Corrosion Inhibition of Carbon Steel in HCl Solution by Some Plant Extracts

The strict environmental legislations and increasing ecological awareness among scientists have led to the development of “green” alternatives to mitigate corrosion. In the present work, literature on green corrosion inhibitors has been reviewed, and the salient features of our work on green corrosion inhibitors have been highlighted. Among the studied leaves, extract Andrographis paniculata showed better inhibition performance (98%) than the other leaves extract. Strychnos nuxvomica showed better inhibition (98%) than the other seed extracts. Moringa oleifera is reflected as a good corrosion inhibitor of mild steel in 1 M HCl with 98% inhibition efficiency among the studied fruits extract. Bacopa monnieri showed its maximum inhibition performance to be 95% at 600 ppm among the investigated stem extracts. All the reported plant extracts were found to inhibit the corrosion of mild steel in acid media.


Introduction
Among the several methods of corrosion control and prevention, the use of corrosion inhibitors is very popular.Corrosion inhibitors are substances which when added in small concentrations to corrosive media decrease or prevent the reaction of the metal with the media.Inhibitors are added to many systems, namely, cooling systems, refinery units, chemicals, oil and gas production units, boiler, and so forth.Most of the effective inhibitors are used to contain heteroatom such as O, N, and S and multiple bonds in their molecules through which they are adsorbed on the metal surface.It has been observed that adsorption depends mainly on certain physicochemical properties of the inhibitor group, such as functional groups, electron density at the donor atom, π-orbital character, and the electronic structure of the molecule.Though many synthetic compounds showed good anticorrosive activity, most of them are highly toxic to both human beings and environment.The use of chemical inhibitors has been limited because of the environmental threat, recently, due to environmental regulations.These inhibitors may cause reversible (temporary) or irreversible (permanent) damage to organ system, namely, kidneys or liver, or disturbing a biochemical process or disturbing an enzyme system at some site in the body.The toxicity may be manifest either during the synthesis of the compound or during its applications.These known hazardous effects of most synthetic corrosion inhibitors are the motivation for the use of some natural products as corrosion inhibitors.Plant extracts have become important because they are environmentally acceptable, inexpensive, readily available and renewable sources of materials, and ecologically acceptable.Plant products are organic in nature, and some of the constituents including tannins, organic and amino acids, alkaloids, and pigments are known to exhibit inhibiting action.Moreover, they can be extracted by simple procedures with low cost.In the present work, the authors have reviewed literature on green corrosion inhibitors.Many authors such as E. E. Ebenso, B. Hammouti, A. Y. El Etre, P. C. Okafor, E. Oguzie, and P. B. Raja, have contributed significantly to the green mitigation by investigating several plants and their different body parts as corrosion inhibitors.The reviews of the literature along with salient features are summarised in Table 1.
In a previous work, the authors have investigated the extract of plants, namely, Azadirachta indica (leaves), Punica The aqueous extract of the leaves of henna (lawsonia) as the corrosion inhibitor was reported in C steel, nickel and zinc in acidic, neutral and alkaline solutions, using the polarization technique [1] (2)

92.2
The temperature effects were investigated on mild steel corrosion in 2.0 M of HCl and H 2 SO 4 in the absence and presence of aqueous extract of fenugreek leaves (AEFLs) with the help of gravimetric method [2] (

97.4
The efficacy of an acid extracts of leaves of Sida rhombifolia L. as the corrosion inhibitor for mild steel in 1 M phosphoric acid medium using weight loss measurements, polarization, and electrochemical impedance spectral studies were investigated.It was found to be an effective corrosion inhibitor [9] (11) Ammi visnaga The inhibitive effect of the extract of Khillah (Ammi visnaga) seeds, on the corrosion of SX 316 steel in HCl solution using weight loss measurements as well as potentiostatic technique, was assessed.Negative values were calculated for the energy of adsorption indicating the spontaneity of the adsorption process [ (36)

Heinsia crinita
The paper provides information on the use of ethanol extract of Heinsia crinita as a corrosion inhibitor.Electrochemical studies such as polarisation and AC impedance spectra will throw more light on the mechanistic aspects of the corrosion inhibition [35] (37)

