In order to understand the mechanisms behind corrosion of reinforcing steel in concrete, one has to examine the chemical reactions involved. In concrete, the presence of abundant amount of calcium hydroxide and relatively small amounts of alkali elements, such as sodium and potassium, gives concrete a very high alkalinity-with pH of 12 to 13. It is widely accepted that, at the early age of the concrete, this high alkalinity results in the transformation of a surface layer of the embedded steel to a tightly adhering film, that is comprised of an inner dense spinel phase in epitaxial orientation to the steel substrate and an outer layer of ferric hydroxide. As long as this film is not disturbed, it will keep the steel passive and protected from corrosion.
When a concrete structure is often exposed to deicing salts, salt splashes, salt spray, or seawater, chloride ions from these will slowly penetrate into the concrete, mostly through the pores in the hydrated cement paste. The chloride ions will eventually reach the steel and then accumulate to beyond a certain concentration level, at which the protective film is destroyed and the steel begins to corrode, when oxygen and moisture are present in the steel-concrete interface.
In 1962, it was reported that the required minimum concentration of chloride in the concrete immediately surrounding the steel to initiate corrosion, the chloride corrosion threshold, is 0.15% soluble chloride, by weight of cement. In typical bridge deck concrete with a cement factor of 7, this is equivalent to 0.025% soluble chloride, by weight of concrete, or 0.59 kg soluble chloride per cubic meter of concrete. Subsequent research at FHWA laboratories estimated the corrosion threshold to be 0.033% total chloride, by weight of concrete.
There are indications that the chloride corrosion threshold can vary between concrete in different bridges, depending on the type of cement and mix design used, which can vary the concentrations of tricalcium aluminate (C3A) and hydroxide ion (OH-) in the concrete. In fact, it has been suggested that because of the role that hydroxide ions play in protecting steel from corrosion, it is more appropriate to express corrosion threshold in terms of the ratio of chloride content to hydroxide content, [Cl-] / [OH-], which was recently established to be between 2.5 to 6.
Once corrosion sets in on the reinforcing steel bars, it proceeds in electrochemical cells formed on the surface of the metal and the electrolyte or solution surrounding the metal. Each cell is consists of a pair of electrodes (the anode and its counterpoint, the cathode) on the surface of the metal, a return circuit, and an electrolyte. Basically, on a relatively anodic spot on the metal, the metal undergoes oxidation (ionization), which is accompanied by production of electrons, and subsequent dissolution. These electrons move through a return circuit, which is a path in the metal itself to reach a relatively cathodic spot on the metal, where these electrons are consumed through reactions involving substances found in the electrolyte. In a reinforced concrete, the anode and the cathode are located on the steel bars, which also serve as the return circuits, with the surrounding concrete acting as the electrolyte.
Corrosion can also occur even in the absence of chloride ions. For example, when the concrete comes into contact with carbonic acid resulting from carbon dioxide in the atmosphere, the ensuing carbonation of the calcium hydroxide in the hydrated cement paste leads to reduction of the alkalinity, to pH as low as 8.5, thereby permitting corrosion of the embedded steel:
The rate of carbonation in concrete is directly dependent on the water/cement ratio (w/c) of the concrete, i.e., the higher the ratio the greater is the depth of carbonation in the concrete. In concrete of reasonable quality, that is properly consolidated and has no cracking, the expected rate of carbonation is very low. For example, in concrete with w/c of 0.45 and concrete cover 25 mm, it will require more than 100 years for carbonation to reach the concrete immediately surrounding the steel. Carbonation of concrete or mortar is more of an issue in Europe than in North America. (reference)