Dec. 23, 2024
Machinery
A popular metal finishing process, electroplating is used in a variety of industries for a wide range of applications. The electroplating process uses an electric current to deposit a thin layer of material on top of an object. It&#;s primarily used to increase wear resistance, protect against corrosion, provide conductivity , or change the aesthetic appeal of an object. In aerospace, automotive, computer, military, space exploration, medical device, healthcare, telecom, and other industries, it&#;s also used to add conductivity, heat resistance, help prevent oxidation, and to meet the demands of engineering teams for unique material combinations.
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Electroplating can be done using several metals, including gold, rhodium, nickel, copper, tin, and, along with alloys made with a combination of these metals with others.
Nickel is the most common electroplated material. It&#;s typically used in a multi-layered process, to increase wear and corrosion resistance. With heat treatment, this wear resistance can be improved even more. Nickel is also used in alloys, to increase elemental resistance, electrical conductivity, and strength/hardness. Recent developments include aesthetic coatings in satin, pearl, and black for use in automotive, motorcycle, and other vehicles, along with consumer goods like bathroom faucets, cupboard fixtures, furniture, and consumer electronics.
Aircraft Parts and components are highly susceptible to atmospheric corrosion. Corrosion can negatively impact titanium, aluminum, steel, and magnesium components, weakening their structural integrity. Plating with a nickel-zinc alloy can help address this common concern among aircraft and aerospace manufacturers.
Copper is second in electroplating popularity only to nickel. An exceptional electrical conductor, it is often used as an undercoat for other metals that are more durable. Copper is sometimes used to smooth out imperfections when the base piece has pitting or other surface imperfections. It can also be used for its antibacterial properties or when it&#;s desirable for the object to have a bright, shiny finish.
Electroplating using gold adds high corrosion, tarnish and wear resistance, conductivity, and aesthetic appeal. It also offers these additional functional benefits in industrial use:
Gold naturally conducts electricity, so it&#;s a good choice for parts in electronics manufacturing.
Gold increases resistance to normal wear and tear, which can be critical in electronic or heavy industry use.
Adding a gold coating to an industrial piece helps reduce corrosion and lengthens the time an object is useful.
Electroplating using gold creates a heat shield effect, protecting the substrate in high-temperature environments.
Gold can be electroplated to several other metals, as well as polymers. In many cases, pieces may be plated with copper, silver, or nickel, adding a final gold layer for aesthetics. Compared to other electroplating metals, gold has a higher cost, which changes with the prices on the fluctuating gold market.
Why is electroplating used in so many industries, for both industrial and consumer applications? Because it&#;s incredibly versatile. Here are some of the benefits of manufacturing and product applications:
Electroplated parts can last longer with the protective barrier that&#;s applied during electroplating. They can hold up better under extreme heat and cold conditions, and more readily resist corrosion.
Electroplating is commonly used to enhance the appearance of products, from jewelry to automotive interiors. It&#;s cost-effective and can be used to create a look of luxury.
Electroplating can improve performance, by reducing friction in products like electrical connectors. Nickel plating is frequently used for this purpose.
Silver plating enhances electrical conductivity, a cost-effective and efficient option for manufacturers of electrical components, and electronic products.
Electroplating an item with palladium absorbs excess oxygen from the manufacturing of automotive catalytic converters, improving their performance.
An alloy of zinc and nickel can help prevent the formation of whiskers, which are sharp protrusions that can occur during manufacturing operations. These whiskers can cause damage from arcing and shorts in electrical parts and components. Electroplating with this zinc-nickel alloy can significantly reduce this type of damage.
Gold and zinc-nickel alloys can be electroplated onto engine parts and components to reduce damage from extreme temperatures. This increases the parts/components lifespan and means they can better withstand extremely high temperatures.
Electroplating can be used to make surfaces harder, making brittle materials much stronger and extending the lifespan of the plated object. Plated surfaces are also less susceptible to damage from being dropped or struck.
