Mechanisms of Corrosion of Steel in Concrete

The corrosion of metals in concrete is a multibillion dollar problem which affects the construction, transportation, and many other industries. While much has been written about the problem, and numerous reports have appeared which discuss how this corrosion can be controlled, little has appeared on the mechanism whereby this corrosion occurs. It is well known that highway structures corrode, for the most part, due to the influence of deicing salts or marine environments. Relatively little has appeared about the equally severe corrosion problems which occur in buildings and other nonhighway structures. Most of these structures are not exposed to the salts present in highway structures, yet they still corrode. This paper discusses how all forms of concrete and similar cementitious environments protect embedded metals. It discusses the three protection mechanisms—passive film formation, changes in the conductivity of the electrolyte, and mineral scales—that have been used to explain how corrosion in concrete occurs. It also presents evidence to explain how, and why, corrosion occurs in these structures, even in the absence of deicing salts, marine environments, or other salt sources. Examples are discussed including buildings, marine piers, foundations, and highway structures.