Study On Process, Microstructures And Properties Of Electroless Plating Ni-P Film On Die-casting Magnesium Alloy Components

More eyes are focused on magnesium alloy because of its great deal of advantages in recent years. The products of Magnesium alloy have been used widely especially in electron industry, but their poor corrosion resistances are the main hindrance. To improve the corrosion resistance of magnesium alloy, surface treatment is taken into use to form a protective layer. This paper carries on technology, structure and properties of electroless Ni-P on die-cast magnesium alloys AZ91D by TEM, SEM, DSC, GDA, salt fog, electrochemical apparatus; The conclusions are obtained as following:1.The P content of electroless coating on magnesium alloys is mild (6.22wt%), the structure of depositing coatings are mixtures of amorphous and small amount of nano-crystals. The layers' average thickness is 18.5 μ m after depositing for 120min. It's shown that coating is coherent to matrix very well by curling experiment.2.The process of electroless plating can be divided into two steps: metal Ni precipitates at firstly, because of the reaction between Ni ions and matrix Mg. Second, with the increasing and growth of Ni nucleus, catalyzed layers of Ni are produced, then Ni and P deposits simultaneously.3.The layer containing 6.22wt% P begin to crystallize at 300 ℃, the crystallizing process finished at 450℃. The process ofcrystallization has three steps, the spreading of atoms in the limit specific areas, nano-grain appearing and growing up; New metastable phases appearing and changing into the stable phase; the grain forming and growing up. Calculating activation energy Ec is 298.7KJ.4.After electroless Ni-P on matrix of magnesium alloys, annealed at temperatures between 200°C and 450°C, the adhesion force show slightly drop before crystallizing. When crystallizing course began, adhesion force is rising fast reaching 3.7 Mpa at 350 °C. When the temperature exceed 350 degrees, adhesion force tended to dropping.5.The hardness of Ni-P coatings have the same variety with adhesion force. The maximal hardness reached 825 + 5HVS1000 at 450 °C.6. With the rising of heat treatment temperature, the impedance value of layer drops.7.Polarized experiment indicated that: the lowest corrosive current of initial coating is 4.52 u A/cm2, and corrode electric potential Ecorr is -250mv. While the annealed temperature is lower than 200 °C, the corrosion-resistant performance of plating layer improves to some extent, annealed at higher than 200 °C , corrosion-resistant performance has a downward trend wholly.