Corrosion of Reinforcement

Concrete's high alkalinity, around pH 13 from calcium hydroxide, creates a protective iron oxide layer on steel through a process called passivity. This passive layer shields the reinforcement from corrosive agents. However, this protection can be compromised by carbonation, chloride ingress, and concrete cracking over time.

Hydrated cement paste containing salts acts as an electrolyte, bridging anodic and cathodic regions on the steel surface. At the anode, iron dissolves as positively charged ions. At the cathode, electrons react with water and oxygen to form hydroxyl ions, which combine with dissolved iron to create ferric hydroxide that oxidizes into rust.

Pitting corrosion is localized corrosion caused by chloride ions in cement paste forming hydrochloric acid at anodic spots. This acid breaks down the protective iron oxide layer, creating small pits on the steel surface. Chloride-induced pitting is more aggressive than general corrosion and can rapidly compromise reinforcement integrity.

When steel rusts, it increases in volume significantly. This expansion exerts pressure on the surrounding concrete, causing internal stress. Eventually, the concrete cracks and spalls as it cannot withstand the outward pressure from the expanding rust layer on the reinforcement.

Protective coatings such as epoxy or zinc applied to steel reduce corrosion by creating a barrier against corrosive agents. Additionally, reducing the permeability of concrete hinders the ingress of chloride ions and oxygen, slowing electrochemical reactions. Both approaches work to maintain the passive layer and extend reinforcement lifespan.

Steel has areas with different electrochemical potentials, creating anodic and cathodic regions. At anodic regions, iron oxidizes and dissolves as positively charged ferrous ions into the electrolyte. At cathodic regions, electrons are consumed in reactions with water and oxygen. These regions drive the electrochemical corrosion process.

Chloride ions in cement paste form hydrochloric acid at anodic spots on the steel surface. This acid directly attacks and breaks down the protective iron oxide layer, exposing bare steel to further corrosion. The resulting pitting corrosion is more damaging than uniform corrosion across the reinforcement surface.