Adsorption and theoretical investigations of a Schiff base for corrosion inhibition of mild steel in an acidic environment

Corrosion is a major economic issue all around the world. Corrosion inhibitors, organic chemicals in particular, are among the most effective ways to protect metal surfaces from corrosion. A Schiff base, namely 2-((mercaptoquinolin-3-yl)methylene)hydrazinecarbothioamide (MQT), was investigated at concentrations ranging from 0.0001 to 0.0005 M as a mild steel corrosion inhibitor in 1.0 M sulfuric acid environment at various temperatures (303, 313, 323, and 333 K). The inhibition efficiency (IE%) of the investigated Schiff base was evaluated using the weight loss technique. The weight loss technique shows that increasing the concentration of the tested inhibitor enhances inhibition efficacy (from 43 to 93%), with the highest protection performance found at a concentration of 0.0005 M. According to the weight loss measurements, the Langmuir isotherm was found to be the best suitable model of adsorption, with negative values of free energy between -20 kJ.mol(-1) and -40 kJ.mol(-1) suggesting a spontaneous nature of simultaneous physical and chemical adsorption. As a result, the high percent IE of the examined compound was interpreted as substantial adsorption of the organic molecules on the mild steel surface. This resulted in the formation of a shielding layer between the mild steel surface and the acidic environment. Furthermore, all of the chemical descriptors generated from DFT show that the tested inhibitor is well absorbed by mild steel sites. Quantum chemistry computations were used to determine the molecular geometry and electronic structure of the molecule. The energy difference between the HOMO (highest occupied molecular orbital) and LUMO (lowest unoccupied molecular orbital) has been checked using a theoretical approach to indicate a chemical reactivity and kinetic stability. Finally, the mechanisms of mild steel corrosion in the H2SO4 environment and prevention of mild steel corrosion by the tested Schiff base were examined.