Research On Electroless Plating On AZ91D Magnesium Alloy Substrate

In order to improve the corrosion resistance and decrease the surface resistance ofAZ91D magnesium alloys, the direct electroless Ni-P plating on AZ91D magnesiumalloys has been studied, and Ni-P/Ag double-layer plating process has beenexplored.The electroless Ni-P plating in this research used nickel sulfate as main salt,sodium phosphite as reducing agent. The direct electroless Ni-P plating process onAZ91D magnesium alloy has been optimized by orthogonal test. The order andcontribution rate of the four factors including main salt concentration, reducingagent concentration, pH of the solution and the plating temperature has beendetermined by means of range analysis. The result also indicates how the factorseffects the plating process. Ni-P coating obtained by using the optimised processhave been studied by SEM, XRD and Potentiodynamic polarization curves. Theresults show that the optimized Ni-P coating is a low phosphorous coating and it hasexcellent corrosion-resistance.AZ91D magnesium alloy can not be Ni-P plated directly. Activation is the mostimportant step before the Ni-P plating process on AZ91D magnesium alloy. Goodactivation contributes to good Ni-P layer. In this research, fluorating has been usedto activate the surface. Different activation time helps bring about different surfaceconditions, thus results in Ni-P layers with different properties. By comparing theproperties of the Ni-P coatings on the differently activated surface, we find out thatthe lower the F/O ratio of the surface is, the better the Ni-P layer it had.The optimized activation and Ni-P plating process has been used to preparing thesample before Ag plating. Then the Ag plating process has been studied. Ag platingcan be directly processed on the Ni-P coating. A compact Ni-P coating is needed topreparing the Ni-P/Ag double layer. If not, the AZ91D substrate will be crroded inthe solution during Ag plating. A twenty minutes Ag plating is enough for having acorrosion resistant Ag layer with face centered cubic structure. The Ni-P/Ag doublelayer is then studied by SEM, XRD and Potentiodynamic polarization curves. Theresults show that the Ni-P/Ag double layer is better than Ni-P layer in corrosionresistance.