Experimental and computational studies onpropanone derivatives of quinoxalin-6-yl-4,5-dihydropyrazole as inhibitors of mild steel corrosion in hydrochloric acid.

Two quinoxaline-based propanones, 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) were tested as inhibitors of mild steel corrosion in 1 M HCl using both experimental and computational approaches. Both compounds were found to retard corrosion rate of mild steel in the studied medium. Mt-3-PQPP and Cl-4-PQPP exhibited mixed-type inhibitive action, reducing the rate of anodic and cathodic corrosion reactions, as suggested by Tafel polarization measurements. Adsorbed molecules of Mt-3-PQPP and Cl-4-PQPP formed pseudo-capacitive film on mild steel surface in 1 M HCl as proposed by electrochemical impedance spectroscopy (EIS) measurements. Adsorption surface coverage data were 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 showed better corrosion inhibition potentials than that of Cl-4-PQPP. Monte Carlo simulation of the adsorption of Mt-3-PQPP and Cl-4-PQPP molecules on Fe(1 1 0) surface also confirmed higher adsorption energy for the former.