Sintered ndfeb magnet since 80 s, is currently the strongest permanent magnetic material, high magnetic energy product and high coercive force, high remanence and other excellent performance and high cost performance, and is widely used in computer, microwave communications, telecommunications engineering, audio and video technology, high-energy micro-motor, medical, aviation, environmental protection and other high-tech fields.
But in a Nd -
B the neodymium permanent magnet is a kind of rare earth elements, its content is higher,
36% ~ 38%)
And Nd metal is a very lively, the standard equilibrium potentials is -
4 v 31 in easily in the wet air oxidation, contact with water and acid will release hydrogen and corrosion;
Second, by rich Nd2Fe14B NdFe B magnet phase, rich, rich Nd phase B phase of three-phase.
Interaction potential of each phase is different, will inevitably cause the electrochemical reaction, which form a corrosion cell.
All of these make the corrosion resistance of the material.
At present, to prolong the service life of the permanent magnet, the protective measures taken by the sintered ndfeb magnet are electroplating, electroless plating, chemical conversion coating, electrophoresis, spraying, etc.
Among them, electroplating and chemical plating is a commonly used method.
But in a Nd -
B magnet surface plating has certain difficulty.
Main problems of plating on the NdFeB magnet is: NdFeB of neodymium oxide extremely easily, former improper handling will cause the loss of the coating binding force;
Is made of powder sintered NdFeB magnet, rough surface loose and there are a large number of pore, seeping into the acid, alkali in the process of plating and the plating solution, causing flooding point after plating and bubbling, even leading to the corrosion of the substrate and coating;
Surface uneven, due to the magnet will cause plating coating porosity increases, the lower protective coating.
Aiming at these problems, we must choose the appropriate pretreatment process, to ensure that the adhesion strength of the plating and anti-corrosion performance.
1 the corrosion phenomenon of sintered ndfeb magnets in use process and cause analysis material decides the particularity of sintered ndfeb magnet is vulnerable to corrosion in use process.
Show is bubbling, rust, leather, color, etc.
Analysis the cause of corrosion products, internal cause and external cause.
1 internal cause,
The difference between materials of various material properties;
The uneven between each phase composition;
Materials in different grain size differences between the parts;
The complex structure of the porous;
A defect in a material.
2 external cause major penetration of erosion of the outside world into the substrate, the substrate from corrosion or after coating corrosion and the corrosion of substrate.
All kinds of corrosion at the end of the day, from electrochemical point belongs to galvanic corrosion;
Tell from the corrosion morphology, belong to intergranular corrosion.
2 and the relationship between the surface treatment of sintered ndfeb magnet material is the basis of the surface treatment.
In the process of production materials, not only should consider whether magnetic parameters standard but also for the future lay a good foundation for surface treatment, at the same time, it should be pay attention to the adverse influence on the material in the processing course.
Thus, to improve the quality of coating, the first thing to understand the nature of the material, material of weightlessness, density, oxygen content and surface residual magnetism, energy product, coercive force and the average particle size distribution of powder will affect the quality of the plating.
Loss of major products, easy to appear when do the PCT test coating bubbling phenomenon, serious when coating completely broke, the underlying pulverization completely.
Hydrogen evolution when the density is small products, electroplating products, and create conditions for the residual plating solution of pore, coating surface roughness, porosity increase, affect the corrosion resistance of coating.
Large area of the product, to minimize the edge and the density differences.
Products in the high oxygen content, in storage, chamfering, degreasing, pickling, moisture corrosion environments, such as easy to corrode, the plating product surface pockmark, porosity and other bad phenomenon, serious impact on the corrosion resistance of the coating.
Products in the high energy product, high coercive force, hardness and brittleness is big, will appear in the process of handling, chamfering, electroplating missing Angle phenomenon, the phenomenon such as porosity, sand holes also increases, and the material is so lively, easy oxidation, bring greater difficulties for electroplating.
Uneven distribution of powder, crystal uneven density of products, easy in weak places appeared when pickling corrosion phenomenon, appear even local small cracks.
Surface residual magnetism products both in theory and practice prove that the coating quality is bad.