Studies on Ervatinine – The anticorrosive phytoconstituent of Ervatamia coronaria

M.G. Sethuraman; V. Aishwarya; C. Kamal; T. Jebakumar Immanuel Edison

doi:10.1016/j.arabjc.2012.10.013

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Original article

10 (

1_suppl

); S522-S530

doi:

10.1016/j.arabjc.2012.10.013

Studies on Ervatinine – The anticorrosive phytoconstituent of Ervatamia coronaria

M.G. Sethuraman^⁎, V. Aishwarya, C. Kamal, T. Jebakumar Immanuel Edison

Department of Chemistry, Gandhigram Rural Institute – Deemed University, Gandhigram 624 302, Dindigul District, Tamil Nadu, India

⁎Corresponding author. Tel.: +91 451 2452371; mobile: +91 944 3021565. mgsethu@gmail.com (M.G. Sethuraman)

Received: 2011-8-22, Accepted: 2012-10-15, Published: 2012-02-01

Disclaimer:
This article was originally published by Elsevier and was migrated to Scientific Scholar after the change of Publisher.

Peer review under responsibility of King Saud University.

Abstract

The inhibition of corrosion of mild steel in 1M HCl and H₂SO₄ acid solutions by Ervatamia coronaria has been evaluated by a weight loss method. The results indicate that the plant extract possesses a significant anticorrosion effect. Ervatinine, the alkaloid present in the leaves of the plant has been isolated and its anti-corrosive potential has been investigated using weight loss, electrochemical impedance, Tafel polarization, scanning electron microscope and X-ray diffraction techniques. The results suggest that ervatinine acts as a good corrosion inhibitor. The adsorption of ervatinine on mild steel surface obeyed Langmuir adsorption isotherm following physisorption mode. The thermodynamic parameters such as adsorption equilibrium constant, standard free energy of adsorption and activation energy have been calculated to determine the mechanism of corrosion inhibition. Results of electrochemical measurements such as potentiodynamic polarization and electrochemical impedance spectroscopy revealed the mode of inhibitive action and adsorption of inhibitor molecules. Further, surface morphological examination supports the protective film formation by ervatinine on mild steel surface. The results confirmed the influencing role of ervatinine in the inhibition of corrosion of mild steel in acid media by extract of E. coronaria.

Keywords

Ervatamia coronaria

Ervatinine

Mild steel

Langmuir isotherm

Impedance

Polarization

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1 Introduction

Mild steel (MS) is a major construction material, which is extensively used in many food, petroleum, power production, chemical and electrochemical industries. But it suffers from severe corrosion in acidic environments. The use of inhibitors to control the destructive attack of acids finds widespread applications in these industries (Wahdan et al., 2002). The inhibition of MS corrosion in acid solutions by different types of organic inhibitors has been extensively studied (Bentis et al., 2000). Nitrogen and sulfur containing organic compounds are known to be efficient corrosion inhibitors in acidic solutions whose toxic effects are well documented (Rafiquee et al., 2007). Also the synthetic inhibitors are more expensive. To overcome the drawbacks of synthetic corrosion inhibitors, now the research is focused toward the development of eco-friendly and cost-effective corrosion inhibitors. Plant extracts can serve as non-toxic and cost-effective corrosion inhibitors (Bothi Raja and Sethuraman, 2008; Xiong et al., 2003). In the recent past, there are several reports documented on the development of green corrosion inhibitors (Kamal and Sethuraman, 2012b; Eduok et al., 2012; Al-Otaibi et al., 2014; Fernando Sílvio de Souza et al., 2012 ).

Ervatamia coronaria (E. coronaria) belonging to Apocynaceae family has been used in the indigenous system of medicine for treatment of ophthalmia, for application on wounds and for a variety of ailments. A number of alkaloids have been reported previously from the leaves, stem-bark and roots of the plant. About 14 indole alkaloids and three tri-terpenoids have been isolated from E. coronaria (Atta-Ur-Rahman et al., 1983, 1984a,b, 1986a,b; Atta-Ur-Rahman and Anjum Muzaffar, 1985 ). In the present study the authors have analyzed the anti-corrosion property of acid extract of leaves of E. coronaria as well as that of ervatinine, an alkaloid isolated from the leaves of E. coronaria.

