Investigation Of Direct Electroless Ni-P Plating On AZ91D Magnesium Alloy

Magnesium alloys are the lightest metal structure materials in the practicalapplication. They have been widely used in aerospace, automobile and electronics industryfor its outstanding characteristics, such as high strengh-to-weight ratio, highelectromagnetic shieldlessness, good dampness, good damping property and machinability,etc. However, the chemical properties of magnesium alloys are very active, poor corrosionresistance has often prevented it from further exploitation. Before used magnesium alloyworkpiece must be protected by certain surface treatment processes, Electroless plating onthe surface of Magnesium alloys is an effective method. Ni-P coating with high corrosionresistance and wear resistance can be obtained on magnesium alloys and uniform coatingcan be gained especially on complex castings.This article mainly focused on electroless plating on AZ91D magnesium alloy.Electroless Ni-P plating obtained on AZ91D magnesium alloy is smooth and bright, andhas a high adhesion strength. Then of pretreatment process before electroless has beenoptimized. The effects of some factors including main salt, reducing agent, pH andtemperature to electroless rate are studied. The deposit coating morphology,microstructure, composition and crystallization temperature are determained and analysedby using SEM observation and EDS analysis, XRD analysis, DSC analysis. Finally, theproperties of electroless plating before and after heat treated has been studied by salt sprayexposure and electrokinetic potential polarization curve research techniques.The hardness of electroless Ni-P plating is about decuple as the bare alloy. Highhardness could extend its using life. The electroless Ni-P plating has high corrosionresistance in the5%sodium chloride solution. The corrosion potential of electroless Ni-Pplating shifts about0.8V to the positive potential direction than the corrosion potential ofthe bare alloy, and no peeling occurs on surface of the plated sample until96hours lasted.It shows that the corrosion resistance of the plated sample improvs a lot. After thecrystallization at annealing condition, the hardness of electroless Ni-P plating furtherimproves, but the corrosion resistance drops with the plated sample before the crystallization.