Preparation Of Electroless Ni-P On Magnesium Alloys And Study Of Corrosion-resistance

Magnesium and its alloy which are better than aluminium alloys are of many characteristics, such as small density, high specific strength, good stationary property of dimensions and fine magnetic shielding. Because of its outstanding mechanics and physical performance, it has already been applied widely in aerospace, automobile industry, electronics industry and so on, becoming one kind of most hopeful material in the modern high and new technology domain. However, application of magnesium alloy has been restricted for poor corrosion-resistance. How to improve its corrosion-resistance is an urgent topic of the day.Today, many surface treatment techniques are used for enhancing the corrosion resistance of magnesium alloy, like chemistry oxidation, electrochemistry oxidation, coloring, souring salty transformation membrane and so on. Because of thin and soft chemistry oxide film,brittle and porous electrochemistry oxide film, these oxidation methods are only used in the decoration. In view of the limitations of other surface modified technologies, this article introduces one kind of electroless nickel plating technology for enhancing corrosion -resistance of the AZ91D.In predecessor's foundation, the research on electroless nickel plating on the AZ91D surface was proceeding through consulting related literature material. Through orthogonal experiment, it was studied that the Ni-P layer was directly obtained on the surface of magnesium alloy by electroless technique. The structure of the Ni-P layer and its electrochemistry performance were also studied. The studies indicated that Ni-P layer on the surface of magnesium alloy may be directly formed by electroless plating technique. The experimental disposition and the operation are simple, avoiding the trouble of plating the intermediate layer. Through the orthogonal experiment the best process parameter of prepared Ni-P layer had been obtained(optimal process: degreasing (10min)- acid pickling (10min) - activation (12min), NiSO4 6H₂O (10g/L), Na2HPO4 H2O (20g/L), HF40% (10ml/L), Na3C6H5O7 · 2H2O (7g/L), NH4HF2 (8g/L), a few buffer, PH =5.5, 80 + 2 ℃ ). The coating thickness, the microhardness, the plating condition, microstructure and the surface morphology were measured. The heat shock and the scratchexperiments indicated that a good adhesive strength of the coating and the substrate had been obtained. The scratch appearance of the coating shows that the coating basically does not peel off.Contrast with the base material electrochemistry corrosion property under three kind of different corrosions conditions, the result showed that the corrosion electric potential of the Ni-P coating obtained a very big enhancement. In order to further examine the corrosion resistance of electroless plating Ni-P, a contrast experiment was carried for Ni-P coating, stainless steel and treated coating. Result showed that the corrosion resistance of electroless plating nickel layer lies between that of the stainless steel and that of the treated coating.The electrochemistry and the soaking experiments were carried in 3.5%NaCl solution for the coatings of the different phosphorus contents, results indicated that the corrosion resistance of high phosphorus coating is bigger than that of low phosphorus coating. Therefore, electroless nickel plating technology provided an ideal method of the protection AZ9 ID from the corrosion.