The Process And Performance Of Direct Electroless Nickel On AZ91D Magnesium Alloy

Magnesium alloy are widely used in automotive industry, aerospace,electronics industry because of a number of desirable features, including lowdensity, high specific strength, excellent castability. However magnesium alloyusually has poor corrosion resistance, which is one of the main limits to theapplication of magnesium alloy in practical environment. Than electroless nickelplating on magnesium alloy is a proper protection because the deposited nickelplating is very resistant to corrosion and abrasion. Becauce of that, electrolessnickel plating has become hot spots among various surface treatment technology.This paper studies pretreated before electroless nickel plating on die casting AZ91Dmagnesium alloy such as chemical etching and acid cleaning activation. Thealkaline electroless nickel plating, composition and process of electroless nickelplating, and bath life are studied as well. The performance of electroless coating onmagnesium are analyzed by three-dimensional video microscope system, scanningelectron microscopy (SEM) and energy dispersive spectrometer(EDS), X-raydiffraction(XRD), electrochemical workstation. The feasibility of this process in themarket is discussed. The experimental results are as follows:During the etching, phosphoric acid played the role of corrosion and adhesivefilm. The optimum polishing process is: H3PO4(85%)650mL/L, etching time2-4min, etching temperature30-40℃. After etching, the coating of electroless nickelis very flat and high bonding strength. Acid cleaning activation composition andprocess iare optimized by single factor and orthogonal experiments, The optimumacid cleaning activation composition and process is: H2O60mL/L, C3H6O350mL/L,H3PO4(85%)10mL/L, HNO320mL/L, time120s, temperature15-25℃. After acidcleaning activation, the content of Mg(α) is reduced, Mg12Al17(β) is increased. Thecoating of electroless nickel is amorphous phase, with high phosphorus content (forabout10.31w%). The thickness of the coating is more than25um after electrolessnickel plating90min. The hardness of electroless coating is412.2HV, three timesmore than magnesium substrate. The corrosion potential of electroless nickelcoating is shift to positive direction1V, as compared to magnesium alloy substrate.The corrosion current is three orders of magnitude smaller compared to magnesiumalloy.The surface of alkaline electroless coating is uneven, with low phosphoruscontent (5.66w%), corrosion potential is shift to positive direction0.85V comparedto magnesium alloy, corrosion current is one orders of magnitude smaller comparedto magnesium alloy. The surface of electroless coating by alkaline+normal electroles is more flat than alkaline electroless coating, it also can improvecorrosion potential and reduce corrosion current. The composition and process ofelectroless nickel plating is optimized by single factor. The optimum acid cleaningactivation composition and process is: NiSO4·6H2O18-30g/L, NaH2PO216-24g/L,CH3COONa6-9g/L, HF(40%)12-18mL/L, temperature80-85℃, time60min,loading capacity60-100cm2/L. From the bath life study, the deposition rateundulated in the scope of17-24μm/h with the period increasing, but bath stability isdeclined. With the periods increasing, the phosphorus content of electroless platingreducing also.The cost of this process is fewer one thirds than Dow process in every Squaremeters. The pretreated of the process can also reduce toxic substances and harm ofhuman body and environment.