Persian Liquorice extract as a highly efficient sustainable corrosion inhibitor for mild steel in sodium chloride solution

Accepted Manuscript Persian Liquorice extract as a highly efficient sustainable corrosion inhibitor for mild steel in sodium chloride solution

E. Alibakhshi, M. Ramezanzadeh, S.A. Haddadi, G. Bahlakeh, B. Ramezanzadeh, M. Mahdavian PII:

S0959-6526(18)33453-X

DOI:

10.1016/j.jclepro.2018.11.053

Reference:

JCLP 14812

To appear in:

Journal of Cleaner Production

Received Date:

20 June 2018

Accepted Date:

05 November 2018

Please cite this article as: E. Alibakhshi, M. Ramezanzadeh, S.A. Haddadi, G. Bahlakeh, B. Ramezanzadeh, M. Mahdavian, Persian Liquorice extract as a highly efficient sustainable corrosion inhibitor for mild steel in sodium chloride solution, Journal of Cleaner Production (2018), doi: 10.1016/j.jclepro.2018.11.053

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Persian Liquorice extract as a highly efficient sustainable corrosion inhibitor for mild steel in sodium chloride solution E. Alibakhshi1*, M. Ramezanzadeh1, S. A. Haddadi2, G. Bahlakeh3, B. Ramezanzadeh1, M. Mahdavian1 1

Surface Coating and Corrosion Department, Institute for Color Science and Technology, Tehran, Iran

2

Chemical and Petroleum Engineering Department, Sharif University of Technology, Tehran, Iran 3

Department of Engineering and Technology, Golestan University, Aliabad Katool, Iran

Abstract: The Persian Liquorice was introduced as a sustainable corrosion inhibitor with excellent inhibition action for mild steel in sodium chloride solution. Persian Liquorice is a root of Glycyrrhiza glabra including many active compounds like Glycyrrhizin (GL), 18βGlycyrrhetinic acid (GA), Liquritigenin (LTG), Licochalcone A (LCA), Licochalcone E (LCE), and Glabridin (GLD). The Fourier transform infrared (FT-IR) spectroscopy was utilized to track various active components exist in the Persian Liquorice extract. Electrochemical impedance spectroscopy, potentiodynamic polarization and electrochemical current noise measurements were conducted to investigate the corrosion inhibition role of various concentrations of Persian Liquorice extract toward mild steel corrosion in sodium chloride solutions. Surface analysis, molecular dynamics and quantum mechanics simulation methods

* Corresponding authors at: Surface Coating and Corrosion Department, Institute for Color Science and Technology, Tehran, Iran E-mail: [email protected], [email protected]

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were combined to get insights of inhibitor molecules adsorption on the mild steel surface. The electrochemical

investigations

revealed

that

in

the

extract the maximum inhibition efficiency of 98.8% was obtained, which is connected to the build-up of a protecting layer over the mild steel surface, blocking the pathway of harsh ions diffusion. There is no report on use of any sustainable corrosion inhibitor for mild steel in chloride solution with this high level of inhibition power even after long immersion time (72 h). The formation of protecting layer on the mild steel surface was proved by scanning electron microscope and atomic force microscope results. The results derived from MD simulations and QM calculations revealed the adsorption of Persian Liquorice components on the steel substrate via donor-acceptor interactions. Keywords: Persian Liquorice; sustainable corrosion inhibition; mild steel; electrochemical methods; MD simulation; QM computation.

1. Introduction Corrosion is a prohibitive issue for a diversity of industries. Detection and discounting the expenditure of metals corrosion has been of great interest to the corrosion engineers and scientists for multitude decades and it is still snowballing. Corrosion-resistant materials, corrosion inhibitors, anodic/cathodic protection, protecting coatings, corrosion inspection and monitoring tools have been fiercely used in abundant applications to protect the metallic structures from corrosion and reduce its cost (Khamseh et al., 2017; Maia et al., 2012; Plawecka et al., 2014). The popular and conventional corrosion protection strategy for safekeeping of metals is application of corrosion inhibitors. Despite the good inhibition activity of various

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inhibitors most of them are not environmentally safe (Bastos et al., 2006; Montemor, 2014; Sinko, 2001). Corrosion inhibitors are defined as the ability of substances to effectively reduce the metal corrosion rate when added at low concentrations to harsh environment (Samiee et al., 2018; Zheludkevich et al., 2010). They can be categorized into two domains of organic and inorganic inhibitors (Bauxbaum and Pfaff, 2005; Zaki Ahmad and Ahmad, 2006) according to chemical structure. Recently, many investigations have dealt with development of the synergistic action between the inorganic-organic corrosion inhibitors in various media (Alinejad et al., 2017; Bahlakeh et al., 2017b; Palimi et al., 2018, 2017). However, the inhibitors that have been mostly utilized in the industries are composed of some toxic compounds with a lot of criticisms due to their threat to human and environments. Therefore, the use and production (industrial scale) of various sustainable inhibitors have become the topic of many researches in recent years (Asipita et al., 2014; Qiang et al., 2017; Shubina et al., 2016). The modern context is to expand the eco-friendly and inexpensive corrosion inhibitors based on the components taken from the plant extracts as a ready renewable source. The significant inhibition performance of the sustainable corrosion inhibitors extracted from the plants for corrosion control of metals in acidic media has been studied by several research groups (Eiman Alibakhshi et al., 2018b; Ehsani et al., 2017; Liao et al., 2017). Nevertheless, most of these inhibitors do not adumbrate impressive inhibition performance in neutral saline mediums. Up to now, only a few studies have demonstrated the inhibition properties of the plant extracts in saline solutions (Salehi et al., 2017; Z. Sanaei et al., 2017). Sanaei et al. investigated the inhibition performance of Cichorium intybus L leaves extract in the 3.5 wt.% sodium chloride solution on mild steel (Z. Sanaei et al., 2017) and demonstrated that the Cichorium intybus L can provide

