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.

The corrosion inhibition of mild steel in 1.0 M HCl solution by some selected imidazolium-based ionic liquids, namely 1-propyl-3-methylimidazolium bis(trifluoromethyl-sulfonyl) imide (), and 1-propyl-2,3-methylimidazolium bis(trifluoromethyl-sulfonyl) imide ([PDMIM][NTf 2 ]) was investigated using weight loss, electrochemical measurements, and quantum chemical calculations. All ionic liquids showed appreciable inhibition efficiency. Among the ionic liquids studied, [PDMIM][NTf 2 ] exhibited the best inhibition efficiency. The results from the weight loss, electrochemical measurements and quantum chemical calculations show that the order of inhibition efficiency by the ionic liquids follow the order. At 303 K, polarization measurements indicated that all the studied compounds are mixed-type inhibitors. The adsorption of the studied ionic liquids obeyed the Langmuir adsorption isotherm. There is good correlation between a composite index of quantum chemical parameters and experimentally determined inhibition efficiency of the inhibitors. The quantitative structure activity relationship (QSAR) approach has provided a good indication that an optimum of at least two quantum chemical parameters is required for a good correlation with the experimentally determined inhibition efficiency of the ionic liquids.

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The inhibition performances of some selected azine and thiazine dyes, namely, Neutral Red (NR), Azure A Eosinate (AAE), Toluidine Blue (TB), phenosafranin (PS), and Rhodanile Blue (RB), on mild steel corrosion in hydrochloric acid solution was studied the using electrochemical impedance spectroscopy (EIS) and Tafel polarization techniques. Quantum chemical calculations based on the density functional theory (DFT) and semiempirical (PM3) methods were used to investigate the reactivities and selectivities of the studied cationic dyes. The effects of inhibitor concentration on the inhibition efficiency have been studied. Inhibition efficiency increased with increase in concentration of all the studied cationic dyes within the concentration range 100−500 ppm. Potentiodynamic studies revealed that all the inhibitors are of mixed type. The results obtained from the EIS studies showed good agreement with the results from potentiodynamic polarization techniques. The quantitative structure−activity relationship (QSAR) approach was also used to correlate the quantum chemical parameters with the experimentally determined inhibition efficiencies. The results show that thiazine dyes are better corrosion inhibitors than azine dyes; however, when azines contain more electron donor centers than thiazines, they are preferred as corrosion inhibitors to thiazine. Hydrogen bonding could be one of the possible physisorption mechanisms for the adsorption of the selected dyes onto the metal surface because of the many hydrogen bond donor centers in the studied compounds. QSAR results show good correlations between a number of quantum chemical parameters and the determined inhibition efficiency.

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