Copper is often electroplated onto a piece when it needs to have a more smooth and uniform finish. It&#;s an ideal way to provide an undercoating for adhesion or for additional plating with other materials.
There are times when a product needs to have an added thickness for the overall quality and longevity of the finish. Copper-nickel plating is a popular choice in manufacturing situations that call for higher thicknesses.
EMI Shielding: Electromagnetism, especially in aircraft, can pose a threat when it begins interfering with the operations of other components.
EMI-shielded housings can successfully isolate electromagnetic interference
where it counts the most.
Electroplating has an incredible number of benefits to many industries, including aerospace, defense, and automotive corporations. However, the electroplating process is one of precision, and not every company has the ability to meet the levels of diligence and experience to do it correctly. Perfection is required and solid electroplating is dependent on both exacting specifications of the electrochemical process and consistent, thorough preparation. It is truly a science.
The electroplating process is rife with opportunities for errors that could prove expensive and even catastrophic if not executed correctly. The process is complex and at times, the electroplating defects come after issues that are already present on the object to be electroplated, before the electroplating process starts. Those problems can include pitting, sharp edges, cold shuts, unclean manufacturing, and cleavage points, which we will dive into further later in this article.
Electroplating involves both chemistry and metallurgy, relying very heavily on proper, impeccable execution. With plating being such a critical part of the operations and products of so many industries &#; like aerospace, automotive, telecommunications, and more &#; getting the job done by a firm with a stellar reputation and a long list of references is important.
Electroplating requires precise preparation in order to be properly executed. Many of the problems arise because of problems with pre-coating defects, while others occur after plating. To help you better understand electroplating and its challenges, here are some of the defects that can present themselves during the electroplating process:
This is evident as small holes on the surface of the object. This can be related to non-permanent chemical fume suppressants used during production, though good quality permanent fume suppressants don&#;t seem to cause the same pitting problem.
Pitting is usually caused by preparation mistakes, like inadequate cleaning or flaws on the piece. Pre-plating polishing, blasting, and grinding can also cause an issue, leaving very small particulate debris on the object. Oxidation, microscopic holes and fissures, and small pieces of non-metallic substances can also contribute to pitting. Additionally, wire brush debris, blaster remnants, and oil can also cause pitting in electroplated materials.
Objects receive electroplating based on their geometry. The process relies on electrical current, which then creates a chemical reaction on the piece&#;s surface, attracting
cations
. Some geometric shapes will receive electroplating better, attracting more cations than others.
More specifically, when the electrical current passes through a to-be-plated object that has a sharp edge, the current density increases at a sharp edge. This can result in a thicker distribution of the coating metal, giving you a brittle plating layer that is likely to crack.
Most manufacturing operations will grind and deburr edges on the objects to be plated, until they aren&#;t as sharp, with a common rule of thumb is you should round off all edges until the radius measures 0.4 to 0.8 millimeters.
When a solid splits along a structural plane, it&#;s called cleavage. When electroplating an object with cleavage points, these areas can cause issues with structural integrity.
The cleanliness of the object to be plated is critical to successful electroplating. Unclean surfaces can result from manufacturing problems, handling, contamination of the object, or contamination of the equipment. An experienced and well-qualified electroplating tech should always examine objects closely for cleanliness before plating. An improperly prepared surface will not allow the right final adhesion to happen between the object and the coating, which can result in bubbling and blistering.
Loss of adhesion between the object and the electroplated coating is one of the most common types of electroplating failure. In most cases, it is a result of the surface of the object to be coated not being properly prepared. The surface must be &#;active,&#; aka ready to receive the plating. Oils, die-release agents, oxides, alloying substances, and oils can compromise this adhesion.
To help avoid this issue, most credible electroplating operations have a pretreatment system built into their plating process. This pretreatment generally involves presoaking the object in alkaline liquid, electro-cleaning the object, pickling it in acid, deoxidizing it, descaling it with chemicals, cleaning it ultrasonically, and activating strikes. The process varies depending on the materials and the object being plated, as well as between electroplating facilities.
