Thermodynamic and Kinetic Studies of Powerful Eco Friendly Green Inhibitors; Costus afer, Uvaria chamae and Xylopia aethiopia for the Control of Mild Steel Corrosion in HCl Solution.

The inhibitive action of the methanol extracts of three powerful eco-friendly green Inhibitors; Costus afer (COA), Uvaria chamae (UVC), and Xylopia aethiopica (XYA) leaves on the corrosion of mild steel in 2.5 M HCl solutions with inhibitor concentrations of 0.5 g/L, 1.0 g/L, 2.0 g/L and 4.0 g/L, at the temperatures of 30°C and 60°C was studied using gravimetric (weight loss) and gasometric (hydrogen evolution) techniques to determine their inhibition efficiencies as well as to characterize the mechanism of inhibition of these three green inhibitors. The gravimetric technique was done for 5 days (120 Hours). Results indicate that the leave extracts inhibit the corrosion process powerfully. COA, UVC and XYA extracts showed inhibition efficiency of (83.7, 84.6 and 87.0) and (85.0, 62.5 and 76.1) for gravimetric (weight loss) and gasometric analysis respectively. The Inhibition efficiency was found to increase with an increase in the extract concentration and decrease with an increase in time(days) and temperature. The inhibition efficiencies followed the trend XYA>UVC>COA and COA>XYA>UVC in gravimetric and gasometric analysis respectively. Thermodynamic considerations revealed that the activation energy, Ea increased in the presence of the plant extracts. The kinetic data confirmed the reaction process to be first order. Adsorption of the plant extracts on mild steel surface is an exothermic process and spontaneous as deduced by mostly negative Qads mean values of -7.40 KJ/mol, -2.14 KJ/mol and -32.18 KJ/mol for COA, UVC and XYA and negative ΔGads values of -9.28 KJ/mol and -12. 0 KJ/mol for COA, -9.46 KJ/mol and -11.23 KJ/mol for UVC and -7.73 KJ/mol and 6.29 KJ/mol for XYA at 30°C and 60°C respectively. The mechanism of adsorption proposed for the plant extract on the mild steel surface is physical adsorption. Experimental data obtained fit the Langmuir adsorption isotherm.