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promising corrosion inhibition action because of the multitude corrosion inhibitive compounds, like Caffeic acid, Flavone and Chicoric acid, existed in its structure. The corrosion inhibition action of the Nettle leaves extract with inhibitive molecules like Quercetin, Histamine, Caffeic acid and Serotonin has been also proved by Salehi et al. (Salehi et al., 2017). The inhibition power for Cichorium intybus L and Nettle leaves extract was 26% and 51%, respectively. It is noted that most of the green inhibitors are not effective in chloride solution except in the case of adding inorganic inhibitors (Salehi et al., 2017; Z. Sanaei et al., 2017). Although there are some reports on the application of plant extracts as a powerful source of corrosion inhibitors for metals in various medium but most of them are not readily available in large scale and cost-effective. Thus, it would not be economically acceptable to use them in industrial applications. In view of the above discussion, several factors including high inhibition efficiency, cost, easy availability and most importantly the safety to environment must be considered when choosing an inhibitor for a specific application. In our previous study, we have evaluated the inhibition action of Glycyrrhiza glabra extract, also known as Liquorice, sweet root or licorice, on mild steel in 1 M hydrochloric acid (Eiman Alibakhshi et al., 2018b). The results revealed that the green corrosion inhibitor present in this extract could remarkably reduce the corrosion rate of substrate. Deyab (Deyab, 2015) also studied the inhibition behavior of Egyptian licorice extract on the corrosion of copper in hydrochloric acid. The inhibition power for Egyptian licorice extract was 86.6 %. Persian Liquorice is a root of Glycyrrhiza glabra, which has been used in hepatoprotective, anti-inflammatory, antimicrobial and antiviral applications (Anagha et al., 2012; Bahmani et al., 2014). The active components found in the Persian Liquorice extract are useful for liver conservation in tuberculosis remedy and other medicinal therapies. Forasmuch as the above

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considerations and in perpetuation of our investigations on the expansion of green corrosion inhibitors, we have introduced the Persian Liquorice extract as a cheap, raw and highly efficient sustainable corrosion inhibitor for mild steel in 3.5 wt.% NaCl solution. Green corrosion inhibitors can be extracted from different parts of plants, i.e leaves, fruit, seeds and flower. However, only a small portion of the mentioned parts of plants includes active components with inhibitory value. Large efforts have been made in recent years for achieving high inhibition capacity by using corrosion inhibitor from green sources. However, the high cost of inhibitor extraction process has made the process too expensive which is not acceptable by industries. Not only the cost of plant but also the extraction efficiency can be important parameters when selecting a green inhibitor. Generally, the extraction efficiency is almost below 20% for most of the green inhibitors. This is due to poor solubility of most parts of the plants and also the poor inhibitory value of major parts of the plants. This means that large amount of plant is required to achieve acceptable amount of inhibitor powder. In addition, to extract the components which are not soluble in water the ethanol has been frequently used instead of water in extraction process. Apart from the negative impact of ethanol on environment the extraction process by water is more cost-effective than the ethanol extraction process. However, the Persian Liquorice is soluble in water, providing large amount of inhibitor powder during one-step extraction process, confirming the cost-effectiveness of Persian Liquorice application as inhibitor over all sustainable inhibitors reported in literature. Further, the importance of mild steel corrosion in a neutral saline medium is related to its industrial applications in heat exchangers and recirculating water systems. Also, it is worth noting that mild steel corrosion causes huge economic losses, energy losses, and safety issues. Therefore, it is significant and necessary to study the corrosion inhibition in neutral saline

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medium. Both electrochemical investigations and surface analyses are employed to interpret the inhibition mechanism of Persian Liquorice extract in chloride solution. Alongside the experiments, computational studies applying molecular dynamics (MD) and quantum mechanics (QM) calculations were done to assess the interfacial adsorption of sustainable inhibitive compounds on the mild steel surface.

2. Experimental 2.1. Raw materials and sample preparation Persian Liquorice and sodium chloride were obtained from the local market. Mild steel panels were provided from Foolad Mobarake steel Co. (Iran) and the corresponding chemical composition (wt.%) is shown in Table 1. Samples were mechanical polished by sand papers of different grades (from 400 to 1200), degreased by acetone (Merck Co.), washed with distilled water and dried in air before immersion. The 3.5 % NaCl solutions including 200, 400 and 600 ppm Persian Liquorice leaves extract (obtained from Glandulifera) was prepared for electrochemical and surface analysis tests. Table 1 2.2. Techniques The chemical structure of the Persian Liquorice leaves was investigated by FT-IR analysis by a Perkin Elmer Spectrum One. The corrosion inhibition performance of the various concentrations of Persian Liquorice on the mild steel corrosion in sodium chloride solution was studied by electrochemical impendence spectroscopy (EIS), polarization and electrochemical noise (EN) techniques. EIS and polarization measurements were implemented by a Compactstat electrochemical work station (Ivium, Netherlands) in a three electrode assembly with mild steel

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as working, Ag/AgCl (3M KCl) as reference and graphite as counter electrodes, respectively. The EIS studies were performed using an AC signal of 10 mV in the frequency sweep of 10 kHz10 mHz at the open circuit potential (OCP). Tafel plots were obtained in the potential window of -200 to +200 mV at OCP while the scan rate was 0.5 mV/s. The electrochemical current noise (ECN) was measured using 10 Hz frequency sampling for 800 s at 0.05 s intervals. The ECN measurements were monitored by Ivium Compactstat using two nominally identical samples (1 cm2) and reference electrode. The signal processing was carried out using a multi-resolution wavelet technique. A Philips scanning electron microscopy (SEM) Model XL30 and a Dualscope atomic force microscopy (AFM) model DS 95-200 were used to study the surface morphology of mild steel after immersion in 3.5% NaCl-Persian Liquorice solutions.

