Microstructure and property of zinc phosphate coating on die-casting magnesium alloy AZ91D
(1.吉林省长春市吉林大学材料科学与工程学院教育部汽车材料重点实验室2.浙江省温州市浙江工贸职业技术学院先进材料研究发展中心3.吉林省长春市吉林大学汽车材料系材料科学与工程系重点实验室)
Abstract: A surface treatment method was described, which can form a uniform and dense phosphate conversion coating on the die -casting magnesium alloy AZ91D in a non-chromate and non-nitrite bath. The coating consists of Zn3(PO4)2？4H2O, Zn, AlPO4 and MgZn2(PO4)2 analyzed by XRD. The SEM results show that the microstructure of the zinc phosphate coating transfers from flower-like to slab-like crystals with the increase of immersion time of magnesium alloy samples in the phosphating bath. The zinc phosphate coating formed in the bath with immersion time of 1 min is denser because metallic Zn and insoluble phosphate crystals co-deposit on the magnesium alloy surface and the growth of the crystals are restricted by each others. The zinc phosphate coating on the magnesium alloy is used as the base layer for further cataphoric and powder paintings. The cataphoric painting on AZ91D alloy based on phosphate coating has similar adhesion and corrosion-resistance to that based on the chromate conversion coating. But for powder painting, the former exhibits better adhesion property than the latter, due to the uneven microstructure and the enough thickness of the phosphate coating.
Key words: magnesium alloy; AZ91D; phosphate coating; adhesion; anticorrosive