The inhibitory effect of two 5-alkylthio-8-hydroxyquinoline salts on steel C22E in a molar electrolyte of hydrochloric acid: Experimental and theoretical studies

Abstract Two new salts of 8-hydrxyquinoline namely: sodium 5-methylthio-8-hydroxyquinolinolate (SMQ) and sodium 5-propylthio-8-hydroxyquinolinolate (SPQ) have been prepared and identified via elemental analyses, Fourier-transform infrared spectroscopy (FTIR) and NMR spectroscopic. Both salts also have been assessed as new organic inhibitors for the corrosion of steel C22E in a molar electrolyte of hydrochloric acid utilizingelectrochemical, gravimetric and theoretical calculation studies. The weight reduction investigation indicated that the rate of corrosion regularly decreased by increasing the concentration of 5-alkylthio-8-hydroxyquinolines in acidic electrolyte. The polarization study data demonstrate that 5-alkylthio-8-hydroxyquinolines have an excellent inhibitive performance for C22E steel in 1 M HCl and behave like mixed-type inhibitors. However, the efficiency of SPQ and SMQ achieved up to 95.8% and 92.1% at 10–3 M, respectively. The impedance experimental results show the frequency distribution of the capacitance, simulated as a constant phase element. The adsorption of synthesized salts complied with the Langmuir's adsorption isotherm and the estimated thermodynamic parameters proposed a combination of physisorption and chemisorption mechanisms. The calculated value of the adsorption-desorption equilibrium constant (797,073 L/mol and 377,182 L/mol) reveals a strong bonding between SPQ and SMQ molecules, respectively, and the steel surface. The inhibition performance for SPQ and SMQ was confirmed by SEM. The results found from theoretical calculations, including the density functional theory (DFT) method and molecular dynamics (MDS) simulations, support the experimental results and explain in a general way how anionic forms adsorb on the iron surface.