adsorbed extracts is enough on the surface, the inhibition efficiency would not be further enhanced with the elongated immersion time. The corrosive products on the surface is determined by the FTIR spectrum to testify that the inhibitor is actually adsorbed on the Mg alloys surface. As shown in Fig. 1b, the characteristic absorption peaks
Cdl (mV1 Sn cm2)
x2
13.1 15.8 15.1 15.6 12.9 14.4 13.2 12.6
9.60 £ 104 3.51 £ 104 1.51 £ 104 6.32 £ 104 7.27 £ 105 2.97 £ 104 5.23 £ 104 1.37 £ 103
hE
u
(%) 60.0 83.2 84.5 85.7 81.5 71.4 53.1
0.600 0.832 0.845 0.857 0.815 0.714 0.531
included in the OPE appears in the corrosive products indicating that the OPE has been adsorbed on the metal surface. Moreover, there is slight peak shift for the absorption peaks suggesting that there are interactions between OPE and Mg alloys, which is helpful to form the more dense film than the adsorption. 3.4. Comparison with single components According to the previous report, three main components, ascorbic acid, naringin, and neohesperidin, would play the critical role in suppressing the corrosion. They are also confirmed to be
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Fig. 3. Potentiodynamic polarization curves for AZ91D Mg alloy electrode in 0.05 wt.% NaCl solution (a) without and with different concentrations of orange peel extracts (b) after different immersion times in 0.030 g L 1 orange peel extract-ethanol solution (c) in the absence and presence of differently mixed inhibitors with the same concentrations of 0.030 g L 1. (A is ascorbic acid, B is neohesperidin and C is naringin).
Table 2 Electrochemical parameters obtained from potentiodynamic polarization curves for AZ91D Mg alloy electrode in 0.05 wt.% NaCl without and with different concentrations of orange peel extracts. C (g L 1)
Ecorr (V)
icorr (mA cm2)
bc (V dec1)
ba (V dec1)
hp (%)
Blank 0.005 0.010 0.020 0.030 0.050 0.100 0.200
1.45 1.44 1.46 1.47 1.48 1.44 1.45 1.45
12.20 5.21 2.10 2.00 1.95 2.33 4.33 6.07
0.096 0.072 0.068 0.068 0.068 0.068 0.069 0.072
0.098 0.076 0.082 0.084 0.086 0.071 0.080 0.075
57.3 82.8 83.6 84.0 80.9 64.5 50.2
included in this work. The corrosion performance of pure compound is compared with OPE under the same concentration. The corresponding results of EIS and potentiodynamic polarization curves are shown in Fig. 2c, Fig. 3c, Fig. S3c, and Tables 5-6. The EIS curves display the similar depressed curves for all tested samples indicating the similar corrosion mechanism controlled by charge transfer process. The Tafel curves also testified the similar conclusion that all of them are mixed corrosive behavior with a shift
towards cathodic direction less than 85 mV. Since EIS and potentiodynamic polarization curves have the consistent result, only the EIS results are discussed. When only single compound is included in 0.05 wt.% NaCl solution with the same concentration, the corrosion inhibition efficiency is only in a range of 74.478.7%, which is much less than that of OPE. If two of them are mixed together, there would be synergistic effect between them. The largest corrosion inhibition efficiency is enhanced to 83.2%, however, it is still less than that of OPE. For the single complex, naringin (78.7%) displays the better corrosion inhibition effect than other two complexes, ascorbic acid (74.4%) and neohesperidin (74.9%). The corrosion inhibition property is better when the naringin is combined with the other compound, such as ascorbic acid + neohesperidin (76.3%), naringin + ascorbic acid (78.6%), and naringin + neohesperidin (83.2%). Naringin is the key component to retard the corrosion, however, the difference between naringin and neohesperidin is too small to be deviate them. When three complexes are mixed together, the corrosion inhibition efficiency is increased to 87.2%, which is slightly larger than that of OPE by 1.5%. The price of naringin is expensive due to the complicate synthesis and purification steps. It is also difficult to extract it from
Please cite this article as: Y. Wu et al., Orange peel extracts as biodegradable corrosion inhibitor for magnesium alloy in NaCl solution: Experimental and theoretical studies, Journal of the Taiwan Institute of Chemical Engineers (2020), https://doi.org/10.1016/j.jtice.2020.10.010
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Y. Wu et al. / Journal of the Taiwan Institute of Chemical Engineers 00 (2020) 112 Table 3 Electrochemical parameters obtained from Nyquist and Bode plots for AZ91D Mg alloy electrode in 0.05 wt.% NaCl solution after different immersion times in 0.030 g L 1 orange peel extract-ethanol solution. Time (h)
Rs (V cm2)
Rp (V cm2)
CPEdl (mV1 Sn cm2)
a
Blank 12 24 36 48 72
514.9 441.1 546.9 451.1 619.3 508.0
682.4 1125.0 2739.0 3943.0 3921.0 3917.0
27.9 21.0 18.9 16.9 18.9 14.8
0.84 0.87 0.88 0.89 0.88 0.90
Table 4 Electrochemical parameters obtained from potentiodynamic polarization curves for AZ91D Mg alloy electrode in 0.05 wt.% NaCl solution after different immersion times in 0.030 g L 1 orange peel extract-ethanol solution. Time (h)
Ecorr (V)
icorr (mA cm2)
bc (V dec1)
ba (V dec1)
hp (%)
Blank 12 24 36 48 72
1.45 1.45 1.44 1.45 1.46 1.46
12.20 7.36 2.62 1.98 2.07 2.15
0.096 0.072 0.067 0.070 0.068 0.076
0.098 0.075 0.081 0.089 0.082 0.098
39.7 78.5 83.8 83.0 82.4
Cdl (mV1 Sn cm2)
x2
hE
13.2 12.0 12.6 12.2 13.2 10.8
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u
(%) 39.3 75.1 82.7 82.6 82.6
0.393 0.751 0.827 0.826 0.826
obtained for Langmuir isotherm model with the R2 of 0.99622 (See Fig. 4). The Kads is calculated according to the following equation [31-34]: Cinh
u
¼
1 þ Cinh Kads
ð5Þ
in which u is surface coverage, Cinh is inhibitor concentration. Consequently, the standard adsorption free energy ðDG0ads Þ is calculated on the basis of relationship of the following expression [32-34]: ! DG0ads 1 Kads ¼ exp ð6Þ CH2O RT
the orange peel. To utilize the OPE is the more advisable choice than to use the pure compounds.
The DG0ads is 32.41 kJ mol1 indicating the adsorption behavior is dominated by both physical and chemical adsorption.
3.5. Adsorption isotherms modeling
3.6. Surface studies
To further elucidate the adsorption behavior of OPE in NaCl solution, the electrochemical data is fitted by Temkin, Freundlich, Frumkin, and Langmuir models (See Fig. S4 and Fig. 4). The best fit is
The surface morphology would not only provide the intuitive result for the inhibition but also is favorable to make a direct justification. As shown in Fig. 5a, the fresh magnesium surface is smooth and
Table 5 Electrochemical parameters obtained from Nyquist and Bode plots for AZ91D Mg alloy electrode in 0.05 wt.% NaCl solution in the absence and presence of differently mixed inhibitors with the same concentrations of 0.030 g L 1.
Blank ascorbic acid neohesperidin naringin ascorbic acid+neohesperidin ascorbic acid+naringin neohesperidin+naringin ascorbic acid+ neohesperidin+naringin OPE
Rs (V cm2)
Rp (V cm2)
CPEdl (mV1 Sn cm2)
n
514.9 496.8 471.3 387.2 483.7 353.8 366.8 363.2 524.9
682.4 2667.0 2716.0 3201.0 2880.0 3189.0 4064.0 5343.0 4786.0
27.9 29.5 24.0 18.9 22.9 21.4 19.5 15.4 18.5
0.84 0.82 0.88 0.87 0.84 0.86 0.86 0.91 0.87
Cdl (mV1 Sn cm2)
x2
hE
13.2 16.5 16.5 12.5 13.6 13.8 12.6 11.9 12.9
9.60 £ 104 2.75 £ 104 7.18 £ 104 6.99 £ 104 9.30 £ 104 4.85 £ 104 4.58 £ 103 2.07 £ 102 7.27 £ 105
u
(%)
74.4 74.9 78.7 76.3 78.6 83.2 87.2 85.7
0.744 0.749 0.787 0.763 0.786 0.832 0.872 0.857
Table 6 Electrochemical parameters obtained from potentiodynamic polarization curves for AZ91D Mg alloy electrode in 0.05 wt.% NaCl solution in the absence and presence of differently mixed inhibitors with the same concentrations of 0.030 g L 1.
