The present work studied aqueous extracts of castor beans as a natural inhibitor of the corrosion of mild steel in 1 M HCl using gravimetric tests, potentiodynamic polarization curves, linear polarization method, electrochemical impedance measurements, surface analysis by scanning electron microscopy (SEM) and chemical characterization by Fourier transform infrared spectroscopy (FTIR). Gravimetric tests showed that inhibition efficiency (IE) increased with immersion time and concentration of the extract, reaching 97.8% in the presence of 800 mg L −1 inhibitor after 24 h immersion time. For the gravimetric tests varying the temperature, a decrease of the activation energy (E a) was observed, suggesting chemical adsorption of extract components on the metal surface. The extracts from the castor beans acted as a good corrosion inhibitor, and the macromolecules present in the high molecular weight fraction (HMWF) have a fundamental participation in this process since it presented 93.7% of IE for 200 mg L −1 .

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