Nickel coatings are a type of metal coatings that are applied to various objects for different purposes, such as decoration, corrosion resistance, wear resistance, or electrical conductivity.
Nickel coatings can be produced by different methods, such as electroplating, electroless plating, or spraying.
Compared to other types of coatings, nickel coatings have some features that make them suitable for certain applications.
- Nickel plating is the most frequently used surface treatment for iron and steel products.
- It accounts for 50% of the global nickel production.
1. Anti-Corrosion:
Neodymium magnets are susceptible to rust when exposed to humid environments. Nickel plating serves as a protective layer on the magnet’s surface, creating a barrier that prevents air and water from directly contacting the iron core. This significantly reduces the risk of corrosion.
2. Improved Aesthetics:
Uncoated neodymium magnets possess an unattractive dark gray appearance. Nickel plating enhances their visual appeal, providing a silver-white finish that is more appealing and aesthetically pleasing.
3. Enhanced Safety:
Before nickel plating, neodymium magnets are chamfered, resulting in smoother edges. This modification makes them less likely to cause injuries or harm to users, ensuring a safer experience.
4. Increased Surface Hardness:
Neodymium magnets have relatively low hardness, making them susceptible to scratches and wear. Nickel plating adds a hard protective layer that effectively reduces wear and scratches, enhancing surface hardness and overall durability.
5. Improved Processability and Assembly:
The nickel-plated layer exhibits a certain degree of lubricity, reducing the friction coefficient on the magnet’s surface. This property makes neodymium magnets easier to process and assemble. Consequently, it contributes to improved productivity and product quality.
It’s important to note that not all magnets require nickel plating. Some low-end applications may forego plating to reduce costs, opting for alternatives like galvanization. Additionally, the choice of coating or treatment may vary depending on the specific application and requirements of the magnet in question.
Nickel plating for neodymium magnets not only enhances their appearance but also provides crucial protection against corrosion, improves safety, and increases durability. These benefits make nickel-plated neodymium magnets a popular choice across a wide range of applications.
Advantages
Some of the possible advantages of nickel plating are:
It can provide superior corrosion resistance, especially against salt spray and acidic environments. Nickel plating forms a protective layer on the substrate metal that prevents oxidation and other corrosive elements from reaching the surface. This can reduce wear and tear on the metal parts over time, which can extend their lifespan and performance. Nickel plating is particularly effective for automotive bolts, electrical connectors, and marine applications that are exposed to harsh conditions.
It can enhance the hardness and durability of the substrate metal, increasing its resistance to wear and abrasion. Nickel plating can increase the strength and toughness of the metal by creating a dense and uniform coating that can withstand high stress and friction. Nickel plating can also improve the surface finish and smoothness of the metal, reducing friction and noise. Nickel plating is ideal for mechanical parts that require high wear resistance, such as gears, bearings, valves, and pistons.
It can improve the aesthetic appeal and value of the metal object, giving it a shiny and lustrous appearance. Nickel plating can produce different colors and finishes, depending on the process and additives used. The normal nickel-plated surface is dull gray in color with a matte finish, but it can also be made whiter, brighter, or more reflective by using brightening agents or post-treatments. Nickel plating can also be colored or chromated to create different effects and styles, such as blue, yellow, green, black, or iridescent. Nickel plating is widely used for decorative purposes, such as jewelry, coins, trophies, musical instruments, and household items.
Drawbacks
Nickel plating has some drawbacks that may make it less suitable for certain applications or environments.
Some of the possible drawbacks of nickel plating are:
Cost
Nickel plating can be more expensive than other coating options, such as zinc plating.
This can be a disadvantage for applications where cost is a significant factor.
Brittleness
Neodymium magnets are brittle, and the application of a coating like nickel can increase the risk of cracking or chipping.
This can be a disadvantage in applications where the magnet is subjected to mechanical stress or impact.
Limited Corrosion Resistance
While nickel plating provides a protective barrier against corrosion, it may not be suitable for highly corrosive environments.
In such cases, other coatings like zinc or gold may be more effective.
Types
Nickel coatings are thin layers of nickel that are applied to metal objects for various purposes, such as decoration, corrosion resistance, wear resistance, or electrical conductivity. Nickel coatings can be produced by different methods, such as electroplating, electroless plating, or spraying. There are different types of nickel coatings that have different characteristics and applications. Some of the most common types are:
Bright nickel plating
This is a type of electroplating that uses an aqueous electrolyte solution and an electric current to deposit a shiny layer of nickel onto a metal object. Bright nickel plating is typically used for decorative purposes and corrosion protection. It can also be combined with other metals, such as chromium or gold, to form a more attractive finish.
