A Comparative Study of the Inhibitory Effect of the Extracts of Ocimum sanctum, Aegle marmelos, and Solanum trilobatum on the Corrosion of Mild Steel in Hydrochloric Acid Medium

A comparative study of the inhibitory effect of plant extracts, Ocimum sanctum, Aegle marmelos, and Solanum trilobatum, on the corrosion of mild steel in 1N HCl medium was investigated using weightloss method, electrochemical methods, and hydrogen permeation method. Polarization method indicates plant extracts behave as mixed-type inhibitor. The impedance method reveals that charge-transfer process mainly controls the corrosion of mild steel. On comparison, maximum inhibition efficiency was found in Ocimum sanctum with 99.6% inhibition efficiency at 6.0% v/v concentration of the extract. The plant extracts obey Langmuir adsorption isotherm. The SEM morphology of the adsorbed protective film on the mild steel surface has confirmed the high performance of inhibitive effect of the plant extracts. From hydrogen permeation method, all the plant extracts were able to reduce the permeation current. The reason for the reduced permeation currents in presence of the inhibitors may be attributed to the slow discharge step followed by fast electrolytic desorption step. Results obtained in all three methods were very much in good agreement in the order Ocimum sanctum > Aegle marmelos > Solanum trilobatum.


Introduction
Mild steel is a structural material widely used in automobiles, pipes and used in most of the chemical industries.Mild steel suffers from severe corrosion in aggressive medium of acids and pickling processes.Hydrochloric acid is widely used for pickling, descaling, and chemical cleaning processes of mild steel.90% of pickling problems can be solved by introducing appropriate pickling inhibitor to the medium.Generally, organic compounds containing O, N, and S atoms are normally used as inhibitors to reduce the corrosion of mild steel in hydrochloric acid medium [1,2].Environmental concerns worldwide are increasing and are likely to influence the choice of corrosion inhibitors in the present and in future.Environmental requirements are still being developed, but some elements have been established.One of the methods to protect metals against corrosion is addition of species to the solution in contact with the surface in order to inhibit the corrosion rate.Unfortunately, many of the inhibitors used are inorganic salts or organic compounds with toxic properties or limited solubility.Increasing awareness of health and ecological risks has drawn attention to find more suitable inhibitors which are nontoxic.Accordingly, greater research efforts have been directed towards formulating environmentally acceptable inhibitors.
Due to the diversity of their structures, many extracts of common plants have been used as corrosion inhibitors for materials in pickling and cleaning processes.Plant materials contain proteins, polysaccharides, polycarboxylic acids, tannin, alkaloids, and so forth.These compounds are potential acid corrosion inhibitors for many metals [3].The cost of using green inhibitors is very low when compared to that of organic inhibitors which require a lot of chemicals and also time for its preparation.Chemical inhibitors are more expensive and cause more hazard effects.Nowadays due to strict environmental legislation, emphasis is being focused International Journal of Corrosion on usage of natural products that are corrosion inhibitor.The recent and growing trend is using plant extracts as corrosion inhibitor.Recently, many plant extracts have been reported as effective corrosion inhibitors within India and outside India [4][5][6][7][8][9][10][11][12][13][14][15][16][17][18][19][20].In this study, leaf extracts of three medicinal plants, namely, Ocimum sanctum (Tulasi), Aegle marmelos (Vilvam), and Solanum trilobatum (Thuthuvalai), have been selected to study the inhibition effect on the corrosion of mild steel in 1N hydrochloric acid medium using weight loss method, the potentiodynamic polarization method, electrochemical impedance method, and hydrogen permeation method.

Experimental Procedure
2.1.Preparation of Mild Steel Specimen.Mild steel strips were mechanically cut into strips of size 4.5 cm × 2 cm × 0.2 cm containing the composition of 0.14% C, 0.35% Mn, 0.17% Si, 0.025% S, 0.03% P, and the remainder Fe and provided with a hole of uniform diameter to facilitate suspension of the strips in the test solution for weight loss method.For electrochemical studies, mild steel strips of the same composition but with an exposed area of 1 cm 2 were used.Mild steel strips were polished mechanically with emery papers of 1/0 to 4/0 grades, subsequently degreased with trichloroethylene or acetone and finally with deionised water, and stored in the desiccator.Accurate weight of the samples was taken using electronic balance.

