Electrochemical properties and structure of vanadium-based conversion coatings on electro-galvanised steel

The corrosion resistance of vanadium-based conversion coatings on electro-galvanised steel (EG) was examined in terms of the different valence species V⁵⁺ and V⁴⁺ as the raw material in the treatment solution. The effects of mixing phosphoric acid with the vanadium-based conversion coating have also been investigated. Corrosion performance was tested by neutral salt spray test and the mechanism of corrosion resistance was analysed by electrochemical impedance spectroscopy and X-ray photoelectron spectroscopy. It was found that conversion coatings formed from V⁵⁺ or V⁴⁺ did not clearly show different corrosion behaviour, but that the addition of phosphoric acid to the solution including V⁵⁺ improved corrosion protection. These results indicate that mixing phosphoric acid lowers the pH of the solution and improves corrosion resistance. The role of phosphoric acid is also explained by the formation of hydrogen bonds at the interface between the vanadium species and EG substrate.