Electroless nickel plating
This is a type of plating that uses a chemical reaction to deposit a uniform layer of nickel onto a metal object without using an electric current. Electroless nickel plating is more resistant to corrosion and wear than electroplated nickel. It can also cover complex shapes and hard-to-reach areas. Electroless nickel plating can be further classified into different subtypes, such as nickel-phosphorus, nickel-boron, or composite coatings, depending on the composition and properties of the coating.
Dull nickel plating
This is a type of electroplating that uses an aqueous electrolyte solution and an electric current to deposit a matte layer of nickel onto a metal object. Dull nickel plating is usually used for engineering applications where high corrosion resistance, ductility, or electrical conductivity is important, and a high luster is not required. Dull nickel plating can also be used as an undercoat for other metals, such as chromium or silver, to improve their adhesion and durability.
Advantages over other coatings
Longevity and Protection
Nickel plating is generally required to ensure the longevity of the magnet, cleanliness, and to prevent oxidation.
It provides a protective barrier against corrosion and other environmental factors.
Increased Surface Hardness
Nickel plating can increase the surface hardness of neodymium magnets, making them more durable.
Resistance to Chipping and Wear
A multilayered metal plating like nickel improves the magnet’s resistance to chipping and wear, especially around corners.
It enhances the magnet’s durability in demanding applications.
Polishing and Appearance
Nickel-plated neodymium magnets have a brighter appearance, similar to the color of coins, as nickel is better in polishing compared to other coatings like zinc.
This can be advantageous for applications where aesthetics are important.
Widespread Method
Galvanic nickel plating is a widespread method used to coat sintered neodymium magnets.
It is a well-established and reliable process for providing a protective layer to the magnets.
Factors
These are some of the quality issues that can affect nickel coatings if they are not properly applied.
Porosity
One of the quality issues that can affect nickel coatings is porosity, which is the presence of tiny holes or gaps in the coating layer. Porosity can reduce the corrosion resistance, wear resistance, and appearance of the coating. Porosity can be caused by various factors, such as impurities in the plating solution, improper cleaning or pretreatment of the substrate, insufficient agitation or filtration of the solution, or excessive current density or temperature during plating.
Cracking
Another quality issue that can affect nickel coatings is cracking, which is the formation of fissures or fractures in the coating layer. Cracking can compromise the integrity and performance of the coating, as well as expose the substrate to corrosion. Cracking can be caused by various factors, such as high internal stress in the coating, thermal expansion or contraction of the substrate or coating, mechanical deformation or fatigue of the substrate or coating, or improper heat treatment or annealing of the coating.
Adhesion failure
A third quality issue that can affect nickel coatings is adhesion failure, which is the loss of bond between the coating and the substrate. Adhesion failure can result in peeling, flaking, blistering, or delamination of the coating. Adhesion failure can be caused by various factors, such as poor surface preparation or activation of the substrate, contamination or oxidation of the substrate or coating, mismatch of thermal expansion coefficients between the substrate and coating, or excessive thickness or hardness of the coating.
Common quality issues
Some frequently occurring plating problems that can occur if nickel is not properly applied are:
Pitting
This is when small holes or pits form on the surface of the nickel coating, creating a rough or uneven appearance. Pitting can affect the corrosion resistance, wear resistance, and aesthetic quality of the nickel coating. Pitting can be caused by gas bubbles, impurities, or contaminants in the plating solution or on the substrate.
To prevent pitting, the plating solution and the substrate should be clean and free of air, water, or organic matter before applying the nickel coating. The plating solution should also be agitated and filtered regularly to remove any particles or gas pockets.
Dark deposits
This is when the nickel coating has a dark or dull color, instead of a bright or shiny one. Dark deposits can affect the reflectivity, conductivity, and appearance of the nickel coating. Dark deposits can be caused by low pH, high temperature, high current density, or excessive organic additives in the plating solution.
To prevent dark deposits, the plating solution should be maintained at the optimal pH, temperature, and current density for the type of nickel coating being applied. The plating solution should also be monitored and adjusted for the amount of organic brighteners or levelers used.
