Evaluation of Vitis vinifera as a Corrosion Inhibitor for Mild Steel in 1 M HCl

 

R Saratha^1*, P Thilagavathy¹ and N Kasthuri²

¹Department of Chemistry, Avinashilingam University for Women, Coimbatore, Tamilnadu

²Department of Chemistry, Sengunthar College of Engineering for Women, Tiruchengode, Tamilnadu

*Corresponding Author E-mail:  saratha63@gmail.com  

ABSTRACT:

The leaves of Vitis vinifera has been tried as a green corrosion inhibitor for mild steel in 1 M HCl. Its inhibitive performance was studied using mass loss method and Electrochemical methods namely potentiodynamic polarization and AC impedance method. The performance of the inhibitor at different time periods, different concentrations of the inhibitor and at various temperatures has been studied. The studies revealed that Vitis vinifera act as a good inhibitor in 1M HCl medium and the maximum efficiency was 97.31% at 2.5%v/v concentration in 24 h period and 89.42% at 323K for the same concentration of the inhibitor. Polarization study revealed that the inhibitor is mixed type with predominant anodic polarization. Activation energies of the process in absence and presence of Vitis vinifera were obtained which suggested that physisorption takes place and inhibitor is more active at lower temperatures. The adsorption isotherm models were tried and it was found to obey Langmuir, Freundlich and Temkin models.

 

 

 

INTRODUCTION:

Corrosion is the primary means by which metals deteriorate. The problem of corrosion is universal and its consequences are of catastrophic. The corrosion of iron and mild steel is of fundamental, academic and industrial concern that has received a considerable amount of attention. The consequences of corrosion are many and varied and the effects of these on the safe, reliable and efficient operation of equipment or structure are often more serious than the simple loss of a mass of metal. Enormous losses occur due to corrosion every year.

 

Inhibitors have always been considered to be the first line of defense against corrosion. Inhibitors are chemicals that react with a metallic surface or the environment this surface is exposed to, giving the surface a certain level of protection. Inhibitor often works by absorbing themselves on the metallic surface protecting the metallic surface by forming a film.

 

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 in finding more suitable inhibitor, which is non toxic.

 

Accordingly greater research efforts have been directed towards formulating environmentally acceptable inhibitors¹. Green inhibitors displaying substantially improved environmental properties are widely used at present.

 

Due to the diversity of their structures, many extracts of common plants have been used as corrosion inhibitors for metals in pickling and cleaning processes. The successful uses of naturally occurring substances like Black pepper², Calendula officinalis flower³, Eugenia jambolans⁴, Lawsonia inermis⁵, Ricinus communis leaves⁶, Andrographis paniculata⁷, Acacia Arabica⁸, Zenthoxylum alatum⁹, Poinciana pulcherrima¹⁰, Cassia occidentalis¹⁰, Datura stramonium¹⁰, Carica papaya¹¹, Musa species peels¹², Hibiscus subdariffa¹³, Lawsonia¹⁴, Nypa fruticans wurmb¹⁵, Michelia champaca¹⁶, Citrus aurantiifolia¹⁷, Emblica officinalis¹⁸,  Nyctanthes arbortristis¹⁸ and fenugreek leaves¹⁹ to inhibit the corrosion of metals have been reported. Vitis vinifera, Common grapevine belongs to the Family Vitaceae. In the present research the acid extract of leaves of Vitis vinifera was investigated for its inhibitive property for mild steel in 1M HCl medium.

 

MATERIALS:

Preparation of the specimens:

The sheets of cold rolled mild steel, which is commercially available in the market were machined into coupons of area 5 ´ 1 Cm². Holes were drilled on the center of the one end of all the coupons for suspension and the coupons are numbered for identification. These coupons were degreased, cleaned with emery paper and washed with distilled water and stored in desiccators in the absence of moisture before their use for the investigation.

 

The composition of the sample has been analyzed using ARL 3460 metal analyzer (Optical emission spectrometer). The metal was found to have the following elemental composition. Carbon-0.0715%, Silicon-0.0920%, Manganese-0.1747%, Phosphorus-0.0169%, Sulphur-0.0162%, Chromium-0.0095%, Molybdenum-0.0020%, Nickel-0.0048%, Vanadium-0.003%, Aluminium-0.0370%, Copper-0.0060%, Titanium-0.0008%, Niobium-0.0006%, Tungsten-0.0006%, Lead-0.0004% Boran-0.0007%, Antimony-0.0001%, Bismuth-0.0020%, Calcium-0.0005%, Zinc-0.0004%, Cerium-0.0001% and Iron-99.5618%.

