A method of depositing a layer of nickel on the surface of a metal part by oxidation reduction without external current. Used to improve corrosion resistance and wear resistance, increase gloss and aesthetics. The method of depositing a layer of nickel on the surface of a metal part by oxidation reduction using no external current. Used to improve corrosion resistance and wear resistance, increase gloss and aesthetics. Bright nickel plating suitable for tubular or complex small parts, no need to be polished. Generally, the plated part is immersed in a mixed solution prepared by using nickel sulfate, sodium dihydrogen phosphate, sodium acetate and boric acid, and changes in a certain acidity and temperature, and the nickel ion in the solution is reduced by sodium dihydrogen phosphate. Deposited on the surface of the part for the formation of a fine nickel coating. Steel parts can be directly nickel plated. Tin, copper and copper alloy parts are first contacted with aluminum sheets on their surfaces for 1-3 minutes to accelerate electroless nickel plating.
Electroless plating is a method of obtaining a metal alloy by using a redox reaction on a plated member having a catalytic surface without being energized. It is a new technology that has recently been developed.
A Brief History of Electroless Nickel Plating The history of electroless nickel plating is relatively short compared to electroplating. In foreign countries, its real application to industry was only in the late 1970s and early 1980s. In 1844, A. Wurtz discovered that metallic nickel can be deposited from the aqueous solution of metallic nickel salts by reduction of hypophosphite. The true discovery of electroless nickel plating technology and its application to date was discovered in 1944 by the American National Bureau of Standards A. Brenner and G. Riddell, clarifying the catalytic properties of the coating and discovering the deposition of non-powdered nickel. The method makes the application of electroless nickel plating technology possible. However, the electroless nickel plating solution at that time was extremely unstable, so there was no practical value in the strict sense. The application of the electroless nickel plating process is nearly ten years behind the laboratory research results. After the Second World War, the US General Transportation Company became interested in this process. They wanted to plate nickel on the inner surface of the caustic soda, and ordinary plating methods could not be realized. Five years later they studied the development of electroless nickel plating. Many patents have been published on the technology of phosphorus alloys. In 1955, they produced their first test line and made a commercially useful electroless nickel plating solution. The commercial name of this electroless nickel plating solution is "Kanigen". At present, electroless nickel plating in foreign countries, especially in the United States, Japan, and Germany, has become a very mature high-tech, and has been widely used in various industrial sectors.
China's industrial production of electroless nickel plating started late, but in recent years, it has developed very rapidly. Not only has a large number of papers been published, but also a national electroless plating conference, according to the statistics of the article published at the 5th Electroplating Annual Meeting. There are already more than 300 manufacturers, but this number should be extremely conservative at the time. It is speculated that the current annual total market for electroless nickel plating in China should be around 30 billion yuan, and it will develop at a rate of 10% to 15% per year.
Electroless nickel plating classification
All chemical deposition methods can be divided into three categories (general classification):
Displacement plating
1. Displacement plating (ion exchange or charge exchange deposition): A metal is immersed in a metal salt solution of a second metal, a local dissolution occurs on the surface of the first metal, and a second metal is spontaneously deposited on the surface thereof. In the case of ion exchange, the base metal itself is the reducing agent. The most widely used base metals (Me1) are copper, iron and nickel, while the most used plating metals (Me2) are gold and copper. If a nail is immersed in a copper sulfate solution, a thin layer of copper is applied to the nail. However, its practical application is limited because the surface of the base metal is once stopped by the metal (Me2) in the solution, and the process is stopped immediately. Therefore, its maximum thickness is small, and the bonding force is not as good as true electroless plating. Due to the poor quality of the coating and limited thickness, the application is very limited.
2. Contact Plating: A method of depositing a metal to be plated with another metal or another piece of the same metal and immersed in a salt solution of the deposited metal. Contact deposition occurs when the surface of the conductive substrate to be plated is in contact with a more active metal than the metal to be deposited in the solution. A pair of galvanic cells is formed between the substrate and the contact metal, wherein the contact metal is an anode, and dissolution occurs, and the substrate to be plated functions as a cathode, and the metal is deposited thereon. This method is the same as the electrodeposition reaction except that the current is derived from the chemical reaction and not from the external power source. This method has almost no practical significance, but it is of great significance for initiating chemical deposition on a non-catalyzed active substrate and acting as a "reaction starter".