2.3. Theoretical details 2.3.1. Ab initio QM optimization Fig. 1 depicts the structure of some organic components comprising the Persian Liquorice corrosion inhibitor (Cheel et al., 2013; Nassiri Asl and Hosseinzadeh, 2008; Wang et al., 2015). The geometries of these compounds were optimized to find their lowest energy structures. For this purpose, the potential energies of all substances were minimized by the use of density functional theory in combination with B3LYP functional (Becke, 1993; Lee et al., 1988; Mclean and Chandler, 1980) and 6-311G** basis set, as successfully used in our recent researches (Bahlakeh et al., 2017b; Zahra Sanaei et al., 2017). The solvent effects were considered using PCM model and SCRF theory (Tomasi et al., 2005). The Gaussian 09 program package was used for QM calculations (Frisch et al., 2009). Subsequently, the HOMO-LUMO, partial atomic

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charges and Fukui indices were evaluated. The ChelpG method was utilized to calculate the inhibitors charges (Breneman and Wiberg, 1990). Fig. 1 2.3.2. Building dissolved inhibitor-iron cell The inhibitors adhesion onto steel was studied through MD simulations. The steel substrate was displayed using a (14 × 14 ) iron (110) supercell as utilized in preceding works (Bahlakeh et al., 2017a; Fu et al., 2012; Saha et al., 2015). The energy-minimized geometries of all six corrosion inhibiting substances were then placed above the highest atomic row of constructed Fe (110) substrate. Afterwards, a water layer containing 600 H2O molecules was inserted into the cell. 2.3.3. Classical MD simulations All constructed cells including Fe (110) surface, single inhibitor molecule and water molecules were initially optimized for 5000 steps by Smart minimizer algorithm and Forcite module implemented in Materials Studio software (S.D. Accelrys Software Inc., 2009). Then, the optimized cells were subjected to a 2000 ps (i.e., 2 ns) MD simulation which was carried out in NVT ensemble at 298 K and using time step of 1 fs. This simulation time was adopted so as to better equilibrate the simulation cells. The energy parameters required for modeling of potential energy were taken from COMPASS force field (Sun, 1998; Sun et al., 1998), with the exception of partial charges of all six inhibitor molecules determined from QM computations. The nonbonded interactions based on electrostatic and van der Waals terms were computed by atombased cutoff and Ewald schemes, respectively.

3. Results and discussion 3.1. Characterization of the Persian Liquorice structure

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FT-IR measurements have been done to identify the functional groups present in the chemical structure of the components present in the Persian Liquorice extract. The FT-IR spectrum is illustrated in Fig. 2. The intense peak observed at 3403 cm-1 is linked to the O-H stretching mode, which is an indicative of the presence of many hydroxyl groups in the Persian Liquorice. A peak appeared at around 1617 cm-1 is a signature of the C=O stretching. The presence of several peaks between 400-900 cm-1 are related to the ≡CH stretching vibration (Gerengi and Sahin, 2012). The other characteristic bands can be seen around 1050 cm-1 (C-O stretching), 1419 cm-1 (C=C stretching) and 2930 cm−1 (C-H stretching) (Liao et al., 2016). These observations clearly reveal that the Persian Liquorice is composed of many chemical compounds with hydroxyl, carboxylic and carbonyl functional groups, which may be involved in the mitigation of corrosion through coordination with iron atoms present on the mild steel surface. Fig. 2. 3.2. Corrosion inhibition evaluation 3.2.1. Electrochemical investigations The influence of Persian Liquorice extract on the inhibition performance of the sample in 3.5 wt.% sodium chloride electrolyte was investigated by EIS, polarization and EN tests. Mild steel specimens were exposed to the solutions with and without Persian Liquorice extract at different exposure times, and the inhibition behavior of the samples was studied in details by EIS analysis. Fig. 3 shows the Nyquist plots for various samples. The Nyquist plots reveal that the samples immersed in the solutions including Persian Liquorice exhibited larger semicircles during all immersion times as compared with those of blank solution, revealing the inhibition of the samples by Persian Liquorice.

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Fig. 3 Fig. 4 illustrates the Bode diagrams, reflecting the responses of the impedance of the specimens during 72 h exposure to the solutions with and without Persian Liquorice extract. The sample exposed to the solution without inhibitor (Fig. 4a) presents a slight decrement of the low frequency impedance as exposure time increased and the related plateau of the phase angle at high frequency declined. This could be attributed to the initiation of corrosion and the gradual formation of the corrosion products on the surface, which provides some level of protection against ongoing dissolution of the mild steel (E. Alibakhshi et al., 2018). The sample immersed in the 200 ppm Persian Liquorice containing solutions reached an impedance value close to 104 Ω.cm2 after 48 h of exposure, and this value reduced sharply after 72 h immersion. The sample exposed to 400 ppm Persian Liquorice containing solutions (Fig. 4c) showed an increase of impedance after 6 h and after that showed a reducing trend. However, the impedance modulus value at the lowest frequency after 72 h immersion is higher than that for the sample exposed to 200 ppm Persian Liquorice. This higher low frequency impedance is concerned to the Persian Liquorice inhibitor capability to form protecting layers over the surface. In the case of 600 ppm Persian Liquorice containing solutions, the impedance responses in the low frequency present an increasing trend over time. It seems that more inhibiting species are available at the active zones of the mild steel surface with increasing the Persian Liquorice concentration up to 600 ppm. This means that the addition of 600 ppm Persian Liquorice to 3.5% NaCl led to a higher corrosion inhibition performance. The more negative phase angle at high frequency of the sample exposed to 600 ppm Persian Liquorice is another result approving its better corrosion inhibition than other samples,

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indicating the more capacitive behavior of this system. It is worth noting that two time-constants were observed for the samples exposed to the chloride solution containing Persian Liquorice extract during the period of 72 h (except 200 ppm after 1 and 72 h immersion). Observation of two time-constants indicates that the active ingredients existed in the Persian Liquorice extract could form a protecting layer over the mild steel surface. Fig. 4 The electrical equivalent circuits (EEC) used to fit the EIS spectra are shown in Fig. 5. Fig. 5 Two EEC models were used to simulate the EIS data. The first model has been used to describe the corrosion process of the mild steel surface over the 72 h exposure to the solution without Persian Liquorice (blank) and the one containing 200 ppm Persian Liquorice after 1 and 72 h immersion. The second EEC model, as a general equivalent circuit, can be used to analyze the electrical behavior of the mild steel samples in the presence of Persian Liquorice. In these models, Rs represents solution resistance, Rf is the film resistance and CPEf is the constant phase element (CPE) representing the non-ideal capacitance of the protective layer. The electrochemical processes at the mild steel/electrolyte interface are described by Rct and CPEdl, where Rct is the charge transfer resistance and CPEdl represents the double layer capacitance. CPE is commonly used to depict the non-ideal capacitive behavior as a result of roughness and defects formed on the surface. Therefore, the CPE is used to account for the inhomogeneous structure of the inhibitor layer covering the surface or corrosion products. The CPE is defined using Y (a constant of the CPE element), and exponent n which indicates an ideal capacitor when is equal to one. To calculate the capacitance values, Eq. 1 was used: 𝐶 = (𝑌 . 𝑅

