The Investigation On Effect Of Pickling And Activation On Electroplating And Electroless Plating On Magnesium Alloys

Magnesium alloys have excellent comprehensive performances,they are recognized as "metal of the 21st century" and are widely applied in automotive, aerospace,electronic,communication industries,etc.However,magnesium alloys have poor corrosion resistance due to their high chemical reactivity,which has restricted their widespread development and applications in many areas. Electroplating and electroless plating on magnesium alloys are techniques that have bright prospect for development and application because they can greatly improve the corrosion and wear resitance,solderability,electrical conductivity and decorative appearance of magnesium alloys.However,magnesium alloys are classified as metal substrates that difficult to plate.Thus,appropriate pretreatments play very important roles in getting coatings of high quality.Pickling and activation are key processes of pretreatments.However,traditional pickling and activation processes have the limitations of using toxic reagents,which are very harmful to human health and environment.Environmental-friendly pickling and activation processes before nickel electroplating and electroless Ni-P plating on magnesium alloys have been studied in this paper.Interface reactions and mechanisms of AZ91D and AM60 magnesium alloys in various pickling and activation solutions have been analyzed.The effects of the compositions and operation conditions of pickling and activation baths on the quality of the coatings have been investigated in order to establish the optimal pickling and activation processes before nickel electroplating and electroless Ni-P plating.The experimental results are as follows:1.The processes and conditions of pickling can strongly influence the corrosion rate,surface morphologies of the Mg substrates and the quality of the coatings. HNO₃+H₃PO₄ or HNO₃+CrO₃ pickling can produce coarse surface,which can augment the mechanical occlusive force between the subsequent coatings and the substrates,so coatings with good adhesion and corrosion resistance can be obtained. HNO₃+H₃PO₄ or HNO₃+CrO₃ pickling is a good choice for nickel electroplating or electroless Ni-P plating respectively.2.HF activation can produce a compact fluoride film on the surface of the specimens,which is more compact than the fluoride film created by NH₄HF₂ activation;K₄P₂O₇ activation can remove the oxides and hydroxides and expose the fresh Mg substrates through the complexing action of K₄P₂O₇,a thin and porous film of Mg(OH)₂ is synchronously produced on the surface;A mixed film of manganese oxides and phosphates is formed on the surface via H₃PO₄+KMnO₄ one-step pickling-activation,which can provisionally protect the magnesium substrates.The components and properties of the activation films can significantly affect the quality of the coatings.3.HF activation and NH₄HF₂ activation can produce compact activation films, but the zinc layers are uneven because the activation films are too compact,so the corrosion resistance of the nickel electroplating coatings is unsatisfied.The activation film formed via K₄P₂O₇ activation is thin and even,so the zinc layers formed via K₄P₂O₇ activation are uniform,both the corrosion resistance and adhesion of the nickel electroplating coatings are excellent.The activation film formed via H₃PO₄+KMnO₄ one-step pickling-activation will be partially dissolved in zinc immersion bath,the zinc layers are symmetrical.The nickel electroplating coatings are shown to have excellent corrosion resistance,but the adhesion is not very good because the molecule sizes of manganese oxides and phosphates are too large. Therefore,K₄P₂O₇ activation is the optimum activation process for nickel electroplating.The optimum compositions and the conditions of K₄P₂O₇ activation bath are as follows:120～200 g·dm^-3 K₄P₂O₇·3H₂O,10～30 g·dm^-3 Na₂CO₃,11 g·dm^-3 KF·2H₂O,activated for 2～3 min at 70±5℃.4.The optimum activation process of electoless Ni-P plating is the two-step activation:HNO₃+H₃PO₄ pickling→K₄P₂O₇ activation→NH₄HF₂ activation.The coatings obtained via two-step activation are shown to have comparable excellent corrosion resistance and good adhesion with the coatings obtained by traditional HNO₃+CrO₃ pickling and HF activation.The two-step activation is an environmental-protective process due to not using poisonous chromic compounds and volatile hydrofluoric acid.