Abstract
In this study, crude aqueous-ethanolic extract and n-butanol fraction of Quercus robur stem bark were obtained, and their corrosion-inhibition performance on mild steel in 1 M HCl was evaluated while their performances are compared to the commercial tannic acid used as standard. Corrosion inhibition was assessed by potentiodynamic polarization at 200 mg L⁻¹ after 1 hour of immersion. The n-butanol fraction exceeded the 71.9 % efficiency of the crude extract, though remaining below the 98.2 % maximum efficiency observed for the tannic acid standard at 1000 mg L⁻¹. Polarization curve shifts support classifying the tannin-containing samples as mixed-type inhibitors. Adsorption-isotherm analysis showed model-dependent behavior: the Freundlich model provided the highest R2 for the crude extract, while the Langmuir model provided the highest R2 for the n-butanol fraction and tannic acid. These fits represent empirical descriptions of concentration-dependent inhibition rather than direct molecular mechanisms. For commercial tannic acid, the standard Gibbs free energy of adsorption as approximately -33.3 kJ mol⁻¹, indicating thermodynamically favorable adsorption. Overall, solvent fractionation improved inhibition performance. These findings indicate the n-butanol fraction's potential as a plant-derived inhibitor. Further phytochemical investigation, isolation and characterization are required to properly identify the active constituents underlying the adsorption mechanisms.