Inside a concrete mass, high alkalinity of calcium hydroxide with a pH of around 13 shields steel reinforcement from corrosion by creating a protective iron oxide layer, a state known as passivity.
Due to varying electrochemical potentials, anodic and cathodic regions are formed on the steel surface.
These regions are bridged by the salt solution in hydrated cement, acting as an electrolyte.
Positively charged ferrous ions from the anode dissolve into the salt solution.
In contrast, negatively charged electrons move to the cathode, are absorbed by the electrolyte, and react with water and oxygen to form hydroxyl ions.
Ferric hydroxide is formed when the hydroxyl ions bond with the ferrous ions. Subsequent oxidation of ferric hydroxide leads to the formation of rust.
Further, chloride ions in the surrounding cement paste form hydrochloric acid at anodic spots, breaking down the protective layer of steel and causing pitting corrosion.
After the steel rusts, it increases in volume and exerts pressure that can lead to concrete cracking and spalling.
Using epoxy-coated or zinc-coated steel or reducing the permeability of concrete can reduce steel corrosion.
