Adsorption, thermodynamic and density functional theory investigation of some sulphonamides as corrosion inhibitors for some selected metals in acidic medium

Abstract

In this thesis, nine sulphonamides derivatives namely, sulphanilamide (SNA), sulphamethoxazole (SMX), sulphadimethoxine (SDM), sulphisoxazole (SSZ), sulphamethazine (SMT), sulphachloropyridazine (SCP), sulphabenzamide (SBZ), sulphaquinoxaline (SQX) and sulphamethizole (SMZ) were investigated as corrosion inhibitors for three different metals namely, mild steel, aluminium and zinc in 1.0 M hydrochloric acid solutions at 30-50 °C. The corrosion inhibition characteristics including corrosion mechanism, corrosion inhibition efficiencies and inhibitor-metal adsorption/desorption behaviour were studied using electrochemical impedance spectroscopy, potentiodynamic polarization and gravimetric analysis. Fourier transform infrared spectroscopy (FTIR) was used to gain more insight into the functional groups that formed or disappeared during the adsorption/desorption of the inhibitor molecules on the alloy surfaces. The adsorption film that resulted on these metal surfaces were further investigated using scanning electron microscopy (SEM) coupled with energy dispersive spectroscopy (EDS). Density functional theory (DFT) was used to compute all theoretical studies. Quantum chemical calculations and quantitative structure activity relationship (QSAR) were also used to establish correlations between experimentally determined inhibition efficiencies and molecular quantum chemical descriptors. All nine sulphonamide compounds inhibited the corrosion of mild steel, aluminium and zinc in 1.0 M hydrochloric acid at 30-50 °C through adsorption of the inhibitor molecules on the metal surfaces without altering the corrosion mechanism. The comparison on the effect of temperature on the corrosion of mild steel, aluminium and zinc showed that mild steel is affected the most with the lowest inhibition efficiencies at the highest temperature of 50 °C. Potentiodynamic polarization results indicated that the use of all nine sulphonamide compounds as corrosion inhibitors significantly reduced the corrosion current densities for both anodic and cathodic half-reactions which suggest that both anodic dissolution and cathodic reduction of the hydrogen ions were inhibited. The obtained potentiodynamic polarization parameters revealed that all nine inhibitors studied acted as mixed-type inhibitors that protected the mild steel, aluminium and zinc surfaces through spontaneous adsorption. The aluminium Tafel plots showed a more pronounced passive region (the elongated section) than both mild steel and zinc. Electrochemical impedance spectroscopy showed that all nine inhibitors protected mild steel, aluminium and zinc surfaces through the adsorption at the metal/hydrochloric acid interface. The adsorption process of all studied inhibitors on mild steel, aluminum and zinc in 1.0 M HCl solution followed the Langmuir adsorption isotherm and a mixed-type mechanism. Analysis of the SEM micrographs and their respective EDS spectra showed that the surfaces of the alloys prior immersion in 1.0 M HCl solution exhibit a smooth nature with minor damages that might have resulted due to the abrasion with various emery papers. After the immersion in 1.0 M HCl solutions, all these metals showed a more degraded nature. Fourier transform infrared spectroscopy studies revealed that all sulphonamide compounds studied interacted with Fe (in mild steel), aluminium and zinc resulting in Fe – inhibitor, Al – inhibitor and Zn – inhibitor complexes. Quantum chemical studies showed that all nine sulphonamide compounds were active inhibitors for mild steel surfaces in acidic medium. Analysis of the HOMO densities for the neutral species showed that the highest HOMO densities occurs at C2, C4, C5 and C6, the amino N1, O9 and O10 atoms for all the sulphonamides studied. The effect of the protonated sulphonamide species was also studied and some protonated species were found to be among the enhanced adsorbates. The best QSAR equation correlating theoretical inhibition efficiency with the experimental inhibition efficiency corresponded to the combination of Ѡ, ELUMO, µ, ŋ and logP quantum chemical parameters. The results from the weight loss and electrochemical measurements show that the order of inhibition efficiency by the sulphonamides on mild steel followed the order: SDM > SMT > SBZ > SCP > SNA > SQX > SSZ > SMX > SMZ and SNA > SBZ > SMX > SMZ > SSZ > SMT > SQX > SDM > SCP for aluminum while the order SBZ > SMX > SNA > SSZ > SCP> SMT > SDM > SQX > SMZ was followed for zinc.