3. True electroless plating: depositing metal from a solution containing a reducing agent. The electroless nickel plating we have mentioned below is this.
(2) Classification of electroless nickel plating:
1. According to the PH value of the plating solution: there are three types of acid, neutral and alkaline.
2. Classified according to deposition temperature: there are three types of low temperature, medium temperature and high temperature.
3. Classified by alloy composition: there are three types of low phosphorus, medium phosphorus and high phosphorus.
4. Classified according to the reducing agent used: Ni-P, Ni-B, etc.
Nickel plating mechanism and characteristics
(1) Mechanism of electroless nickel plating
Electroless nickel plating is the reduction deposition of nickel ions in a solution onto a catalytically active surface with a reducing agent. Electroless nickel plating can use a variety of reducing agents. Currently, the most common industrial application is the electroless nickel plating process using sodium hypophosphite as a reducing agent. The reaction mechanism is generally accepted as "atomic hydrogen theory" and "hydride theory". ".
1) Atomic hydrogen theory
According to atomic hydrogen theory, Ni2+ in solution is reduced to metallic nickel by the atomic active hydrogen released by the reducing agent sodium hypophosphite (NaH2P02), instead of H2PO2- directly interacting with Ni2+.
First, under heating, sodium hypophosphite hydrolyzes on the catalytic surface to release atomic hydrogen, or H 2PO2-catalytic dehydrogenation produces atomic hydrogen. Then, H atoms adsorbed on the surface of the active metal reduce Ni2+ to metal Ni deposition. On the surface of the plated parts. At the same time, the hypophosphite is reduced by phosphorus by atomic hydrogen, or phosphorus is deposited by self-oxidation and reduction reaction, and the precipitation of H2 may be caused by hydrolysis of H2POf or by hydrogen bonding of atomic state.
2) hydride theory
According to the hydride theory, sodium hypophosphite decomposition does not release atomic hydrogen, but releases a more reducing hydride ion (hydrogen anion H), which is reduced by hydrogen negative ions.
In the acidic plating solution, H2PO2- reacts with water on the catalytic surface, and in the alkaline plating solution, the nickel ions are reduced by the hydride ions, that is, the hydrogen anion H can simultaneously react with H20 or H+ to release hydrogen: Phosphorus is reduced and precipitated.
(2) Characteristics of electroless nickel plating
To date, the development of electroless nickel plating has been around for more than 50 years. After more than half a century of research and development, electroless nickel plating has entered a mature stage, and its current status can be summarized as: mature technology, stable performance, diverse functions and wide range of uses.
Coatings deposited by electroless nickel plating have some properties different from those of electrodeposited layers.
1 When sodium hypophosphite is used as a reducing agent, phosphorus and nickel are co-deposited due to phosphorus precipitation. Therefore, the electroless nickel plating layer is a nickel-phosphorus alloy coating in which phosphorus is in a dispersed state, and the mass fraction of phosphorus in the plating layer is 1% to 15 %, the nickel-phosphorus coating obtained by controlling the phosphorus content is dense, non-porous, and the corrosion resistance is much better than electroplating nickel. When borohydride or aminoborane is used as the reducing agent, the electroless nickel plating layer is a nickel boron alloy plating layer, and the boron content is 1% to 7%. Only the coating obtained by using hydrazine as a reducing agent is a pure nickel layer, and the nickel content can reach 99.5% or more.
2 high hardness and good wear resistance. The hardness of the electroplated nickel layer is only l60-180 HV, and the hardness of the electroless nickel plating layer is generally 400-700 HV. After proper heat treatment, the hardness of the electroplated nickel layer can be further increased to be close to or even exceed the hardness of the chrome plating layer, so the wear resistance is good and more rare. It is an electroless nickel plating layer with good corrosion resistance and wear resistance.
