Corrosion inhibition, surface adsorption and computational studies of Momordica charantia extract: a sustainable and green approach

The aerial parts extract of Momordica charantia plant were used for the corrosion resistance of carbon steel in the acidic medium (0.5 M H2SO4) utilizing weight loss method, Tafel and Electrochemical Impedance Spectroscopy. The state of mixed inhibitor adsorption on the carbon steel surface is shown by potentiodynamic polarization. M. charantia achieved the extraordinary inhibition efficiency of 93.51% at 500 mg/L of inhibitor concentration. Scanning electron microscopy and atomic force microscopy were used to know about the thin layer which was formed on the surface of carbon steel for its protection from corrosion and the adsorption of inhibitor was shown by UV–vis. spectroscopic technique. Fourier Transform Infrared Spectroscopy technique confirmed the existence of functional groups and the heteroatoms exhibit in the inhibitor. Adsorbance by the inhibitory molecules on the carbon steel surface followed the Langmuir adsorption isotherm. Hypothetical investigations (computational) showed a very valuable report. All acquired outcomes ensure that M. charantia extract can procedure an effectual preventing layer and restrict the corrosion procedure.


Introduction
Carbon steel is an alloy which can be easily found and have a lot of mechanical properties in its development, industrial process and its production [1]. We have used carbon steel here as carbon steel shows specific properties when it comes in contact of acidic medium [2, 3]. As we know that acidic medium is used by different modern processes like pickling, descaling and cleaning and in all these processes carbon steel get corroded. Green inhibitors adsorbed on the metal surface using π-electronic systems, and S, P, N, O [4]. They got adsorbed on the metal surface physically (physisorption) or/and chemically (chemisorption) and protect the metal from being corroded. This adsorption of inhibitory molecules can be chemical by creating a physical or covalent bond (chemisorption) or/and through electrostatic interaction (physisorption) [5]. Corrosion inhibitors are accepted to act by engrossing and making a defensive film structure on the metal surface. As corrosion is an electrochemical procedure, the adsorbed layer can be considered as a twofold layer framed at the interface between the electrolyte and the outside of the metallic electrolyte [6]. The idea of the dealing of adsorbed particles on the carbon steel surface is evaluated utilizing adsorption and thermodynamic models. Sulfuric corrosive has been utilized in acidifying, formation fracture and improving oil recuperation was used to imitate the corroding media. Genuine alloys and metals respond chemically or electrochemically with the destructive media to make a steady compost where metal misfortune happens [7]. The manure in this manner made is known as a corrosion product, and the metal surface becomes rusts. Corrosion includes the exchange of metal particles into the solution at active zones (anode), expulsion of electrons from metal in less dynamic regions (cathode), ionic current in the solution, and electric flow in metal. The cathodic procedure is required to give an electron acceptor as oxygen or oxidizing operators or hydrogen particles [8]. Right now, because of its inexhaustible, biodegradable, cheap and non-harmful properties, biomaterials of plant can be utilized as a natural and sustainable corrosion inhibitors, like fruits, seeds, plant waste, and so on [9]. The phytochemicals in plant have incredible potential as economical, nonpoisonous and inexhaustible sources of a wide scope of industrially significant natural chemical compounds. The yield of described green products as well as the corrosion reticence capabilities of the plant extracts differ extensively depending on the part of the plant and its location. Natural phytochemicals extracted from several portions of the plants are environmentally friendly, effective and economical [10]. A few investigations have been accounted for by different scientist utilizing such characteristic items as consumption inhibitor for various metals in various media [11][12][13][14][15][16][17][18][19][20]. But the vast majority of the natural corrosion inhibitors took higher inhibitor concentration to show their highest inhibition performance.
M. charantia generally grows up to 5 m (16 ft). This herb bears simple, alternate leaves 4-12 cm across, with 3-7 deeply separated lobes. It belongs to the Cucurbitaceae family and known as Bitter gourd, Bitter melon, Bitter squash, and Karela. It is commonly found all throughout India, China, East Asia, South Asia and Southeast Asia. Aerial parts of M. charantia are very easily available in the market. This plant contains Kuguacin-F, Cucurbitane triterpenoid and Pentanorcucurbitacin as its key phytochemicals [21,22]. Figure 1 shows the picture of M. charantia's prime phytochemical constituents.
The current observation is expected for the M. charantia extraction and study the anti-corrosion assets in the acidic corrosive media which depends upon weight loss measurements followed by electrochemical estimations. In addition, the inhibitor adsorption on the carbon steel surface has been examined employing SEM and AFM. The UV-vis. and FT-IR spectroscopic measures have been implied to verify the adsorption phenomenon and the presence of various functional groups. Toward confirm the experimental results, quantum parameters were generated per the density functional theory (DFT) method.

