Electrochemical Behavior Of Steel With Carbon Fiber Coupling In Simulated Concrete Environment

Carbon fiber reinforced concrete is a new-type building structural materialwith excellent performances such as good flexural stress, good compresivestress, strong wearability and high impermeability property. Besides, carbonfiber reinforced concrete has new electrical property compared with thetraditional concrete material. However, because carbon fiber is a goodelectrical conductor, galvanic corrosion might happen if carbon fiber andsteels contact in concrete, which might have adverse effect on steel, so as tocause the durability of concrete structures reduce. So far, few researchers havestudied the influence of carbon fiber on electrochemical corrosion behavior ofsteel in simulated concrete environment because of the galvanic couplebetween carbon fiber and steel. So this thesis was aimed at the influences ofcarbon fiber, Cl-and pH values on electrochemical behavior of carbon steel insimulated concrete environment by electrochemical tests, weight-loss test. Also, the effect of carbon fiber coupling on the protective property of passivefilm on steel surface was analyzed by XPS.In pH12.5simulated concrete pore solution, electrochemical tests andweight-loss tests results show that after carbon fiber coupling, double layercapacity (C_dl) of steel surface is significantly increased, meanwhile chargetransfer resistance (R_ct) decrease. The changes of Cdland Rctparametersindicate that the passive film formed on steel surface in pore solution becomesthiner when steel is coupled with carbon fiber, and the protective property ofthe steel surface film get weak. Meanwhile, after carbon fiber coupling, thecorrosion potential of steel shifts positively, and corrosion current densityincreases, thus the corrosion rate of steel is accelerated. The results of XPSsupports the conclusion from electrochemical and weight-loss tests: after steelcoupling with carbon fiber, the ratio of Fe³⁺to Fe²⁺on steel surface decreases,which indicates the stability of passive film on steel gets decreased. Althoughcarbon fiber coupling has a negative effect on steel passivation, carbon steelstill could keep good passivation in12.5pH concrete pore solution withoutCl-.The influence of carbon fiber volumes on electrochemical behavior of steelwas studied. With volume amounts increasing, Rctof steel descends,meanwhile C_dlascends, and theFe³⁺ Fe²⁺to Feratio in the passive film on steelsurface decreases. These suggests that with the volume of carbon fibercoupling increasing, the protective property of passive film on steel surface descends, also the film thickness reduces. The corrosion rate of steel showsincreasing tendency when the volumes of carbon fiber coupling increases.The pH value of pore solution is a very significant influence factor on theelectromechanical behavior of steel, so some investigations were focused onthe influence of pH values （13.3,12.5and11.6） on the electrochemicalbehavior of steel coupling with carbon fiber. In the higher pH value poresolutions （pH13.3）, the passivation of steel surface is very good; as the pHvalue decreases （12.5and11.6）, good passivation gets a little weaker. Whensteel coupled with carbon fiber in lower pH pore solution （pH11.6）, thecorrosion susceptibility of steel increases apparently and the corrosion rate ofsteel accelerates.Cl-is the most important factor to induce initiation of corrosion on steelin concrete, so experiments were carried out to study the influence of Cl^-onthe corrosion behavior of steel coupling carbon fiber in various pH value poresolutions （pH13.3,12.5and11.6）. With the concentration of Cl-increases, Rctof steel decreases slightly, the protective property of passive film decends,meanwhile the corrosion rate of steel ascends. In pH13.3pore solution,passivation of steel still maintained even when the Cl-concentration is veryhigh （2.65%）. As pH value of pore solution is decreased to12.5, corrosionoccurrs when the Cl^-concentration is0.7%and1.39%. As pH value falls to11.6, corrosion of steel initiates when the concentration of Cl^-is in acomparatively low level （0.3%）. When coupling with carbon fiber, carbon still is sensitive to corrosion in low pH value pore solutions （pH11.6） evencontaining low content of Cl-.The electrochemical behavior of steel in cement mortar mixed withcarbon fiber has been studied. In cement mortar, as the mixing mass of carbonfiber increases （0.2%,0.5%and1%, the mass ratio of carbon fiber andcement）, impedance of steel firstly increases and then decreases. When thecarbon fiber mass is0.2%, impedance of rebar ascends, which means steel isbetter protected. When the mass of carbon fiber increases to0.5%and1%, theimpedance decreases, which is even lower than the impedance of carbon steelin cement mortar without cabon fiber. So, when mixing mass of carbon fiber isexcessive, it is adverse to the protection of steel rabar in cement mortar.