Ekemini B. Ituen, Edidiong A. Essien, Uwemedimo E. Udo and Ogede R. Oluwaseyi
A new approach to characterizing the mechanism of adsorption of inhibitor molecules has been proposed from the findings of this study based on the degree of responsiveness of inhibition efficiency to changes in temperature. The adsorption and inhibition of mild steel corrosion in 1.0 M H2SO4 using different concentrations of zinc metallic soap of Cucumeropsis mannii N. was investigated at 303 - 333K via weight loss, hydrogen evolution and computational techniques using DFT at the B3LYP/6-31G*basis set level. Corrosion rate increased with increase in temperature both for the free acid and inhibited solutions, with marked reluctance to increase in corrosion rate for the inhibited solutions-reluctancy increasing with increase in concentration of the metallic soap of zinc. Inhibition efficiency increased with increase in the concentration of zinc soap. Addition of iodide ions further increased the inhibition efficiency indicating synergistic inhibition. The maximum inhibition efficiency of 70.1 % was obtained for 10 x 10-5 M zinc soap but increased to 85.3 % on addition of 0.001 M iodide ion. The adsorption was best described by the Langmuir adsorption isotherm from where negative values of Gibbs free energy change were obtained indicating spontaneity of the adsorption phenomena. Activation Energy was deduced from the Arrhenius equation while thermodynamic approach was used to elucidate the enthalpy and entropy of adsorption. Quantum chemical calculations reveal large energy gap between the LUMO and HOMO of the constituent fatty acids indicating a theoretically significant possibility of adsorption and mild steel-zinc soap molecules interactions. A new equation for temperature coefficient of inhibition efficiency (μ) was derived and applied for the first time using a classical physics concept and negative values of μ were obtained and interpreted as indicative of physical adsorption mechanism