3 high chemical stability and good adhesion of coating. In the atmosphere and in other media, the chemical stability of the electroless nickel layer is higher than the chemical stability of the electroplated nickel layer. It has good bonding with ordinary steel, copper and other substrates, and the bonding force is not lower than the bonding force between the electroplated nickel layer and the substrate.
4 Due to the difference in the amount of phosphorus (boron) in the electroless nickel plating layer and the heat treatment process after plating, the physical and chemical properties of the nickel plating layer, such as hardness, corrosion resistance, wear resistance, electromagnetic properties, etc., have various changes. It is rare in other plating types. Therefore, the industrial application and process design of electroless nickel plating is characterized by diversity and specificity.
Treatment of nickel plating waste liquid
1. Removal of nickel ions
In the electroless nickel plating waste liquid, if there is no amount of complexing agent or complexing agent, the pH value can be directly adjusted by using sodium hydroxide (concentration: 6 mol/L), and the nickel ion concentration in the waste liquid is added according to the concentration of nickel ions in the waste liquid. An appropriate amount of NaOH is used to precipitate nickel ions as Ni(OH)2. For the removal of nickel from the complex waste liquid, first adjust the pH of the waste liquid to about 8 with CaO, remove most of the organic acid complexing agent, and then add CaO or NaOH to the waste liquid to adjust the pH of the waste liquid. The value is 11-12, which causes most of the nickel ions and other heavy metal ions in the waste liquid to precipitate, and then add appropriate amount of high-molecular flocculant to accelerate the sedimentation of insoluble matter. During the sedimentation process, a suitable and appropriate amount of oxidant is added. Potassium permanganate, hydrogen peroxide or chlorine, etc., to remove secondary and phosphite in the waste liquid, is conducive to the precipitation of nickel ions and reduces the chemical oxygen demand (COD) of the wastewater.
2. Removal of organic acids
In order to improve the quality of the plating layer, the stability of the plating solution, and the deposition rate of the metallic nickel, various complexing agents are added to the electroless nickel plating solution. If the waste liquid contains complexing agents such as malic acid, tartaric acid and citric acid, the pH value of the waste liquid can be adjusted by using CaO, so that the organic acid forms a corresponding calcium salt precipitate and is removed, because the solubility of calcium malate is relatively large, except In addition to the calcium salt method, the method of further increasing the pH value is used to increase the sedimentation amount of the waste liquid, thereby promoting the removal of the calcium salt precipitate formed by the malate; and the solubility of the calcium tartrate is small, when the pH is about 8, 95% of the tartaric acid salt will be removed; as with calcium tartrate, the solubility of calcium citrate is also small, and at pH 8 or so, 98% of the citrate will be removed.
3. Removal of hypophosphite and phosphite
The nickel-plated waste liquid contains a large amount of hypophosphite and its oxidized product phosphite. Due to the high solubility of calcium hypophosphite, the CaO precipitation method cannot effectively remove the hypophosphite, but is added when the nickel ion is removed. CaO increases the pH of the waste liquid. At this time, if the temperature of the waste liquid is increased, the hypophosphite in the solution can reduce nickel ions and other heavy metal ions, and the hypophosphite is oxidized to the phosphite. If the waste liquid contains a large amount of hypophosphite, it can be removed by adding an appropriate oxidizing agent (such as potassium permanganate, hydrogen peroxide, etc.). When the pH of the waste liquid is about 7, the solubility of calcium phosphite will drop sharply. The test shows that the removal rate of phosphite in the plating solution is 95% or more at a pH of 5.5 to 7. For the unremoved phosphite, sodium tungstate can be used as a catalyst, and phosphite can be oxidized to phosphate by using hydrogen peroxide; or potassium permanganate can be directly used as an oxidant to oxidize excess hypophosphite and phosphite to phosphoric acid. salt. CaO is added to the waste liquid containing phosphate to adjust the pH of the waste liquid to 9.5 or more, and the solubility of calcium phosphate is small, and the precipitate formed is easily removed by filtration. At this time, the phosphorus content in the waste liquid can be reduced to 2-7 mg/L, which meets the requirements of wastewater discharge.
