Electrochemical formation of green rusts in deaerated seawater-like solutions

Carbon steel electrodes were polarised at a potential similar to 150 mV higher than the open circuit potential, in a deaerated seawater-like electrolyte (0.5 mol dm(-3) NaCl, 0.03 mol dm(-3) Na2SO4, 0.003 mol dm(-3) NaHCO3). X-ray diffraction and mu-Raman analysis demonstrated that a layer mainly composed of GR(SO42-) had grown on the steel surface. GR(SO42-) was accompanied by traces of GR(CO32-). Similar experiments performed in a solution composed of 0.3 mol dm(-3) of Na2SO4 and 0.03 mol dm(-3) of NaHCO3 led to the same result. The nature of the GR forming on steel is thus mainly linked to the sulphate to carbonate concentration ratio. Finally, carbon steel coupons immersed for 11 years in the harbour of La Rochelle (Atlantic coast) were removed from seawater for analysis. The inner part of the rust layer proved to be mainly composed of magnetite, GR(SO42-) and iron sulphide FeS. This definitively confirms that GR(SO42-), as Fe3O4 and FeS, can form from steel in O-2-depleted environments. (C) 2011 Elsevier Ltd. All rights reserved.