Abstract
The Inhibitive action of the extracts of Adathoda vasica, Eclipta alba, and Centella asiatica on the corrosion of mild steel in 1N HCl has been studied using weight loss method, electrochemical methods, and hydrogen permeation method. Polarization method indicates that the plant extracts are under mixed control, that is, promoting retardation of both anodic and cathodic reactions. The impedance method reveals that charge-transfer process controls the corrosion of mild steel. The plant extracts obey Langmuir adsorption isotherm. Theoretical fitting of the corrosion data to the kinetic-thermodynamic model was tested to show the nature of adsorption. Physisorption mechanism has been proposed for the inhibition action of these plant extracts. The protective film formed on the surface was confirmed by SEM. From hydrogen permeation method, all the plant extracts were able to reduce the permeation current. Results obtained in all three methods were very much in good agreement in the order Eclipta alba > Adathoda vasica > Centella asiatica, and, among the three plant extracts studied, the maximum inhibition efficiency was found in Eclipta alba which showed 99.6% inhibition efficiency at 8.0% v/v concentration of the extract.
1. Introduction
Mild steel was the material of choice due to its characteristics of wide application in motor car bodies, machines, gears, pipes, tanks, and so forth and in most of the chemical industries. Hydrochloric acid and sulphuric acids are the medium generally being used for pickling mild steel. About 90% of pickling problems can be solved by introducing appropriate pickling inhibitor to the medium. The recent and growing trend is using plant extracts as corrosion inhibitor. Owing to strict environmental legislation, emphasis is being focused on development of naturally occurring substances as corrosion inhibitors [1]. Recently, many plant extracts have been reported to be very effective corrosion inhibitors for the protection of mild steel in acidic media [2–19]. In this study, the inhibition effect of the leaf extracts of Adathoda vasica (Adathodai), Eclipta alba (Karisalankanni), and Centella asiatica (Vallarai) on the corrosion of mild steel in 1N hydrochloric acid was investigated using weight loss method, electrochemical methods, and hydrogen permeation method. There was no literature report on the studies of corrosion inhibition effect of the above plant extracts on mild steel in hydrochloric acid medium previously. From literature survey, it were found that the six plants selected for investigation was found to contain some alkaloids or hydroxyl organic compounds like sterols, tannins, and so forth. The aqueous extracts of these plants were prepared because alkaloids or hydroxyl organic compounds are easily soluble in water, and moreover due to the biodegradability, ecofriendliness, less toxicity, cost-effectiveness, easy availability, environmentally safe, and highly stable nature in acidic solutions, it was used to study the corrosion inhibition effect on mild steel in acid medium.
2. 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 cm2 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 deionized water, and stored in the desiccator. Accurate weight of the samples was taken using electronic balance.
2.2. Preparation of the Plant Extract
The leaves of the Adathoda vasica, Eclipta alba, and Centella asiatica 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 stock solution was prepared from the collected filtrate and prepared the desired concentrations by dilution with 1N HCl 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 1 N hydrochloric acid. The aqueous extracts of these plants were prepared because alkaloids/hydroxyl organic compounds present in the leaves of these plants are easily hydrolysable and moreover have highly stable nature in acidic solutions. Similar kind of preparation has been reported in studies using aqueous plant extracts in the recent years [20–28].
2.3. Weight Loss Method
The pretreated specimens’ initial weights were noted and were immersed in the experimental solution (in triplicate) with the help of glass hooks at 30°C for a period of 3 h. The experimental solution used was 1N HCl in the absence and presence of various concentrations of the plant extracts. 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 () of plant extracts was calculated using the formula where (constant), is weight loss in is area in cmm2, is time in hours and is density in gm/cmm3 (7.86), where and are corrosion rates in the absence and presence of the inhibitors.
2.4. Potentiodynamic Polarization Method
Potentiodynamic polarization measurements were carried out using electrochemical analyzer. The polarization measurements were made to evaluate the corrosion current, corrosion potential, and Tafel slopes. Experiments were carried out in a conventional three-electrode cell assembly with working electrode as mild steel specimen of 1 sq.cm.area which was exposed and the rest being covered with red lacquer, a rectangular Pt foil as the counter electrode, and the reference electrode as SCE. Instead of salt, bridge a luggin capillary arrangement was used to keep SCE close to the working electrode to avoid the ohmic contribution. A time interval of 10–15 minutes was given for each experiment to attain the steady-state open-circuit potential. The polarization was carried from a cathodic potential of −800 mV (versus SCE) to an anodic potential of −200 mV (versus SCE) at a sweep rate of 1 mV per second. From the polarization curves, Tafel slopes, corrosion potential, and corrosion current were calculated. The inhibitor efficiency was calculated using the formula, where and are corrosion current in the absence and presence of inhibitors.
2.5. 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 () and the imaginary part () were measured at various frequencies in the range of 100 KHz to 10 MHz. A plot of versus was made. From the plot, the charge transfer resistance () was calculated, and the double layer capacitance was then calculated using where is charge transfer resistance, and 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 and are the charge transfer resistance in the presence and absence of inhibitors.