1‒𝑛

)

1 𝑛

(1) 11

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The fitting values of various elements in the EEC models are listed in Table 2. Table 2 The fitting results for the sample immersed in solution without Persian Liquorice (blank) show a decreasing trend of Rct with time. This suggests that the corrosion is taking place on the surface. Also, the corrosion products layer formed on the surface and the electrolyte diffusion to this layer led to the increment of Cdl. The rapid increase of Cdl and the correlated decrease in Rct can be observed for the samples exposed to 200 and 400 ppm Persian Liquorice containing solution during 72 h exposure. This reveals significant increase of dielectric constant with the rapid infiltration of electrolyte through the porous protective film to the mild steel substrate. It is noted that the Rct for the sample exposed to the 600 ppm Persian Liquorice containing solutions increased significantly from around 14.8 kΩ.cm2 to 50.8 8 kΩ. This could be linked to the formation of a compact inhibitor film on the metal surface, resulting in significant blocking of the corrosion processes. Moreover, this sample shows a significant decrease of Cdl to nearly 11.9 μF. cm-2. The further decline of Cdl could be resulted from the increase of double layer thickness due to the formation of complex compounds on the metal/solution interface (Nemati et al., 2018). The sample exposed to 600 ppm Persian Liquorice shows that the Rf increases over time and after 72 h exposure, the value is significantly higher than the Rf value obtained for the sample exposed to 400 ppm Persian Liquorice. This behavior can be related to the gradual densification of the inhibitor film by deposition of inhibitive components, increasing the corrosion inhibition performance. In the case of the solutions containing 600 ppm Persian Liquorice, the lower Cf obtained after 72 h exposure compared to its value for the solutions containing 400 ppm Persian Liquorice might be a result of thicker inhibitor film adsorbed. In other words, this behavior also

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could indicate the gradual replacement of water molecules by inhibitor molecules, forming a protective film at the mild steel/solution interface. The significant differences of the results derived from the EIS measurements (Rct, Cdl, Rf , and Cf) suggest that the highest level of persistently corrosion inhibition performance was obtained at 600 ppm Persian Liquorice extract concentration. The values of corrosion inhibition efficiency (η%) was also calculated according to Eq. 2 and the results are tabulated versus immersion time at different concentrations in Table 2.

 %  100  (1 

R p ,b R p ,i

(2)

)

where Rp is the polarization resistance (summation of Rct and Rf ) in the presence (i) and absence of inhibitor (b), respectively. As shown in Table 2, an increase of the Persian Liquorice concentration from 200 ppm to 600 ppm resulted in a continuous increase of inhibition efficiency. This suggests that the higher values of inhibitive compounds existed in the solution containing 600 ppm Persian Liquorice resulted in an excellent inhibition performance on the mild steel surface. It is interesting to note that most of the green extracts reported in the literature are not effective inhibitors in neutral saline solutions and show inhibition affects less than 50% in sodium chloride solution. Notably, the η% value after 72 h for this sample was ca. 98.8%, which is much higher than that reported for Urtica dioica leaves extract (around 26%) (Z. Sanaei et al., 2017), Nettle extract (around 69.9%) (Izadi et al., 2017), Morinda Citrifolia (around 76.9%) (Kusumastuti et al., 2017). So, Persian Liquorice is the most unique one and this result proves that the Persian Liquorice act as a highly efficient sustainable corrosion inhibitor for mild steel in sodium chloride solution.

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In this part, an inhibition mechanism was recommended for the Persian Liquorice based on the polarization measurements after 24 h exposure. Fig. 6 shows representative polarization plots for the samples after 24 h immersion in the absence and presence of various concentrations of Persian Liquorice. Corrosion current density (icorr), anodic Tafel slope (βa), cathodic Tafel slope (βc) and corrosion potential (Ecorr) values were obtained through Tafel extrapolation method (Table 3). The Tafel plots reveal that with the increase in Persian Liquorice concentration the anodic and cathodic current densities decreased, the Ecorr shifted to more negative values and both cathodic and anodic Tafel slopes were affected. These results confirm the mixed inhibition activity of the inhibitor. Accordingly, the Ecorr shifted to the cathodic branch direction but the changes in the anodic Tafel slopes were more significant, implying the inhibitor role on the anodic steel dissolution reaction mechanism change. The lowest icorr value was obtained at maximum concentration of inhibitor. In compliance with the EIS test results, the polarization data revealed that 600 ppm Persian Liquorice offered higher corrosion inhibition performance compared to other concentrations. These results could show the high capability of Persian Liquorice in mild steel corrosion rate reduction through adsorption and/or film formation on the active sites. Fig. 6 Table 3 ECN measurements were performed to assure sufficient elucidation of the corrosion inhibition performance of the samples dipped in the solution containing Persian Liquorice. Fig. 7 shows the undecimated wavelet transform (UWT) spectrum for the ECN signal of the samples after 72 h immersion. Low-frequency contributions can be seen for all of the samples, while Fig. 7 reveals the presence of high frequency component for the Persian Liquorice. The presence of

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these high frequency transients might be due to the localized corrosion on the metal during the corrosion action. This reflects that a protective film is formed on the surface, which is in agreement with EIS analysis results. Fig. 7 The relative energy contribution ( E dj ) of each crystal to the overall signal can be calculated by Eq. 3 (Eiman Alibakhshi et al., 2018a).