Blank ascorbic acid neohesperidin naringin ascorbic acid+neohesperidin ascorbic acid+naringin neohesperidin+naringin ascorbic acid+neohesperidin +naringin OPE
Ecorr (V)
icorr (mA cm2)
bc (V dec1)
ba (V dec1)
hp (%)
1.45 1.48 1.48 1.48 1.49 1.49 1.49 1.51 1.48
12.20 3.01 2.93 2.11 2.36 2.12 2.08 1.93 1.95
0.096 0.088 0.069 0.057 0.068 0.067 0.067 0.058 0.068
0.098 0.108 0.084 0.070 0.082 0.083 0.084 0.069 0.086
75.3 76.0 82.7 80.6 82.6 83.0 84.2 84.0
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Fig. 4. Langmuir adsorption isotherm for orange peel extracts on AZ91D Mg alloy in 0.05 wt.% NaCl solution at 298 K.
polished except for some visible scratches due to the abrading by emery paper in the sample preparation process. Deep cavities as well as lots of cracks are obviously visualized because of the
7
heavy corrosion in the solution free of inhibitor (See Fig. 5b). It is also consistent with the conclusion that corrosion of magnesium alloys has the pit corrosion behavior. AFM (See Fig. 6) reveals the great roughness of Mg surface with rolling high-altitude valley. There is great fluctuation with the average roughness of 53.9 nm. In contrast, the damage of surface is greatly decreased with average roughness of 8.0 nm (See Fig. 7) with 0.030 g L 1 OPE. The longitudinal view presents almost the straight curve. For SEM image (See Fig. 5c), abrasive scratches could also be observed with some small cracks, which would be deposited corrosive products or inhibitors. It is similar for the sample in 0.05 wt.% NaCl after immersing in 0.030 g L 1 OPE for 36 h except that the abrasive scratches are even more with slightly enhanced average roughness (See Fig. 5d and Fig. 8). Aforementioned three samples are also performed the XRD measurements (See Fig. 9). There are many Mg characteristic signals with the predominant one at 36.7°, which is almost the same for three samples. The inhibition influence is not obviously presented from the variation of Mg signals. The peaks related with Mg(OH)2 at 50.8° and 69.2° are obviously reduced suggesting that protection is formed with the addition of inhibitors. Since there are lots of components in OPE, other peaks are not easy to be detected. It is difficult to determine whether OPE are adsorbed on the Mg alloys surface. Perhaps, the theoretical calculations would help us to elucidate it.
Fig. 5. SEM images for the surfaces of AZ91D Mg alloy samples (a) after polishing and immersed in 0.05 wt.% NaCl solution for 6 days (b) without inhibitor; (c) with 0.030 g L 1 orange peel extracts; (d) after coating for 36 h in 0.030 g L 1 orange peel extract-ethanol solution.
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Fig. 6. AFM images in 2D (a) and 3D (b) formats of the AZ91D Mg alloy immersed in 0.05 wt.% NaCl solution for 3 days in the absence of inhibitors. Panel (c) is the height profile of the AZ91D Mg surface made along the marked lines on panel (a) by the Nanoscope v 1.80 software.
Fig. 7. AFM images in 2D (a) and 3D (b) formats of the AZ91D Mg alloy immersed in 0.05 wt.% NaCl solution for 3 days in 0.030 g L 1 orange peel extracts. Panel (c) is the height profifile of the AZ91D Mg alloy surface made along the marked lines on panel (a) by the Nanoscope v 1.80 software.
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Fig. 8. AFM images in 2D (a) and 3D (b) formats of the AZ91D Mg alloy immersed in 0.05 wt.% NaCl solution for 3 days after coating for 36 h in 0.030 g L 1 orange peel extract-ethanol solution. Panel (c) is the height profile of the AZ91D Mg surface made along the marked lines on panel (a) by the Nanoscope v1.80 software.
3.7. Theoretical calculation
Fig. 9. XRD patterns for AZ91D Mg alloy immersed in 0.05 wt.% NaCl solution for 6 days (a) without inhibitor; (b) with 0.030 g L 1 orange peel extracts; (c) after coating for 36 h in 0.030 g L 1 orange peel extract-ethanol solution.
Theoretical calculation would help us to deeply understand the interaction between inhibitor and magnesium surface. The single components, ascorbic acid, naringin, neohesperidin, and mixture of three components are chosen as the representative of OPE. It is also favorable to elucidate that the corrosion inhibition performance of naringin is better than other two components. The structures of single component, Mg ð1010Þ surface, and component@Mg ð1010Þ surface are optimized by PBE/GGA method (See Fig. S5). In our previous work, the different Mg surfaces are compared to determine the more stable one. The Mg ð1010Þ surface is the more stable surface, which is employed to represent the Mg alloys. Based on the optimized geometries, the corresponding PDOS is plotted in Fig. 10. Around the Femi energy level, the peak of PDOS is more flat after the inhibitor is adsorbed on the Mg surface, which indicates that there is strong interaction between inhibitor and Mg surface. The strong interaction would also be detected by the adsorption energy, which is 20.56 kJ mol1 for ascorbic acid@Mg, 57.45 kJ mol1 for neohesperidin@Mg, 74.12 kJ mol1 for naringin@Mg, and 100.00 kJ mol1 for mixture (ascorbic acid + naringin + neohesperidin)@Mg. The negative adsorption energy indicates that the adsorption is spontaneous, which is also consistent with aforementioned experimental results. The adsorption energy of mixture@Mg is much larger than that of singlecomponent@Mg suggesting the stronger adsorption. The stronger adsorption energy is beneficial for the formation of protective film. Correspondingly, the corrosion performance of mixture is better than
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Fig. 10. The upper panel calculated partial density of states for clean Mg (101(-)0) surface and total density of states for isolated (a) ascorbic acid, (b) naringin, (e) neohesperidin and (f) ascorbic acid + naringin + neohesperidin. The lower panel calculated partial density of states of s and p orbitals for Mg (101(-)0) surface and (c) ascorbic acid, (d) naringin, (g) neohesperidin and (h) ascorbic acid + naringin + neohesperidin for inhibitor@Mg(100) systems.
