Electrochemical study on the behaviour of steel for prestressed concrete in alkaline solutions: Influence of chloride ions

摘要

The paper presents the results of study on the electrochemical behaviour of prestressing steel in aqueous suspensions of cements, with various levels of chloride concentration by using the cyclic potentiodynamic polarization technique. The electrochemical measurements were performed on two types of high-strength steel used as tendons for prestressed concrete: cold-drawn steel and thermally treated (quenched and tempered). The anodic potentiodynamic polarization curves of steel specimens in aqueous suspension of Portland cement CEM I (pH = 12.7) with 0.0001 ÷ 1.0 M [Cl~-] were traced starting from the potential of -1000 mV/SCE with a forward (in the noble direction) scan rate of 0.556 mV/s. The polarization direction was reversed with the same backward scan rate once the oxygen release potential (E_(O2)) was reached in order to establish whether or not the steel surface remains in the passive state. Depassivation action of chloride ions was established by determining the potential of that observe at a sudden increase of current density. The "passivity breakdown (pitting) potential" (E_R) or pitting potential (E_(pit)) of steel 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 cement mass). The potentiodynamic polarization method is a fast, selective and efficient method for the comparative study of the influence of cements, additives and admixtures on electrochemical corrosion behaviour of different types of prestressing steel in concrete.