A Study On Tin And Babbitt Metal Coating On Magnesium Alloy (AZ91D) Substrate

As the lightest structural and recycled easily metal, magnesium and its alloys are increasingly being utilized in many fields such as aerospace and computer industries. However, their applications are limited severely for their low friction/wear property and corrosion resistance. A protecting tin and Babbitt metal coating on the surface of AZ91D magnesium alloy was obtained by hot-dipping process, based on Ni（p） transition layer. The interface morphology affect by anneal treatment, Corrosion resistance and friction property were studied in this paper. The obtained conclusions are summarized as follows:Ni（P） transition layer can form during 10-15min by electroless plating, Ni（P） layer can protect the surface of work piece not be oxidized and reduce the free energy of interface. Fluxing agent mainly with NH₄HCl₂ and ZnCl₂ can effectively remove the oxide film generated in the process of hot-dip, and increase the wetting properties between substrate and solution.We get the tin plating layer and Babbitt coating successfully on the surface of AZ91D magnesium alloy by technology of hot dip. Detection the coating interfaces between Ni（P）-Sn by electron microscope, there have been significant diffusion phenomenon. Resulting in intermetallic compound buffer layer, the interface binding force is good. The studies of intermetallic compounds have shown that heat treatment was a clear role in promoting on the Ni（P）-Sn compound. The compound mainly is Ni₃Sn₄ and it showed scallop-shaped in-depth to the coating, the intermetallic compounds possess corresponding thickness, and distribute with state of "layer", Ni3P layer along the interface acted as a good diffusion barrier Sn diffuse into Ni. this behavior can ensure the protection of layer thickness and substrate performance.Appropriate anneal treatment can increase corrosion resistance coating, tin layer after anneal treatment for 1h does not appear corrosion points in neutral salt spray test more than 72 hours, electrochemical polarization curves show the corrosion potential of tin layer increased about 0.6V than the magnesium alloy, this makes clearly that the hot-dipping Sn coating has excellent corrosion resistance. But the corrosion resistance of work piece treated by 2h significantly decreased, The test shows that this is because Ni-Sn compounds destruction of single-phase structure of the coating after spread to the surface, and micro-cracks were observed on surface.Friction tests show that Pure tin plating on friction only at the beginning of tens of seconds time the coating shows good antifriction effect. Babbitt metal coatings exhibited excellent friction/wear properties under low load 0.5N show a low friction coefficient （average 0.145）, and the matrix is 0.397. On the 2N load conditions, the average friction coefficient is 0.25, also has good reduction friction performance. However, when the test load is 0.6N,0.75N, 1N, The coating exhibit a friction reducing properties at the beginning of the test, the average coefficient of friction even more than the matrix.This study results show that technology of hot dip can be applied to magnesium alloy, the process with high production efficiency and does not require expensive equipment. Babbitt metal obtained by hot-dip coating both with corrosion-resistant and wear resistance, coating a high binding force.There are few literature relating to the present studies, the conclusions obtained in present researches provide a scientific basis for preparing high quality films and thick coating on Magnesium alloy substrate.