CORROSION INHIBITION OF Acalypha wilkesiana L. LEAVES, Acalypha godseffiana J. LEAVES AND Cola nitida A. HUSK EXTRACTS IN ACIDIC MEDIUM

Abstract

The toxic effects of synthetic corrosion inhibitors have led to the search for naturally occurring substances which are not only readily available but also environmentally friendly. Therefore, the corrosion inhibition efficiencies of ethanol extracts of Acalypha wilkesiana leaves, Acalypha godseffiana leaves and Cola nitida husk extracts on mild steel in acidic medium were investigated in this study. The study evaluated the potentials of Acalypha wilkesiana leaves, Acalypha godseffiana leaves and Cola nitida husks as green inhibitors. The materials were collected at Adeyemi College of Education, Ondo. They were washed, sun-dried and air-dried, powdered, sieved and stored in desiccators at room temperature. 2 kg of the powdered leaves and husks was soaked in ethanol in different containers for 72 hours to obtain inhibitor extracts. Weight loss, inhibition efficiency (IE) and corrosion rate were studied using gravimetric and electrochemical methods. Models that relate concentration of inhibitor and temperature to IE were proposed, established and evaluated using statistical methods and theoretical calculations was done with Becke’s three – parameter hybrid function employing Lee-Yang-Parr correlation function (B3LYP) with 6-31G** basis set. The study revealed the presence of saponin, tannin, phlobatannin, flavonoid, steroid, terpenoid and cardiac glycoside in the extracts and alkaloids were only present in Cola nitida husks while none of the samples had anthraquinone. FT-IR results revealed that ethanol extracts contained O-H, C-H, C-O, C=C and C=O which meets the general structural consideration of the compounds expected to be used as good corrosion inhibitors. The SEM images of the mild steel samples showed that the metal was protected in the presence of the extracts. Pseudo-first order kinetic model best represented the corrosion of the mild steel in the presence of the ethanol extracts of AWL, AGL and CNH. Langmuir isotherm was found to be the best model that fitted the adsorption data most. The best models for sun-dried (R2 = 0.978) and air-dried (R2 = 0.971) extracts were linear with interaction. Also, thermodynamic studies showed that activation energies decreased in the presence of the extracts. The values for enthalpy of activation were positive, entropy and Gibb’s free energy were negative.. The electrochemical studies revealed that the ethanol extracts of the plants act as mixed inhibitors as they retard both cathodic and anodic reactions. The computational results revealed that the compound with the lowest band gap is the most easily adsorbed, and therefore has the highest IE. It was also revealed that inhibition efficiency increased with increase in the concentration of the extracts, 60.43 to 82.89 % for AWL, 58.72 to 88.37 % for AGL and 60.11 to 84.57 % for CNH at 0.2-1.0 g/L within the temperature range of 303-333 K for the air- and sun-dried extracts. The inhibition efficiency also increased with increasing temperature of the reaction system suggesting a chemical adsorption mechanism. The cost analysis revealed that it is economical to utilize plant extracts as effective green corrosion inhibitors. It was concluded that the three extracts could be considered as effective green corrosion inhibitors for mild steel in acidic media.