The effect of the degree of substitution (DS) of carboxymethylated starch (CMS) as green corrosion inhibitor of carbon steel on 200 mgL~(-1) NaCl solution was investigated. Physical/chemical structure of CMS was studied by Fourier Transform Infrared Spectroscopy (FTIR) and Nuclear Magnetic Resonance (~(13)C-NMR). DS of this bio-polymer was determined by back-titration. Its inhibitive properties were evaluated using polarization curves and electrochemical impedance spectroscopy. CMS acts as corrosion inhibitor and the protection level is highly dependent on the degree of substitution. The formation of a chelate between carboxylate groups and ferrous cations allows the inhibition mechanism of CMS to be explained.
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