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one of component. Actually, the real condition is more complicate including Mg, MgO, Mg(OH)2, and other components. It is impossible to build a model to include every components. However, it is enough to determine the relative sequence of each components and mixture of them by employment of pure Mg surface. When the MgO surface is employed, the adsorption energies are positive. The component could not be adsorbed spontaneously with the value of 233.15 kJ mol1 for ascorbic acid@MgO, 52.41 kJ mol1 for neohesperidin@MgO, and 15.92 kJ mol1 for naringin@MgO. The relative sequence among them is still kept, which is in good agreement with that calculated on the pure Mg surface. The adsorption of naringin is easier than other two components with the less energy cost. The inaccuracy is inevitable in the calculation, the absolute value is no meaning to do the judgment. Only the comparison could be performed among various components. 3.8. Inhibition mechanism According to the above results, the following inhibition mechanism is conjectured. The OPE move towards the Mg alloys and is adsorbed on it to form the thin film, which has been testified by the FTIR and theoretical results. Taken three known components as example, all of them have the ring structure in their structures, which would form the p-p* interaction with the Mg alloys. Moreover, the O atom in them would form the interaction with Mg alloys by affording the lone pair electrons. The corrosion of Mg would increase the local acidity, which would arouse the prontonation of OPE. Then, the prontonated composition is favorable to interact with Mg/MgO/Mg(OH)2 to form the more solid interactions, which affords the stronger protection as compared with the adsorbed inhibitors. As compared with ascorbic acid, naringin and neohesperidin have the more rings and -OH groups leading to the stronger interactions with Mg surface. Correspondingly, they play the more important role in inhibition, which has been confirmed by the electrochemcial measurements and theoretical calculations. However, the performance of naringin and neohesperidin is comparable with the similar electrochemcial results because of their similar structures. When they are blended with other components, the situation is more complex since there are cooperation and competition among them. Conclusion EIS and polarization curves display that OPE would effectively suppress the Mg alloys corrosion up to the threshold of 85.7% with the relative low concentration of 0.030 g L 1. Further increasing concentration would not be beneficial for the refinement of inhibition efficiency. As compare with the pure three selected compounds, OPE presents the better inhibition performance. When three compounds are mixed together, their inhibition efficiency would be comparable with that of OPE at the same concentration of 0.030 g L 1. However, the cost for pure compounds is much high. After the magnesium alloys is immersed in OPE for 36 h, the inhibition efficiency is 82.7% suggesting that the protective film has been formed on the surface. The electrochemical results is further confirmed by SEM, AFM, XRD, and FTIR measurements. The adsorption obeys the Langmuir adsorption behavior with both chemical and physical adsorption feature. DFT results indicate that the mixture of three compounds is easier to form stronger interaction with magnesium surface as compared with single one, which supports the electrochemical results. In summary, utilization of plants waste as inhibitor is much favorable than synthesized organic compounds with cheap, easy available, and environmental acceptable. It is deserved to be greatly prompted in future, which would ultimately replace the organic inhibitors.
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Declaration of competing interest None Acknowledgments We thank National Center for Quality Supervision and Inspection of Magnesium and Magnesium Alloy Products for providing Mg alloys and the National Supercomputing Center in Shenzhen (Shenzhen Cloud Computing Center) for providing computational resources and softwares. This work was supported by the National Natural Science Foundation of China (21975064), Program of Henan Center for Outstanding Overseas Scientists (GZS2020011), Henan University's firstclass discipline science and technology research project (2018YLTD07, 2018YLZDYJ11, 2019YLZDYJ09), and the Excellent Foreign Experts Project of Henan University. Supplementary materials Supplementary material associated with this article can be found, in the online version, at doi:10.1016/j.jtice.2020.10.010. References [1] Esmaily M, Svensson JE, Fajardo S, Birbilis N, Frankel GS, Virtanen S, et al. Fundamentals and advances in magnesium alloy corrosion. Prog Mater Sci 2017;89:92–193. [2] Zeng Z, Stanford N, Davies CHJ, Nie J-F, Birbilis N. Magnesium extrusion alloys: a review of developments and prospects. Int Mater Rev 2018;64:27–62. [3] Atrens A, Song G-L, Liu M, Shi Z, Cao F, Dargusch MS. Review of recent developments in the field of magnesium corrosion. Adv Eng Mater 2015;17:400–53. [4] Lyon SB, Bingham R, Mills DJ. Advances in corrosion protection by organic coatings: what we know and what we would like to know. Prog Org Coat 2017;102:2–7. [5] Dinodi N, Shetty AN. Alkyl carboxylates as efficient and green inhibitors of magnesium alloy ZE41 corrosion in aqueous salt solution. Corros Sci 2014;85:411–27. [6] Feng Z, Hurley B, Li J, Buchheit R. Corrosion inhibition study of aqueous vanadate on Mg alloy AZ31. J Electrochem Soc 2018;165:C94–C102. [7] Jayaraj J, Raj SA, Srinivasan A, Ananthakumar S, Pillai UTS, Dhaipule NGK, et al. Composite magnesium phosphate coatings for improved corrosion resistance of magnesium AZ31 alloy. Corros Sci 2016;113:104–15. [8] Su H, Liu Y, Gao X, Qian Y, Li W, Ren T, et al. Corrosion inhibition of magnesium alloy in NaCl solution by ionic liquid: synthesis, electrochemical and theoretical studies. J Alloy Compd 2019;791:681–9. [9] Deyab MA. Inhibition activity of Seaweed extract for mild carbon steel corrosion in saline formation water. Desalination 2016;384:60–7. [10] Deyab MA. Egyptian licorice extract as a green corrosion inhibitor for copper in hydrochloric acid solution. J Ind Eng Chem 2015;22:384–9. [11] Deyab MA. Corrosion inhibition of aluminum in biodiesel by ethanol extracts of Rosemary leaves. J Taiwan Inst Chem E 2016;58:536–41. [12] Qiang Y, Zhang S, Tan B, Chen S. Evaluation of Ginkgo leaf extract as an ecofriendly corrosion inhibitor of X70 steel in HCl solution. Corros Sci 2018;133: 6–16. [13] Bahlakeh G, Ramezanzadeh B, Dehghani A, Ramezanzadeh M. Novel cost-effective and high-performance green inhibitor based on aqueous Peganum harmala seed extract for mild steel corrosion in HCl solution: detailed experimental and electronic/atomic level computational explorations. J Mol Liq 2019;83:174–95. [14] Bouknana D, Hammouti B, Jodeh S, Bouyanzer A, Aouniti A, Warad I. Aqueous extracts of olive roots, stems, and leaves as eco-friendly corrosion inhibitor for steel in 1 MHCl medium. Int J Ind Chem 2015;6:233–45. [15] Abiola OK, Otaigbe JOE. The effects of Phyllanthus amarus extract on corrosion and kinetics of corrosion process of aluminum in alkaline solution. Corros Sci 2009;51:2790–3. [16] Umoren SA, Solomon MM, Obot IB, Suleiman RK. A critical review on the recent studies on plant biomaterials as corrosion inhibitors for industrial metals. J Ind Eng Chem 2019;76:91–115. [17] M’hiri N, Veys-Renaux D, Rocca E, Ioannou I, Boudhrioua NM, Ghoul M. Corrosion inhibition of carbon steel in acidic medium by orange peel extract and its main antioxidant compounds. Corros Sci 2016;102:55–62. [18] Perdew JP, Burke K, Ernzerhof M. Generalized gradient approximation made simple. Phys Rev Lett 1996;77:3865–8. r R, Berg M, Carle n L, Jakobsson B, Nore n B, Oskarsson A, et al. K+ emission in [19] Elme symmetric heavy ion reactions at subthreshold energies. Phys Rev Lett 1996;77:4884–6. [20] Goerigk L, Grimme S. A thorough benchmark of density functional methods for general main group thermochemistry, kinetics, and noncovalent interactions. Phys Chem Chem Phys 2011;27:6670–88. [21] Grimme S, Antony J, Ehrlich S, Krieg H. A consistent and accurate ab initio parametrization of density functional dispersion correction (DFT-D) for the 94 elements H-Pu. J Chem Phys 2010;132:154104 (1-15).
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[29] Cao C. On electrochemical techniques for interface inhibitor research. Corros Sci 1996;38:2073–82. [30] Sekine I, Nakahata Y, Tanabe H. The corrosion inhibition of mild steel by ascorbic and folic acids. Corros Sci 1988;28:987–1001. [31] El Hamdani N, Fdil R, Tourabi M, Jama C, Bentiss F. Alkaloids extract of Retama monosperma (L.) Boiss. seeds used as novel eco-friendly inhibitor for carbon steel corrosion in 1M HCl solution: electrochemical and surface studies. Appl Surf Sci 2015;357:1294–305. [32] Schapink FW, Oudeman M, Leu KW, Helle JN. The adsorption of thiourea at a mercury-electrolyte interface. Trans Faraday Soc 1960;56:415–23. [33] Ikeda O, Jimbo H, Tamura H. Adsorption of tetramethylthiourea at the mercury/ water interface. J Electroanal Chem 1982;137:127–41. [34] ElAwady AA, Abd-El-Nabey BA, Aziz SG. Kineticthermodynamic and adsorption isotherms analyses for the inhibition of the acid corrosion of steel by cyclic and open-chain amines. J Electrochem Soc 1992;139:2149–54.
