Adsorption of Organic Anions on Low-Carbon Steel in Saturated Ca(OH)[sub 2] and the HSAB Principle

Abstract

This paper represents a fundamental study of corrosion inhibition behavior of selected organic anions on low-carbon steel in saturated Ca͑OH͒ 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 Fe͑II͒ surface sites. The proposed mechanism of inhibition involves formation of surface chelates which tend to stabilize iron in Fe͑III͒ state and increase the Tafel slope of the actual potential dependence of the ͓Fe͑II͔͒ 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.The overwhelming majority of literature on corrosion inhibitors deals with the effect of inhibitors on uniform corrosion, e.g., of steel in acidic or neutral solutions while the inhibitors for pitting corrosion are far less studied. 1,2 Since the adsorption of an aggressive anion on the film surface is associated with the breakdown of a passive film, cation inhibitors may suppress pitting corrosion via electrostatic interaction with the aggressive anion. 3 On the other hand, the action of the anion corrosion inhibitors for bare or oxidecovered metal surfaces can be interpreted in terms of the hard and soft acid and base ͑HSAB͒ principle. 4-6 In a corrosion system containing the aggressive anions and the inhibitor anions, oxidized metal acts as a hard Lewis acid and anions act as Lewis bases. Hence, the inhibitor anions should be harder bases than the aggressive anions, since they must preferentially bind to the highly oxidized metal ions on the film surface via replacement of oxide or hydroxide groups.In particular, compounds containing carboxylic group have been reported to be effective inhibitors of passive film breakdown with chloride on the surface of various metals and alloys by means of their competitive adsorption on the oxide film, since the carboxylate ions are harder bases than chloride. 7 The aim of the present work is to assess the adsorption behavior of eight amino acids ͑proline, PRO; cystine, CYS-SS-CYS; arginine, ARG; phenylalanine, PHE; tryptophane, TRP; tyrosine, TYR; alanine, ALA; and glutamic acid, GLU͒ and two carboxylic acids ͑ox-alic, OXA and tartaric, TAR͒ on steel in sat. Ca͑OH͒ 2 solution in the presence of chloride ions. Ascorbic acid ͑AA͒ and mimosa tannin ͑MT͒ have been investigated also for comparative purposes as some prev...

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