Dacryodis edulis
The inhibition of low-carbon-steel corrosion in 1 M HCl and 0. International Journal of Corrosion granatum (shell), and Momordica charantia as corrosion inhibitors on mild steel in 3% NaCl solution by chemical and electrochemical methods.Maximum inhibition efficiency of 86%, 82%, and 79% was obtained at a concentration of 6 mL/L, 3 mL/L and 1.2 mL/L, respectively.Azadirachta indica showed 97% antiscaling properties [39].Aqueous extracts of Cordia latifolia and Curcumin were investigated as corrosion inhibitors for mild steel in industrial cooling systems.The extracts showed maximum inhibition efficiency of 97.7% and 60%, respectively [40].
The inhibitive effect of the aqueous extract of Jasmin (Jasminum auriculatum) on corrosion of mild steel in 3% NaCl was investigated.It showed inhibition efficiency of 80%.It was found to be predominantly the anodic corrosion inhibitor [41].
The inhibitive effects of aqueous extracts of Eucalyptus (leaves), Hibiscus (flower), and Agaricus on the corrosion of mild steel for cooling-water systems, using tap water, have been investigated by means of weight loss (under static as well as dynamic conditions) and polarization methods.All the plant extracts were found to inhibit corrosion of mild steel following and their inhibitive efficiencies were in the order: Agaricus (85%), Hibiscus (79%), and Eucalyptus (74%) under the static test conditions.The inhibition efficiencies remain almost the same under the dynamic test conditions, which are nearer to field conditions.All the inhibitors (extracts) were found to follow Langmuir as well as Freundlich adsorption isotherms, that is, they inhibit corrosion through adsorption.Polarization measurements gave a similar order of inhibition efficiencies of plant extracts as that determined using the weight loss technique.Agaricus extract was found to be predominantly a cathodic inhibitor, while the extracts of Eucalyptus and Hibiscus were found to be mixed inhibitors [40].
Ascorbic acid in combination with DQ-2000 (aminotrimethyl phosphonic acid) and DQ-2010 (1-hydroxyethylidine 1,1-diphosphonic acid) was used to reduce the concentration of zinc in the blowdown of the cooling systems.All the inhibitors used were found to be effective.The maximum inhibition efficiency 99.2% was obtained with DQ-2010 100 ppm + Ascorbic acid 200 ppm concentration.Inhibitors follow Langmuir isotherm which showed that they inhibit corrosion through adsorption [42].

Experimental
Prior to all measurements, the mild steel specimens, having composition (in wt%) 0.076 C, 0.012 P, 0.026 Si, 0.192 Mn, 0.050 Cr, 0.135 Cu, 0.023 Al, 0.050 Ni, and the remainder iron, were polished successively with fine grade Emery papers from 600 to 1200 grades.The specimens were washed thoroughly with double-distilled water and finally degreased with acetone and dried at room temperature.The aggressive solution 1 M HCl was prepared by dilution of analytical grade HCl (37%) with double-distilled water, and all experiments were carried out in unstirred solutions.
AC impedance (EIS) measurements and potentiodynamic polarization studies were carried out using a GAMRY PCI 4/300 electrochemical work station based on ESA 400.Gamry applications include EIS 300 (for EIS measurements) and DC 105 software (for corrosion) and Echem Analyst (5.50 V) software for data fitting.All electrochemical experiments were performed in a Gamry three-electrodes electrochemical cell under the atmospheric conditions with a platinum counter electrode and a saturated calomel electrode (SCE) as the reference electrode.The working electrode mild steel (7.5 cm long stem) with the exposed surface of 1.0 cm 2 was immersed into aggressive solutions with and without inhibitor, and then the open circuit potential was measured after 30 minutes.EIS measurements were performed at corrosion potentials, E corr , over a frequency range of 100 kHz to 10 mHz with an AC signal amplitude perturbation of 10 mV peak to peak.Potentiodynamic polarization studies were performed with a scan rate of 1 mVs −1 in the potential range from 250 mV below the corrosion potential to 250 mV above the corrosion potential.All potentials were recorded with respect to the SCE.

Leaves Extract as Corrosion Inhibitors.
The leaves extract of Andrographis paniculata, Murraya koenigii, and Aegle marmelos were investigated as corrosion inhibitors by weight loss and electrochemical methods in the present study.Among the studied leaves extract, Andrographis paniculata showed better inhibition performance than the other leaves extract.The result is summarized in Table 3 and Figure 1.The order of their inhibition efficiency has been found as follows: Andrographis paniculata > Murraya koenigii > Aegle marmelos. ( The higher inhibitive performance of Andrographis paniculata is due to the presence of delocalized π-electrons.This extensive delocalized π-electrons favours its greater adsorption on the mild steel surface, thereby giving rise in very high inhibition efficiency (98.1%) at a concentration of 1200 ppm the relatively better performance of Murraya koenigii (96.7%) at 600 ppm than Aegle marmelos (96.2%) at 400 ppm.The most pronounced effect and the highest R ct value (491.0 ohm cm 2 ) was obtained by inhibitor Andrographis paniculata at 1200 ppm concentration.The lowest R ct value (264.8 ohm cm 2 ) was obtained by inhibitor Aegle marmelos.The high R ct values are generally associated with a slower corroding system.These data revealed that R ct values increased after the addition of inhibitors, and on the other hand, C dl values decreased.This situation was a result of the adsorption of inhibitors at the metal/solution interface.Terminalia arjuna 88.9 (1) HO A decrease in local dielectric constant and/or an increase in the thickness of the electrical double layer can cause this decrease in C dl values, suggesting that the water molecules (having high dielectric constant) are replaced with inhibitor molecules (having low dielectric constant).It is worth noting that the percentage inhibition efficiencies obtained from impedance measurements were reasonably in a good agreement with those obtained from weight loss measurements.