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Orange peel extracts as biodegradable corrosion inhibitor for magnesium alloy in NaCl solution: Experimental and theoretical studies Yaxu Wua,b,c, Yuanmi Zhanga,b,c, Yumiao Jianga,b,c, Yafeng Qiand, Xugeng Guoa,b,c,*, Li Wanga,b,c,*, Jinglai Zhanga,b,c,* a
Henan Engineering Research Center of Green Anticorrosion Technology for Magnesium Alloys, Henan University, Kaifeng, Henan 475004, PR China Henan Engineering Research Center of Corrosion and Protection for Magnesium Alloys, Henan University, Kaifeng, Henan 475004, PR China c College of Chemistry and Chemical Engineering, Henan University, Kaifeng, Henan 475004, PR China d National Center for Quality Supervision and Inspection of Magnesium and Magnesium Alloy Products, Hebi, Henan 458030, PR China b
A R T I C L E
I N F O
Article History: Received 27 July 2020 Revised 8 October 2020 Accepted 12 October 2020 Available online xxx Keywords: Corrosion AZ91D magnesium alloy Biodegradable inhibitor Electrochemistry Theoretical study
A B S T R A C T
Performance of orange peel extracts (OPE) to retard the magnesium alloys corrosion is thoroughly evaluated by Electrochemical impedance spectra (EIS) and potentiodynamic polarization curves. OPE would be an effective inhibitor with the efficiency of 85.7% at very low concentration of 0.030 g L 1. The inhibition effect of selected three pure components including in OPE is much inferior or comparable with that of OPE, whereas the cost of pure compounds is too expensive to be afforded. The protective self-assembly film would be formed on the magnesium surface by immersion in OPE. Structure characterization is performed by scanning electron microscopy (SEM), atomic force microscopy (AFM), and X-ray diffractometer (XRD). The adsorption of OPE on magnesium surface follows the Langmuir adsorption rule with both chemical and physical interactions. The interaction between three selected pure compounds and magnesium alloys is further calculated by density functional theory (DFT). Moreover, the possible inhibition mechanism is constructed on the basis of above results along with the FTIR measurements. The low cost and green inhibitor is explored in this work, which would replace the expensive inhibitors with the similar or better performance. © 2020 Taiwan Institute of Chemical Engineers. Published by Elsevier B.V. All rights reserved.
1. Introduction In past decades, magnesium alloys have been mainly applied in the region of aerospace and military [1,2]. Recently, their applied region is extended to automobiles, energy storage, infrastructure materials, and electronics due to the ideal weight-to-strength ratio, excellent electromagnetic shielding, and good machinability [2]. They are regarded as the promising candidate to replace heavy metal alloys in future. The predominant barrier to realize the aforementioned applications is that magnesium is vulnerable to corrosion. Numerous pathways have been developed to prevent the magnesium from corrosion [3]. Addition of inhibitors is a classical method to provide protection for metal with the advantages of easy operation, without special equipment, and economy. Organic inhibitors including N, O, S, and hetero rings in their structures and inorganic inhibitors, normally chromates and phosphates, have been applied for magnesium alloys [4-7]. Recently, ionic liquids also have been developed as the corrosion inhibitors [8]. Although all of them would suppress the corrosion with different * Corresponding author. E-mail addresses: [email protected] (X. Guo), [email protected] (L. Wang), [email protected] (J. Zhang).
degree, they have some common disadvantages, such as expensive and complicate synthesis and purification. More importantly, they are toxic and hazardous for human health and environment. Lots of efforts have been made to explore the abundant, easily available, biodegradable materials with non-environmental pollution due to more and more stringent restriction on environmental protection. Plant extracts including leaves, seeds, and stems, have been employed as the inhibitors for copper, steel, and aluminum alloy [9-11]. Ginkgo leaf, Peganum harmala seed, and Olive stem have been utilized as the inhibitors for steel in HCl solution reaching the inhibition efficiency of 92.5%, 95.0%, and 88.8%, respectively [12-14]. Similarly, Phyllanthus amarus extract has been reported as inhibitor for aluminum alloy in 2 M NaOH solution with the inhibition efficiency of 76.0% [15]. As compared with organic or inorganic inhibitors, plant extracts present better biocompatibility along with “zero” pollution. However, if they are large scale used, such as, roots, stems, and other plants, the forest and environment would be suffered resulting in other pollution problems [16]. If inhibitors would be extracted from the plant waste, they would not only be favorable to disposing them but also be beneficial to purifying the environment. Orange is popular fruit in the world market with delicious taste and high nutrition. However orange peel is discarded as waste, which is almost 50% of the total fruit mass. The thrown orange peel would
https://doi.org/10.1016/j.jtice.2020.10.010 1876-1070/© 2020 Taiwan Institute of Chemical Engineers. Published by Elsevier B.V. All rights reserved.
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result in serious environmental problem because of the rotting. Orange peel extracts (OPE) have presented inhibition behavior for carbon steel in acidic medium [17]. Three main components, ascorbic acid, naringin, and neohesperidin, (See Fig. S1) are determined by Nouha M’hiri et al., which is helpful to elucidate their role in the corrosion inhibition. Unfortunately, it is not discussed in previous literature. Their inhibition effect on the carbon steel is simply attributed to the antioxidant activities. There are O atoms, conjugated rings, and hydroxyl groups in OPE, which is similar with the reported organic inhibitors. The inhibitor would adsorb on the metal surface by donated electrons of O atoms and unoccupied orbitals of metal. In addition, the hydroxyl groups would react with magnesium on the surface, which would retard the corrosion of internal magnesium. Therefore, their inhibition behavior is deserved to be further determined. Moreover, the application scope of OPE should be further enlarged to other metals rather than only carbon steel. In this contribution, the inhibition behavior of OPE on AZ91D magnesium alloys has been investigated by electrochemical test along with scanning electron microscope (SEM), atomic force microscope (AFM), fourier transform infrared spectrometer (FTIR), and X-ray diffraction (XRD). The inhibition performance of OPE is compared with three major pure components included in OPE by both experimental and theoretical calculations. When magnesium alloys are immersed in OPE, the protective self-assembly film would be formed on the surface. Its inhibition performance is also investigated as compared to samples with addition of inhibitor directly. Finally, the possible inhibition mechanism is conjectured on the basis of the experimental and theoretical studies. It is expected that a novel green inhibitor for magnesium alloys would be confirmed from the waste, which possesses lots of incomparable advantages. 2. Experimental and theoretical details 2.1. Materials The employed working electrodes were prepared by AZ91D magnesium alloy with major composition of Mg as well as rest: Al, 8.7, Zn, 0.64, Mn, 0.023, Si, 0.014, Fe, 0.002, and Cu, 0.002 (wt.%). The electrodes were sealed in epoxy, leaving around 1 cm2 area exposed to the aggressive solution for electrochemical tests. Prior to each measurement, the samples (1.0 cm £ 1.0 cm £ 0.5 cm) were abraded by a series of metallographic abrasive paper with 100, 800, 1200, and 3000 grades to remove the oxides from the surface. After rinsed with distilled water, the electrodes were washed ultrasonically for 3 mins with anhydrous ethanol. Finally, the clean electrodes were dried in the air quickly. Raw orange (orange color) were bought from the local market in Henan province, China. The extract is obtained by refluxing. Then, OPE was dissolved in 0.05 wt.% NaCl solution and prepared for electrochemical and surface analysis tests. The extract was characterized by FT-IR (VERTEX 70, Bruker, USA) in the range of 4000500 cm1 through KBr disk technique. The 0.05 wt.% NaCl solution is employed to be corrosive environment, which is utilized to simulate the marine atmospheric environment. 2.2. Electrochemical tests The electrochemical measurements including EIS measurements and potentiodynamic polarization tests were performed with RST5000F electrochemical station. The traditional three electrode cell was employed including saturated calomel electrode (SCE) as reference, Platinum electrode as counter electrode (CE), and magnesium alloys as working electrode (WE). Before each electrochemical test, the electrode was allowed to be corroded freely and its open circuit potential (OCP) was recorded as a function of time up to 1000s to stabilize the system. The frequency range of 10 kHz to 1 Hz was applied