𝑑 𝐸𝑗 =

∑𝑛

2 𝑑 𝑘 = 1 𝑗,𝑘

∑𝑛

(3)

2

𝑥 𝑘=1 𝑘

where d insinuates the detail coefficient and x is the n-sample electrochemical signal. The E dj diagrams are provided in Fig. 8. For all of the samples, the maximum relative energy contributions are obtained at the position of the detail crystals d7 and d8, indicating the general corrosion for them. Another striking feature inferred from Fig. 8 is that the relative energy contribution of detail crystal d1 is bigger for the solution containing Persian Liquorice especially at concentration of 600 ppm compared to the blank solution. This could be linked to the localized corrosion arising from the formation of an uniform film on the surface (Aballe et al., 1999). Fig. 8 The total absolute energy of detail crystals (ET) of ECN signal can be written by Eq. 4 (Aballe et al., 1999). 8 𝑛 2 𝐸𝑇 = ∑𝑗 = 1∑𝑘 = 1𝑑𝑗,𝑘

(4)

The ET values follow the sequences: blank (18760 pA2) > 200 ppm (1664 pA2) > 400 ppm (1102 pA2 > 600 ppm (189.9 pA2). The much lower ET was obtained for the solution containing 600

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ppm Persian Liquorice in comparison with other samples, offering the lower ECN signal energy; therefore, the best corrosion inhibition performance. The higher inhibition performance at 600 ppm Persian Liquorice than other concentrations might be related to the higher values of inhibitive components, which concerned its capability to precipitate a protecting layer over the surface. In other words, these results suggest that the higher values of inhibitive components existed in the 3.5 % NaCl solution result in a high corrosion inhibition property on the mild steel. An inhibitor with excellent inhibition properties could be managed in blocking the path of aggressive ions intrusion into the surface. 3.2.2. Surface analysis investigations The morphology of the mild steel samples after 72 h immersion in 3.5% NaCl solutions with and without Persian Liquorice extract was observed by SEM and the results are displayed in Fig. 9. Fig. 9a demonstrates plane view of SEM micrograph of the blank (without inhibitor) sample which is characterized by a highly rough surface with corrosion products on the surface. This means that the blank steel surface is highly defective and depicted obvious degradation. Compared with the blank sample, the surface was effectively protected with introduction of Persian Liquorice inhibitor and the surface damage was not observed. In other words, the SEM observations revealed that an inhibitive layer was stranded on the surface that showed lower permeability for aggressive ions. Consequently, more protective and thicker film was observed as the concentration increased, validating the EIS and EN results. Fig. 9 The surface roughness values of the uninhibited and inhibited specimens exposed to 3.5 wt.% NaCl solution were further investigated by AFM. Fig. 10 illustrates 2D and 3D AFM micrographs of the samples immersed in blank solution and the one including 600 ppm Persian

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Liqorice. As shown, the mild steel surface was extremely damaged and corroded in the absence of Persian Liqorice because of the corrosive ions attack. The rough surface structure can also be observed for this sample. However, there are remarkable difference between the surface morphology of the samples in presence of Persian Liqorice and that of the blank sample, which presents that the inhibitive components from Persian Liqorice structure form protective films on the surface. This shows that these films separate the metal surface from aggressive environment and protect it from corrosion. The arithmetic average roughness (Sa) values for the mild steel samples exposed to the blank and 600 ppm Persian Liqorice containing solutions are 364 and 113 nm, respectively. In the presence of Persian Liqorice, the smooth surface covered with some particles was obtained. This could be related to the inhibitive components adsorption on the surface, which protects the mild steel from corrosion. These results confirmed the SEM and electrochemical tests results. As a result, the electrochemical test results confirmed the inhibition effects of Persian Liqorice in chloride solution on the corrosion of mild steel. Also, it was observed that the inhibitor concentration significantly influenced the inhibition performance. Fig. 10 3.3. Economic investigation of Persian Liquorice The search for innovative, environmental friendly and cost efficient corrosion inhibitors is still an ongoing process. As discussed above the Persian Liquorice extract (600 ppm) provided high inhibition efficiency of 98.8% toward mild steel substrate in 3.5 wt.% NaCl solution. This high inhibition capacity has not been reported for other green inhibitors in neutral chloride containing solution. In fact, most of the green compounds based on plants are not effective inhibitors for metal in chloride containing solutions and only provide high inhibition power in acidic media. In

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order to reach high inhibition power in chloride containing solution large amount of inhibitor is needed which make the process too expensive. In this section, the economic aspect of using Persian Liquorice as a sustainable corrosion inhibitor has been discussed. Liquorice plant is farmed in different geographical areas such as southern Europe and Asia. The Glycyrrhiza glabra root from which flavors and other components are extracted has a price between 0.3 – 0.5 $ per kg. The total price of Liquorice extraction process, including the cost of water and energy, is about 3 -3.5 $ per kg. It is worth noting that, as the Liquorice is 100% soluble in water, the Persian Liquorice extract preparation process has not negative impact on environment due to using water instead of ethanol. In addition, unlike most of other plants, the Persian Liquorice plant is easy-accessible and its extraction process is cost-effective and eco-friendly which makes it more appropriate to be effectively used as a source of an efficient sustainable inhibitor. In comparison with the Persian Liquorice extract, the synthetic (Bentiss et al., 2000; Frignani et al., 1999; Ravichandran et al., 2004) and conventional (Aljourani et al., 2009; Haddadi et al., 2018; Mahdavian and Ashhari, 2010) corrosion inhibitors which are considerably toxic in nature are more expensive and are effective inhibitors for acidic corrosion of metals not in chloride containing solution (Table 4). As can be seen in Table 4, the price of the synthesized corrosion inhibitors is much more than the green inhibitors. Consequently, despite the environmental considerations, the usage of the green inhibitors such as Persian Liquorice extract for inhibition of the metallic objectives in the harsh media is more cost-effective than synthetic inhibitors. Table 4 3.4. Theoretical calculations