Preparation of the Plant Extract. The leaves of the plants
Ocimum sanctum, Aegle marmelos, and Solanum trilobatum were taken and cut into small pieces, and they were dried in an air oven at 80 • C for 2 h and ground well into powder.From this, 10 g of the sample was refluxed in 100 mL distilled water for 1 h.The refluxed solution was then filtered carefully, the filtrate volume was made up to 100 mL using double distilled water which is the stock solution, and the concentration of the stock solution is expressed in terms of % (v/v).From the stock solution, 2-10% concentration of the extract was prepared using 1N hydrochloric acid.Similar kind of preparation has been reported in studies using aqueous plant extracts in the recent years [21][22][23][24][25][26][27][28][29][30].

Weight Loss Method.
The pretreated specimens' initial weights were noted and were immersed in the experimental solution with the help of glass hooks at 30 • C for a period of 3h.The experimental solution used was 1N HCl in the absence and presence of various concentrations of the inhibitors.After three hours, the specimens were taken out, washed thoroughly with distilled water, and dried completely, and their final weights were noted.From the initial and final weights of the specimen, the loss in weight was calculated and tabulated.From the weight loss, the corrosion rate (mmpy), inhibition efficiency (%), and surface coverage (θ) of plant extracts were calculated using the formula

Corrosion rate mmpy
where K = 8.76 × 10 4 (constant), W is weight loss in g, A is area in cmm 2 , t is time in hours, and D is density in gm/cmm 3 (7.86): where CR B and CR I are corrosion rates in the absence and presence of the inhibitors.

Potentiodynamic Polarization
where I Corr and I * Corr are corrosion current in the absence and presence of inhibitors.

Electrochemical Impedance Method.
The electrochemical AC-impedance measurements were also performed using electrochemical analyzer.Experiments were carried out in a conventional three-electrode cell assembly as that used for potentiodynamic polarization studies.A sine wave with amplitude of 10 mV was superimposed on the steady open circuit potential.The real part (Z ) and the imaginary part (Z ) were measured at various frequencies in the range of 100 KHz to 10 MHz.A plot of Z versus Z was made.From the plot, the charge transfer resistance (R t ) was calculated, and the double layer capacitance was then calculated using where R t is charge transfer resistance, and C dl is double layer capacitance.The experiments were carried out in the absence and presence of different concentrations of inhibitors.The percentage of inhibition efficiency was calculated using where R * t and R t are the charge transfer resistance in the presence and absence of inhibitors.

Hydrogen Permeation Method.
When metals are in contact with acids, atomic hydrogen is produced.Before they combine to produce hydrogen molecules, a fraction may diffuse into the metal.Inside the metal, the hydrogen atoms may combine to form molecular hydrogen.Thus, a very high internal pressure is built up.This leads to heavy damage of the metal.This is known as "hydrogen embrittlement".This phenomenon of hydrogen entry into the metals can occur in industrial processes like pickling, plating, phosphating, and so forth.An inhibitor can be considered as completely effective only if it simultaneously inhibits metal dissolution and hydrogen penetration into the metal [31].Hydrogen permeation study has been taken up with an idea of screening the inhibitors with regard to their effectiveness on the reduction of hydrogen uptake.Hence, the hydrogen permeation study was carried out using an adaptation of the modified Devanathan-Stachurski twocompartment cell assembly [32,33] in 1N HCl medium in the absence and presence of optimum concentration of the extracts.Similar kind of study is reported in the works of Quraishi and Rawat [34].

Surface Examination Studies.
Surface examination of mild steel specimens in the absence and presence of the optimum concentration of the extracts immersed for 3 h at 30 • C was studied using JEOL-Scanning electron microscope (SEM) with the magnification of 1000x specimens.

Weight Loss Studies.
The weight loss studies were done in 1N hydrochloric acid in the absence and presence of various concentrations of the plant extracts ranging from 2% to 10% v/v.Using the weight loss data, the corrosion rate, inhibition efficiency, surface coverage, and the optimum concentration of the extract have been calculated.The corrosion parameters obtained in the weight loss method are listed in Table 1.
From Table 1, it was found that with the addition of the plant extract to 1N hydrochloric acid, the weight loss of mild steel decreased, the corrosion rate also decreased, while the inhibition efficiency increased.The optimum concentration for Ocimum sanctum was found to be 6% v/v with maximum inhibition efficiency of 99.6%, Aegle marmelos at 8% v/v with maximum inhibition efficiency of 97.5%, and Solanum trilobatum at 10% v/v with maximum inhibition efficiency of 90.2% for a period of 3 hours of immersion time.This result indicated that the plant extracts could act as effective corrosion inhibitors for mild steel in 1N HCl.The effect of immersion time studied for a period of 3 h to 24 h as given in Table 2 reveals that the plant extracts showed maximum efficiency at 3 h of immersion time which is sufficient for pickling process.The order of inhibition effect among the three plant extracts on mild steel in 1N HCl is found to be Ocimum sanctum > Aegle marmelos > Solanum trilobatum.

