The Study Of Electroplating Pretreatment On AZ91D Magnesium Alloy

Magnesium, the lightest one of industrial metal materials, is low price and can be restored. Due to its high specific strength, good shock absorption and relatively high stiffness, Mg alloy is called green metal structural material in 21st century. Mg alloy is presently used in a wide range of structural applications such as aerospace, automobiles, electronics and other industries. Therefore, finding an appropriate surface protection method to improve the corrosion resistance is a practical significance.However, magnesium with high activity and low corrosion resistance is difficult to be electroplated or chemical plated directly. It's necessary to be pretreated before plating. Firstly, this work investigated two pretreatments (zinc immersion and nickel electroless plating) on Mg alloy. Secondly, to choose an appropriate formulation, copper electroplating was employed by Hull Cell test, and the copper electroplating layer was examined by the porosity test and binding test. Finally, we examined the capability of nickel-double copper layer (copper pyrophosphate and acid copper plating) obtained by nickel electroless plating. The results show that:(1) To get a good coating, the corrosion of substrate through pickling and activation should not be too strong, and the obtained coating must be no porosity. Otherwise, the final coating layer obtained by electroplating and chemical plating will be more porosity and uneven.(2) The compound composition (C₂H₂O₄·2H₂O+NaF) bath is better than the single composition (C₂H₂O₄·2H₂O) bath in the pickling-activation. The appropriate temperature of pickling-activation is room temperature. The optimal formulations of picking-activation are 12 g/L C₂H₂O₄·2H₂O, 50 g/L NaF, T= 25³5℃, t = 60³00s. Both NaF and Na₂CO₃ in zinc immersion solution play an important role in the open circuit potential (OCP) and the surface microstructure of magnesium alloy AZ91D. The zinc immersion film obtained in the condition of continuously varied bath temperature was better than that obtained in the isothermal condition. The optimal temperature is that the operation temperature was varied continuously from 30℃to 40℃. The appropriate time of zinc immersion is 10-15min.(3)Hydrogen emitting at the process of pickling and activation plays a key role in self-catalytic process, the rate of the chemistry plating and the coating morphology. Serious hydrogen emission can cause fast deposition rate, poor adhesion of coating and the spontaneous decomposition of the bath. The optimal formulations of chemistry plating are electrolyte polishing (25³0 g/L Na₂CO₃, 20²5 g/L Na₃PO₄ T:65℃, DA/A·dm^-2:80⁹0, t:1²min), hot water splashing, cold water splashing, electrolyze degreasing(14¹2g/L H₂SO₄, 88⁸6 g/L H₃PO₄ T:70⁸0℃, DA/A·dm^-2:15²0, t:2³min), hot water splashing, cold water splashing, acid pickling(24ml/L H₂SO₄, 78ml/L HNO₃ room temperature, t:1²min), hot water splashing, cold water splashing, activation(375ml/L HF room temperature, t:10²0s), cold water splashing and nickel electroless plating.(4) The Hull Cell test shows that copper pyrophosphate electroplate has good capability, the optimal formulations are 60g/L Cu₂P₂O₇, 300g/L K₄P₂O₇·3H₂O, 40 g/L K₂HPO₄·3H₂O, 0.2 g/L phytic acid, 40 g/L C₄H₄O₆KNa·3H₂O, 0.05⁰.1 g/L vanillin, T:50℃, pH=7.89.(5) The porosity of the multi-layer coating obtained from the processes of pickling, activation, nickel electroless plating and double copper electroplating is reduced apparently. The coating layer, which has no bubbling after 20 times thermal shock test, is bound on the substrate strongly. Finally, the coating obtained by acid copper plating is so even that another metal can be plated on it.(6) This paper is the fruit of Technology Bureau of DaLian item (NO.2007A10Gx122).