What Is Involved in Electroless Nickel Plating?

The fastest growing type in electroless plating. The plating solution generally uses nickel sulfate, nickel acetate, etc. as the main salt, hypophosphite, sodium borohydride, borane, hydrazine, etc. as reducing agents, and various additives are added. The operation is performed in an acidic solution at 90 ° C, a neutral solution or an alkaline solution near normal temperature. It is divided into electroless nickel-phosphorus plating and electroless nickel-boron plating according to the difference in the use of reducing agents. The coating shows superiority in terms of uniformity, corrosion resistance, hardness, solderability, magnetic properties, and decorative properties.

Electroless Nickel

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Chinese name: Electroless Nickel
nickname: Electroless plating

Sexual performance: Corrosion resistance, hardness, solderability
reducing agent: Hypophosphite, sodium borohydride

Electroless plating is also called electroless plating, and it can also be called Autocatalytic plating [1]. The specific process refers to a process in which metal ions in an aqueous solution are reduced by a reducing agent and precipitated on the surface of a solid substrate under certain conditions. As defined in ASTM B374 (ASTM, American Society for Materials and Testing), Autocatalytic plating is "deposition of a metallic coating by a controlled chemical reduction that is catalyzed by the metal or alloy being deposited". This process is different from replacement plating. The plating layer can be continuously thickened [2], and the metal plating itself has catalytic ability.

Under the catalysis of the catalyst Fe, the hypophosphite in the solution is catalytically dehydrogenated on the catalytic surface to form an active hydride and is oxidized to phosphite; the active hydride reacts with nickel ions in the solution to deposit nickel, It itself oxidizes to hydrogen. which is:

2H2PO2- + 2H2O + Ni2 + Ni0 + H2 + 4H ++ 2HPO32-.

At the same time, part of the hypophosphite in the solution was reduced to elemental phosphorus by the hydride and entered the coating. which is:

H2PO2-+ [H +] (catalytic surface) P + H2O + OH-, the electroless coating formed is NiP alloy, which has an amorphous sheet structure. ^[1]

No external current, borrow

After years of continuous exploration and research, electroless nickel plating has developed extremely well in recent years. Electroless nickel plating water is suitable for almost all metal surfaces. Such as: steel nickel plating, stainless steel nickel plating, aluminum nickel plating, copper nickel plating, etc., it is also suitable for non-metallic nickel plating. For example: ceramic nickel plating, glass nickel plating, diamond nickel plating, carbon plate nickel plating, plastic nickel plating, resin nickel plating and so on. The range of use is very wide.

History of Electroless Nickel Plating

All chemical deposition methods can be divided into three categories (general classification):

Since the electroless nickel plating has excellent comprehensive physical and chemical properties such as uniformity, hardness, wear resistance and corrosion resistance, this technology has been widely used, and it is almost difficult to find an industry that does not use electroless nickel plating. According to reports, the proportion of electroless nickel plating in various industries is roughly as follows: aerospace industry: 9%, automotive industry: 5%, electronic computer industry: 15%, food industry: 5%, machinery industry: 15%, nuclear industry : 2%, petroleum industry: 10%, plastic industry: 5%, power transmission industry: 3%, printing industry: 3%, pump manufacturing industry: 5%, valve manufacturing industry: 17%, others: 6%. The application of chemical nickel plating in industrialized countries in the world has experienced unprecedented development in the 1980s, with an average annual net increase rate of 10% to 15%; it is expected that the application of chemical nickel plating will continue to develop and the average annual net value rate will decrease to about 6%. , And entered the mature stage of development. In the booming East and Southeast Asia, including China, the application of electroless plating is on the rise, and it is expected to maintain unprecedented high-speed development.

