Adsorption and corrosion inhibition properties of: N -{ n -[1-R-5-(quinoxalin-6-yl)-4,5-dihydropyrazol-3-yl]phenyl}methanesulfonamides on mild steel in 1 M HCl: Experimental and theoretical studies

Six quinoxalinyl-dihydropyrazolyl-phenyl-methanesulfonamides were investigated for their adsorption characteristics and inhibition of mild steel corrosion in 1 M HCl medium. Tafel polarization measurements revealed that all the studied compounds are mixed-type inhibitors. Electrochemical impedance spectroscopy showed that the compounds form a pseudo-capacitive protective film on mild steel surface and protect the steel from direct acid attack. The inhibitors adsorb on mild steel in 1 M HCl via competitive physisorption and chemisorption mechanisms and their adsorption obeyed the Langmuir adsorption isotherm model. UV-vis spectra confirmed that the inhibitors interact with mild steel in solution to form Fe-inhibitor complexes. Scanning electron microscope (SEM) images also confirmed the protective efficacy of the studied compounds on mild steel in the acid. Quantum chemical calculations and quantitative structure activity relationship (QSAR) studies proposed good correlations between molecular quantum chemical descriptors and experimental inhibition efficiencies. Descriptors for protonated species correlate better than those of neutral species. Adsorption of the studied molecules was simulated on Fe(110) surface and the binding energies derived from molecular dynamics simulations corroborate experimental results. Compounds in which the sulfonamido group is attached to position 3 on the phenyl ring showed higher corrosion inhibition activities.