A Theoretical Approach to the Study of Some Plant Extracts as Green Corrosion Inhibitor for Mild Steel in Hcl Solution

Introduction

Inhibitors are added to many systems, namely, cooling systems, refinery units, chemicals, oil and gas production units, boiler, and so forth. Most of the effective inhibitors are used to contain heteroatom such as O, N, and S and multiple bonds in their molecules through which they are adsorbed on the metal surface. Plant extracts have become important because they are environmentally acceptable, inexpensive, readily available and renewable sources of materials, and ecologically acceptable. Plant products are organic in nature, and some of the constituents including tannins, organic and amino acids, alkaloids, and pigments are known to exhibit inhibiting action [1]. Moreover, they can be extracted by simple procedures with low cost. Many authors such as E. E. Ebenso, B. Hammouti, A. Y. El Etre, P. C. Okafor, E. Oguzie, and P. B. Raja, have contributed significantly to the green mitigation by investigating several plants and their different body parts as corrosion inhibitors [2-23]. In present work, we have worked on the extracts of (Kalmegh) Andrographis paniculata, (Meethi Neem) Murraya koenigii, (Bael) Aegle marmelos, (Kuchla) Strychnos nuxvomica, (Shahjan) Moringa oleifera, (Orange) Citrus aurantium, and (Arjun) Terminalia arjuna as corrosion inhibitors [24-29]. The active constituents and inhibition efficiencies of the extracts used are shown in Figure 1.

 

Scheme 1 Click here to View scheme

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Experimental

Theoretical study

All the calculations were performed with Gaussian 03 for windows. The molecular structures of the species were fully and geometrically optimized using the functional hybrid B3LYP (Becke, three-parameter, Lee-Yang-Parr exchange-correlation function) Density function theory (DFT) formalism with electron basis set 6-31G (*, *) for all atoms. The quantum chemical parameters obtained were EHOMO, ELUMO, EHOMO-LUMO (ΔE), Dipole moment (μ), and Mulliken charges on heteroatoms (O,N,S).

Results and Discussion

Quantum Chemical Calculations

Quantum chemical calculations were carried out in order to investigate adsorption and inhibition mechanism of various inhibitor molecules in the extract. Figure 2 show full geometry optimization of the inhibitor molecules with Mulliken charges. The Frontier molecular orbital (FMO) density distributions of the inhibitor molecules present in Plant extract are shown in Figure 3 (a-b). Frontier orbital theory is useful in predicting adsorption centers of the inhibitor molecules responsible for the interaction with surface atoms. Excellent corrosion inhibitors are usually those compounds who not only offer electrons to unoccupied orbital of the metal, but also accept free electrons from the metal. It is important to focus on the parameters that directly influence the electronic interaction of the inhibitor molecules with the metal surface. These are mainly: E_HOMO, E_LUMO, ∆E (E_LUMO-E_HOMO), and dipole moment µ.  The values of calculated quantum chemical parameters such as E_HOMO, E_LUMO, ∆E (E_LUMO-E_HOMO) and µ of Plant extracts are listed in Table 1. Higher values of the E_HOMO facilitate adsorption (and therefore inhibition) by influencing the transport process through the adsorbed layer. Therefore, the energy of the E_LUMO indicates the ability of the molecule to accept electrons; hence these are the acceptor states.

The lower the value of E_LUMO, the more probable, it is that the molecule would accept electrons. As for the values of ∆E (E_LUMO-E_HOMO)concern; lower values of the energy difference ∆E will cause higher inhibition efficiency because the energy to remove an electron from the last occupied orbital will be low. For the dipole moment (µ), higher values of µ will favor strong interaction of the inhibitor molecules with metal surface and lower values favor the accumulation of inhibitor molecules around electrode surface. Quantum chemical parameters presented in Table 1 confirmed strong interaction of extracts molecules with the metal surface and thereby forming protective adsorption layer at mild steel/acid solution interface.

Table 1: Calculated Quantum chemical parameters of studied inhibitor.      

Figure 1  Click here to View Figure

 

Figure 2Click here to View Figure

 

Mechanism of Inhibition

Inhibition performance of Plant extracts for mild steel in 1 M HCl interface depends on the extent of adsorption and adsorption depends on several factors such as the number of adsorption sites, molecular size, and mode of interaction with the metal surface and extent of formation of metallic complexes. The neutral molecules may be adsorbed on the mild steel surface through the chemisorption mechanism, involving the displacement of water molecules from the mild steel surface and the sharing electrons between the hetero atoms and iron. The inhibitor molecules can also adsorb on the mild steel surface on the basis of donor–acceptor interactions between π-electrons of the aromatic ring and vacant d-orbitals of surface iron atoms.

Conclusions

The inhibitors studied had an excellent inhibition effect for the corrosion of mild steel in 1 M HCl. The high inhibition efficiencies of were attributed to the adherent adsorption of the inhibitor molecules on the mild steel surface.
Quantum chemical approach is adequately sufficient to predict the structure and molecule suitability to be an inhibitor.

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