Corrosion-Inhibiting Admixtures
Concrete reinforcing bars (commonly referred to as rebar) typically consist of uncoated carbon steel, and as such have a tendency to corrode over time, particularly when exposed to air, water or chloride ions (salt). In order to counter this tendency, corrosion inhibitors may be added to the concrete.
There are two goals behind the addition of corrosion inhibitors, both with the objective of reducing the sensitivity of the steel to environmental factors. The first goal is to extend the period between the installation of the concrete and the onset of the reinforcement corrosion, and the second goal is to reduce the overall amount of corrosion, if it does occur.
There are several types of corrosion inhibitors, each with a different mechanism of operation. Calcium nitrite has been used extensively for many years to mitigate the impact of chloride ion migration in concrete. Chloride ions are introduced into concrete commonly through exposure to sea water or deicing salts, and may make their way to the reinforcing steel. Normally, the surface of the reinforcing steel inside typical concrete is coated with a compound known as ferrous oxide that results from exposure to air. The ferrous oxide is in an unstable state, and will attract nearby chloride ions, which will lead to steel degradation. The calcium nitrite reacts with the surface layer of the steel and converts it to ferric oxide, which is in a passive or neutral state. Encroachment of chloride ions into the concrete will then have little to no effect on the reinforcing steel as they cannot react with stable molecules
Other types of corrosion inhibitors, such as those comprised of amino alcohols, create a coating over the steel that prevents intrusion of chloride ions, as well as disrupting the effects of oxygen and water on the steel.