4, other processing
If the nickel in the electroless nickel plating waste liquid and most of the organic acid are precipitated and removed, and the COD in the waste liquid does not reach the discharge standard, the waste liquid needs to be further treated, and the chlorine gas is introduced under the condition that the pH value is greater than 9. At this time, Cl2 mainly exists in the form of CLO-, which has strong oxidizing ability. If a small amount of copper ions is added as a catalyst in the waste liquid, the waste liquid can be deeply treated, and the COD can be easily reduced to (100 mg/L). Direct discharge standards. At the same time, the organic acid can be oxidized to CO2 and water, and the hypophosphite and phosphite are also easily oxidized to phosphate, and the phosphate is easily removed by precipitation.
Nickel plating application and development Because the electroless nickel plating layer has excellent physicochemical properties such as excellent uniformity, hardness, wear resistance and corrosion resistance, this technology has been widely used, and it is almost impossible to find an industry that does not use electroless nickel plating technology. . According to reports, the proportion of electroless nickel plating in various industries is as follows: aerospace industry: 9%, automotive industry: 5%, electronic computer industry: 15%, food industry: 5%, machinery industry: 15%, nuclear industry : 2%, oil industry: 10%, plastics industry: 5%, power transmission industry: 3%, printing industry: 3%, pump manufacturing: 5%, valve manufacturing: 17%, others: 6%. The application of electroless nickel plating in the industrialized countries of 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 electroless nickel plating will continue to develop and the average annual net value rate will be reduced to around 6%. And enter the mature stage of development. In the booming East Asia and Southeast Asia, including China, electroless plating applications are on the rise and are expected to maintain unprecedented high-speed development.
I. Aerospace industry
The aerospace industry is one of the largest users of electroless nickel plating. The most prominent application examples are: Oklahoma Aviation Logistics Center, USA. Since 1979, Northwest Airlines has used electroless nickel plating technology to repair aircraft since 1983. Engine parts. Although the Pratt-Whitney JT8D jet engine has been discontinued, there are still thousands of such engines on the Boeing 727 and the McDonnell Douglas DC-9 because of a high phosphorus, compressive stress. Electroless nickel plating technology is used to repair the impellers of the JT8D six-type jet engine, ensuring the re-use of this engine. On the blades of turbines or compressors of aero-engines, the nickel-phosphorus alloy is usually electrolessly plated to a thickness of 25 to 75 um to prevent gas corrosion, and the fatigue strength is reduced by 25% less than that of electroplated chromium. The Oklahoma Aviation Logistics Center uses ultra-thick chemical nickel plating to repair aircraft parts with a plating thickness of 275 to 750 um. The rework rate of the original electroplating process was 50%, and the pass rate after electroless nickel plating was over 90%, showing obvious economic benefits. After the electroless nickel plating on the aircraft's auxiliary generator (APU), its life 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 starting and stopping.
In order to reduce the weight, the aviation industry uses a large number of aluminum alloy parts. After being strengthened by electroless nickel plating, it is not only corrosion-resistant, wear-resistant, but also weldable. For example, the piston head of a stroke engine is chemically plated to increase the service life. Others include titanium alloy parts and niobium alloy parts, which are protected by low-stress and compressive stresses.
Nickel + lanthanum + boron ternary alloy electroless plating (NTB) is specified as a surface strengthening process for more than 160 parts of the Pratt-Whitney jet engine to resist scratches and fretting wear, such as NTB electroless plating Sealed on the jet engine shaft. The US Air Force requires engine manufacturers to provide 4000 tactical cycles, at which point the wear amount is 0.178mm, so it must be disassembled and rebuilt. After NTB electroless plating, the wear of the spindle sealing surface is significantly reduced. After 4000 tactical cycles, the wear is about 0.008. Mm.
The aerospace system is widely used in metal light mirrors. Its base is made of high-strength and light-weight tantalum or aluminum. It is strengthened by special electroless nickel plating. The electroless nickel plating with a phosphorus content of 12.2%~12.7% can be polished to 9? Such high precision has excellent performance in a space requiring low inertia.
