The Investigation Of Fabrication And Performances Of Zinc Phosphate Coatings On Magnesium Alloy

Magnesium alloy has low density, high strength-to-weight ratio and other good physical and mechanical characteristics. However, the application of magnesium alloys has been limited due to their undesirable properties including poor corrosion and low wear resistance. Thus, the formation of anticorrosion and high wear-resistance coatings on the surface of Mg or Mg alloys is necessary in practical applications. Since magnesium is one of the most electrochemically active metal, ordinary coatings, such as nickel, copper and zinc coatings, can only provide a physical barrier to corrosion attack of magnesium substrate. So, any coatings on magnesium alloys should be as uniform, adherent and porosity free as possible.Chromate conversion coatings have been used to prevent the magnesium alloy from corrosion and provide a base for the further paint. However, the hexavalent chromium used in the treatments bath is carcinogenic. In consideration of the environment friendly technology, zinc phosphate coating should be one promising method for replacing the chromate conversion treatment. 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. However, it is difficult to obtain Phosphate coating on magnesium alloys because of the high electrochemically activity of magnesium. Although a lot of investigations focus on magnesium alloys, there are only limited studies about phosphate treatments on the surface of magnesium alloy. Phosphate coatings of Mn3(PO4)2 were obtained in phosphate-permanganate baths. Han et al obtained a phosphate coating of Mn3(PO4)2 on AZ31D magnesium alloy in a bath containing phosphate and manganese. Kouisni et al studied the growth and the electrochemical behavior of zinc phosphate coatings containing mainly hopeite crystals on magnesium alloy AM60.The preparing processes, influence factors, formation mechanisms andperformances of the zinc phosphate coatings have been studied in detail. The dense and fine complex zinc phosphate coatings containing 20-30nm nanocrystalline zinc particles have been prepared.The compositions and micro-structure of the coating were examined by XRD and the morphography of the coating was analyzed with SEM. Dense and crack-free complex zinc phosphate coatings have been prepared The compositions of the coating are Zn3(PO4)2?4H2O, Zn, AlPO4 and MgZn2(PO4)2 analyzed by XRD patterns. SEM has shown that very fine zinc particles surrounded phosphate crystals and filled in the interstice of the insoluble phosphate. The adhesion of magnesium alloy and the zinc phosphate coating was better than that of chromate conversion coating, due to the uneven structure of the phosphate coating.sodium metanitrobenzene sulphonate, SMBS(the abridge will be used in the follows)was used to the accelerating reagent for replacing nitrite and its influence on the characteristics of the zinc phosphate coatings on AZ91D magnesium alloy was investigated. The results of this study showed that addition of sodium metanitrobenzene sulphonate greatly shortened the formation time of phosphate film from 55 minutes to about 4～5 minutes. The phosphating process can be accelerated because sodium metanitrobenzene sulphonate absorbed on the anode areas of the magnesium alloy to restrain the resolving of the substrate magnesium and to facilitate the nucleation of zinc phosphate. As a result, denser and fine phosphate crystals formed on the magnesium alloy substrate.The pH of the phosphating bath strongly affects the microstructure and phase chemical composition of the zinc phosphate coating on AZ9lD magnesium alloy. SEM and XRD indicate that fully compact and dense phosphate coatings were formed in the phosphate baths with pH in the range of 2.15～2.5. When the pH of the phosphating bath is lower than 2.15, the edges of slabs of crystal became round and the substrate was not fully covered by phosphate crystals. That is because the formed phosphate crystalstend to resolve in the strong acidic bath。When the pH of the phosphating bath is more than 2.50, phosphate crystals is difficult to form because the resolve of magnesium is too slow. As the pH of the bath decreases,zinc phosphate crystals of slab-like in the coating increases.The growth process of phosphate film on the magnesium alloy substrate was investigated by SEM observation and EDS and XRD analysis. Phosphate crystals deposit only on theαphases of AZ91D magnesium alloy in a non-additive phosphating bath. In phosphating bath containing additives, however, additives are absorbed on the surface of magnesium alloy and a great number and uniform nucleation of Zn3(PO4)2?4H2O crystals and metallic zinc particles deposit on bothαandβphases simultaneously. Zn3(PO4)2?4H2O crystals deposit on the"β-microcathodes"and the mixtures of Zn3(PO4)2?4H2O and zinc deposit on the"α-microcathodes". Only a small part ofαphases are the microanodes in the electrochemical formation reaction. In the early stage(0-60s) of phosphatization, both hopeite and metallic zinc nucleated and grew together on the substrate to form flower structure. A great number and uniform nucleation of hopeite and metallic zinc formed on the substrate simultaneously. Afterwards, because magnesium alloy substrate was almost fully coverd, only hopeite deposited continuously to form slab-like structure. During phosphatization as soon as the metal (or alloy) sample is soaked in the phosphating bath, the surface of sample divided into micro anode sites and micro cathode sites. The hydrogen moved towards micro cathode, and hydrogen given out from microcathode sites simultaneously. The reduction of hydrogen ions results in the increase of local pH at metal-solution interface, which facilitated the precipitation of insoluble phosphate and formed the phosphate coating.