Study On Electroless Ni-P Plating And Electroless Ni-P-PTFE Composite Plating On Magnesium Alloys

In recent years, magnesium industry has obtained a well developing in China, and production capacity, yield and export of magnesium were all in the front rank of the world. Magnesium alloys are more and more applied in automobile industry, aerospace and 3C parts. Magnesium alloys have the most developing prospect in the 21st century considered as "green materials". The principal drawback of magnesium alloys as a structural material is its property of high chemical activity which leads to a low corrosion resistance in many cases, so their application is limited. Electroless plating is a effective method to improve corrosion resistance of magnesium alloys.In this thesis, direct electroless nickel plating on magnesium alloys was studied in details, which included effects of MgF2 in Ni-P coating by traditional direct electreless plating, optimizing process of electreless plating, a uniform nickel film on magnesium alloys and subsequent Ni-P coating, electroless Ni-P-PTFE composite plating and the electroless plating dynamics.The coating on magnesium alloys by traditional direct electroless plating was studied by means of Scanning Electron Microscopy (SEM), Energy Dispersive X-ray Spectroscopy (EDS) and X-ray Diffraction (XRD). The changes in pretreatments, the concentration of each component in bath and the other process parameters were emphatically studied and the optimized technology conditions were achieved. The results showed that a protective layer of MgF2 is formed after activation which could protect magnesium substrate from corrosion of plating bath and the MgF2 has no reactivity. The MgF2 layer exists between the magnesium alloy substrate and the Ni-P coating. The physical model of effects MgF2 on the porosity of Ni-P coating was build. Excessive activation could raise the porosity of Ni-P coating and lead to deterioration of corrosion resistance of the Ni-P coating.On the base of the above studies, the metal nickel film made by a novel and simple pretreatment method for electroless plating on magnesium alloys has been studied by means of SEM, EDS, XRD, the Glow Discharge Optical Emission Spectrometry (GDS) and electrochemical potentiodynamic polarization. The uniform nickel film generated on magnesium alloys could protect magnesium alloys from excessive corrosion by bath, and the consequent electroless plating could process on the nickel film successfully. The results show that the directional alignment of surfactant molecules could strongly protected the magnesium alloys from excessive corrosion by replacement solution. A uniform film of nickel metal formed on the surface of magnesium alloys. The nickel metal film could provide enough protection for magnesium alloys without harm to the environment and enough activity which made electroless plating to carry out directly. Ni-P coating on nickel film presented good corrosion resistance properties. In the replacement solution, the concentration of each component and the other process parameters were emphatically discussed. If the concentration of nickel salt increased, the reaction rate of replacement was increased. But if the concentration was too high, the stability of replacement would decrease and floc precipitates would occur. If the concentration of surfactant was too low, the sample could not gain enough protection from replacement solution. But if was too high, the adhesion between sample and nickel metal film should deteriorate. The increase of pH value was helpful for increasing the reaction rate of replacement, but if it was too high, the dissolution rate of Mg should slower and the appearance of nickel metal film should deteriorate. Ni-P coating is silvery gray color, semi-bright, smooth, uniform and compact. The deposition rate is about 15～18μm·h-1. Ni-P coating belongs to medium phosphorous coating and shows good adhesion. The hardness of Ni-P coating could reach 703.34HV. The diffusion of Ni-P coating and substrate after annealed was studied by SEM, a clear transition layer was formed which was helpful to increase adhesion. During the formation of nickel metal film on magnesium alloy, Mg is dissolved and turned to ions and at the same time Ni2+ is reduced to nickel atoms. These atoms deposited on substrate by adsorption. The alignment effect of surfactant could decrease potential difference of the surface of magnesium alloy, and also protect magnesium alloys from corrosion. The preparation of nickel film replaces two previous procedures of acidic etching and activation and has no harm to the environment and prolongs applied time of bath.Ni-P-PTFE coating on magnesium alloys was prepared on the base of above studies. The results show that only when the ratio of PTFE and FC-4 is within a proper range, the plating process could be normal proceeding. The operation flexibility of composite plating process turned to narrow because of the join of FC-4 and PTFE. Ni-P-PTFE coating took on bright black color, smooth, uniform and compact. The deposition rate is about 12～14μm·h-1, Ni-P-PTFE coating belongs to medium phosphorous coating, shows good adhesion and its corrosion resistance is about equal to the Ni-P coating.. The hardness of Ni-P-PTFE coating could reach 231.83HV which decreased comparing with Ni-P coating. The surface energy of Ni-P-PTFE coating decreased clearly because of the join of particles of PTFE. The friction characteristics of Ni-P coating and Ni-P-PTFE coating were studied by friction and wear test. The friction coefficient of Ni-P-PTFE coating decreases clearly due to the join of particles of PTFE, its average friction coefficient is about 0.15 and change fluctuation of the friction coefficient of Ni-P coating is bigger than Ni-P-PTFE coating.The physical model of electrless plating nickel on magnesium alloys was studied. Influence rules of every process parameters were described. According to linear regression method, the dynamics equation of the electroless plating on magnesium alloys was obtained, which was described as following:lgv= 12+0.33751g[H2PO2-]-1.21201g[L]+0.35981g[F-]+0.0985pH-3963/TThe calculated results from the dynamics equation were in accordance with the experiment results.