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3.4.1. MD simulation results To realize atomic-level intuitions regarding the inhibition ability of Persian Liqorice, MD simulations of inhibitors adsorption onto Fe (110) surface representative of mild steel sheets were carried out. Before analyzing the MD simulation results the equilibration of simulated cells were assessed by using thermodynamics properties including temperature and potential energy. Fig. 11 depicts the time evolution of these properties within the last 2 ns of NVT MD simulations for all molecules. It can be seen from this figure that temperature and potential energy remained unchanged and their fluctuations throughout the 2 ns MD simulation time are slight, which can serve as an indication for equilibrium attainment (Chen et al., 2008; Sepahvandi et al., 2013). Fig. 12 illustrates the pictures for the final snapshots of each inhibitor molecules of LCA (Licochalcone A), LCE (Licochalcone E), LTG (Liquiritigenin), GA (18βGlycyrrhetinic acid), GL (Glycyrrhizin) and GLD (Glabridin) over the chosen crystallographic iron suface. Based on these visualized snapshots it is visible that all six corrosion inhibiting agents present in sustainable Persian Liquorice inhibitor are able to adsorb on the Fe (110) substrate, suggesting their propensity to shackle to surface and thereby form a corrosionprotecting layer. It may be noted that by continuing MD simulations for longer time periods all sustainable inhibitor compounds emerged as attached to metallic surface, implying their stabilized adhesion to mild steel adsorbent. According to the top and side views of these last snapshots, it can be seen that the molecular determination of all six adsorbed inhibitors embraced a parallel orientation relative to plane (110) of iron surface, which augments their surface coverage capability, which itself could lead to improved surface protection characteristics. Fig. 11 Fig. 12

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To quantitatively assess the surface attachment and corrosion-protective layer formation tendency of Persian Liqorice, the binding energy parameter (ΔEbinding), was determined for all inhibitors using the following formula of: ΔEbinding = Einhibitor/Fe – (Einhibitor + EFe). The quantity of Einhibitor/Fe shows the potential energy of whole system (i.e., inhibitor bound to iron surface), and the Einhibitor and EFe quantities are respectively indicative of the energy of the isolated inhibitor molecule and isolated Fe (110) surface. The obtained ΔEbinding values are presented in Fig. 13a. It is clear that the ΔEbinding of all six inhibitors are negative quantitatively signifying the adhesion of chosen sustainable corrosion inhibiting substances. It could be observed that the highest ΔEbinding founded in case of Glycyrrhizin which proposes its major role in protecting of mild steel in corrosive environments. To further analyze the interactions of inhibitor molecules with metallic Fe (110) substrate, the radial distribution function (RDF, g(r)) (Bahlakeh et al., 2017a) was examined for Fe atoms in the uppermost layer with regards to the oxygen atoms of all investigated inhibitors. The RDF results for each inhibitor are provided in panels (b) and (c) in Fig. 13. Based on the obtained results it is obvious that the RDFs of Fe atoms against inhibitors oxygen atoms yielded an intensified first peak. Such an observation points to the strong affinity of inhibitors oxygenated sites towards the iron atoms located in the outermost layer of metallic substrate. Fig. 13 3.4.2. QM calculation results To comprehend further detailed electronic-level details concerning the adhesion sites of inhibiting substances the first-principle QM calculations were conducted. It is well known that from an electronic point of view surface binding of corrosion inhibitors takes place through electronic donor-acceptor interactions of electron-rich regions of inhibitors with electron-

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deficient sites of metallic adsorbent. Within such interfacial binding mechanism, inhibitor electron-containing groups (e.g., aromatic rings, double and triple bounds and O, S, and P heteroatoms) give their electrons to empty d orbitals of metal atoms (Musa et al., 2012). According to the frontier molecular orbital, the ability of electron rich groups of inhibitor molecules to share electrons with surface metal atoms is linked to HOMO and LUMO distribution. The HOMO determines the active sites with the immense capability of electron donation, and on the other hand the LUMO reflects the inhibitor regions accepting the electrons of occupied metal orbitals. Fig. 14 provides the DFT/B3LYP/6-311G** optimized geometries as well as HOMO and LUMO of all six selected sustainable components acting as corrosion inhibitor. It is evident from the pictures for molecular orbitals in LCA and LCE inhibitors that the HOMO distributed on the benzene ring, methoxy, hydroxyl and carbonyl oxygen atoms, and carbon-carbon double bound. Such an observation suggests the ability of these active sites to share their electrons with surface Fe atoms, and thereby involve in interfacial donor-acceptor interactions. The carbon atoms of these reactive sites formed the LUMO, and thus could receive electrons from filled orbitals of iron atoms. In case of LTG, it is evident that the HOMO located over whole aromatic backbone and all three oxygenated moieties, implying Liquiritigenin attachment to steel surface via interaction of these active regions. Similar to LCA and LCE substances, the LUMO occurred on C atoms of one of the aromatic rings. Additionally, as shown in geometry-optimized structures of GA and GL species, the middle carbonyl oxygen along with its neighboring rings emerged as HOMO regions, while carbon atoms of these rings behaved as active zones for LUMO, evidencing their greatest capacity of electron sharing with d orbitals of surface iron atoms. Also, in Glabridin (GLD) compound of sustainable corrosion inhibitor, the aromatic benzen cycle together with its neighboring oxygen-bearing heterocyl acted as HOMO

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sites, and thus adhesion of Glabridin onto Fe (110) could happen via donor-acceptor interaction mechanism of these active centers. Fig. 14 In order to further explore the active sites of inhibitors affecting in donor-acceptor interactions, the Fukui indices for electrophilic and nucleophilic manner was analyzed and the graphical results are given in Fig. 15. As illustrated in this figure, in LCA and LCE materials, the one of the benzene cycles, O atom of oxygenated hydroxyl, methoxy and carbonyl moieties, and C=C double bond appeared as sites with electrophilic attacks, which in agreement with HOMO distribution in LCA and LCE indicates their propensity to supply delocalized π electrons and lone pair of electrons towards the empty d orbitals in Fe cations. In the lowest energy structure of LTG inhibitor, the electrophilic behavior happened almost on the one of aromatic rings, carbonyl oxygen and hydroxyl atoms. On the other hand, C atoms of the other ring and carbonyl fragment yielded nucleophilic feature. As a consequence, all backbone rings and oxygenated centers of LTG species are able to contribute in electron transfer required for inhibitor adsorption to metallic surface. Furthermore, on the basis of Fukui indices in GA and GL compounds it is clearly observed that the carbonyl O and its neighboring C atoms showed electrophilic interactions, an observation which is identical to corresponding HOMO outcomes in these materials. These theoretical findings further declare the strong propensity of carbonyl functionality to involve in donor-acceptor adhesion mechanism of inhibitors. In addition, as displayed in case of GLD component of sustainable Persian Liquorice inhibitor the electrophilic attack located on benzene cycle and oxygen-containing heterocycle, while carbon atoms of these two rings emerged as a nucleophilic site. As a result, identical to HOMO and LUMO pictures from the graphical distribution of Fukui indices it is concluded that the aromatic skeleton of

