Corrosion inhibition of zinc in 0.5 M HNO₃ using <i>Azadirachta indica</i> extract: Experimental and computational study

Abstract

Azadirachta indica (AI) leave extract was investigated as a zinc corrosion inhibitor in nitric acid solution using experimental weight loss measurements and computational studies. Inhibition efficiency and corrosion rate was studied using effect of immersion time (1 – 4 h), effect of temperature (30 – 60 ℃) and inhibitor concentration (0.20 – 0.80 g/L). Corrosion rate increases with increase in temperature and decrease with increased in immersion time. Inhibition efficiency of the extract increases with increased in inhibitor concentration reaching 53.09% at 4 hours immersion time. Adsorption isotherm studies show that the experimental data was best fit into Freundlich isotherm (R₂ = 0.9986 − 0.9995) and the adsorption mechanism can be described as physisorption (-5.75 to -9.48 kJmol^-1). The adsorption process indicated feasible and spontaneous process from the large negative values of ΔG_ads. The values of enthalpy are all positive indicating endothermic process of adsorption. Activation energy was found to increase with increase in inhibitor concentration which increase the energy barrier of the corrosion reaction. Computational studies were carried out using density functional theory (DFT) and molecular dynamic simulation (MDS) and the result revealed that ADE has strong interaction with Zn (1 1 0) surface than AME and ADME as such it has higher adsorption energy (39.29, 11.32 and 10.02 kcal/mol) and inhibition efficiency. The experimental inhibition efficiency can be correlated to the adsorption of such molecules of inhibitor on metal surface that facilitate in reducing the corrosion of zinc in nitric acid solution. Hence, AI extract was found to be good inhibitor for the corrosion of zinc in nitric acid medium.