Influence of organic matter/bacteria on the formation and transformation of sulfate green rust

Corrosion processes of carbon steel permanently immersed in natural seawater are in any case influenced by microorganisms due to the important biological activity. The influence of sulfide-producing bacteria is well known but various microorganisms, the biofilm itself, and the numerous organic species associated with biological activity may also play a role. Because the sulfate green rust GR_SO4 is one of the main corrosion products of carbon steel in seawater, the role of organic matter/bacteria on its formation and transformation was addressed. GR_SO4 was precipitated from Fe(II) and Fe(III) salts in the presence of various marine bacterial species. Abiotic experiments were performed for comparison, a first one without any organic species, a second one with sodium acetate added. The obtained aqueous suspensions were aged at room temperature for 1 week. The number of bacteria (UFC/mL) was followed over time and the solid phases were characterized by XRD. Whatever the fate of the bacteria (no activity, or activity and growth), the formation of GR_SO4 was favored and its transformation to magnetite completely inhibited. This effect is attributed to the adsorption of organic molecules on the surface of the GR_SO4 crystals. A similar effect, though less important, was observed with acetate.