Weight loss studies
Weight loss (gravimetric) assessments were carried out following the ASTM G 31-72 method for 24 h [23]. All the assessments of weight loss were made at 298 ± 0.5 K utilizing a water circulated thermostat (PPI Fini X48). The size of the corrosive solution was 500 mL for the weight loss experiments. The pre-weighted carbon steel coupons were dipped in the corrosive media. After the immersion time, the carbon steel coupons were left out, washed through acetone, dehydratedusing nitrogen flow and finally weighted utilizing Shimadzu electronic balance (BL-220H/D455006313).

Electrochemical studies
The electrochemical techniques (Tafel and EIS) were conducted using the CHI760C electrochemical workstation. Examines were get done at 298 ± 0.5 K in 0.5 M sulfuric corrosive, including a few inhibitor concentrations. The size of the corrosive media was 250 mL for the electrochemical tests. A regular three-electrode assembly was applied. The carbon steel with a working territory of 1 cm 2 was utilized as the working electrode (WE). The platinum terminal utilized as an auxiliary electrode and the saturated calomel anode (SCE) fixed to a luggin capillary was utilized as the reference electrode. Prior to each investigation, the WE was drenched in the test solution for an hour to settle corrosion potential values ( E corr ). The Tafel bends were acquired at ± 250 mV versus SCE with an scanning rate of 1 mV/s. The EIS spectra were examined with a frequency change between 100 kHz to 0.01 Hz through an amplitude perturbation of 0.005 V [24].

Surface examinations
To know about the morphology carbon steel surface, AFM-Model: NT-MDT-INTEGRA followed by SEM-Model: LEO-435 VP techniques were used. The micrographs of pre-treated carbon steel coupons in 0.5 M sulfuric acid were examined, without and with the inhibitor extract [25]. After immersing the pre-treated carbon steel specimens from 0.5 M H 2 SO 4 solution without and with 500 mg/L extract for 24 h, the surfaces of carbon steel specimens were rinsed with ultrapure water and acetone, and dried.

Spectroscopic studies
The carbon steel specimen before and after the immersion in the test solution has been investigated by Shimadzu UV-1800 UV-visible absorption spectrophotometer. The obtained spectrums was used to explain the inhibition mechanism. The information of the presence of various functional groups was also obtained using FT-IR technique.
The plant extract blended with KBr pallet for FT-IR investigation in the Shimadzu FTIR 8400S spectrophotometer with wavenumber 400-4000 cm −1 [26].

Theoretical investigation
Theoretical findings were conducted to a meaningful understanding of the adsorption mechanism. Quantum chemical calculations were operated as theoretical investigations. It is well known that plant extracts contain several phytochemicals. For theoretical studies, we monitored the presence of the main phytochemical component in the M. charantia extract. Theoretical studies were performed using DFT (Density Functional Theory) in combination with the Hyperchem software package. Key factors are extracted from molecules whose structure is optimized [27]. The MD simulation was performed employing the Forcite module of Materials Studio 8.0 software developed by BIOVIA Inc. They

Spectroscopic analysis
FT-IR technique has been conducted to verify the several functional groups existing in the plant extract. FT-IR spectra of pure inhibitor extract has been shown in Fig. 2. Peaks obtained for different functional groups have been shown in Table 2. In pure inhibitor, peaks for different functional groups like; -OH, C-O and CH 2 confirmed the presence of heteroatoms in the plant extract.
The adsorption fact was also checked with UV-visible spectroscopy procedure. The analysis was done twice that is before and after the corrosion examination. The UV-vis. spectra of inhibitor in both the modes shown in Fig. 3. Prior to the corrosion measurement, the inhibitor solution shows an adsorption peak at 276.00 nm which shows the n-π* transition. It is clear from Fig. 3 that this peak reallocated afterwards the corrosion assessment. When looking at both spectra, a huge change in the adsorption band was discovered, which is associated with the inhibiting molecules adsorption on the outside of carbon steel [28].