Seed Extracts as Corrosion Inhibitors.
We have used seed extracts of Strychnos nuxvomica, Pongamia pinnata, and Syzygium cumini in our present study.The result is concluded in Table 4 and Figure 2. The order of their inhibition efficiency has been found as follows: Str ychnos nuxvomica > Pongamia pinnata > Syzygium cumini. ( The best performance of Strychnos nuxvomica as the corrosion inhibitor can be attributed to the presence of three methoxy groups attached to the benzene nucleus.These extensive groups favor its greater adsorption on the mild steel surface, thereby giving rise to very high inhibition efficiency (98.2%) at a concentration as low as 350 ppm.The next  best performance of Pongamia pinnata (97.6%) has been found at 400 ppm concentration.It was found that R ct values increased to a maximum of 264 (Ω cm 2 ) at an optimum concentration of Strychnos nuxvomica.This situation was a result of the adsorption of inhibitors at the metal/solution interface.In the present study, maximum displacement was 48 mV, suggesting that tested seeds extract belonged to the mixed-type inhibitors.

Fruits Extracts as Corrosion Inhibitors.
We have used fruits extract of Moringa oleifera, Piper longum and Citrus aurantium in our present study.The result is depicted in Table 5 and Figure 3.The inhibition efficiency of fruits extract follows the order Moringa olei f era > Piper longum > Citrus aurantium (3) Good performance of fruits extract as corrosion inhibitors for mild steel in 1 M HCl solutions may be due to the presence of heteroatoms, π-electrons, and aromatic rings in their structures.The highest inhibition efficiency shown by Moringa oleifera is 98.2% at 300 ppm due to the presence of imine (C=N) group, four N atoms, and long alkyl chain and least efficiency of Citrus aurantium is 88.1% at 1200 ppm attributed to the presence of electron withdrawing COOH group.The R ct values were found to increase, and on the other hand, C dl values decreased in the presence of all fruits extract.This is due to the adsorption of these compounds at the metal/solution interface.The values of I corr were found to decrease in the presence of inhibitors.The decrease in I corr values can be due to the adsorption of fruits extract on the mild steel surface.It was observed that there is a small shift towards the cathodic region in the values of E corr .In  the present study, maximum displacement in E corr value was 69 mV, which indicates that all studied fruits extract were mixed-type inhibitors.

Stem Extracts as Corrosion Inhibitors. Stem extracts of
Bacopa monnieri, Ficus religiosa, and Terminalia arjuna were used as corrosion inhibitors.Bacopa monnieri showed its maximum inhibition performance 95.2% at 600 ppm, while Ficus religiosa shows 88.7% at 1200 ppm.The better performance of Bacopa monnieri can be attributed to the presence of more O atoms in its structure.Terminalia arjuna has been found to give its maximum inhibition efficiency 83.4% at 1200 ppm.The R ct values were found to increase and on the other hand, C dl values decreased in the presence of all stem extract as in Table 6 and Figure 4.This may be due to the adsorption of these compounds at the metal/solution interface.Decrease in C dl values, caused by a decrease in local dielectric constant and/or an increase in the thickness of the electrical double layer, suggests that the water molecules are replaced by inhibitor molecules.It was observed that the values of I corr decrease in the presence of inhibitors.The decrease in I corr values can be due to the adsorption of stems extract on the mild steel surface.The b c and b a values remained more or less identical in the absence and presence of stems extract studied, suggesting that the effect of inhibitors is not as large as to change the mechanism of corrosion.
All the studied plant extracts obtained from leaves, seeds, fruits, and stem showed good inhibition efficiency (>95%) at their optimum concentrations for mild steel in 1 M HCl.The optimum concentration is considered as a concentration beyond which increase in extract concentration showed no significant change in the inhibition efficiency.The good performance may be attributed to the synergism between the different compounds present in the extracts.Andrographis paniculata leaves extract showed 98% inhibition efficiency due to the presence of delocalized π-electrons as compared to those of Strychnous nuxvomica seed extract which can be attributed to the presence of three methoxy groups attached to the benzene nucleus favoring its greater adsorption on the mild steel surface, thereby giving rise to very high inhibition efficiency (98.2%) and Moringa oleifera fruit extract (98.1%) due to the presence of imine (C=N) group, four N atoms and long alkyl chain.Also, the low inhibition efficiency of Bacopa monnieri as compared to Andrographis paniculata, Strychnous nuxvomica, and Moringa oleifera can be attributed to the presence of O atoms in its structure.