2 Materials and methods

2.1

2.1 Preparation of E. coronaria extract

About 500 g of dried and powdered E. coronaria leaves was refluxed with 10% HCl and 10% H₂SO₄ separately for 6 h, with a view to extract the active components. The extracts were then filtered off and concentrated up to 250 ml. From this solution, different concentrations of the plant extract were made and used for the weight loss studies.

2.2

2.2 Isolation of EVT

From the leaves of E. coronaria, ervatinine (EVT) was isolated as given in the literature (Atta-Ur-Rahman et al., 1985). Then its purity was checked by TLC and its IR and NMR spectra were recorded which correlated well with the previous report.

2.3

2.3 Specimen preparation

The corrosion tests were performed on MS specimens of the following composition (wt.%): C = 0.016, Mn = 0.34, P = 0.08, and remainder Fe. MS specimens of size 1.5 cm × 5 cm × 0.2 cm were used for weight loss and a specimen with an exposed area of 1 cm² was used for electrochemical study. The surface preparations of the mechanically polished specimens were carried out using different grades of emery paper and subsequent degreasing with acetone was also done.

2.4

2.4 Weight loss measurements

Weight loss measurements were performed using MS specimens of size (1.5 cm × 5 cm × 0.2 cm) immersed in 1M HCl and 1M H₂SO₄ solutions separately with and without addition of different concentrations of the extract and EVT separately for a period of 2 h. Weight loss studies were performed at three different temperatures viz. 303, 313 and 323 K. After immersion, the surface of the specimen was cleaned by acetone, rinsed and the sample was weighed again in order to calculate the Inhibition efficiency (IE) (Xianghong Li et al., 2008).

2.5

2.5 Electrochemical measurements

Electrochemical impedance (EIS) measurements and potentiodynamic polarization studies were carried out using a CH 650B electrochemical analyzer. All electrochemical experiments were performed in a conventional three electrode electrochemical cell at 303 K under atmospheric conditions with platinum as counter electrode and a saturated calomel electrode (SCE) as the reference electrode. The working electrode MS encapsulated with Teflon with the exposed surface of 1 × 1 cm², was placed into the test solution and then the open circuit potential was measured after 30 min. EIS measurements were performed at corrosion potentials, E_corr, in the frequency range of 100 kHz to 10 mHz with a signal amplitude perturbation of 5 mV. The potentiodynamic polarization measurements were started from cathodic to the anodic direction (OCP ± 300 mV) with a scan rate of 1 mVs⁻¹ after 30 min of immersion. All potentials were recorded with respect to the SCE (Vracar and Drazic, 2002).

2.6

2.6 Surface analysis

The surface (1 × 1 cm²) of MS specimens immersed in 1 M HCl and 1 M H₂SO₄ separately in the absence and presence of 50 ppm concentration of EVT for 2 h at 303 K was analyzed by scanning electron microscopy (SEM). The specimens were taken out, cleaned with distilled water, rinsed with acetone, dried and finally used for SEM. SEM experiments were performed with instrument model VEGAS-TESCAN scanning electron microscope.

2.7

2.7 XRD measurements

The film formation on MS surface exposed to corrosive medium with and without inhibitor concentration was studied after 24 h using INEL XRD spectrometer.

3 Results and discussion

3.1

3.1 Weight loss measurements

The results of the weight loss studies for MS in 1 M HCl and 1 M H₂SO₄ solutions in the absence and presence of different concentrations of extract of E. coronaria as well as the isolated EVT are summarized in Table 1. The IE was calculated by using the following equation,

(1)

IE (%) = \frac{(W_{0} - W_{inh})}{W_{0}} \times 100

where W₀ and W_inh are the weight loss values of MS in the absence and presence of the inhibitor, respectively. The results indicate that the plant extract produces a significant anticorrosive effect. The EVT isolated from E. coronaria, when subjected to weight loss studies, could produce the IE to the extent of 82% even at a concentration of 50 ppm. It can be observed that the IE increases with increasing the concentrations of the inhibitor (see Fig. 1.)