Manufacturers can help electroplating companies avoid these and other issues by doing the following:
Inform the plater exactly what alloy is being used. Even a variance of 5% in the alloys make-up can require a different pretreatment and plating technique.
Use oils that are less difficult to remove. Organic oils like vegetable and animal oils are more easily removed than silicon wax and lubricants.
Heat-treat in an inert environment. While it&#;s more expensive, it requires significantly less pretreatment so it can save money.
Use plating-grade materials. A finished plating job cannot turn out defect-free if the materials used aren&#;t of high quality. Materials that don&#;t meet plating standards often include metallic inclusions and residues on their surfaces.
When working with finishing metals, hydrogen can cause them to become brittle, break, and fracture. Unfortunately, hydrogen can work its way into parts without the manufacturer&#;s knowledge. This type of embrittlement typically shows up after the plating process has been completed and when the part/object is put under stress. To help minimize hydrogen cracking, engineers may try several measures, including stress relief baking and peening the object with a shot. Both of these processes help promote strength.
Hazy or dull spots on a plating surface may have several potential causes, including:
Insufficient levels of Dura Additive or an excessive level of sulfate, chromic acid, or contaminants in the object bath can result in dull or hazy deposits in the product plating. It can be caused by any of the following factors.
Improper rectifier functioning: The rectifier you&#;re using in the electroplating process must be in proper working order, or you risk dull deposits. A credible electroplating company will perform tests regularly to look for and perform, any necessary repairs.
Inadequate preheating: If the parts being electroplated aren&#;t hot enough, it can lead to defects. The objects need to be warmed to the bath temperature consistently, all through their mass.
Interrupted current: When the electroplating current isn&#;t consistent or gets interrupted, the results will be adversely affected.
Incorrect bath temperature: While plating bath parameters can differ, the bath temperature should typically be between 130 and 140 degrees Fahrenheit. It&#;s critical to keep the temperature consistent and stay within two degrees Fahrenheit.
Incorrect current density: The correct current density is two ASI. Any deviation from this level can lead to haziness and dullness in the finish.
Incorrect current distribution: The anodes must be the correct distance from the object or the current won&#;t flow correctly. The use of conforming anodes helps to solve this issue.
Inadequate rinse: Objects must be rinsed thoroughly, ensuring the removal of all cleansers, chemicals, solvents, or oils. Any residue left on a part can cause adhesion problems.
Part sitting too high in the solution: The object to be plated must be submerged at least 4 inches below the surface of the solution, in order to receive the proper amount of current.
Organic compound contamination: Over time, organic substances can form a scum on the surface of the bath. That scum can then end up coating the object being plated.
When gas expands in the pores of the object being plated, typically hydrogen or nitrogen, blisters can erupt on the surface of the plating. This typically happens when an object is heated, the gases expand and cause the plating to blister off of the surface.
Objects are often electroplated to prevent oxidation, though manufacturers must be mindful of making sure the object hasn&#;t already oxidized before it is plated. Oxidation results in poor adhesion of the plating to the object, contributing to lift.
Check out these common and best practices for electroplating.
Solid electroplating requires prepping to ensure the piece you&#;re plating is &#;active&#; and ready for the coating. Manufacturers use techniques like blasting, washing, rinsing, and acid baths to eliminate all surface imperfections.
Different solutions can affect alloys in different ways. Paying close attention to the solution that&#;s recommended for the specific object material will result in the best appearance and functionality of the plating.
Over time, byproducts of chemical reactions can degrade the chemical baths being used in electroplating. Don&#;t allow the bath to degrade below the desired level, or the quality of the coating will be impacted.
Every time an item is coated in a bath, some of the solutions is used and the volume of the bath is reduced. It&#;s critical to keep measuring the volume and chemistry of the solution before every plating.
