The Study of a Phosphate Conversion Coating on Magnesium Alloy AZ91D: I. Formation, Morphology and Composition

This paper involved a phosphate conversion coating on magnesium alloy AZ91D. The formation of the phosphate conversion coating was studied with potential-time curve and electrochemical impedance spectroscopy (EIS), and the morphology and composition of the phosphate conversion coating were analyzed with scanning electron microscope (SEM), X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS). The results showed that the growth process of the phosphate conversion coating could be divided into three steps. In the first step (0-40 s) the potential fluctuated sharply due to the alternate presence and absence of the metastable crystal nucleuses and the formation of the stable crystal nucleuses composed of the elements of Zn, P and O. In the second step (40-200 s) the potential moved to the negative direction because of the electrochemical inhomogeneity attributed to the growth of the stable crystal nucleuses until the magnesium alloy surface was covered by the slab-like phosphate crystals completely. In the finally step (200-1200 s) the potential move to the positive direction slowly until it leveled off because the thickness of the phosphate conversion coating increased gradually from the slab-like phosphate crystals initially. The magnesium alloy substrate was isolated from the acidic and corrosive phosphating bath by the phosphate conversion coating, which was formed completely at 1200s. The phosphate conversion coating showed a uniform, compact and integral crystal clusters and was composed of Zn3(PO4)2, Zn2Mg(PO4)2 and Zn.