Table 1 Effect of EVT on the acid induced corrosion of MS at various temperatures.

Concentration of inhibitor W/V (%)	Acid Extract of E.coronaria IE (%)						Concentration of inhibitor (ppm)	Isolated EVT IE (%)
	HCl			H₂SO₄				HCl			H₂SO₄
	303 K	313 K	323 K	303 K	313 K	323 K		303 K	313 K	323 K	303 K	313 K	323 K
10	69.86	67.99	65.01	71.95	69.45	68.17	10	66.66	65.50	64.55	67.63	66.03	65.01
20	73.11	71.27	69.38	75.64	73.48	70.99	20	71.11	69.40	68.54	72.05	71.08	70.21
30	79.45	76.64	74.82	81.62	79.37	78.43	30	77.07	73.71	72.81	77.18	76.61	76.01
40	83.97	79.62	77.27	86.97	83.79	81.72	40	81.48	75.97	74.76	82.49	80.46	77.12
50	86.84	82.17	79.35	89.92	86.11	84.19	50	82.22	79.05	77.09	83.79	81.16	77.99

3.2

3.2 Effect of temperature

The effect of temperature on the inhibition efficiency for MS in both the acid media in the absence and presence of different concentrations of inhibitor at 303, 313 and 333 K was studied and the results are given in Table 1. The inhibition efficiencies are found to decrease with increasing the solution temperature from 303 to 333 K. This behavior can be interpreted on the basis that the increase in temperature results in desorption of the inhibitor from the surface of MS. Similar behavior was observed in the case of both plant extract and EVT. From the values of Table 1, it is clear that the inhibition efficiency of E. coronaria is higher in 1 M H₂SO₄ than in 1 M HCl. This is due to the availability of more sites on the metal surface for adsorption in H₂SO₄ medium because of lesser adsorption of the sulfate ions on the metal surface (Ahamad and Quraishi, 2009).

3.3

3.3 Adsorption isotherm

The adsorption behavior of the inhibitor on the electrode surface was investigated to understand its role in corrosion control (Maayta and Al-Rawashdeh, 2004). The values of surface coverage, (θ) computed from the results of the weight loss study, for different concentrations of the EVT have been used to test the best adsorption isotherm. A straight line was obtained on plotting ln θ/(1−θ) vs. ln c (concentration of inhibitor) which suggested that the adsorption of the EVT on MS follows Langmuir adsorption isotherm (Fig. 2).

Langmuir plots of EVT on MS in (a) 1 M HCl (b) 1 M H2SO4.

3.4

3.4 Thermodynamic parameters

The plot of the logarithm of the corrosion rate (mg cm⁻² h⁻¹) of MS obtained from weight loss measurements vs. 1000/T gave a straight line (Fig. 3).

Arrhenius plots for MS in (a) 1 M HCl (b) 1 M H2SO4 solutions in the absence and presence of various concentrations of EVT.

The apparent activation energy (E_a) was calculated by using the following relationship:

(2)

In ({CR}_{corr}) = \frac{- E_{a}}{RT} + A

where E_a is the apparent activation energy for the corrosion of MS in 1 M HCl and 1 M H₂SO₄ solutions, R the general gas constant, A the Arrhenius pre-exponential factor and T is the absolute temperature. The values of E_a obtained from the slope of the lines are given in Table 2.

Table 2 Thermodynamic parameters for mild steel in 1 M HCl and 1 M H₂SO₄ in the presence of EVT (obtained from weight loss measurements).

Medium	Concentration of inhibitor (ppm)	E_a (kJ/mol)
HCl	0	87.21
	10	89.20
	20	89.87
	30	93.28
	40	99.10
	50	95.86
H₂SO₄	0	56.98
	10	60.24
	20	59.50
	30	59.42
	40	67.93
	50	70.34

The data show that the thermodynamic activation function (E_a) of the corrosion of MS in 1 M HCl and 1 M H₂SO₄ solutions in the presence of EVT is higher than that in the free acid solution. The values of E_a increase with increasing inhibitor concentration, indicating that there is an energy barrier to mass and charge transfer due to the adsorption of inhibitor on the metal surface (Zhang and Hua, 2009).

