Green Rusts and Their Relationship to Iron Corrosion; a Key Role in Microbially Influenced Corrosion

Abstract

Mossbauer spectroscopy (MS) has often been used to characterise double-layered hydrox-ysalts usually named green rusts (GR) and to follow their Fe(II)/Fe(III) ratio during the oxidation process of Fe(OH)2 in the presence of aggressive anions such as CI−, SO 4 2− , CO 3 2− ,.... They are intermediate compounds between the initial metal Fe(0) via the Fe(II) and the final Fe(III) (oxy-hydr)oxides constituting the usual rusts. E-pH Pourbaix diagrams of iron for predicting the aqueous corrosion conditions of iron-based materials are determined by monitoring the electrode potential E h and pH vs. time. The crystal structure of GRs, in any case constituted of layers of [Fe (1−x) II Fe x III (OH)2] x+ that alternate with interlayers [(x/n)A n− · (mx/n)H2O] x− made of A n− anions and water molecules, are presented. Several examples of the role of GRs are discussed, from chloride pitting of concrete reinforcing bars to bacterial corrosion of cast iron in water pipes or steel sheet piles in harbours. The efficiency of corrosion inhibitors like phosphate and their relationship to the oxidation of GRs are presented from basic MS studies. But most importantly, the evidence by MS of the dissimilatory reduction of a common ferric oxyhydroxide, γ-FeOOH lepidocrocite, into a GR by the action of a bacterium, Shewanella putrefaciens, opens the path through which microbially influenced corrosion (MIC) operates. A cycling of aerobic and anaerobic conditions is necessary where GRs but also magnetite play likely the key role.