Inhibitive action of ethanol extracts from Nauclea latifolia on the corrosion of mild steel in H2SO4 solutions and their adsorption characteristics

I.E. Uwah; P.C. Okafor; V.E. Ebiekpe

doi:10.1016/j.arabjc.2010.10.008

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Original article

6 (

); 285-293

doi:

10.1016/j.arabjc.2010.10.008

Inhibitive action of ethanol extracts from Nauclea latifolia on the corrosion of mild steel in H₂SO₄ solutions and their adsorption characteristics

I.E. Uwah, P.C. Okafor^*, V.E. Ebiekpe

Corrosion and Electrochemistry Research Group, Department of Pure and Applied Chemistry, University of Calabar, P.M.B. 1115, Calabar, Nigeria

*Corresponding author. Tel.: +234 803 429 5604 pcokafor@chemist.com (P.C. Okafor) pcokafor@gmail.com (P.C. Okafor)

Received: 2010-9-7, Accepted: 2010-10-11, Published: 2010-07-01

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.

Available online 18 October 2010

Abstract

The inhibitive action of ethanol extracts from leaves (LV), bark (BK) and roots (RT) of Nauclea latifolia on mild steel corrosion in H₂SO₄ solutions at 30–60 °C was studied using weight loss and gasometric techniques. The extracts were found to inhibit the corrosion of mild steel in H₂SO₄ solutions and the inhibition efficiencies of the extracts follow the trend: RT > LV > BK. The inhibition efficiency increased with the extracts concentration but decreased with temperature rise. Physical adsorption of the phytochemical components of the plant on the metal surface is proposed as the mechanism of inhibition. The adsorption characteristics of the inhibitor were approximated by the thermodynamic-kinetic model of El-Awady et al.

Keywords

Mild steel

Acid corrosion

Corrosion inhibition

Nauclea latifolia

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1 Introduction

Mineral acids of various concentrations are extensively employed in various industrial processes including chemical cleaning, acid pickling and acid descaling as well as in oil well acidification which result in corrosion. Due to the aggressiveness of these acids many organic compounds have been studied and employed as corrosion inhibitors. It is interesting to note that in the field of corrosion inhibition we are going back to the past as a result of the fact that increasing awareness of health and ecological risks is drawing attention to finding more suitable non-toxic inhibitors, which are found mostly among the class of natural products. The first patent in corrosion inhibition was to Baldwin, British Patent 2327, which specified the use of a natural product, molasses and vegetable oils for pickling sheet steel in acids (Putilova et al., 1960). US Patent 640491 in 1900 to Robinson and Sutherland used starch, a biodegradable material. Present trends in research on environmental friendly corrosion inhibitors are concentrating on natural products origin due principally to their low cost and eco-friendliness. Among these natural products are plants extracts. They constitute several organic compounds which have corrosion inhibiting abilities. The yield of these compounds as well as the corrosion inhibition abilities vary widely depending on the part of the plant (Khuzhaeu and Aripova, 2000; Okafor and Ebenso, 2007; Okafor et al., 2008 ) and its geographical location (Ogan, 1971). The use of natural products as corrosion inhibitors have been widely reported by several authors. The extracts from the leaves, seeds, heartwood, bark, roots and fruits of plants have been reported to inhibit metallic corrosion in acidic media (Ebenso and Ekpe, 1996; Ebenso et al., 2004a,b; Gunasekaran and Chauhan, 2004; Okafor et al., 2005, 2006, 2008, 2009, 2010; El-Etre et al., 2005; Oguzie, 2006; Oguzie et al., 2006; Abdel-Gaber et al., 2006; Umoren et al., 2006; Okafor and Ebenso, 2007; Abiola et al., 2009; Abiola and Otaigbe, 2009; Abiola and James, 2010 ). The use of extracts from plants as corrosion inhibitors has been reviewed extensively in our previous work (Okafor et al., 2008).

Nauclea latifolia commonly known as Pin Cushion Tree is a straggling shrub or small spreading tree that belongs to the family Rubiaceae and to the class Magnoliopsida (Dicotyledons). It is a widely distributed savanna plant native to Africa and Asia. It is found in the forest and fringe tropical forest. It bears an interesting flower, a large red ball with long projecting stamens. The fruit is red, edible, but not appealing. The mode of propagation is by seed. In many African communities, the plant enjoys huge popularity on account of its diverse potential for treating various ailments. For instance, numerous applications of this plant have been reported in traditional medicinal practices of the East and West African sub-regions of continental Africa (Dalziel, 1957), where various extracts of the plant is used for the therapeutic management of malaria (Gamaniel et al., 1997); hypertension (Akubue and Mittal, 1982); prolonged menstrual flow (Elujoba, 1995); cough, gonorrhoea, stomach disorders, diarrhea, dysentery, ulcers and liver ailments (Lamidi et al., 1995; Traore-Keita et al., 2000). Phytochemical investigation of the plant revealed the presence of naturally synthesized biological compounds including, monoterpene, triterpene, indole alkaloid, saponins and traces of inorganic compounds (Brown et al., 1979; Hussain and Deeni, 1991; Abreu and Pereira, 1998, 2001 ). However, to the best of our knowledge, the plant has not been studied for corrosion inhibition abilities. This study was, therefore, carried out to investigate the probable corrosion inhibition abilities of the ethanol extracts of the leaves (LV), barks (BK) and roots (RT) of N. latifolia on mild steel corrosion in H₂SO₄ solutions using gasometric and gravimetric techniques.

2 Experimental

The mild steel sheets used in this present work have the composition presented in Table 1. Before measurements, the mild steel coupons were mechanically polished with series of Emery paper of variable grades starting with the coarsest and proceeding in steps to the finest (600) grade, degreased with absolute ethanol, dipped into acetone and air dried. The hydrogen evolution experiments were conducted on mild steel coupons of dimension 2.0 × 0.08 × 5.0 cm (with a surface area of 21.12 cm²) and for the weight loss study, mild steel specimens of size 4.0 × 0.08 × 5.0 cm (with a surface area of 41.44 cm²) were used.

Table 1 Chemical composition of the mild steel.

Element	C	Si	Mn	S	P	Ni	Cr	Mo	Cu
Composition	0.19	0.26	0.64	0.05	0.06	0.09	0.08	0.02	0.27

2.1

2.1 Preparation of plant extracts

The parts (bark (BK), leaves (LV) and root (RT)) of N. latifolia were collected as required from a local bush in Calabar, Nigeria. These were cut into small chips, dried in an N53C – Genlab laboratory oven at 50 °C, and ground into powder form. 80.0 g of the powder was extracted continually with 250 cm³ of absolute ethanol in a soxhlet extractor for 24 h. After recovering most of the ethanol, the extract was heated on a water bath (at 60 °C) until most of the ethanol evaporated. 4.0 g of the ethanol extract was digested in 1.0 liter of 5.0 M H₂SO₄ and 1.0 M H₂SO₄ solutions for hydrogen evolution and weight loss measurements, respectively. The resultant solution was kept for 24 h, filtered and stored. From the stock solution (4.0 g/l), inhibitor test solutions (concentrations of 0.1, 0.5, 1.0, 2.0 and 4.0 g/l) were prepared.

