Study Of Pretreatment Process On AZ91D Magnesium Alloy

Magnesium alloys, which have many special advantages, have been widelyused in many fields such as automobile, aerospace and3C products. However thefurther applications of magnesium alloys are restricted by its poor corrosionresistance. Electroless nickel plating on magnesium alloy can improve hardness,anti-corrosion, and provide a variety of decorative appearance. However, theadhesion and protective performance of the coatings have been a difficult issue toresolve.In this thesis, AZ91D magnesium alloys which are used widely in industry areused as the research material. A technique of pretreatment process on magnesiumalloys is developed in order to obtain a basic coating with good adhension. Acidicelectroless nickel plating, nickel electroplating are implemented on the basic coating,in order to obtain a coating with protective finally.Pretreatment process before electroless nickel plating was studied in this paper.E-t curve, Corrosion rate, film weight, components, morphology of the zincate layerand binding force of nickel layer after zincate were researched by electrochemicaltests, XRF, EDS and SEM.Multi-metal zincate solution was optimized by studying K₄P₂O₇, ZnSO₄, NiSO₄,auxiliary complexing agent A and KF, and the effects of Ni, Fe and Cu in zincatesolution were also researched. It is shown that thin and dense zincate layers, withhigh stable potential, could be attained in the multi-metal （Zn, Ni）zincate solution.The corrosion rate in3.5%NaCl solution was slow, and high binding force wasfound to the sequent layers. Zincate mechanism was discussed simply in the paper,the process is studied.The effect of composition and operating conditions of pre-electroless nickelsolution on the adhension of the coating have been investigated. The bestpre-electroless nickel technology is chosen by comparison. XRF is used toinvestigated the deposition rate of the pre-electroless nickel; SEM is used toobserves the surface morphology of the coating; thermal shock test and cross test areused to test the adhension of the coating; polarization curve and a porosity test areused to evaluate the anti-corrosion of the coating. The results is shown that the surface of pre-electroless nickel coating is evenly, good adhesion to the substratecoating, and show excellent anti-corrosion. When acidic electroless nickel platingand nickel electroplating on the pre-electroless nickel coating, porous of the coatingis not observed.The process can obtain a high anti-corrosion and good adhesion coating. Thetest of polarization curve shows that：The corrosion potential of the coating of thenickel is about-0.347V in3.5%NaCl. This process has environmental significancewithout using cyanide copper. Better society and economic benefit can be obtainedwith further developing and using．...