Study On Surface Protection Of Sintered Nd-Fe-B Permanent Magnets

The corrosion resistance of Sintered Nd-Fe-B was the focus in the world.Because it was relation to the use of Nd-Fe-B magnets.Electroless Ni-P alloy and Zinc-Chromium coating were applicated in anti-corrosion of Nd-Fe-B magnets.As for the loose and porous problem,different pretreatable processes were studied.and the effect on anti-corrosion for Nd-Fe-B magnets was studied under the best process.Besides effects of coating thickness on porosities and the composition and structure of coating were studied. Zinc-Chromium film,called Dacromet in foreign country,has high corrosion resistance,as well as no hydrogen brittle,no pollution,good combination with substrate and the organic coat,uniformity,and etc,so it is used widely.The production,performance,structure and composition of the film were investigated in this article systematically.The reseach result show that griding-alcohol rinsing-blocking holes-chemical degreasing-chemical degreasing-hot water rinsing-pickling-cold water rinsing-activating-cold water rinsing-electroless Ni-P-hot wate rinsing-cold water rinsing-passivating-hot water rinsing-cold water rinsing-dring are the more fitting pre-treatable process for Nd-Fe-B substrate. Under the best pre-treatable.process,acid coating was not directly deposited onthe surface of Nd-Fe-B magnets.but alkali was .The low porosites and best corrosion resistance amorphous Ni-P coating can be obtained.by the double coating of acid (topcoat) /alkali (undercoat).The optimum content of Zn-Cr film coating bath have been concluded as follows; Zinc powder 250g/L, Aluminium powder 80g/L,CrO3 50g/L, H3BO3 10g/L,baking temperating 300"C, baking time 20 min through the quadrature experiment.The analysis of X-ray diffractometer show that the Zn-chromium film has lots of Zinc and Aluminium powders,which are combined by oxide.The structures have efficient protection to Nd-Fe-B substrate.Comparism amony Zinc-chromium, Ni-P coating and Zinc plating was made.It is show that Zn-chromium film is much better than Ni-P coating and Zinc plating.