2.2

2.2 Phytochemical screening

Phytochemical screening was carried out on the ethanol extracts from N. latifolia following the methods described by Harbone (1973), Sofowora (1980), Trease and Evans (1996). The plant extracts were screened for alkaloids, saponins, tannins, flavonoids, polyphenols, cardiac glycosides, anthraquinins, and phenobutinon.

2.3

2.3 Weight loss and gasometric measurements

The weight loss and gasometric measurements were carried out as previously described ((Ekpe et al., 1995; Ita and Offiong, 1997). However, experiments were conducted at 30 °C for weight loss, and at 30, 40, 50 and 60 °C for gasometric measurements.

For the weight loss measurements, the mild steel coupons were each suspended and completely immersed in the test solutions (1.0 M H₂SO₄ with and without different concentrations of the plant extracts) with the help of glass hooks and rods for 5 days at 30 ± 1 °C. However, the coupons were retrieved at 24 h intervals, washed several times in 20% sodium hydroxide solution containing 200 g/l of zinc dust until clean, dipped into acetone, air dried and reweighed (Ekpe et al., 1995; Ita and Offiong, 1997). From the weight loss data, the corrosion rates (R) were calculated from Eq. (1):

(1)

R (mg {cm}^{- 2} h^{- 1}) = \frac{w}{At}

where w is weight loss in mg, A is the specimen surface area (of 41.44 cm²) and t the immersion period in hours (120 h). From the corrosion rate, the surface coverage (θ) as a result of adsorption of the components of the extracts, and inhibition efficiencies of the plant extracts (η%) were determined using Eqs. (2) and (3), respectively.

(2)

θ = \frac{R_{0} - R_{i}}{R_{0}}

(3)

η = \frac{R_{0} - R_{i}}{R_{0}} \times 100

where R₀ and R_i are the corrosion rates in the absence and presence of the plant extracts, respectively.

Gasometric technique is based on the principle that corrosion reactions in aqueous media are characterized by the evolution of gas resulting from the cathodic reaction of the corrosion process, which is proportional to the rate of corrosion (Okafor et al., 2005). The rate of evolution of the gas (R_H) is determined from the slope of the graph of volume of gas evolved (V) versus time (t) and the inhibitor surface coverage (θ) and efficiencies (η%) determined using Eqs. (4) and (5), respectively.

(4)

θ = \frac{R_{H 0} - R_{Hi}}{R_{H 0}}

(5)

η = \frac{R_{H 0} - R_{Hi}}{R_{H 0}} \times 100

where R_H₀ and R_Hi are the rates of hydrogen evolution in the absence and presence of the plant extracts, respectively.

This gasometric technique has been corroborated by other well established corrosion rate determination techniques, including weight loss, thermometric and electrochemical techniques (El-Etre, 2003; Abdallah, 2004; Okafor et al., 2006 ). The data presented here represents the average of two to three measurements from the weight loss and gasometric techniques.

3 Results and discussion

3.1

3.1 Gravimetric results

The material loss expressed as the corrosion rate (mg cm⁻² h⁻¹) for the mild steel coupons in 1.0 M H₂SO₄ solutions containing different concentrations of N. latifolia extracts (NLE) as a function of inhibitor concentration is presented in Fig. 1. It is observed that the corrosion rates decreased with the increase in concentration of NLE for all the plant’s part used, indicating that the extent of inhibition is dependent on the amount of extract present. The values of the percentage inhibition efficiency were determined for 120 h immersion period using Eq. (3). The results obtained are shown in Table 2 and indicate that the plant extracts show a significant inhibitive effect on mild steel in H₂SO₄ solutions. From Fig. 2 (variation of inhibition efficiency as a function of extract concentration), it is observed that the inhibition efficiencies increase with increase in NLE concentration. This indicates that the phytochemical components of the extracts are adsorbed onto the mild steel surface resulting in the blocking of the reaction sites, and protection of the mild steel surface from the attack of the corrosion active ions in the acid medium. Comparing the inhibition efficiencies of the different parts of the extracts, it is observed that the efficiencies follow the trend: RT > LV > BK. This shows that the yield of the phytochemicals in the plant extracts responsible for the inhibition of the corrosion reactions depend on the part of the plant. This finding is in line with previous reports (Khuzhaeu and Aripova, 2000; Okafor and Ebenso, 2007; Okafor et al., 2008; Abiola et al., 2009 ).

Variation of corrosion rates with extract concentration for mild steel coupons in 1.0 M H2SO4 solutions containing the different parts of Nauclea latifolia using weight loss technique.

Table 2 Calculated values of the corrosion rate, inhibition efficiency and surface coverage for mild steel coupons in 1.0 M H₂SO₄ solutions containing ethanol extracts from Nauclea latifolia (using the weight loss technique).

Plant’s part	Extract concentration (g/l)	Corrosion rates (mg cm⁻²/h⁻¹)	Inhibition efficiency (%)
Plant’s part	Blank	1.13	–
BK	0.1	1.10	2.42
	0.5	0.97	13.90
	1.0	0.73	35.20
	2.0	0.47	58.67
	4.0	0.28	75.26

LV	0.1	1.10	2.55
	0.5	0.79	30.10
	1.0	0.33	70.54
	2.0	0.18	83.93
	4.0	0.09	91.58

RT	0.1	0.63	43.88
	0.5	0.27	75.77
	1.0	0.12	89.16
	2.0	0.09	91.71
	4.0	0.06	94.26

Variation of inhibition efficiency with extracts concentration for mild steel coupons in 1.0 M H2SO4 solutions containing the different parts of Nauclea latifolia using weight loss technique.

To explain the kinetics of the corrosion of mild steel in 1.0 M H₂SO₄ solutions in the absence and presence of NLE, linear plots of the logarithm of the measured weight (in g) of the mild steel coupons after post treatment (W_f) against time (t) (Fig. 3) were obtained which confirm a first order kinetics with respect to the mild steel (Atkins and Paula, 2002; Okafor et al., 2007; Abiola and James, 2010 ). It could, therefore, be assumed that the inhibition of acid corrosion of mild steel by NLE does not influence the anodic reaction order (Abiola and James, 2010).

Plot of log Wf against time for mild steel coupons in 1.0 M H2SO4 solutions containing: (a) bark (BK), (b) leaves (LV), and (c) roots (RT) extracts from Nauclea latifolia using weight loss technique.