Although the chemical plating industry in China started late, since the 1990s, through the unremitting efforts of various scientific research units, it now has mature technology and experience. In China, Luoyang has built an electroless nickel plating processing line for aircraft parts.
Second, the automotive industry
Solving the problem of using ethanol and gasoline blended fuel is one of the development trends of the automotive industry. The use of mixed fuels, in addition to performance problems, also creates corrosion problems in the fuel system. In Brazil, the use of ethanol as a fuel, the application of electroless nickel plating technology to protect zinc die-casting parts, such as vaporizers from corrosion has become a process specification. In the United States, when methanol or methanol and gasoline are widely used, the automotive industry is bound to use electroless nickel plating as a surface protection means for vaporizers and fuel pumping systems.
The differential planetary gear is an important part of the car. It is coated with a 25um thick electroless nickel plating layer to improve the wear resistance. Some automobile manufacturers use a PTFE composite electroless nickel plating process on the shaft. The coating has both a certain hardness and good lubrication properties, which improves the service life of the shaft.
The automotive industry takes advantage of the very uniform electroless nickel plating layer, which is protected by electroless plating on parts with complex shapes such as gears, radiators and injectors. An aluminum heat sink coated with an electroless nickel plating layer of about 10 μm has good brazing properties. The dimensional error after electroless plating on the gear is easily maintained at ±0.3 to 0.5 um. If the electroplating process is used, it must be machined after plating to achieve a qualified working range. The electroless nickel plating on the injector provides good resistance to fuel corrosion and wear. In general, fuel corrosion and wear can lead to an increase in the fuel injection hole, so the fuel injection amount increases, making the horsepower of the automobile engine exceed the design standard and speeding up the engine damage. The electroless nickel plating layer can effectively prevent the corrosion and wear of the injector and improve the reliability and service life of the engine.
Third, the chemical industry
Chemical industry application research Electroless nickel plating technology replaces expensive corrosion-resistant alloys to solve corrosion problems, in order to improve the purity of chemical products, protect the environment, improve operational safety and reliability of production and transportation, thereby obtaining more favorable technical and economic competitiveness.
Electroless plating is widely used for the protection of the inner wall of large reaction vessels. An application example that was very interesting at first was: In 1955, the General Transportation Company of America (GATX) used electroless plating to protect the inner wall of the tanker to prevent corrosion of caustic. Today, electroless nickel plating technology has made great strides to provide reliable protection in a variety of chemical corrosive environments.
The most widely used chemical nickel plating is the valve manufacturing industry. Ball valves, gate valves, rotary valves, check valves and butterfly valves made of steel, etc., with high-phosphorus electroless nickel plating of 25 to 75 um, can improve corrosion resistance and service life. The effect of electroless nickel plating on chemical pumps is also significant. For valves operating under caustic corrosion conditions, low-phosphorus electroless nickel plating with a phosphorus content of 1% to 2% should be used. Because the corrosion rate of the low-phosphorus electroless nickel layer is about 2.5um under caustic corrosion conditions, it is better than the medium phosphorus or high-phosphorus electroless nickel layer. The electroless nickel plating layer is not resistant to corrosion in a strong oxidizing acid such as concentrated nitric acid or concentrated sulfuric acid. Although the corrosion rate in hydrochloric acid is lower than that of austenitic stainless steel "target=_blank> stainless steel, the corrosion resistance is still insufficient. Therefore, it is not suitable for the above-mentioned strong acid medium or medium which may hydrolyze the above strong acid. Nickel layer. Carbon steel fasteners are coated with a high-phosphorus electroless nickel plating layer of 25-50um thick instead of stainless steel "target=_blank> stainless steel fasteners, which overcomes the stress corrosion of austenitic stainless steel "target=_blank> stainless steel. The cracking problem saves a lot of money. The inner wall of the pressure vessel for producing low-density polyethylene is 25um to prevent iron pollution and the resulting discoloration of polyethylene. If stainless steel "target=_blank> stainless steel is used, the price is about electroless plating. Twice the method.
