Green rust 
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The " rusts green " (" fougerite ") is an unstable iron compound. "Fougerite" is a natural rock, whereas the "green rust" is a product formed by a metal corrosion. But in fact it deals with the same compounds.

For its corrosion process , iron starts by a disolution, then it reacts with the aqueous medium and forms ferrous hydroxide Fe(OH)2 where iron is divalent (Fe
II). Then, this compound is transformed into products of a green color, called " green rusts ", which is stable only in the absence of oxygen. These " green rusts " have at the same time divalent (FeII) and trivalent (FeIII) iron. The composition of " green rust " formed in the presence of chloride is [FeII3 FeIII (OH)8]+ [Cl. H2 O]-. Concerning the crystal structure, chlorine ion is included inserted " green rust ".

The thermodynamic study of " green rust " made it possible to define its domain of existence in the POURBAIX diagram. " Green rust " due to chlorides is stable when pH is about 7 or 9. This is not in contradiction with the fact that a concrete with a pH of about 12, but polluted by chlorides, can induce reinforcement corrosion. In fact, the experimentations have shown that such a liquid induces steel depassivation, and its pH has spontaneously a lower value.



POURBAIX diagram of iron in water in the presence of chlorides
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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.


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