Transport Of Chloride Ions In Concrete And Effect On Reinforing Steel Corrosion Behavior

As the most widely used infrastructural materials,corrosion induced failure is one of the main factors of the damage of steel-concrete composite structure.Once the steel which wrapped in concrete suffers the serious corrosion,it will lead to the failure of the whole structure,resulting in extremely large economic losses.The corrosion of steel is mainly due to the intrusion of chloride ion,which accelerates the electrochemical corrosion process on the surface of the steel bar.Therefore,it is particularly important to prevent the chloride ion from transporting in the steel structure.This study was carried out from two aspects.One is adding the admixture in concrete to block the chloride ion in concrete transmission.The other is prepare a kind of conversion film to impede the transfer of Cl~-to the internal reinforcement.Based on the theory of the formation of phosphating film,a double-layer film with high corrosion resistance was prepared on the surface of steel bar which have a good shielding effect on Cl~-,thus postponing the process of corrosion of steel.The transmission behavior of chloride ions through concrete samples with two different additives and the original samples were characterized and compared by electromigration test and electrochemical impedance monitoring.Firstly,the reliability of the experimental device for monitoring the concentration of chloride ion in the steel interfacial interface was verified to be reliable and stable.Then the effect of the addition of polyhydroxy acid water-reducing agent and sodium fluoride addition has no obvious influence on the performance of blocking the chloride ion transmission behavior,while the addition of polyhydroxy acid water-reducer could markediy impede chloride ions through the concrete layer in the early stage of the immersion test(46h).However,more chloride ions still penetrated the concrete layer when immersion time was prolonged(231h).this indicates that the addition of polyhydroxy acid water reducer can effectly benefit life-extension of the reinforced concrete structure.The optimal pH of Zn-Ni phosphate film was prepared under the guidance of the theory of phosphate film formation.The results show that the Zn-Ni phosphate film obtained at pH=5.0 has lower self-corrosion current density and higher corrosion potential due to its enhanced impedance to chloride ions comparing to the bare counterpart.Zn-Ca conversion film is more dense,and has higher breakdown potential and lower passive current density.But the corrosion potential is lower than that of bare samples.In order to combination the excellent properties of these two single film,a two-step synthesis method by immersion for the preparation of Zn-Ni-Ca film was developed.The chloride ion impedance and corrosion resistance of Zn-Ni-Ca film was characterized by EIS,polarization curve,PZFC,potentiostatic polarization SEM and EDS analysis.The results show that the dual-layer conversion film really combines the excellent properties of Zn-Ni and Zn-Ca monolayers with the strong shielding effect on Cl~-.and the strongest protective effect on the substrate.