doi: 10.4152/pea.201902115
-->Study of Bitter Leaves Extract as Inhibitive Agent in HCl Medium for the Treatment of Mild Steel through Pickling
O.D. Onukwulia and M. Omotiomab*
a Department of Chemical Engineering, Nnamdi Azikiwe University, Awka, Nigeria
b Department of Chemical Engineering, Enugu State University of Science and Technology, P.M.B. 01660, Enugu, Nigeria
Abstract
Bitter leaves extract as inhibitive agent in HCl medium for the treatment of mild steel through pickling was studied. Thermometric, gravimetric and potentiodynamic polarization methods were employed in the corrosion inhibition study. The bitter leaves extract was analyzed using gas chromatography-mass spectrometer. The analysis of the extract revealed the presence of C6H8O (96 g/mole: 2,4-Hexadienal); C7H12(96 g/mole: 3,4-Heptadiene; 1,3-Diethylallene) and C10H18O2 (170 g/mole: 2-Decenoic acid) as the predominant chemical constituents. The activation energy for the corrosion inhibition process ranged from 39.831 to 77.533 kJ/mol, while the heat of adsorption ranged from -16.093 to -30.224 kJ/mol. These values showed that exothermic and spontaneous adsorption of the extract on the mild steel followed the mechanism of physical adsorption. Maximum inhibition efficiency of 85.4% was obtained. The extract was highly efficient in the corrosion inhibition function. The plant-based inhibitor of bitter leaves extract is a suitable additive for pickling, cleaning and descaling operations.
]]> Keywords: Bitter leaves, treatment, mild steel, pickling.
Introduction
Mild steel is a vital metallic material used in engineering, transportation and construction industries. It has good machining properties and can be hardened by heat treatment. Besides carbon, steel contains many chemical elements which are added into iron to form steel of different kinds having various physical properties [1]. Mild steel is a medium carbon steel with carbon content of 0.2 – 0.5% [2]. It is weldable, which expands its possible applications. It is used for the production of lightly stressed machine fittings, turbine motors, railways axels, pipes and drums. Mild steel is widely used in the manufacturing of installations for chemical and allied industries. Most often processes within these industries such maintenance operations involve contact between mild steel and aggressive solution.
Mild steel structures corrode as a result of electrochemical reaction with its environment. In most industries, maintenance operations such as pickling, cleaning and descaling are carried out to prolong the life span of the mild steel structures. But aggressive hydrochloric acid used for such operations often corrodes the mild steel structures. Considering the viability of mild steel and its high cost of production and installation, several steps are taken to prolong its life span. The synthetic chemicals commonly used as corrosion inhibitors are harmful to the environment. There is need for the corrosion inhibition using eco-friendly inhibitors of plant origin. Bitter leaves extract has been found useful in pharmaceutical applications [3, 4]. The chemical constituents of bitter leaves extract include several organic compounds with polar atoms capable of exhibiting electrochemical activity of strong adsorption onto mild steel surface. Quantum chemical methods have been employed in determining the molecular structure as well as describing the electronic structure and reactivity. The chemical techniques apply the knowledge of the density for a complete determination of all ground state molecular properties. The basic relationship of the density functional theory of chemical reactivity is the one that links the chemical potential of density functional theory with the first derivative of the energy with respect to the number of electrons, and therefore with the negative of the electronegativity Χ [5].
where μ is the chemical potential, E is the total energy, N is the number of electrons, and ν(r) is the external potential of the system.
For detail understanding of the chemical constituents of plant extract, sophisticated device (such as gas chromatography-mass spectrometer) is needed for the characterization of the plant extract. Gas chromatography-mass spectrometer helps in identifying the molecular compositions of the extract for the corrosion inhibition functions.
]]> Experimental methods
Leaves of bitter leaf (Vernonia amygdalina) were collected from Akpugo, Enugu State, Nigeria. Sheet of mild steel with composition of P (0.02%), Mn (0.11%), Si (0.02%), S (0.02%), Cu (0.01%), C (0.23%), Ni (0.02), Cr (0.01%) and Fe (99.56 %) were cut into coupons (5 cm × 4 cm). In the extraction of the bitter leaves extract and surface preparation of the mild steel, the method used in the previous study was adopted [6]. Chemical analysis of the bitter leaves extract was carried out using gas chromatography-mass spectrometer (GCMS-QP2010 PLUS, SHIMADZU). The method used by previous authors was adopted [7, 8].
