Pitting Corrosion in Reinforced Concrete

Different forms of corrosion occurs in steel, such as general corrosion, stress corrosion cracking, pitting corrosion, hydrogen embrittlement and corrosion fatigue but when steel is embedded in concrete then steel is effected mainly by two different forms of corrosion i.e. general (uniform) corrosion and pitting (localized) corrosion. Concrete researchers generally use uniform corrosion to study the effect of corrosion on RC structures, which is not accurate because concrete structures are subjected to chloride attack causing pitting corrosion.

Corrosion starts when the passive layer on the metal surface is destroyed and there is enough oxygen and water to maintain the corrosion reaction and this passive layer is destroyed in the presence of chloride ions. The first stage of deterioration is corrosion initiation that is defined as time required for the chlorine concentration to reach the threshold chlorine concentration, which is sufficiently required to destroy the passive layer on the steel. The second stage is called corrosion propagation, where steel starts corroding which in turn reduces the area causing the failure of the structure. However, in concrete the mechanism by which corrosion of steel takes place in the presence of chloride is not stated properly even though there is a local failure of the passive layer that is formed in the highly alkaline condition of the concrete, which is the most common reason for the corrosion of steel. Also, it is assumed that the pitting corrosion initiates when critical chloride content reaches the rebar surface [8] but specific conditions that allows the chlorine to reach this critical condition for the corrosion to start are not recognized, otherwise can be used to prevent the use of a characteristic valve in the calculation of life of concrete structures. According to Lianfang Li, A.A. Sagues the chloride level should be minimum of 0.004M [Cl-] in Ca(OH)2 for the steel to undergo pitting corrosion.