Adsorption of organic anions on low-carbon steel in saturated Ca(OH)2 and the HSAB principle

This paper represents a fundamental study of corrosion inhibition behavior of selected organic anions on low-carbon steel in saturated Ca(OH)(2) solution containing chlorides. The compounds studied include eight amino acids, two carboxylic acids, ascorbic acid and a mimosa tannin extract. All the inhibitors suppressed the reduction of O-2, a process that is known to be strongly catalyzed on passive iron, the essential step being O-2 reaction with Fe(II) surface sites. The proposed mechanism of inhibition involves formation of surface chelates which tend to stabilize iron in Fe(III) state and increase the Tafel slope of the actual potential dependence of the [Fe(II)] surface centers. In accordance with the hard and soft acid and base principle, the corrosion inhibition effect was well correlated with the absolute electronegativity of the anions obtained from the semiempirical quantum molecular modeling calculations. The lack of correlation of the inhibition effect with the absolute hardness of the inhibitor anions has been explained by considering the charge of the anions as the companion parameter for absolute hardness. The highest degree of inhibition was observed for ascorbic acid that also showed a clearly pronounced increase in the pitting initiation potential.