ABSTRACT
The kinetics and optimization of inhibitive action of leaves extract of pawpaw, Azadirachta indica (neem tree) and bitter leaf on corrosion of mild steel in acidic medium was studied by weight loss technique using central composite design of response surface methodology for optimization. The metal was analyzed using Atomic Absorption Spectrophotometer to determine its elemental compositions. The extract was obtained by liquid extraction using ethanol and was analyzed to determine phytochemical components and functional groups using American Society of Testing Materials and Fourier Infra red Spectrometer respectively. The effect of process variables such as concentration of inhibitors, time, temperature and concentration of medium on corrosion rate in the acidic (HCl) medium was studied. The kinetics of inhibitive action of leaves extract were carried out using zero order, first order and second order kinetic models. The result obtained showed that the major composition of the mild steel is iron and the presence of phytochemical components and functional groups made the extracts performed as good inhibitors. The corrosion rate was found to decrease with increase in concentration of inhibitor, time and concentration of the medium and increase with increase in temperature showing that the adsorption was by physical mechanism. The activation energy of the inhibition obtained indicated that there was inhibition. The optimal conditions are: concentration of inhibitor 18.94mg/100ml, time, 5.5h, temperature 48oC, concentration of medium 1.74M in PPLE inhibited acidic medium; concentration of inhibitor 12.1mg/100ml, time, 7.5h, temperature 47.3oC, concentration of medium 2.36M in AILE inhibited medium; and concentration of inhibitor 16.37mg/100ml, time, 6.55h, temperature 48oC in BLE inhibited medium. The optimal conditions obtained gave inhibition efficiencies of 96.4%, 84.85% and 88.4% in PPLE, AILE and BLE inhibited acid media respectively. This indicates that leaves extracts can inhibit corrosion of mild steel in acidic medium. The inhibition of corrosion using the three leaves conform to zero order kinetic models. It was found out that inhibitive action of the leaves extracts will last beyond 18hours. This shows that the pawpaw, azadirachta indica and bitter leaves extracts have the potential to inhibit corrosion of metal in acidic medium.