Electroless nickel aerospace

The aerospace industry is one of the major users of electroless nickel plating. The most prominent application example is the Oklahoma Aviation Logistics Center in the United States. Since 1979, and Northwest Airlines has used electroless nickel plating to repair aircraft since 1983. Engine parts. Although Pratt & Whitney's JT8D jet engine has been discontinued, so far there are still thousands of such engines used on Boeing 727 and McDonnell Douglas DC-9 aircraft because of a high phosphorus, compressive stress Electroless nickel plating technology is used to repair the impellers of six types of JT8D jet engines, ensuring the reuse of this engine. On the blades of turbines or compressors of aviation engines, the nickel-phosphorus alloy is usually electrolessly plated to a thickness of 25 to 75um to prevent gas corrosion, and its fatigue strength is reduced by 25% less than electroplated chromium. The Oklahoma Aviation Logistics Center uses ultra-thick layers of chemical nickel plating to repair aircraft parts, with a plating thickness of 275-750um. The original rework rate was 50% when the electroplating process was used, and the pass rate was more than 90% after electroless nickel plating. It can be seen that significant economic benefits have been achieved. After the auxiliary generator (APU) on the aircraft is electroless nickel plated, its life span is increased by 3 to 4 times. The main bearing surface of the 8.2-ton turbine engine is electroless nickel plated 100um to prevent vibration damage caused by startup and shutdown.

In order to reduce weight, the aviation industry uses a large amount of aluminum alloy parts. After chemical nickel plating, the surface is not only corrosion-resistant and wear-resistant, but also weldable. For example, the piston head of a stroke engine is chemically nickel-plated to increase its service life. Others include titanium alloy parts and beryllium alloy parts, which use low and compressive stress electroless nickel plating to protect the surface.

Nickel + rhenium + boron ternary alloy electroless plating (NTB) is designated as the surface strengthening process for more than 160 parts on Pratt-Whitney jet engines to resist abrasion and fretting wear. For example: NTB electroless plating Sealed on the main shaft of the jet engine. The United States Air Force requires the engine manufacturer to provide 4,000 tactical cycles, at which time the wear amount reaches 0.178mm. Therefore, it must be disassembled and repaired. After the NTB electroless plating, the wear of the spindle sealing surface is significantly reduced. mm.

Metal light mirrors are widely used in aerospace systems. The substrate is high-strength, light-weight beryllium or aluminum, which is strengthened by a special electroless nickel plating surface. This electroless nickel plating with a phosphorus content of 12.2% to 12.7% can be polished to 9? Such high accuracy has excellent performance in cosmic space that requires low inertia.

Although China's chemical plating industry started late, after the unremitting efforts of various scientific research units since the 1990s, it now has mature technology and experience, and has established an electroless nickel plating processing line for aircraft parts in Luoyang, China.

Electroless nickel plating automotive industry

Solving the problem of using ethanol and gasoline mixed fuels is one of the development trends of the automobile industry. In addition to performance problems, the use of mixed fuels also causes corrosion problems in the fuel system. In Brazil, the use of ethanol as a fuel and the application of electroless nickel plating to protect zinc die-castings, such as vaporizers, from corrosion has become a process specification. In the United States, when methanol or a mixture of methanol and gasoline is widely used, the automotive industry is bound to use electroless nickel plating as a surface protection method for carburetor and fuel pumping systems.

The differential planetary gear is an important part of the car. It is plated with a 25um thick chemical nickel plating layer to improve wear resistance. Some automobile manufacturers use polytetrafluoroethylene composite chemical nickel plating on the shaft. The coating has both a certain hardness and good lubricity, which improves the service life of the shaft.

The automotive industry takes advantage of the very uniform electroless nickel plating and uses electroless plating to protect parts with complex shapes, such as gears, radiators and injectors. The aluminum heat sink plated with an electroless nickel plating layer of about 10um has good solderability. After the electroless plating on the gears, the dimensional error is easily maintained at ± 0.3 to 0.5um. If the electroplating process is used, it must be machined after plating to reach the acceptable tolerance range. Electroless nickel plating on the injector provides good resistance to fuel corrosion and wear. Generally, fuel corrosion and wear will lead to the expansion of the injection hole, so the amount of fuel injection will increase, causing the horsepower of the car engine to exceed design standards and speeding up engine damage. Chemical nickel plating can effectively prevent the corrosion and abrasion of the injector, and improve the reliability and service life of the engine.

