Table-1Inhibition Efficiency of MS in 1 N H2SO4 at various temperatures in the presence and absence of different concentrations of Malachra capitata leaves extract.

Temprature (K)	Concentration (ppm)	W (µg/cm² h)	Inhibition Efficiency IE (%)	θ
303	1 N H₂SO₄	16.25	-	-
	25	3.69	77.29	0.7729
	50	2.64	83.75	0.8375
	100	2.09	87.14	0.8714
	150	1.74	89.29	0.8929
	200	1.42	91.26	0.9126
313	1 N H₂SO₄	17.31	-	-
	25	4.22	75.62	0.7562
	50	3.24	81.28	0.8128
	100	2.61	84.92	0.8492
	150	2.14	87.63	0.8763
	200	1.86	89.25	0.8925
323	1 N H₂SO₄	18.03	-	-
	25	4.90	72.82	0.7282
	50	3.92	78.26	0.7826
	100	3.32	81.59	0.8159
	150	2.81	84.41	0.8441
	200	2.29	87.29	0.8729

Table-2 Effect of Malachra capitata leaves extracts on corrosion of MS in 1N H2SO4 solution studied by Tafel polarization method

Concentration of inhibitor (ppm)	E_corr V	Tafel Constant (mV/decade) ba bc		Corrosion Current Density i_corr(mA/cm²)	Inhibition Efficiency IE (%)
0	-0.4997	143	160	5.914	-
25	-0.4883	105	141	1.136	81
50	-0.4750	96	142	0.939	84
100	-0.4724	92	142	0.603	90
150	-0.4644	87	143	0.536	91
200	-0.4627	83	143	0.394	93

Fig.1. Langmuir adsorption plots for mild steel in 1N H₂SO₄at different temperatures.

were washed, dried and weighed. From the weight loss data, percent inhibition efficiency (IE %) was calculated.

Electrochemical measurements:

A three-electrode cell of Borosil glass, consisting of a working electrode (WE) of MS, a pure platinum counter electrode (CE), and Saturated calomel electrode (SCE) as a reference electrode, was used for the measurements.

The electrolytes used were acidic solutions maintained at 30^oC. The AC impedance measurements are shown as Nyquist plots and polarization data as Tafel plots. CH Electrochemical analyzer model 608 C (USA) was used for this purpose. SEM analysis was carried out using HITACHI model S3400 N scanning electron microscope.

RESULTS AND DISCUSSION:

Based on weight loss measurements, the corrosion rate (W_corr) and the values of inhibition efficiency (E_w%) for various concentrations of Malachra capitata leaves extract after 2 h of immersion at 303K, 313K and 323K are given in Table - 1. The relation equation determines the inhibition efficiency (E_w%).

….. (1)

Where, W_corrand W₀ are the corrosion rates of steel with and without the inhibitor, respectively.

From the table it is clear that the E_w% increases with the increase of concentration reaching a maximum value of 91.26% at a higher concentration of 200 ppm at 303K. This suggests that increase in the inhibitor concentration increases the number of molecules adsorbed over the mild steel surface, blocking the active sites of acid attack and thereby protecting the metal from corrosion. Generally the corrosion increases with the rise of temperature and that we can see from the Table – 1.

The degree of surface coverage θ for different concentrations of the inhibitor in acidic media has been evaluated from weight loss using the equation:

..… (2)

Where, W_m is the smallest corrosion rate. From Table – 1, the increase of W₀ is more pronounced with the rise of temperature for the blank solution. In the presence of Malachra capitata leaves extract θ decreases slightly with increasing experimental temperature, which could be caused by, desorption of the inhibitor from the steel surface. The slight decrease of θ suggests that the efficiency of Malachra capitata leaves extract is independent of temperature. The result shows that Malachra capitata leaves extract effectively protects the steel even at high temperature.

Fig.2. Tafel plots showing effect of Malachra capitata leaves extracts on corrosion of MS in H₂SO₄ medium

The adsorption of the inhibitor is influenced by the nature and the charge of the metal, the chemical structure of the inhibitor, distribution of the charge in the molecule, and the type of electrolyte^16–20. Important information about the interaction between the inhibitor and steel surface can be provided by the adsorption isotherm. In the above work, it could be concluded that θ increases with the inhibitor concentration; this is attributed to more adsorption of inhibitor molecules onto the steel surface. Now, assuming that the adsorption of Malachra capitata leaves extract belonged to the monolayer adsorption, then the Langmuir adsorption isotherm is applied to investigate the mechanism by the following equation:

….. (3)

Where, C is the inhibitor concentration in the electrolyte and K is the equilibrium constant for the adsorption/ desorption process.

Fig.3. Nyquist plots showing effect of Malachra capitata leaves extracts on corrosion of MS in H₂SO₄ medium

Three representative Langmuir adsorption plots at different temperatures are shown in Fig.1. Linear plots are obtained with slopes equal to 1.10, 1.12 and 1.14 for the experimental temperature at 303, 313 and 323 K, respectively. These results indicate that some constituent of Malachra capitata leaves unit occupies more than one adsorption site on the steel surface. A modified Langmuir adsorption isotherm ^21-22 could be applied to this phenomenon, which is given by the corrected equation:

.…. (4)

The potentiodynamic polarization data are shown as the Tafel plots for MS in 1 N H₂SO₄ with the addition of various concentrations of the additive in Fig.2. The corrosion kinetic parameters such as corrosion potential (E_corr), corrosion current density (I_corr), anodic and cathodic Tafel slopes (b_a and b_c) were derived from these curves and are given in Table-2. The values of E_I% are calculated using the following equation.

