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The Inhibition of Mild Steel Corrosion in an Acidic Medium by the Juice of Citrus Paradisi (Grapefruit) A.K. Olusegun, N. C. Oforka, E.E. Ebenso* Department of Pure and Industrial Chemistry, University of Port Harcourt, P.M.B. 5323, Port Harcourt, Rivers State Nigeria. abiolaolusegun @yahoo.com *Department of Pure and Applied Chemistry, University of Calabar, P.M.B. 1115, Calabar, Nigeria ABSTRACT The corrosion inhibition of mild steel in HCl solution in the presence of the juice of citrus paradisi (grapefruit) in the temperature range 30-50°C was studied using the weight loss technique. The juice of citrus paradisi was found to act as a corrosion inhibitor in the acid environment. The inhibition efficiency increases with increase in inhibitor concentration but decreases with an increase in temperature. The inhibition is attributed to the adsorption of a component in the juice onto the surface of the mild steel. Keywords: Citrus paradisi; mild steel; corrosion inhibition; adsorption INTRODUCTION Iron and its alloys (e.g. steels) are exposed to the action of acids in industry. Processes in which acids play a very important part are acid pickling, industrial acid cleaning, cleaning of oil refinery equipment, oil well acidizing and acid de-scaling [1,2]. The exposures can be most severe but in many cases, corrosion inhibitors are widely used in industry to prevent or to reduce the corrosion rates of metallic materials in these media. Because of the toxic nature and high cost of some chemicals currently in use it is necessary to develop environmentally acceptable and less expensive inhibitors. Natural products can be considered as a good source for this purpose . The possible replacement of some expensive chemicals as corrosion inhibitors for metal in acid cleaning process by naturally occurring substances of plant origin has been studied, amongst others by Hosary and Saleh . 1 Natural products of plant origin contain different organic compounds (e.g. alkaloids, tannins, pigments, organic and amino acids) and most are known to have inhibitive action [3,4]. In our earlier communication  it was found that cocos nucifera (coconut) juice inhibits the corrosion of mild steel in hydrochloric acid solution (HCl). Ekpe et al.  and Saleh et al.  used the aqueous extracts of some natural products (fruits, fruit shells, leaves, seeds) as corrosion inhibitors of some metals. Citrus species are utilized in many industries for the production of the various brands of citrus juices . The juice is also rich in vitamin C, folic acid and significant quantities of other vitamins, pectins, flavonoids among others [7,8]. In particular, nootkatone contributes to the bitter flavour of the grapefruit . Nitrogenous compounds are also present to the extent of 0.05 1.0% mostly as free amino acids asparagines, alanine, arginine, aspartic acid, glutamine, glutathione, histidine, betaine, cysteine, proline, serine and stachydrine [5,7, 8]. The principal acid in citrus fruits is citric acid (80 90% of the total acids). Others are malic, tartaric, benzoic, succinic, quinic, oxalic and formic acid . The fruit juice of citrus paradisi may be of use in the production of non-toxic inhibitors to replace toxic corrosion inhibitors. For example, species present in the juice of citrus paradisi, especially nitrogen containing organic compounds, may adsorb on the metal and block the active sites on the surface, thereby reducing the corrosion rate in acid environment. The aim of this study was, therefore, to determine the inhibition efficiency of the fruit juice of citrus paradisi as an inhibitor for the corrosion of mild steel in 0.5M HCl. Weight loss technique was employed to carry out the measurements. EXPERIMENTAL PROCEDURES Material Preparation The composition and preparation of mild steel coupons are described in detail as reported previously . All test solutions were prepared from analytical grade reagents and doubledistilled water. The citrus paradisi juice was used as an additive for this investigation. The additive concentration of 0.5, 1.0, 1.5, 2.0 and 2.5% volume/volume percent (v/v) were prepared in 0.5M HCl (corrodent) solutions at 30, 40 and 50°C. 2 Weight loss measurement Previously weighed mild steel coupons were immersed in 250ml open beakers containing 0.5M HCl solution (blank) without additive, and with additive concentrations of 0.5, 1.0, 1.5 2.0 and 2.5% in 0.5M HCl. The variation of weight loss was followed at 3 hour intervals progressively over15 hours at 30, 40 and 50°C. The procedure for weight loss determination was similar to that reported previously [9,10]. The inhibition efficiency (1%) was determined from : I% = (Wb Wi) / Wb x 100% ... (1) Where Wb and Wi are the weight loss of mild steel per unit area (mg cm-2) of coupons in the corrodent (blank) and corrodent-inhibitor systems. RESULTS AND DISCUSSION Effect of citrus paradisi juice on the corrosion of mild steel in HCl solution Figure 1 shows the variation of the weight loss with time for mild steel corrosion in 0.5M HCl and 0.5M HCl with various concentrations of citrus paradisi juice at 30°C. Similar trends were observed at 40 and 50°C. From the variation of weight loss with time of immersion in HCl solution without additive, compared with mild steel in HCl solution containing the additive at 30°C (Fig. 1), there is a general decrease in weight loss, signifying the inhibition of the acid corrosion of mild steel. The extent of the decrease in weight