Weight loss (gravimetric)and electrochemical measurements
The weight loss technique is the easiest procedure for obtaining a basic idea of the relation between the inhibitor concentration and corrosion resistance efficiency of the inhibitor. These experiments have been carried out at 298 ± 0.5 K for 24 h with various inhibitor concentrations.
The following comparison was employed to obtain the corrosion rate [29]: (1)   The outcomes acquired with the weight loss measurements are introduced in Table 3, which show an expansion in the inhibitor concentration, an abatement in the consumption rate and an expansion in the restraint effectiveness. The most noteworthy corrosion resistance value of 90.80% was accomplished at 500 mg/L. The outcomes were utilized to follow the Langmuir adsorption isotherm. Langmuir adsorption isotherm shows a chart between C/θ and C, appeared in Fig. 4(a), which shows a straight line with a regression coefficient (R 2 ) near 1. It verifies the development of a monolayer on the outside of the carbon steel. This can be depicted as [30]: Here • θ denotes the surface coverage • C denotes inhibition concentration • K ads denotes the equilibrium adsorption constant The polarization curves were obtained for the carbon steel in 0.5 M H 2 SO 4 at 298 ± 0.5 K nonappearance and existence of different concentrations of M. charantia. Figure 4b demonstrates the Tafel curves, starting with zero to different concentrations of M. charantia inhibitor in 0.5 M H 2 SO 4 . Obtained Tafel plots give the values for Corrosion current density ( i corr ), Corrosion potential ( E corr ), Cathodic and Anodic Tafel slopes ( c and a ), and corrosion inhibition efficiency (IE) applying the following equitation [31]: corr represents the corrosion current density without inhibitor • i i corr represents the corrosion current density with inhibitor Table 3 represents the polarization results for the carbon steel in the nonappearance and appearance of several inhibitor concentrations. The corrosion potential ( E corr ) is within 85 mV in relation to the blank, suggesting its behaviour as a type of mixed inhibitor. The Tafel bends show abatement in the incline of the anode and cathode bends with the expansion of the inhibitor. This is because of the way that the particles of the active inhibitor molecules are get adsorbed on the outside of the carbon steel and the reduction of the steel. With every expansion of the inhibitor concentration, the thickness of the corrosion current density ( i corr ) decreases constantly. a and c changed essentially with inhibitor concentration. This  demonstrates the adsorption of the dynamic segments of M. charantia was impacted both by the components of iron disintegration and by the systems of the oxygen decrease response. This is because of the solid coordination bond between the free iron orbital and the free electrons, which are available in the active parts of the To examination the impact of M. charantia extract concentration for the carbon steel in 0.5 M H 2 SO 4 at 298 ± 0.5 K, the electrochemical impedance spectroscopy technique was performed to obtain a stable state. E OCP (open circuit potential) was showing a stable steady state following an hour of immersion as shown in Fig. 4f. A charge transfer resistance ( R ct ), a double layer capacitance(C dl ) and a solution resistance ( R s ) are portions of the circuit. The outcomes are appeared in Table 3. Figure 4c shows Nyquist outlines for carbon steel without and with specific concentration of M. charantia. Figure 4d shows the Bode outlines for carbon steel and Fig. 4e shows the used equivalent circuit. The impedance values were acquired utilizing the accompanying equation [32]: Here • (R ct ) shows the charge transfer resistance with inhibitor • (R 0 ct ) shows the charge transfer resistance without inhibitor As the inhibitor concentration expands, C dl reduces and R ct increments, as appeared in Table 3. These outcomes affirm the high adsorption level of M. charantia on the outside of carbon steel. M. charantia indicated 93.51% hindrance effectiveness at 500 mg/L with R ct (242.22 Ω cm 2 ) and confirms its better performance against the corrosive medium.

Quantum chemical calculations
As it was discussed earlier that the plant extract contains various phytochemicals. Here, in the present study, three main phytochemicals of the M. charantia were selected for the computational studies. Figure 5 shows the optimized, HOMO and LUMO orbitals of the Kuguacin-F, Cucurbitane triterpenoid and Pentanorcucurbitacin. Followings equations have been used to calculate different quantum chemical parameters [33]: Here; • χ inh and η inh represents the electronegativity and hardness of inhibitor molecule • χ inh and η Fe means the electronegativity and hardness of iron Table 4 covers the fundamental assets of Kuguacin-F, Cucurbitane triterpenoid and Pentanorcucurbitacin, which are of enormous significance due to their impact on the collaboration of electrons among the inhibitor molecule and the carbon steel surface. As a conclusion, as the various molecule shows different values for different parameters, suggesting a mixture of different interactions may take place and the inhibitor as a mixture of various phytochemical performances as a mixed kind of inhibitor and support the experimental outcomes.