 

Preparation of the extract:

The leaves of Vitis vinifera were shade dried and powdered. The extract was prepared by refluxing 50g of powdered dry leaves in 1000 ml of 1M HCl for 3 hours and kept overnight. Then filtered and the volume of filtrate was made up to 1000ml using the same acid. This solution was taken as the stock solution for further dilutions.

 

METHODS:

The methods used for the evaluation are Mass loss, Potentiostatic polarization and AC impedance.

 

Mass loss method:

Mild steel specimens were accurately weighed using Denver M 220 D digital balance and fully immersed in 100ml of 1M HCl in absence and presence of different concentrations of the inhibitor and at different time intervals. Test specimens were removed after the definite intervals of time and dipped in sodium bicarbonate solution for neutralization of remaining acid on the surface of the specimen, then washed with distilled water, dried and reweighed.

 

The loss in weight was determined in triplicate and the results were averaged. The corrosion rate in mpy and Inhibition efficiency are calculated using the following formulae.

               Corrosion Rate (CR) = W₀ – W_i /W₀

 

Where W_{0 is} the weight loss in the absence of the plant extract and W_i is the weight loss in the presence of different concentrations of the plant extract.

 

Inhibition Efficiency (IE) % =( CR_b – CR_i / CR_b ) * 100

 

Where CR_b is the Corrosion rate in the absence of the plant extract and CR_i is the Corrosion rate in the presence of different concentrations of the plant extract.

 

The parameters used for the present study are given below

·        Time – 1/2 h, 1 h,  3 h,  7 h,  24 h,  48 h,  168 h

·        Concentration of the inhibitor – 0.005%, 0.01%, 0.05%, 0.10%, 0.15%, 0.2%, 0.5%, 1.0%, 1.5% and  2.5%(v/v)

·            Temperatures – (303 K , 313 K , 323 K ,  333 K , 343 K)  ±  2 K

 

Potentiodynamic Polarization technique:

A frequency response analyzer 1284 (Solartron) and an IBM personal computer which automatically controls linear polarization and Tafel polarization was used for the polarization study. The data were analysed using computer software. The cell for the polarization studies was a glass beaker containing the aerated unstirred test solution with a platinum electrode as the counter electrode, a saturated calomel electrode as the reference electrode and the mild steel as the working electrode. For potentiostatic polarization studies, mild steel strips of same composition (as in the weight loss method) coated with lacquer with an exposed area of 1 Cm² was used.

 

AC impedance method:

AC impedance measurements started as soon as working temperature was achieved and polarization had stopped. Applied sinusoidal signal amplitude was 10mV in the nominal frequency range from 10 kHz to 0.02Hz. Combined frequency response analyzer and potentiostat (Solartron 1284) was used to make A.C impedance measurements under open circuit potential during the test. During each test the data were displayed as Nyquist plots (Z_im vs Z_real) via Z plot software.

 

Surface analysis:

Surface features of mild steel specimen were examined before and after exposure to 1M HCl solution in the absence and presence of a certain concentration of the extract. Optical microscope (NIKON - model EPI-PHOT) was used for this investigation.

 

RESULTS AND DISCUSSION:

Effect of concentration:

Table 1 gives the values of inhibition efficiency calculated from weight loss measurements for different concentrations of the extract ranging from (0.005 to 2.5%) in 1M HCl at different immersion times at room temperature. The increase in inhibition efficiency with the increase in the concentration of the extract was evident from the table and it was found to have maximum inhibition efficiency of 97.31% at 2.5%v/v concentration of the inhibitor in 24h period of immersion.  From the table, it is noted that the addition of inhibitor increases the Inhibition efficiency irrespective of the time of immersion. This may be due to the inhibition of corrosion of metals by the phytochemical constituents present in the acid extract of Vitis vinifera which is usually attributed to the increase in surface coverage by the inhibitor molecules that blocks the active sites in which direct acid attack takes place. Further increase in concentration of the extract increases the inhibition efficiency.

 

Effect of time of immersion:

To investigate the effect of inhibitor with period of immersion, experiments were carried out at various time intervals (1/2h, 1h, 3h, 7h, 24h, 48h and 168h) in absence and  in presence of various concentrations of the inhibitor. The results are shown in Table 1.

