Unexpected deterioration of fusion-bonded epoxy-coated rebars embedded in chloride-contaminated concrete environments

Abstract

Attempts have been made to understand the causes of the very fast deterioration of fusion-bonded epoxy-coated rebars (FBECR) in chloride-contaminated alkaline environments and the fairly stable behavior of FBECR in neutral chloride solution. To accomplish this, the corrosion studies of FBECR directly exposed to a water solution of 3.5% sodium chloride (NaCl), chloride-contaminated simulated concrete pore solution (SPS), and solid-cast concrete mortars have been performed for more than 1,000 days of exposure. The FBECR under embedded conditions in concrete deteriorate at an unexpectedly faster rate and require only 100 days to 150 days of a wet/dry exposure period before the onset of corrosion. It was observed that fusion-bonded epoxy coating (FBEC) is more prone to the absorption of moisture and chloride in contact with alkaline solution but is very resistant to their diffusion in neutral solution. The presence of defects/cracks in the coating easily allows the penetration of chloride ions at the substrate surface. The results further show that the FBECR are very stable in contact with aqueous solutions having no trace of chloride ions. In the presence of this ion, the deterioration of the coating is quite fast, especially in alkaline environments. During the test period of 558 days, the FBECR with pinholes deteriorated very fast and the whole undercoating surface was observed to be covered with black and powdery oxides. The coating with no pinholes tested under identical test conditions, however, remained intact except at a few spots where tiny spots of rust appeared. The mortars prepared using defect-free FBECR withstood the test conditions without any sign of distress, even after exposure of 1,000 days. To understand the causes of the above observations and the characterization of corrosion products, direct current (DC) polarization, alternating current electrochemical impedance spectroscopy (EIS), scanning electron microscopy (SEM), energy-dispersive x-ray analysis (EDXA), and x-ray diffraction studies (XRD) were performed. The black corrosion products accumulated under the coating after exposure in contaminated concrete were found to be magnetite (Fe3O4) and akaganite (beta-FeO[OH]). Very strong peaks of chloride were also recorded in EDXA studies, indicating the role of chloride ions in the destabilization of the coating-steel interface.