The experimental aspect of the corrosion inhibition potentials of Brachystegia eurycoma seed extract was carried out using Phytochemical Screening and gravimetric techniques as well as scanning electron microscopy (SEM) and Fourier transform infrared spectroscopy (FTIR), whereas the theoretical aspect was studied using the density functional theory calculations to be performed and modeling the electronic structures of some extract constituents, including physiosorptive interactions with the mild steel surface. The analysis of the weight loss results showed that the Inhibition efficiency of the Brachystegia eurycoma seed extract is directly proportional to its concentration and inversely proportional to solution temperature and mild steel contact time in the test solu¬tion. The inhibition efficiency increases gradually reaching a maximum value of 96% within the first 24 hours at a concentration of 0.5g/l. The kinetic study shows that the inhibitory action is a first order kinetics with the concentration of the seed extract of Brachystegia eurycoma which is best fitted with the Langmuir adsorption isotherm. This was further supported by the thermodynamic parameters which indicate that the adsorption of Brachystegia eurycoma seed extract onto the metal surface was spontaneous, exothermic and has contributed to physical adsorption process. FTIR results showed that the inhibition mechanism was an absorption process through the functional groups present in the seed extract. Surface morphology also revealed that corrosion product confirmed the protection offered by the extract on the surface of the metal immersed in the acid media. Quantum chemical studies indicated that inhibition was due to adsorption of active molecules leading to formation of a protective layer on surface of mild steel. Quantum chemical parameters such as highest occupied molecular orbital (HOMO), lowest unoccupied molecular orbital (LUMO) energy levels, HOMO–LUMO energy gap and electronic density were virtually identified. Quantum analysis demonstrated reactive centres of electrophilic and nucleophilic attack and strong inhibition properties of bioactive molecules of BES extract.