3.2

3.2 Gasometric results

Hydrogen evolution technique was employed to access the inhibition efficiency of the plant extracts at higher concentration. The weight loss method is most suitable at a low concentration, while the hydrogen evolution method suffices for a high corrodent concentration (Ekpe et al., 1995; Ebenso et al., 1999). The acid corrosion of mild steel is characterized by evolution of hydrogen and the rate of corrosion is proportional to the amount of hydrogen evolved (Okafor et al., 2004, 2008). The volume of hydrogen evolved, V_H, during the corrosion of mild steel in 5.0 M H₂SO₄ solutions in the absence and presence of NLE was assessed and the rates of hydrogen evolution obtained. The rate of hydrogen evolution values are recorded in Table 3 and the variation of rate of hydrogen evolved as a function of N. latifolia bark extract concentration is depicted in Fig. 4. It is observed that the presence of the extracts decreases the volume of hydrogen evolved during the corrosion reaction compared to the blank system and that the rate of hydrogen evolution decreased with the increase in NLE concentration and increased with temperature for all systems studied (Fig. 4). Also, the increase in the rate of hydrogen evolution at all the concentrations and temperatures studied followed the trend: BK > LV > RT. This observation also confirms that the yield of the organic compounds in the plant extracts and consequently, the corrosion inhibition ability of the plant extract is dependent on the parts of the plant.

Table 3 Calculated values of the rate of hydrogen evolution, inhibition efficiency, activation energy, change in enthalpy and change in entropy for mild steel coupons in 5.0 M H₂SO₄ solutions containing extracts from N. latifolia (using hydrogen evolution technique).

Plants part	Extract concentration (g/l)	Rate of hydrogen evolution (cm³/min)				Inhibition efficiency (%)				E_a (kJ/mol)	ΔH^o (kJ/mol)	ΔS^o (J mol⁻¹ K⁻¹)
	Extract concentration (g/l)	30 °C	40 °C	50 °C	60 °C	30 °C	40 °C	50 °C	60 °C	E_a (kJ/mol)	ΔH^o (kJ/mol)	ΔS^o (J mol⁻¹ K⁻¹)
	Blank	0.51	1.34	2.43	4.35	–	–	–	–	58.98	−56.34	−63.98
BK	0.1	0.46	1.22	2.28	4.22	10.71	8.73	5.97	3.01	61.30	−58.66	−57.27
	0.5	0.35	0.97	1.82	3.36	31.12	27.31	25.10	22.86	62.09	−59.45	−56.76
	1.0	0.26	0.71	1.56	2.94	50.32	47.15	35.76	32.53	68.32	−65.68	−39.09
	2.0	0.14	0.42	0.91	1.91	72.89	68.40	62.52	56.15	72.50	−69.87	−30.25
	4.0	0.10	0.31	0.63	1.30	80.25	77.19	73.90	70.08	70.56	−67.92	−30.25

LV	0.1	0.39	1.08	1.97	3.78	23.29	19.27	18.76	13.11	62.15	−59.52	−55.70
	0.5	0.34	0.94	1.77	3.56	33.71	29.67	26.97	18.29	64.53	−61.89	−49.17
	1.0	0.23	0.65	1.27	2.55	56.12	51.67	47.83	41.41	66.31	−64.24	−44.78
	2.0	0.16	0.48	0.98	2.00	68.69	64.23	59.47	53.96	69.72	−67.08	−38.19
	4.0	0.10	0.30	0.62	1.24	79.82	77.22	74.29	71.60	68.60	−65.96	−45.51

RT	0.1	0.32	0.91	1.77	3.51	38.04	31.79	27.08	19.26	66.33	−63.69	−43.72
	0.5	0.26	0.76	1.41	2.86	48.24	43.28	41.98	34.34	65.27	−62.63	−48.73
	1.0	0.20	0.56	1.10	2.36	61.37	57.91	54.61	45.70	68.27	−65.63	−41.43
	2.0	0.13	0.37	0.72	1.53	75.29	72.24	70.37	64.80	68.54	−65.90	−44.16
	4.0	0.10	0.29	0.58	1.22	80.20	78.13	75.97	72.05	68.55	−65.91	−45.98

Variation of inhibition corrosion rate with extract concentration for mild steel coupons in 5.0 M H2SO4 solutions containing ethanol extracts from bark (BK) of Nauclea latifolia using hydrogen evolution technique.

From the rate of hydrogen evolution values, the inhibition efficiencies of the plant extract on the corrosion of mild steel in 5.0 M H₂SO₄ solutions in the absence (blank) and presence of the NLE were determined. The results obtained (Table 3) indicate that the inhibition efficiencies increase with increase in NLE concentration and decrease with increase in temperature. Similar trend was obtained at lower concentration of the acid via the weight loss method. The increase of efficiency with concentration opines that the phytochemicals in the plants are adsorbed on the surface of the metal and the decrease with temperature suggests a physical adsorption mechanism for the corrosion inhibition process (Okafor et al., 2007).

3.3

3.3 Activation parameters

The activation parameters for the dissolution of mild steel coupons in 5.0 M H₂SO₄ solutions in the absence and presence of NLE were calculated from the Arrhenius-type plot (Eq. (6)) and the transition state Eq. (7):

(6)

k = A \exp (- E_{a} / RT)

(7)

k = RT / Nh \exp (Δ S^{o} / R) \exp (- Δ H^{o} / RT)

where R is the universal gas constant, N is the Avogadro’s number, h is the Plank’s constant, E_a is the activation energy, T is the absolute temperature, and ΔH^o and ΔS^o are the standard enthalpy and entropy of activation, respectively. The values of k were taken to be equal to the rate of hydrogen evolution (Putilova et al., 1969; Bockris and Reddy, 1977; El-Awady et al., 1992; Gomma and Wahdan, 1994; Gomma, 1998; Oguzie et al., 2005; Okafor et al., 2007 ). A plot of log k versus 1/T, for mild steel in 5.0 M H₂SO₄ containing NLE (Fig. 5 for bark extracts) gave straight lines. The calculated activation energies obtained from the slopes of the plots are listed in Table 3. The results showed that E_a values increased in the presence of the NLE compared to the blank system which is typical of physical adsorption (Putilova et al., 1969; Gomma and Wahdan, 1994; Martinez and Stern, 2002 ). The average activation energy values of the inhibitor reported: 66.95 kJ/mol (for BK extract), 66.38 kJ/mol (for LV extract) and 67.38 kJ/mol (for RT extract) also confirmed the assertion that the inhibition of steel corrosion is by physical adsorption mechanism which is in agreement with those of Ebenso (2003) and Barrow (1983). According to the authors, the activation energy for physical adsorption mechanism should be less than 80 kJ/mol. The higher E_a values in the presence of inhibitor compared to the blank solution indicates that the inhibitor will be effective at low temperatures, but efficiencies will be diminished at higher temperatures (Umoren et al., 2006).

Arrhenius plot for mild steel coupons in 5.0 M H2SO4 solutions containing ethanol extracts from the bark of Nauclea latifolia using hydrogen evolution technique.

