A green vanadium-based formulation for the conversion of steel

The electroless deposition of a novel vanadium-based conversion coating (VCC) on carbon steel was investigated varying parameters as vanadium salt concentration, immersion time, bath pH and temperature. The novelty resides on the deposition of a protective layer from a V4+ solution comprising green reducing agents like ascorbic and citric acids. The VCC was found to be amorphous, composed by closely packed particles rich in vanadium oxides/hydroxides. Corrosion resistance was evaluated by linear polarization resistance, Tafel extrapolation and electrochemical impedance spectroscopy. Such conversion coating provided an optimum corrosion performance for steel substrates immersed in sulphates and chlorides rich solutions. An optimum was achieved with a 0.1 M KVO3 solution held at 45 & DEG;C, at pH = 3 and a deposition time of 10 min. Higher values of the latter parameters did not provide any further improvements: the formation of cracks during the dehydration process compromised the coating integrity.