Electroplating with metal can be drastically impacted by temperature. Lower temps make the process take longer, though if the temperature is too high, the process happens too quickly. Both of these effects cause problems in the plating. Temperatures must be monitored consistently during the plating process, preferably via an automated process, to help avoid these problems.
Post-treatment, heat can be used to improve the hardness of a nickel or alloy coating.
Plastics have been engineered for use in innumerable industries and products around the globe. Plastics made of modern polymers are popular in aerospace, automotive, aeronautics, health care equipment, and other high tech industries for their easy machinability, excellent surface finishes, lightweight, and overall versatility. Electroplating plastics also allows engineering teams to take advantage of the economies of scale that have made injection molding immensely popular for large-scale production.
While the majority of electroplating is done on metal, plating metal onto plastics and polymers allows engineers to use polymer/metal combinations to create unique physical characteristics. Electroplating plastics adds additional properties that make it even more valuable for use in industry, like:
Electromagnetic Interference shielding (EMI)
Wear protection
Corrosion resistance
Increased surface hardness
Electrical conductivity
Improved appearance
Add solderability
Chemical resistivity
Increased thermal range
Electroplating transforms a non-conductive plastic surface to one that conducts electricity. This means that manufacturers of electronic components used in automobiles, planes, spacecraft, and a myriad of other products can build lighter products that conduct electricity. The metal coating can also be used to reflect damaging light away from the surface of a plastic object, and serve as a barrier against detrimental substances, wear, corrosion, and EMI. A metal coating can also help reduce energy dissipation.
Manufacturers get plastic parts electroplated in order to protect the parts from chemicals that could damage them, slow down corrosion, and increase wear and strength. The process can also be used to create an impression of increased quality and luxury.
Thermoplastics are commonly used in 3D printing components and parts, specifically polycarbonate, polystyrene, polyacetal, polypropylene, and blended nylons. However, some 3D printing methods are also able to extrude parts using titanium, steel, and wax, materials that are very rarely plated.
The use of 3D printed parts has become critical to the medical, engineering design, and automotive industries. The ability to extrude complete prototypes and early models of equipment and vehicles from scratch has quickly made this process invaluable to engineering teams around the globe.
When plating 3D printed parts, the type of material to be used for the plating is an important decision. If the material being used for the component is a chemically-resistant plastic, it will be more difficult to plate. Plating plastics requires a chemical surface treatment as a pretreatment to improve adhesion.
Plating for 3D printed parts is done for both aesthetic and functional reasons. Silver, for example, can enhance a product&#;s aesthetic while making it more resistant to corrosion. Metal plating for 3D printed parts provides functional benefits, too, improving resistance to heat, corrosion, and chemicals, shielding from EMI and RFI, and increasing strength.
When manufacturing injected molded plastic parts for electroplating, there are several processes that need to be carefully followed, including:
The use of palatable resin compounds can minimize the difficulties in surface adhesion that comes with high molded-in stresses
Minimizing the residence time of the resin in the injection barrel can help avoid the potential degradation that can negatively affect the ability to plate the plastic.
The resin must be properly dried to avoid blistering and moisture-related surface issues after plating.
Pristine injection molding processes are essential. All pieces and areas around the molding machine must be free of contaminants like cleaning chemicals, oil, and grease.
When handling injection molded parts, wear the proper gloves to reduce the oils that can come with fingerprints.
All molded parts must be put into protective packaging immediately, to avoid dust and protect it until it is plated.
Unfortunately, plastics aren&#;t electrically conductive, so they can be challenging to plate and proper adhesion can be difficult to achieve. Achieving excellent adhesion and RF performance is critical, as many of these plated plastics will be utilized in extreme temperature conditions.
Plastic objects must be properly prepared before being coated. Often, this involves being chemically cleaned and stripped of contaminants, then layered with electroless copper, which doesn&#;t require electrical conductivity for adhesion. Additional metals or alloys, like nickel or silver, can then be adhered to the copper plating to create the desired properties in the final plated object.
