Investigation On Film Formation Mechanism, Microstructure And Performances Of Complex Zinc Phosphate Coatings Of Magnesium Alloy

Chromate conversion coating was usually performed on magnesium alloy substrates for preventing from corrosion. In consideration of the environment friendly technology, zinc phosphate coating should be one promising method for magnesium alloys anticorrosion and pretreatment process before paint, tends to replace chromate treatment where the environment polluted hexad chromium was involved.Zinc phosphate coatings have been successfully used to underneath the paint of steel and aluminum for many years because they can enhance the adhesion of the paint and substrates. Phosphatization of magnesium alloy is rather difficult in comparison with the traditional phosphatization of steel and aluminum because of high electrochemically activity of magnesium. In the acid phosphating bath resolving rate of magnesium must be restrained in order to obtain dense and high quality phosphate coating. Although the growing interests in magnesium alloys, there are only limited studies about phosphate treatments on the surface of magnesium alloy.Han et al. obtained a phosphate film of Mn₃(PO₄)₂ on AZ31D magnesium alloy in a bath containing phosphate and manganese in 2003. Phosphate films were gained from phosphate-permanganate bath, which have micro cracks,similar microstructure to chromate conversion. A zinc phosphate (Zn₃(PO₄)₂.4H₂O)...