Electroless Nickel Plating
Electroless nickel plating (ENP) is a surface finishing process that places a nickel alloy coating using chemical reduction. In the process, nickel ions are reduced to metallic nickel using a chemical reduction agent...
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This article will take an in-depth look at types of metal plating.
The article will bring more detail on topics such as:
This chapter will discuss what metal plating is, its types of processes, and the materials used.
Metal plating is a process where a thin layer of metal coating is applied on the surface, or on the substrate of a metal part, product, or component. The process of metal plating can consist of electroplating, which makes a deposition of metal ions on the surface of the substrate by making use of an electric current.
Electroless plating applies metal ions on the workpiece as well without making use of any electrodes. Despite the process each possesses many benefits with the main purpose of improving the resistance to corrosion of a metal.
Even though metal plating can give protection to a part or component from environmental factors that can potentially damage it, it also improves the appearance of the part. There are different types of coatings which can either be industrial or commercial.
Commercial decorative coating is utilized on items that are used every day to provide them with improved appearance and strength. Most commercial coatings are used on certain tools, silverware, and jewelry. Industrial coatings are applied to metal parts to give them endurance and to improve their wear resistance so that they can withstand harsh conditions. This helps in protecting and strengthening machine parts and truck parts.
Coatings are also capable of improving the solderability and strength with reduced friction to reduce potential wear. By applying a coating to a metal, the conductivity of the metal is altered or its ability to conduct an electric current. Metals that are coated can be painted and magnetic properties are improved.
The various metal plating processes include:
Electroplating is a process where electrodeposition is utilized to coat the workpiece in a thin layer of metal. Controlled electrolysis is utilized by engineers for the transfer of the desired metal coating from the anode (the part that contains the metal that will be made use of as the plating) to the cathode (the part that is to be plated). Both the cathode and the anode are immersed in an electrolyte chemical bath and a continuous electrical charge is applied.
The electric current causes the movement of negatively charged ions (anions) to the anode and the movement of positively charged ions (cations) to the cathode, forming a thin layer around or plating the desired workpiece in an even metal coating. By the process of electroplating, a substrate material is taken and encapsulated in a thin shell of metal like copper or nickel.
The process of electroplating is most commonly used on other metals, due to the basic requirement that the substrate is a conductive material. There are other less common autocatalytic pre-coating processes that have been developed that produce a conductive interface that is ultra-thin, allowing different metals, most notably nickel and copper alloys, to get plated on parts made of plastic.
Both processes of electroplating and electroforming are carried out using electrodeposition. The major difference between these two processes is that electroforming makes use of a mold that is then removed after the forming of a part.
The process of electroforming is utilized for the creation of solid metal pieces, while electroplating is utilized for the covering of an existing workpiece, which is made from a material that is different in metal.
A single metal can be electroplated onto an object, or a combination of metals. Many manufacturers opt for layering metals like nickel and copper to increase the strength and conductivity. The most commonly used materials in the process of electroplating are listed below:
Almost any material can be used in the making of substrates, from stainless steel and other different types of metals to plastics. Artisans have organic materials that are electroplated, like soft fabric ribbons and flowers. It must be noted that substrates that are non-conductive, like plastic, glass, and wood must be made conductive prior to their electroplating process. This can be achieved by coating a substrate that is non-conductive in a layer of conductive spray or paint.
Electroless plating is a metal plating method that uses chemicals instead of electricity. The workpiece is immersed in a reducing agent with a catalyst that turns metal ions to metal and deposits the ions on the surface of the workpiece. It is widely used for metal plating of plastics, such as printed electronic circuits. Consumer goods are metal plated to make them more durable and give them an enhanced appearance.
Electroless plating is non-galvanic and includes simultaneous reactions in an aqueous solution without the use of electrical current. The reaction occurs when a reducing agent releases hydrogen, usually sodium hypophosphite, and is oxidized to produce a negative charge on the surface of the workpiece. Electroless plating evenly deposits metal ions inside holes, along the edges, and over irregular shapes, which are difficult to metal plate using electroplating.
There are various non-conductive materials that require conductivity as part of their function in an application. Such materials include plastics and various types of metal. The electroless metal plating process deposits conductive metals on non-conductive surfaces to make it conductive such that it can be electroplated or used for a conductive application.
Nickel plating is the most commonly used electroless plating technique, although copper, gold, and silver layers can also be applied in a similar manner. The process of electroless plating is also called auto-catalytic plating or chemical plating.
