Investigation of the adsorption characteristics of some selected sulphonamide derivatives as corrosion inhibitors at mild steel/hydrochloric acid interface: Experimental, quantum chemical and QSAR studies

Five sulphonamides derivatives namely, sulphanilamide (SNA), sulphamethoxazole (SMX), sulphadimethoxine (SDM), sulphisoxazole (SSZ), and sulphamethizole (SMZ) have been selected as corrosion inhibitors for mild steel in hydrochloric acid solutions utilizing electrochemical techniques as well as theoretical method (using DFT). These compounds were found to protect mild steel through spontaneous adsorption on to mild steel surface, thereby blocking the active sites. The obtained thermodynamic and kinetic parameters deduced that sulphonamides adopt a Langmuir adsorption isotherm mechanism. Results from electrochemical impedance and potentiodynamic polarization showed exceptional correlation and revealed that these compounds are mixed-type inhibitors. The formation of the adsorption film on the mild steel surface was confirmed by SEM, EDS and FTIR. Quantum chemical calculations and quantitative structure activity relationship (QSAR) were also utilized to investigate the preferred reactive sites on the molecules of the sulphonamides and to correlate the experimentally determined inhibition efficiency with the theoretically determined inhibition efficiency. The results indicate that the preferred sites for interaction between the inhibition and metal surface would be the aromatic ring as well as the heteroatoms present in each sulphonamide. Five quantum chemical parameters are sufficient in the development of QSAR to provide a strong correlation between inhibition efficiency obtained theoretically and that obtained experimentally.