Investigation of Citrus aurantiifolia Leaves Extract as Corrosion Inhibitor for Mild Steel in 1 M HCL

The inhibition efficiency of acid extract of leaves of Citrus aurantiifolia [CAL] plant on the corrosion of mild steel in 1 M HCl was investigated by weight loss measurements and electrochemical studies. The corrosion rate of mild steel and the inhibition efficiencies of the extract were calculated. The results obtained show that the extract could serve as an effective inhibitor for the corrosion of mild steel in HCl media. Inhibition was found to increase with increasing concentration of the plant extract. The inhibitive action of plant extract is discussed on the basis of adsorption of stable complex at the mild steel surface. Theoretical fitting of different isotherms, Langmuir, Temkin, Freundlich, Frumkin, Flory-Huggins and the kinetic thermodynamic model, were tested to clarify the nature of adsorption. Polarisation curves revealed that this inhibitor act as a mixed type inhibitor and the inhibition efficiency of up to 97.51% can be obtained. The surface analysis study confirms the corrosion of mild steel and its inhibition by the inhibitor CAL.


Introduction
Mild steel (MS) has been extensively used under different conditions in chemical and allied industries in handling alkaline, acid and salt solutions.Chloride, sulphate and nitrate ions in aqueous media are particularly aggressive and accelerate corrosion.One way of protecting MS from corrosion is to use corrosion inhibitors.The known hazardous effects of most synthetic corrosion inhibitors are the motivation for the use of some natural products.The recent trend is towards environmentally friendly inhibitors.Most of the natural products are non-toxic, biodegradable and readily available in plenty.Several investigations have been reported using such naturally occurring substances as corrosion inhibitor for several metals in different media [1][2][3][4][5][6][7][8][9][10][11][12][13][14][15][16] .The aim of the present work is to find a naturally occurring cheap and environmentally safe substance that could be used for inhibiting the corrosion of mild steel.The use of natural product will establish, simultaneously, the economic and environmental goals.

Sample preparation
Mild steel specimens of following compositions were used for the experiment (wt %).The composition of the sample was analyzed using ARL 3460 Metal Analyzer, Optical Emission Spectrometer.The composition of the specimen was analyzed and given in Table 1.
Table 1 Rectangular samples of area 5x1 cm 2 have been cut from a large sheet of mild steel.The samples were polished, drilled a hole at one end and numbered by punching.During the study the samples were polished with 400 grade emery papers, degreased in a solution of non-toxic detergent, washed with distilled water, dried, weighed and stored in desiccators for further use.

Choice of acid medium
Acids are widely used in industry, the most important areas of application being acid pickling, industrial acid cleaning, acid descaling and oil well acidizing 17 .HCl is widely used to pickle iron and steel because HCl quickly removes oxide scale and iron rust and filth during acid pickling.Side effects of these processes are corrosion of the substrate metal and atmospheric pollution caused by acid vapour.Inhibitors are generally used in these processes to control the metal dissolution as well as acid consumption.

Selection of the inhibitor
Use of inhibitors is an important task in the protection of metals from corrosion.Till now the majority of metal corrosion inhibitors used is toxic for human being and environment.The choice of the present inhibitors is based on the following considerations: • Less-expensive • Non toxic • Possess no threat to the environment • Easy availability For the present study, extract of CAL is used as corrosion inhibitor for mild steel in 1 M HCl.The leaves were collected from a farm in Duraiyur, Trichy.

Extraction of plant materials
The leaves were collected, shade dried and powdered.Plant materials are dried in shade so as to enrich the active principles in them, by reducing their moisture content.The extract was prepared by refluxing 25 g of powdered dry leaves and seeds in 500 mL of 1 M HCl for 3 h and kept overnight.Then it was filtered and the volume of the filtrate was made up to 500 mL using the same acid and this was taken as stock solution.

Weight loss measurements
Weighed samples are immersed in 100 mL of the acid (1 M HCl) without and with different concentrations of the inhibitor for various intervals of time.They are then taken out and immersed in saturated sodium bicarbonate solution to remove residual acids and then washed thoroughly with tap water, rinsed with distilled water, dried, stored in desiccators and reweighed.The parameters used for the present study are given below 1.Time :1/2 h, 1 h, 3 h, 7 h, 24 h, 48 h, 168 h 2. Concentration of the inhibitor :0.005%, 0.05%, 0.15%, 0.5%, 2.5%(v/v) 3. Temperatures : (303 K , 313 K , 323 K , 333 K , 343 K) ±2 K

Potentiodynamic polarisation measurement
Potentiodynamic polarisation studies were carried out using Solartron 1280 B. 100 mL of 1 M HCl without and with different concentrations of the inhibitor was taken in an electrochemical cell.The polished electrode was then introduced.The electrode was placed at 0.8 mV to its open circuit potential.Thus the potential was scanned at -0.2 mV/sec towards the anodic direction in Tafel extrapolation.Applied potential vs. current was plotted and on extrapolation of linear portion to the corrosion potential gives the corrosion current.In anodic and cathodic plot, the slope of the linear portion gives Tafel constants 'b a ' and 'b c ' respectively.According to the Stern-Geary equation, the steps of the linear polarization plot are substituted to get corrosion current.
Where, R p is polarization resistance.