There are several appealing reasons for electroplating metals. Some are more expensive than others or have different desirable qualities. The process has been in use since the early 19th century.
For example, gold and silver are considered top metals, both rare and expensive. Jewelers create less expensive pieces by electroplating a thin layer of gold or silver over a less valuable metal. Thin layers of chromium, another shiny metal, have been used for decades to add shine and appeal to consumer products like cars and home decor.
Metal is often electroplated onto another metal to protect the surface. For example, copper, nickel, and chromium are often used to provide a protective coating (that is less reactive) for items made of the more reactive substances zinc and cadmium.
In the electronics industry, electroplating is used to add conductivity to electronic components and circuits made of less expensive metals. Gold and silver, which are excellent (but expensive) conductors of electricity, can be electroplated onto those components, saving money for the manufacturers of cell phones, computers, and other electronic consumer products.
A composite material is made of two or more distinct constituent materials. Though the constituent ingredients remain distinct within the composite, the final product material has new properties &#; such as greater strength or heat resistance &#; as a result of the combination.
Generally, composite materials are formed by adding a reinforcement component to a matrix material, like mixing gravel with cement to create stronger material concrete. Indeed, building materials such as concrete and plywood are familiar examples of composites.
Some composites are more carefully structured than concrete, however. Engineers often weave small reinforcement fibers into a matrix, using intentional patterns to increase the material&#;s performance in various capacities. The matrix holds the reinforcement fibers in place, and the reinforcement provides improved properties to the matrix, preventing breaking or bending, for example.
Composites are popular for industrial applications because the wide variety of available components make composites easily customized to fit any industry or product&#;s unique physical requirements. A composite material can be lighter, stronger, more durable, or even less expensive than a single homogeneous material.
Composite materials are becoming increasingly popular in manufacturing and that trend is sure to continue for some time into the future. Let&#;s review three of the composite materials that manufacturers frequently electroplate with metal.
Glass-reinforced plastics (GRP): Glass-reinforced plastics are overall more economical than carbon alloy steels, stainless steels, high nickel alloys, or titanium materials, with a density of about one-quarter of the steel material. GRP is also light in weight, easy to repair, and allows electrical conductivity to be safely used, where it would typically not be tolerated.
Fiber-reinforced plastics (FRP): FRP is created by using fibers as reinforcement for a plastic matrix. These fibers can be carbon or glass. Electroplating FRP typically involves a layer of electroless nickel, to provide a base for electroplating.
Metal matrix composites (MMCs): Metal matrix composites are made up of a metal reinforcement material embedded into a metal matrix. These materials get plated using a similar process to other electroplating projects.
Ceramic matrix composites (CMCs): CMCs are a combination of ceramix matrix and ceramic fibers. These can include carbon and carbon fibers. To electroplate CMCs, the base will go through electroless plating so that it will conduct electricity. Typically, electroless nickel is used in preparation for traditional electroplating.
Composite materials are frequently used in the creation of parts in the aerospace and military industry sectors, where advanced composites can add strength, reduce weight (and thereby become more eco friendly), minimize cost, and provide a wide range of additional benefits in both form and function, depending on the materials used in the composite.
For example, a silver door handle is significantly more expensive than a door handle made of plastic that&#;s plated with bright nickel and chrome. A shiny metal (electroplated) finish provides a beautiful aesthetic for less.
In another scenario, some industries use composite materials because they can be used in high-performance situations. Standard ceramics aren&#;t as strong as ceramic composites. Ceramic composites are heat tolerant to extreme temperatures, so they can be used as heat shields for spaceships. And when you electroplate these ceramic composites with metals, you create a very heat tolerant object with electrical conductivity.
Electroplating is everywhere. From automotive trim to the parts of the last rocketship you watched get launched into space, electroplated materials are used for their versatility, their conductivity, their appearance, and more. Here are a few examples of common applications:
Electromagnetic interference (EMI) is a concern in any situation involving electronic systems. EMI shielding using electroplating can protect a device or cabling, so that conflicting signals and interference are blocked. EMI shielding can also be used to help prevent power degradation issues in cables.
