ABSTRACT

The inhibition efficiencies of the two Schiff bases 2[2-diethylamino) ethyl methyl amino)-4-methy1-5-3 (3-methyl sulfanyl propy1 amino) methyldiene cyclohexdien-1-one (DEMS) and [1-(azepan-1-yl)2-2-[4-(2-tert-butyl sulfanyl ethyl piperazin-1-yl] ethanone (ATSP) synthesised from  linoleic and benheric acids on mild steel and aluminised steel in 1 M HCl solutions were investigated. Their inhibition performance was tested by gasometric, weight loss, potentiodynamic polarization, electrochemical impedance spectroscopy (EIS) and quantum chemical techniques and were characterised using FTIR spectroscopy. The FTIR spectra showed that the synthesised Schiff bases contained heterocyclic compounds with heteroatoms (N, S, and O) which had conjugated double bonds, indicating the presence of π- electrons. The results obtained from the weight loss method showed that inhibition efficiencies increase with increase in concentration of the Schiff base molecules and with the highest inhibition efficiency observed at the optimum concentration of 2.0 g/l. The inhibitor, DEMS showed inhibition efficiency of 81.14% while the inhibitor, ATSP showed inhibition efficiency of 71.21% for mild steel while for the aluminised steel, inhibition efficiencies were 88.64% (DEMS) and 86.90% (ATSP) respectively. Inhibition efficiencies were also found to decrease with increase in temperature. The following adsorption isotherms were employed in the study: Freundlich, Temkin and Langmuir isotherms with Langmuir showing best fit to the experimental data (R²=0.99). Thermodynamic studies of the inhibition process showed that the ΔG_ads values for DEMS and ATSP were both negative indicating the spontaneity of the reaction and the values were less than -20 kJmol,^-1 indicating that the mechanism of adsorption is physiosorption. The negative entropy values for both inhibitors on mild steel and aluminised steel denote increase in disorderliness on the surface of the coupons as concentration of inhibitors increases. The enthalpy values were positive showing that the reaction was endothermic. The potentiodynamic polarisation results showed that the inhibitors were mixed type because the displacement obtained in the E_corr value for both DEMS and ATSP were -15 mV and -63 mV respectively and these values were less than 85 mV. The highest inhibition efficiency values of the Nyquist plots obtained at 2.0 g/l from EIS for mild steel were 80.40% (DEMS) and 66.59% (ATSP) while 82.13% and 81.09% were obtained for aluminised steel for the inhibitor molecules, DEMS and ATSP respectively. These suggest the formation of protective layer by the inhibitor molecules. Molecular simulation studies were carried out using semi- empirical PM3 method to optimize the structure of the inhibitors. The results obtained from the chemical studies showed that there exists a relationship between the inhibitors’ efficiency and the energy gap (ΔE), energy of the highest occupied molecular orbital (E _Homo), energy of the lowest occupied molecular orbital (E _Lumo), dipole moment (μ), softness (σ) as well as the hardness (η) of the inhibitor molecules. Fukui function was used to determine the site where adsorption can occur on the inhibitor molecules. The inhibition efficiency trend of the Schiff base molecules follows DEMS > ATSP.