Thermodynamic and Optimization Studies of Castor Leaf Extract as Corrosion Inhibitor on Stainless Steel (301)

Abstract

Inhibition of stainless steel corrosion in acidic medium with castor leaf extract was studied using the Gravimetric measurement. The mechanisms of inhibition, influence of temperature on inhibition efficiency and weight loss were determined for temperature range 40 oC – 80 oC at 7 hours immersion time. An increased in temperature showed a decreased in the inhibition efficiency of the castor leaf extract which resulted to increase in weight loss of the stainless steel. The value of rate constant for the corrosion process ranges from 0.333 – 1.225 hr−1, this is seen to be directly relative to the inhibitor concentrations. Activation energy, enthalpy of activation, and entropy values ranges from 74.000 - 136.377 kJ/mol, 71.820 – 133.620 kJ/mol, and 9.860 – 178.110 J/molK respectively. Rise in activation energy with inhibitor concentration confirmed physisorption adsorption mechanism for stainless steel surface corrosion. In order to obtain the optimum weight loss, optimization of the process variables was carried out using the Box – Behnken Design plan and desirability function of response surface methodology (RSM). Four parameters were varied viz; time of immersion, HCl concentration, inhibitor’s concentration, and temperature alongside their effects on weight loss of the stainless steel were verified. The optimal conditions predicted from the second order quadratic model were time (9.10 hours), HCl concentration (3.97 M), concentration of inhibitor (240.90 ppm), and temperature (78.67 oC) with 2.978 g as the weight loss. Statistically, the results showed that 95.03% of the variation in total weight loss of stainless steel can be connected to the experimental variables examined.
 Keywords: Weight loss, activation parameters, adsorption, optimization, stainless steel, castor leaf