Pore Solution Composition and Chloride Effects on the Corrosion of Steel in Concrete

The corrosion mechanism of steel in concrete and the so-called threshold chloride content are discussed based on the results obtained by electrochemical measurement, pore solution analysis, and scanning electron microscopy (SEM) observation. It is shown that mortar (or concrete) provides better protection to steel than alkaline solutions. One of the protective mechanisms provided by mortar is found to be the pH control action of calcium hydroxide crystals located at the steel-mortar interface. For the protective mechanisms provided by mortar to operate, adhesion between the steel and the mortar is necessary and formation of voids at the steel-mortar interface is essential for active corrosion to start. The critical threshold chloride content is not determined simply by a parameter of the pore solution such as [Cl−]/[OH−] ratio; it is shown that the threshold value depends on the steel-mortar interface conditions.