Definition - What does Macrocell mean?

A macrocell is a type of defect in reinforced steel bars (rebars) that is primarily caused by corrosion. Reinforced steel bars are commonly used to construct buildings that are later covered by concrete. The primary reason for corrosion in rebars is an oxidation process occurring between the concrete and the steel, which breaks down the passive film on the steel due to the presence of chloride ions or carbon oxides.

A macrocell is sometimes referred to as macrocell corrosion.

Corrosionpedia explains Macrocell

In macrocell corrosion, reinforced steel bars (rebars) become corroded when they are surrounded by concrete, which tends to form a corrosion circuit or a macrocell circuit across the steel rebars. This circuit is formed only when a steel object is enclosed under pressure in the soil or inside a material that is rich in chloride ions or carbon oxides, or when impurities are present in the concrete. The soil or other material acts as an electrolyte solution in which an anode and a cathode are separated from each other. Electrons can flow from the cathode to the anode, resulting in the anode side of the steel being sacrificed and becoming corroded.

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