Corrosion Mechanisms

Corrosion of Steel in Concrete

Unless protected, steel will corrode or rust when exposed to air. However, reinforcing steel embedded in concrete does not rapidly corrode as the concrete has a high pH and forms a passive layer that protects the steel from further corrosion. This passive film can be disrupted if the concrete’s pH decreases, through a process called carbonation, or if sufficient chloride ions are present, most commonly due to deicing salts or sea-water environments.

During the corrosion process, the iron metal converts to iron ions at the anode, releasing electrons. The iron ions then react with oxygen to form rust or iron oxides. As the iron oxides occupy more space than iron, pressure builds and cracking, spalling and delamination of concrete may occur.

The electrons released at the anode move to the cathode. In concrete, the electrons are consumed by a reaction between water and oxygen, forming hydroxide. The speed of this cathodic reaction frequently limits the overall corrosion rate.

Both anodic and cathodic reactions need to be present for corrosion to continue, but they do not need to be adjacent. In bridge decks, the bottom mat bars may be cathodic to the top bars, which are subjected to increased chloride exposure.

Corrosion Protection Using Epoxy-Coated Steel Reinforcing Bar

Epoxy-Coating protects the steel using several mechanisms. If undamaged, the coating prohibits the passage of chloride ions, thus protecting the steel from corrosion damage. If minor holes or damage are present in the coating and sufficient chloride ions are available in the concrete, then localized corrosion may occur. For the corrosion reaction to proceed, a cathode is required. If all bars are coated, then only very small cathodes are present, substantially reducing the corrosion rate. Tests conducted for the FHWA demonstrated that if both top and bottom mats were coated, the corrosion rates of Epoxy-Coated Steel Reinforcing Bars were 40 to 50 times less than that of similar uncoated bars.