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The Passivity of Mild Steel in Borate Buffer Solution Containing Tannin

The effect of mimosa tannin on the electrochemical behaviour of carbon steel in borate buffer solution (pH 8.4) was studied using potentiodynamic and electrochemical impedance spectroscopy techniques. Mimosa tannin was found to increase the dissolution rate of CS in the active range by forming a soluble iron(II)tannin complex. In this way tannin suppresses electrochemical oxidation of Fe^2+ ions and formation of a deposited porous outer layer with a composition similar to iron(III) oxyhydroxide (FeOOH). The results of both techniques showed that mimosa tannin contributes to CS passivation by forming an insoluble chelating complex that precipitates onto the electrode surface, resulting in a decrease in the passive current density.

carbon steel; impedance spectroscopy; iron-tannin complexes; mimosa tannin; passivity

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