Influence of lactate ions on the formation of rust

The formation of rust can be simulated by oxidation of aqueous suspensions of Fe(OH)2 obtained by mixing solutions of NaOH and a Fe(II) salt. The aim of this study was to investigate the influence of organic species associated with microbially influenced corrosion. The lactate anion, often used as a carbon and electrons source for the development of microorganisms, was chosen as an example. Then, in the first part of the study, Fe(OH)2 was precipitated using iron(II) lactate and NaOH. Its oxidation process involved two stages, as usually observed. The first stage led to a Fe(II–III) intermediate compound, the lactate green rust, GR(C3H5O3-). This compound has never been reported yet. Its existence demonstrates that the GR structure is able to incorporate a very wide range of anions, whatever the size and geometry. The second stage corresponded to the oxidation of GR(C3H5O3-). It led to ferrihydrite, the most poorly ordered form of iron(III) oxides and oxyhydroxides. In the second part of the study, the formation of rust in seawater was simulated by oxidation of Fe(OH)2 in an aqueous media containing both Cl− and SO42- anions. The first stage led to the sulphate green rust, GR(SO42-), the second stage to lepidocrocite γ-FeOOH. Small amounts of iron(II) lactate were added to the reactants. Lactate ions did not modify the first stage but drastically perturbed the second stage, as ferrihydrite was obtained instead of γ-FeOOH.