A novel thiophene Schiff base as an efficient corrosion inhibitor for mild steel in 1.0 M HCl: Electrochemical and quantum chemical studies
Introduction
The corrosion of mild steel in numerous industrial operations is a severe problem that must be addressed and prioritized for economic purposes as well as for machine durability. The utilization of mild steel in many cleaning processes such as pickling, industrial acid cleaning, acid descaling, and oil wells is highly susceptible to corrosion, particularly in acidic environments. In a significant number of these operations, hydrochloric acid is used due to low-cost, ease of use, and efficiency as opposed to mineral acids. However, owing to the strong corrosive behaviour of hydrochloric acid, a protective surface layer for mild steel is inevitable [1]. It is imperative to reuse mild steel for industrial applications in order to preserve its commercial value. It is impossible to entirely stop the corrosion; nonetheless, taking suitable precautionary measures can slow down the corrosion rate. Along these lines, researchers are focusing on designing protective layers on steel surface aiming to reduce the rate of corrosion amid extreme mechanical activities including pipeline systems, drilling, and processing of oil and gas. Therefore, a variety of organic inhibitors for example, N,N′-bis(1-phenylethanol) ethylenediamine, 2-phenyl-benzothiazole derivatives, 2-((thiazole-2-ylimino)methyl)phenol, and 1H-pyrrole-2,5-dione derivatives have been used [[2], [3], [4], [5], [6], [7], [8], [9], [10], [11], [12], [13], [14], [15], [16], [17], [18], [19]]. For the most part, these organic compounds incorporate sulphur, nitrogen, and oxygen in their chemical structures, which adsorb to the surface of steel by a nucleophilic attack and forms a protective surface layer [8,[20], [21], [22], [23], [24], [25]]. It is well understood that among the various inhibitors, heterocyclic organic compounds containing heteroatoms such as N, O, and S, and aromatic rings demonstrate high inhibition activities [26]. Additionally, inhibitor molecules containing both nitrogen and sulphur demonstrate superior inhibition efficiency than those containing only one of these atoms [[27], [28], [29], [30], [31], [32], [33]]. Adsorption to the surface may be characterized as a physical or chemical process, which tends to exhibit a synergic interaction between the occupied p-orbitals of the organic compounds and the unoccupied d-orbitals of metals. For physical adsorption, the following two things must be present: an electrically charged metal surface, and charge carrying species in bulk of the solution. During chemical adsorption, formation of chemical bonds occur involving the transfer of electrons from an inhibitor molecule to the metal surface due to the presence of heteroatoms (i.e., N, P, S, and O) in the structure of the inhibitor that possess lone pairs of electrons, or the presence of aromatic rings. Therefore, an immense interest has been shown in Schiff bases, which have nitrogen and oxygen atoms in their chemical structures. Moreover, the adsorption capability increases with the existence of π-bonds, conjugated bonds, and aromatic rings. Similarly, Schiff bases are potential materials for mild steel corrosion inhibition in acidic media as they demonstrate an excellent inhibition activity through their adsorptive >CN-group exhibiting lone pair of electrons on N atoms [5,[34], [35], [36], [37], [38], [39]]. Additionally, Schiff bases are inexpensive and non-toxic compounds that can be produced on a large scale using simple processing methods, therefore, they are applicable for commercial purposes. The synthesis strategy of Schiff bases is simple, involving the linking of an amine group with a carbonyl group in an appropriate solvent [40].
Previously, various thiophene Schiff bases have been reported exhibiting good corrosion inhibition behaviour in acidic media [16,19,31,32]. Along these lines, this study aims to design new sulphur and nitrogen-containing Schiff base for corrosion inhibition of mild steel in aggressive conditions. The inhibition efficiency of thiophene Schiff base is examined on mild steel in 1.0 M HCl solution by a solution assay analysis using inductive coupled plasma-optical emission spectrometer (ICP-OES), potentiodynamic polarization, and EIS method. Moreover, quantum chemical calculations have been applied to understand the adsorption of inhibitor molecules to the metal surface in more depth. Quantum chemical calculations help in designing new inhibitor molecules by successfully correlating the molecular structure, and chemical reactivity and selectivity during the corrosion inhibition process. This allows us to understand the mechanism of the adsorption process on the molecular level.
Section snippets
Preparation of electrode
We used mild steel with an elemental composition of C: 0.17, Si: 0.59, Mn:1.6, P: 0.040, and Fe: 97.6 by weight% for the electrochemical measurements and solution assay experiments. First, mild steel (5.0 cm in length) was acquired by mechanical cutting, and a copper wire was connected to one of the surfaces to make an electrical contact. The electrode surface was covered with polyester except the bottom face. Then, the exposed surface of the electrode was polished with grit emery papers (320
Characterization of 4-TAB
The chemical structure of the synthesized 4-TAB was characterized by FT-IR and 1H NMR. The FT-IR spectra (Fig. 2.a.) of (4-((thiophene-2-ylmethylene)amino)benzamide) indicates band stretching around 3343, 3183, 2979, 1651, 1556, and 1400 cm−1 which corresponds to NH stretching, aromatic CH stretching, aliphatic CH stretching, CO stretching, CH stretching, and asymmetric characteristic thiophene CC stretching, respectively. Similarly, the peaks around 1047, 778, 660 cm−1 are associated with
Conclusion
A new Schiff base 4-((thiophene-2-ylmethylene)amino) benzamide was successfully synthesized by soft chemical route and characterized with FT-IR and 1H NMR measurements to understand the molecule's structure. The inhibition behaviour of 4-TAB on mild steel was investigated using solution assay analysis, electrochemical measurements, and FESEM images. The experimental results are further contemplated by quantum chemical calculations. EIS measurements show that the inhibition efficiency of 4-TAB
Acknowledgments
This work was supported by Çukurova University Scientific Research Project (grant numbers: FBA-2017-7056).
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