Corrosion Inhibition Efficiency, Adsorption and Thermodynamic Studies of Ocimum Gratissimum on Carbon Steel in 2 M Sodium Chloride Solution

Abstract

The gravimetric method was applied to investigate the corrosion inhibition efficiency of ocimum gratissimum leaf extract on carbon steel, in 2 M NaCl solution. Concentration of the extract was varied from 0.5 to 3.0 g/L, using a step size of 0.5 g/L. The corrosion test was performed at temperature values of 308 K, 318 K, 328 K and 338 K for 3 h, 6 h and 9 h intervals. Six adsorption isotherm models were tested to study the mechanism of adsorption of the leaf extract onto the metal surface. Kinetic and thermodynamic parameters such as activation energy (Ea), enthalpy (∆H), entropy (∆S) and Gibb’s free energy change of adsorption (∆Gads) were also estimated. Results obtained revealed that inhibition efficiency increased with increase in concentration and time, but decreased with increased temperature. Maximum inhibition efficiency of 95.8%, was obtained at inhibition concentration of 3.0 g/L after a -9 h exposure period, at 308 K. The Langmuir model was found to have the best fit. While all the values of Ea were positive, the lower spectrum of Ea reflected the absence of ocimum gratissimum leaf extract. Also, ∆H values were positive and increased with increase in concentration and time. ∆S values were negative with decreasing magnitude as time and concentration increased. Similarly, ∆Gads values were shown to be negative and with magnitude less than 20 kJ/mol. These results obtained, show that ocimum gratissimum leaf extract can be a good corrosion inhibitor for carbon steel in 2 M NaCl solution and that its adsorption onto the metal surface is realistic, spontaneous and occurred by physisorption.