Chemical nickel plating chemical industry

The chemical industry applies research on chemical nickel plating technology instead of expensive corrosion-resistant alloys to solve the problem of corrosion, in order to improve the purity of chemical products, protect the environment, improve the safety of operations and the reliability of production and transportation, so as to obtain more favorable technological and economic competitiveness.

Electroless plating is widely used to protect the inner walls of large reaction vessels. At first, a very interesting application example was: In 1955, the United States General Transportation Company (GATX) used electroless plating to protect the inner walls of tank cars to prevent caustic corrosion. Nowadays, the electroless nickel plating technology has made great progress and can provide reliable protection in a variety of chemical corrosive environments.

The most widely used chemical nickel plating is valve manufacturing. Steel ball valves, gate valves, rotary valves, check valves and butterfly valves, etc., are plated with high phosphorus chemical nickel 25 ~ 75um, which can improve corrosion resistance and service life. The effect of chemical nickel plating for chemical pumps is also significant. Valves working under caustic corrosion conditions should use low-phosphorus electroless nickel plating with a phosphorus content of 1% to 2%. Because under caustic corrosion conditions, the annual corrosion rate of the low-phosphorus electroless nickel plating layer is about 2.5um, which is better than that of the medium or high-phosphorus electroless nickel plating layer. Electroless nickel plating is not resistant to corrosion in strong oxidizing acids such as concentrated nitric acid and concentrated sulfuric acid. Although the corrosion rate in hydrochloric acid is lower than that of austenitic stainless steel, the corrosion resistance is still insufficient. Therefore, for the above-mentioned strong acid medium, or the medium which may be hydrolyzed to generate the above-mentioned strong acid, it is not suitable to use the electroless nickel plating layer. Carbon steel fasteners are plated with a 25-50um thick high-phosphorus chemical nickel plating layer instead of stainless steel fasteners, which not only overcomes the problem of stress corrosion cracking of austenitic stainless steel, but also saves a lot of costs. The inner wall of the low-density polyethylene pressure vessel is 25um to prevent iron contamination and the resulting polyethylene from discoloring. If it is made of stainless steel, its price is about twice that of the electroless plating method.

Chemical nickel plating oil and gas

Oil and gas are one of the important markets for electroless nickel plating. Electrochemical nickel plating technology is widely used in oil field production and pipeline equipment. The typical corrosive environment of the oil and gas industry is downhole brine, carbon dioxide, hydrogen sulfide, with temperatures up to 170-200 ° C, accompanied by erosion of sand and other abrasive particles, etc. The corrosive environment is quite harsh. Under such severe conditions, the low-carbon steel oil and gas pipeline has a life of only 2 to 3 months. After being protected by a 50-100um thick high-phosphorus electroless nickel plating layer, its corrosion rate is reduced to a level comparable to that of Hastelloy. Considering the high cost of corrosion-resistant alloys, from a performance-price ratio perspective, carbon steel pipelines have the best technical and economic performance of chemical nickel plating protection.

Pump housings, impellers, and outlet pipes for oil and gas pumps are chemically nickel-plated to a thickness of 25 to 75um, depending on the corrosion environment. Electroless nickel plating of oil pumps is an ideal application example: In the West Texas oil field, the oil pumps protected by chemical nickel plating have a service life of more than 4 years, and the life of the unprotected oil pumps does not exceed 6 months. The chemical nickel plating layer is resistant to corrosion and abrasion, and because of the high uniformity of the chemical nickel plating layer, the oil pumping cylinder can be made as a whole, thereby significantly improving the quality of the oil pumping pump and reducing the production cost. In oil fields, high-phosphorus chemical nickel plating is also widely used on the surface of heaters in oil-water separation devices to prevent corrosion. The plating thickness is usually 25 to 75um. Valves, pipe joints, pipe clamps, etc. of the oil collecting and oil transmission devices are also protected by chemical nickel plating.

