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51317--9708-Electrochemical Behavior of Prestressing Steel in Alkaline Electrolytes: Influence of Chloride Ions

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 The paper presents the study on the electrochemical behavior of prestressing steel in alkaline electrolytes, with various levels of chloride concentration by using the cyclic potentiodynamic polarization technique.

Product Number: 51317--9708-SG
ISBN: 9708 2017 CP
Author: Ioan Pepenar
Publication Date: 2017
Industry: Science of Corrosion
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It is generally known that the steel in concrete is protected against corrosion due to the high pH-value of the pore solution of the concrete which leads to the formation of a passive oxide layer on the steel surface preventing its further corrosion. In the presence of an over critical chloride concentration in the concrete at the steel surface this protective passive layer is locally destroyed (depassivation) and the steel corrosion starts. Therefore understanding the mechanism of chloride induced breakdown of steel passive layer in concrete is significantly important in order to prevent the initiation of corrosion in prestressed concrete structures.The paper presents the results of research on the electrochemical behavior of prestressing steel in alkaline electrolytes with various levels of chloride concentration by using cyclic potentiodynamic polarization technique. After each potentiodynamic polarization test which led to pitting corrosion the steel specimens were examined visually and microscopically for evidence of pitting initiation and propagation. The research was performed on two types of high-strength steel used as tendons for prestressed concrete: cold-drawn steel with pearlitic microstructure and thermally treated (quenched and tempered) steel with martensitic microstructure. The passivity breakdown (pitting) potential of steel in aqueous suspension of Portland cement CEM I (pH = 12.7) with 0.0001…1.0 M [Cl-] was determined depending on the shape of the polarization curves chloride ions concentration and type of steel. The obtained results allow the definition of a critical chloride ion concentration (chloride threshold) below which no prestressing steel depassivation occurs with a value of 0.01 M [Cl-] respectively 0.0355% Cl- by mass of cement. The passivity breakdown (pitting) potential decreases linearly with the logarithmic values of chloride ion concentration. The potentiodynamic polarization method is a fast and efficient method for the comparative study of the influence of additives and admixtures on electrochemical corrosion behavior of the steel in concrete.

Key words: prestressing steel, cement aqueous solution, chloride ions, potentiodynamic method, anodic polarization curve, pitting potential, chloride threshold

 

It is generally known that the steel in concrete is protected against corrosion due to the high pH-value of the pore solution of the concrete which leads to the formation of a passive oxide layer on the steel surface preventing its further corrosion. In the presence of an over critical chloride concentration in the concrete at the steel surface this protective passive layer is locally destroyed (depassivation) and the steel corrosion starts. Therefore understanding the mechanism of chloride induced breakdown of steel passive layer in concrete is significantly important in order to prevent the initiation of corrosion in prestressed concrete structures.The paper presents the results of research on the electrochemical behavior of prestressing steel in alkaline electrolytes with various levels of chloride concentration by using cyclic potentiodynamic polarization technique. After each potentiodynamic polarization test which led to pitting corrosion the steel specimens were examined visually and microscopically for evidence of pitting initiation and propagation. The research was performed on two types of high-strength steel used as tendons for prestressed concrete: cold-drawn steel with pearlitic microstructure and thermally treated (quenched and tempered) steel with martensitic microstructure. The passivity breakdown (pitting) potential of steel in aqueous suspension of Portland cement CEM I (pH = 12.7) with 0.0001…1.0 M [Cl-] was determined depending on the shape of the polarization curves chloride ions concentration and type of steel. The obtained results allow the definition of a critical chloride ion concentration (chloride threshold) below which no prestressing steel depassivation occurs with a value of 0.01 M [Cl-] respectively 0.0355% Cl- by mass of cement. The passivity breakdown (pitting) potential decreases linearly with the logarithmic values of chloride ion concentration. The potentiodynamic polarization method is a fast and efficient method for the comparative study of the influence of additives and admixtures on electrochemical corrosion behavior of the steel in concrete.

Key words: prestressing steel, cement aqueous solution, chloride ions, potentiodynamic method, anodic polarization curve, pitting potential, chloride threshold

 

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