Corrosion of steel is an electrochemical process with different reactions occurring at the anodic and cathodic sites. A supply of water and oxygen is required to maintain the reaction.

In new concrete the background pH is about 12.5. In these conditions a stable oxide “passive” layer is formed on the surface of the steel which provides protection from corrosion. If the concrete carbonates to the depth of the steel and/or chlorides are present the protection can be lost.

Products of corrosion generally occupy a larger volume than the uncorroded steel. Unhydrated ferric oxide is about twice the volume, once hydrated it may be up to 6 to 10 times the volume.

Steel fibres have been used in external concrete slabs for some 30 years. The steel will be protected by the background alkalinity of the cement paste. It has been reported by researchers that corrosion is less active as compared to steel bar reinforcement (King and Adler, Bernard, Lambrechts). A fibre, being discontinuous, is not capable of giving rise to galvanic corrosion. If they do corrode, the small volume of the fibre is insufficient to create the bursting stresses associated with the corrosion of larger diameter reinforcement bars.

For well compacted concrete the corrosion of fibres is restricted to the surface of the concrete. These will rapidly corrode and may be unsightly.