Research On Bath Stability Of Electroless Plating Nickel On Magnesium Alloy

Magnesium alloys are the lightest metal structure materials in the practical application, which have many merits such as high strength and stiffness, high electromagnetic shieldlessness, good dampness and excellent cutting characteristics, etc. Therefore, they have prosperous applications in many fields such as aerospace, automobile and electronic components, and so on. However, the chemical properties of magnesium alloys are very active. The standard electrode potential is -2.36V and poor corrosion resistance restricts the application of magnesium alloys. Chemical nickel coating with high corrosion resistance and wear resistance can be obtained on magnesium alloys and uniform coating can be gained especially on complex castings. However, electroless nickel plating solution is an instable system in thermodynamic. In the industrialized bath particles, solid microparticles exist inevitably. Because these particles have a larger active surface, a large number of nickel ions have been consumed. When there are many particles in the solution, it is even decomposed. Consequently, it is an urgent issue to solve life of electroless nickel plating solution.Protection methods of magnesium alloys had been summarized in this article. The latest research progresses of pre-treatment before electroless nickel, electroless nickel plating were introduced. The influence of stabilizers on the stability of electroless nickel plating solution of magnesium alloy was studied. The effect of thiourea and potassium iodate on the stability of electroless nickel plating solution was tested. The thermodynamic properties of electroless nickel were analyzed. The kinetic equation of reaction rate of electroless nickel plating was determined. There was an important meaning for research of deposition mechanism of electroless nickel plating and production practice. The effect of complexing agents on the stability of electroless nickel plating solution was studied and the effect of the states of magnesium alloy substrate on the initial deposition mechanism of electroless nickel plating was explored. The research results show that:1. The effect of thiourea on the bath stability and deposition rate is more significant than potassium iodate. 0.5 mg·dm^-3 thiourea in the bath can improve not only the deposition rate, but also increase its stability. The catalytic efficiency of deposition on magnesium alloy surface is raised.2. The deposition rate equation and the apparent activation energy of reaction were found through the study of reaction kinetics. The apparent activation energy E_a was 38.03 kJ·mol^-1, which was a smaller resistance of deposition of nickel particles.3. Compared to a single ligand and binary ligands, ternary ligands did not affect the performance of the coating, but affected the stability of the solution. The stability of bath containing binary ligands of citric acid and succinic acid was slightly higher than that in the single ligand bath, but there was higher stability in ternary ligand bath.4. Compared pyrophosphate potassium process, the better qualitive coating was obtained through hydrofluoric acid activation. The initial deposition mechanism of electroless nickel plating on magnesium alloys was: the first replacement deposition happened onβphase on magnesium alloy and then autocatalytic deposition progressed on first deposited nickel particles onβphase.5. The Ni²⁺ concentration in the bath was measured by EDTA complexometry. The measured concentration Ni²⁺ was close to real value. There was smaller deviation, so nickel sulfate was added in the bath according to the requirement. Sodium hypophosphite was complemented according to the ratio of Ni²⁺ and H₂PO₂^-consumption in the reaction. Consequently, the life of bath was maintained.