The enthalpy and the entropy of activation values for the dissolution of mild steel in 5.0 M H₂SO₄ solutions obtained from linear square fit of log (k/T) data versus 1/T (Fig. 6 for bark extract) are also presented in Table 3. The higher values for ΔH^o in the presence of the plant extracts are indicative of the higher protection efficiency observed for the system (El-Awady et al., 1992). Generally, negative values of ΔH^o were obtained indicating an exothermic adsorption process. The exothermic adsorption process signifies either physical or chemical adsorption while endothermic adsorption process is attributable to chemisorption. In an exothermic process, physical adsorption is distinguished from chemisorption by considering the absolute value of adsorption enthalpy. Typically, enthalpy of physical adsorption process is lower than 80 kJ/mol while the enthalpy of chemisorption process approaches 100 kJ/mol (Martinez and Stern, 2002). The values of the obtained enthalpy, therefore, suggest physical adsorption of the components of the plant extracts on the surface of the metal. The ΔS^o values in the absence and presence of NLE are negative. This implies that, the activation complex in the rate determining step represents association rather than dissociation step, meaning that a decrease in disordering takes place on going from reactants to the activated complex (Abd El Rehim et al., 2001).

Transition state plot for mild steel coupons in 5.0 M H2SO4 solutions containing ethanol extracts from the bark of Nauclea latifolia using hydrogen evolution technique.

3.4

3.4 Adsorption isotherm

It is necessary to determine empirically which adsorption isotherm fits best to the surface coverage data in order to use the corrosion rate measurements to calculate the thermodynamic parameters pertaining to inhibitor adsorption. Different adsorption isotherms were found to describe the adsorption of the inhibitors on steel. From the result, it was found that the experimental data fitted the thermodynamic-kinetic model of El-Awady et al. (1992) (Fig. 7). The model may be formulated as Eq. (8):

(8)

\log (\frac{θ}{1 - θ}) = \log K^{'} + y \log c

where c is the concentration of the adsorbate, θ is the coverage degree and 1/y is the number of inhibitor molecules occupying one active site. The binding constant K is given by:

(9)

K = K^{' (1 / y)}

The data are given in Table 4. The efficiency of a given inhibitor is a function of both the magnitude of its binding constant K and the number of active sites (1/y) it is capable of blocking, with the former being the most important (El-Awady et al., 1992). Large values of K mean better and strong interaction, while small values of K mean that the interaction between the inhibitor molecules and the metal is weaker (Okafor and Ebenso, 2007). Hence according to the numerical values of K obtained, the inhibition efficiency of different NLE could be arranged in the order: RT > LV > BK. Inspection of Table 4 shows that K values decrease with increase in temperature. This is interpreted to mean that weaker surface interactions result at higher temperatures. The values of 1/y obtained were mostly greater than one. This implies that a given inhibitor molecule will occupy more than one active site on the surface of the metal (El-Awady et al., 1992).

El-Awady et al. thermodynamic-kinetic model for mild steel coupons in 1.0 M H2SO4 solutions containing extracts from Nauclea latifolia (using weight loss technique).

Table 4 Adsorption parameters for mild steel in H₂SO₄ solutions containing NLE.

Plant’s part	Temperature (°C)	Hydrogen evolution technique		Weight loss technique
Plant’s part	Temperature (°C)	K	1/y	K	1/y
BK	30	1.08	1.01	0.51	0.60
	40	0.90	1.00
	50	0.69	0.96
	60	0.57	0.85

LV	30	1.39	1.39	1.48	1.26
	40	1.07	1.35
	50	0.87	1.43
	60	0.61	1.27

RT	30	3.01	1.86	6.03	8.18
	40	2.01	1.72
	50	1.60	1.65
	60	0.99	1.50

3.5

3.5 Inhibition mechanism

The use of corrosion inhibitors increases continually (Antropov et al., 1976). The mechanism of their action can be different, depending on metal, the medium and the structure of the inhibitor. One possible mechanism is the adsorption of the inhibitor, which blocks the metal surface and thus do not permit the corrosion process to take place (Christov and Popova, 2004).

The observed corrosion inhibition of mild steel in H₂SO₄ solutions can be explained by the adsorption of components of NLE on the metal surface (Okafor et al., 2007). Various mechanisms have been proposed for the inhibition or acceleration of metallic corrosion in acid media (Antropov, 1977; Oguzie, 2004; Abdel-Gaber et al., 2006; Okafor et al., 2008 ). A general mechanism for the inhibition of steel corrosion would be similar to that reported by Abdel-Gaber et al. (2006) and Okafor et al. (2008). According to the mechanism, if a molecule or ion, X⁻, is adsorbed on the steel surface, a surface complex forms in the anodic process, and the complex is then desorbed from the surface, Eqs. (10)–(13):

(10)

Fe + X^{-} \Leftrightarrow ({FeX}^{-}) s

(11)

({FeX}^{-}) s \Leftrightarrow (FeX) s + e

(12)

(FeX) s \to FeX + + e

(13)

{FeX}^{+} \Leftrightarrow {Fe}^{2 +} + X^{-}

where s represents ion or compound at the surface. In general, if the adsorbed molecule/ion on the surface complex is stable, the corrosion of steel is inhibited. Therefore, it can be concluded that the adsorption of the phytochemical components of the plant extract leads to the formation of stable surface complex, which blocks the active sites on the surface of the metal thereby reducing the corrosion rate. Increase in the concentration of the phytochemicals increases the amount of the surface complex resulting in greater inhibition of the corrosion. From the observed results, it can be inferred that the insoluble Fe–NLE complexes dominate the adsorbed intermediate and thus the resultant inhibitive effects. This conclusion is in line with those of Jaen et al. (1999), Oguzie (2004) and Okafor et al. (2008). Studies on the phytochemical constituents of the ethanolic extracts of N. latifolia revealed the presence of appreciable quantities of flavonoids, polyphenols and glycosides. Also detected in the plant extracts are alkaloids (in the root and leaves extract) and tannins (in the bark extract) (Table 5). A closer inspection of Table 5 revealed the absence of alkaloids in the bark extracts of the plant, while the extracts from the leaves and roots show the presence of alkaloids which in the present system may be responsible for the observed inhibition efficiency trend (RT > LV > BK). The alkaloids (some of the structures are shown in Fig. 8 (Sun et al., 2008) contain nitrogen, oxygen and aromatic ring in their molecular structures. Organic heterogeneous compounds containing these elements have been reported to be efficient corrosion inhibitors (Ekpe et al., 2001; Ebenso et al., 2004a,b; Khaled and Hackerman, 2003; Okafor et al., 2003, 2004; Ebenso and Oguzie, 2005 ). Thus, in the system studied, the adsorption of these compounds on the surface of the metal is assumed to be mostly responsible for the inhibition of the corrosion reaction. However, the mutual effects of the phytochemical components of plant may also contribute to its inhibition properties.

Table 5 Phytochemical screening of the ethanol extracts from root (RT), leaves (LV) and bark (BK) of N. latifolia.

Chemical constituent	Screening
Chemical constituent	Root	Leaves	Bark
Alkaloids	+	+	−
Saponins	−	−	−
Flavonoids	+	+	+
Tannins	−	−	+
Polyphenols	+	+	+
Reducing compounds	−	−	−
Glycosides	+	+	+
Phenobutinones	−	−	−
Anthraquinones	−	−	−

Notes: +, present in the extracts; −, absent in the extracts.

