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The inhibitive effect of aqueous extract of leaves of Polyalthia longifolia on the corrosion of mild steel was studied using the gravimetric (weight loss) method at the temperatures 303 and 318 K. The results show that the aqueous extract of Polyalthia longifolia leaves inhibited the corrosion of mild steel in dilute H 2 SO 4 solution. It was found that the inhibition efficiency increased with inhibitor concentration and decreased with rise in temperature. Values of the activation energy of the inhibited corrosion reaction range between 45.40 kJ/mol and 62.22 kJ/mol. This is much higher than the 25.74 kJ/mol obtained for the blank. The adsorption of the extract was spontaneous and occurs according to Flory-Huggins adsorption isotherm. The corrosion inhibition of Polyalthia longifolia leaves extract was attributed to the adsorption of phytochemical molecules present in the extract onto the surface of the mild steel. Physical adsorption mechanism has been proposed for the adsorption of the inhibitor.

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