| Neodymium iron boron magnets are widely used in many energy fields due to their excellent magnetic properties,however,their corrosion resistance is poor.After the magnets have been prepared,a coating process is a must before used.Firstly,the coating process will inevitably generate waste gas and waste liquid,which does not meet the requirements of sustainable development;Secondly,as the magnet and the substrate are made up of different components,the coating layer will be peeled off with the change of temperature and humidity in the environment,leading to the failure of the coating.Then substrates with low corrosion resistance will be corroded seriously.Therefore,it is necessary to improve the corrosion resistance of the NdFeB substrate itself.The improvement of corrosion resistance for sintered Nd-Fe-B magnets is usually focused on the modification of grain boundary phase in the previous research,whose effects are not significant.The volume fraction of the main phase in NdFeB magnets is high,and some studies have shown that oxygen enters the main phase,causing it to decompose.Adjusting only the grain boundary phase without paying attention to the corrosion resistance of the main phase may be one of the reasons why the corrosion resistance of NdFeB is not high.Therefore,the role that improving the corrosion resistance of the main phase plays in improving the overall corrosion resistance of NdFeB needs to be investigated and verified.In addition,due to the negative standard electrode potential,Nd is prone to oxidation.Based on the general rule of corrosion resistance enhancement,it is assumed that the corrosion resistance of NdFeB magnets will be improved by adding elements with positive electrode potential,but studies which reported on the correlation between potential modification and corrosion resistance of NdFeB magnets are seldom.Based on the above issues,firstly,this paper verifies the key role of the main phase in enhancing the corrosion resistance of NdFeB substrates,as well as the correlation between potential regulation and the corrosion resistance of magnets;secondly,a super high corrosion resistance NdFeB magnet is designed and prepared to solve the problem of low corrosion resistance of NdFeB substrates,to gaining experience and rules for the preparation of new "rust-free" NdFeB magnets with low corrosion rates;finally,in the process of exploring the rust-free magnets,combined with the analysis of the microstructure and corrosion resistance mechanism,the following conclusions are obtained:1)Cr and Ni have a preferential distribution in the magnet.In the magnets containing Ni and Cr prepared separately,the results show that Ni is distributed in the grain boundary phase and Cr enters the main phase,enabling the preparation of potentially modification magnets in the grain boundary and main phases;2)Results shows that there is correlation between potential modification and the corrosion resistance of magnets.The potential modification of the main and grain boundary phases of the magnets was achieved through the selective distribution characteristics of Cr and Ni in the magnets,and thus the corrosion resistance of NdFeB magnets: The main phase has been potentiostatically modified to improve the corrosion resistance of the magnets,demonstrating the importance of the main phase in improving the corrosion resistance of the magnets;3)In terms of improving the corrosion resistance of magnets,the research results show that when the content of added elements is low,the effect in improving the corrosion resistance of the grain boundary phase modification is better than the main phase;On the other hand,further modification for the grain boundary phase has the limited effect on the overall corrosion resistance of magnets,requiring simultaneous modification of grain boundary phase and main phase phase to achieve the optimal corrosion resistance of magnets;4)The designed NdNi FeCrB magnets exhibit ultra-high corrosion resistance characteristics based on a potential-modified corrosion resistance mechanism: under the same tests conditions,the mass loss of NdNiFeCrB magnet is 91.2% lower than that of NdFeB magnets with the same rare earth content;5)By studying the mechanism of ultra-high corrosion resistance of NdNiFeCrB magnets,the key to improving corrosion resistance lies in: the small potential difference between the grain boundary and the main phase,and the stability of the grain boundary and the main phase have been improved;6)The improvement of magnetic properties of the magnets was achieved by the dual alloy method with NdNiFeCrB with NdFeB powders.The results show that the maximum energy product has been increased to 37.8 MGOe,the remanence to 12.56 k Gs,and it maintains ultra-high corrosion resistance characteristics at the same time;... |
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