Structure of some indole alkaloids present in Nauclea latifolia.

4 Conclusions

Ethanol extracts from the root, leaves and bark of N. latifolia were found to inhibit the corrosion of mild steel in H₂SO₄ solutions. The inhibition efficiency increases with increase in concentration of the extracts and with decrease in temperature suggesting physical adsorption.

The inhibition is due to the adsorption of phytochemicals in the extracts on the surface of the metal, thereby blocking the active corrosion sites. The presence of alkaloids in the root and leaves extracts is assumed to be responsible for the observed inhibition trend RT > LV > BK.

The experimental data obtained in this study fits the thermodynamic-kinetic model of El-Awady et al. (1992) adsorption isotherm.

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Abiola O.K., Otaigbe J.O.E., . The effects of Phyllanthus amarus extract on corrosion and kinetics of corrosion process of aluminum in alkaline solution. Corros. Sci.. 2009;51:2790.
[Google Scholar]
Abreu P., Pereira A., . A new indole alkaloids from Sarcocephalus latifolius. Heterocycles. 1998;48:885.
[Google Scholar]
Abreu P., Pereira A., . Two new indole alkaloids from Sarcocephalus latifolius. Nat. Prod. Lett.. 2001;15:43.
[Google Scholar]
Akubue P.I., Mittal G.C., . Clinical evaluation of a traditional herbal practice in Nigeria: a preliminary report. J. Ethnopharm.. 1982;6:355.
[Google Scholar]
Antropov L.I., . Theoretical Electrochemistry. Moscow: Mir Publishers; 1977.
Antropov L.I., Makushin E.M., Panasenko V.F., . Inhibitors of Metal Corrosion. Moscow: Metallurgia; 1976.
Atkins P., Paula J.D., . Physical Chemistry. Oxford: Oxford University Press; 2002.
Barrow G.M., . Physical Chemistry. Auckland: McGraw Hill; 1983.
Bockris J.O.M., Reddy A.K.N., . Modern Electrochemistry. New York: Plenum Press; 1977.
Brown R.T., Chapple C.L., Lasheord A.G., . Isolation of strictosidine (sovincoside) lactam from Nauclea latifolia. Phytochemistry. 1979;16:1619.
[Google Scholar]
Christov M., Popova A., . Adsorption characteristics of corrosion inhibitors from corrosion rate measurements. Corros. Sci.. 2004;46:1613.
[Google Scholar]
Dalziel J.K., . The Useful Plants of West Tropical Africa (second ed.). London: Crown Agents; 1957.
Ebenso E.E., . Synergistic effect of halide ions on the corrosion of aluminium in H₂SO₄ using 2-acetylphenothiazine. Mater. Chem. Phys.. 2003;79(1):58.
[Google Scholar]
Ebenso E.E., Ekpe U.J., . Kinetic study of corrosion and corrosion inhibition of mild steel in H₂SO₄ using Carica papaya leaves extract. W. Afric. J. Bio. Appl. Chem.. 1996;41:21.
[Google Scholar]
Ebenso E.E., Ekpe U.J., Ita B.I., Offiong O.E., Ibok U.J., . Effect of molecular structure on the efficiency of amides and thiosemicarbazones used for corrosion inhibition of mild steel in hydrochloric acid. Mater. Chem. Phys.. 1999;60:79.
[Google Scholar]
Ebenso E.E., Ibok U.J., Ekpe U.J., Umoren S.A., Jackson E., Abiola O.K., Oforka N.C., Martinez S., . Corrosion inhibition studies of some plant extracts on aluminium in acidic medium. Trans. SAEST. 2004;39(4):117.
[Google Scholar]
Ebenso E.E., Oguzie E.E., . Corrosion inhibition of mild steel in acidic media by some organic dyes. Mater. Lett.. 2005;59(17):2163.
[Google Scholar]
Ebenso E.E., Okafor P.C., Ekpe U.J., Ibok U.J., Onuchukwu A.I., . The joint effect of halide ions on methylene blue on the corrosion inhibition of aluminium and mild steel in acid corrodent. J. Chem. Soc. Nigeria. 2004;29(1):15.
[Google Scholar]
Ekpe U.J., Ibok U.J., Ita B.I., Offiong O.E., Ebenso E.E., . Inhibitory action of methyl and phenyl thiosemicarbazone derivatives on the corrosion of mild steel in hydrochloric acid. Mater. Chem. Phys.. 1995;40(2):87.
[Google Scholar]
Ekpe U.J., Okafor P.C., Ebenso E.E., Offiong O.E., Ita B.I., . Mutual effect of TSC derivatives on the acidic corrosion of aluminium. Bull. Electrochem.. 2001;17:131.
[Google Scholar]
El-Awady A.A., Abd-El-Nabey B.A., Aziz S.G., . Kinetic-thermodynamic and adsorption isotherms analyses for the inhibition of the acid corrosion of steel by cyclic and open-chain amines. J. Electrochem. Soc.. 1992;139(8):2149.
[Google Scholar]
El-Etre A.Y., . The inhibitive action of the mucilage extracted from the modified stems of prickly pears, toward acid corrosion of aluminum. Corros. Sci.. 2003;45:2485.
[Google Scholar]
El-Etre A.Y., Abdallah M., El-Tantawy Z.E., . Corrosion inhibition of some metals using lawsonia extract. Corros. Sci.. 2005;47(2):385.
[Google Scholar]
Elujoba A.A., . Female infertility in the hands of traditional birth attendants in South-western Nigeria. Fitoterapia. 1995;66:239.
[Google Scholar]
Gamaniel K., Wambebe C., Amupitan J., Hussaini I.M., Amos S., Awodogan A., Dunah A.W., Ekuta J.E., Akeju M.O., Usman H., Enwerem N., . Active column fractions of Nauclea latifolia on Plasmodium berghei and rabbit ileum. J. Pharm. Res. Dev.. 1997;2:44.
[Google Scholar]
Gomma K., . Corrosion of low carbon steel in sulphuric acid solution in presence of pyrazole-halides mixture. Mater. Chem. Phys.. 1998;55:241.
[Google Scholar]
Gomma K., Wahdan M.H., . Effect of temperature on the acidic dissolution of copper in the presence of amino acids. Mater. Chem. Phys.. 1994;32(2):142.
[Google Scholar]
Gunasekaran G., Chauhan L.R., . Eco-friendly inhibitor for corrosion inhibition of mild steel in phosphoric acid medium. Electrochim. Acta. 2004;49(25):4387.
[Google Scholar]
Harbone J.B., . Phytochemical Methods. London: Chapman and Hill; 1973.