 

Table 1: Inhibition Efficiency values for different concentrations of the extract for the corrosion of mild steel at various immersion period

CONC (%)  (v/v)	Inhibition Efficiency (%)
	½ h	1 h	3 h	7 h	24 h	48 h	168 h
0.005 0.010 0.050 0.100 0.150 0.200 0.500 1.000 1.500 2.500	30.57 37.64 42.51 47.45 54.37 56.50 58.48 61.14 63.27 64.41	32.43 52.88 57.59 61.34 61.98 64.86 70.94 72.68 74.20 75.00	35.56 59.87 65.34 79.10 79.96 82.11 83.62 85.89 87.55 89.70	46.47 73.03 80.02 82.20 83.30 84.05 88.46 90.35 90.99 91.85	71.63 73.73 81.22 82.94 85.45 86.25 88.56 91.35 91.83 97.31	57.49 64.08 68.68 71.43 73.99 76.90 79.64 81.28 86.19 89.00	33.29 44.36 56.63 65.12 66.23 69.21 71.64 72.59 74.60 77.80

 

 

The table reveals that for all the concentrations of the inhibitor, the inhibition efficiency increases from ½ h to 24 h period of immersion of mild steel in the corrosive medium (64.41% to 97.31 at 2.5%v/v). After 24 h period of immersion, the inhibition efficiency decreased with time. As the time of immersion increases the process of adsorption of phytochemical constituents of the extract on the metal surface takes place gradually. After 24 h period of immersion, desorption of the constituents from the metal surface results in depletion of inhibition efficiency.

 

Effect of temperature:

The effect of temperature on the inhibitory action of the extract was determined by weight loss method for various concentrations at different temperatures (303K, 313K, 323K, 333K and 343K, ± 2) for a fixed immersion time of 1/2 h. The tabulated data (Table 2) reveal that as the concentration of the inhibitor increased there is no regular trend in the change of inhibition efficiency with increase in temperature

 

Table 2: Protection performance of Vitis vinifera on mild steel in 1M HCl at different temperatures

Conc % (v/v)	IE %
	303K	313K	323K	333K	343K
Blank	-	-	-	-	-
0.005	30.57	31.24	53.51	50.74	54.16
0.010	37.64	48.50	60.15	57.76	55.24
0.050	42.51	58.14	62.92	59.73	56.77
0.100	47.45	63.31	67.23	62.06	60.02
0.150	54.37	63.37	68.90	65.21	62.07
0.200	56.5	69.72	73.48	69.27	65.37
0.500	58.48	72.14	75.23	72.21	67.47
1.500	61.14	81.45	83.54	78.37	71.66
2.000	63.27	82.28	87.13	82.55	73.46
2.500	64.41	83.45	89.42	85.55	77.35

 

It can be seen from the table that, as the temperature increased from 303K to 323K there is an increase in inhibition efficiency after which slight depletion in the inhibition efficiency is noted and the maximum efficiency of the extract was 89.42 % at 2.5 % (v/v). The change in IE with increase in temperature is due to the different mechanism of action of the inhibitor on the metal surface.

 

The thermodynamic parameters:

Activation energy (E_a) and the thermodynamic parameters like change in free energy of adsorption (ΔG_ads), enthalpy (ΔH), and entropy (ΔS) are purposeful to find out whether the system

 

undergoes physical or chemical adsorption. The calculated values of E_a and thermodynamic parameters are given in Table 3.

 

Energy of activation:

The activation energy was calculated from the plot of log corrosion rate as a function of inverse of temperature (Figure-1). The values of slope obtained could give the activation energy.

 

Inspection of the data given in table reveals that the activation energy (E_a) in 1M HCl solution in the absence of the extract was 33.44 kJ/mol. The addition of the inhibitors to the acid solution increases the activation energy to 41.32 kJ/mol till 0.2% and then decreases the activation energy to 24.03 kJ/mol at 2.5% (v/v). The increase in activation energy in the presence of the extract indicates that the inhibition probably occurs via formation of a physisorbed monolayer on the metal surface. It indicates that the inhibitor is more effective at lower temperatures for lower concentration and higher temperature for higher concentration.

 

Fig.1-Arrhenius plot for mild steel dissolution process in 1M HCl:

Table 3: Activation energy (e_a) and Thermodynamic parameters for the corrosion of mild steel in 1M HCl

