In the atmosphere, in water or in the ground iron is normally subject to corrosion.
Within reinforced concrete castings though, it protects itself, thanks to the high alkalinity provided by the hydration reactions of the cement constituents, with a protective film that is usually very resistant.
In spite of this protection, even concrete reinforcement may be subject to corrosion, even though it is immersed within the cement.
This phenomenon occurs when, within the concrete, a carbonation process appears, initiated by the diffusion of carbon dioxide or the presence of chloride ions resulting in a decrease of the pH (pH value is about 8.5). There are several forms of iron hydroxide made up by the process of oxidation of metallic iron. In very basic environments, i.e. with pH > 11 iron hydroxide is formed that is stable, dense, compact and adherent to the metallic substrates that protect it from the oxidation process by developing an impenetrable barrier to oxygen and moisture. When the environment’s basicity drops, that is when the pH < 11, the iron hydroxide formed becomes porous, permeable to oxygen and moisture. This implies a non-depassivation of the iron and can trigger the corrosion process. The corrosion of concrete reinforcement bars can lead to a reduction of a part of the round bars and the detachment of the cover (spalling) due to the expansions that follow the rust formation. The concrete cover because of its thickness and its compactness should theoretically be the best protection against substances potentially aggressive towards iron. It is important to cast concrete with optimal proportions and an appropriate choice of the concrete cover thickness to ensure a satisfying lifelong service.
To stop and prevent corrosion of reinforcement bars, passive or active protection can be used.
Passive protection isolates the metal surfaces from the external environment by means of a suitable cladding carried out with a suitable application cycle (protective paint film, bituminous coverings, anodic oxidation products, galvanization, chrome plating etc.). The active protection instead is used to eliminate corrosion from buried artefacts through cathodic protection.