In situ oxidation of green rusts by deprotonation; wet corrosion and passivation of weathering steels

Intermediate compounds that belong to the layered double hydroxide family and bear the common name of green rusts (GR) get oxidized through two modes determining material wearing for steel corrosion: either the classical aerial mode where the GR gets dissolved before precipitating into several types of orange ferric oxyhydroxides FeOOH rusts, or the in situ deprotonation of OH- ions within the GR. Hydroxycarbonate, GR(CO32-), [FeII4 FeIII2 (OH)12]2+ . [CO32- . 3H2O]2- produced e.g. in carbonated medium, becomes [FeII6(1-x) FeIII6x O12 H2(7-3x)]2+ . [CO32- . 3H2O]2- which is an oxyhydroxycarbonate where ferric molar ratio x = [FeIII / Fetotal] belongs to the [0, 1] interval; it ends into the ferric green rust, GR(CO32-)*, [FeIII6 O12 H8]2+ . [CO32- . 3H2O]2- which is in fact orange. These two modes of oxidation of GR that depend upon the flux of oxygen that is used give rise to two different corrosion behaviors. The first one corrodes the material since the GR layer covering the steel surface gets dissolved before precipitating ferric oxyhydroxide. The second mode occurs within the GR layer without destruction; thus, the metal is passivated as for weathering steels.