….. (5)

Where, I_corr and I_corr(inh)are the values of corrosion current densities of MS without and with the additive, respectively, which were determined by extrapolation of the cathodic and anodic Tafel lines to the corrosion potential E_corr.

Fig.4. SEM images of MS a) in 1 N H₂SO₄ media b) with Malachra capitata leaves extract (200 ppm)

From the Table-2, it is observed that the I_corr values gradually decreased with gradual increase in the concentration of additive up to 200 ppm from 5.914 to 0.394 mA/cm² leading to 93% of IE. Further, there was anodic shift of the E_corr value of -0.5V (blank) to -0.46V at 200 ppm indicating that the Malachra capitata leaves extracts acted as an anodic inhibitor²³ for MS in 1 N H₂SO₄ which was supported by the gradual and significant decrease of anodic Tafel slope, b_a is 143 mV / decade of blank to 83 mV/ decade at 200 ppm. It could be derived from this decrease that the rate of anodic dissolution was much retarded in comparison to that of cathodic hydrogen evolution. There was decrease of only 17 mV/ decade in the corresponding values of cathodic Tafel slopes. This means that the extract must have acted predominantly by blocking anodic sites, and also cathodic sites to some extent, and the extract contained the active molecules which behaved as mixed-type of the acid corrosion inhibitors.

The corrosion behavior of MS in 1N H₂SO₄, in absence and the presence of various concentrations of Malachra capitata extract were also investigated by EIS technique. The resultant Nyquist plots are shown in Fig.3. The existence of a single semicircle in each plot shows that there was only single charge transfer process during the anodic dissolution of MS and remained unaffected in the presence of inhibitive molecules of the extract added in the acid. An isolated Nyquist plot for the blank system is shown in the window in Fig.3 and the value of real impedance (Z’) was only 7 Ohms which indicated that there was least charge transfer resistance (R_t) of the corrosion reactions. There was gradual increase in the diameter of each semicircle of the Nyquist plot due to increase in the number of inhibitive molecules in the extract when the concentration was raised from 25 to 200 ppm. This increase of the diameters clearly reflected that the R_t values also increased from 7 to 225 Ohms at highest concentration of 200 ppm due to formation and gradual improvement of the barrier layer of the inhibitive molecules (IE 97 %), and as a result the acid corrosion rate of MS gradually decreased.

Table-3 Data from electrochemical impedance measurements of mild steel in 1 N H2SO4 for various concentrations of Malachra capitata leaves extract

Concentration of inhibitor (ppm)	R_t Ohm.cm²	Cdl µF/cm²	Inhibition Efficiency IE (%)
0	07	155.27	-
25	40	124.13	83
50	96	92.65	93
100	128	84.23	95
150	173	73.59	96
200	225	69.83	97

Table-3 embodies various parameters such as R_t and double layer capacitance (C_dl). There was a gradual decrease in values of C_dl from 155 to 70 µF/cm² with increase in the concentration of the extract. This considerable change indicated that the inhibitive molecules of the extracts have been adsorbed on the MS surface and decreased the roughness of the MS surface. The values of E_R% were calculated by the equation as follows.

….. (6)

Where R_t and R_t(inh) are charge-transfer resistance values in the absence and presence of the inhibitor, respectively.

To obtain the double layer capacitance (Cdl), the frequency at which the imaginary component of the impedance is maximum –Z_im(max) is found and Cdl values are obtained from the following equation:

….. (7)

The SEM photograph (Fig. 4a) shows that the surface is extremely damaged in the absence of the inhibitor while (Fig. 4b) shows a formation of a film by the active Malachra capitata leaves constituent on the metal surface which may be responsible for the corrosion inhibition.

The inhibition properties of Malachra capitata must be due to the presence of nitrogenous compounds²⁴ or tannins²⁵ in the extract the leaves. However, tannins are complex astringent aromatic acidic glycosides found in various plants and their presence can be ruled out as they are made up of the polyphenols and their acidic and heterocyclic derivatives because such constituents would not have been extracted in the acid.

CONCLUSION:

The active molecules present in the extract of Malachra capitata leaves have effectively inhibited corrosion of mild steel in 1 N H₂SO₄ at various temperatures by forming a protective barrier layer. It adsorbs on the steel surface according to the modified Langmuir adsorption isotherm. The inhibition efficiency of the extract increased gradually with increase in its concentration.

Polarization measurements have shown that the extract of Malachra capitata leaves has acted as a mixed- inhibitor, retarding predominantly anodic dissolution of steel in 1N H₂SO₄.

The results of the EIS show that inhibition efficiency increases with the concentration to attain a maximum value of 97% at 200 ppm.

The inhibition efficiency of the extract decreases marginally with the rise of the temperature.

ACKNOWLEDGEMENT:

The authors thank Dr. P. D. Porey, Director and Dr. S. Jauhari, Head of Applied Chemistry Department of S. V. National Institute of Technology, Surat for the scholarship, encouragement and facilities and Dr. T. G. Gohil, Head of Bioscience Department of B. K. M. Science College, Valsad for providing necessary help.

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Received on 07.05.2009 Modified on 11.07.2009

Asian J. Research Chem. 2(4):Oct.-Dec. 2009 page 427-431