Fourth, oil and gas
Oil and natural gas are one of the important markets for electroless nickel plating, and electroless nickel plating technology is widely used in oil field production and pipeline equipment. The typical oil and gas industry corrosive environment is underground salt water, carbon dioxide, hydrogen sulfide, the temperature is as high as 170 ~ 200 ° C, accompanied by sediment and other abrasive particles, etc., the corrosion environment is quite bad. Under such harsh conditions, low carbon steel oil and gas pipelines have a life span of only 2 to 3 months. After being protected by a 50-100 um thick high-phosphorus electroless nickel plating layer, the corrosion rate is reduced to a level comparable to Hastelloy. Considering the high price of corrosion resistant alloys, the technical and economic performance of carbon steel pipes using electroless nickel plating is the best in terms of performance and price.
Pump parts for oil and gas, such as pump casing, impeller and outlet pipe, vary in corrosion environment, and are electroplated with nickel plating thickness of 25 to 75 um, which has excellent anti-corrosion effect. The electroless nickel plating of the oil pump is an ideal application example: in the West Texas oil field, the oil pump protected by electroless nickel plating has a life span of more than 4 years, and the life of the pump with the same protection is not more than 6 months. The electroless nickel plating layer is resistant to corrosion and wear, and due to the high uniformity of the electroless nickel plating layer, the pumping cylinder can be made into a single piece, thereby significantly improving the quality of the pump and reducing the production cost. In the oil field, high-phosphorus electroless nickel plating is also widely used in the surface of heaters of oil-water separation devices to prevent corrosion, and the plating thickness is usually 25 to 75 um. Valves, pipe joints, pipe clamps, etc. for oil collection and oil transfer devices are also protected by electroless nickel plating.
V. Food processing industry
The food processing industry offers a huge potential market for the application of electroless nickel plating; it is called a potential market because of the current obstacles in the widespread use of electroless nickel plating in the food industry. For example, in the United States, the FDA (United States Food and Drug Administration) has established regulatory standards for the application of electroless nickel plating in the food industry; in general, the FDA takes cases for the application of electroless nickel plating to direct food contact. The way of handling is approved. The reason is mainly because the classic electroless plating solution contains toxic lead ions such as lead and cadmium as stabilizers. However, heavy metal ions have not been used as stabilizers in many modern electroless nickel plating solutions; obviously, this obstacle will be removed sooner or later. At present, parts of food packaging machinery that are not in 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.
In the food processing process, it will involve problems such as salt water, nitrite, citric acid, acetic acid, natural wood smoke, corrosive medium such as volatile organic acid; food processing temperature range is 60-200, the relative humidity in the production environment is very high. High; under such conditions, food processing equipment has problems such as metal corrosion, fatigue and wear. For metal surfaces that come into contact with food, the traditional protection method is to plate hard chrome; however, in the acidic medium containing chloride ions, the corrosion resistance of the chrome plating layer is not good; however, the electroless nickel plating has the uniform plating ability and high corrosion resistance. , anti-adhesive, mold release and other aspects have obvious advantages. The electroless nickel plating on the parts in contact with food on the kneading machine is one of the successful examples; other in the food filling machine, screw feeder, mixing pot, food mold, baking tray, drying box, bread holding furnace, etc. More electroless nickel plating is used in food machinery.
6. Mining industry
The mining industry has harsh environmental conditions, and the downhole machinery inevitably comes into contact with salt water and mineral acid to withstand the test of corrosion and wear. Therefore, mining machinery requires surface protection.
In the mine roof support system, the corrosion-resistant and wear-resistant protective layer is often used as a hydraulic prop. However, since the hard chrome surface is cracked and porous, it is often inoperable because the corrosion is so severe that the hydraulic struts are bitten. This problem of high pressure hydraulic cylinders is even more serious. Under high pressure operation, the coating is stretched, which further exacerbates the crack of the high internal stress hard chrome layer. In this case, the high-phosphorus electroless nickel plating layer with 25um thick compressive stress is used for protection. When the hydraulic prop is subjected to high-pressure drawing, 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, for example, phosphate rock for fertilizer selection, high pressure pump and jet pump nozzle are used. At this time, the corrosion and erosion problems are quite serious, but the application of corrosion resistant and wear resistant electroless nickel plating layer is applied. Premature damage to mechanical parts can be prevented.