Chemical nickel plating food processing industry

The food processing industry provides a huge potential market for the application of electroless nickel plating; the reason why it is called a potential market is that there are obstacles in the widespread application of electroless nickel plating in the food industry. For example, in the United States, the FDA (United States Food and Drug Administration) has not yet formulated regulatory standards for the application of electroless nickel plating in the food industry; usually, for cases where electroless nickel plating is used in direct contact with food, the FDA takes a case The method of processing is approved. The reason is mainly because the classic formula of electroless plating solution contains toxic heavy metal ions such as lead and cadmium as stabilizers. However, many modern electroless nickel plating solutions no longer use heavy metal ions as stabilizers; obviously, this barrier will be removed sooner or later. Parts in food packaging machinery that do not come into direct contact with food, such as bearings, rollers, conveyor belts, hydraulic systems, and gears are typical applications of electroless nickel plating in the food industry.

During food processing, there will be problems such as saline, nitrite, citric acid, acetic acid, fumigation of natural wood, volatile organic acids and other corrosive media; food processing temperature ranges from 60 to 200, and relative humidity in the production environment is very High; under such conditions, food processing equipment suffers from metal corrosion, fatigue, and wear. For metal surfaces that come into contact with food, the traditional protection method is electroplated hard chromium; however, in acidic media containing chloride ions, the corrosion resistance of the chromium plating layer is not good; however, chemical nickel plating has a uniform plating ability and high corrosion resistance. , Anti-sticking, mold release and other aspects have obvious advantages. Electroless nickel plating on parts in contact with food on the kneading machine is one of the successful applications; others are used in food filling cans, screw feeders, mixing pots, food molds, baking trays, drying boxes, bread holding furnaces Food machinery is increasingly using electroless nickel plating.

Chemical nickel plating industry

The mining industry has harsh environmental conditions, and underground machinery will inevitably come into contact with salt water and mineral acids to withstand the test of corrosion and wear. Therefore, mining machinery needs surface protection.

In mine roof support systems, electroplated hard chrome is often used as a corrosion-resistant and wear-resistant protective layer for hydraulic props. However, due to the cracks and porosity of the hard chromium surface, the hydraulic props are often bitten and unable to operate due to severe corrosion. This problem is exacerbated by high-pressure hydraulic cylinders. Under high pressure operation, the coating is stretched, which makes the crack of the hard chromium layer with high internal stress further intensified. In this case, a 25um thick compressive high-phosphorus electroless nickel plating layer is used for protection. When the hydraulic prop is stretched under high pressure, the electroless nickel plating layer will not crack and can withstand the corrosion and wear of the underground coal mine environment .

In some open-pit mining production, such as the phosphate ore used for fertilizer selection, high-pressure pumps and jet nozzles are used. At this time, the corrosion and erosion problems are quite serious, but the application of corrosion-resistant and wear-resistant chemical nickel plating This prevents premature damage to mechanical parts.

Electroless nickel plating military industry

Electroless nickel plating technology is widely used in the military. Prominent examples are the chemical nickel plating protection of the catapult hoods and rails of aircraft carriers. The working environment of the catapult is very harsh. The high-temperature airflow scours the track when the aircraft is launched, the huge force during the catapult, and the corrosion of the marine climatic conditions make the catapult system useable for only 6 to 12 months. The current surface treatment process is: the ejector hood after correct pretreatment, after electroplating nickel, electroless nickel plating of 100um, and then electroplating of cadmium 12.5um, and passivation with chromic acid. Such a composite coating protective layer has good abrasion resistance and anti-vibration wear performance, and the service life of the ejection system can be extended to 14-18 years, that is, 18 times.

The trunnion of military vehicles has been protected by chemical nickel plating for many years to prevent corrosion and wear of road mud and salt water.

