The inhibitive effect of leaf extract of Euphorbia hirta on aluminium corrosion in HCl and NaOH solutions were studied using gravimetric technique at 30 and 60°C. Results obtained showed that the leaf extract functioned as effective and excellent inhibitors in the acidic and alkaline media. Also, the leaf extract suppressed the corrosion reaction in both media. Inhibition efficiency of the extract was found to vary with concentration, temperature and exposure time. The leaf extracts of E. hirtademonstrated better inhibition efficiency in the acidic medium than in the alkaline medium. The adsorption characteristics of the leaf extract were best described by the Langmuir isotherm. Values of the free energy of adsorption Gads were determined from the isotherm parameters and they showed that the adsorption was spontaneous. The molecular interaction parameter (α) between the molecular species of the inhibitors and the metal surface they cover was also determined. The heat of adsorption (Qads) and apparent activation energy (Ea) were estimated from the temperature dependence of inhibition efficiency. Physical adsorption mechanism is proposed from the obtained thermodynamic parameters.
Key words: Adsorption isotherm, corrosion inhibition, aluminium, leaf extracts, thermodynamics.
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