If a steel is initially covered with passivating products (case of sound concrete), this protection is deteriorated by introducing too much chloride. Then a depassivation occurs, its mechanism includes the following stages :

    the passivating layer is damaged locally, where chloride content is very high,
    " green rust " is formed where passivation disappeared
    " green rust " is transformed into porous oxides (or hydroxides), if the oxygen content is rather       high.

The transformations of the " green rust " formed in the presence of chlorides were not well known yet. However, they explain the following practical observations :

    When reinforcement corrosion under a protective layer occurs in the absence of oxygen, the       corrosion products are of a very dark colour. This corresponds to the transformation of green       rust into magnetite.

    When the products of corrosion are rather thick, they form layers, where the chlorine content is       variable. This corresponds to the various transformations of green rust. Indeed, because of the       transformations of green rust, the chloride ions are mainly ejected from the crystal structure and       they are dissolved in the solution.

    The chloride content in concrete is higher in the vicinity of a corroding reinforcement than       elsewhere. This corresponds, as indicated above, at the exit of chloride ions out of the corrosion       products. So, this produces a kind of recycling of chloride ions in the corrosion process of iron.

When the concrete pH is lower than approximately 9 the critical chloride or carbonates contents, inducing steel depassivation, are very low and correspond to natural, drinkable water. This is why, in practice, this pH is often regarded as being the critical value of steel depassivation.