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Guanine was investigated as a potential green corrosion inhibitor to API 5L X65 carbon steel in HCl 0.1 mol.dm-3 solution, at pH = 2.0, under hydrodynamic conditions using electrochemical, weight loss and Raman spectroscopy experiments. Cyclic voltammetry and potentiodynamic polarization curves suggested that Guanine does not change the metal/solution interface appreciably and weight loss tests confirmed the low inhibition efficiency (IE) of Guanine in protecting the X65 steel in this corrosive medium. An IE = 22% was determined after 48 h of exposure, a value lower than that reported in the literature on the mild steel/Guanine/HCl system. Raman spectroscopy was employed to gain insight on the nature of the steel/Guanine interaction and the results pointed out to a weak physisorption of the molecule, thus explaining the very low IE values obtained.