Electroless plating is a method of metal plating, that is non-galvanic, that includes several reactions that are simultaneous, in an aqueous solution. The reactions occur with no use of external power supply.
The reaction is achieved when a reducing agent releases hydrogen, usually sodium hypophosphite, and is oxidized, thus a negative charge is produced on the part’s surface.
Electroless plating allows the bathing of all parts of the object by a consistent metal ion concentration, and evenly deposits metal inside holes, along the edges, and over objects that are irregularly shaped which are difficult to metal plate evenly by the process of electroplating.
The application of electroless plating is also done for the deposition of a conductive surface on an object that is non-conductive to allow it to be metal coated by the process of electroplating.
Immersion plating adheres layers of nobler metals to the surface of another metal by dipping in an ion solution of nobler metal for the production of a replacement reaction. It causes a metallic coating to be deposited on a base metal from solutions containing the coating metal. In this process, there is typical displacement of one metal by metal ion that has lower oxidation potential levels, relative to the displaced metal ion.
This process is also used for improving electrical properties and also for the enhancement of adhesive coatings or organic coatings bonding to the substrate. Immersion plating is also called dip plating or metal replacement. Immersion plating is different from different electroplating processes in the sense that no external current is present. It follows the principle that when metal components like copper are immersed into an electrolyte having ions of nobler metal, the component or metal that is less noble will undergo dissolution.
This results in the electrons being released, which allows metals that are highly noble to settle down. Unlike the process of electroless plating, the metals being deposited are halted once the object being plated is totally coated with higher nobility metals. This type of metal plating occurs at temperatures that are high such as in gold immersion occurs at 80°C to 90°C whereas in silver immersion takes place at 50°C to 60°C.
Electroplating and electroless plating improve the toughness of a component and its resistance to corrosion. They make a part more attractive such as the plating of jewelry or other consumer goods. The main difference between these processes is the use of electric current, which is used for electroplating but not used with electroless plating.
Electroplating uses a power source, like a rectifier or battery, to provide electric current to a component in a chemical solution. The application of the current alters the chemical composition of metals and deposits a hard and durable metal coating on a component’s surface. It is more complex than electroless plating, requires exceptionally clean conditions, relies on potentially hazardous equipment and may require multiple applications to achieve the necessary plating thickness.
Electroless plating is a process that is much less complex. In the process of electroless plating, the component is cleaned first with chemical cleansers for the removal of corrosive elements and oils, then is dipped in the aqueous solution and anti-oxidation chemicals are added. The resulting component is plated and exhibits high resistance to corrosion and friction.
Electroless nickel plating has no requirements of complex filtration or other equipment, and because of the absence of electricity, there is no risk of accidents that are electricity related.
This chapter will discuss the different types of metal plating based on the metal used.
This is a form of treatment by means of an alloy designed for the increase of hardness and resistance of a metal or plastic. The process of electroless nickel plating is simpler than that of its counterpart electroplating. In this process, no passing of electric current is required through the solution in the chemical bath in order to start the plating process.
Rather, the metal surface is put through a series of autocatalytic reactions and cleaning, which has been perfected by electro-coatings. Below are the stages of electroless nickel plating:
The first step is the treatment of the surface, whereby it is cleaned with a series of chemicals for the removal of oils and grease. Thorough cleaning is required in the preparation of the component in order for it to be plated properly. The cleaning of each component is done meticulously based on the material’s surface.
Once the substrate has been cleaned, it is activated by means of a proprietary solution or acid etch, preparing the surface so that nickel-phosphorus can be deposited on it.
After the end of the plating process, the electroless process of nickel renders more resistance to friction and corrosion to the component.
The deposition of electroless nickel plating can be done at a rate of 5 microns per hour up to 25 microns per hour. Since the process is continuous and builds on itself, the process provides a thickness that is essentially limitless. However, as the thickness increases, minute imperfections become clearly seen. Depending on the specific requirements, one of five different coating options is applied by electro-coating.
Zinc is a material that is inexpensive and it is utilized to provide a galvanized coating on different metal substrates. In addition to being electroplated, the application of the element is done by a way of the sherardizing process, by dipping in a molten bath and by spraying. In the cold or electrolytic process, the part to be plated is set up as the cathode in a bath containing an electrolyte of soluble zinc salts together with metallic zinc as the anode.
This process produces a coating of pure zinc that is highly ductile, whose uniformity and thickness can be controlled precisely. The sherardizing process is utilized for the coating of small hardware items like nails and screws. The items are loaded together with zinc dust in a barrel and heated to a temperature of approximately 500℉.