Potentiodynamic Polarization Studies.
The potentiodynamic polarization parameters for different concentrations of the plant extracts are given in Table 3, and the polarization curves are given in Figure 1.Potentiodynamic polarization studies revealed that the corrosion current density (I corr )    markedly decreased with the addition of the extract and the corrosion potential shifts to less negative values upon addition of the plant extract.Moreover, the values of anodic and cathodic Tafel slopes (b a and b c ) are slightly changed indicating that this behavior reflects the plant extracts' ability to inhibit the corrosion of mild steel in 1N HCl solution via the adsorption of its molecules on both anodic and cathodic sites, and, consequently, the extracts act through mixed mode of inhibition [15,16].It was observed that with increase in concentration of the plant extract from 2% to 10%, the maximum inhibition efficiency of 99.7% was observed for Ocimum sanctum extract at 6% v/v, for Aegle marmelos with 97.5% at 8% v/v, and for Solanum trilobatum with 90.8% at 10% v/v of the extract.

Electrochemical Impedance Studies.
Impedance measurements were studied to evaluate the charge transfer resistance (R t ) and double layer capacitance (C dl ), and through these parameters, the inhibition efficiency was calculated.Figure 2 shows the impedance diagrams for mild steel in 1N HCl with different concentrations of the plant extract, and the impedance parameters derived from these investigations are given in Table 4.As noticed from Figure 2, the obtained impedance diagrams are almost in a semicircular appearance, indicating    that the charge-transfer process mainly controls the corrosion of mild steel.Deviations of perfect circular shape are often referred to the frequency dispersion of interfacial impedance.This anomalous phenomenon may be attributed to the inhomogeneity of the electrode surface arising from surface roughness or interfacial phenomena.In fact, in the presence of the plant extracts, the values of R t have enhanced and the values of double-layer capacitance are also brought down to the maximum extent.The decrease in C dl shows that the adsorption of the inhibitors takes place on the metal surface in acidic solution.
For Ocimum sanctum extract, the maximum R t value of 358.80 Ω cm 2 and minimum C dl value of 6.00 μF/cm 2 are obtained at an optimum concentration of 6% in v/v with a maximum inhibition efficiency of 97.9%.For Aegle marmelos extract, the maximum R t value of 224.80 Ω cm 2 and minimum C dl value of 9.62 μF/cm 2 are obtained at an optimum concentration of 8% in v/v with a maximum inhibition efficiency of 96.6%.For Solanum trilobatum extract, the maximum R t value of 87.86 Ω cm 2 and minimum C dl value of 24.52 μF/cm 2 are obtained at an optimum concentration of 10% in v/v with a maximum inhibition efficiency of 91.4%.A good agreement is observed between the results of weight loss method and electrochemical methods (potentiodynamic polarization method and impedance method).
Inspection of the chemical structures of the phytochemical constituents reveals that these compounds are easily hydrolysable and the compounds can adsorb on the metal surface via the lone pair of electrons present on their oxygen atoms and make a barrier for charge and mass transfer leading to decrease the interaction of the metal with the corrosive environment.As a result, the corrosion rate of the metal was decreased.The formation of film layer essentially blocks discharge of H + and dissolution of metal ions.Acid pickling inhibitors containing organic N, S, and OH groups behave similarly to inhibit corrosion [38,39].
It follows that inhibition efficiency (IE) is directly proportional to the fraction of the surface covered by the adsorbed molecules (θ).Therefore, (θ) with the extract concentration specifies the adsorption isotherm that describes the system.Adsorption isotherm gives the relationship between the coverage of an interface with the adsorbed species and the concentration of species in solution.The use of adsorption isotherms provides useful insight into the corrosion inhibition mechanism.The values of the degree  3.6.Hydrogen Permeation Studies.The behaviour of the inhibitors with regard to hydrogen permeation can be understood by measuring the permeation current with and without inhibitors.Those inhibitors which reduce the permeation current are good at inhibiting the entry of hydrogen into the metal concerned [31].There are basically two reaction schemes.(ii) discharge D, followed by electrolytic desorption, ED, For transition metals, it has been reported that the electrolytic desorption is the rate determining step.A part of the atomic hydrogen liberated during these processes enters the metal, when the remainder is evolved as hydrogen gas [40].Permeation current versus time curves for mild steel in 1N HCl in the absence and presence of inhibitors are shown in Figure 13, and their corresponding permeation are given in Table 5.
From the hydrogen permeation studies on mild steel in 1N HCl in the absence and presence of inhibitors, it was observed that all the prepared extracts were able to reduce the permeation current compared to the control.The decrease in the permeation current follows the order Ocimum sanctum > Aegle marmelos > Solanum trilobatum.The reason for the reduced permeation currents in presence of the inhibitors can be attributed to the slow discharge step followed by fast electrolytic desorption step The reduction of hydrogen uptake could be attributed to adsorption of the phytochemical constituents present in the plant extracts on the mild steel surface, which prevented permeation of hydrogen into metal.

