The Research On New Process Of Electroless Plating Nickel On AZ91D Magnesium Alloys Via Twice Activation

Owing to their low density, excellent anti-shock resistance, good dampness and high strength-to-weight, magnesium alloys have been extensively applied in military, automobile, motorcycle, electronic communication industry, etc. However, magnesium alloys have poor corrosion resistance, which restricts its application. Electroless nickel is an effective surface treatment method of magnesium alloys. However, there are toxic substances in traditional pretreatment. The performances of electroless nickel plating coatings of AZ91D magnesium alloys obtained via different pretreatments have been studied in this article, and obtained an appropriate, environmental-friendly pretreatment. The interface reaction mechanism of the environmental-friendly pretreatment was analyzed. The main factors of electroless nickel plating were studied. And the concentration of nickel source in the plating bath with the number of plating was studied. The experimental results are as follows:1. Different degrees of corrosion were produced on the surface of magnesium alloys, there were some deep cavities on the surface and a coarse surface was all formed. The coarse surface can augment the mechanical occlusive force between the coatings and the substrates. A compact fluoride film was formed on the surface of the specimen via HNO3+CrO3picklingâ†'HF activation or HNO3+H3PO4picklingâ†'K4P2O7activationâ†'NH4HF2activation. The fluoride film can hinder the magnesium alloys from further dissolution. And a zinc film was produced on the substrate via HNO3+CrO3picklingâ†'K442O7activationâ†'zinc immersion,which can also effectively protect the magnesium alloys from corrosion.2. The surfaces via different pretreatments were low porosity of nickel coating. No obvious surface damages were observed, which ensure the good corrosion resistance of the coatings. Ni-P coatings had typical cellular shape. There were higher corrosion potential of the coating obtained via HNO3+H3PO4picklingâ†'K44P2O7activationâ†'NH4HF2activation and lower corrosion current, which ensure that its coating had perfect density. Monoever, the adhesion of the coating was well via the pretreatment.3. There was suitable F/O (1.2:1) on the substrate surface via HNO3+H3PO4picklingâ†'K4P2O7activationâ†'NH4HF2activation, which can provide appropriate active points, and effectively protect the magnesium alloys from corrosion in the bath 4. The Ni-P coating had good appearance, good corrosion resistance and excellent adhesion. And the toxic substances were avoided in this pretreatment. Therefore, it was a chromium-free, low fluoride and environmental-friendly pretreatment technology.5. Deposition rate and performance of coating under various concentrations of nickel salt and reducing agent were determined. Effect of plating temperature on deposition rate and performance of coating were studied, too. Through synthetical analysis of the deposition rate and the economical efficiency, both nickel salt and reducing agent were generally maintained in between20～28g·dm-3. Electroless nickel plating temperature was controlled to80℃～90℃. The consumption of Ni2+concentration in the bath was measured by EDTA complexometry. The deposition rate reduced to8.127μm·h-1and the concentration of nickel sulfate reduced to16.56g·dm-3after more than twice platings. The plating bath started to become instable and the obtained coating was poor.