Corrosion Inhibitors for Reinforced Concrete

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Title: Corrosion Inhibitors for Reinforced Concrete

Author(s): H. Justnes

Publication: Special Publication

Volume: 234

Issue:

Appears on pages(s): 53-70

Keywords: chloride; concrete; corrosion; inhibitor; nitrate; nitrite; steel reinforcement

Date: 3/22/2006

Abstract:
Inhibitors against chloride induced corrosion of reinforcement in concrete have been briefly reviewed with emphasis on anodic inhibitors. A program designed to compare the performance of the common anodic corrosion inhibitor calcium nitrite, Ca(NO2)2, with the more recently discovered anodic inhibitor calcium nitrate, Ca(NO3)2, is discussed in detail. Calcium nitrate has been proven to inhibit reinforcing steel corrosion initiated by both intruded and intermixed chloride in long term tests, and to out-perform calcium nitrite in an accelerated test. The effect of the two admixtures on concrete properties is compared as well. It is shown by theory that the mechanism of nitrate and nitrite as corrosion inhibitors is similar in alkaline conditions like concrete. The kinetics for the nitrate reaction are slower than for nitrite, but this is only relevant for rapid tests since reinforcing steel 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 a corrosion inhibitor are that it is cheaper, less harmful and more available. Between 3 - 4 % calcium nitrate of cement by weight seems sufficient to protect the reinforcing steel against corrosion due to intruded chlorides from the environment or intermixed chlorides from, for example, contaminated aggregate.