Investigation On Electroplating Of Magnesium Alloy Products And Mixing Nickel Plating

Magnesium alloys are regarded as one kind of green metallic structural materials in 21st century. More and more attentions are paid to their wide applications, such as land transportation, aerospace aircraft, 3C products, home goods et al. However, magnesium alloys have high chemical activity and poor corrosion resistance, which have limited their widespread applications.In recent years, there are many reports about electroplating and electroless nickel plating on magnesium alloys, but most of them are merely applied in laboratory research, the results have not been tested in production practice. In this thesis, many laboratory achievements of electroplating and electroless nickel plating on magnesium alloys in the past few years were put into production of real workpieces of magnesium alloys. According to the production practice, a new pickling bath without toxicity and a new mixing nickel plating method were developed to make up for the faults of electroplating and electroless nickel plating. The results show that:(1) The products of magnesium alloys are plated by either electroless nickel plating or nickel electroplating as a rendering layer. The subsequent composite Cu/Ni/Cr coatings, which are applied in rigorous environment, are electroplateded. At the same time, The causes of several disfigurements produced in production practice and solving measures during electroless nickel plating process and nickel plating process were found. During electroless nickel plating, blisters on the chemical nickel coating are mainly produced by corrosion of the magnesium alloy surface in the plating bath. It is very important to control pickling and activation processes so that the surface of magnesium substrate has a suitable protective film. Uneven zinc film will affect conductivity of magnesium alloys in process of electroplating nickel and make the coating coarse and poriferous. Pin-pores on the nickel coating will influence the subsequent plating processes because the next acidic plating bath would infiltrate to magnesium substrate and corrode it. Residual impurities in the plating bath would cause burrs on nickel coating. Filtering plating bath can avoid the production of burrs.(2) The pickling bath, which is composed of 20 g·dm^-3Na₂MoO₄·2H₂O, 300 cm³Â·dm^-3 85%H₃PO₄, 30 cm³Â·dm^-3 65%HNO₃, is developed before electroplating nickel on magnesium alloys. The developed bath lowers the dosage of H₃PO₄ and it strengthens the protection for Mg substrate after oxide film is removed. Different type casts of magnesium alloys have different time, 20-30 s for AZ91D, 50-70 s for AM60. The nickel coating obtained through developed pickling pretreatment has higher corrosion potential in 3.5% NaCl bath than that in the pickling bath only containing H₃PO₄ + HNO₃. That is, Na₂MoO₄·2H₂O can improve some performances of pickling bath, so it is an effective method to enhance the adhesion of nickel coating.(3) When the concentration of NaH₂PO₂ reaches 25 g·dm^-3 in mixed nickel plating bath, NaH₂PO₂ can improve the dispersive ability of plating bath, so highly qualitive nickel coating can be obtained. The pH 9 bath is an alkaline solution. In pH 9±1 plating bath, the good nickel coating is obtained when electroplating is conducted by the density of 1-2 A·dm^-2 at 65-70℃. The pulse plating can further improve the quality of nickel coating. Good nickel coating with high adhesion and low porosity can be obtained through controlling appropriate the parameters of pulse plating.