Determination of inhibition efficiency By Tafel method
where, I corr is corrosion current without inhibitor I corr (1) is corrosion current with inhibitor.

X100
Where, Rp(inh) is the polarization resistance in the presence of inhibitor.Rp(blank) is the polarization resistance in the absence of inhibitor.

Impedance studies
Impedance measurements were carried out at various corrosion potentials.An ac sine wave of

Surface examination
The mild steel specimens were immersed in various test solutions for a period of 168 h.The specimens were taken out and dried.Optical microscope (NIKON-Model EPI-PHOT) was used for this investigation.

Weight loss data
The inhibitor was tested for six different concentrations and their corresponding weight loss data are presented in Table 2.The addition of inhibitors increases the IE, irrespective of the time of immersion.This may be due to the adsorption of phytochemical constituents of the extracts on the metal surface.The IE is 97.51 % at a concentration of 2.5% v/v.The results concerned with the effect of period of immersion at various concentrations of the inhibitors on mild steel in 1 M HCl are also shown in Table 2.The IE increases from ½ h to 24 h period of immersion and decreases in IE from 24 h to 48 h period of immersion.The decrease in IE with time may be attributed to various factors such as an increase in the ferrous ion concentration.

Effect of temperature
The effect of temperature on the inhibitory action of the inhibitor was determined by weight loss method at various concentrations at different temperatures (303, 313, 323, 333 and 343) ±2 K for a fixed immersion time of ½ h.The tabulated data (Table 3) reveal that, as the concentration of the inhibitor increases the corrosion rate has decreased at all temperatures.Though the C.R has increased with temperature for a given concentration of the inhibitor, there is no regular trend in the change of inhibition efficiency.This may be explained on the basis of the time lag between the process of adsorption and desorption.However, the inhibitor could be effectively used at 343 K and maximum efficiency being 86.61%.

Interpretation of thermodynamic data
Activation energy (E a ) and thermodynamic data, such as change in free energy (∆Gº ads ), enthalpy (∆Hº), and entropy (∆Sº) for mild steel in 1 M HCl in the presence and in the absence of the inhibitor was calculated and listed in Table 4.The activation energy at different concentration of the inhibitor in HCl is calculated by plotting log C.R vs. 1/T (Figure 1).E a values for inhibited systems are lower than those for uninhibited system indicating that all the inhibitors exhibit high I.E at elevated temperatures.The range of E a is 32.77-20.12kJ/mol for CAL in 1 M HCl.The negative values of ∆Gº ads (9.22-27.90kJ) suggest the strong interaction of the inhibitor molecules whereas low value of ∆Gº ads indicated spontaneous adsorption of inhibitors on MS surface.The negative values of ∆Hº (6.65-27.59kJ/mol) indicate that the adsorption of inhibitor molecules is an exothermic reaction.The change in entropy was found to be greater than zero (0.09-0.15).This indicates that the reaction is irreversible.It is clear that, the complete desorption of the inhibitor is not possible.

Applicability of adsorption isotherms
The surface coverage (θ) values for different concentrations of the inhibitors in both medium have been evaluated from the weight loss data.The data were tested graphically to find a suitable adsorption isotherm.A plot of log (θ/ (1-θ)) against log C (Figure 2) shows a straight line (R>0.9)indicating that adsorption follows the Langmuir adsorption isotherm.It is observed that although these plots are linear, the gradients are never unity, contrary to what is expected for ideal Langmuir adsorption isotherm equation.Organic molecules having polar atoms or groups which are adsorbed on the metal surface may interact by mutual repulsion or attraction and this may be advocated as the reason for the departure of the slope values from unity.The number of active sites of the surface occupied by one molecule of the inhibitor is given by the value of (1/y).A straight line was obtained when the surface coverage was plotted against log C for the inhibitor.This shows that the adsorption obeys a Temkin adsorption isoltherm, which is graphically represented in Figure 3.The plots of log θ vs. log C are shown in Figure 4.The linearity shows that the adsorption of the inhibitor on mild steel surface follows Freundlich isotherm.The plots of IE against log C are shown in Figure 5.The sigmoidal shape shows that the adsorption of the inhibitor on mild steel surface follows Frumkin isotherm.
-   The equation of Flory Huggins Isotherm, log θ/C = log XK+X log (1-θ) where, θ is the degree of coverage X is the number of active sites occupied by one inhibitor molecule or number of water molecules replaced by one molecule of the adsorbate.The value of X>1, implied that one inhibitor molecule replaces more than one water molecule.The plots of log (θ/C) against log (1-θ) are shown in Figure 6.

