The Research On Electroless Nickel Plating Process Of Magnesium Alloy AZ91D In Nickel Sulfate Bath

Magnesium alloys have many advantages such as lightness, good thermal conductivity, high strength to weight ratio and impact strength, ability to dampen electromagnetism and shock waves and being reclaimed easily, so they have been widely used in many fields such as aerospace, automobile, optics apparatus, telecommunication, acoustics materials and so on. However poor corrosion resistance is one of main causes to restrict their applications.Recent advances in corrosion control measures and electroless nickel plating of magnesium alloys were reviewed in this thesis. The condition and flow of eletroless nickel plating technology on magnesium alloy AZ91D as well as the main influencing factors of bath pH, plating temperature, main salt of nickel sulfate, reducing agent sodium hypophosphite, complexing agent citric acid and so forth were studied. Techniques, such as horizontal metallographic microscopy (HMM), inductively coupled plasma atomic emission spectroscopy (ICP-AES), X-ray diffraction (XRD), microhardness tester (MHT) and so on, were employed to determine the properties of the coating. The results are indicated as follows:(1) The formulations of acid pickling, activation, immersing zinc, copper plating and electroless nickel plating in electroless nickel plating process on magnesium alloys were studied. The optimal formulations were gained as well as the operation sequence of electroless nickel plating, i.e. ultrasonic degreasing, acid pickling, activation, immersing zinc, stripping in activation solution, then the second immersing zinc, electroless nickel plating , finally passivation treatment(water rinse with each other).(2) Electroless nickel plating was investigated at the plating temperature from 70 C to 95 C and the pH from 3.0 to 7.0. It showed that the optimal pH value and plating temperature were 4.0 and 95 C respectively. The deposition rate increased at first and decreased subsequently with the increasing concentrations of main salt nickel sulfate and reducing agent sodium hypophosphite. The deposition rate reached the maximum when the concentrations of nickel sulfate and sodium hypophosphite were 25 g dm-3 and 30 g-dm3 respectively. The deposition rate and the coating thickness decreased with the increasing concentration of complexing agent citric acid, and therefore the corrosion resistance of the coating became worse. Prolonging the plating time could improve the corrosion resistance of the coating. The plating time was usually 1 to 2hours. When ultrasonic was performed during electroless nickel plating, it could diminish and disperse uniformly the grain, improving the uniformity and the corrosion resistance of the coating accordingly.(3) The optimal condition of eletroless nickel plating on magnesium alloys was obtained by the orthogonal test. The phosphorus content of the coating gained under this condition was 7.9% and the coating was non-crystal. The Vickers microhardness value of it was 640. The specimen showed good adhesion and better corrosion resistance than magnesium alloys.(4) Magnesium alloys substrate was not corroded in the bath of nickel sulfate at plating temperature. The reaction mechanism of electroless nickel plating on magnesium alloys was described based on the principles of thermodynamics and chemical dynamics.(5) The concentrations of nickel ion and hypophosphite in the electroless nickel bath were determined with the methods of the EDTA complexing titration and indirect iodimetry respectively. The life-span of bath could be evaluated by palladium chloride test and determination of steady constant of the bath.Investigation results showed that the coating obtained in the bath of nickel sulfate is better than that in the bath of basic nickel carbonate. Meanwhile, the cost of nickel sulfate is lower. So the plating process of nickel sulfate has more practical significance in the production.