The standard free energy of adsorption (ΔG_ads) at different temperatures is calculated from the equation,

(3)

Δ G_{ads} = - RT ln (55.5 K_{ads})

where K_ads is the equilibrium constant and is given by:

(4)

K_{ads} = \frac{θ}{C (1 - θ)}

where θ is the degree of surface coverage of the metal surface and C the concentration of EVT.

The equilibrium constant and standard free energy for MS in 1 M HCl and 1 M H₂SO₄ solutions in the presence of 50 ppm EVT are given in Table 3. The value of standard free energy of adsorption (ΔG_ads) calculated in the presence of EVT is found to be −24.5 kJ mol⁻¹ which indicates that the inhibitor is adsorbed on the metal surface by both physical and chemical processes and the negative sign of ΔG_ads indicates spontaneous interaction of EVT molecule with the corroding MS surface (Fernando Sílvio de Souza and Almir Spinelli, 2009). But as it can be seen from Table 1, the values of ΔG_ads decreases with increasing temperature which indicated that EVT is adsorbed physically to a large extent on the surface of MS. Further, the equilibrium constant decreases with increasing temperature, also suggesting that the molecules of inhibitor are physically adsorbed on the metal surface and desorption process increases with elevating temperatures (Obot et al., 2011) (see Fig. 4).

Table 3 Equilibrium constant and standard free energy for mild steel in 1 M HCl and 1 M H₂SO₄ in the presence of 50 ppm of EVT at different temperatures.

Medium	Temperature (K)	K_ads mol/L	−ΔG kJ/mol
HCl	303	231	24.7
	313	188	24.0
	323	175	23.8
H₂SO₄	303	258	24.6
	313	215	24.4
	323	174	24.1

Plot of ln(CR/T) vs. 1,000/T on MS in (a) 1 M HCl (b) 1 M H2SO4 solutions in the absence and presence of various concentrations of EVT.

3.5

3.5 Electrochemical impedance spectroscopy

The effects of the inhibitor concentration on the impedance behavior of MS in 1 M HCl and 1 M H₂SO₄ solutions have been studied. Nyquist plots and the representative Bode diagram are given in Figs. 5 and 6 respectively. The impedance spectra are obtained in semi-circular shape and only one time constant was observed in Bode diagram. This indicates the corrosion of the MS in acid solution is mainly controlled by a charge transfer process. An equivalent circuit was introduced to explain the EIS data as shown in Fig. 5(a). For the iron/acid interface model this circuit is generally used to describe EIS data. The charge transfer resistance (R_t) must be corresponding to the resistance between the metal and OHP (outer Helmholtz plane) and can be calculated from the difference in impedance at lower and higher frequencies, as suggested earlier (Tsuru et al., 1978). The frequency at which the imaginary component of the impedance is maximum (−Z_max) is found out and C_dl values are calculated from the following equation,

(5)

f (- Z_{max}) = \frac{1}{2 π C_{dl} R_{t}}

Nyquist plots of EVT on MS in (a) 1 M HCl (b)1 M H2SO4.

(a) Bode plots of EVT on MS in 1 M HCl. (b). Bode plots of EVT on MS in 1 M H2SO4.

The results of the impedance study are given in Table 4. It is clear that, the corrosion of MS in acid medium is obviously inhibited in the presence of the EVT. It is apparent that, the impedance response for MS in acid solution changes significantly with increasing inhibitor concentration. The inhibition efficiency (IE) is calculated using charge transfer resistance as follows,

(6)

IE (%) = \frac{R_{t (inh)} - R_{t}}{R_{t (inh)}} \times 100

where R_t and R_t_(inh) are the charge transfer resistance values without and with inhibitor for MS in 1 M acid solutions, respectively. As the inhibitor concentration increased, the R_t values increased, while the C_dl values decreased. The decrease in C_dl value is due to the adsorption of inhibitor on the metal surface. The inhibition efficiency increased with increasing inhibitor concentration.