Hussain H.S.N., Deeni Y.Y., . Plants in Kano Ethnomedicine, screening for antimicrobial activity and alkaloids. Ind. J. Pharmacol.. 1991;29:51.
[Google Scholar]
Ita B.I., Offiong O.E., . Inhibition of steel corrosion in HCl by pyridoxal-(4-methylthiosemicarbazide), pyridoxal-(4-methylthiosemicarbazone) and its Zinc (II) complex. Mater. Chem. Phys.. 1997;48(2):164.
[Google Scholar]
Jaen J.A., Garcia de Saldana E., Hernandez C., . Characterization of reaction products of iron and iron salts and aqueous plant extracts. Hyperfine Interact.. 1999;122:139.
[Google Scholar]
Khaled K.F., Hackerman N., . Investigation of the inhibitive effect of ortho-substituted anilines on the corrosion of iron in 0.5 M H₂SO₄. Mater. Chem. Phys.. 2003;82:949.
[Google Scholar]
Khuzhaeu V.U., Aripova S.F., . Pseudocarpaine from Carica papaya. Chem. Nat. Comp.. 2000;36(4):418.
[Google Scholar]
Lamidi M., Olivier E., Faure R., Debrauwer L., Nze-Ekekang L., Balansard G., . Quinovic acid glycosides from Nauclea diderichii. Planta Med.. 1995;61:280.
[Google Scholar]
Martinez S., Stern I., . Thermodynamic characterization of metal dissolution and inhibitor adsorption processes in the low carbon steel/mimosa tannin/sulfuric acid system. Appl. Surf. Sci.. 2002;199:83.
[Google Scholar]
Ogan A.U., . The basic constituents of the leaves of Carica papaya. Phytochem. Rep.. 1971;99(2/3):441.
[Google Scholar]
Oguzie E.E., . Influence of halides on the inhibitive effect of Congo red dye on the corrosion of mild steel in sulphuric acid solution. Mater. Chem. Phys.. 2004;87:212.
[Google Scholar]
Oguzie E.E., . Studies on the inhibitive effect of Occimum viridis extract on the acid corrosion of mild steel. Mater. Chem. Phys.. 2006;99(2–3):441.
[Google Scholar]
Oguzie E.E., Onuchukwu A.I., Okafor P.C., Ebenso E.E., . Corrosion inhibition and adsorption behaviour of Ocimum basilicum extract on aluminium. Pigm. Res. Tech.. 2006;35(2):63.
[Google Scholar]
Oguzie E.E., Onuoha G.N., Onuchukwu A.I., . The inhibition of aluminium corrosion in potassium hydroxide by Congo red dye, and synergistic action with halide ions. Anti-Corros. Meth. Mater.. 2005;52(5):293.
[Google Scholar]
Okafor P.C., Ebenso E.E., . Inhibitive action of Carica papaya extracts on the corrosion of mild steel in acidic media and their adsorption characteristics. Pigm. Res. Tech.. 2007;36(3):134.
[Google Scholar]
Okafor P.C., Ebenso E.E., Ekpe U.J., . Inhibition of the acid corrosion of aluminium by some derivatives of thiosemicarbazone. Bull. Chem. Soc. Ethiop.. 2004;18:181.
[Google Scholar]
Okafor P.C., Ebenso E.E., Ekpe U.J., . Azadirachta indica extracts as corrosion inhibitors for mild steel in acid medium. Int. J. Electrochem. Sci.. 2010;5:978.
[Google Scholar]
Okafor P.C., Ebenso E.E., Ibok U.J., Ekpe U.J., Ikpi M.I., . Inhibition of 4-acetamidoaniline on corrosion of mild steel in HCl solution. Trans. SAEST. 2003;38:91.
[Google Scholar]
Okafor P.C., Ekpe U.J., Ebenso E.E., Oguzie E.E., Umo N.S., Etor A.R., . Effect of Allium cepa and Allium sativum as corrosion inhibitors of mild steel in HCl solution. Trans. SAEST. 2006;41:82.
[Google Scholar]
Okafor P.C., Ekpe U.J., Ebenso E.E., Umoren E.M., Leizou K.E., . Inhibition of mild steel corrosion in acidic medium using Allium sativum extracts. Bull. Electrochem.. 2005;21(8):347.
[Google Scholar]
Okafor P.C., Ikpi M.E., Uwah I.E., Ebenso E.E., Ekpe U.J., Umoren S.A., . Inhibitory action of Phyllanthus amarus extracts on the corrosion of mild steel in acidic media. Corros. Sci.. 2008;50:2310.
[Google Scholar]
Okafor P.C., Osabor V.I., Ebenso E.E., . Eco-friendly corrosion inhibitors: inhibitive action of ethanol extracts of Garcinia kola for the corrosion of mild steel in H₂SO₄ solutions. Pigm. Res. Technol.. 2007;36(5):299.
[Google Scholar]
Okafor P.C., Uwah I.E., Ekerenam O.O., Ekpe U.J., . Combretum bracteosum as Eco-friendly corrosion inhibitor for mild steel in acid medium. Pigm. Resin Technol.. 2009;38(4):236.
[Google Scholar]
Putilova, I.N., Balezin, S.A., Barannik, V.P., 1960. Metallic Corrosion Inhibitors (Translated from the Russian by Ryback). Pergamon Press, New York (As cited by Gupta, D.V. and Green, S., 2004, Inhibitors – Where are we. Proceedings of Corrosion 2004, paper number 04406. NACE International, Houston, 2004).
Putilova I.N., Balezin S.A., Barannik V.P., . Metallic Corrosion Inhibitors. New York: Pergamon Press; 1969.
Sofowora E.R., . Guidelines for research promotion and development in traditional medicine. Nigerian J. Pharm.. 1980;11:117.
[Google Scholar]
Sun J., Lou H., Dai S., Xu H., Zhao F., Liu K., . Indole alkoloids from Nauclea officinalis with weak antimalarial activity. Phytochemistry. 2008;69:1405.
[Google Scholar]
Traore-Keita F., Gasquet M., Di Giorgio C., Olivier E., Delmas F., Keita A., Doumbo O., Balansard G., Timon-David P., . Antimalarial activity of four plants used in traditional medicine in Mali. Phytotherapy Res.. 2000;14:45.
[Google Scholar]
Trease G.E., Evans W.C., . A Textbook of Pharmacology. London: Baillier Tidally; 1996.
Umoren S.A., Obot I.B., Ebenso E.E., Okafor P.C., Ogbobe O., Oguzie E.E., . Gum arabic as a potential corrosion inhibitor for aluminium in alkaline medium and its adsorption characteristics. Anti-Corros. Meth. Mater.. 2006;53(5):277.
[Google Scholar]

Introduction

Experimental

Preparation of plant extracts
Phytochemical screening
Weight loss and gasometric measurements

Results and discussion

Gravimetric results
Gasometric results
Activation parameters
Adsorption isotherm
Inhibition mechanism