Conc (%) (v/v)	Activation energy (Ea) KJ/mol	Free energy of adsorption (-∆Gads) KJ/mol					Heat of adsorption (∆H) KJ/mol	Entropy changes (∆S) KJ/deg/mol
		303 K	313 K	323 K	333 K	343 K
Blank 0.005 0.010 0.050 0.100 0.150 0.200 0.500 1.500 2.000 2.500	33.44 34.97 37.60 39.96 40.38 41.32 39.67 38.45 33.80 28.23 24.03	- 21.36 20.97 17.66 16.46 15.86 15.60 13.53 11.64 11.22 10.81	- 22.86 22.17 18.57 17.27 16.57 16.30 14.18 12.21 11.83 11.54	- 24.35 23.36 19.48 18.07 17.29 17.01 14.83 12.79 12.44 12.27	- 25.85 24.55 20.40 18.87 18.00 17.72 15.48 13.36 13.05 13.00	- 27.35 25.74 21.31 19.67 18.72 18.42 16.13 13.93 13.67 13.73	- - 23.959 0.092 0.053 0.039 0.044 0.031 0.023 -0.014 0.061 0.073	- 0.149 0.119 0.091 0.080 0.071 0.070 0.065 0.057 0.061 0.073

 

Table 4: Electrochemical parameters for mild steel IN 1M HCl with various concentrations of the extract

Conc (%) (v/v)	-Ecorr mV	I corr µA cm-2	Ba mV/dec	Bc mV/dec	I.E (%)	Rp (Ω cm2)		I.E (%)
Blank 0.005 0.05 0.15 0.5 2.5	521.94 521.29 513.45 512.63 508.52 511.06	4.492 3.839 0.098 0.061 0.037 0.012	213.46 203.81 123.95 107.70 120.47 128.67	137.82 132.84 87.58 84.96 85.38 86.80	- 14.54 97.82 98.64 99.18 99.73		8.16 9.12 18.26 25.87 58.31 174.56	- 10.53 55.31 68.46 86.00 95.33

 

 

Free energy of adsorption (ΔG), Entropy (ΔS), Enthalpy (ΔH):

The low and negative values of -10.81 kJ/mol for ΔGads (Table 3) indicate the spontaneous adsorption of inhibitor on the surface of the mild steel. It also suggests the strong interaction of inhibitor molecules on to the mild steel surface. The ΔG values at all concentrations are found to be below 40 kJ/mol, which indicate physical adsorption of the extract over the surface of mild steel. The negative value of enthalpy showed that the metal dissolution process is accompanied by the release of higher amount of heat energy (ie) the process is exothermic. The change in entropy (ΔS) was found to be greater than zero. This indicates that the reaction is irreversible.

 

Adsorption Isotherms:

The phenomenon of interaction between the metal surface and inhibitor can be better understood in terms of adsorption isotherm. An adsorption isotherm is a mathematical expression, which relates the bulk concentration of an adsorbing species to its surface coverage at constant temperature. The surface coverage (θ) values were evaluated using corrosion rate values (k) obtained from weight loss method. The θ values for different inhibitor concentration were tested by fitting various isotherms such as Langmuir, Freundlich and Temkin.

 

Langmuir Adsorption Isotherm:

Langmuir adsorption isotherm has been conveniently used to study inhibitor characters. Assuming that the percentage area covered by the inhibitor is directly related to the retardation in C.R, the compound should obey the Langmuir adsorption isotherm in which log (θ/1- θ) is a linear function of log C according to the equation.

               Log C  =  log (θ/1- θ) – log K

C = bulk concentration of the inhibitor

K = adsorption equilibrium constant

               K = exp (-ΔG_ads/ RT)

The data obtained give a straight line at all temperatures. Figure 2 illustrates the Langmuir plot for the inhibitor at various times. It is observed that the plot is linear.

 

Freundlich Adsorption Isotherm:

The plot of log θ vs log C is shown in Figure 3. The linearity shows that the adsorption of the inhibitor on mild steel surface follow Freundlich adsorption isotherm.

 

Fig.– 2 Langmuir Isotherm for the adsorption of inhibitor on mild steel surface in 1M HCl

 

Fig.- 3 Freundlich Isotherm for the adsorption of inhibitor on mild steel surface in 1M HCl

 

Temkin Adsorption Isotherm

The surface coverage (θ) values for different concentration of the inhibitor have been evaluated from the weight loss data. The data were tested graphically to find suitable adsorption isotherms. 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 isotherm, which is graphically represented in Figure 4.

 

Fig.– 4 Temkin Isotherm for the adsorption of inhibitor on mild steel surface in 1M HCl

 

Potentiodynamic Polarization Studies:

The values of various electrochemical parameters are presented in the Table 4 and corresponding Tafel plot in Figure 5. It is observed from the table that the I_corr values decrease in the presence of the inhibitor. These suggest that the adsorption of the inhibitor molecules on the metal surface reduces the uncovered surface area for the anodic as well as cathodic reactions. This is also seen from the increase in the R_p values. The fact there is no significant change in the E_corr values 521.29 mV to 508.52 suggests that the inhibitor function as a mixed type. 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.