for EIS measurements with zero amplitude sinusoidal voltage of 7 mV at OCP. The potentiodynamic polarization was performed with a scan rate of 10 mV s 1 from 400 mV to 100 mV versus OCP. Each experiment was repeated at least three times to ensure the reproducibility. 2.3. Surface studies The structure characteristics of AZ91D Mg alloy samples were observed by SEM (SU3500, HITACHI, Japan) and AFM (Dimension ICON, Bruker, USA) techniques. The corrosion products on the surface was analyzed by XRD (SmartLab SE, Rigaku, Japan). During XRD analysis, the step size is 0.01° and the scan range is 580° along with a scanning rate of 10° min1. With exception of fresh sample, other samples were washed by chromic acid solution (200 g L1 CrO3 + 10 g L 1 AgNO3) in an ultrasonic bath for 1520 mins to remove the corrosion products. Following it, the sample are ultrasonically washed by three times in absolute ethanol for 5 mins/each time to remove chromic acid. Finally, they were cleaned with distilled water and dried in the air for test. To further confirm the possible groups existed on the surface, the corrosive products were also determined by FTIR. Before test, the Mg alloy species was immersed in the NaCl solution including 0.030 g L 1 OPE for ten days to get the enough corrosive products. 2.4. Theoretical details To explore the interfacial properties between inhibitor and the metal surface atom. The structures of inhibitor (three major components in OPE), Mg ð1010Þsurface, and inhibitor@Mg ð1010Þsurface were optimized by generalized gradient approximation (GGA) along with Perdew-Burke-Ernzerhof (PBE) method [18,19]. Then, the Grimme’s D3 dispersion correction was employed to further refine the energy [20,21]. All calculations were implemented using Vienna ab-initio software package (VASP) with energy cut-off of 500 eV and the force on each atom of 0.1 eV A1. Projected density of states (PDOS) were calculated at the same level based on the optimized geometries [22-25]. 3. Results and discussion 3.1. Characterize of orange peel extracts Three components, ascorbic acid, naringin, and neohesperidin, have been testified to be included in OPE in previous literature [17], which plays a critical role in inhibition. By the mass spectrum (MS), they are confirmed to be included in our OPE. Further, they are further measured by FTIR. The FTIR spectroscopy of OPE is shown in Fig. 1a. The strong and broad peak at 3400 cm1 is attributed to the OH stretching vibration. The -OH group is plentiful in three components, which is consistent with the strongest strength. The second strong peak at 1065 cm1 is associated with CO stretching vibration. The small sharp peaks at 2929 and 1642 cm1 correspond to CH and C=O stretching vibration. The vibration of aromatic ring is related with the small peaks around 1515 and 1270 cm1, while the vibration of aromatic CH group is related with peaks below 1000 cm1. The appearance of O atoms, conjugated rings, and -OH groups ensures the basic requirement for corrosion inhibitors. 3.2. Corrosion inhibition 3.2.1. Electrochemical impedance spectroscopy (EIS) The EIS spectra with and without different concentrations of extracts obey the one-time constant, which is fitted by the equivalent electronic circuit described in Fig. S2. The Cdl is replaced by CPE in equivalent electronic circuit because of the inhomogeneous metal
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3
Fig. 1. The FT-IR spectra of OPE (a) and the corrosive products on the metal surface immersed in 0.05 wt.% NaCl solution with 0.030 g L 1 OPE after ten days (b).
surface. The detailed definition of CPE and Cdl please refer to refs [26-28] a ZCPE ¼ Y1 0 ðjvÞ
ð1Þ
where Y0 is the CPE magnitude constant, a is the CPE exponent, j is an imaginary number, and v is the angular frequency. Cd1 ¼ Y0 ð2pf max Þa1
ð2Þ
where 2pfmax equals v, fmax is the maximum frequency. As shown in Fig. 2a, 0.030 g L 1 OPE is the threshold. Below it, the diameter of capacitive loops is increased with the enhancement of concentration indicating the larger inhibition performance. After that, the inhibition performance is decreased when the concentration is further increased. The similar conclusion could be deduced from Bode plots (Fig. S3a). The appearance of depressed semicircle indicates that capacitive layer is formed on the metal surface due to the adsorption of extract along with the roughness of metal surface. The detailed data fitted by EIS results are tabulated in Table 1. The variation of solution resistance (Rs) is slight and irregular for all concentrations indicating that the negligible effect of electrolyte on the corrosion. Polarization resistance (Rp) presents the similar variation rule with the change of semicircle diameter since the corrosion process is predominantly controlled by charge transfer process. The CPE exponent (a) is around 0.8 with slight differentiation, which is attributed to the similar inhibition mechanism with and without OPE. The highest inhibition efficiency is 85.7% with addition of 0.030 g L 1 OPE in 0.05 wt.% NaCl solution, which is defined by the following expression [29]: nE % ¼
RpðinhÞ Rp 100 RpðinhÞ
ð3Þ
where Rp(inh) and Rp are polarization resistance with and without OPE. The more extracts would be adsorbed on the metal surface with the increasing concentrations leading to the larger surface coverage (u ). However, the further more extracts would not be adsorbed on the metal surface due to the competition among them. Alternatively, some of them would react with the corrosive product resulting in some soluble products. As a result, the coating surface is reduced in contrast. The adsorption is a key item to suppress the corrosion.
3.2.2. Tafel polarization Polarization curves obtained for AZ91D Mg alloys measured in the absence and presence of OPE in 0.05 wt.% NaCl solution are presented Fig. 3a along with the fitting data in Table 2. With the addition of OPE, the corrosion current density (icorr) is greatly reduced in different extent. The corrosion potential (Ecorr) also varies with concentration of OPE, however, no clear trend is followed. The maximum shift of Ecorr happens for 0.030 g L 1 OPE and the minimum shift of Ecorr appears for 0.005 g L 1 OPE. All of Ecorr(s) present the mixed inhibition behavior with the shift less than 85 mV [30], however, the shift direction is not the totally same with both negative and positive directions. There is only slight change for both anodic and cathodic Tafel slopes (ba and bc) indicating that anodic and cathodic reactions are suppressed with the similar mechanism. Following the relationship of expression: np % ¼
icorr icorrðinhÞ 100 icorr
ð4Þ
where icorr and icorr(inh) are the corrosion current density in the absence and presence of inhibitors, respectively. The best inhibition efficiency of 84.0% is obtained for 0.030 g L 1 OPE in 0.05 wt.% NaCl solution. There is a good coherence between EIS and potentiodynamic polarization test for both the inhibition efficiency and inhibition behavior. 3.3. Immersion time According to the EIS and potentiodynamic polarization test, the involvement of 0.030 g L 1 OPE in 0.05 wt.% NaCl solution would get the best inhibition efficiency. For magnesium alloys, the inhibitor used in the solution is not our ultimate goal. The ideal inhibitor would form the film on the metal surface to provide more durable resistance. The fresh Mg alloys are immersed in 0.030 g L 1 OPE for 12 h, 24 h, 36 h, 48 h, and 72 h, respectively. Then, they are put in 0.05 wt.% NaCl solution without OPE to do the EIS and potentiodynamic polarization test along with the date presented in Fig. 2b, Fig. 3b, Fig. S3b, and Tables 3-4. The immersion samples present the similar results for both EIS and potentiodynamic polarization curves which is not stated out in detail. The inhibition performance is improved with the elongation of immersed time reaching the maximum value of 82.7% for 36 h. The inhibition efficiency is similar with that measured by addition of OPE indicating the formation of
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Fig. 2. Nyquist plot for AZ91D Mg alloy electrode in 0.05 wt.% NaCl solution (a) without and with different concentrations of orange peel extracts. (b) after different immersion times in 0.030 g L 1 orange peel extract-ethanol solution. (c) in the absence and presence of differently mixed inhibitors with the same concentrations of 0.030 g L 1. (A is ascorbic acid, B is neohesperidin and C is naringin).
