Research On Mechanism And Process Of Electroless Nickel Plating On Al And Mg Alloys

Aluminum and magnesium alloys are widely uesd in many fields: military,automobile, aerospace, mechanical and electronic components, etc, because they havemany attrative properties, such as: low density, high intensity and easy to pressureprocessing, etc. However, aluminum and magnesium alloys have some commonshortcomings. Aluminum and magnesium alloys are intrinsically high reactive, andare easily oxidized to form oxidation film in the dry air. They have poor corrosionresistance and wearability, low hardness, which are actually one of the main obstaclesto the application of aluminum and magnesium alloys in practical environments.Electroless nickel plating is one of the effective surface modifications for aluminum,magnesium and its alloys. It not only improves the wear resistance and corrosionresistance, but also improve the hardness, wear resistance and electrical contactperformance. Electroless nickel plating has widely been applied in many industries.Ni-P coating exhibits all sort of new functions, such as magnetic property, lubricationperformance, etc. Therefore, the performances of electroless nickel plating coatingson AZ91D magnesium alloys obtained via different pretreatments have been studiedin this dissertation. We aim to develop an appropriate, environmental-friendlypretreatment technique for meeting the needs of industrial production. The interfacereaction mechanism of the environmental-friendly pretreatment was analyzed. Themain factors of electroless nickel plating were studied. At the same time, thedynamics equation of the acidic electroless plating on magnesium alloys was studied.In addition, the reaction mechanism and process of direct electroless nickel plating on6063aluminum alloy in temperature （70℃） were studied. The additive （fluoridecompound） was used and the main complexing agent was adjusted in the plating bathto improve the deposition rate of the electroless nickel plating. As far as aluminumand magnesium alloys are concerned, the main salts of electroless plating solutionsmostly focus attentions on basic nickel carbonate or nickel acetate, which result inhigh-cost, low-efficiency, instability of electroless plating solutions and littleapplications. There have important theoretic and practical signification of theelectroless plating on aluminum and magnesium alloys. Additionally, the platingmechanism and process are studied, by using low-cost, long-life NiSO₄·6H2O as themain salt. In this dissertation, the electroless nickel plating on aluminum, and magnesiumalloys was studied in detail by means of weight method, Scanning ElectronMicroscope （SEM）, X-ray Diffraction （XRD）, Energy Dispersed X-ray spectrometer（EDX） Environmental Scanning Electron Microscope （ESEM） and electrochemistrymethods, etc. These measurement techniques were applied in the investigations ofpre-treatment process, the surface morphology and the cross section views of thecoatings, the analysis of the coating surface, the coating porosity and its evaluationmethod, the new electroless plating process，and so on.Experimental results showed that:1. Three different pre-treatments were studied, and the surface properties of thepretreated magnesium alloys were characterized by means of SEM, EDX andelectrochemical method. The results showed that the different degrees of corrosionwere produced on the surface of magnesium alloys. There had some deep cavities onthe surface and a coarse surface was all formed. The coarse surface can augment themechanical occlusive force between the coatings and the substrates. A compactfluoride film was formed on the surface of the specimen via （1） HNO₃+CrO₃picklingâ†'HF activation or （2） HNO₃+H₃PO₄picklingâ†'K₄P₂O₇activationâ†'NH₄HF₂activation. The fluoride film can hinder the magnesium alloys from furtherdissolution. A zinc film was produced on the substrate via （3） HNO₃+CrO₃picklingâ†'K₄P₂O₇activationâ†'zinc immersion, which can also effectively protect themagnesium alloys from corrosion.2. After the surfaces of magnesium alloys were pretreated via HNO₃+H₃PO₄picklingâ†'K₄P₂O₇activationâ†'NH₄HF₂activation. There was suitable F/O （1.2:1）on the substrate surface, which can provide appropriate active points, and effectivelyprotect the magnesium alloys from corrosion in the bath. The Ni-P coating had goodappearance, good corrosion resistance and excellent adhesion. The usage of the toxicsubstances in this pretreatment was avoided. Therefore, it was a chromium-free, lowfluoride and environmental-friendly pretreatment technology.3. In the electroless plating acidic solutions using NiSO₄·6H2O as the main salt,both nickel salt and reducing agent were generally maintained between20²8g·dm^-3.The temperature of electroless nickel plating was controlled to be80℃⁹0℃. Thedeposition rate was reduced to8.127μm·h-1and the concentration of nickel sulfatewas reduced to16.56g·dm^-3after more than twice platings. We need to supplenmentthe main salt and reducing agent arc needed in order to carry out normal electrolessnickel plating bath. 4. Appropriate concentrations of additive can accelerate the rate of chemicallyinduced nickel deposition, and realize direct electroless nickel plating on6063aluminum alloy in medium temperature （70℃）. The additive can play important rolesin accelerating the deposition rate and improving the comprehensive performance ofcoatings.5. Aminoacetic acid as a main complexing agent in the bath can controlreasonable plating rate of electroless nickel plating, and produce high quality coatingwith high corrosion resistance, good adhesion. The resistance of color degenerationalso got obvious improvement. When the additive was adjusted to2g·dm^-3and theaminoacetic was controlled to8g·dm^-3, the Ni–P coating with better comprehensiveperformance was obtained.