Steel embedded in an alkaline environment does not corrode as a result of the formation of a passive oxide layer on its surface. Instead, corrosion can be initiated if the passive oxide layer is destabilized – a condition that can occur in a number of ways. Aggressive elements such as chlorides in de-icing salts, sea water and marine environments can penetrate the masonry and cause carbonation. 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.