The formation of "
green rust ", which corresponds to steel depassivation,
occurs when the the concentrations ratio(here chlorides) [Cl-]
/ [OH-] > 1. Thus,
a) this depassivation does not occur if alkalinity
([ OH- ] content).of the environment is high enough.
This means, in practice, that
reinforcement does not corrode if the
concrete alkalinity is high enough to avoid the
effect of the aggressive agents.
b) in a medium with a pH lower than 9,5 (approximately),
the critical content chloride (inducing depassivation
) is in the order of magnitude of chloride content in a drinking
water. Thus, reinforcements
corrosion in a carbonated concrete occurs in the absence of chloride.
In a carbonated concrete, which is not completely dry, a reinforcement
can corrode according to a process similar to that of corrosion
in the presence of chlorides. The intermediate product is a green
rust whose composition is [FeII4 FeIII2
(OH)12]2+ [CO3 2H2O]2-
. The domain of existence in the POURBAIX diagram is almost superimposed
on that which corresponds to chlorides. The end products are those
of traditional rust or those due to corrosion in the presence of
chloride, except for the akaganeite (β - FeOOH). When the concrete
carbonated and polluted by chlorides, the experiment showed that
it is green rust due to the carbonate which is most stable and which
is formed firstly.
Generally, by oxidation, the " green rust " is transformed
into made up more stable compound, in which iron is trivalent, lepidocrocite
(γ - FeOOH), goethite (α - FeOOH), akaganeite (β
- FeOOH) or the magnetite (Fe3 O4) The nature
of these products depends on chlorine content and on other factors
such as temperature.