SOURCE:

Faculty: Engineering
Department: Chemical Engineering

CONTRIBUTORS:

Emembolu, L.N.
Onukwuli, O.D

ABSTRACT:

ABSTRACT
This work assessed the corrosion inhibition of plant leaf extracts on aluminum and mild steel in acidic and alkaline media using different corrosion techniques. The six plant leaf extracts utilized in this work include Aspilia africana, Chromolena odorata, Newbouldia leavis, Dennettia tripetala, Dialium guineense and Vitex doniana plant leaf extracts. These plant leaf extracts were characterized using Fourier transform infrared spectroscopy (FTIR), Gas chromatography mass spectrophotometer (GC MS) and phytochemical analyses. The effect of temperature on the corrosion behaviour of aluminum and mild steel in the presence and absence of the plant leaf extracts were studied in the temperature range of 303K–343K. The gravimetric, thermometric, potentiodynamic polarization, electrochemical impedance spectroscopy measurements were employed in studying the adsorption, kinetics and thermodynamics behaviour of corrosion inhibition of aluminum and mild steel. Response Surface Methodology (RSM) was used to optimize the inhibition efficiency. Quantum chemical calculations using density functional theory were used to calculate some electronic properties of the molecule in order to ascertain any correlation between the inhibitive effect and molecular structure of the leaf extracts. FTIR and SEM were used to determine the shifting mechanisms of the inhibitors’ functional groups and the surface morphology of aluminum and mild steel, respectively. FTIR results revealed that the six plant leaf extracts contain oxygen and nitrogen atoms (O–H, N–H, C–N, C–O, C=O and C≡N) and aromatic rings in their functional groups. There were reductions in peaks of the functional groups of the corrosion products compared to the peaks of the pure leaf extracts from FTIR analysis. SEM micrographs revealed that the inhibitors’ molecules formed good protective film on the aluminum and mild steel. Phytochemical analysis showed the presence of tannins, saponins, phytates, phenolics, flavonoids, cardiac glycosides and alkaloids. The GC MS revealed the molecular compounds present in the inhibitors were phytol (C20H40O), sitosterol (C24H30O). Adsorption of the plant extracts on the surfaces of the metals was spontaneous and occurred according to the mechanism of physical adsorption. The adsorption isotherm followed Temkin isotherm model, with regression coefficient (R²) = 1.000. The values of activation energy ranged from 4.26 to 74.4KJ/mol and other thermodynamic parameters like enthalpy of adsorption 2.92 to 67.8 KJ/mol, entropy of adsorption -211.0 to 0.05J/mol/K and adsorption equilibrium constant 0.0004 - 376 M-1 and free energy of adsorption – 0.12 to -35.23 KJ/mol, were used to elaborate the mechanism of corrosion inhibition respectively. The values of free energy of adsorption ranged from -0.1243 to -35.23 kJ/mol. The optimum inhibition efficiency of the leaf extracts of Aspilia africana, Chromolena odorata, Newbouldia leavis, Dennettia tripetala, Dialium guineense and Vitex doniana ranged from 50.07% to 83.37%, 48.22% to 78.29%, 48.83% to 82.93%, 42.13% to 85.26%, 50.80% to 78.61% and 52.14% to 80.29% respectively. The gravimetric, thermometric, potentiodynamic polarization and electrochemical impedance spectroscopy measurements were in reasonable agreement with the experimental results. The results revealed that the inhibitors concentration increased with increase in the inhibition efficiency and decrease with temperature. The potentiodynamic polarization results revealed that the plant leaf extracts were mixed–type inhibitors. Electrochemical impedance spectroscopy results revealed that the addition of inhibitors increased the charge- transfer resistance of the corrosion process and hence the inhibition performance. Quantum chemical parameters confirmed that the active constituents of the inhibitor molecules retarded the corrosion of aluminum and mild steel. The leaf extracts of Aspilia africana, Chromolena odorata, Newbouldia leavis, Dennettia tripetala, Dialium guineense and Vitex doniana exhibited good performance. Hence the plant leaf were recommended as corrosion inhibitors for aluminum and mild steel in alkaline and acid solutions.