Surface treatment and plating of NdFeB

We all know that the sintered NdFeB permanent magnet material is produced by powder metallurgy technology. It is a kind of material with relatively active chemical properties. There are tiny pores inside, and the Nd-rich phase is easy to oxidize in the air. After the material is corroded or damaged by the components, the magnetic properties will be attenuated or even lost over time, thereby affecting the performance and life of the whole machine, so strict anti-corrosion treatment must be carried out before use. Previous articles have introduced several common coatings for you. Today we will systematically sort out and compare the common coatings of NdFeB.

Surface protection treatment can be divided into two categories: wet and dry surface treatment

The wet method is the surface protection treatment of the magnet in pure water, inorganic solution or organic solution, such as electroplating, electroless plating, electrophoresis, spraying and dipping. Dry method is a surface protection treatment performed by physical or chemical processes on the magnet without contacting the solution, such as physical vapor deposition (PVD) and chemical vapor deposition (CVD). Among them, physical vapor deposition includes vacuum evaporation plating, vacuum sputtering plating and ion plating (IVD). Different coating methods have their own strengths and need to be selected for different application environment requirements.

Plating & Coatings

Plating neodymium magnets is an important process to protect the magnet against corrosion. The substrate NdFeB (Neodymium, Iron, Boron) will oxidize quickly without a protective layer. Below is a list of platings and their effectiveness in certian environments.

All of the Neodymium Iron Boron we stock are protected. The majority are Ni-Cu-Ni plated which consists of 3 layers (Nickel, Copper, and Nickel).

Note: All coatings / platings must be done at the factory before the magnet is saturated (charged). The magnetic field will disrupt the electroplating process, while the heat required for other types of platings would demagnetize the magnet.

SuperiorExcellent > Good > Bad

 

Nickel (Ni-Cu-Ni)

Excellent  Good Good Bad Bad Bad  Bad 10-20 Microns  Low  


Zinc (Zn)

Excellent Excellent Excellent Good Good Bad Good 8-10 Microns Low  


Gold (Ni-Cu-Au)

Excellent Good Good Bad Bad Bad Bad 10-20 Microns High
 

Black Epoxy (Ni-Cu-BE)

Superior Bad Superior Superior Excellent Excellent Good 15-25 Microns Medium  
 

Raw Epoxy (BE)

Superior Good Superior Superior Excellent Excellent Superior 10-20 Microns Medium  
 

Plastic (ABS)

Superior Superior Superior Superior Superior Superior Excellent >800 Microns High  
 

Teflon (PTFE)  

Superior Superior Superior Superior Superior Superior Bad 250-500 Microns High  
 

Everlube-6155

Superior  Superior Superior Superior Superior Superior Good  8-25 Microns High   
 

Xylan

Superior  Superior  Superior Superior Superior Superior Good 15-30 Microns High 

 
 

Parylene

Superior  Superior  Superior Superior Superior Superior Good 15-30 Microns High   
  from: amazingmagnets.com

 

Nickel (Ni-Cu-Ni)

The most common plating. Ni-Cu-Ni is a durable 3 layer (nickel, copper, nickel coating). Great for indoor use. They may be used outdoors if protected from rain and humidity. Good abrasion resistance.
Nickel - Surface treatment and plating of NdFeB
 

Zinc Plating (Zn)

Zinc is a standalone (one layer) plating. Zinc is self- sacrificing, meaning when it starts to oxidize the outside turns white creating a durable layer of protection.
Zinc - Surface treatment and plating of NdFeB
 

Gold (Ni-Cu-Ni-Au)

A very thin / cosmetic layer of gold. Usually produced with an underlying layer of nickel to create a brighter finish.
Gold - Surface treatment and plating of NdFeB  

Black Epoxy (Ni-Cu-BE)

Black Epoxy plating is consists of 3 layers Nickel, Copper, and Epoxy exposed as the top layer. Great for outdoor applications. However, black epoxy is not as abrasion resistant as other platings. In harsh conditions the epoxy layer may be scrached off to expose the copper layer underneath.


Epoxy - Surface treatment and plating of NdFeB
 

Raw Epoxy (BE)

Raw Epoxy is designed specifically for gluing applications where the magnet requires a good adhesion to epoxy glue. This coating consists of one layer of epoxy covering the magnet.
RawEpoxy - Surface treatment and plating of NdFeB  

Plastic (ABS)

ABS Plastic is highly corrosion resistant. This coating requires an injection mold to be created in order to encase the magnet. The plastic shell needs to be thicker than other coatings so the plastic can flow during the injection process. Multiple colors available.
 Plastic - Surface treatment and plating of NdFeB  

Teflon (PTFE)

Teflon is very resilient and can withstand harsh environments. It is highly corrosion resistant. An injection mold is required to create the shell around the magnet. There will be a noticeable mold seam around the magnet where the injection mold separates.
 Teflon - Surface treatment and plating of NdFeB  

Everlube 6155

Everlube 6155 is a aluminized barrier coating specially formulated to maximize adhesion and corrosion protection when applied to rare earth magnets. The coating is very durable and provides excellent chemical and corrosion resistance.
 Everlube - Surface treatment and plating of NdFeB  

Xylan

Xylan coatings are fluorocarbon coatings that contain dry lubricants. Slippery and highly corrosion resistant. Multiple colors available.