The parts are then tumbled inside the barrel for the production of a coating that consists of 10% iron and 90% zinc. The application of molten zinc may also be done by dipping or by manual coating of large items. The addition of a small amount of aluminum to the bath is done sometimes for the improvement of fluidity and to improve the coating of shapes that are odd.
This type of plating is often used on miscellaneous automotive items. The manufacturers of aircrafts specify it for its sacrificial protection characteristics and also its lubricity that is natural, for components that are removed frequently and reinstalled. This type of metal plating is suited to marine environments where it holds up well against salt and freshwater.
Because of safety reasons, the use of this type of plating has diminished over the years even though it is still available. Multiple manufacturers of aircrafts now use zinc-nickel alloy plating as a substitute for cadmium plating.
This type of plating is only used in decorative purposes but it also exhibits high resistance to corrosion and hardness, making it useful for industrial applications where there is a concern of wear. It is used sometimes for the restoration of tolerances on worn parts. Chromium is plated mostly over nickel in the production of furniture made from steel and automotive trims.
The process of chrome plating is an electroplating process that involves the utilization of a chromic acid called hexavalent chromium. For industrial purposes, trivalent chromium baths which consist of chromium chloride or chromium sulfate are another option.
Aluminum has a wide assortment of alloys some of which have electrical and mechanical properties that make them possible replacements for copper as a plating material. The 1000 series of aluminum has exceptional electrical and thermal conductivity with excellent resistance to corrosion and outstanding workability. On the other hand, the 7000 series of aluminum is alloyed with zinc and small amounts of magnesium, which makes it heat treatable and very strong.
The stability of the cost of aluminum and its ever expanding use is another factor that makes it attractive as a plating material. It is a less noble metal and easily forms an oxide layer, which limits its use as a plating metal. Additionally, the plating of aluminum is limited due to the inconsistencies in the forming of the grain structure and the tenuousness of its bond with the base metal.
The processes used to plate aluminum are immersion, electroplating, and electroless plating. The main concern when plating on aluminum is its oxide layer that interferes with the adhesion of the added layer. A zinc immersion film is used to prevent the development of the oxide layer with zincation being the primary method.
Copper plating is another metal that is popular for plating for applications that require cost efficiency and high conductivity. Copper plating is a popular plating metal for components used in the electronics field, like printed circuit boards. Copper is made one of the less expensive plating metals because of its low material cost and high-plating efficiency.
There are three types of process utilized in copper plating which are: acid, alkaline, and mild alkaline. Higher levels of alkaline provide superior throwing power but they require lower densities of current and safety precautions that are increased. It is important that these levels are monitored because cyanide has been linked in alkaline copper baths by health inspectors.
Gold has a high resistance to oxidation and also electrical conductivity, therefore it is prized. Gold plating, which is different from gliding because gold is not a foil, is one of the easiest ways of imparting these characteristics on metals like silver and copper. The process is mostly utilized for the decoration of jewelry and for the improvement of the conductivity of the parts used in electronics like electrical connectors.
When copper is gold plated, tarnishing can be an issue, which can be easily resolved by preceding the deposition with a strike of nickel. When determining factors like length of immersion and optimal bath mixture, consider the hardness and purity of gold.
The wide use of silver as a plating material is due to its exceptional thermal and electrical conductivity, which is far better than gold. It is used in applications that require high current power transmission and low power connector applications. The lubricity of silver makes it ideal for high temperature anti-galling applications in the bearing and fastener industries.
Silver plating of copper creates a functional surface for transferring heat and electricity. The silver plating process is over 200 years old when it was used to plate switch gears and other forms of electrical components. Recently, silver plating has grown with the increased use of EVs.
In the silver plating process, silver is applied in several layers since it can wear away if it is continuously used. Aside from its practical manufacturing use, silver is also used with applications that require decorative appeal with electrical conductivity. It is a cost effective replacement for gold, palladium, and rhodium. Its use is limited due to its tendency to be susceptible to galvanic corrosion and the effects of humidity.
Silver plating of stainless steel is used for nuts, fasteners, slip rings, thrust washers, and bushings. Stainless steel parts plated with silver have lubricity at high temperatures and gives parts the properties of anti-galling and anti-seizing. The various positive properties of stainless steel makes it difficult to plate due to its oxide layer. Manufacturers use various methods to remove the oxide layer prior to plating.
