Corrosion Inhibitors for Concrete

Synopsis - Corrosion of rebars in reinforced concrete structures is probably the most common  reason for distress and repair, and chloride ingress is more often the cause than carbonation. The addition of an inhibitor to the concrete recipe for structures prone to chloride attack is one measure that may enhance service life or time to necessary expensive repair. Corrosion inhibitors for reinforced concrete have been briefly reviewed with a special emphasis on anodic inhibitors. A special designed program is discussed in detail, where the performance of the common anodic corrosion inhibitor calcium nitrite, Ca(NO₂)2, is directly compared with that of the more recently discovered calcium nitrate, Ca(NO3)2. Calcium nitrate has been proven to work against rebar corrosion initiated by both intruded and intermixed chloride in long term tests, and to out-perform calcium nitrite in an accelerated electrically forced migration/corrosion test (the so called “lollipop” method). The effect of the two admixtures on concrete properties is compared as well. It is shown by theoretical considerations that the mechanism of nitrate and nitrite as corrosion inhibitors is similar in alkaline conditions like concrete. The kinetics for the nitrate reaction is slower than for nitrite, but this is only relevant for rapid tests since rebar corrosion in practice is a rather slow process. According to theory, calcium nitrate offers a larger buffer than calcium nitrite. Other advantages of using calcium nitrate rather than nitrite as corrosion inhibitor is that it is cheaper, less harmful and more available. Between 3 - 4 % calcium nitrate of cement weight seems sufficient to protect the rebar against corrosion due to intruded chlorides from the environment or intermixed chlorides.