2.6. Hydrogen Permeation Method
The behaviour of the inhibitors with regard to hydrogen permeation can be understood by measuring the permeation current with and without inhibitors. An inhibitor can be considered as completely effective only if it simultaneously inhibits metal dissolution and hydrogen penetration into the metal [29]. 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 two compartment cell assembly [30, 31] 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 [32].
2.7. 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.
3. Results and Discussion
3.1. Weight Loss Studies
The various corrosion parameters such as corrosion rate (CR), inhibition efficiency (IE), and surface coverage () were obtained from weight loss method 1N hydrochloric acid in the absence and presence of various concentrations of the plant extracts ranging from 2% to 10% v/v and listed in Table 1.
It was found that the optimum concentration for Adathoda vasica was found to be 6% v/v with maximum inhibition efficiency of 99.0%, Eclipta alba, at 8% v/v with maximum inhibition efficiency of 99.6% and Centella asiatica at 10% v/v with maximum inhibition efficiency of 85.3% for a period of 3 hours of immersion time. This result indicates that the plant extracts could act as good corrosion inhibitors.
3.1.1. Effect of Immersion Time at 30°C
The effect of immersion time on corrosion rate and inhibition efficiency of the plant extracts with an optimum concentration at 30°C studied as given in Table 2 shows that the inhibition efficiency of the extract slightly decreased with the increase of immersion time from 3 to 24 h and reveals that the plant extracts showed maximum efficiency at 3 h of immersion time which is sufficient for the pickling process.
3.2. Potentiodynamic Polarization Studies
Electrochemical corrosion kinetic parameters such as corrosion potential (), corrosion current (), anodic and cathodic Tafel slopes ( and ), and percentage efficiency (IE) for the corrosion of mild steel in 1N HCl at 30°C in the absence and presence of different concentrations of the plant extract are given in Table 3, and its corresponding polarization curves are shown in Figure 1. Potentiodynamic polarization studies revealed that the corrosion current density () 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 ( and ) 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 [20, 21]. It was observed that with increase in concentration of the plant extract from 2% to 10%, the maximum inhibition efficiency of 99.2% was observed for Adathoda vasica at an optimum concentration of 6% in v/v, for Eclipta alba extract with 99.7% at 8% v/v, and Centella asiatica with 85.7% at 10% v/v of the extract.
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3.3. Electrochemical Impedance Studies
impedance measurements were studied to evaluate the charge-transfer resistance () and double-layer capacitance (), 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.
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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 have enhanced and the values of double-layer capacitance are also brought down to the maximum extent. The decrease in shows that the adsorption of the inhibitors takes place on the metal surface in acidic solution.
For Adathoda vasica extract, the maximum value of 285.23 Ω cm2 and minimum value of 7.65 μF/cm2 are obtained at an optimum concentration of 6% in v/v with a maximum inhibition efficiency of 97.3%. For Eclipta alba extract, the maximum value of 358.80 Ω cm2 and minimum value of 6.00 μF/cm2 are obtained at an optimum concentration of 8% in v/v with a maximum inhibition efficiency of 97.9%. For Centella asiatica extract, the maximum value of 54.32 Ω cm2 and minimum value of 39.88 μF/cm2 are obtained at an optimum concentration of 10% in v/v with a maximum inhibition efficiency of 86.0%. A good agreement is observed between the results of weight loss method and electrochemical methods (potentiodynamic polarization method and impedance method) in the order Eclipta alba > Adathoda vasica > Centella asiatica.
3.4. Effect of Temperature
The effect of temperature on the corrosion rate of mild steel in free acid and in the presence of the optimum concentration of the inhibitors (plant extracts) was studied in the temperature range of 30°C to 80°C, using the weight loss measurements and given in Table 5. It was found that the rates of mild steel corrosion, in free and inhibited acid solutions, increase with increase in temperature, but the corrosion rate is much decreased for inhibited acid solution than the uninhibited acid solution. Consequently, the inhibition efficiency of the extract decreases with the increasing temperature. This result suggests a physical adsorption of the extract compounds on the mild steel surface. It also revealed that the extract was adsorbed on the mild steel surface at all temperatures studied. A similar observation was seen in the studies of El-Etre [3].
3.5. Kinetics and Mechanism of Corrosion Inhibition
The major phytochemical constituents present in Adathoda vasica are the alkaloids Vasicine and Vasicinone (Figure 3), the major phytochemical constituent present in Centella asiatica is Asiaticoside, a triterpene glycoside (Figure 4), and the major phytochemical constituent present in Eclipta alba are Wedelolactone, β-sitosterol, Stigmasterol (Figures 5(a), 5(b), and 5(c)), and also an alkaloid Ecliptine [33–35]. 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 decreasing 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. Due to electrostatic interaction, the protonated constituent’s molecules are adsorbed (physisorption) and high inhibition is expected. Acid pickling inhibitors containing organic N, S, and OH groups behave similarly to inhibit corrosion [36, 37].