MD simulations
The dynamic process was performed and the whole system was balanced until the temperature and energy of the system were balanced [34]. The low energy adsorption configurations of the three inhibitors adsorbed on the Fe (110) surface are shown in Fig. 6. As can be seen in Fig. 6, the adsorption centres of the examined inhibitors on the surface of Fe (110) are the electrons of the benzene rings, the oxygen, sulphur and nitrogen atoms. Inhibitor molecules are adsorbed in a nearly flat orientation on the iron surface to maximize coating and surface contact, providing a strong interaction for the adsorbate/substrate system [35,36]. The strength of corrosion inhibitors absorbed on iron surface can be expressed by the adsorption energy (E ads ), so it will be very interesting to study the adsorption energies of inhibitors absorbed on iron surface when considered the inhibition performance. The adsorption energy in solution can be calculated by the following equations [37]:  where E total is the total potential energy of the system, which include iron crystal, the adsorbed inhibitor molecule and solution; E surf+water and E inh+water are the potential energies of the system without the inhibitor and the system without the iron crystal, respectively; E water is the potential energy of the water molecules [38]. The adsorption energies in this work were calculated from the average adsorption energy of the obtained equilibrium configurations. The E ads obtained are − 336.9, − 380.3 and − 413.4 kJ/mol for THA-H, THA-Br and THA-OCH3, respectively [39]. It is seen that the adsorption energies are negative and therefore spontaneous adsorption can be expected [40]. In general, the higher the absolute value of Eads, the stronger the interaction between the inhibitor and the metal surface [41]. It is obvious that THA-OCH3 has higher absolute values of E ads than THA-H and THA-Br, and, therefore, exhibits better inhibitory properties for carbon steel, which is in agreement with experimental results [42].  Figure 7b shows the corroded surface of carbon steel after a corrosion test without inhibitors. Rough micrographs can be seen because metals are very sensitive to corrosion and shows the highest surface roughness at (138.81 nm) and maximum height (2100 nm). • Figure 7c shows the carbon steel surface after a corrosion test with the inhibitor. This shows that the pres- This is much less than the results obtained without inhibitors. This shows that M. charantia absorbs on the metal surface, creating a protective layer to prevent corrosion on carbon surfaces [43].

The proposed mechanism
To decide the impact of resistance of M. charantia on the carbon steel surface with the presence of 0.5 M sulfuric acid, it is essential to compute the consequences of this test and associate them with the chemical, electronic and structural properties of the inhibitor particles [44]. The M. charantia contains numerous phytochemical substances that contain heteroatoms. These molecules behave as Lewis bases and structure coordination bonds with free d-orbital of iron and adsorb on the carbon steel surface, framing a defensive covering on the outside of the carbon steel to secure against corrosive media. Thus, it tends to be said that the connection between inhibitor particles and iron can be interceded by chemisorption and physisorption. Retro-donation can likewise be made utilizing electronic pai-electrons [45]. This phenomenon likewise been demonstrated by hypothetical examinations and the after effects of polarization [46][47][48][49]. Figure 8 shows the proposed system for carbon steel surface adsorption strategy by Cucurbitane triperpenoid.

Conclusion
The result of the anti-corrosion properties of M. charantia in the 0.5 M H 2 SO 4 corrosive medium for carbon steel was investigated by electrochemical analysis followed by weight loss tests, SEM &AFM. Electrochemical examination evaluations verified that M. charantia signifies inhibition efficiency of more than > 93% at 500 mg/L to prevent corrosion of the carbon steel. Authentication of numerous functional groups including heteroatoms and unsaturation in the plant extract constituents was done with the help of FT-IR technique. Verification of the formation of coordination bonds between inhibitor molecules and Fe 2+ was done with the help of UV-vis. technique. The surface adsorption of M. charantia inhibitor was investigated by SEM supported by AFM technique. Computational investigation has been done using DFT and MD simulations for the main constituents.
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