 

Fig.-5 Potentiodynamic polarization curves for mild steel in 1M HCl in presence of diffferent concentrations of the extract at room temperature

 

Impedance Measurements:

Impedance diagrams (Nyquist plots) obtained for mild steel in 1M HCl in the presence of various concentrations of the inhibitor is depicted in the Figure 6. It is seen from these figures that the impedance diagrams for the inhibitor are not perfect semicircles. The difference has been attributed to the frequency dispersion.

 

Fig.-6:Nyquist plot for mild steel in 1 M HCl containing different concentrations of the extract at room temperature

Impedance parameters derived from Nyquist plots are tabulated in table 5. It can be seen that the presence of the inhibitor enhances the values of R_ct and reduces the C_dl in at 2.5% (v/v) of the extract. The decrease in C_dl may be due to the adsorption of the inhibitor to form an adherent film on the metal surface and suggest that the coverage of the metal surface with this film decreases the double layer thickness.

 

Table 5: Impedance Parameters for the corrosion of mild steel in 1M HCl containing different concentrations of the extract at room temperature

Conc (%)  (v/v)	Rct (Ω cm2)	Cdl (µF cm-2)	I.E (%)
Blank 0.005 0.05 0.15 0.50 2.5	79.35 45.90 109.06 114.80 237.71 439.86	160.14 150.23 127.17 119.88 96.61 71.32	- - 42.16 27.24 30.88 66.66 81.96

 

Mechanism of Corrosion Inhibition

The acid extracts investigated in the study are organic in nature and found to contain the following compounds.

 

 (+) – catechin                              Alpha-terpineol

          

anthocyanin                                 Ascorbic acid

 

beta- amyrin                               Caffeic Acid

 

Chlorogenic acid                           D- Catechin

      

Flavone                                             Geraniol

                    

Glycolic acid                               Kaempferol

                  

Pectin                                               Quercetin

   

The probable mechanism can be explained on the basis of adsorption process and structure of the constituents present in the extract. Most of the phytochemical constituents present are Oxygen containing compounds. The compounds may be adsorbed on the mild steel surface through their oxygen atom of the hydroxyl group, oxygen atom on the ring and the π electrons in the ring which inhibits the corrosion of mild steel. The protective effect may also be due to the presence of N atoms as amide group. In most of the molecules free electrons exist on O atom. The free electrons on both the O and N atoms form bonds with electrons on the metal surface. The double bond and the –CO group activate free electrons on the O atom. The activated free electrons form a strong bond with electrons on the metal surface. A hydrogen atom attached to the C in the ring when replaced by a substituent group (- COOH) improves inhibition. A nucleophilic group (-COOH) reduces the charge to less than one electronic charge distributed over the whole ring. The aromatic compounds with large cross sectional area are able to pack close enough to make an impervious covering on the metal. Hence the corrosion inhibition may be due the formation of a complex, which can cause blocking of micro anodes and micro cathodes that are generated on the metal surface when the metal comes in contact with the corrodent and hence can retard the dissolution of the metal.

 

Surface Analysis:

The polished specimen and the test specimens which are immersed in the blank (1M HCl) and in certain concentrations of the inhibitor VV for 24h were observed under a metallurgical microscope and photomicrographs are shown in the plates 1- 4.

 

1. Photomicrographs of Mild Steel Samples

 

2. Sample Immersed in 1M HCl

 

3.  Sample Immersed in 0.2%  inhibitor solution

 

4. Sample immersed in 2.5% inhibitor solution

 

Photograph 1 shows the polished mild steel surface before exposure to the acid medium, which is associated with polishing scratches. It is clear from the photograph 2, that the surface of the mild steel was heavily corroded in 1M HCl medium, whereas in the presence of inhibitor, the surface condition was comparatively better (photographs 3 and 4). This 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:

From the results of the present study it can be concluded that Vitis vinifera acts as a good inhibitor for mild steel in 1 M HCl medium. Vitis vinifera inhibited the corrosion of mild steel by physical adsorption on the surface. Based on the variations in the inhibition efficiencies with temperature, values of activation energy and free energy changes, the reaction is found to be spontaneous physical adsorption. The inhibitor is found to be of mixed type.

 

ACKNOWLEDGEMENT:

The authors would like to express their deep gratitude to the authorities of Avinashilingam University for Women, Coimbatore for providing facilities to carry out this research work.

 

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Received on 27.09.2009        Modified on 29.11.2009

Accepted on 22.12.2009        © AJRC All right reserved