THERMODYNAMIC INVESTIGATION AND QUANTUM CHEMICAL EVALUATION OF n-HEXANE EXTRACTS OF Costus lucanusianus AS CORROSION INHIBITORS FOR MILD STEEL AND ALUMINUM IN 1 M HCl SOLUTION

n-Hexane extracts of Costus lucanusianus stem and leaves were used for corrosion inhibition of mild steel and aluminium in 1 M HCl at concentrations of 0.1 g/L, 0.2 g/L, 0.5 g/L, 0.7 g/L and 1 g/L using thermo-gravimetric analysis (303 K – 333 K). In 1g/L, n-Hexane stem extract had maximum inhibition efficiency of 85 % at 333 K and 78% at 303 K for mild steel and aluminium respectively. Addition of extracts to acid solution increased values of Ea. Positive values of ΔH indicated endothermic adsorption process. Negative values of ΔS indicated decrease in disorderliness of the system. Negative values of ΔG (-2.95 KJ/mol to -4.10 KJ/mol) indicated physisorption mechanism. The adsorption was found to obey Freundlich isotherm. GCMS analysis showed major components of the extracts labelled n-Hexane stem extract (n-HSE) and n-Hexane leave extract (n-HLE) as 2,2'-methylenebis-1H–isoindole-1,3(2H)-dione (33.29%) and triacontanoic acid methylester (61.01%) respectively. Quantum chemical calculations were performed using Density Functional Theory with complete geometry optimization for calculations of EHOMO, ELUMO and energy gap. The energy gaps were 3.03 eV and 5.34 eV for n-HSE and n-HLE respectively. The softness (σ) values indicated that components of n-HSE are soft molecules; and are more reactive than n-HLE. This observation was consistent with results obtained from experimental inhibition efficiencies.