Comparative corrosion study of zinc phosphate and magnesium phosphate conversion coatings on AZ31 Mg alloy

In this study, zinc phosphate and magnesium phosphate conversion coatings were formed on AZ31 Mg alloy (AZ31) by immersion treatment in zinc phosphate and magnesium phosphate conversion coating (ZPCC and MPCC) solutions. The coatings were characterized by a scanning electron microscope (SEM), energy dispersive X-ray spectroscopy (EDS) and X-ray diffraction (XRD). The corrosion behavior and corrosion resistance of coatings were studied by immersion test, potentiodynamic polarization curves and salt-spray tests (SST) with and without paints coated by electrophoretic deposition. During the immersion test, corrosion initiated much slowly on the MPCC than ZPCC. The ZPCC-coated samples showed pitting corrosion while filiform corrosion was observed on the MPCC-coated samples. Although ZPCC showed thicker coating, more positive corrosion potential and lower corrosion current density than MPCC, but it was very susceptible to corrosion under salt-spray conditions comparing with MPCC. The electropainted MPCC also showed much longer stability under salt-spray conditions than electropainted ZPCC.