Study On The Influence Mechanism Of PH Value On Electroless Nickel Plating On Magnesium Alloy

Magnesium alloys have excellent comprehensive performances, they arerecognized as “metal of the21st century” and are widely applied in automotive,aerospace, electronic, communication industries, etc. However, magnesium alloyshave poor corrosion resistance due to their high chemical reactivity, which hasrestricted their widespread development and applications in many areas. Electrolessplating on magnesium alloys can greatly improve the corrosion and wear resitance,solderability, electrical conductivity and decorative appearance of magnesium alloys.However, the electroless plating solution is a thermodynamically unstable system andthe bath composition was changing in the process of electroless nickel plating,especially the dramatic changes of the pH value will cause strong corrosion ofmagnesium alloy in nickel sulfate bath, also have a strong impact on the electrolessnickel plating. Consequently, it is an urgent issue to maintain the pH value of the bath.In this article, the influence of pH value on the electroless plating, the changeregulation and distribution of these ions during the electroless plating, methods andprinciples of the stability of the pH value and the effection of different pH buffers ofacidic and alkaline electroless nickel plating solution were studied in detail, whileoptimal solution was determined, and the effects of the electroless nickel bath withhigh stability were displayed. The experimental results are as follows:1. The pH value played an important part on the microstructure, electrolessnickel plating and electrochemical characteristics of magnesium alloy. With theincrease of pH, the deposition rates of the Ni-P coating increased linearly, phosphoruscontent in the coating reduced, the structure of the electroless Ni-P coatingtransformed from amorphous to crystalline, the morphology of the coating, thecorrosion potential and the corrosion current changed, too.2. During the electroless nickel plating, the concentration of H+would increaseand the pH value would reduce. Without buffer agent, excess H+could not beconsumped, the stability of the bath could not maintain. Therefore it was necessary toadd some kinds of buffer agents to stable the bath pH in the proper range.3. With the comparison of the buffer capacity of several buffer agents, it wasfound that the buffer capacity of ammonium acetate was good. The quality of thecoating and corrosion resistance of ammonium acetate was better than that of succinate ammonium. So it was the best choice to choose ammonium acetate as thebuffer agent. After the investigation of the concentration of buffer agents, it wasoptimal to add ammonium acetate with20g·dm^-3, considering the buffer capacity ofthe bath, disposition rate and corrosion resistance of coatings. In the MTO, with theaddition of ammonium acetate with20g·dm^-3, pH of the bath had been maintained,the stability of the bath had been improved and the bath life had been extended fromfive to ten MTOs, almost increased by one time.4. The electroless nickel-plating on AM60magnesium alloy was successfullycarried out in an environmentally friendly non-fluoride plating bath withNiSO₄·6H₂O as the main salt. In addition, Na₂CO₃was found to be an appropriatebuffer for the electroless plating on magnesium alloy by examining the corrosion rateof magnesium alloy immersed in the test solutions. It was found to have a stronginfluence on the plating bath. The addition of Na₂CO₃not only promoted the growthrate of the plating coating, but also affected the adhesion between the electrolessplating coating and the substrate. The addition of Na₂CO₃also helped maintain thepH value of the plating bath within the optimum range. At70℃, disposition rate wasfast enough, the bath was stable, can be used. In contrast with acid electroless plating,basic electroless plating still had to be improved.