The mechanism by which chlorides initiate corrosion is by locally breaking down the passive film which forms on steel in the highly alkaline concrete pore solution. However, the breakdown of passivity requires a certain concentration of chlorides. The aim of the project described in the paper has been to determine the influence of a number of factors on the critical concentration of Cl- necessary for initiation of corrosion of steel embedded in concrete. The variables investigated include hardening conditions, water/cement ratio, cement type, reinforcing steel surface condition, and salt type.
Mortar samples containing a steel rod have been cast, hardened, and subsequently exposed to a sodium chloride or calcium chloride solution. The corrosion current of the embedded steel has been monitored electrochemically and initially was of the order of 10-4 A/m2, corresponding to a corrosion rate of approximately 0.1 μm/year from the steel surface. After a period of time, the corrosion current increased by several orders of magnitude indicating that the chloride had penetrated to the steel surface and had initiated corrosion. The rate of this penetration, the chloride concentration in the mortar adjacent to the steel at the onset of corrosion, and the subsequent corrosion rate have all been measured to determine the influence of the preceding variables.