Investigations on corrosion resistance of steel coated with chromium-free ZTM conversion coating

Chromium-free conversion coatings based on Zr/Ti/Mo (ZTM) compounds via chemical conversion technology were investigated to improve the corrosion performance of SPCC-JISG 31 steels. A preliminary study was carried out to evaluate the influences of critical parameters, including the concentrations of active species Zr, Ti and Mo, and the conversion bath's pH, on the protective efficiency using a Statistical design of experiments (DoE) methodology. Passivation layer with compositional Mo, Ti and Zr elements has been employed on the steels by dipping in mixed solutions containing 17 g/l Na₂MoO₄, 1 g/l K₂TiF₆ and 7 g/l K₂ZrF₆ under pH = 5. The morphology and elemental analysis of Zr/Ti/Mo on surface of the steels was studied by the employment of scanning electron microscopy with energy dispersive spectroscopy (SEM/EDS). Pull-off adhesion test was performed, indicating that the ZTM coatings enabled to increase the adhesion strength (6.0 MPa) of mild steel to the organic coating than that of traditional phosphate (ZrP) coating. The results of electrochemical impedance spectroscopy and the salt spray test (529 h) evidenced the higher corrosion resistance of the Zr/Ti/Mo coating compared with theconventional phosphate coating.