Electrochemical formation of green rusts in deaerated seawater-like solutions

Carbon steel electrodes were polarised at a potential ∼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 μ-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 O2-depleted environments.

► Sulphated green rust could be electro-generated on carbon steel in anoxic seawater-like electrolytes. ► Rust layers grown during 11 years on carbon steel in natural seawater were thoroughly characterised by μ-Raman spectroscopy. ► The mechanism of marine corrosion of carbon steel in anoxic conditions could be specified.