7. Military industry
Electroless nickel plating technology is widely used in the military, such as the aircraft's catapult hood and the electroless nickel plating of the track. The working environment of the catapult is very bad. The high-temperature airflow when the aircraft is launched washes the track, the huge force during the ejection, and the corrosion of the marine weather conditions, so that the ejection system can only be used for 6 to 12 months. The surface treatment process currently used is: the correct pre-treatment ejector cover, after electroplating nickel, electroless nickel plating 100um, and then electroplating cadmium 12.5um, and passivated by chromic acid. Such a composite coating protective layer has good wear resistance and anti-vibration wear performance, and the life of the ejection system can be extended to 14 to 18 years, that is, an increase of 18 times.
The trunnions of military vehicles have been protected by electroless nickel plating for many years to prevent corrosion and wear of road mud and salt water.
The tank's rearview mirror is made of aluminum. After fine grinding and polishing, electroless nickel is used as a corrosion-resistant and wear-resistant protective layer. Technical requirements Rearview mirrors have an 80% reflectance in the visible spectrum, and electroless nickel plating readily meets these optical requirements. The aluminum radar waveguide is plated with a 25 um thick electroless nickel layer to prevent corrosion on land and at sea. The uniformity of the electroless nickel plating layer can meet the technical requirements of various waveguides.
Eight, electronics and computer industry
Electroless nickel plating is the most widely used in the electronics and computer industries, and involves almost every electroless nickel plating technology and process. Many new electroless nickel plating processes and materials have been developed based on the needs of the electronics and computer industries. In terms of technical performance, in addition to corrosion resistance and wear resistance, it also has requirements for solderability, non-diffusion, electrical properties and magnetic properties.
Some countries have established regulations: electronic equipment must be shielded to prevent electromagnetic and radio frequency interference. The plastic housing of the electronic device is plated with copper and then electrolessly plated with nickel. Such a bimetallic structural coating is recognized as one of the most effective shielding methods. Electroless nickel plating is one of the key steps in the manufacture of computer film hard disks. The surface of the finely processed 5086 magnesium aluminum is plated with a 12.5 um thick nickel-phosphorus alloy layer for the subsequent vacuum sputtering magnetic recording film. The electroless nickel plating layer has a phosphorus content of 12% by weight (about 20.5% by atom). The coating must be low stress and compressive. After heating at 250 ° C or 300 ° C for 1 h, it remains non-magnetic at this time, that is, the remanence is less than 0.1 × 10 -4 T. The coating must be uniform and smooth, and any defects and protrusions on the surface must not exceed 0.025 um. Because of the high technical requirements, it is necessary to use high-quality and high-cleaning special electroless plating bath, fully automatic plating control equipment and high cleanliness workshop environment. Computer film hard disk electroless nickel plating is a typical representative of high-tech electroless nickel plating, which has a very important market share.
The application of electroless nickel plating technology in the manufacturing of microelectronic devices is growing rapidly. It is reported that Xerox uses selective nickel-phosphorus alloy electroless plating technology in the interconnection and over-hole filling and leveling process of VLSI multi-layer chips; Tests for shear strength, tensile strength, high and low temperature cycles, and various electrical properties. Practice shows that the application of electroless nickel plating technology improves the technical economy and product reliability of the microelectronic device manufacturing process.
9. Other industries
Various types of molds such as injection molding machines and die-casting molds are widely used in the mechanical and light industry. Due to the complex geometry of the mold, when the surface of the mold is reinforced by electroplating, in order to enable each surface to be plated, it is necessary to design and install complex auxiliary anodes and hangers. Moreover, it is necessary to perform post-plating machining to ensure Dimensional accuracy and surface roughness requirements; and the electroless nickel plating layer has a low coefficient of friction and outstanding release properties, 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 the electroless nickel-nickel surface protection, the quality of the casting model and the core box is improved, and the service life is significantly improved.
