| Magnesium alloy has been hailed as the 21 st century "green engineering material" duo to the excellent mechanical and physical properties, and it is widely used in automotive, aerospace, 3 C electronic products and medical devices and so on. But the further development and application of magnesium alloy are seriously limited as the high chemical activity of magnesium extremely. Therefore it must be take some protection process es on the surface of magnesium alloys. People pay more and more attention to the direct electroless nickel plating(ENP) due to the excellent properties of the coating and the simple process. This paper adopted a chromium-free, low fluorine and environmentally friendly pretreatment process. It can provide theoretical basis for effective control of plating quality by studying the acid picking and activation reaction mechanism. At the same time, this work also studied the growth regularity of coating and the effect of bath composition the on deposition rate, which can further guide the industrial production of coating. The results obtained as follows:1. The surface of magnesium alloy is etched uniformly and covered with a layer of phosphating film after treated by H3PO4 and HNO3. The activation with K4P2O7 and NH4HF2 solution not only removes the substrate pickling product and surface dusting, but also insures magnesium alloy covered by fluoride. This pretreatment process is chromium-free and low fluorine, which is harmless to the environment. Compared with HF, it is less health threat to operators and belongs to the environmental ly friendly pretreatment process.2. Magnesium alloy surface is covered a layer of fluoride membrane containing oxide or hydroxide after activation. The naked magnesium alloy provide active center for nickel deposition after oxide or hydroxide dissolved in the plating bath, and fluoride prevent magnesium alloy from excessive corrosion. After secondary activation process, the F/O ratio(1.12~1.49) is moderate on the activation coating. The Ni-P coating with uniform density, excellent adhesion, good corrosion resistance, can protect the magnesium alloy for a long time.3. With the F/O ratio(3.52) of activation coating activated by HF is the highest, the initial deposition rate of nickel is slow, and the Ni-P coating is loose with poor adhesion. At the same time, the reduction efficiency of sodium phosphite is reduced with the seriously decomposing of plating bath, which does not conform to the subject of the economic social. With the F/O ratio(0.84) of activation coating activated by NH 4HF2 is minimum, large amount of hydrogen gas produce and then converge to airstream on the substrate, which would cause the high porosity and poor adhesion coating.4. The growth of Ni-P coating experiences Mg-Ni displacement reaction and Ni-P self-catalytic reaction two separate reaction stages. The Ni nuclei is firstly produced and regarded as the active center of Ni-P deposition, then the plating process is completed step by step, covers the whole magnesium alloy at last. Nickel deposition begins from below the fluoride, therefore part of the fluoride may be wrapped in the middle of the Ni-P coating.5. When the concentration of main salt solution is less than 2.08 g?dm-3, deposition rate increases with the increasing of nickel ion concentration. Conversely it decreases; Deposition rate increases with the increas ing of reducing agent concentration, p H and temperature; The higher the complexing agent concentration, the smaller the deposition rate. |
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