Mechanism of Corrosion Inhibition.
In acidic solutions, transition of the metal/solution interface is attributed to the adsorption of the inhibitor molecules at the metal/solution interface, forming a protective film.The rate of adsorption is usually rapid, and hence, the reactive metal surface is shielded from the acid solutions [49].The adsorption of an inhibitor depends on its chemical structure, its molecular size, the nature and charged surface of the metal, and distribution of charge over the whole inhibitor molecule.In fact, adsorption process can occur through the replacement of solvent molecules from the metal surface by ions and molecules accumulated near the metal/solution interface.Ions can accumulate at the metal/solution interface in excess of those required to balance the charge on the metal at the  operating potential.These ions replace solvent molecules from the metal surface, and their centres reside at the inner Helmholtz plane.This phenomenon is termed specific adsorption, contact adsorption.The anions are adsorbed when the metal surface has an excess positive charge in an amount greater than that required to balance the charge corresponding to the applied potential.The exact nature of the interactions between a metal surface and an aromatic molecule depends on the relative coordinating strength towards the given metal of the particular groups present [50].Generally, two modes of adsorption were considered.In one mode, the neutral molecules of leaves extract can be adsorbed on the surface of mild steel through the chemisorption mechanism, involving the displacement of water molecules from the mild steel surface and the sharing electrons between the heteroatoms and iron.The inhibitor molecules can also adsorb on the mild steel surface based on donor-acceptor interactions between π-electrons of the aromatic/heterocyclic ring and vacant d-orbitals of surface iron.In another mode, since it is well known that the steel surface bears the positive charge in acidic solutions [51], so it is difficult for the protonated leaves extract to approach the positively charged mild steel surface (H 3 O + /metal interface) due to the electrostatic repulsion.Since chloride ions have a smaller degree of hydration, thus they could bring excess negative charges in the vicinity of the interface and favour more adsorption of the positively charged inhibitor molecules, the protonated leaves extract adsorbed through electrostatic interactions between the positively charged molecules and the negatively charged metal surface.
Since all the different parts of plant extract possess several heteroatoms containing active constituents, therefore there may be a synergism between the molecules accounting for the good inhibition efficiencies.

Figure 1 :
Figure 1: Nyquist plots and Tafel plots for mild steel in 1 M HCl in the absence and presence of different inhibitors at their optimum concentration.

Figure 2 :
Figure 2: Nyquist plots and Tafel plots for mild steel in 1 M HCl in the absence and presence of different inhibitors at their optimum concentrations.

Figure 3 :
Figure 3: Nyquist plots and Tafel plots for mild steel in 1 M HCl in the absence and presence of different inhibitors at their optimum concentrations.

Figure 4 :
Figure 4: Nyquist plots and Tafel plots for mild steel in 1 M HCl in the absence and presence of different inhibitors at their optimum concentrations.

Table 1 :
Plant extracts investigated as corrosion inhibitors by other authors.
[8] inhibitive action of ethanol extracts from leaves (LV), bark (BK), and roots (RT) of Nauclea latifolia on mild steel corrosion in H 2 SO 4 solutions at 30 • and 60 • C was studied using weight loss and gasometric techniques[8]
[23]ella asiatica was studied as the corrosion inhibitor on mild steel in 1 N hydrochloric acid by weight loss method, gasometric method, potentiodynamic polarization method and AC impedance method[23]

Table 2 :
Plant extracts used by us as corrosion inhibitors.

Table 3 :
Electrochemical impedance and Tafel data at 308 K.

Table 4 :
Electrochemical impedance, Tafel, and linear polarization resistance data at 308 K.

Table 5 :
Electrochemical impedance, Tafel, and linear polarization resistance data at 308 K.

Table 6 :
Electrochemical impedance, Tafel, and linear polarization resistance data at 308 K.