An inconsequential down side of silver is its tendency to tarnish, which is perceived as a defect. Silver tarnish is unlike other forms of tarnish that create an oxide layer. The tarnish that appears on silver is due to its exposure to hydrogen sulfide or sulfur to form a layer of silver nitrate, which is conductive and can be easily removed by wiping the surface.
Specifically, silver plating is not a good solution to applications that are subjected to humid conditions because silver is more susceptible to flaking and cracking, which may expose the base substrate.
Tin is a soft, ductile, silvery white metal that is a good conductor, does not oxidize easily in the air, and is corrosion resistant. Since it is non-toxic and non-carcinogenic, it is widely used for the storage and preparation of food. For centuries, tin was used in the manufacture of bronze weapons and primitive cooking containers.
Tin plating places a thin layer of tin on the surface of a metal to improve the metal’s solderability and conductivity as well as enhance a metal’s resistance to corrosion and improve its appearance. Since tin is a good conductor of electricity, tin plating is used to coat electrical terminals, battery connectors, and copper components that are used to pass current such as bus bars made of copper that transfer electrical power.
The tin plating process begins with the cleaning of the surface of the metal to remove impurities or contaminants. A layer of tin is applied using electroplating which involves immersing the metal in a solution with tin ions. Electric current is passed through the solution to deposit the tin onto the surface of the metal. The thickness of the tin layer is determined by how long the metal remains in the electroplating solution.
Rhodium is a type of platinum that offers resistance to tarnish, resistance to scratch, and a shiny lustrous white appearance. This type of plating is commonly utilized in the production of jewelry, especially in situations that require the plating of white gold. Copper, silver, and platinum are also popular base metals that are used in rhodium plating.
The disadvantage of rhodium plating is the eventual wearing of the protective barrier of rhodium in applications that involve high wear levels. This will eventually lead to discoloration and a second round of plating after a few years may be necessary.
Electroplating and electroless plating are used to deposit metal on plastics in order to give the plastic material extra protection for additional manufacturing processes or to make the plastic conductive. In several industries, plating of plastic is used to enhance the appearance of plastic and give it a smooth even finish. The types of plastics that are ideal for metal plating are thermoplastics that become malleable when heated.
The plastic that is plated the most is acrylonitrile butadiene styrene (ABS), which is a tough, durable, and affordable plastic that is resistant to chemicals. Since ABS is vulnerable to environmental factors and has poor mechanical strength, plating it is used to overcome its shortcomings. Several experiments were completed on ABS to determine how it could be plated. The development of a chemical for preparing ABS for plating was the major advancement that led to the plating of other plastics.
The key to plating plastic is proper preparation, which includes etching the plastic in a chromic acid based solution to improve adhesion. After any excess chromic acid is removed, a solution of palladium and tin salts is applied to make the plastic ready for the plating process. Copper or nickel is applied in an electroless plating solution using the palladium and tin salt solution as a catalyst. Once electroless plating is completed, the plastic is ready for standard electroplating.
|Plastics That can be Metal Plated
|Acrylonitrile Butadiene Styrene (ABS)
|Polylactic Acid (PLA)
|Polyether Sulfone (PES)
|Phenol formaldehyde resin
|Polyether ether Ketone (PEEK)
|Polyphenylene oxide (PPO)
|Polyphenylene sulfide (PPS)
|Polyvinyl Chloride (PVC)
|Vinyl ester resin
|Polyvinylidene Fluoride (PVDF)
This chapter will discuss the applications and benefits of metal plating.
The applications of metal plating include:
Multiple components of airplanes are electroplated for the addition of a sacrificial coating which improves the lifespan of the parts by lowering the rate of corrosion. Due to the subjection of aircraft components to high and extreme changes in temperature and also environmental factors, an additional layer of metal is applied to a metal substrate.
This is meant to not compromise the functionality of a part by tear and wear. Many fasteners and steel bolts designed for the industry of aerospace are chromium electroplated.
Biodegradable items like branches, flowers, and even bugs are often turned into durable pieces of art that are long lasting with the process of metal plating. Electroplating can be employed for the showing off and preservation of fine details in items that can decompose easily otherwise. Electroplating is sometimes used by digital designers for the production of sculptures.
Designers are capable of printing 3D substrates using a 3D desktop printer and then they electroplate the design in gold, silver, copper, or any metal chosen to achieve the finish that is desired. The combination of electroplating and 3D printing in this way produces pieces that are easier and cheaper to manufacture, while the look is not changed.
