Computational and adsorption investigation of some quinoxaline derivatives on selected metals in acidic media

MSc (Chemistry)

Department of Chemistry

This research study reports the inhibition of mild steel (MS), zinc (Zn) in 1.0 M HCl and 1.0 M H2SO4, and the inhibition of aluminium (Al) in 0.5 M HCl by three selected quinoxaline derivatives namely, quinoxalone-6-carboxylic acid (Q6CA), 3-hydroxy-2-quinoxaline carboxylic acid (H2QCA), and Methyl quinoxaline-6-carboxylate (MQ6CA) at 303 – 333 K. The corrosion inhibition characteristics including corrosion mechanism, corrosion inhibition efficiencies, and inhibitor-metal adsorption/desorption behavior were analyzed using gravimetric analysis, electrochemical impedance spectroscopy (EIS), and potentiodynamic polarization (PDP). Fourier transform infrared spectroscopy (FTIR) was utilized to give more insight into the functional groups that formed or disappeared during the adsorption/desorption of the studied quinoxaline molecules on the metal surfaces. Atomic absorption spectroscopy (AAS) was employed to determine the amount of MS and Zn ions that remained in the solutions after gravimetric analysis. Density functional theory (DFT) was utilized to compute all theoretical studies. The gravimetric analysis for mild steel show that the inhibition efficiency increased with the increase in the concentrations of the studied quinoxalines and decreased with the increase in temperature of the corrosive environment, whereas for zinc the inhibition efficiency increased with the increase in the temperature of the corrosive environment for all the quinoxaline compounds. The compounds inhibited the mild steel and zinc corrosion by adsorption on the active sites on the surfaces without altering the mechanism of the adsorption process. The studied compounds obeyed the Langmuir isotherm, and this isotherm indicated the adsorption mechanism which was mixed-typed adsorption with chemisorption dominant for both mild and zinc. The trend of inhibition efficiency for both mild steel and zinc varied in the order: MQ6CA>Q6CA>H2QCA. PDP results indicated that the studied quinoxalines shifted the polarization curves towards the region of low current densities as compared to the uninhibited system, which suggested that the inhibitor molecules reduced the anodic dissolution of mild steel, zinc, and aluminium and also suppressed the hydrogen evolution reaction. The obtained potentiodynamic polarization parameters revealed that all three inhibitors studied acted as mixed-type inhibitors, that is, anodic and cathodic inhibitor that protected the mild steel, zinc, and aluminium surfaces through spontaneous adsorption. Moreover, the increase in the concentration of the inhibitors increased the inhibition efficiency. EIS results showed that the studied quinoxalines retarded the rate of corrosion of mild steel, zinc, and aluminium surfaces through the adsorption process. For all the investigated metals the charge transfer resistance values increased with the increase in concentration of the inhibitors. iii | P a g e The AAS analysis revealed a decrease in the concentration of iron and zinc ions in the presence of the studied inhibitors as compared to the blank solutions. The inhibition efficiency increased with an increase in the concentration of the inhibitors. The FTIR spectra confirmed the formation of the inhibitor-Fe2+ and inhibitor-Zn2+ complexes. The obtained adsorption energies from the DFT results revealed that the studied quinoxalines exhibit a mixed-type adsorption mechanism, with the domination of the chemisorption process.

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