The corrosion of steel reinforcement within concrete is a process influenced by the material's inherent properties and external factors. The high pH level of around 13, provided by calcium hydroxide present in concrete, initially protects the steel reinforcement by promoting the formation of a passive iron oxide layer on its surface.
However, over time and under certain conditions like carbonation, chloride ingress, and cracking this protective state can be compromised. Steel has areas with different electrochemical potentials, known as anodic and cathodic regions. The hydrated cement paste, containing salts, acts as an electrolyte that connects these regions. At the anodic regions, iron from the steel dissolves as positively charged ions into the electrolyte, while at the cathodic regions, electrons are consumed in reactions that involve water and oxygen from the surrounding concrete to produce hydroxyl ions.
These hydroxyl ions combine with the dissolved iron to create ferric hydroxide, which further oxidizes to become rust. The presence of chloride ions exacerbates this process, leading to localized pitting corrosion by forming hydrochloric acid that attacks the protective iron oxide layer. This corrosion can expand the steel, causing the concrete to crack and spall.
Preventative measures against the corrosion of steel reinforcement include using protective coatings such as epoxy or zinc on the steel itself or improving the concrete's quality to reduce its permeability, thereby hindering the ingress of corrosive agents.
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.
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