Conclusions

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[1] Abd El Rehim S.S., Ibrahim M.A.M., Khaled K.F., . The inhibition of 4-(2′-amino-5′-methylphenylazo) antipyrine on corrosion of mild steel in HCl solution. Mater. Chem. Phys.. 2001;70:268.
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[2] Abdallah M., . Antibacterial drugs as corrosion inhibitors for corrosion of aluminium in hydrochloric solution. Corros. Sci.. 2004;46(8):1981.
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[3] Abdel-Gaber A.M., Abd-El-Nabey B.A., Sidahmed I.M., El-Zayady A.M., Saadway M., . Inhibitive action of some plant extracts on the corrosion of steel in acidic media. Corros. Sci.. 2006;48(9):2765.
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[4] Abiola O.K., James A.O., . The effects of Aloe vera extract on corrosion and kinetics of corrosion process of zinc in HCl solution. Corros. Sci.. 2010;52:661.
[Google Scholar]

[5] Abiola O.K., Otaigbe J.O.E., Kio O.J., . Gossipium hirsutum extracts as green corrosion inhibitor for aluminum in NaOH solution. Corros. Sci.. 2009;51:1879.
[Google Scholar]

[6] Abiola O.K., Otaigbe J.O.E., . The effects of Phyllanthus amarus extract on corrosion and kinetics of corrosion process of aluminum in alkaline solution. Corros. Sci.. 2009;51:2790.
[Google Scholar]

[7] Abreu P., Pereira A., . A new indole alkaloids from Sarcocephalus latifolius. Heterocycles. 1998;48:885.
[Google Scholar]

[8] Abreu P., Pereira A., . Two new indole alkaloids from Sarcocephalus latifolius. Nat. Prod. Lett.. 2001;15:43.
[Google Scholar]

[9] Akubue P.I., Mittal G.C., . Clinical evaluation of a traditional herbal practice in Nigeria: a preliminary report. J. Ethnopharm.. 1982;6:355.
[Google Scholar]

[10] Antropov L.I., . Theoretical Electrochemistry. Moscow: Mir Publishers; 1977.

[11] Antropov L.I., Makushin E.M., Panasenko V.F., . Inhibitors of Metal Corrosion. Moscow: Metallurgia; 1976.

[12] Atkins P., Paula J.D., . Physical Chemistry. Oxford: Oxford University Press; 2002.

[13] Barrow G.M., . Physical Chemistry. Auckland: McGraw Hill; 1983.

[14] Bockris J.O.M., Reddy A.K.N., . Modern Electrochemistry. New York: Plenum Press; 1977.

[15] Brown R.T., Chapple C.L., Lasheord A.G., . Isolation of strictosidine (sovincoside) lactam from Nauclea latifolia. Phytochemistry. 1979;16:1619.
[Google Scholar]

[16] Christov M., Popova A., . Adsorption characteristics of corrosion inhibitors from corrosion rate measurements. Corros. Sci.. 2004;46:1613.
[Google Scholar]

[17] Dalziel J.K., . The Useful Plants of West Tropical Africa (second ed.). London: Crown Agents; 1957.

[18] Ebenso E.E., . Synergistic effect of halide ions on the corrosion of aluminium in H₂SO₄ using 2-acetylphenothiazine. Mater. Chem. Phys.. 2003;79(1):58.
[Google Scholar]

[19] Ebenso E.E., Ekpe U.J., . Kinetic study of corrosion and corrosion inhibition of mild steel in H₂SO₄ using Carica papaya leaves extract. W. Afric. J. Bio. Appl. Chem.. 1996;41:21.
[Google Scholar]

[20] Ebenso E.E., Ekpe U.J., Ita B.I., Offiong O.E., Ibok U.J., . Effect of molecular structure on the efficiency of amides and thiosemicarbazones used for corrosion inhibition of mild steel in hydrochloric acid. Mater. Chem. Phys.. 1999;60:79.
[Google Scholar]

[21] Ebenso E.E., Ibok U.J., Ekpe U.J., Umoren S.A., Jackson E., Abiola O.K., Oforka N.C., Martinez S., . Corrosion inhibition studies of some plant extracts on aluminium in acidic medium. Trans. SAEST. 2004;39(4):117.
[Google Scholar]

[22] Ebenso E.E., Oguzie E.E., . Corrosion inhibition of mild steel in acidic media by some organic dyes. Mater. Lett.. 2005;59(17):2163.
[Google Scholar]

[23] Ebenso E.E., Okafor P.C., Ekpe U.J., Ibok U.J., Onuchukwu A.I., . The joint effect of halide ions on methylene blue on the corrosion inhibition of aluminium and mild steel in acid corrodent. J. Chem. Soc. Nigeria. 2004;29(1):15.
[Google Scholar]

[24] Ekpe U.J., Ibok U.J., Ita B.I., Offiong O.E., Ebenso E.E., . Inhibitory action of methyl and phenyl thiosemicarbazone derivatives on the corrosion of mild steel in hydrochloric acid. Mater. Chem. Phys.. 1995;40(2):87.
[Google Scholar]

[25] Ekpe U.J., Okafor P.C., Ebenso E.E., Offiong O.E., Ita B.I., . Mutual effect of TSC derivatives on the acidic corrosion of aluminium. Bull. Electrochem.. 2001;17:131.
[Google Scholar]

[26] El-Awady A.A., Abd-El-Nabey B.A., Aziz S.G., . Kinetic-thermodynamic and adsorption isotherms analyses for the inhibition of the acid corrosion of steel by cyclic and open-chain amines. J. Electrochem. Soc.. 1992;139(8):2149.
[Google Scholar]

[27] El-Etre A.Y., . The inhibitive action of the mucilage extracted from the modified stems of prickly pears, toward acid corrosion of aluminum. Corros. Sci.. 2003;45:2485.
[Google Scholar]

[28] El-Etre A.Y., Abdallah M., El-Tantawy Z.E., . Corrosion inhibition of some metals using lawsonia extract. Corros. Sci.. 2005;47(2):385.
[Google Scholar]

[29] Elujoba A.A., . Female infertility in the hands of traditional birth attendants in South-western Nigeria. Fitoterapia. 1995;66:239.
[Google Scholar]

[30] Gamaniel K., Wambebe C., Amupitan J., Hussaini I.M., Amos S., Awodogan A., Dunah A.W., Ekuta J.E., Akeju M.O., Usman H., Enwerem N., . Active column fractions of Nauclea latifolia on Plasmodium berghei and rabbit ileum. J. Pharm. Res. Dev.. 1997;2:44.
[Google Scholar]

[31] Gomma K., . Corrosion of low carbon steel in sulphuric acid solution in presence of pyrazole-halides mixture. Mater. Chem. Phys.. 1998;55:241.
[Google Scholar]

[32] Gomma K., Wahdan M.H., . Effect of temperature on the acidic dissolution of copper in the presence of amino acids. Mater. Chem. Phys.. 1994;32(2):142.
[Google Scholar]

[33] Gunasekaran G., Chauhan L.R., . Eco-friendly inhibitor for corrosion inhibition of mild steel in phosphoric acid medium. Electrochim. Acta. 2004;49(25):4387.
[Google Scholar]

[34] Harbone J.B., . Phytochemical Methods. London: Chapman and Hill; 1973.