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GLD species could participate in its binding to steel substrate via electronic charge sharing at the metal-inhibitor interface. Fig. 15 4. Conclusion The excellent corrosion inhibition properties of the Persian Liquorice as a cheap sustainable inhibitor for mild steel in sodium chloride solution were shown by experimental studies. The EIS and ECN analyses justified that the Persian Liquorice mitigate metal corrosion through adsorption at the electrolyte/mild steel interface, thereby forming a protective layer on the surface. Corrosion inhibition performance significantly increased as the inhibitor concentration elapsed. It was found that in the presence of Persian Liquorice the maximum corrosion inhibition of about 98.8% was obtained even after 72 h immersion. According to these results one novel achievement of this study compared to the previous ones is that many of the synthetic and/or green corrosion inhibitors used for mild steel protection from corrosion in sodium chloride solution did not provide high inhibition efficiency at long exposure times but the green compounds of Persian Liquorice provided very high inhibition efficiency even at long exposure times. SEM and AFM analyses proved the film-forming capability of the Persian Liquorice molecules over the mild steel surface. Moreover, the theoretical outcomes confirmed the strong tendency of Persian Liquorice molecules adsorption on the surface.

Acknowledgment The authors gratefully thank the use of School of Computer Science, Institute for Research in Fundamental Science (IPM) as the computations were done there.

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Figure Captions: Fig. 1. Chemical structures of some of the active components existed in the Persian Liquorice composition. Fig. 2. FT-IR spectra of the Persian Liquorice. Fig. 3. Nyquist plots of mild steel in 3.5% NaCl solutions as a function of time; (a) without inhibitor (blank), (b) including 200 ppm, (c) 400 ppm and (d) 600 ppm of the Persian Liquorice extract. Fig. 4. Bode plots of the mild steel samples immersed in 3.5% NaCl solutions as a function of time; (a) without inhibitor (blank), (b) including 200 ppm, (c) 400 ppm and (d) 600 ppm of the Persian Liquorice extract. Fig. 5. Electrical equivalent circuit models used to fit EIS data for samples exposed to the blank and the inhibited solutions: (a) one time-constant (b) two time-constants. Fig.6. Polarization plots of the mild steel samples immersed in 3.5% NaCl solutions with and without Persian Liquorice extract after 24 h immersion. Fig. 7. UWT spectra of the ECN signals obtained for the blank (a), solutions containing 200 ppm (b) 400 ppm (c) and 600 ppm Persian Liquorice. Fig. 8. Energy distribution plot of the detail crystals obtained from ECN signals. Fig. 9. SEM micrographs of the mild steel in 3.5 % NaCl solution in absence and presence of Persian Liquorice after 72 h exposure. (a) blank (without Persian Liquorice), (b) solutions containing 200 ppm (c) 400 ppm and (d) 600 ppm Persian Liquorice. Fig. 10. 2D (left) and 3D (right) AFM micrographs of mild steel specimens after72 h exposure to the 3.5% NaCl solution (a) with 600 ppm Persian Liquorice and (b) without Persian Liquorice.

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Fig. 11. The time evolution of temperature and potential energy of chosen compounds of green inhibitor during the 2 ns NVT MD simulations. Fig. 12. The final snapshots of inhibitors over Fe (110) surface obtained at the end of 2 ns NVT MD simulations. Fig. 13. (a) The binding energies of inhibitor molecules adsorbed to Fe (110) substrate and (b) and (c) the RDFs of surface iron (Fe) atoms relative to inhibitors oxygen atoms. Fig. 14. The B3LYP/6-311G** optimized geometry, HOMO and LUMO of inhibitors. Fig. 15. The simulated Fukui indices of optimized inhibitors.

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Fig.1. Chemical structures of some of the active components existed in the Persian Liquorice composition (Cheel et al., 2013; Nassiri Asl and Hosseinzadeh, 2008; Wang et al., 2015).

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Fig.2. FT-IR spectra of the Persian Liquorice.

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Fig.3. Nyquist plots of mild steel in 3.5% NaCl solutions as a function of time; (a) without inhibitor (blank), (b) including 200 ppm, (c) 400 ppm and (d) 600 ppm of the Persian Liquorice extract.

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Fig.4. Bode plots of the mild steel samples immersed in 3.5% NaCl solutions as a function of time; (a) without inhibitor (blank), (b) including 200 ppm, (c) 400 ppm and (d) 600 ppm of the Persian Liquorice extract.

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Fig. 7. Bode Fig.5. Electrical equivalent circuit models used to fit EIS data for samples exposed to the blank and the inhibited solutions: (a) one time-constant (b) two time-constant.

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Fig.6. Polarization plots of the mild steel samples immersed in 3.5% NaCl solutions with and without Persian Liquorice extract after 24 h immersion.

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Fig.7. UWT spectra of the ECN signals obtained for the blank (a), solutions containing 200 ppm (b) 400 ppm (c) and 600 ppm Persian Liquorice.

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Fig.8. Energy distribution plot of the detail crystals obtained from ECN signals.

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Fig.9. SEM micrographs of the mild steel in 3.5 % NaCl solution in absence and presence of Persian Liquorice after 72 h exposure. (a) blank (without Persian Liquorice), (b) solutions containing 200 ppm (c) 400 ppm and (d) 600 ppm Persian Liquorice.

