Corrosion Performance of Steel Rebars Embedded in Chloride Contaminated Concrete Mortars-Role of Surface Pre-Treatments

Abstract

This paper incorporates the studies related to the role of surface condition (surface having black oxide, brown rust and bright & polished) of thermo mechanically treated (TMT) low carbon mild steel rehars embedded in chloride contaminated concrete mortars on their corrosion resis-tance. It is established that the presence of brown rust or black oxide scale on rebars' surface has an accel-erating effect on uniform and localised corrosion rates. The mortars embedded with rebars having mill scale cracked within 450 days of exposure in the test environment whereas the mortars with descaled rebars took 760 days of exposure for cracking. The presence of black and brown oxides on the rebars surface and embedded in concrete accelerated the corrosion which has been ascertained by weight loss, electrochemical direct current polarisation and alternating current impedance spectroscopy (EIS) studies. The tests of such specimens exposed directly to cement slurry also provi,-Ic the same conclusion. Cyclic polarisation plots indicate that the steel rebars having oxide on their surface are prone to crevice corrosion, a widely reported localised type of corrosion attack in concrete structures. XRf) (X-ray diffraction) studies of the rusts generated on rebars surface during their expo-sure show the formation of unstable akaganite ; I1-FeO (Oil)) and hematite (Fe203) phases. SEM (scanning elec-tron, microscopy) studies show that nodular types of corrosion products are formed on the pre-scaled surface of embedded rebars. The corrosion rate of descaled surface exposed under identical test conditions exhibit compara-tively lower rate of corrosion than the oxide-covered surfaces. This difference in corrosion rate is attributed to the cathodic de-polarisation of corrosion reaction caused due to the presence of Fe3 ions of oxides on the corroding surface. The pre-rusted surface (under contr-olled conditions) exhibits almost the same behaviour as the rebars with scale. The resulis have been discussed in light of kinetics of polarisation of reduction reaction and film formation at the steel surface.