Green rusts in electrochemical and microbially influenced corrosion of steel

Green rusts have been identified as corrosion products of steel in neutral or slightly alkaline aqueous media. They were mainly observed in carbonated media, where the carbonated green rust is obtained, and in seawater, where the sulphated variety is obtained. In the first case, the formation of the carbonated green rust competes with that of siderite FeCO 3 . It is favoured when the dissolution of iron is accompanied by the reduction of dissolved oxygen and the formation of OH − ions. In the second case, the formation of the sulphated variety competes with that of the chlorinated variety. The sulphated green rust is obtained since the layered structure of green rusts is characterised by a strong affinity for divalent anions. Finally, the oxidation of green rusts leads to the various constituents of ‘common’ rust. The conditions favouring the formation of a ferric compound keeping the crystal structure of green rusts is discussed.