Preparation and characterization of coatings on carbon steel obtained by PEO in silicate/carbonate electrolyte

• Coatings containing SiO2, Fe2O3 and Fe3O4 were obtained on carbon steel, by PEO.
• XRD analysis revealed the structure of amorphous SiO2 and polycrystalline structure of iron oxides.
• In-depth distribution of Si and Fe have been revealed by SEM and GIXRD.
• The corrosion protective properties were illustrated by salt spray tests and potentiodynamic polarization measurements.

Coatings containing amorphous silicon dioxide have been obtained by plasma electrolytic oxidation (PEO) of carbon steel S235JR, in two water-based solutions containing sodium silicate/carbonate, with pH=11.2 and pH=10.5, respectively. The aim of this work was to obtain surface anticorrosion protective layers. The coatings have been analysed by X-ray Photoelectron Spectroscopy (XPS), Scanning electron microscopy (SEM), Energy dispersive spectroscopy (EDS), X-ray diffraction (XRD), metallography, salt spray tests and potentiodynamic polarization measurements. The coatings comprised a mixture of the amorphous phase of SiO2 with polycrystalline iron oxides Fe2O3 and Fe3O4 phases, with the thickness in the range of 10–198 μm. During PEO formation of the anticorrosion layer, the surface became uneven and heterogeneous and present a structure of clusters mainly formed from silicon dioxide and iron oxides.