Some Quinoxalin-6-yl Derivatives as Corrosion Inhibitors for Mild Steel in Hydrochloric Acid: Experimental and Theoretical Studies

The inhibition of mild steel corrosion in 1 M HCl by some quinoxalin-6-yl derivatives namely 1-[3-phenyl-5-quinoxalin-6-yl-4,5-dihydropyrazol-1-yl]butan-1-one (PQDPB), 1-(3-phenyl-5-(quinoxalin-6-yl)-4,5-dihydro-1H-pyrazol-1-yl)propan-1-one (PQDPP), and 2-phenyl-1-[3-phenyl-5-(quinoxalin-6-yl)-4,5-dihydropyrazol-1-yl]ethanone (PPQDPE) has been investigated using electrochemical studies and quantum chemical calculations. The results showed that PQDPP is the best corrosion inhibitor among the three compounds studied and the inhibition efficiency increases with increase in concentration for all the inhibitors. The adsorption of inhibitor molecules on mild steel surface was found to be spontaneous and obeyed the Frumkin adsorption isotherm. Scanning electron microscopy (SEM) images confirmed the formation of protective films of the inhibitors on mild steel surface. Quantum chemical calculations showed that the inhibitors have the tendency to be protonated in the acid and the results agree with experimental observations. Monte Carlo simulations were applied to search for the most stable configuration and adsorption energy for the interaction of inhibitors on Fe(110)/100 H2O interface. The results of the Monte Carlo simulations accord with the experimentally determined inhibition efficiencies. Different carbonyl substituents on the common nucleus of the three compounds obviously contributed to the difference in inhibition efficiency.