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The inhibition of the corrosion of mild steel in 1N HCl medium with addition of different concentrations of the extract can be explained by the adsorption of the components of the plant extracts on the metal surface. Inhibition efficiency (IE) is directly proportional to the fraction of the surface covered by the adsorbed molecules (). Therefore, () with the extract concentration specifies the adsorption isotherm that describes the system and gives the relationship between the coverage of an interface with the adsorbed species and the concentration of species in solution. The values of the degree of surface coverage () were evaluated at different concentrations of the inhibitors in 1N HCl solution. Attempts were made to fit values to various adsorption isotherm. An inhibitor is found to obey Langmuir, if a plot of versus is linear. Similarly, for Temkin plot versus , for BDM plot () versus 3/2 and for Frumkin plot versus will be linear. On examining, the adsorption of different concentrations of Adathoda vasica, Eclipta alba, and Centella asiatica extracts on the surface of mild steel in 1N hydrochloric acid was found to obey Langmuir adsorption isotherm. The Langmuir adsorption isotherm plot for the adsorption of various concentrations of the plant extracts is shown in Figure 6.
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Theoretical fitting of the corrosion data to the kinetic-thermodynamic model was tested to show the nature of adsorption. The standard free energy of adsorption which can characterize the interaction of adsorption molecules and metal surface was calculated where one molecule of water is replaced by one molecule of inhibitor and the numerical value (1/55.5) in the equation stands for the molarity of water.
The value of can be calculated using The enthalpy of adsorption () was calculated and the entropy of adsorption () was calculated using The calculated values of activation energy (), enthalpy of adsorption (), free energy of adsorption (), and entropy of adsorption () are shown in Table 6.
The activation energy was found to be 31.38 KJ mol−1 for (1N HCl) and increased to 62.41 KJ mol−1 in the presence of the Adathoda vasica extract and 93.48 KJ mol−1 for Eclipta alba which shows that the adsorbed organic matter has provided a physical barrier to charge and mass transfer, leading to reduction in corrosion rate. The higher value of in the presence of the inhibitor compared to that in the absence of the inhibitor was attributed to physisorption [4]. For Centella asiatica, was found to be 31.33 KJ mol−1 and remained almost same as blank suggesting inhibition efficiency had not changed with temperature variation for Centella asiatica.
The values of around −20 KJ mol−1 or lower are consistent with the electrostatic interaction between organic charged molecules, and the charged metal (physisorption) and those around −40 KJ mol−1 or higher involved charge sharing or transfer from the organic molecules to the metal surface to form a coordinate type of bond (chemisorption) as discussed by Moretti et al. [38]. In this case, the negative sign of free energy of adsorption for the plant extracts indicates that the adsorption of the plant extracts on mild steel surface was a spontaneous process and the adsorption could be physisorption. Studies of El-Etre [3] and Li et al. [5] reported similar kind of results. The positive value of enthalpy of adsorption () suggests that the reaction was endothermic and the adsorption of the inhibitor on the metal surface has taken place. Positive value of entropy of adsorption () indicates that the reaction was spontaneous and feasible. Earlier work of Bhajiwala and Vashi [39] supports this.
3.6. Surface Examination Studies
Surface examination of the mild steel specimens was made using JEOL—scanning electron microscope (SEM) with the magnification of 1000x. The mild steel specimens after immersion in 1N HCl solution for three hours at 30°C in the absence and presence of optimum concentration of the plant extracts were taken out, dried, and kept in a dessicator. The protective film formed on the surface of the mild steel was confirmed by SEM studies. The SEM images of mild steel immersed in 1N HCl in the absence and presence of the optimum concentration of the plant extracts are shown in Figures 7, 8, 9, and 10. From the SEM images, it was found that more grains were found in SEM image of mild steel immersed in 1N HCl solution in the absence of the inhibitor, whereas no grains were found in the SEM image of mild steel immersed in 1N HCl solution in the presence of the plant extracts, which shows the presence of a protective film over the surface of the mild steel in the presence of the inhibitors and the protective film is uniform in the order Eclipta alba > Adathoda vasica > Centella asiatica. 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.
3.7. Hydrogen Permeation Studies
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 [40]. 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. The behaviour of the inhibitors with regard to hydrogen permeation can be understood by measuring the permeation current with and without inhibitors [30].
There are basically two reaction schemes. Common to both schemes, the first step is the diffusion of few hydrogen atoms that get onto the electrode surface. Hydrated protons are reduced to form neutral hydrogen atoms upon those areas of the surface, which are unoccupied. One can say protons are discharged on to free sites on the electrode to form adsorbed hydrogen atoms where M is the cathodic metal surface. The second step is the desorption step. The two basic reaction paths are(i)discharge D, followed by chemical desorption, CD, (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]. 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 Eclipta alba > Adathoda vasica > Centella asiatica. Permeation current versus time curves for mild steel in 1N HCl in the absence and presence of inhibitors are shown in Figure 11, and their corresponding permeation are given in Table 7.
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.