Zinc Dipping Solution And The Mechanism On Mg-Li Alloy

Zinc dipping technique is often used as pretreatment for the plating of Mg alloy. When traditional zinc dipping solution is applied to the surface treatment of Mg-Li alloy, heat treatment process and long zinc dipping time are necessary, and asymmetric zinc-dipped layers are often obtained. Therefore, it is very important to investigate a suitable zinc dipping solution for Mg-Li alloy.In this paper, five kinds of organic acid salt complexing agents were used to prepare zinc dipping solutions, in which fine, compact and homogeneous zinc-dipped layers were formed on Mg-Li alloy surface. SEM images, X-ray diffractometer and electrochemical analyzer were employed to analyze the micromorphology, the surface composition and electrochemical performance of the zinc-dipped layers. The results showed that a uniform and compact zinc-dipped layer was obtained under normal temperature. Corrosion rate of the zinc-dipped layer was low in 3.5wt% NaCl solution. At the same time, the zinc-dipped layer had excellent adhesion with the sequent plating layer.Zinc dipping solutions containing different organic acid salt complexing agents were investigated during zinc dipping process. Effects of zinc dipping time on microscopic morphology and electrochemical performance were analyzed. The optimum zinc dipping time was confirmed. Moreover, the sodium gluconate was verified to be an excellent complexing agent. Then nickel layer was electroplated on the zinc-dipped layer, which showed good corrosion resistance and good adhesion with Mg-Li alloy matrix.Zinc dipping mechanism was discussed by analyzing kinetics, electrochemical parameters during zinc dipping process and effects of zinc dipping time on microscopic morphology. The whole growing process of zinc crystals was analyzed by SEM, and zinc dipping model was established. That is, zinc layer firstly grew on crystalline boundary of Mg-Li alloy surface, and the growth at crystalline boundary was easier than that on crystalline grain at the beginning of zinc dipping process. With the increase in zinc dipping time, zinc crystals diffused from crystalline boundary to the center of the crystalline grain until the substrate was fully covered. Zinc-dipped layers were finer, more uniform and denser after the second dipping process than the first dipping process. With an excessively long zinc dipping time, morphology of zinc crystal would grow more aggregatively, which led to the formation of gaps between crystals.