loss was found to depend on the concentration of additive. Figure 2 also confirms that the additive is a corrosion inhibitor; since there was a general decrease in corrosion rate (mg cm-2 h-1). The corrosion rate as a function of additive concentration and at different temperatures (30 50°C) is shown in Figure 2. The corrosion rate decreases with increasing concentration of citrus paradisi juice at each of the temperatures. This confirms that the presence of the additive in 0.5M HCl solution inhibits the corrosion of mild steel by HCl and that the degree of corrosion inhibition depends on the amount of the citrus paradisi juice present. Figure 3 illustrates the variation of the inhibition efficiency, I%, versus the concentration of the additive at 30, 40 and 50°C. The inhibition efficiency increases with increasing the concentration of the inhibitor. As shown in figure 3 inhibition efficiency increases with increase in concentration of the inhibitor up to 2.5%v/v at a maximum efficiency of 45.6%, 37.3% and 24.1% at 30%, 40 and 50°C respectively. 3 Figure 3 shows the effect of increasing temperature from 30°C to 50°C on the inhibition efficiency. From the plot of the inhibition efficiency with concentration of the inhibitor and from the result given in Table 1, it was observed that the efficiency decreased with increasing temperature. On this basis it is suggested the inhibition mechanism is physisorption of the inhibitor on the metal surface. Adsorption considerations The surface coverage, , at each concentration of inhibitor was evaluated using the equation: = 1 Wb/Wi ... (2) Where Wb and Wi are the weight loss in corrodent and corrodent-inhibitor systems respectively at constant temperature. The surface coverage data and corrosion rate are recorded in Table 2. The experimentally observed linear decrease in corrosion rate as seen in Table 2 with surface coverage, therefore supports the observation that the inhibitor inhibits corrosion by being adsorbed at the reaction sites on the mild steel surface . A curtailment of these reaction sites would therefore lead to a reduction in the corrosion rate and this may be precisely how the inhibitor achieves inhibition by being adsorbed on the mild steel surface at the reaction sites Figure 4 shows the plot of logarithm inhibition efficiency (I%) versus logarithm inhibitor concentration for the additive at 30, 40, and 50°C a linear plot is obtained which obeys the Freundlish isotherm. CONCLUSIONS CONCLUSIONS From the present investigation, the following conclusions can be drawn. 1. The efficiency of inhibition of mild steel in HCl is a function of the concentration of citrus paradisi juice. 2. The inhibition by this additive increased with increased additive concentration and decreased temperature. 4 3. Citrus paradisi fruit juice is a corrosion inhibitor for mild steel in HCl solution and can be used to replace toxic chemicals provided only modest (<50%) inhibition is required. References 1. G.I Gardner in, "Corrosion inhibitors", C. C. Nathan Ed., NACE, p.156. 2. S.S. Abd El Rehim, M.A.M. Ibrahim and K.F. Khalid. The Inhibition of 4 (2`- amino5' Methylphenylazo) antipyrine on corrosion of mild steel in HCl solution. Material Chemistry and Physics, 70 p.268, 2001 70, 3. "The Inhibitive action of molasses on the corrosion of mild steel in acidic media", R. Hosary and H. Salem, Corrosion Eng. 1, p.63, 1984 4. "Inhibitory action of Azadirachta indica leaves extract on corrosion of mild steel in Tetraoxosulphate (VI) acid", U.J. Ekpe, E.E. Ebenso and U.J. Ibok, Journal West African Science Association, 37 p.13, 1994 37, 5. "The corrosion inhibition effect of Cocos Nucifera (coconut) water on mild steel in HCl solution", O.K. Abiola and N.C. Oforka. Proceedings of the Chemical Society of Nigeria, 25th International Conference, 2002. 6. "Corrosion Inhibition by naturally occurring substances", R.M. Saleh, A.A. Ismail and A.A. ElHosary, British Corrosion Journal , 17 p.131, 1980 17, 7. "Nutritional quality of plant foods", U.O. Anthony and U.E. Offiong, Post Harvest Research Unit Dept. of Biochemistry University of Benin, p. 96 1998. 8. "Integrated Food Science and Technology for the Tropics" A.I. Ihekoronye and P.O. Ngoddy, pub. Macmillan p.305, 1984 9. "Inhibition of the Corrosion of Mild Steel in Hydrochloric Acid by (4-Amino 2 Methyl-5-Pyrimidinyl Methylthio) Acetic Acid and its Precursor", O.K. Abiola and N.C. Oforka, Journal of Corrosion Science and Engineering 3, Paper 21, 2002 10. U.J. Ekpe, U.J. Ibok, B.I. Ita, O.E. Offiong and E.E. Ebenso. Inhibitory action of Methyl and Phenylthiosemicarbazone derivatives as the Corrosion of Mild Steel in hydrochloric acid. Material Chemistry and Physics, 48 p.87, 1995. 48, 5 11. "Mutual Effects of Ihiosemicarbazone derivatives on the Acidic Corrosion of Aluminum", U.J. Ekpe, P.C. Okafor, E.E. Ebenso, O.E. Offiong and B.I. Ita Bulletin Electrochemistry 3, p.131, 2001. 1: TABLE 1 The effect of grapefruit (citrus paradisi ) juice on mild steel corrosion in 0.5M HCl Inhibitor Conc. v/v 0.0% 0.5% 1.0% 1.5% 2.0% 2.5% 30°C 30.6 33.3 41.4 43.1 45.6 Inhibitor efficiency (I%) 40°C 18.2 21.4 29.0 34.1 37.3 50°C 9 10.3 15.6 20.3 24.4 2: Surface coverage, and corrosion rate during corrosion of mild steel in 0.5M TABLE 2 HCl containing various concentrations of grapefruit (citrus paradisi ) juice at 30°C Concentration of inhibitor v/v% 0.5% 1.0% 1.5% 2.0% 2.5% Surface Coverage, 0.31 0.33 0.41 0.43 0.46 Corrosion rate (mg cm-2 h-1) 0.174 0.167 0.147 0.142 0.136 6 7 8 9 10