Enhancement of Adhesion Properties, Corrosion Resistance and Cathodic Disbonding of Mild Steel-Epoxy Coating Systems By Vanadium Conversion Coating

In this study, vanadium conversion coating(VCC) was deposited on the surface of soft-cast steel (St-37).  A thermoset coating was enforced on the VCC and blank substrates. The surface was characterized by field emission scanning electron microscope (FE-SEM), elemental mapping of energy dispersive X-ray (EDX) and atomic force microscopy (AFM). The adhesion strength of the thermoset coating on the surface of the treated samples was measured before and after 35 days of submergence in 3.5 wt. % NaCl solution via a pull-off device. Meanwhile, the effect of VCC treatment on the thermoset coating (epoxy) was examined by electrochemical impedance spectroscopy (EIS) and cathodic delamination (CD) tests. FE-SEM, EDX and AFM studies of treated surfaces showed a homogeneous vanadium oxide/hydroxide layer precipitated on the metal which increased the surface roughness. It was shown that VCC significantly improved the corrosion resistance of the epoxy coating. The vanadium compounds also reduced the cathodic activity on steel resulting in lower cathodic disbandment. The lower adhesion loss was also observed on the vanadium treated sample in comparison with the blank one.