Study Of Micro-arc Oxidation And Electroless Plating Ni On The MAO Coating Of Magnesium Alloy AZ91D

The technologys of micro-arc oxidation(MAO) and electroless depositon nickelphosphorus alloy(EN) all are one of the research hotspots of magnesium surface treatment at present.In this paper, basing on the study of micro-arc oxidation process, a smooth composite coating which had been prepeared on AZ91D magnesium alloy by electroless depositing Ni-P alloy on the MAO caremic film.The composite coating had very compact interior structure and high anti-corrosion capability,and it could provid more perfect protection for magnesium alloy from corroding.On the basic of researching the primary components of micro-arc oxidation electrolyte, the influences of metallic oxysalt, polyacid salt and surfactant on the anti-corrosion property and micro structure of the micro-arc oxidation ceramic film were studyed.The orthogonal design method was applicated to optimize the MAO electrolytes formulas which included composite electrolyte component, then two good micro-arc oxidation electrolyte formulas were obtained. The one was composed of Na2SiO3 14g/L,(NaPO3)6 10g/L,Na2WO4 1g/L,NaOH 9g/L,NH4F 2g/L,Na2EDTA 1g/L;the other was composed of NaAlO2 10g/L,(NaPO3)6 10g/L,Na2WO4 1 g/L,NaOH 9g/L,Na2EDTA 1g/L,C12H25O4SNa 0.5g/L。Because of accession of composite additives, the micro-arc's striking and termination voltage were decreased obviously, so the structure and corrosion resistance of oxidation film were improved greatly.The dynamic potential polarization researchs showed that ceramic oxidation film's corrosion current density had been reduced about four orders of magnitude and the corrosion potential increased about 0.34-0.37V in the dynamic potential polarization curve,and it's AC impedance had been increased about 13000-15000?/cm2,.The texts of brine corrosion and electrochemistry performance indicated that the ceramic oxidation film which was prepared during longer micro-arc discharging time had the better property.The low concentration Sn2+ sensitization and Pd2+ activation process were used to pretreat the micro-arc oxidation film,then a crystal structure Ni-P coating could be prepared on it in alkaline electroless plating solution.The single factor optimization experiments were applied to obtain a good alkaline electroless plating Ni-P alloy process as follows: NiSO₄·6H₂O 28g/L,NaH₂PO₂·H₂O 28g/L,Na₂C₄H₆O6·2H₂O 20 g/L,NH4F 20 g/L,pH 8.5-9.0,temperature 78℃. The Ni-P alloy's depositing speed of the alkaline electroless plating solution could reach 19μm/h,but its stability and plating speed all descended quickly while the metal turn over of plating solution increased. On the crystal structure Ni-P alloy coating,a amorphism structure Ni-P alloy coating could be prepared successfully in a acidic electroless plating solution. The orthogonal design method was applicated to optimize the acidic electroless plating Ni-P process and a good one could be got as follows: NiSO₄·6H₂O 20g/L,NaH₂PO₂·H₂O 28g/L,Na₃C₆H₅O₇·2H₂O 10 g/L,CH₃COONa 15 g/L,C₂H₅OCOOH 10 ml/L,temperature 87℃,pH 4.6-4.8.The average depositing speed of Ni-P alloy could reach to 22μm/h in the acid electroless plating solution。The dynamic potential polarization measurements showed that the composite coating system which was made up of micro-arc oxidation ceramic film and amorphism structure electroless plating nickel coating only presented the nickel-phosphorus alloy coating's corrosion information. The sample with MAO ceramic film and amorphism structure Ni–P alloy coating exhibited lower corrosion current density and more positive corrosion potential than the sample only with ceramic oxidation film. The heat treatment during 350-400 degree celsius could improve the micro-hardness of amorphism structure Ni–P alloy coating greatly. However,It would also decrease the Ni-P alloy coating's anticorrosion capacity.