Technology And Application Of High Phosphor Electroless Plating Ni-P On Aluminum Alloy At High Temperatur

The reaction mechanisms of Ni-P electroless plating and Ni-P-SiC composite electroless plating are researched, and the reaction thermodynamics are analyzed. The appropriate pre-treatment and electroless solution (JXD) are suitable for aluminum(YL12) and magnesium(AZ91) alloys. By reading a large number of journals, composite complexing agent, accelerant and buffer are applied in the solution. Technology and performance of the coating were researched through experiments. The results as follows:(1)Alkaline-acid electroless Ni- P Plating technics was adopted, the proper solution prescription and parameters of electroless plating have been gained:Alkaline Pre-Plating: NiSO₄·6H₂O 30g/L, NaH₂PO2·H₂O 25g/L, (NH₄)₃C₆H₂O₇ 100g/L, pH8.2, 65℃and 6min.Acid plating: NiSO₄·6H₂O 30g/L , NaH₂PO2·H₂O 25g/L, (NH₄)₃C₆H₂O₇ 10g/L, C₃H₆O₃ 30ml/L, NaAC 10g/L, pH5.0, 85℃. The time is determined by coating thickness.(2) The amorphous and high-phosphorus Ni-P coating was gained by"JXD"electroless solution, and the coating has a great salt spray corrosion resistance; Phosphorus content of the coating is more than 12%, the microhardness reaches 670HV, these two indicators are higher than ordinary high-phosphorus coating. Cellular structure of the coating has larger size with pH increases.(3) The study indicates: Immersing zinc is one of the key processes of eleetroless Ni-P alloy plating. The proper Solution prescription and parameters of immersing zinc have been gained: ZnSO₄ 40g/L, NaOH 90g/L, NaF 1g/L Fecl₃ 1g/L, KNaC₄O₄H₄O₆ 10g/L, 40℃, 90 seconds for the firs time and 18 seconds for the second time. The replaced zinc film is light gray and it is rather uniform and compact.(4) "JXD" solution applies to AZ91 magnesium alloy, the stability of plating solution reaches 25 minutes with PdCl2 experiment and the service life reaches 5MTO.(5)Contrast with the coating of Lingming aero-enging corporation, the coating of"JXD"had the better density, uniform and microhardness.The composition, microstructure, microhardness, friction and wear properties, corrosion-resistant property and service life of Ni-P composite electroless plating are studied with the help of modern analytical instrument, such as the scan electron microscope, the photo electron spectroscopy, and so on. The results were as follows:(6) The coating was typical of the amorphous structure, and the coating takes on obvious crystal structure as the temperature reaches 500℃with the crystallized product Ni and Ni3P.(7) Corrosion resistance of the coating has a great impact by heat treatment, and corrosion resistance is improved with heat treatment temperature 100²00℃or higher than 450℃; but the corrosion resistance is rapid deteriorated with heat treatment temperature 200³50℃.(8)Under heat treatment temperature 200℃, microhardness of the coating is improved with temperature knock down; the microhardness is improved slowly with temperature 200³00℃; the microhardness is improved largely when heat treatment temperature surpass 300℃.(9)SiC particles turn up agglomeration in the non-vacuum condition for a long time. SiC particles is controlled the size into 1³Î¼m by disperse 5 minute, then, SiC particles can be easily deposited on the surface of the coating to form a Ni-P-SiC composite coatings.(10)The coating is affect by the coating microhardness, coating and the substrate bonding, the plastic, the lubricity of SiC. Ni-P-SiC composite coatings have the smallest friction coefficient(0.2), the best wear resistance; The wear resistance of Ni-P coating is better than aluminum alloy(YL12); The wear resistance of the coating is improved when heat treatment temperature at 500℃. During 144 hours salt spray corrosion, resistance of the coating has declined largely.