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

Abstract

In the presence of various anions such as Cl −, SO 4 2− or CO 3 2−, 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(SO 4 2−) or GR1(CO 3 2−). First, the competition between SO 4 2− and CO 3 2− was studied by oxidising Fe(OH) 2 in the simultaneous presence of both anions. It appears that GR1(CO 3 2−) forms preferentially and GR2(SO 4 2−) 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 CO 3 2− 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 SO 4 2− or CO 3 2− are provided, GR1(Cl −), owning an average oxidation number for Fe of + 2.25, oxidises into GR2(SO 4 2−) or GR1(CO 3 2−), 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.