Thermometric, gravimetric and potentiodynamic polarization methods were used in the corrosion inhibition study. The method of the thermometric measurements used by previous authors was adopted with slight modification [9, 10]. The inhibitor efficiency was determined using Equation (2).
where RNfree and RNadd are the reaction numbers for the metal dissolution in free and inhibited corrosive medium, respectively.
The potentiodynamic polarization study was carried out according to the method used by previous authors [11, 12]. For the gravimetric method, the weight loss (Δw), corrosion rate (CR), inhibition efficiency (IE) and degree of surface coverage, were determined using the standard Equations [6, 13]. Considering the corrosion rates of the metal at T1 and T2 as CR1 and CR2, the activation energy, Ea, was obtained using Equation (3) [14, 15, 16].
The heat of adsorption Qads(kJmol-1) was calculated using Equation (4) [15, 17].
]]>
where R is the gas constant, Θ1 and Θ2 are the degree of surface coverage at temperatures T1 and T2, respectively.
Different adsorption isotherms (The Langmuir, Frumkin, Temkin and Flory- Huggins isotherms) were used to determine the mechanism of the adsorption of the extract on the metal surface [6, 15, 18, 19, 20].
The free energy of adsorption (ΔGads) was calculated according to Equation (5) [6, 15, 18].
where R is the gas constant, T is temperature and K is the adsorption equilibrium constant.
Results and discussion
]]> Analysis of the bitter leaves extractIn Fig. 1, the GC MS chromatogram of the bitter leaves extract shows various levels of peaks.
The analysis revealed the presence of C6H8O (96 g/mole: 2,4- Hexadienal; Sorbaldehyde , n-Hex-2,4-dienal, Hexa-2,4-dienal; Sorbic aldehyde 1,3-Pentadiene-1-carboxaldehyde; 2,4-Hexadien-1-al; 2,4-Hexadienal; trans,trans-2,4-Hexadienal; 3-Propyleneacrolein); C7H12(96 g/mole: 3,4- Heptadiene; 1,3-Diethylallene); C10H18O2(170 g/mole: 2-Decenoic acid).
Other compounds present in the extract include C9H16O2(156 g/mole: 2- Nonenoic acid; trans-2-Nonenoic acid; Nonylenic acid, 2-Nonenylic acid); C12H14N2O6(282g/mole: Phenol, 2-(1-methylpropyl)-4,6-dinitro-, acetate; Acetic acid, 2-(sec-butyl)-4,6-dinitrophenyl ester); C12H18O (178 g/mole: Benzene ethanol; 2-(3-Isopropylphenyl)-1-propanol); C18H34O2 (282 g/mole: Oleic Acid, 9-Octadecenoic acid; cis-9-Octadecenoic Acid); C18H34O (266 g/mole: 13-Octadecenal; cis-13-Octadecenal) and C16H30O (238g/mol: cis-9- Hexadecenal; 9-Hexadecenal).
Results of the thermometric method
Table 1 presents the effect of concentration on the reaction number and inhibition efficiency. The reaction number decreases with increase in concentration, while the inhibition efficiency increases with increase in concentration.
]]> Results of the gravimetric method
The results of corrosion inhibition of mild steel in HCl medium with bitter leaves extract are presented in Table 2.
The corrosion rate decreases with increase in concentration of the extract. This is in agreement with previous studies [6, 15]. Maximum inhibition efficiency of 85.4% was obtained. It showed that the extract of bitter leaf can be used as additive for pickling, cleaning and descaling operations. The extract is efficient for corrosion control of mild steel in the hydrochloric acid medium.
The activation energy and heat of adsorption
The activation energy and heat of adsorption are presented in Table 3.
The activation energy for the corrosion inhibition process ranged from 39.831 to 77.533 kJ/mol, while the heat of adsorption ranged from -16.093 to -30.224 kJ/mol. These values showed that exothermic and spontaneous adsorption of the extract on the mild steel followed the mechanism of physical adsorption.
]]> Adsorption parameters for the corrosion inhibition processAdsorption parameters for the corrosion inhibition of mild steel in HCl by bitter leaves extract are shown in Table 4.