Table 1 Electrochemical parameters obtained from Nyquist and Bode plots for AZ91D Mg alloy electrode in 0.05 wt.% NaCl without and with different concentrations of orange peel extracts. C (g L 1)
Blank 0.005 0.010 0.020 0.030 0.050 0.100 0.200
Rs (V cm2) 514.9 604.3 520.0 477.4 524.9 556.0 516.3 469.1
Rp (V cm2)
CPEdl (mV1 Sn cm2)
a
682.4 1705.0 4060.0 4391.0 4786.0 3693.0 2382.0 1455.0
27.9 24.2 21.7 22.5 18.5 19.4 19.2 20.3
0.84 0.88 0.87 0.86 0.87 0.90 0.89 0.88
protective film on the magnesium surface. After that, the inhibition efficiency is almost kept with slight fluctuation even if the immersion time is further elongated. Initially, the OPE does not have enough time to reach the Mg surface resulting in the less inhibition efficiency within 24 h. Therefore, the inhibition efficiency would be improved with the elongated immersion time. When the adsorbed extracts is enough on the surface, the inhibition efficiency would not be further enhanced with the elongated immersion time. The corrosive products on the surface is determined by the FTIR spectrum to testify that the inhibitor is actually adsorbed on the Mg alloys surface. As shown in Fig. 1b, the characteristic absorption peaks
Cdl (mV1 Sn cm2)
x2
13.1 15.8 15.1 15.6 12.9 14.4 13.2 12.6
9.60 £ 104 3.51 £ 104 1.51 £ 104 6.32 £ 104 7.27 £ 105 2.97 £ 104 5.23 £ 104 1.37 £ 103
hE
u
(%) 60.0 83.2 84.5 85.7 81.5 71.4 53.1
0.600 0.832 0.845 0.857 0.815 0.714 0.531
included in the OPE appears in the corrosive products indicating that the OPE has been adsorbed on the metal surface. Moreover, there is slight peak shift for the absorption peaks suggesting that there are interactions between OPE and Mg alloys, which is helpful to form the more dense film than the adsorption. 3.4. Comparison with single components According to the previous report, three main components, ascorbic acid, naringin, and neohesperidin, would play the critical role in suppressing the corrosion. They are also confirmed to be
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5
Fig. 3. Potentiodynamic polarization curves for AZ91D Mg alloy electrode in 0.05 wt.% NaCl solution (a) without and with different concentrations of orange peel extracts (b) after different immersion times in 0.030 g L 1 orange peel extract-ethanol solution (c) in the absence and presence of differently mixed inhibitors with the same concentrations of 0.030 g L 1. (A is ascorbic acid, B is neohesperidin and C is naringin).
Table 2 Electrochemical parameters obtained from potentiodynamic polarization curves for AZ91D Mg alloy electrode in 0.05 wt.% NaCl without and with different concentrations of orange peel extracts. C (g L 1)
Ecorr (V)
icorr (mA cm2)
bc (V dec1)
ba (V dec1)
hp (%)
Blank 0.005 0.010 0.020 0.030 0.050 0.100 0.200
1.45 1.44 1.46 1.47 1.48 1.44 1.45 1.45
12.20 5.21 2.10 2.00 1.95 2.33 4.33 6.07
0.096 0.072 0.068 0.068 0.068 0.068 0.069 0.072
0.098 0.076 0.082 0.084 0.086 0.071 0.080 0.075
57.3 82.8 83.6 84.0 80.9 64.5 50.2
included in this work. The corrosion performance of pure compound is compared with OPE under the same concentration. The corresponding results of EIS and potentiodynamic polarization curves are shown in Fig. 2c, Fig. 3c, Fig. S3c, and Tables 5-6. The EIS curves display the similar depressed curves for all tested samples indicating the similar corrosion mechanism controlled by charge transfer process. The Tafel curves also testified the similar conclusion that all of them are mixed corrosive behavior with a shift
towards cathodic direction less than 85 mV. Since EIS and potentiodynamic polarization curves have the consistent result, only the EIS results are discussed. When only single compound is included in 0.05 wt.% NaCl solution with the same concentration, the corrosion inhibition efficiency is only in a range of 74.478.7%, which is much less than that of OPE. If two of them are mixed together, there would be synergistic effect between them. The largest corrosion inhibition efficiency is enhanced to 83.2%, however, it is still less than that of OPE. For the single complex, naringin (78.7%) displays the better corrosion inhibition effect than other two complexes, ascorbic acid (74.4%) and neohesperidin (74.9%). The corrosion inhibition property is better when the naringin is combined with the other compound, such as ascorbic acid + neohesperidin (76.3%), naringin + ascorbic acid (78.6%), and naringin + neohesperidin (83.2%). Naringin is the key component to retard the corrosion, however, the difference between naringin and neohesperidin is too small to be deviate them. When three complexes are mixed together, the corrosion inhibition efficiency is increased to 87.2%, which is slightly larger than that of OPE by 1.5%. The price of naringin is expensive due to the complicate synthesis and purification steps. It is also difficult to extract it from
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Y. Wu et al. / Journal of the Taiwan Institute of Chemical Engineers 00 (2020) 112 Table 3 Electrochemical parameters obtained from Nyquist and Bode plots for AZ91D Mg alloy electrode in 0.05 wt.% NaCl solution after different immersion times in 0.030 g L 1 orange peel extract-ethanol solution. Time (h)
Rs (V cm2)
Rp (V cm2)
CPEdl (mV1 Sn cm2)
a
Blank 12 24 36 48 72
514.9 441.1 546.9 451.1 619.3 508.0
682.4 1125.0 2739.0 3943.0 3921.0 3917.0
27.9 21.0 18.9 16.9 18.9 14.8
0.84 0.87 0.88 0.89 0.88 0.90
Table 4 Electrochemical parameters obtained from potentiodynamic polarization curves for AZ91D Mg alloy electrode in 0.05 wt.% NaCl solution after different immersion times in 0.030 g L 1 orange peel extract-ethanol solution. Time (h)
Ecorr (V)
icorr (mA cm2)
bc (V dec1)
ba (V dec1)
hp (%)
Blank 12 24 36 48 72
1.45 1.45 1.44 1.45 1.46 1.46
12.20 7.36 2.62 1.98 2.07 2.15
0.096 0.072 0.067 0.070 0.068 0.076
0.098 0.075 0.081 0.089 0.082 0.098
39.7 78.5 83.8 83.0 82.4
Cdl (mV1 Sn cm2)
x2
hE
13.2 12.0 12.6 12.2 13.2 10.8
9.60 £ 104 1.90 £ 103 2.15 £ 103 1.33 £ 103 5.30 £ 103 1.09 £ 103
u
(%) 39.3 75.1 82.7 82.6 82.6
0.393 0.751 0.827 0.826 0.826
obtained for Langmuir isotherm model with the R2 of 0.99622 (See Fig. 4). The Kads is calculated according to the following equation [31-34]: Cinh
u
¼
1 þ Cinh Kads
ð5Þ
in which u is surface coverage, Cinh is inhibitor concentration. Consequently, the standard adsorption free energy ðDG0ads Þ is calculated on the basis of relationship of the following expression [32-34]: ! DG0ads 1 Kads ¼ exp ð6Þ CH2O RT
the orange peel. To utilize the OPE is the more advisable choice than to use the pure compounds.
The DG0ads is 32.41 kJ mol1 indicating the adsorption behavior is dominated by both physical and chemical adsorption.
3.5. Adsorption isotherms modeling
3.6. Surface studies
To further elucidate the adsorption behavior of OPE in NaCl solution, the electrochemical data is fitted by Temkin, Freundlich, Frumkin, and Langmuir models (See Fig. S4 and Fig. 4). The best fit is
The surface morphology would not only provide the intuitive result for the inhibition but also is favorable to make a direct justification. As shown in Fig. 5a, the fresh magnesium surface is smooth and
Table 5 Electrochemical parameters obtained from Nyquist and Bode plots for AZ91D Mg alloy electrode in 0.05 wt.% NaCl solution in the absence and presence of differently mixed inhibitors with the same concentrations of 0.030 g L 1.