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Fig.10. 2D (left) and 3D (right) AFM micrographs of mild steel specimens after72 h exposure to the 3.5% NaCl solution (a) with 600 ppm Persian Liquorice and (b) without Persian Liquorice.

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Fig. 11. The time evolution of temperature and potential energy of chosen compounds of green inhibitor during the 2 ns NVT MD simulations.

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LCA

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Fig.12. The final snapshots of inhibitors over Fe (110) surface obtained at the end of 2 ns NVT MD simulations.

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Fig.13. (a) The binding energies of inhibitor molecules adsorbed to Fe (110) substrate and (b) and (c) the RDFs of surface iron (Fe) atoms relative to inhibitors oxygen atoms.

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Fig.14. The B3LYP/6-311G** optimized geometry, HOMO and LUMO of inhibitors.

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LCE

LCA

Electrophilic attack

Fig.15. The simulated Fukui indices of optimized inhibitors.

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

Persian Liquorice was introduced as a green corrosion inhibitor for mild steel.



Electrochemical, surface characterization methods are used for inhibition study.



The Persian Liquorice extract behaves as a mixed-type inhibitor in NaCl solution.



Active species in inhibitor structure is responsible for its excellent performance.



Inhibition performance of inhibitor was explained by theoretical calculations.

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Table 1. Composition of the mild steel sample Elements

Fe

C

Si

Mn

P

S

Cr

Mo

Co

Cu

wt%

balance

0.19

0.415

1.39

<0.005

< 0.005

0.026

0.018

0.0559

0.0429

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Table 2: Electrochemical parameters extracted from EIS data for the mild steel immersed in the blank solution and the Persian Liquorice extracts for various exposure times. Solution

Time (h)

Rcta (Ω.cm2)

CPEdl Y0b

nc

Cdl

Rfd

(µF.cm-2)

(Ω.cm2)

(μsn.Ω-1. cm-2) Blank (without inhibitor)

200 ppm

400 ppm

600 ppm

CPEf Y0b

Cf (µF.cm-2)

nc

η (%)

( μsn.Ω-1. cm-2)

1

1602

591.6

0.81

584.2

-

-

-

-

-

6

1390

702.1

0.80

697.8

-

-

-

-

-

48

810.4

1218.2

0.80

1214.3

-

-

-

-

-

72

720

1276.5

0.80

1249.8

-

-

-

-

-

1

6212

309.6

0.74

389.6

-

-

-

-

74.2

6

11240

204.6

0.91

222.2

2648

369.1

0.52

361.4

90

48

14650

120.6

0.73

148.9

75.7

64.5

0.71

7.3

94.5

72

6117

308.7

0.74

385.9

-

-

-

-

88.2

1

15820

69.4

0.80

71

66.8

19.1

0.73

1.6

90

6

16420

21.1

0.79

15.9

693.8

51.3

0.75

16.9

91.9

48

13950

99.3

0.76

110.1

802.9

144.2

0.64

42.8

94.6

72

9691

228

0.78

285.1

221.2

516

0.66

168.7

92.7

1

14780

92.8

0.76

102.5

124

16.5

0.66

0.7

89.2

6

29580

137.8

0.65

293.7

894.5

92.5

0.77

43.9

97.3

48

35420

130.1

0.79

195.3

1822

91.1

0.64

33.2

97.8

72

50810

12.2

0.95

11.9

9370

105.7

0.65

105.1

98.8

a. The standard deviation range for Rct values is between 1.5% and 6.5.0%. b. The standard deviation range for Y0 values is between 0.7% and 7.2%. c. The standard deviation range for n values is between 0.5% and 1.5%. d. The Standard deviation range for Rct values is between 1.8% and 6.4%.

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Table 3: Electrochemical parameters derived from Tafel plots of the mild steel samples immersed in the blank solution and the Persian Liquorice extracts after 24 h immersion. Solution

Ecorr vs. Ag/AgCl (mV)

icorr (µA/cm2)

βa (V/dec)

-βc (V/dec)

blank solution

-718±32.1

18.6 ± 0.4

0.21±0.008

0.43±0.03

200 ppm

-680±22.3

6.7 ± 0.3

0.09±0.006

0.21±0.03

400 ppm

-621±17.8

5.3 ± 0.3

0.10±0.006

0.17±0.04

600 ppm

-590±14.4

4.6±0.2

0.10±0.002

0.15±0.02

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Table 4: The cost and inhibition efficiency comparisons for some traditional corrosion inhibitors.

Type of corrosion inhibitor

Water solubility (mg L-1)

Cost per kg of inhibitor ($)

Type of substrate

Max corrosion inhibition efficiency (%)

Ref.

Benzimidazole

~ 2010

250

Mild steel

~ 74

(Aljourani et al., 2009)

2mercaptobenzimi dazole

3.5 wt. % NaCl

800

Mild steel

~ 53

(Haddadi et al., 2018)

2mercaptobenzox azole

100 - 140

1 M HCl

150

Mild steel

~ 91

(Mahdavian and Ashhari, 2010)

130

Austenitic stainless steel

~ 97

(Frignani et al., 1999)

Type of solution

Concent ration (mg L-1)

240

1 M HCl

~ 300

85 - 110

~ 117

1,2,3benzotriazole

~ 20000

220 - 270

0.1 M H2SO4

N-[1(benzotriazol1yl)methyl] aniline

Insoluble

3400 4000

3 wt. % NaCl

150

Brass

~ 85

(Ravichandran et al., 2004)

1-hydroxy methyl benzotriazole

1000 5000

3500 4000

3 wt. % NaCl

150

Brass

~ 91

(Ravichandran et al., 2004)

2,5 bis(2pyridyl)-1,3,4oxidazole

< 20

10000 ≤

0.5 M H2SO4

80

Mild steel

~ 64

(Bentiss et al., 2000)

2,5 bis(2pyridyl)-1,3,4oxidazole

< 20

10000 ≤

1 M HCl

80

Mild steel

~ 95

(Bentiss et al., 2000)