Studies Of Novel Electroless Nickel Plating On AZ91D Magnesium Alloys

Magnesium alloys have a number of excellent characteristics, including low density, high strength-to-weight ratio, excellent castability, machining and damping performance. So they have good social and economic benefits and hence are extensively applied in many fields such as aerospace, automobile, mechanical and electronic components. However, poor corrosion resistance has often baffled it from further applications.The processes of chemical Ni-P coating and Ni-W-P coating are investigated to protect the substrate of AZ91D magnesium alloys in our experiments. In the experiments, SEM (Scanning Electron Microscope) was used to observe the surface morphology of the two coatings; EDX (Energy Dispersed X-ray Spectrometer) was applied to mensurate contents of the coating; 3.5% NaCl solution immersion was employed to assess the corrosion resistance of the two coatings. The nicking testing and thermal shock testing were applied to determine the adhesion of plating and PdCl₂ testing was used to detect the stability of plating solution.The characteristic and application of AZ91D magnesium alloy, the recent advances in metal protection coating and the modern technics of electroless nickel plating on magnesium alloy were summarized. The mechanism of sodium molybdate in the pickling procss of electroless nickeling plating was investigated. The effects of the developed chrome-free pickling on the following activation and eletroless nickel plating were systemically studied. At the same time, electroless Ni-W-P coating on AZ91D magnesium alloy was also tested. The experimental results are shown as follows:(1) Sodium molybdate in phosphoric acid medium is a cathodic inhibitor for the AZ91D magnesium alloy corrosion. The inhibition ability of Na₂MoO₄·2H₂O is stronger than CrO₃. And the dosage of Na₂MoO₄·2H₂O in pickling bath is much smaller than that of CrO₃. In the phosphoric acid medium, anions are adsorbed on the surface of AZ91D magnesium alloys to form insoluble products of multi-molybdate and multi-molybdate-phosphate. These insoluble products reduce active sites on the surface of the cathode, the dissolution rate slows down on the conjugated anode surface. A compact film composed of the corrosion products of multi-molybdate and Mg(OH)₂ can provide a relative even surface for the subsequent chemical coating. (2) The Ni-P coating on AZ91D magnesium alloys with high adhesion and good corrosion resistance is obtained by the pretreatment of pickling in the H₃PO₄ bath containing 1-5 g·dm^-3 Na₂MoO₄·2H₂O and activation in 200 g·dm^-3 NH₄HF₂ and then electroless nickel plating. The developed pre-treatment of pickling in molybdate-phosphoric acid and activation in NH₄HF₂ solution is feasible environment-friendly process since the new pretreatment procedure does not use Cr⁶⁺ and HF any longer. The production environment is greatly improved and social efficiencies are increased.(3) The compositions and operation conditions of chemical Ni-W-P coating are: NiSO₄·6H₂O 21 g·dm^-3, C₆H₈O₇·H₂O 20 g·dm^-3, Na₂WO₄·2H₂O 15 g·dm^-3, NH₄HF₂ 26 g·dm^-3,NaH₂PO₂·H₂O 20 g·dm^-3,Thiourea 1 mg·dm^-3,pH 5.5 - 6.0,80℃-85℃。A uniform compact Ni-W-P coating can be obtained by the developed electroless plating, so it is a satisfying protective coating. Ni-W-P coating can significantly improve the wear resistance of AZ91D magnesium alloy products.