Table 4 Electrochemical impedance parameters for mild steel in 1 M HCl and 1 M H₂SO₄ in the presence of EVT at 303 K.

Medium	Concentration of inhibitor (ppm)	R_t Ωcm⁻²	C_dl μFcm⁻²	I.E. (in%)
HCl	Blank	64.50	172.34	–
	10	109.30	130.15	40.90
	20	134.30	93.65	51.90
	30	150.20	78.09	57.05
	40	161.90	48.22	60.16
	50	169.90	38.49	62.03
H₂SO₄	Blank	167.84	172.34	–
	10	345.20	130.15	51.37
	20	399.18	93.65	57.95
	30	513.20	78.09	67.29
	40	578.30	48.22	70.97
	50	597.70	38.49	71.91

3.6

3.6 Potentiodynamic polarization test

The potentiodynamic polarization curves for MS in 1 M HCl and 1 M H₂SO₄ solutions with different concentrations of EVT at 303 K after 30 min of immersion time are shown in Fig. 7. It could be observed that addition of EVT alters both anodic and cathodic reactions especially in a higher concentration range. Therefore, the inhibitor could be classified as mixed type. Electrochemical corrosion kinetics parameters, i.e., corrosion potential (E_corr), cathodic and anodic Tafel slopes (b_c and b_a) and corrosion current density (j_corr) obtained from the Tafel extrapolation of the polarization curves, are given in Table 5. The IE was calculated using the following equation

(7)

IE (%) = \frac{j_{corr} - j_{corr (inh)}}{j_{corr}} \times 100

where j_corr and j_corr₍_inh₎ are the corrosion current densities without and with addition of the inhibitor respectively (Lowmunkhong et al., 2010). The results show that the inhibition efficiency increases, while the corrosion current density decreases when the concentration of the inhibitor (EVT) is increased in the case of both the acid media. The results obtained from the polarization measurements are in good agreement with those obtained from the EIS method. The variation of IE between those obtained from weight loss and electrochemical measurements could be attributed to the difference in the immersion period (Kamal and Sethuraman, 2012a).

Tafel plots of EVT on MS in (a) 1 M HCl (b) 1 M H2SO4.

Table 5 Electrochemical polarization parameters for mild steel in 1 M HCl and 1 M H₂SO₄ in the presence of EVT at 303 K.

Medium	Concentration of inhibitor (ppm)	b_a mV dec⁻¹	b_c mV dec⁻¹	−E_corr mV	j_corr μ A cm⁻²	I.E. (in%)
HCl	Blank	185.2	218.8	482	481.8	–
	10	92.8	150.3	488	225.0	53.30
	20	91.7	143.8	502	162.6	66.25
	30	98.4	134.9	509	150.1	68.86
	40	114.9	132.0	506	145.4	69.82
	50	131.6	128.4	509	140.2	70.90
H₂SO₄	Blank	601.6	130.2	490	1176	–
	10	107.2	146.1	495	776.5	33.97
	20	109.7	145.5	490	773.3	34.26
	30	100.9	140.6	489	572.3	51.36
	40	100.7	139.4	490	53.6	61.42
	50	100.6	135.5	492	238.6	79.71

3.7

3.7 Surface analysis study

SEM micrographs Fig. 8(a and b) show that the surface is highly damaged in the absence of the inhibitor while there is a formation of film on the metal surface in the presence of EVT (Fig. 8c and d).

SEM image of MS in (a) 1 M HCl (b)1 M H2SO4; SEM image of MS in (c) 1 M HCl (d) 1 M H2SO4 in the presence of EVT at a concentration of 50 ppm.

