Electrochemical formation of carbonated corrosion products on carbon steel in deaerated solutions

Highlights: ► Green rust is electro-generated at low NaHCO3 concentration (0.003 mol dm−3). ► Chukanovite and carbonated green rust are obtained in NaHCO3 + Na2SO4 deaerated electrolytes. ► The mechanisms of formation of carbonated corrosion products of carbon steel are specified. - Abstract: To investigate the nature and properties of carbonated rust layers, carbon steel electrodes were polarised anodically at a potential ∼100–200 mV higher than the open circuit potential in NaHCO3 solutions (0.003, 0.1 and 1 mol dm−3) continuously deaerated by an argon flow. X-ray diffraction and μ-Raman spectroscopy were used to identify the electro-generated compounds. GR(CO32−) (=FeII4FeIII2(OH)12CO3·4H2O) is observed at 0.003 and 0.1 mol dm−3 NaHCO3 whereas FeCO3 is obtained at the largest concentration (1 mol dm−3). GR(CO32−) is accompanied by magnetite Fe3O4 at the lowest NaHCO3 concentration. The current density decreases to negligible values in each case, indicating that a passive film also forms independently of the nature of the carbonated compound. Experiments were performed similarly in solutions of NaHCO3 and Na2SO4. Chukanovite Fe2(OH)2CO3 could be obtained in solutions containing 0.03 mol dm−3 of each salt. In contrast with the results obtained in the solutions free of sulphate, the current density remains important during the formation of the rust layer