Study On The Low-pollution Process Of Pre-electroless Ni-P And The Deposition Mechanism On AZ91D Magnesium Alloys

Magnesium alloys,which have excellent mechanical properties,physical properties special advantages characteristics as low density,high strength to weight ratio,have been widely used in many fields such as automobile, aerospace and3C products. Magnesium alloys are named as the "environmental engineering material in the21st century". A metallic coating to magnesium can enhance corrosion resistance, wear resistance,solder ability, conductivity and improve artistic appearance of magnesium alloys.However, the adhesion and protective performance of the coatings have been a difficult issue to resolve.In this thesis, AZ91D magnesium alloys which are used widely in industry are used as the research material. A technique of novel pre-electroless nickel on magnesium alloys is developed in order to obtain a basic coating with good adhension for the follow-up planting. Based on special characteristics of the AZ91D magnesium alloys and the protection of environment, the new technique does not us does not use Cr(VI) and HF in its pretreatment.Therefore is a much environmentally friendly process.Weighing method is used to investigated the deposition rate of the pre-electroless nickel; metallographic microscopy and SEM are used to observes the surface morphology of the coating;Thermal shock test and bending test are used to test the adhension of the coating; Salt test and a porosity test are used to evaluate the anti-corrosion of the coating; Use EDS and XRD to analyse the element and form,studying the effect of pretreatment conditions,the best technological conditions of the pre-electroless Ni-P on AZ91D magnesium alloy, and discusses the pre-electroless nickel phosphorus deposition mechanism. The results of the experiments show that: (1) Pretreatment can strongly influence the corrosion rate,surface morphologies of the Mg substrates and the quality of the coatings.By comparing four kinds of pickling and activation processes to determine that (C2H2O4+H3PO4)+NH4HF2is the best pretreatment process. After this process, there are better occlusive force and corrosion resistance between the Ni-P coating and the substrate.(2) Determine the best of main component of the Pre-electroless Ni-P on AZ91D magnesiu malloy.Themain-salt:20g/L,NaH2PO2:15g/L,(NH4)3C6H5O7·2H2O:8g/L, K4P2O7·10H2O:30g/L,temperature:75-80℃, pH:9, time:50min.The surface of novel pre-electroless nickel coating is evenly,good adhesion to the substrate coating,and show excellent anti-corrosion. The novel technology is simple and the solution is without Cl-, SO42-, avoiding the erosion of magnesium substrate.(3) Through studying on three areas of the morphologies of the Ni-P coating of the different deposition time,the catalytic activity and the deposition and the phosphorus content coating of the surface to analyze the deposition mechanism of pre-electroless Ni-P on AZ91D magnesium alloy.The catalytic activity of magnesium alloy mainly comes from that the oxide or hydroxide of the surface that is generated in the activation dissolved in the bath caused replacement reaction between the substrate and nickel ions of the bath. In deposition initial,phosphorus content is fluctuating trend but tend to a fixed value eventually.