Electrochemical, Spectroscopic and Quantum Chemical Calculation Studies on Some Quinoxaline Derivatives As Corrosion Inhibitors for Mild Steel in Hydrochloric Acid Medium

Mild steel is considered a material of choice in diverse industrial and structural applications. It is used to make a wide range of equipment due to its relatively low cost and good mechanical strength [1]. But acid solutions used in many industrial processes such as acid cleaning and oil well acidizing constitute strong corrosion media that enhance the rate of mild steel corrosion. As a result, corrosion of steel has been identified as a common problem that consumes large maintenance costs in many industries. It has been established that organic molecules containing heteroatoms such as nitrogen, oxygen and sulphur usually exhibit good anticorrosion activities [2]. Quinoxaline and its derivatives are among organic compounds that have been reported to be good corrosion inhibitors [3]. Quinoxaline units are parts of many materials including dyes and pharmaceuticals. They are biodegradable and non-toxic. Thus, the purpose of this work is to investigate the corrosion inhibition activities of some quinoxaline derivatives on the corrosion of mild steel in hydrochloric acid medium. To the best of our knowledge, the set of organic compounds selected for this work has not been investigated for the same purpose in any previous work.

In this study, the corrosion inhibition and adsorption characteristics of four quinoxaline derivatives (Me-4-PQPB, Mt-3-PQPB, Mt-4-PQPB and Oxo-1,3-PQPB) on mild steel surface in 1 M HCl were investigated. Potentiodynamic polarization (PDP) and electrochemical impedance spectroscopy (EIS) techniques were used in the study. Fourier transform infrared (FTIR) and ultraviolet-visible (UV-Vis) spectroscopic techniques were used to investigate possible formation of Fe/inhibitor complex and/or adsorption of the inhibitors on steel surface. Quantum chemical calculations were carried out on the molecules to correlate experimental results with quantum molecular parameters. The results showed that the studied molecules inhibit corrosion of mild steel in HCl medium with Me-4-PQPB having the best inhibition property. The inhibitors are mixed-type in their activities. All the inhibitors physisorb and chemisorb spontaneously on mild steel surface and their adsorption behaviour obeyed Langmuir equation. Some quantum chemical parameters support the observed trend of inhibition potencies and suggest that the inhibition effects were attributed to the protonated forms of the quinoxaline derivatives. The graphical surfaces of condensed Fukui indices suggest possible adsorption sites on the inhibitors.

References

1. De la Fuente D., Diaz I., Simancas J., Chico B., Morcillo M. “Long-term Atmospheric Corrosion of Mild Steel, Corrosion Science (2010), doi:10.1016/j.corsci.2010.10.007.
2. Obi-Egbedi N.O., Obot I.B., El-Khaiary M.I., Umoren S.A., Ebenso E.E. “Computational Simulation and Statistical Analysis on the Relationship between Corrosion Inhibition    Efficiency and Molecular Structure of Some Phenanthroline Derivatives on Mild Steel       Surface.” Int. J. Electrochem. Sci., 6 (2011): 5649 – 5675.
3. Obi-Egbedi, N.O. and Obot, I.B. Indeno-1-one-[2,3-b]-quinoxaline as an effective inhibitor for the corrosion of mild steel in 0.5M H₂SO₄ solution. Materials Chemistry and Physics 122 (2010) 325–328.