What is neodymium iron boron electroplating technology
With the rapid development of rare earth NdFeB permanent magnet materials, the problems in NdFeB plating are becoming more and more obvious, which has become an important factor restricting its development. This paper summarizes several difficulties of NdFeB electroplating, then takes the current technology of NdFeB electroplating as the line, analyzes and introduces the production status, existing problems and production equipment of NdFeB electroplating technology in detail. Then, based on these processes, the development direction of improvement problems, process and equipment is put forward.
Since the advent of rare earth Nd2Fe14B permanent magnet materials in the mid-1980s, they have been rapidly applied in many industries due to their unique characteristics of high magnetic energy product, high coercivity and high remanence.
Since the end of 1980s, China began to produce Nd-Fe-B materials in batches, and then increased rapidly year by year, reaching 52.9% of the world output in 2001 and 78.6% of the world output in 2008.
Nd-Fe-B material is a fine alloy. Its main components are Fe (about 65%), B (about 1%) and re (about 33%).
The potential difference between the nd rich phase, B rich phase and the main phase of the magnet alloy makes it easy to form galvanic cell and cause electrochemical corrosion on the surface of the material.
In addition, the Nd-Fe-B magnet is produced by powder metallurgy process, which makes the actual density of the material unable to reach the theoretical density. There are micro pores and voids in the material. It is easy to absorb oxygen in the atmosphere to oxidize the rare earth elements and destroy the alloy components. After material corrosion or component damage, the magnetic properties will be attenuated or even lost for a long time, which will affect the service performance and life of the whole machine. Therefore, strict anti-corrosion treatment must be carried out before use.
At present, electroplating, electroless plating, chemical conversion film, electrophoresis, spraying and other methods are widely used in the anti-corrosion treatment of NdFeB, among which electroplating is widely used as a mature means of metal surface treatment.
Difficulties in NdFeB electroplating
The Nd-Fe-B magnets are mostly small workpieces (mass 0.25g-20g), so the plating production is mainly barrel plating, supplemented by hanging plating. However, the barrel plating of NdFeB parts is much more difficult than that of ordinary steel parts.
The neodymium chemical activity in the material is very strong
Nd-Fe-B parts even contact with water will produce hydrogen and corrode:
- ① Before plating, acid and alkali should not be used too strongly, otherwise the material matrix is easy to be corroded, and because chlorine in hydrochloric acid reacts strongly with neodymium, hydrochloric acid should not be used;
- ② When pre plating or direct plating, simple salt plating solution (such as watt nickel, potassium chloride zinc plating) is selected, which is easy to oxidize the parts and affect the adhesion between the coating and the substrate, and the parts are corroded and the plating solution is polluted;
- ③ It is difficult to select the large-size drum (which will affect the production capacity), otherwise the mixing cycle will have a greater impact and the parts will be seriously oxidized.
NdFeB magnets are magnetic functional materials
The metal coating will affect its magnetic properties, so how to coordinate the relationship between the type, combination and thickness of the coating (these properties are related to the corrosion resistance of the coating) and the magnetic properties of the product is the difficulty of NdFeB electroplating.
The surface of NdFeB magnet is porous and rough
The micro surface area is much larger than the macro surface area
- ① The surface dirt is not easy to clean, and the burden of pretreatment is heavy;
- ② When pre plating or direct plating, it is impossible to select complex plating solution with good adhesion and corrosion resistance (because the current efficiency of complex plating solution is low, it is not easy to deposit on the surface of porous parts), such as alkali copper, alkali zinc, etc.
Nd-Fe-B material is brittle
The surface is easily damaged, so:
- ① It is more difficult for workers to operate;
- ② It is difficult to select the large-size drum (otherwise, the parts are easily bumped due to strong overturning), which will affect the improvement of labor production efficiency.
Production status of NdFeB electroplating technology
At present, the production of NdFeB electroplating mainly adopts three processes or process combinations, namely zinc plating, nickel + copper + nickel plating and nickel + copper + electroless nickel plating. The others, such as gold plating, silver plating, tin plating and black nickel plating, are generally plated on the basis of the above three processes.
Pretreatment before plating
Due to the special properties of Nd-Fe-B materials (strong chemical activity, loose and porous surface, etc.), the pretreatment of Nd-Fe-B electroplating technology has always been a major difficulty. However, after years of efforts, this problem has been largely solved.
- 1. Chamfering, i.e. polishing. This process can make the surface of the parts flat, smooth and reduce the micro area, which is conducive to the rapid, uniform and continuous deposition of the coating. The horizontal planetary calender and vibration polishing machine (generally referred to as chamfering machine) are used in the equipment, which respectively adopt the planetary motion and vibration principle, and can achieve the finishing purpose without damaging the parts. Horizontal planetary calender is mostly used for the finishing of Nd-Fe-B small-size parts, while vibration calender is mostly used for the finishing of Nd-Fe-B large-size parts.
