Electrochemical, Surface Morphology and Computational Studies on Some Quinoxalin-6-Yl-4,5-Dihydropyrazolylpropanones As Inhibitors of Mild Steel Corrosion in 1 M HCl Solution

Abstract

Corrosion of mild steel in 1 M HCl in the presence of 1-[3-(3-methoxyphenyl)-5-(quinoxalin-6-yl)-4,5-dihydropyrazol-1-yl]propan-1-one (Mt-3-PQPP) and 1-(3-(4-chlorophenyl)-5-(quinoxalin-6-yl)-4,5-dihydro-1H-pyrazol-1-yl)propan-1-one (Cl-4-PQPP) was investigated electrochemically using Tafel polarization and AC impedance techniques. Mild steel corrosion was observed to be retarded by the studied compounds such that an inhibition efficiency of up to 80% was achieved even at very low concentration (< 30 µM), especially with Mt-3-PQPP. Both compounds inhibit mild steel corrosion in 1 M HCl and the inhibition efficiency increases with increase in concentration. The Tafel curve for steel corrosion suggested that the studied compounds are mixed-type corrosion inhibitors. The protective adsorbed film of the inhibitors is pseudo-capacitive as revealed by the electrochemical impedance spectroscopy (EIS) analysis. Mt-3-PQPP molecules appeared to chemisorb onto the steel surface, while the adsorption of Cl-4-PQPP molecules involved both physisorption and chemisorption mechanisms. Comparison of mild steel surface immersed in 1 M HCl in the absence and presence of Mt-3-PQPP and Cl-4-PQPP revealed that the inhibitor-free acid solution damaged the steel surface more badly than the inhibitor-containing solutions. This further suggests that the inhibitor molecules protected the steel surface from unhindered attack by the corrosion acid ions. Quantum chemical calculations were carried out on the studied molecules and relevant reactivity parameters were used to provide molecular based explanations on the inhibitive behaviour of the Mt-3-PQPP and Cl-4-PQPP. Adsorption of inhibitor molecules on mild steel surface was simulated using the molecular dynamics method while the metal substrate was represented by Fe(110) cleaved surface. Theoretical quantum chemical calculations and molecular dynamics simulation results corroborate experimental electrochemical studies.