engleski

Effect of Cystine on the Semiconductive and Protective Properties of Oxide Film Formed on Steel in sat. Ca(OH)2

Process of pitting corrosion initiation is known to be highly dependent on the protective and semiconductive properties of the oxide film. Initiation of localized corrosion proceeds through the competitive adsorption of hydroxide and chloride ions. The adsorption of organic molecules on the oxide film surface may significantly change the properties of the oxide film and postpone the pitting initiation process through the competitive adsorption of organic anion and chloride ions. In this study, cystine was investigated as a potential inhibitor of the reinforcing steel pitting corrosion. The effect of different concentrations of cystine, on the pitting initiation process on non alloyed steel was investigated by linear polarization measurements in sat. Ca(OH)2 with addition of 0.14 M NaCl. In presence of 2.5 × 10-3 mol dm-3 cystine, pitting corrosion was postponed for 465 mV in the anodic direction. The possible explanation for such behaviour is competitive adsorption between the chloride ions and cystine through the surface complex formation. Cyclic voltammetry measurements showed that presence of cystine in sat. Ca(OH)2 solution enhanced the oxidation and reduction peaks as well the current in the passive region giving the maximal values in the presence of 2.5 × 10-3 mol dm-3 cystine. This may be the consequence of the enhanced accumulation of the oxidation products at the steel surface and thickening of the oxide film. Electrochemical impedance spectroscopy gave response with two time constants in the presence of cystine where the high – frequency time constant was attributed to the double layer capacitance and charge transfer resistance while the low – frequency time constant was attributed to the redox processes in the oxide film. Total resistance maximum was obtained in the presence of 2.5 × 10-3 mol dm-3 cystine. ATR FTIR measurements confirmed presence of the adsorbed cystine molecules at the steel surface. Mott-Schottky experiments performed after 20 minutes of the oxide film formation at the anodic potential in the passive region revealed the influence of cystine on the semiconductive properties of the oxide film. The electronic character of passive film formed on steel in sat. Ca(OH)2 with and without cystine presence was found to be n-type semiconductor which is consistent with the dominant defects being cation interstitials. Using dielectric constant assumption, semiquantitative defect densities, flatband potentials and space charge layer thickness were determined. The defect concentration was found to be enhanced with adsorption of cystine on the oxide surface while the flatband potential was shifted towards positive direction. The space charge layer thickness was lowered in the presence of cystine. The addition of chloride ions into the solution enhanced the donor concentration and shifted the flatband potentials to negative direction. Such behaviour could be ascribed to the chloride ions adsorption onto the oxide film surface. In the presence of cystine the effect of chloride ions was significantly reduced due to the competitive adsorption of cystine and chloride ions.

steel; oxide film; pitting; semiconductive properties; cystine

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