3.8

3.8 XRD measurements

In view of excellent corrosion inhibition potential of EVT, XRD patterns for MS specimens exposed to acid medium containing 50 ppm of EVT were recorded. From Fig. 9 it could be observed that 2θ values for Fe (43.5°, 64.7°, and 82.5°) seen in polished MS specimens were not seen in the MS specimen exposed to inhibitor in HCl medium. The non-appearance of characteristic peaks of Fe clearly shows the formation of protecting film over the metal surface (Ramesh et al., 2003). Similar observation could also be made in the case of 1 M H₂SO₄ (Fig. 10). This clearly proves the influencing role of EVT on the acid corrosion of MS.

XRD pattern (a) MS in 1 M HCl (b) MS in 1 M HCl in the presence of EVT at 50 ppm.

XRD pattern (a) MS in 1 M H2SO4 (b) MS in 1 M H2SO4 in the presence of EVT at 50 ppm.

4 Conclusions

►

The acid extract of E. coronaria shows a significant corrosion inhibitive effect as evident from the results of weight loss studies.
►

Both E. coronaria and EVT showed good inhibition efficiency for the corrosion of MS in 1 M HCl and 1 M H₂SO₄ in a dose dependent manner.
►

The adsorption of inhibitor on mild steel surface followed Langmuir adsorption isotherm.
►

The inhibitor altered both anodic and cathodic Tafel slopes, which revealed the mixed mode action.
►

The increase in R_t values with increasing concentration of inhibitor confirms the adsorption of inhibitor on MS surface.
►

The results of SEM and XRD studies proved the formation of the protective layer over MS surface.
►

The study carried out with EVT clearly revealed that the corrosion inhibitive effect of E. coronaria could be correlated to the presence of EVT along with synergistic influence of other phytoconstituents.

Acknowledgements

Authors thank the University Grants Commission (UGC), New Delhi for the financial assistance through Major Research Programme (MRP) and Special Assistance Programme (SAP).

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Show Sections

[1] Ahamad I., Quraishi M.A., . Bis (benzimidazol-2-yl) disulphide: An efficient water soluble inhibitor for corrosion of mild steel in acid media. Corros. Sci.. 2009;51:2006-2013.
[Google Scholar]

[2] Al-Otaibi M.S., Al-Mayouf A.M., Khan M., Mousa A.A., Al-Mazroa S.A., Alkhathlan H.Z., . Corrosion inhibitory action of some plant extracts on the corrosion of mild steel in acidic media. Arab. J. Chem.. 2014;7:340-346.
[Google Scholar]

[3] Atta-Ur-Rahman, Anjum Muzaffar, . The isolation and structure of ervaticine. Heterocycles. 1985;23:2975.
[Google Scholar]

[4] Atta-Ur-Rahman, Anjum Muzaffar, Daulatabadi N., . The isolation and structure of mehranine – a new indole alkaloid from Ervatamia coronaria. Z. Naturforsch.. 1983;38b:1700.
[Google Scholar]

[5] Atta-Ur-Rahman, Daulatabadi N., Anjum Muzaffar, . The isolation and structure of lahoricine – a new indolenine alkaloid from Ervatamia coronaria. Z. Naturforsch.. 1984;39:1289.
[Google Scholar]

[6] Atta-Ur-Rahman, Daulatabadi N., Anjum Muzaffar, . Isolation of apparicine from the leaves of Ervatamia coronaria. J. Nat. Prod.. 1984;57:900.
[Google Scholar]

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Studies on Ervatinine – The anticorrosive phytoconstituent of Ervatamia coronaria

Abstract

Keywords

Ervatamia coronaria

Ervatinine

Mild steel

Langmuir isotherm

Impedance

Polarization

1 Introduction

2 Materials and methods

2.1 Preparation of E. coronaria extract

2.2 Isolation of EVT

2.3 Specimen preparation

2.4 Weight loss measurements

2.5 Electrochemical measurements

2.6 Surface analysis

2.7 XRD measurements

3 Results and discussion

3.1 Weight loss measurements

3.2 Effect of temperature

3.3 Adsorption isotherm

3.4 Thermodynamic parameters

3.5 Electrochemical impedance spectroscopy

3.6 Potentiodynamic polarization test

3.7 Surface analysis study

3.8 XRD measurements

4 Conclusions

Acknowledgements

References

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