Modified cassava starches as corrosion inhibitors of carbon steel: An electrochemical and morphological approach

The physical and chemical modification of cassava starch was carried out to prepare compounds that were evaluated as corrosion inhibitors of carbon steel under alkaline conditions in 200 mg L-1 NaCl solutions. Two species were tested: an activated starch (AS) and a carboxymethylated starch (CMS) with two different degrees of substitution (DS). The success of the chemical procedure was confirmed by C-13 NMR. The DS was determined by back titration and products with 0.13 (CMS0.13) and 0.24 (CMS0.24) were obtained. The inhibitive properties were studied by means of electrochemical impedance spectroscopy. It was found that modified starches have corrosion inhibitive properties and that their protection level depends on the type and amount of active groups present in the molecules. AS showed better performance than CMS, whose inhibition ability increases with the degree of substitution. This result was explained by the strong ionic interaction between AS and ferrous cations, which was confirmed by the electrostatic potential mapping of the monomeric units. After the corrosion experiments, the surfaces also were investigated through atomic force microscopy. It was found that a densification of the inhibitive layer was responsible for the higher protection level afforded by AS after 24 h of immersion. (C) 2010 Elsevier Ltd. All rights reserved.