Chemical constituents of ethanol extract of Tapinanthus bangwensis (TB) leaves as a green corrosion inhibitor for mild steel and aluminum have a significant influence on the mechanism of corrosion inhibition by the extract. Therefore, the corrosion inhibition efficiency of ethanol extract of TB leaf was investigated using GCMS analysis (to identify the chemical constituents of the extract), potentiodynamic polarization (to obtained polarization data), and thermometric/gravimetry (to obtain inhibition efficiency) techniques. Molecular mechanics, quantum mechanics and molecular simulation calculations were carried out to support the experiment data. Ethanol extract of Tapinanthus bangwensis (TB) leaf contains 1,6,10-dodecatriene (42.56%), 3-chlorophenol (41.50%), pentadecanoic acid (10.23%), 2,3-diphenyl-1,4-naphthoquinone (2.12%), 1-monolinoleoyl glycerol trimethylsilyl ether (2.02%) and stigmasterol (1.57%). Weight loss results indicated that the inhibition efficiency of the extract ranged from 68.23 to 81.52% and from 63.24 to 76.87% for mild steel and aluminum at 303 K, respectively. At 333 K, the ranges were 41.96 to 59.51% and from 48.14 to 57.29%, respectively. Calculated inhibition efficiencies were observed to increase with increase in extract concentration but decreased with temperature and period of contact. Molecular and quantum mechanics simulation results gave values of EHOMO, ΔE, χ, η, EBind and steric energy parameters indicated that the activity of the chemical constituents of the extract towards corrosion inhibition responded to the following trend: 3-chlorophenol>1,6,10-dodecatriene>pentadecanoic acid. The adsorption of the inhibitor on the metal surfaces is endothermic, spontaneous and progressed with increasing degree of orderliness. However, the adsorption mechanism favoured physiosorption mechanism and obeyed the adsorption models of Langmuir and Freundlich.
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