The Research On Electroplating Of Magnesium Alloy Using Pretreatment Of Pickling-Activation Without Chromium And With Low Fluorine

Magnesium alloys are regarded as the lightest metal structure materials, which have a number of desirable features and prosperous applications in many fields such as aerospace, automobile, mechanical and electronic components etc. So they are called as green engineering materials with most developing potential and beautiful future in 21st century. However, magnesium has poor corrosion resistance and are easily oxidized in ambient owing to their low electrochemical potentials (-2.36 V). The loose oxide film formed on the surface of magnesium alloys in the atmosphere has poor protective performance and magnesium alloys are made to be ceaselessly corroded in damp atmosphere, soil and seawater so that their wide applications are limited. Plating is one of the most important means for the treatment of surface protection of magnesium alloys. It can make magnesium alloy surface with high luster decoration, good wearability and heat-resistance. However, an appropriate pretreatment process is the key factors to obtain the satisfactory metal coatingsThe production and application of magnesium alloys in domestic markets are summarized and the recent advances in metal protection coating and the pretreatment of magnesium alloy electroplating are reviewed in this dissertation. A pretreatment process of one-step pickling-activation before magnesium alloy electroplating has been developed. The two-step process of pickling and activation applied in magnesium alloy pretreatment which contain chromium and fluorine can be replaced by one-step pickling-activation without chromium and fluorine. The bath composition, operation conditions and mechanism of pickling-activation of magnesium alloy were investigated. At the same time, zinc immersion solution was improved by adding metal complexer. The process of zinc immersion process, mechanism of zinc alloy immersion on magnesium alloy and stabilization of zinc immersion solution were also studied. Moreover, the copper, zinc and nickel electroplating after zinc immersion with the developed pretreatment were studied. The research results show that:1. One-step pickling-activation can replace the two-step process of pickling and activation containing fluorine and chromium harmful to human body. Activation film composed of manganese oxides and other compounds formed by pickling-activation effectively improves the corrosion resistance of the substrate in the soulution. The activation protective film is produced when the specimens are marinated in pH 4～6 pickling-activation solution for over 450 s. The dissolution and the formation of activation film on magnesium alloy surface simultaneously hapen in the pickling-activation solution. At the beginning, the corrosion rate of specimens exceeds the formation rate of activation film. Afterward, the formation of the activation film rate increases ceaselessly and restrains the corrosion reaction. The formation of activation film undergoes three stages of crystallization, growth and thickening in pickling-activation. The activation film formed in the growth stage is the optimum stage for immersing zinc.2. Zinc immersion needs a small amount of fluorine ion because the displacement reaction between magnesium and zinc ion was depressed by adding fluorine ion in zinc immersion solution. Magnesium fluoride deposited on the surface of substrate will restrains the displacement reaction between magnesium and zinc ion and decrease the rate of zinc deposition. The crystal structure of zinc film is improved by adding nickel complex in the immersion bath. The more compact and uniform zinc-nickel film with fine crystal grain is obtained. The stabilization of zinc immersion solution is improved by adding nickel complexer. The optimum temperature of zinc immersion are 55～75℃and operation time 30～300 s.3. The pretreatment of one-step pickling-activation can been appled in copper, zinc and nickel electroplating. Their platings with good corrosion resistance and adhesion are obtained. The copper platting obtained by one-step pickling-activation treatment is more compact than that by Dow process. The developed pretreatment is a simple and chrome-free process.