The rearview mirror of the tank is made of aluminum. After fine grinding and polishing, electroless nickel plating is used as a corrosion-resistant and wear-resistant protective layer. Technical requirements The rearview mirror has a reflectance of 80% in the visible spectrum, and the chemical nickel plating layer easily meets these optical requirements. The aluminum radar waveguide is plated with a 25um thick electroless nickel coating to prevent corrosion on land and at sea. The uniformity of the electroless nickel plating can meet the technical requirements of various waveguides.

Electroless nickel plating computer

Electroless nickel plating is the most widely used in the electronics and computer industries, involving almost every electroless nickel plating technology and process. Many new electroless nickel plating processes and materials have been developed according to the needs of the development of the electronics and computer industries. In terms of technical performance, in addition to corrosion and abrasion resistance, it also has requirements for weldability, diffusion resistance, electrical properties and magnetic properties.

Some countries have established regulations: Electronic equipment must be shielded to prevent electromagnetic and radio frequency interference. The bi-metal structure coating of the electronic device's plastic shell is copper-plated and then electroless-nickel-plated, which is considered to be one of the most effective shielding methods. Electroless nickel plating is one of the key steps in the manufacture of computer thin-film hard disks. Mainly, a 12.5um thick nickel-phosphorus alloy layer is plated on the finely processed 5086 magnesium aluminum surface to prepare for the subsequent vacuum sputtering magnetic recording film. The electroless nickel plating layer has a phosphorus content of 12% wt (about 20.5% atomic percent). The coating must be low and compressive. After being heated at 250 ° C or 300 ° C for 1 hour, it still remains non-magnetic at this time, that is, the residual magnetism is less than 0.1 × 10-4T. The coating must be uniform and smooth, and any defects and protrusions on the surface must not exceed 0.025um. Because of the high technical requirements, it is necessary to use a high-quality and clean special chemical plating solution, fully automatic plating control equipment and a high-clean workshop environment. Computer film hard disk electroless nickel plating is a typical representative of high-tech electroless nickel plating, occupying a very important market share.

The application of electroless nickel plating technology in the manufacturing of microelectronic devices is growing rapidly. Xerox reportedly uses selective nickel-phosphorus alloy electroless plating technology in the filling and leveling process of interconnects and via-holes of VLSI multilayer chips; all of its products have passed the resistance Tests for shear strength, tensile strength, high and low temperature cycling, and various electrical properties. Practice shows that the application of electroless nickel plating technology improves the technical economy and product reliability of microelectronic device manufacturing processes.

Electroless nickel plating other industries

Various types of molds, such as injection molding machines and die-casting molds, are large-scale products in the machinery and light industry industries. Due to the complicated geometry of the mold, when the surface of the mold is strengthened by electroplating, in order to enable plating on all sides, complex auxiliary anodes and hangers must be designed and installed; moreover, post-plating machining must be performed to ensure Requirements for dimensional accuracy and surface roughness; moreover, the electroless nickel plating layer has a low coefficient of friction and outstanding mold release performance, making it one of the most cost-effective mold surface treatment technologies.

Casting models and core boxes are usually cast iron or cast aluminum parts, which are subject to abrasive wear during use and are quickly scrapped. After adopting electroless nickel-plated nickel surface protection, the quality of the casting model and core box is improved, and the service life is significantly improved.

Textile machinery rotates at a high speed, and various fiber yarns are very serious for the wear of mechanical parts. Electroless nickel plating, especially synthetic polycrystalline diamond composite electroless plating technology, has successfully solved the problem of wear of textile machinery parts.

Various rollers and components on the printing press are protected by a 25-50um thick electroless nickel plating layer to prevent corrosion of printing inks and whitening liquids. The high uniformity of the electroless nickel plating layer can ensure the dimensional accuracy of the printing roller without the need for mechanical processing after plating.

Some medical devices, such as surgical forceps, dental drills, and medical molds, have been electroless nickel-plated instead of electroplated chromium.