Blank ascorbic acid neohesperidin naringin ascorbic acid+neohesperidin ascorbic acid+naringin neohesperidin+naringin ascorbic acid+ neohesperidin+naringin OPE
Rs (V cm2)
Rp (V cm2)
CPEdl (mV1 Sn cm2)
n
514.9 496.8 471.3 387.2 483.7 353.8 366.8 363.2 524.9
682.4 2667.0 2716.0 3201.0 2880.0 3189.0 4064.0 5343.0 4786.0
27.9 29.5 24.0 18.9 22.9 21.4 19.5 15.4 18.5
0.84 0.82 0.88 0.87 0.84 0.86 0.86 0.91 0.87
Cdl (mV1 Sn cm2)
x2
hE
13.2 16.5 16.5 12.5 13.6 13.8 12.6 11.9 12.9
9.60 £ 104 2.75 £ 104 7.18 £ 104 6.99 £ 104 9.30 £ 104 4.85 £ 104 4.58 £ 103 2.07 £ 102 7.27 £ 105
u
(%)
74.4 74.9 78.7 76.3 78.6 83.2 87.2 85.7
0.744 0.749 0.787 0.763 0.786 0.832 0.872 0.857
Table 6 Electrochemical parameters obtained from potentiodynamic polarization curves for AZ91D Mg alloy electrode in 0.05 wt.% NaCl solution in the absence and presence of differently mixed inhibitors with the same concentrations of 0.030 g L 1.
Blank ascorbic acid neohesperidin naringin ascorbic acid+neohesperidin ascorbic acid+naringin neohesperidin+naringin ascorbic acid+neohesperidin +naringin OPE
Ecorr (V)
icorr (mA cm2)
bc (V dec1)
ba (V dec1)
hp (%)
1.45 1.48 1.48 1.48 1.49 1.49 1.49 1.51 1.48
12.20 3.01 2.93 2.11 2.36 2.12 2.08 1.93 1.95
0.096 0.088 0.069 0.057 0.068 0.067 0.067 0.058 0.068
0.098 0.108 0.084 0.070 0.082 0.083 0.084 0.069 0.086
75.3 76.0 82.7 80.6 82.6 83.0 84.2 84.0
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Fig. 4. Langmuir adsorption isotherm for orange peel extracts on AZ91D Mg alloy in 0.05 wt.% NaCl solution at 298 K.
polished except for some visible scratches due to the abrading by emery paper in the sample preparation process. Deep cavities as well as lots of cracks are obviously visualized because of the
7
heavy corrosion in the solution free of inhibitor (See Fig. 5b). It is also consistent with the conclusion that corrosion of magnesium alloys has the pit corrosion behavior. AFM (See Fig. 6) reveals the great roughness of Mg surface with rolling high-altitude valley. There is great fluctuation with the average roughness of 53.9 nm. In contrast, the damage of surface is greatly decreased with average roughness of 8.0 nm (See Fig. 7) with 0.030 g L 1 OPE. The longitudinal view presents almost the straight curve. For SEM image (See Fig. 5c), abrasive scratches could also be observed with some small cracks, which would be deposited corrosive products or inhibitors. It is similar for the sample in 0.05 wt.% NaCl after immersing in 0.030 g L 1 OPE for 36 h except that the abrasive scratches are even more with slightly enhanced average roughness (See Fig. 5d and Fig. 8). Aforementioned three samples are also performed the XRD measurements (See Fig. 9). There are many Mg characteristic signals with the predominant one at 36.7°, which is almost the same for three samples. The inhibition influence is not obviously presented from the variation of Mg signals. The peaks related with Mg(OH)2 at 50.8° and 69.2° are obviously reduced suggesting that protection is formed with the addition of inhibitors. Since there are lots of components in OPE, other peaks are not easy to be detected. It is difficult to determine whether OPE are adsorbed on the Mg alloys surface. Perhaps, the theoretical calculations would help us to elucidate it.
Fig. 5. SEM images for the surfaces of AZ91D Mg alloy samples (a) after polishing and immersed in 0.05 wt.% NaCl solution for 6 days (b) without inhibitor; (c) with 0.030 g L 1 orange peel extracts; (d) after coating for 36 h in 0.030 g L 1 orange peel extract-ethanol solution.
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Fig. 6. AFM images in 2D (a) and 3D (b) formats of the AZ91D Mg alloy immersed in 0.05 wt.% NaCl solution for 3 days in the absence of inhibitors. Panel (c) is the height profile of the AZ91D Mg surface made along the marked lines on panel (a) by the Nanoscope v 1.80 software.
Fig. 7. AFM images in 2D (a) and 3D (b) formats of the AZ91D Mg alloy immersed in 0.05 wt.% NaCl solution for 3 days in 0.030 g L 1 orange peel extracts. Panel (c) is the height profifile of the AZ91D Mg alloy surface made along the marked lines on panel (a) by the Nanoscope v 1.80 software.
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9
Fig. 8. AFM images in 2D (a) and 3D (b) formats of the AZ91D Mg alloy immersed in 0.05 wt.% NaCl solution for 3 days after coating for 36 h in 0.030 g L 1 orange peel extract-ethanol solution. Panel (c) is the height profile of the AZ91D Mg surface made along the marked lines on panel (a) by the Nanoscope v1.80 software.
3.7. Theoretical calculation
Fig. 9. XRD patterns for AZ91D Mg alloy immersed in 0.05 wt.% NaCl solution for 6 days (a) without inhibitor; (b) with 0.030 g L 1 orange peel extracts; (c) after coating for 36 h in 0.030 g L 1 orange peel extract-ethanol solution.
Theoretical calculation would help us to deeply understand the interaction between inhibitor and magnesium surface. The single components, ascorbic acid, naringin, neohesperidin, and mixture of three components are chosen as the representative of OPE. It is also favorable to elucidate that the corrosion inhibition performance of naringin is better than other two components. The structures of single component, Mg ð1010Þ surface, and component@Mg ð1010Þ surface are optimized by PBE/GGA method (See Fig. S5). In our previous work, the different Mg surfaces are compared to determine the more stable one. The Mg ð1010Þ surface is the more stable surface, which is employed to represent the Mg alloys. Based on the optimized geometries, the corresponding PDOS is plotted in Fig. 10. Around the Femi energy level, the peak of PDOS is more flat after the inhibitor is adsorbed on the Mg surface, which indicates that there is strong interaction between inhibitor and Mg surface. The strong interaction would also be detected by the adsorption energy, which is 20.56 kJ mol1 for ascorbic acid@Mg, 57.45 kJ mol1 for neohesperidin@Mg, 74.12 kJ mol1 for naringin@Mg, and 100.00 kJ mol1 for mixture (ascorbic acid + naringin + neohesperidin)@Mg. The negative adsorption energy indicates that the adsorption is spontaneous, which is also consistent with aforementioned experimental results. The adsorption energy of mixture@Mg is much larger than that of singlecomponent@Mg suggesting the stronger adsorption. The stronger adsorption energy is beneficial for the formation of protective film. Correspondingly, the corrosion performance of mixture is better than
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Fig. 10. The upper panel calculated partial density of states for clean Mg (101(-)0) surface and total density of states for isolated (a) ascorbic acid, (b) naringin, (e) neohesperidin and (f) ascorbic acid + naringin + neohesperidin. The lower panel calculated partial density of states of s and p orbitals for Mg (101(-)0) surface and (c) ascorbic acid, (d) naringin, (g) neohesperidin and (h) ascorbic acid + naringin + neohesperidin for inhibitor@Mg(100) systems.
