Extent of Cathodic Reaction on Epoxy Coated Rebar with Partially Disbonded Coating

Corrosion of epoxy coated rebar (ECR) in chloride exposed concrete can be enabled by the oxygen reduction cathodic reaction taking place on steel exposed by coating breaks. The total cathodic current would be expected to increase if the reaction locus were to extend into crevices formed at disbonded coating regions around coating breaks. To examine the possible amount of this effect, distress in the form of controlled surface breaks was created on production samples of ECR, some of which were further subject to cathodic disbondment to create surrounding disbonded regions around the breaks. The samples were cast in concrete specimens kept at ~80% RH in air and fitted with electrodes to perform cyclic polarization (CYP) in the cathodic direction and electrochemical impedance spectroscopy (EIS) measurements under open circuit conditions. Measurements performed initially and after 6 years of aging showed for the disbonded specimens a moderately greater extent of cathodic current and greater effective electrode area compared to that of the simply distressed specimens. Results will be presented for experiments in progress where concrete moisture is elevated to increase the electrolyte content in the disbondment crevice. The implications of the findings on the estimated service life of ECR structures will be quantitatively assessed. INTRODUCTION Epoxy-coated steel reinforcement (ECR) is used for corrosion durability improvement of concrete structures in marine and deicing salt environment. The polymer coating is supposed to be a physical barrier to aggressive agents to the steel surface, but defects in the coating can lead to initiation of corrosion activity. These defects are mainly imperfections of the coating production or the result of the rough handling during construction. Once a critical amount of chloride ions reach the defect, the corrosion process starts by extended macro-cell formation with cathodic reaction supported at additional defects elsewhere at the rebar assembly or by micro-cell formation with cathodic and anodic reactions within one defect.1-4 The oxygen reduction produces hydroxyl ions (OH-) which lead to a shift in pH-values at the interface polymer/metal oxide. The cathodic disbondment is explained by a bond breaking between polymer and metal oxide. The radical intermediate stages like -O2H, OH and -O2 during the oxygen reduction can contribute to an oxidative degradation of the coating.5,6 The cathodic disbondment process is influenced by the electrical and chemical characteristics of the metal oxide, the oxygen permeability, the adhesion of the epoxy coating, the type and geometry of the defects as well as the humidity.7-9      «

Corrosion of epoxy coated rebar (ECR) in chloride exposed concrete can be enabled by the oxygen reduction cathodic reaction taking place on steel exposed by coating breaks. The total cathodic current would be expected to increase if the reaction locus were to extend into crevices formed at disbonded coating regions around coating breaks. To examine the possible amount of this effect, distress in the form of controlled surface breaks was created on production samples of ECR, some of which were f...     »