Experimental and Theoretical Studies on Flavonoids from Adansonia digitata Leaf Extract as Sustainable Corrosion Inhibitors for Mild Steel in HCL Medium

The search for green and sustainable corrosion inhibitors has attracted increasing attention as alternatives to toxic synthetic chemicals. In this study, flavonoids extracted from Adansonia digitata (baobab) leaves were investigated as eco-friendly inhibitors for mild steel corrosion in hydrochloric acid medium through a combination of experimental and computational approaches. Weight loss measurements demonstrated a concentration-dependent inhibition effect, achieving a maximum efficiency of 86.4% at 500 ppm, with quercetin exhibiting the highest protection efficiency (90.2%), followed by luteolin (87.5%), kaempferol (83.1%), and apigenin (79.6%). FTIR spectra of the steel surface after exposure confirmed adsorption of flavonoids, showing characteristic shifts in the O-H stretching band (3420-3380 cm-1) and C=O stretching band (1662-1645 cm-1), indicative of coordination between hydroxyl/carbonyl groups and Fe atoms. Quantum chemical calculations using density functional theory (DFT) provided molecular-level insights into the inhibition mechanism: quercetin and luteolin displayed the highest HOMO energies (-5.81 eV and -5.94 eV) and lowest energy gaps ΔE (3.12 eV and 3.25 eV), consistent with their superior electron-donating capacity. Frontier molecular orbital (FMO) distributions and Fukui function mapping further identified hydroxyl and carbonyl sites as the dominant adsorption centers. The agreement between experimental and theoretical findings confirms that A. digitata flavonoids inhibit corrosion primarily via chemisorption through donor–acceptor interactions, offering a sustainable and highly effective alternative to toxic synthetic inhibitors.