The values of free energy of adsorption (ΔGads) were less than the threshold value of -40 kJ/mol required for chemical adsorption. The lateral interaction term (a) gave positive values, indicating attractive behaviour of the extract on the mild steel surface. The attractive parameter value (a) is negative, which implies that repulsion exists in the adsorption layer [16]. The value of the size parameter (x) is positive. The adsorbed species of the bitter leaves extract was bulky. This is in agreement with previous works [6, 15, 21]. The values of the correlation coefficient (R2) are close to unity (1), which implies that the Langmuir, Frumkin, Temkin and Flory- Huggins isotherms of Equations (6) , (7), (8) and (9) were obeyed [15, 22].
]]> where C is the concentration of the inhibitor, K is the adsorption equilibrium constant and Θ is the degree of surface coverage.
Potentiodynamic polarization curves
Potentiodynamic polarization curves for mild steel in HCl in absence and presence of bitter leaves extract are shown in Fig. 2.
The extract inhibited the corrosion of the mild steel in the HCl medium, affecting both the cathodic and anodic reactions, indicating that it is a mixed type inhibitor.
Conclusion
From the analysis of the experimental results the following conclusions can be drawn:
· The bitter leaves extract was highly efficient in the corrosion inhibition of the mild steel in HCl medium.
]]> · It is effective for corrosion prevention of mild steel in the corrosive medium.· The exothermic and spontaneous adsorption of the extract on the mild steel followed the mechanism of physical adsorption.
·The plant-based inhibitor of bitter leaves extract should be applied as additive for pickling, cleaning and descaling operations.
References
1. Uppal MM, Bhatia SC. Engineering Chemistry (chemical technology). 7th ed. New Delhi: Khanna Publ; 2009. [ Links ]
2. Aggarwal OP. Engineering Chemistry. 3rd ed. New Delhi: Khanna Publ; 2010. [ Links ]
3. Masaba SC. Ethiopian J Health Develop. 2000;13:211-216. [ Links ]
4. Nwajo HU. Nig J Physiol Sci. 2005;20:39 - 42. [ Links ]
5. Udhayakala P, Rajendiran TV, Gunasekaran S. J Comp Methods Molec Design. 2012;2:1-15. [ Links ]
6. Omotioma M, Onukwuli OD. Port Electrochim Acta. 2016;34:287-294. [ Links ]
7. El Ouariachi E, Paolini J, Bouklah M, et al. Acta Metal Sin. 2010;23:13-20. [ Links ]
8. Rosaline-Vimela J, Leema RA, Raja S. Der Chemica Sinica. 2012;3:582- 588. [ Links ]
9. Mabrouk EM, Shokry H, Abu Al-Naja KM. Chem Met Alloys. 2011;4:98- 106. [ Links ]
10. Eddy NO, Ita BI, Dodo SN, et al. Green Chem Lett Rev. 2012;5:43-53. [ Links ]
11. Oguzie EE, Enenebeaku CK, Akalezi CO, et al. J Colloid Interf Sci. 2010;349:283-292. [ Links ]
12. Ihebrodike MM, Nwandu MC, Kelechukwu BO, et al. J Mater Sci. 2011;47:2559-2572. [ Links ]
13. Nagm AN, Kandile NG, Badr EA, et al. Corros Sci. 2012;65:94-103. [ Links ]
14. Octave L. Chemical Reaction Engineering. 3rd ed. New York: John Wiley and Sons; 2003. [ Links ]
15. Orubite-Okorosaye K, Oforka NC. J Appl Sci Environ Mgt. 2004;8:57-61. [ Links ]
16. Nwabanne J T, Okafor VN. JETEAS. 2011;2:619-625. [ Links ]
17. Alinno IJ, Ejikeme PM. Am Chem Sci J. 2012;2:122-135. [ Links ]
18. Li X, Deng S. Corros Sci. 2012;65:299-308. [ Links ]
19. Patel NS, Jauhariand S, Mehta GN, et al. Int J Electrochem Sci. 2013;8:2635-2655. [ Links ]
20. Vasudha VG, Shanmuga PK. Res J Chem Sci. 2013;3:21-26. [ Links ]
21. Ndibe OM, Menkiti MC, Ijomah MNC, et al. EJEAFChe. 2011;10:2847- 2860. [ Links ]
22. Nnanna LA, Owate IO, Nwadiuko OC, et al. Int J Mater Chem. 2013;3:10-16. [ Links ]
]]> Acknowledgement
The authors wish to thank National Research Institute for Chemical Technology (NARICT), Zaria, Nigeria, for the GC-MS analysis of the bitter leaves extract.
*Corresponding author. E-mail address: omorchem@yahoo.com
Received October 12, 2016; accepted March 8, 2017
]]>