Flaking or peeling
This is when the nickel coating detaches from the substrate, exposing the bare metal or material underneath. Flaking or peeling can affect the performance, durability, and appearance of the coated product. Flaking or peeling can be caused by poor adhesion, thermal stress, mechanical damage, or contamination of the coating surface.
To prevent flaking or peeling, the substrate should be properly prepared, cleaned, and roughened before applying the nickel coating. The coating should also be cured at the appropriate temperature and time, and protected from abrasion and impact.
Pinholes
These are small holes or gaps in the nickel coating that expose the substrate to the environment. Pinholes can increase the risk of corrosion, oxidation, or contamination of the coated product. Pinholes can be caused by dust, dirt, or foreign particles on the coating surface, or by insufficient coating thickness or coverage.
To prevent pinholes, the substrate and the coating should be clean and smooth before applying the nickel coating. The coating should also be applied in sufficient and consistent thickness, and inspected for defects before curing.
How to verify
One way to tell whether the protective film on a magnet is nickel or not is to:
- Use a spot test with dimethylglyoxime and ammonium hydroxide.
This is a chemical test that can detect the presence of nickel ions in a solution.
- If the coating is nickel, it will react with these chemicals and turn pink.
- This test is quick, easy, and inexpensive.
To perform this test, you will need the following materials:
- A drop of 1% dimethylglyoxime solution
- A drop of 10% ammonium hydroxide solution
- A cotton swab or a small brush
- A clean and dry metal object with the coating you want to test
The steps are as follows:
- Apply a drop of dimethylglyoxime solution to the cotton swab or brush.
- Rub the swab or brush gently on the coated surface of the metal object. You may need to scratch the surface slightly to expose the coating.
- Apply a drop of ammonium hydroxide solution to the same spot where you applied the dimethylglyoxime solution.
- Observe the color change. If the spot turns pink, it means that the coating is nickel. If there is no color change, it means that the coating is not nickel.
3. Features of nickel coatings
Nickel coatings can offer several useful features, and these features make them attractive options for a wide range of applications. These features include:
1). Corrosion resistance
The strong corrosion resistance that nickel coatings can offer is one of the main benefits. The electroplated nickel layer is highly stable in the air. Metallic nickel has a strong passivation ability. It can quickly generate a very thin passivation film on the surface, which can resist the corrosion of the atmosphere, alkali, and certain acids. Therefore, it’s an excellent choice for protecting neodymium magnets for various applications.
2). Surface hardness
Neodymium magnets’ surface hardness can be increased by nickel plating, making them more durable. This can be especially helpful for parts like gears and bearings that experience a lot of scratches.
3). Wear resistance
Nickel coatings can also improve wear resistance in addition to increasing surface hardness. This can be helpful for components like valve seats and pump impellers that experience friction or frequent contact.
4). Enhancement of appearance
Electroplated nickel has extremely fine crystals and excellent polishing properties. Polished nickel coatings can offer a mirror-like glossy surface, while retaining its luster in the atmosphere for a long time. Therefore, electroplated nickel coatings are often used for decoration.
5). Versatility
Nickel coatings are extremely adaptable, and they are applicable to a variety of metals and alloys. This makes them ideal choices for a variety of applications, from aircraft components to automobile parts.
4. Methods for verifying nickel coatings’ quality
The nickel coatings are ignorable surface treatment techniques. But it doesn’t mean they don’t have any quality issues. We still need to be careful about their finished results on neodymium magnets.
There are several methods that can be used to verify the quality of nickel coatings:
• Visual inspection: This is a simple and quick method that involves visual inspections on the surfaces of the nickel coatings on neodymium magnets to ensure that they don’t have defects. Usually, quality engineers can do it with their naked eyes.
• Microscopic examination: This method involves using a microscope to inspect the surface of the nickel coating at a higher magnification. This can help to identify any defects or imperfections in the coating that may not be visible to the naked eye.
• Surface roughness measurement: We measure surface roughness using a device such as a profilometer. This can help to ensure that the nickel coating has a smooth and even finish, which is important for many applications.
• Adhesion testing: Adhesion testing involves applying a force to the nickel coating to see how well it sticks to the neodymium magnets. We use a cross-cut tester or a scratch tester to do this test.
