Inhibition of Acid Corrosion of Mild Steel By Selected Quinoxalinyl-Pyrazolyl-Propanones: Electrochemical and Computational Studies

Abstract

Acid corrosion of mild steel was studied in the presence of two propanone derivatives of quinoxalin-6-yl-4,5-dihydro-1H-pyrazol-1-one, namely 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). The inhibitive potentials of the compounds were assessed by measuring the corrosion rate of mild steel in the electrolyte admixture using electrochemical techniques. Both compounds were found to repress steel corrosion in HCl. Tafel polarization experiments suggested that the compounds are mixed-type inhibitors as they reduce both the anodic and cathodic corrosion reactions. Electrochemical impedance spectroscopy (EIS) measurements revealed that Mt-3-PQPP and Cl-4-PQPP molecules adsorbed on mild steel surface to form pseudo-capacitive film. Surface coverage data was fitted into the Langmuir adsorption isotherm and the evaluated thermodynamic parameters suggested chemisorption for Mt-3-PQPP and competitive physisorption and chemisorption for Cl-4-PQPP. Scanning electron microscopy (SEM) analyses further revealed that adsorbed film of the inhibitor molecules protected the steel from direct exposure to acidic ions. Quantum chemical calculations suggested that higher corrosion inhibition efficiency of Mt-3-PQPP compared to Cl-4-PQPP molecule is due to the higher electron donating tendency of the former. Mt-3-PQPP molecule also showed higher protonation tendency in the acid than Cl-4-PQPP and its protonated form is more reactive than that of Cl-4-PQPP. Monte Carlo simulation of the adsorption of Mt-3-PQPP and Cl-4-PQPP molecules on Fe(110) surface also confirmed higher adsorption energy for the former.