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The inhibition ability of the methanolic extract of Rosmarinus officinalis on the corrosion of mild steel (XC48), in HCl solution (2M), was investigated at various temperatures (298– 328 K) via weight loss measurements, potentiodynamic polarization and electrochemical impedance spectroscopy (EIS). The results showed that the inhibition efficiency increased with the increase of the extract concentration to reach a maximum of 89,18% at 200 ppm. A further augmentation of the extract concentration led to some deterioration of the inhibition efficiency which dropped to 77,18 % at 400 ppm. This high efficacity may be attributed to the production of a protective layer on the surface of the electrode which is confirmed by the scanning electron microscopy (SEM) images. The investigation of the temperature effect on the inhibition efficiency and the determination of the different thermodynamic parameters, allowed us to deduce a physisorption mechanism of Rosmarinus officinalis extract on the surface of the metal. On the other hand, the results of the poten- tiometric polarization tests revealed that the extract components are acting as a mixed- type inhibitor.
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