In the automotive industry, electroplating is very common. It is utilized by multiple major automotive companies for the creation of chrome bumpers and other different metal parts. Electroplating can also be utilized in the creation of custom parts for concept vehicles.
Vehicle customization businesses and restoration companies also utilize electroplating for the application of chrome, nickel, and other finishes to different motorcycle and car parts.
The jewelry industry is associated with electroplating. The designers of jewelry and manufactures rely on this process of metal plating for the enhancement of durability, aesthetic appeal and color of bracelets, pendants, rings, and a broad range of other items.
Jewelry described as silver or gold plated have a high chance that they were electroplated. Various metals are used in combination to achieve finishes that are uniquely hued. For instance copper, silver, and gold are often combined to create rose gold.
The metal plating process called electroplating is used for the addition of resilient exteriors to all sorts of dental and medical elements. Gold planting is employed more often for the creation of tooth inlays and aids in different dental processes.
Parts that are implanted, like screws, plates, and replacement joints are electroplated frequently to make them more resistant to corrosion and also more compatible with pre-insertion sterilization. Surgical tools as well as medical tools such as radiological parts and forceps are also commonly electroplated.
Numerous solar and electrical components are metal coated through the process of electroplating to improve their conductivity. The contacts of solar cells and different types of antennas are manufactured routinely using electroplating. Wires can be metal coated by electroplating them in nickel, silver, and multiple other types of metal.
Gold plating is used more often, in combination with other metals, to improve durability of the workpiece. Gold is also used frequently to improve the parts’ lifespan due to its conductivity, its ductility, and because it doesn’t interact with oxygen.
For the production of custom or low-volume metal parts for the purpose of prototyping, this can be time consuming and very costly with the traditional manufacturing processes. As a result, electroplating and 3D printing are combined by engineers for a time saving and low cost solution.
For the propagation of radio waves, antennas must have electrical conductivity. Even though parts that are 3D printed don’t conduct electricity, they offer a design freedom that is almost infinite and materials having good thermal and mechanical properties. These benefits can be used in combination with electroplating for the achievement of the conductivity desired. This results in a very good solution for custom antennas for the purpose of research and development in the defense, education, medicine, and automotive fields.
The benefits of metal plating include:
Electroplating has many benefits, such as improved strength, conductivity and lifespan of parts. Manufacturers, artists and engineers capitalize on these benefits in different ways. Engineers often utilize the process of electroplating to improve the durability and strength of different designs.
The tensile strength of different parts can be improved by putting a coat of metal, like nickel and copper around them. By placing a metallic skin on the surface of a part, their resistance to environmental factors such as UV light for outdoor, chemical exposure, or corrosive applications is increased.
Artists often make use of the process of electroplating for the preservation of the natural elements susceptible to decay, like leaves, and convert them into works of art that are more durable. In the medical sector, electroplating is utilized for the making of medical implants that are resistant to corrosion and can be sterilized properly.
Electroplating is a way that is effective when adding cosmetic metal finishes to the products of customers, figurines, sculptures, and art pieces. Multiple manufacturers also opt for electroplating a substrate for the creation of parts that are light in weight that are simple and cheaper to ship or move.
Electroplating also has an advantage of conductivity. Due to the inherent conductive property of metals, electroplating is a great way of improving a part’s conductivity. The performance of electrical components, antennas, and other parts can be improved by electroplating.
While electroplating has many benefits, its drawbacks lie in the hazardous and complexity nature of the process itself. Workers that carry out the process of electroplating are most likely to suffer from hexavalent chromium exposure if proper precautions are neglected. Proper ventilation is essential for the workers in the workplace.
Structural plating is quite difficult to execute reliably and it requires multiple baths, long plate times, and compatibility between metals. Due to the dangers involved and expertise required, many designers and engineers opt to hire a third party electroplating manufacturer who specializes in this process.
This article presented a discussion of the different types of processes of metal plating such as electroplating, electroless plating, and immersion plating. It also presented a discussion of the different types of metal plating based on the metal used, for example it presented on nickel plating, zinc plating, rhodium plating etc. each process of metal plating offers its own unique advantages as well as its drawbacks. For example electroplating requires the application of an electrical current from a power source while electroless plating does not require the passing of an electric current. The different types of metal plating based on the metal used offer different benefits and drawbacks, for example gold plating offers an advantage of no interaction with oxygen and high resistance to conductivity. When choosing a type of metal plating, one has to be aware of the requirements of the metal plating process as well as the properties of the metal that is going to be used for the coating.
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