• Salt spray testing: This method involves exposing the nickel coating to a salt spray environment for a period of time and then examining the surface for signs of corrosion or other damage. Usually, the duration time depends on specific applications.
• Thickness measurement: We use a thickness gauge to measure the thickness of the nickel coating. This can help to ensure that the coating is applied to the correct thickness for the specific application.
• Hardness testing: It will need a Rockwell hardness tester to obtain the hardness of the nickel coating. This can help to ensure that the coating has the required level of hardness for the specific application.
5. Disadvantages of nickel coatings
Although nickel coatings have multiple advantages, they still have a few potential disadvantages that you need to beware.
• Sensitivity to heat: Nickel coatings are not very durable to high temperatures and may not be suitable for use in very hot environments.
• Corrosion resistance: Nickel coatings are generally resistant to corrosion, but they may not be as corrosion-resistant as some other types of coatings, such as PTFE and Parylene.
• Complexity of application: Applying a nickel coating can be a complex process, particularly if the coating needs to be applied to a complex shape or in a thin layer. This can make it more difficult to achieve a consistent and high-quality finish.
• For neodymium magnets with complex shapes or small deep holes or blind holes, we can’t obtain better coating surface results with this technique.
2. Different types of nickel coatings
1). Electroplated nickel
This type of nickel coating is applied using an electrolytic process, in which a current is used to deposit a layer of nickel onto the surface of a substrate. Substrates can benefit from electroplated nickel coatings thanks to better wear resistance, corrosion resistance, and aesthetics.
This is the most common technique that we use to apply nickel coatings onto neodymium magnets. Many coating technique suppliers use it for various products every day. It is fully optimized and less likely to have quality issues. Meanwhile, they can offer excellent cost-effectiveness, thanks to their wide applications.
2). Electroless nickel
This is a type of nickel coating that is applied using an autocatalytic chemical process rather than an electrolytic process. It offers an evenly distributed, robust, and corrosion-resistant coating that can be used on a range of substrate materials. This is one of the best surface treatment techniques in the world.
3). Nickel-chrome
This is a type of nickel coating that is applied over a layer of chromium. In applications where high temperatures and harsh conditions are a concern, nickel and chromium work together to provide a strong, corrosion-resistant coating.
4). Nickel-phosphorus
This is a type of nickel coating that is alloyed with phosphorus to improve its corrosion resistance and wear resistance. We usually use it in situations that need a lot of lubricities, like bearings and gears.
5). Nickel-tungsten
This is a type of nickel coating that is alloyed with tungsten to improve its hardness and wear resistance. We typically use it in applications requiring great wear resistance, including cutting tools and dies.
Initially, magnets were coated with a single layer of nickel, but this approach had its limitations.
- The single-layer nickel plating had high porosity, making it inadequate for effectively safeguarding the NdFeB substrate.
- Additionally, this plating method was cathodic in nature, meaning it posed challenges unless the nickel layer was exceptionally thick, typically reaching 20 to 25 micrometers.
However, an excessively thick nickel layer, being ferromagnetic, adversely affected the magnetic properties of the NdFeB magnets.
As a result, NdFeB single-layer nickel plating was not considered an ideal choice.
And then, a vast majority of magnet manufacturers have shifted to a more effective solution:
- Double-layer nickel plating, specifically nickel-copper-nickel plating.
This transition gained momentum in the late 1990s and is now the standard practice among magnet manufacturers.
The double-layer nickel plating provides enhanced corrosion resistance and offers several advantages:
1. Improved Corrosion Resistance: Double-layer nickel plating is significantly more resistant to corrosion compared to single-layer nickel plating, making it a superior choice for protecting NdFeB magnets.
2. Appearance and Cost: Not only does the double-layer nickel-copper-nickel plating offer better protection, but it also enhances the appearance of the magnets. Moreover, it is often more cost-effective than single-layer nickel plating.
In summary, when it comes to plating NdFeB magnets, the recommendation is to use double-layer nickel-copper-nickel plating (NiCuNi).
This approach not only ensures corrosion resistance but also enhances the magnet’s appearance and is cost-effective.
While single-layer nickel plating is still used for specific applications with lower corrosion resistance requirements, the vast majority of NdFeB magnets benefit from the superior protection offered by multi-layer plating combinations like NiCuNi.
PS: Double-layer nickel plating is preferred over single-layer nickel plating due to its superior corrosion resistance, attractive appearance, and cost-effectiveness.