Potentiodynamic polarization results
The various electrochemical parameters calculated from Tafel plot (Figure 7) are given in Table 5.The lower corrosion current density (I corr ) values in the presence of inhibitors without causing significant changes in corrosion potential (E corr ) (522.29-502.15mV) suggests that the compound is mixed type inhibitor (i.e., inhibit both anodic and cathodic reactions) and are adsorbed on the surface thereby blocking the corrosion reaction.In all concentrations b a is greater than b c suggesting that though the inhibition is under mixed control, the effect of the inhibitor on the anodic polarization is more pronounced than on the cathodic polarization.

Constituents present in CAL
The probable mechanism can be explained on the basis of adsorption process and the structure of the constituents present in the extract.The inhibition may be due to the adsorption of phytochemical constituents present in the extract through oxygen atoms on to the surface of the metal.

Surface analysis
The polished specimen and the test specimens which are immersed in the blank (1 M HCl) and in the inhibitor CAL for 168 h, were observed under a metallurgical microscope and photomicrographs are shown in the plate 1 and 2. Photograph 1 shows the polished mild steel surface before exposure to the corrosion solution, which is associated with polishing scratches.It is clear from the photograph 2, the surface of the mild steel was heavily corroded in 1 M HCl, whereas in the presence of inhibitor in 1 M HCl, the surface condition was comparatively better (Photographs 3 and 4) depends on the concentration of the inhibitor solution suggesting that thereby the presence of a protective adsorbed layer of the inhibitor on mild steel surface which impedes corrosion rate of metal appreciably.

Conclusion
• The natural extract CAL was found to be effective inhibitor in the acidic medium giving up to 97.51 % efficiency.The extracts under study resist corrosion at higher temperature in 1 M HCl, the optimum temperature being 343 K. • Adsorption models-Langmuir, Temkin, Freundlich, Frumkin and Flory-Huggins isotherm fit well as evident from the correlation coefficient values (R ≅ 1 in all cases).This proves the applicability of all the models to the process.• The surface analysis study confirms the corrosion of mild steel and its inhibition by the inhibitor CAL.• The results obtained from the polarization study revealed that the extract under study behaved as a mixed type of inhibitor.• The inhibitor CAL can be adsorbed on the metal surface through their oxygen atom of the constituents present in the extracts.
10 mV amplitude was applied to the electrode.The frequency which is varied from 10 KHz to 100 MHz was superimposed at the open circuit potential.The results are presented in the form of Nyquist and Bode plots.All the measurements were presented in the corrosion potential.In Impedance method, the charge transfer resistance (R ct ) is obtained from the plots of Z' vs. Z'' (real part Vs imaginary part; Nyquist plot).The value (R p + R s ) corresponds to the point where the plot cuts Z axis to the point where the semicircle cuts the Z axis at high frequency.The difference gives the R p values, which were substituted in the Stern-Geary equation to get I corr , b a x b c I corr = 2.3(b a + b c ) R ct The b a and b c values were experimentally obtained as mentioned in the case of linear polarization for each system.

Table 2 .
Inhibition efficiency of MS in 1 M HCl in the presence of CAL at 303 K.

Table 3 .
Protection performance of CAL on MS in 1 M HCl (½ h) at different temperatures.

Table 4 .
Activation parameters for the dissolution of mild steel in the presence of CAL in 1 M HCl.

Table 5 .
Electrochemical parameters for mild steel in 1 M HCl containing CAL extract.Impedance diagram (Nyquist plot) obtained for mild steel in 1 M HCl in the presence of various concentrations of the inhibitor is depicted in Figure8.They are perfect semicircles and this was attributed to charge transfer reaction.Impedance parameters derived from Nyquist plots are tabulated in Table6.It can be seen that as the concentration of inhibitor increases, C dl values decrease.Decrease in C dl , which can result from an increase in thickness of electrical double layer, suggests that the inhibitor molecules function by adsorption at the metal-solution interface.

Table 6 .
Impedance parameters for the corrosion of mild steel in 1 M HCl containing different concentrations of CAL.