Study On Process And Performance Of Chromium-free And Direct Electroless Nickel For AZ91D Magnesium Alloy

Magnesium and its alloys are widely used and have huge market prospect in the fields of electronic communication,aerospace and automotive industry because they are the representatives of light alloys and have a variety of excellent performance.However,large-scale application has been limited for its low potential and poor corrosion resistance.Surface technology is one of the approaches to improve the corrosion resistance of materials.As a new superficial process,with cost-effective,energy efficiency and non-pollution,the electroless plating on magnesium alloys is an excellent surface technology to improve comprehensive performance,such as anti-corrosion,wear-resistant.Therefore,it has become an important area of protection technology for material.In this paper,AZ91 D magnesium alloy as the research object,with electroless plating as main film forming technology,Ni-P plating,as the main composition of membrane layer,were deposited on the substrate surface to modify the surface of magnesium alloy.The effects of pretreatment process and Ni-P solution were systematically studied.Main works and conlusions are as follows:1)Study on pretreatment process.On basis of self electroless solution,the effects for Ni-P plating of pickling,acid activation and surface conditioning were analysised in detail.Meanwhile the best formulation are determined,namely pickling : H₃PO₄ 200mL/L,KF0.5¹g/L,10 s,30?;surface conditioning:pyrophosphate 150²00 g/L,Na₂CO₃ 20 g/L,KF 11 g/L,2.5min,80?;activation:NH4HF2 200g/L,10 min,RT?A rough and porous film was formed on the surface of the specimen via H₃PO₄ + KF pickling?NH4HF2activation.However,a compact and uniform Ni-P plating was acquired via H₃PO₄+KF pickling?surface conditioning?NH4HF2 activation and its anti-corrosion was significantly increased compared with pickling + activation film and magnesium substrate.2)Study on process optimization for Ni-P binary alloy electroless plating.Based on the optimized pretreatment process and the main technical indicators of binding force,plating rate,porosity and macro morphology.The effects of main salts,reducing agent,complexing agent and additives were discussed at length.The optimized solution for plating wasdetermined,namely C6H8O7·H₂O 4g/L,NiSO4·6H₂O 25g/L,NaH₂PO2·H₂O 20g/L,NH4HF220g/L,H₂NCSNH₂ 2mg/L;pH=7.00,T=85±1?.The additive of NH4HF2 can not only inhibit the corrosion of magnesium but also accelerate plating rate and buffer the solution pH.3)Typical sample performance testing and microstructure analysis.The typical samples of surface conditioning + activation plating and Dow direct electroless plating were analyzed by EDS,SEM,binding test and potentiodynamic polarization curve.SEM and EDS analysis showed that the coating was made up by amounts of spherricles and had copact structure.Compared with Dow samples,surface conditioning + activation film is more compact and uniform.Dow and optimized samples,containing P 5.04% and 6.49%respectively,belong to medium phosphorous coating.Electrochemical dynamic potential polarization indicated that the corrosion potential of optimized sample was 0.99 V higher than matrix.Corrosion current density reduced 2 orders of the matrix,and was half the Dow sample.The coating of surface conditioning + activation was well bond.