Study On Composite Technology Of Microarc-oxidation And Electroless Plating On Magnesium Alloys

Magnesium alloys have wide application prospects in the automotive, aerospace andconsumer electronics other industries for its excellent comprehensive performance. However,poor corrosion resistance hindered its widely used. As a kind of excellent surface treatingtechnologies, electroless nickel plating can uniform coatings on complicated castings,whichhas been gained wide recognition and application in recent years.The existing electrolessnickel plating on magnesium alloy has complex pretreatment and use Cr (Ⅵ) and HF whichdo great harm to the environment and human body and has the bath stability problem. Tosolve these problems, this paper do the following two main research:1)development ofsulfuric acid ni-based electroless nickel plating solution on magnesium alloy;2) newelectroless nickel plating technology based on micro-arc oxidation pretreatment.Result of single factor test showed that nickel salt, reducing agent, complexing agent andammonium bifluoride were four basic compoents of sulfuric acid ni-based electroless platingsolution. Amminium bifuoride played for both accelerated agent and corrosion inhibitors;citric acid was better than sodium citrate as the complexing agent. The best basis solutionformula of eletroless plating was determined by the orthogonal test. A moderate amout ofstabilizer can improve plating speed and corrosion resistace of the coating. The pH value canimprove dramatically improve plating speed, but too big speed cause poor bonding force andpoor corrosion risistance. The formative mechanics and growth process of Micro-arcOxidation film on magnesium alloy were also explored in the paper, the best parameter ofmicro-arc oxidation for electroless plating was:150V, constant coltage,3min.Electrochemical dynamic potential polarization results of typical sample show that:corrosion potential of MAO-EP sample was0.37higher than the matrix, corrosion currentdensity reduced2orders of the matrix, and was one tenth of the Dow sample. Full immersioncorrosion test showed that: corrosion speed of MAO-EP sample which was0.157mg/(cm2·h)was nearly half of the Dow sample when the test period was24h, and corrosion speed ofMAO-EP sample which was0.312mg/(cm2·h) was nearly one third of the Dow sample whenthe test period was48h. Mcro hardness test result showed that hardness of compound coatingwas7.99times of the matrix(72Hv). SEM and EDS analysis showed that the compoundcoating was made up by amouts of spherricles and had copact structure and the P content was5.61%. The coatings of plating and micro-arc oxidation was well bond like canie teeth.