The speed of textile machinery is very high, and the abrasion of mechanical parts by various fiber yarns is very serious. Electroless nickel plating, especially artificial polycrystalline diamond composite electroless plating technology, has successfully solved the wear problem of textile machinery parts.
Various rollers and components on the press are protected by 25 to 50 um thick electroless nickel plating to prevent corrosion of printing inks and whitening liquids. The high uniformity of the electroless nickel layer guarantees the dimensional accuracy of the printing cylinder without the need for post-plating machining.
Some medical devices such as surgical forceps, dental drills and medical molds have used electroless nickel plating instead of the original chromium plating.
Definition and Characteristics of Electroless Nickel Plating Electroless plating, also known as electroless plating, 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 under a certain condition and precipitated onto the surface of a solid substrate. ASTM B374 (ASTM, American Society for Testing and Materials) is defined as 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 displacement plating, and the coating can be continuously thickened [2], and the metal plating itself also has catalytic ability.
Traditionally, electroless plating has been attributed to electroplating as a surface treatment method and is a plating type for electroplating. However, electroless plating is different from electroplating, mainly because electroless plating does not require an external power supply, and the operation method is different from electroplating, and its characteristics are as follows:
(1) The thickness of the plating layer is uniform, and the degree of dispersion of the electroless plating solution is close to 100%. Electroless plating itself is an autocatalytic redox process. As long as the catalytic substrate is in contact with the solution, it can be plated. It is almost a copy of the shape of the substrate, to the extent of profiling, and does not appear to be unevenly distributed due to electroplating. The phenomenon of uneven plating thickness is caused.
(2) Electroless plating can be carried out not only on the surface of the metal, but also on a non-metallic surface by special activation or sensitization treatment.
(3) The electroless plating equipment is simple, and does not need a power source and an anode. As long as the temperature and pH process parameters are reasonable, the parts to be plated can be immersed in the plating solution.
(4) The bonding strength and anti-corrosion performance of electroless plating are superior to electroplating. Some electroless coatings also have special physicochemical properties.
At present, electroless plating includes many kinds of plating such as nickel plating, copper plating, gold plating, tin plating, etc., but the most widely used one is electroless nickel plating. Compared with the electroplated nickel layer, the performance of the electroless nickel plating layer has the following advantages [3]:
(1) The electroless nickel plating process using sodium hypophosphite as a reducing agent obtains a Ni-P alloy, and the phosphorus content in the plating layer can be controlled to obtain a Ni-P amorphous structure plating layer. The coating is dense, low porosity and corrosion resistance are better than electroplated nickel.
(2) The electroless nickel plating layer has a plated hardness of 450 to 600 HV, and after reasonable heat treatment, it can reach 1000-1100 HV, and in some cases, can even be used instead of hard chrome.
(3) The plating layer can be controlled to be magnetic or non-magnetic depending on the amount of phosphorus contained in the plating layer.
(4) The coating has a low friction coefficient, can achieve oil-free lubrication, and is superior to electroplating in terms of lubricity and metal abrasion resistance.
(5) Low-phosphorus coating has good solderability.
Of course, like many technologies, electroless nickel plating itself has many disadvantages:
(1) Compared with electroplated nickel, the composition of the plating solution is complicated, and some raw materials are more demanding.
(2) The operation of electroless plating is troublesome, and it is necessary to continuously perform analysis and addition and adjust pH in plating.
(3) The electroless plating solution itself is a thermodynamically unstable system, which is prone to decomposition and other accidents.
(4) Compared with electroplating, the plating rate of electroless plating is slow. At present, the plating rate of most electroless plating is between 10-30 μm/h.
(5) Many electroless platings operate at temperatures around 90 °C. Maintaining this temperature also consumes a lot of energy.
(6) The chemical plating is not as decorative as electroplating, and the brightness is insufficient.
Http://news.chinawj.com.cn Editor: (Hardware Business Network Information Center) http://news.chinawj.com.cn
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