 Xylan - Surface treatment and plating of NdFeB

Wet surface treatment

The main methods include phosphating, electroplating, electroless plating, electrophoresis, spraying, and dipping. They can be used alone or in combination.

Phosphating

The phosphating process is a process in which the magnet is placed in a solution containing phosphate, and the solution reacts with the surface of the magnet to form a phosphating film. Phosphating film is generally gray and difficult to contain water, so it can improve the water absorption and corrosion resistance of the magnet, and can be used alone to deal with short-term corrosion resistance requirements or occasions where the use environment is not high.
20210101222328 45101 - Surface treatment and plating of NdFeB

Electroplating

The electroplating process is to connect the magnet to the cathode of a DC power supply and immerse it in a solution containing cations of the coating material. The cations will migrate to the cathode under the action of an electric field and be converted into metal atoms to crystallize on the surface of the magnet.
Sintered NdFeB commonly used zinc plating and nickel plating, and nickel-copper-nickel composite plating.
Zinc is relatively stable in dry air, and a zinc carbonate film is formed in moist air or oxygen-containing water, which can delay the corrosion rate of zinc, but it has better corrosion resistance in acid-base salt solutions, marine atmosphere, and high-temperature and high-humidity air. Poor, passivation treatment can significantly improve the corrosion resistance of the zinc coating.
Nickel easily forms a very thin passivation film with oxygen in the air, and has good corrosion resistance to the atmosphere, alkalis and some acids at room temperature. Therefore, nickel plating has become the most common electroplating method for sintered NdFeB. However, nickel is an excellent soft magnetic material, which will form a shield to the magnetism of the substrate, especially when the magnet is small or thin. The corrosion risk of single-layer nickel is relatively high, and multi-layer plating or composite plating can solve this problem well.
Copper has a lively chemical nature and is easy to rust, so it is generally not used alone, but as a bottom coating or an intermediate layer to improve the bonding force between the substrate and the surface coating.
20210101222419 55180 - Surface treatment and plating of NdFeB

        Nickel plated on the surface                             Galvanized surface

Electroless plating

Electroless plating is the same as electroplating. It also undergoes oxidation-reduction reaction. The metal ions in the plating solution are reduced to atoms and adhere to the surface of the magnet. The difference is that there is no current to attract ions and enhance the adhesion of atoms. Therefore, a reducing agent is required to coexist in the plating solution. , The surface of the substrate also needs catalysis. No power supply is the biggest advantage of electroless plating. It can form a coating with uniform thickness on the surface of a magnet with complex shapes. The coating has high hardness, small voids and high chemical stability.

Electrophoresis

Unlike phosphating and electroplating, the surface protective layer formed by electrophoresis, spraying and dipping is organic. Electrophoresis is a phenomenon in which the charged colloidal particles in the conductive dispersion medium move towards the opposite electrode under the action of an external electric field. Electrophoretic coating uses this feature to make the charged organic paint molecules (usually epoxy resin) firmly adsorb on the opposite electrode. On the surface of the magnet, the magnet is taken out from the electrophoresis tank and then cured to form a dense protective film.
20210101222555 74488 - Surface treatment and plating of NdFeB

Dry surface treatment

In dry coating, the chamfering and degreasing steps of magnets are similar to wet coating, but dry coating does not need to make the chamfer of the magnet too large, because the film thickness of dry coating is more uniform. The film thickness is low under the same corrosion conditions, and the corner effect is relatively small.

Physical vapor deposition (PVD)-vacuum ion evaporation (IVD)

Vacuum ion evaporation is a kind of physical vapor deposition method, developed from the vacuum evaporation process. Vacuum evaporation is to evaporate the metal material to be plated by resistance heating, arc heating or sputtering in a vacuum environment. The atomic metal is deposited on the surface of the magnet, and the crystal grows into a metal film that completely covers the magnet. The process is the most mature It is resistance heating evaporation aluminum plating. Vacuum ion plating accelerates the deposition rate of aluminum vapor by loading a high negative bias between the evaporation source and the magnet, and further improves the adhesion of the magnet to the aluminum film.

Chemical Vapor Deposition (CVD)

The application of chemical vapor deposition para-xylene polymer (Parylene parylene) has been developed into the field of NdFeB surface protection in recent years. The coating process is carried out in a vacuum environment. The parylene is heated and sublimated into a gas-phase monomer. It is deposited on the surface of the lower temperature magnet and polymerized into a solid parylene film. Parylen has a very low water vapor transmission rate and can provide excellent moisture and corrosion protection.

20210101222627 22242 - Surface treatment and plating of NdFeB
Source: China Permanent Magnet Manufacturer – www.ymagnet.com

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