The anionic species competition in iron aqueous corrosion: Role of various green rust compounds

In the presence of various anions such as Cl-, SO42- or CO32-, aqueous suspensions of ferrous hydroxide Fe(OH)(2) usually oxidise into ferric oxyhydroxides via an intermediate Fe(II)-Fe(III) hydroxy-salt designated as Green Rust (GR), e.g. GR1(Cl-), GR2(SO42-) or GR1(CO32-). First, the competition between SO42- and CO32- was studied by oxidising Fe(OH)2 in the simultaneous presence of both anions. It appears that GR1(CO32-) forms preferentially and GR2(SO42-) is clearly observed only when the amount of carbonate anions is not sufficient to transform all the initial ferrous hydroxide. Then a mixture of both GRs is obtained. This confirms the affinity that pyroaurite-type layered hydroxides such as GRs exhibit for CO32- anions, amongst any other type. This statement is supported by a theoretical approach based on the thermodynamic constants previously determined for GRs. Secondly, the possibility that a GR compound can oxidise into another one was definitively established. When SO42- or CO32- are provided, GR1(Cl-), owning an average oxidation number for Fe of + 2.25, oxidises into GR2(SO42-) or GR1(CO32-), compounds owning a larger oxidation number for Fe of + 2.33. The indirect oxidation process via a GR compound always takes place rather than a direct process into ferric oxyhydroxides. (C) 1997 Elsevier Science Ltd.