Neodymium iron boron permanent magnet material plating

- Nov 21, 2017 -

General situation

Nd-Fe-B permanent magnet since the advent of the 80's, is the most magnetic permanent magnet material, with high magnetic energy product, high coercivity, high remanence and other excellent performance and high cost performance, and widely used In the computer, microwave communications, telecommunications engineering, audio-visual technology, high-energy micro-motor, medical, aviation, environmental protection and other high-tech fields. However, Nd in Nd-Fe-B permanent magnets is a rare earth element with a high content (36% to 38%) and Nd is a very active metal with a standard equilibrium potential of -2.4 to 31 V in wet Of the air is easily oxidized, contact with water and acid will release hydrogen and corrosive; Secondly, NdFe B magnet Nd2Fe14B phase rich, Nd-rich phase, B phase rich B phase composition. The potential of each phase in contact with each other is different, will inevitably lead to electrochemical reactions, the formation of corrosion cells. All of these make the material corrosion resistance decreased.

 Currently, in order to extend the service life of permanent magnets, the protective measures taken are plating, electroless plating, chemical conversion coating, electrophoresis and spraying. Among them, electroplating and electroless plating are the more common methods. However, there is some difficulty in plating the surface of the Nd-Fe-B magnet. NdFeB magnet coating on the main problems are: NdFeB Nd easily oxidized, improper pretreatment will result in reduced coating adhesion; NdFeB magnet powder sintering is made of a rough surface loose and there is a large number of pores in the plating In the process of infiltration of acid, alkali and electroplating solution, resulting in pan-point plating and bubbling, and even cause corrosion of the substrate and the coating; due to uneven surface microstructure of the magnet, plating porosity can cause an increase, reducing the protective coating . In response to these problems, we must choose the right pre-treatment process, in order to ensure the bonding of the coating and anti-corrosion performance.

1.1 neodymium iron boron material classification [1]

NdFeB iron content of about 60%, carbon content of about 0.0003% ~ 0.0004%. Sulfur content of about 0.0006% ~ 0.0008%, which should be classified as low-carbon iron-based alloys. The production process of the material melting, crushing, pressing, sintering, cutting, grinding, coupled with loose porous material, which can be zoned NdFeB powder metallurgy materials.

 As the material contains rare earths such as neodymium and praseodymium with very low equilibrium potential, the material is very lively and prone to spontaneous dissolution or displacement during electroplating. Therefore, it should be a very active and easily oxidized but not passivated metallic material.

According to the characteristics of the material, NdFeB can be seen in the surface treatment is difficult to plating substrate, which determines the NdFeB compared with ordinary metal materials are very different, if the treatment of hardware ideas and surface treatment methods For NdFeB does not work.

1.2 neodymium iron boron materials in the course of corrosion and the reasons for the analysis [1]

The particularity of the material determines that NdFeB is highly susceptible to corrosion during use. Performance for the bubbling, rust point, blasting, discoloration and so on. Analysis of the causes of corrosion products, there are internal and external causes

1.2.1 internal factors

(1) the difference between the various material properties in the material;

(2) non-uniformity between the ingredients of each phase;

(3) the difference between the various parts of the material caused by different grain sizes;

(4) porous and complex structure;

(5) defects in the material.

1.2.2 External factors

Mainly to the outside world of infiltration of infiltration substrate, so that the substrate was corroded or corrosion of the coating and then corrosion of the substrate. All kinds of corrosion in the final analysis, from the electrochemical point of view belongs to galvanic corrosion; from the corrosion morphology, belonging to intergranular corrosion.

1.3 neodymium iron boron material and the relationship between surface treatment [1]

Material is the basis of surface treatment. In the process of making materials, not only to consider whether the magnetic properties of the parameters are up to standard but also for the subsequent surface treatment to lay a good foundation, but also pay special attention to the external processing of materials on the adverse effects. Therefore, in order to improve the quality of the coating, we must first understand the nature of the material, weight loss, density, oxygen content, surface remanence, energy product, coercivity, average particle size distribution of the powder will affect the quality of the plating.

(1) heavy weight loss products, prone to PCT test bubble bubbling phenomenon occurs when the coating is completely burst, the underlying completely powdered.

(2) The products with low density have serious hydrogen evolution during electroplating and the pores create the conditions for the residual plating solution. The surface of the coating is rough and the porosity is increased, which affects the corrosion resistance of the coating. Large area products, to minimize the difference between the edge and the central density.

(3) Products with large oxygen content are highly susceptible to corrosion in storage, chamfering, degreasing, pickling, wetting and other corrosive environments. The products on the plated surface have such defects as mane and porosity that they seriously affect the coating Corrosion resistance.

(4) Products with high energy and coercive force show high hardness and brittleness. When carrying, chamfering, nicking phenomenon occurs in the electroplating process. The phenomenon of stomata and blister will also increase, and the material is very lively. Easily oxidized, to bring greater difficulties plating. (5) uneven distribution of powder, uneven crystallization of dense products, pickling prone to over-eclipse in weak places, and even local small cracks.

(6) Surface remanence products, whether theoretically or practically proven to the coating quality are not good.

Article from NdFeB Industry Network

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