Steel embedded in an alkaline environment does not corrode as a result of the formation of a passive oxide layer on its surface. Corrosion can be initiated if the passive oxide layer is destabilized, this can occur in a number of ways. Aggressive species such as chlorides which are found in, for example, de-icing salts, sea water and marine environments penetrate the masonry and react to break down the passive oxide layer, initiating corrosion.
Carbonation of the masonry leads to a neutralisation of the environment around the steel and the breakdown of the passive film. The carbonation reaction occurs when atmospheric carbon dioxide dissolves in the masonry pore solution and reacts to form carbonic acid. This acid reacts with present alkaline calcium hydroxide to form neutral calcium carbonate. As the calcium hydroxide is consumed the pH of the masonry drops from approximately 13 to 9 at which stage the passive film begins to break down. Once the film is sufficiently destabilized corrosion occurs utilizing the available water and oxygen in the pore solution.