Electrochemical and quantum chemical studies of some sulphur-containing organic compounds as inhibitors for the acid corrosion of mild steel

Abstract

The influence of thioacetamide (TAcA), thiourea (TU) and thiobenzamide (TBA) on the corrosion of mild steel in 0.1 M H 2SO 4 solution has been studied in relation to the concentrations. The CS NH 2 group is the common main group for these compounds and they differ only having different groups on the other side of the thiocarbonyl group. Electrochemical impedance spectroscopy (EIS) and polarization resistance measurement ( R p) techniques have been used to obtain experimental data. The results showed that these compounds revealed a good corrosion inhibition, thiobenzamide being the most efficient and thioacetamide the least. The correlation between the molecular structures and corrosion inhibition efficiencies of TAcA, TU and TBA has been investigated using ab initio quantum chemical calculations. The structures of these compounds have been optimized, highest occupied molecular orbital (HOMO), lowest unoccupied molecular orbital (LUMO) energy levels, LUMO–HOMO energy gap and molecular orbital densities have been computed. The relations between the inhibition efficiency and quantum chemical parameters have been discussed. It was seen that there is a clear relation between the increase in corrosion inhibition and the increase of HOMO energy level. The highest values of the HOMO densities were found in the vicinity of the sulphur atom indicating it as most probable adsorption center.