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

To investigate the nature and properties of carbonated rust layers, carbon steel electrodes were polarised anodically at a potential similar to 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 mu-Raman spectroscopy were used to identify the electro-generated compounds. GR(CO32-) (=(Fe4Fe2III)-Fe-II(OH)(12)CO3 center dot 4H(2)O) 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 Fe-2(OH)(2)CO3 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 (c) 2012 Elsevier Ltd. All rights reserved.