- 2. The acidity and basicity of degreasing, pickling, activation degreasing and pickling solutions should not be too strong to avoid corrosion of parts. In addition, it is necessary to add neodymium chelating substances in the treatment solution to prevent oxidation of neodymium. Hydrochloric acid should not be used in pickling solution and activation solution.
- 3. The cavitation effect of ultrasonic treatment can completely remove the oil, acid and alkali in NdFeB micropores. The pretreatment of NdFeB plating mostly adopts ultrasonic treatment technology in small amount or large amount according to the situation. In actual operation, a few parts are usually filled with plastic net for manual shaking cleaning. After ultrasonic degreasing, acid pickling, water washing and other steps are completed, the parts are loaded into the roller for electroplating. Although the labor intensity of this operation is a little higher, the parts are cleaned thoroughly and the effect is good.
Because the potential of Neodymium in the material is very negative, neodymium iron boron zinc plating can not play a very obvious anodic protection effect on the substrate like ordinary steel zinc plating, so the density of the coating is of great significance to the corrosion resistance. The density of alkaline zinc coating is high, but the current efficiency of alkaline zinc plating solution is low, so it is difficult to directly plating on the porous NdFeB surface.
At present, the common potassium chloride zinc plating process is generally used for NdFeB zinc plating, but the process belongs to the type of simple salt bath. If the plating is directly applied on the surface of NdFeB, the problems such as poor adhesion of the coating, corrosion of parts and pollution of the bath will appear. To solve these problems, we should start with how to make the surface of parts plated as soon as possible. The faster the plating, the slower the surface oxidation, and the lighter the problems. At present, we take the following measures:
- ① The bath with high current density upper limit is used;
- ② Small size slender roller is adopted;
- ③ Live into the tank, high current impact, uninterrupted operation between processes, etc.
After 2007, according to the requirements of EU ROHS directive, the traditional heavy pollution hexavalent chromium passivation process was replaced by the new light pollution trivalent chromium passivation process. At present, with the development of trivalent chromium passivation commercial solution, the trivalent chromium passivation film system mainly composed of blue, white and color has been formed.
However, the corrosion resistance of trivalent chromium passivation film decreases greatly compared with hexavalent chromium.
Analysis: hexavalent chromium passivation film is thick and has self-healing ability, trivalent chromium passivation film is thin, and the interference of impurities in the coating is obvious, so continuous coating can be formed on the surface of pure zinc. Therefore, the corrosion resistance of trivalent chromium passivation film will inevitably decline.
Plate with nickel
At present, Ni + Cu + Ni (i.e. “pre Ni + intermediate Cu + surface bright Ni”) composite coating system is generally used for NdFeB nickel plating.
The purpose of pre nickel plating is to provide a positive potential, dense structure of the bottom layer for subsequent copper plating, to ensure the normal application of subsequent copper plating, to prevent the substrate from being corroded by the copper plating solution, mainly focusing on the adhesion between the coating and the substrate, deep plating ability and so on.
The bonding strength and deep plating ability of pre plating copper are better, but it is difficult to be used for the backing of NdFeB substrate, because the current pre plating copper process belongs to the type of complex plating solution, the current efficiency of the solution is low, and the continuous and qualified copper coating can not be obtained on the porous NdFeB substrate.
Watt nickel plating process is often used in NdFeB pre nickel plating, and semi bright nickel additives are used appropriately. The purpose of using additives is not to pursue brightness, but to use large current density, which is conducive to the rapid deposition of the coating. Watts nickel also belongs to the type of simple salt bath, because it needs to be directly plated on the NdFeB substrate, many requirements (such as bath, roller and operation, etc.) are roughly the same as NdFeB zinc plating.
The standard bright nickel plating process is often used for bright nickel coating. The current bright nickel plating process is mature enough and will not be described more. Few manufacturers use sulfamate nickel plating process.
Generally, the average thickness of NdFeB pre nickel plating is required to be no less than 4 ~ 5 μ m to ensure that the low area coating of parts is completely covered and prevent the corrosion of subsequent copper plating solution. The thickness of nickel coating is 8 ~ 10 μ m to ensure the corrosion resistance of the coating. In this way, the total thickness of nickel layer is 12-15 μ M. Nickel belongs to ferromagnetic metal, its coating not only does not produce magnetic output, but also shield the magnetic output of NdFeB magnet. The thicker the coating, the greater the shielding effect.
