Study On The Propagation Of Localized Corrosion And Its Inhibition Mechanism Of Steel Rebar In Reinforced Concrete

The premature degradation of reinforced concrete (RC) related to chloride, CO2, SO42-, oxygen content, relative humidity, microstructure of concrete and construction defects et.al, could lead to heterogeneous corrosion on steel rebar surface. Then, the following expansion of rust will reduce the strength of RC structures by formation of cracking and spalling, and even shorten their service life.Pitting corrosion is generally the initial form of corrosion on rebar surface in the highly alkaline environment in concrete. In order to restrain the initiation of corrosion, inhibitors were pre-mixed in newly built concrete. However, their inhibitive mechanism on pitting corrosion is still controversial. Conventional electrochemical methods fail to obtain the details about the initiation, propagation and rehabilitation of localized corrosion on rebar. Based on a series of experiments and discussion about the migration and inhibition mechanism of MCIs, several notable acknowledges can be concluded:1) The wire beam electrode (WBE) was employed to study the evolution of pitting corrosion of rebar under concrete cover, aiming at simulating the migration and competitive adsorption behavior of aggressive ions and inhibitor at the interface of rebar/concrete. The inhibition mechanisms of enamine-based, aminoalcohol-based and inorganic salt Ca(NO2)2on the pitting corrosion were studied in simulating pore solution and in mortar. It was found that the migration rate of DMEA(Dimethylethanolamine) in concrete was higher than TETA(Triethylenetetramine), but the inhibitive efficiency of DMEA on pitting corrosion was lower than that of TETA. The organic inhibitors may initially promote the anodic dissolution under the rust layer because of its slow penetration rate through the product barrier. A localized corrosion factor (LF) was further presented to quantify the localized corrosion based on galvanic current maps of WBE.2) Wetting-drying cycle experiments showed that the corrosion products such as FeO and Fe3O4can be oxidized to Fe2O3or FeOOH during the drying period due to the adequate oxygen and fast diffusion rate in the mortar pores. However, in the oxygen-deficient wetting period, the Fe (III) instead of oxygen could take part in the cathodic depolarization that will sustain the anodic dissolution. 3) With the help of3D-microscopy, electrochemical noise, EIS and micro-Raman spectroscopy, the mechanism of initiation, growth and death of metastable pits on rebar in simulating concrete pore solution (CPS) was investigated. In addition, the catalytic mechanism of corrosion products on the transition of metastable to stable pits was proposed. It was found the intensity and lifespan of metastable peaks was associated with the breakdown and recovery of passive film. Based on the charge and discharge of passive film capacitance, an equivalent circuit model were established to further explain the acceleration mechanism of the rust layer on metastable pits.4) By using atomic force microscopy (AFM), XPS, ECN and Mott-Schottky measurement, the absorption behaviors of organic inhibitors at the interface between rebar and rust layer were studied. Moss-Schottky (M-S) plots shows that both nitrite and TEPA(tetraethylenepentamine) can reduce the donor density in passive film, leading to a more positive film breakdown potential. The TEPA can preferably adsorb on oxygen vacancy of passive film rather than on the rust layer, which will fail to inhibit the growth of pitting corrosion. Although NO2-has very low diffusivity in concrete, its inhibitive effect on stable pits beneath corrosion products was remarkable. Furthermore, the spectrum of XPS shows a strong coordination bond between N1s of TEPA and the passive film of carbon steel other than on the rust layer after TETA was added, thus suggesting a weaker adsorbability of amino-based inhibitors on corrosion products.