Electrochemical, quantum chemical calculation and molecular dynamic simulation studies on some quinoxaline derivatives as corrosion inhibitors for mild steel in acidic medium

Abstract

This thesis reports the inhibition of mild steel (MS) corrosion in 1 M HCl solution by seventeen (17) selected quinoxaline derivatives that comprise the quinoxalinyl, pyrazolyl and phenyl rings in a single molecular entity. All the compounds investigated in this work contain the 6-(3-phenyl-4,5-dihydro-1H-pyrazol-5-yl)quinoxaline group and only differ in the substituent groups on the pyrazolyl and/or phenyl ring(s). The adsorption characteristics and corrosion inhibition properties of these compounds were investigated for MS in 1 M HCl solution at 303 K using electrochemical, spectroscopic, quantum chemical calculation and molecular dynamic simulation methods. All the studied compounds showed appreciable inhibition performances. The compounds inhibit MS corrosion by adsorbing on the active sites on the steel surface without changing the mechanism of the corrosion process. All the studied compounds are mixed-type inhibitors and their adsorption conforms to the Langmuir isotherm model. The trends of the inhibition efficiency are: Me-4-PQPB > Mt-3-PQPB > oxo-1,3-PQPB > Mt-4-PQPB (for GROUP I); PQDPP > PQBPB > PPQDPE (for GROUP II); MS-3-PQPP > MS-4-PQPP > MS-2-PQPP (for GROUP III); MS-3-PQPMS > MS-2-PQPMS > MS-4-PQPMS (for GROUP IV); and Mt-3-PQPP > Cl-4-PQPP, and also (based on average values of percentage inhibition efficiency) MS-2-PQPA > MS-4-PQPA (for GROUP V). The thermodynamic adsorption parameters for all the studied compounds suggest mixed physisorption and chemisorption mechanisms. Electrochemical impedance spectroscopy measurements revealed that the compounds adsorbed on MS surface to form protective film with pseudo-capacitive characteristics. It was deduced from the studies on the GROUP II compounds that the chain length/type of carbonyl substituent on the pyrazole ring affects the inhibition strength such that the most efficient compound in this group (PQDPP) is the one with the shortest carbonyl chain and the observation is in agreement with the established trend of reactivity of carbonyl compounds. The combination of electronic and steric effects attributed to the methanesulphonamido group appeared to favour higher inhibition efficiency when the group is attached to position 3 compared to position 2 or 4 on the phenyl ring. This accounts for the highest inhibition efficiency observed for MS-3-PQPP (in GROUP III) and MS-3-PQPMS (in GROUP IV) compared to other compounds in their respective groups. Quantum chemical parameters suggest that the studied compounds have the tendency of accepting charges from the metal in a retro-donation step during the donor-acceptor interactions between the inhibitor molecules and MS and that protonated species of the inhibitors might also be involved in the adsorption process. The HOMO, LUMO, and Fukui indices showed that the quinoxaline ring in all the studied compounds is generally electron-deficient, while the pyrazole ring is a π-electron rich centre in conformity with the general notion of the chemical reactivity of heterocyclic compounds. Molecular dynamic simulation studies revealed that the adsorption/binding energies of the inhibitor/Fe (110) systems for the GROUPs II, III, IV and V compounds are in good agreement with the order of inhibition efficiencies of the inhibitor molecules. The non-correlation of order of adsorption energies of the inhibitor/Fe (110) systems for the GROUP I compounds with the observed trend of protection efficiencies might be due to the fact that the theoretical simulation considers only non-covalent interactions, whilst both covalent and non-covalent interactions are most likely to feature in the experiment.