The magnetic properties of small size magnets less than 0.5g can be attenuated by 10-15% with the present Ni + Cu + Ni composite coating system of NdFeB. How to reduce the amount of nickel plating without affecting the subsequent copper plating and coating anti-corrosion is the key to solve or improve the problem, which is a major difficulty of NdFeB nickel plating.
Neodymium iron boron copper plating refers to the intermediate copper plating between the pre nickel plating and the surface nickel plating. The purpose is to increase the total thickness of the coating by relying on the copper layer to reduce the thickness of the surface nickel layer
- ① Copper is a non-magnetic metal, and its magnetic shielding is less than that of nickel. The loss of magnetic properties caused by magnetic shielding of nickel layer can be reduced by replacing part of nickel with copper;
- ② The porosity of copper is lower than that of nickel, which can improve the corrosion resistance of the coating;
- ③ The coating cost can be reduced;
- ④ For small products with larger area volume ratio (also known as specific surface area), especially for ultra-small products, the influence of nickel layer thickness on magnetic properties of magnets is greater, and the significance of reducing nickel layer thickness is greater.
Copper plating is the main hidden trouble of the instability of “Ni + Cu + Ni” combination process for NdFeB. The consensus of the industry is that the cyanide copper plating process is better, the solution is stable, the anti pollution ability is strong, the deep plating ability is good, the brightness of the coating is uniform, soft, the stress is low, and all aspects of the performance are balanced and stable. However, cyanide is a highly toxic substance, and the state has strict management and use restrictions on it. At present, only a few manufacturers use this process.
The acid copper process has very high requirements for pre plating. If it is not well controlled, it may cause the corrosion of NdFeB substrate, and the bonding strength of semi bright nickel backing by acid copper plating is very poor. At present, semi bright nickel backing is generally used for NdFeB plating, so in the current “Ni + Cu + Ni” system of NdFeB plating, at least rolling copper plating is not recommended (hanging copper plating is discussed separately).
At present, pyrophosphate copper plating process (about 85%) is widely used in NdFeB copper plating, followed by citrate copper plating process developed in recent years. Years of production practice shows that under the condition of fine control, the two processes can basically meet the requirements of NdFeB copper plating. However, the two processes are still unsatisfactory.
1. Pyrophosphate copper plating
Pyrophosphate copper plating began to be used in NdFeB electroplating industry in 1997.
- ① If the solution concentration is high, the copper ion content is more than 18G / L, the Baume degree of the solution is about 35, and the viscosity of the solution is high, the amount of the solution is large in the production process, and the process cleaning is complex. It is suitable for manual production line, and is not easy to be used in automatic production line.
- ② The solution parameters change quickly, but the range of process parameters is narrow (the ratio of pyrophosphate to copper ion is between 7.5 and 8.8).
- ③ The analysis of pyrophosphate is difficult and the accuracy is low, so it is often difficult to judge whether the ratio of pyrophosphate to copper ion is in the normal range.
- ④ The application of pyrophosphate copper plating process is narrow, there are few brightener manufacturers, and the brightener technology is not very mature, which makes the adding control of brightener much more difficult than nickel plating.
In a word, the pyrophosphate copper plating process is often difficult to achieve advanced control, always rely on experience to adjust and recover after the coating quality problems, and it is difficult to timely supplement and adjust through the analysis of solution parameters to restore the stability of the solution, so it is easy to cause production quality accidents.
2. Citrate copper plating is a new process developed and applied after 2003. It is rarely used in other industries and almost belongs to the special copper plating process for magnet industry. The process also has difficulties in analysis, lack of control means and other problems, but the main problem is the bacteria in the solution. If the treatment measures are improper, the surface brightness of the workpiece will be partially blurred, which will affect the product quality.
3. The common problem of the above two processes is that pyrophosphate and citrate are weak complexing agents of copper ions, so the two processes will have higher requirements on the quality of the bottom nickel layer (thickness, coverage, missed plating, etc.), otherwise the phenomenon of copper replacement will occur, which will affect the adhesion of the coating and pollute the copper plating solution. At present, the stability of Nd-Fe-B barrel plating coke copper solution is slightly worse than that of steel barrel plating coke copper solution, which indicates that the Nd-Fe-B substrate is gradually corroded by coke copper solution, and the product will pollute the solution.
Generally, NdFeB copper plating is produced by the above two processes, and the average thickness of the bottom nickel layer is required to be no less than 4 ~ 5 μ m, so as to ensure that the bottom coating is completely covered and avoid copper replacement. However, the thickness of the nickel base layer increases the total thickness of the nickel layer, especially for the small-size products.
Source: China Neodymium Iron Boron Magnet Manufacturer www.rizinia.com