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one of component. Actually, the real condition is more complicate including Mg, MgO, Mg(OH)2, and other components. It is impossible to build a model to include every components. However, it is enough to determine the relative sequence of each components and mixture of them by employment of pure Mg surface. When the MgO surface is employed, the adsorption energies are positive. The component could not be adsorbed spontaneously with the value of 233.15 kJ mol1 for ascorbic acid@MgO, 52.41 kJ mol1 for neohesperidin@MgO, and 15.92 kJ mol1 for naringin@MgO. The relative sequence among them is still kept, which is in good agreement with that calculated on the pure Mg surface. The adsorption of naringin is easier than other two components with the less energy cost. The inaccuracy is inevitable in the calculation, the absolute value is no meaning to do the judgment. Only the comparison could be performed among various components. 3.8. Inhibition mechanism According to the above results, the following inhibition mechanism is conjectured. The OPE move towards the Mg alloys and is adsorbed on it to form the thin film, which has been testified by the FTIR and theoretical results. Taken three known components as example, all of them have the ring structure in their structures, which would form the p-p* interaction with the Mg alloys. Moreover, the O atom in them would form the interaction with Mg alloys by affording the lone pair electrons. The corrosion of Mg would increase the local acidity, which would arouse the prontonation of OPE. Then, the prontonated composition is favorable to interact with Mg/MgO/Mg(OH)2 to form the more solid interactions, which affords the stronger protection as compared with the adsorbed inhibitors. As compared with ascorbic acid, naringin and neohesperidin have the more rings and -OH groups leading to the stronger interactions with Mg surface. Correspondingly, they play the more important role in inhibition, which has been confirmed by the electrochemcial measurements and theoretical calculations. However, the performance of naringin and neohesperidin is comparable with the similar electrochemcial results because of their similar structures. When they are blended with other components, the situation is more complex since there are cooperation and competition among them. Conclusion EIS and polarization curves display that OPE would effectively suppress the Mg alloys corrosion up to the threshold of 85.7% with the relative low concentration of 0.030 g L 1. Further increasing concentration would not be beneficial for the refinement of inhibition efficiency. As compare with the pure three selected compounds, OPE presents the better inhibition performance. When three compounds are mixed together, their inhibition efficiency would be comparable with that of OPE at the same concentration of 0.030 g L 1. However, the cost for pure compounds is much high. After the magnesium alloys is immersed in OPE for 36 h, the inhibition efficiency is 82.7% suggesting that the protective film has been formed on the surface. The electrochemical results is further confirmed by SEM, AFM, XRD, and FTIR measurements. The adsorption obeys the Langmuir adsorption behavior with both chemical and physical adsorption feature. DFT results indicate that the mixture of three compounds is easier to form stronger interaction with magnesium surface as compared with single one, which supports the electrochemical results. In summary, utilization of plants waste as inhibitor is much favorable than synthesized organic compounds with cheap, easy available, and environmental acceptable. It is deserved to be greatly prompted in future, which would ultimately replace the organic inhibitors.
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Declaration of competing interest None Acknowledgments We thank National Center for Quality Supervision and Inspection of Magnesium and Magnesium Alloy Products for providing Mg alloys and the National Supercomputing Center in Shenzhen (Shenzhen Cloud Computing Center) for providing computational resources and softwares. This work was supported by the National Natural Science Foundation of China (21975064), Program of Henan Center for Outstanding Overseas Scientists (GZS2020011), Henan University's firstclass discipline science and technology research project (2018YLTD07, 2018YLZDYJ11, 2019YLZDYJ09), and the Excellent Foreign Experts Project of Henan University. Supplementary materials Supplementary material associated with this article can be found, in the online version, at doi:10.1016/j.jtice.2020.10.010. References [1] Esmaily M, Svensson JE, Fajardo S, Birbilis N, Frankel GS, Virtanen S, et al. Fundamentals and advances in magnesium alloy corrosion. Prog Mater Sci 2017;89:92–193. [2] Zeng Z, Stanford N, Davies CHJ, Nie J-F, Birbilis N. Magnesium extrusion alloys: a review of developments and prospects. Int Mater Rev 2018;64:27–62. [3] Atrens A, Song G-L, Liu M, Shi Z, Cao F, Dargusch MS. Review of recent developments in the field of magnesium corrosion. Adv Eng Mater 2015;17:400–53. [4] Lyon SB, Bingham R, Mills DJ. Advances in corrosion protection by organic coatings: what we know and what we would like to know. Prog Org Coat 2017;102:2–7. [5] Dinodi N, Shetty AN. Alkyl carboxylates as efficient and green inhibitors of magnesium alloy ZE41 corrosion in aqueous salt solution. Corros Sci 2014;85:411–27. [6] Feng Z, Hurley B, Li J, Buchheit R. Corrosion inhibition study of aqueous vanadate on Mg alloy AZ31. J Electrochem Soc 2018;165:C94–C102. [7] Jayaraj J, Raj SA, Srinivasan A, Ananthakumar S, Pillai UTS, Dhaipule NGK, et al. Composite magnesium phosphate coatings for improved corrosion resistance of magnesium AZ31 alloy. Corros Sci 2016;113:104–15. [8] Su H, Liu Y, Gao X, Qian Y, Li W, Ren T, et al. Corrosion inhibition of magnesium alloy in NaCl solution by ionic liquid: synthesis, electrochemical and theoretical studies. J Alloy Compd 2019;791:681–9. [9] Deyab MA. Inhibition activity of Seaweed extract for mild carbon steel corrosion in saline formation water. Desalination 2016;384:60–7. [10] Deyab MA. Egyptian licorice extract as a green corrosion inhibitor for copper in hydrochloric acid solution. J Ind Eng Chem 2015;22:384–9. [11] Deyab MA. Corrosion inhibition of aluminum in biodiesel by ethanol extracts of Rosemary leaves. J Taiwan Inst Chem E 2016;58:536–41. [12] Qiang Y, Zhang S, Tan B, Chen S. Evaluation of Ginkgo leaf extract as an ecofriendly corrosion inhibitor of X70 steel in HCl solution. Corros Sci 2018;133: 6–16. [13] Bahlakeh G, Ramezanzadeh B, Dehghani A, Ramezanzadeh M. Novel cost-effective and high-performance green inhibitor based on aqueous Peganum harmala seed extract for mild steel corrosion in HCl solution: detailed experimental and electronic/atomic level computational explorations. J Mol Liq 2019;83:174–95. [14] Bouknana D, Hammouti B, Jodeh S, Bouyanzer A, Aouniti A, Warad I. Aqueous extracts of olive roots, stems, and leaves as eco-friendly corrosion inhibitor for steel in 1 MHCl medium. Int J Ind Chem 2015;6:233–45. [15] Abiola OK, Otaigbe JOE. The effects of Phyllanthus amarus extract on corrosion and kinetics of corrosion process of aluminum in alkaline solution. Corros Sci 2009;51:2790–3. [16] Umoren SA, Solomon MM, Obot IB, Suleiman RK. A critical review on the recent studies on plant biomaterials as corrosion inhibitors for industrial metals. J Ind Eng Chem 2019;76:91–115. [17] M’hiri N, Veys-Renaux D, Rocca E, Ioannou I, Boudhrioua NM, Ghoul M. Corrosion inhibition of carbon steel in acidic medium by orange peel extract and its main antioxidant compounds. Corros Sci 2016;102:55–62. [18] Perdew JP, Burke K, Ernzerhof M. Generalized gradient approximation made simple. Phys Rev Lett 1996;77:3865–8. r R, Berg M, Carle n L, Jakobsson B, Nore n B, Oskarsson A, et al. K+ emission in [19] Elme symmetric heavy ion reactions at subthreshold energies. Phys Rev Lett 1996;77:4884–6. [20] Goerigk L, Grimme S. A thorough benchmark of density functional methods for general main group thermochemistry, kinetics, and noncovalent interactions. Phys Chem Chem Phys 2011;27:6670–88. [21] Grimme S, Antony J, Ehrlich S, Krieg H. A consistent and accurate ab initio parametrization of density functional dispersion correction (DFT-D) for the 94 elements H-Pu. J Chem Phys 2010;132:154104 (1-15).
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Please cite this article as: Y. Wu et al., Orange peel extracts as biodegradable corrosion inhibitor for magnesium alloy in NaCl solution: Experimental and theoretical studies, Journal of the Taiwan Institute of Chemical Engineers (2020), https://doi.org/10.1016/j.jtice.2020.10.010