Conclusion
(i) The leaf extracts of Ocimum sanctum, Aegle marmelos, and Solanum trilobatum act as good and efficient inhibitors for corrosion of mild steel in 1N hydrochloric acid.
(ii) Potentiodynamic polarization studies revealed that the extracts act through mixed mode of inhibition.
(iii) The Nyquist diagrams obtained in impedance method revealed that charge-transfer process mainly controls the corrosion of mild steel.
(iv) The mechanism involved in this study is the phytochemical constituents in the plant extracts that have adsorbed on the mild steel surface forming a protective thin film layer preventing the discharge of H + ions and dissolution of metal ions and has prevented the small corrosion on the surface of the metal.
(v) The plant extracts obey Langmuir adsorption isotherm.
(vi) The SEM morphology of the adsorbed protective film on the mild steel surface has confirmed the high performance of inhibitive effect of the plant extracts.
(vii) From hydrogen permeation method, it was observed that all the plant extracts were able to reduce the permeation current compared to the control.
(viii) The reduction of hydrogen uptake in hydrogen permeation method could be attributed to adsorption of the phytochemical constituents present in the plant extracts on the mild steel surface, which prevented permeation of hydrogen into metal.
(ix) Results obtained in weight loss method were very much in good agreement with the electrochemical methods (potentiodynamic polarization method and impedance method) and hydrogen permeation method in the order Ocimum sanctum > Aegle marmelos > Solanum trilobatum.
(x) Among the three plant extracts studied, the maximum inhibition efficiency was found in Ocimum sanctum which showed 99.6% inhibition efficiency at 6.0% v/v concentration of the extract.

Figure 1 :
Figure 1: Potentiodynamic polarization curves for mild steel in 1N HCl solution in the absence and presence of various concentrations of the plant extracts (a) Ocimum sanctum, (b) Aegle marmelos, and (c) Solanum trilobatum.

Figure 2 :
Figure 2: Impedance diagrams for mild steel in 1N HCl solution in the absence and presence of various concentrations of the plant extract (a) Ocimum sanctum, (b) Aegle marmelos, and (c) Solanum trilobatum.

R 2 =Figure 8 :Figure 9 :
Figure 8: Langmuir adsorption isotherm plot for the adsorption of various concentrations of the plant extracts on the surface of mild steel in 1N HCl solution.

Table 1 :
Corrosion parameters obtained from weight loss measurements for mild steel in 1N HCl containing various concentrations of the plant extracts.Name of the plant extract Conc. of the extract (% in v/v) Corrosion rate (mmpy) Inhibition efficiency (%) Surface coverage (θ)

Table 2 :
Effect of immersion time on percentage inhibition efficiency of mild steel in 1N HCl at 30 • C in the presence of optimum concentration of the plant extracts.Name of the plant extract with optimum conc.

Table 3 :
Potentiodynamic polarization parameters for mild steel in 1N HCl containing various concentrations of the plant extracts.

Table 4 :
Impedance parameters for the corrosion of mild steel in 1N HCl in the absence and presence of various concentrations of the plant extracts at 30 • C.

Table 5 :
Values of hydrogen permeation current for the corrosion of mild steel in 1N HCl alone and in the presence of inhibitors.