[35] Hussain H.S.N., Deeni Y.Y., . Plants in Kano Ethnomedicine, screening for antimicrobial activity and alkaloids. Ind. J. Pharmacol.. 1991;29:51.
[Google Scholar]

[36] Ita B.I., Offiong O.E., . Inhibition of steel corrosion in HCl by pyridoxal-(4-methylthiosemicarbazide), pyridoxal-(4-methylthiosemicarbazone) and its Zinc (II) complex. Mater. Chem. Phys.. 1997;48(2):164.
[Google Scholar]

[37] Jaen J.A., Garcia de Saldana E., Hernandez C., . Characterization of reaction products of iron and iron salts and aqueous plant extracts. Hyperfine Interact.. 1999;122:139.
[Google Scholar]

[38] Khaled K.F., Hackerman N., . Investigation of the inhibitive effect of ortho-substituted anilines on the corrosion of iron in 0.5 M H₂SO₄. Mater. Chem. Phys.. 2003;82:949.
[Google Scholar]

[39] Khuzhaeu V.U., Aripova S.F., . Pseudocarpaine from Carica papaya. Chem. Nat. Comp.. 2000;36(4):418.
[Google Scholar]

[40] Lamidi M., Olivier E., Faure R., Debrauwer L., Nze-Ekekang L., Balansard G., . Quinovic acid glycosides from Nauclea diderichii. Planta Med.. 1995;61:280.
[Google Scholar]

[41] Martinez S., Stern I., . Thermodynamic characterization of metal dissolution and inhibitor adsorption processes in the low carbon steel/mimosa tannin/sulfuric acid system. Appl. Surf. Sci.. 2002;199:83.
[Google Scholar]

[42] Ogan A.U., . The basic constituents of the leaves of Carica papaya. Phytochem. Rep.. 1971;99(2/3):441.
[Google Scholar]

[43] Oguzie E.E., . Influence of halides on the inhibitive effect of Congo red dye on the corrosion of mild steel in sulphuric acid solution. Mater. Chem. Phys.. 2004;87:212.
[Google Scholar]

[44] Oguzie E.E., . Studies on the inhibitive effect of Occimum viridis extract on the acid corrosion of mild steel. Mater. Chem. Phys.. 2006;99(2–3):441.
[Google Scholar]

[45] Oguzie E.E., Onuchukwu A.I., Okafor P.C., Ebenso E.E., . Corrosion inhibition and adsorption behaviour of Ocimum basilicum extract on aluminium. Pigm. Res. Tech.. 2006;35(2):63.
[Google Scholar]

[46] Oguzie E.E., Onuoha G.N., Onuchukwu A.I., . The inhibition of aluminium corrosion in potassium hydroxide by Congo red dye, and synergistic action with halide ions. Anti-Corros. Meth. Mater.. 2005;52(5):293.
[Google Scholar]

[47] Okafor P.C., Ebenso E.E., . Inhibitive action of Carica papaya extracts on the corrosion of mild steel in acidic media and their adsorption characteristics. Pigm. Res. Tech.. 2007;36(3):134.
[Google Scholar]

[48] Okafor P.C., Ebenso E.E., Ekpe U.J., . Inhibition of the acid corrosion of aluminium by some derivatives of thiosemicarbazone. Bull. Chem. Soc. Ethiop.. 2004;18:181.
[Google Scholar]

[49] Okafor P.C., Ebenso E.E., Ekpe U.J., . Azadirachta indica extracts as corrosion inhibitors for mild steel in acid medium. Int. J. Electrochem. Sci.. 2010;5:978.
[Google Scholar]

[50] Okafor P.C., Ebenso E.E., Ibok U.J., Ekpe U.J., Ikpi M.I., . Inhibition of 4-acetamidoaniline on corrosion of mild steel in HCl solution. Trans. SAEST. 2003;38:91.
[Google Scholar]

[51] Okafor P.C., Ekpe U.J., Ebenso E.E., Oguzie E.E., Umo N.S., Etor A.R., . Effect of Allium cepa and Allium sativum as corrosion inhibitors of mild steel in HCl solution. Trans. SAEST. 2006;41:82.
[Google Scholar]

[52] Okafor P.C., Ekpe U.J., Ebenso E.E., Umoren E.M., Leizou K.E., . Inhibition of mild steel corrosion in acidic medium using Allium sativum extracts. Bull. Electrochem.. 2005;21(8):347.
[Google Scholar]

[53] Okafor P.C., Ikpi M.E., Uwah I.E., Ebenso E.E., Ekpe U.J., Umoren S.A., . Inhibitory action of Phyllanthus amarus extracts on the corrosion of mild steel in acidic media. Corros. Sci.. 2008;50:2310.
[Google Scholar]

[54] Okafor P.C., Osabor V.I., Ebenso E.E., . Eco-friendly corrosion inhibitors: inhibitive action of ethanol extracts of Garcinia kola for the corrosion of mild steel in H₂SO₄ solutions. Pigm. Res. Technol.. 2007;36(5):299.
[Google Scholar]

[55] Okafor P.C., Uwah I.E., Ekerenam O.O., Ekpe U.J., . Combretum bracteosum as Eco-friendly corrosion inhibitor for mild steel in acid medium. Pigm. Resin Technol.. 2009;38(4):236.
[Google Scholar]

[56] Putilova, I.N., Balezin, S.A., Barannik, V.P., 1960. Metallic Corrosion Inhibitors (Translated from the Russian by Ryback). Pergamon Press, New York (As cited by Gupta, D.V. and Green, S., 2004, Inhibitors – Where are we. Proceedings of Corrosion 2004, paper number 04406. NACE International, Houston, 2004).

[57] Putilova I.N., Balezin S.A., Barannik V.P., . Metallic Corrosion Inhibitors. New York: Pergamon Press; 1969.

[58] Sofowora E.R., . Guidelines for research promotion and development in traditional medicine. Nigerian J. Pharm.. 1980;11:117.
[Google Scholar]

[59] Sun J., Lou H., Dai S., Xu H., Zhao F., Liu K., . Indole alkoloids from Nauclea officinalis with weak antimalarial activity. Phytochemistry. 2008;69:1405.
[Google Scholar]

[60] Traore-Keita F., Gasquet M., Di Giorgio C., Olivier E., Delmas F., Keita A., Doumbo O., Balansard G., Timon-David P., . Antimalarial activity of four plants used in traditional medicine in Mali. Phytotherapy Res.. 2000;14:45.
[Google Scholar]

[61] Trease G.E., Evans W.C., . A Textbook of Pharmacology. London: Baillier Tidally; 1996.

[62] Umoren S.A., Obot I.B., Ebenso E.E., Okafor P.C., Ogbobe O., Oguzie E.E., . Gum arabic as a potential corrosion inhibitor for aluminium in alkaline medium and its adsorption characteristics. Anti-Corros. Meth. Mater.. 2006;53(5):277.
[Google Scholar]