Hard chrome plating is used to increase the hardness, durability, and corrosion resistance on high wear surfaces. Chromium metal is used to add a hard, durable surface finish with improved wear, even in extreme situations. Parts last longer and resist corrosion, and chromium is less likely to wear away in harsh environments.
Electroplating is a critical part of aerospace and satellite metal finishing, where it is used to:
Protect against corrosion
Increase lifespan and wear of components
Increase heat resistance in extreme temperatures
Add or increase electrical conductivity
Increase resistance to oxidation
Add strength to metal substrates and components
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Add to the visual aesthetic
Spacecraft and satellites face numerous environmental threats, both within the atmosphere and outside of it, that can degrade and damage equipment and materials. Additional threats that can be mitigated or met by electroplating include:
Powerful light: The light from the sun is incredibly powerful and without protection from the earth&#;s atmosphere, satellites and spacecraft are exposed to huge amounts of sunlight, causing heat and glare. Metal insulation plating can help reflect and absorb this damaging light.
Heat: The sun&#;s heat is incredibly stronger outside of the earth&#;s atmosphere. Reflective coatings and metal plating are used to transfer heat to help mitigate damage to electronic equipment.
Temperature Cycling: Satellites and spacecraft can go through many cycles of extreme heat and extreme cold in one day . Metal coatings can be used to minimize the effects of these quick thermal changes on equipment and the crafts themselves.
Space Garbage: Satellites and spacecraft are subject to being hit by small objects in space, whether those objects are natural in the form of micrometeoroids or man-made space junk. Strong metal coatings can help protect the craft or satellite&#;s exterior.
Equipment that&#;s used by our nation&#;s military must be able to withstand extreme temperatures, high and near-constant use, high levels of friction, and other issues that will be encountered in service and combat situations. Most materials in these use situations must be coated with metal finishes to address: electrical conductivity, hardness, oxygen deprivation or saturation, chemical and caustic materials exposure, insulation, and other extremes. In defense applications, electroplating requires strict adherence to provided specifications that are provided by the Department of Defense and defense contract manufacturers.
The need to create lighter and more aerodynamic vehicles for all of the new forms of future mobility has created increasing and renewed interest in electroplating on plastic. Automotive manufacturers are using more electroplated injection-molded parts in electric vehicles, for internal engine components, connector and interconnect device housings, exterior trim and panels, and battery housings, among other uses.
Medical devices and implants that are placed inside the body are required to meet high standards for sanitation, reliability, durability, and safety. Many of these products rely on metal electroplating to ensure the product is of the highest quality, clean, and free of defects that could cause a problem for the patient. Here are just a few examples of electroplating in use in the medical device industry:
Gold is often used due to its high biocompatibility, corrosion resistance, and electrical conductivity. It&#;s used often for arterial stents, pacemakers, and dental crowns, among other uses.
Silver has antibacterial properties, making it a good choice for the medical plating of tools and equipment.
Platinum is hard, corrosion-resistant, and biocompatible, so it makes a great choice for catheters, pacemakers, and implants.
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Electroplating, also known as electrochemical deposition or electrodeposition, is a process for producing a metal coating on a solid substrate through the reduction of cations of that metal by means of a direct electric current. The part to be coated acts as the cathode (negative electrode) of an electrolytic cell; the electrolyte is a solution of a salt whose cation is the metal to be coated, and the anode (positive electrode) is usually either a block of that metal, or of some inert conductive material. The current is provided by an external power supply.
Electroplating is widely used in industry and decorative arts to improve the surface qualities of objects&#;such as resistance to abrasion and corrosion, lubricity, reflectivity, electrical conductivity, or appearance. It is used to build up thickness on undersized or worn-out parts and to manufacture metal plates with complex shape, a process called electroforming. It is used to deposit copper and other conductors in forming printed circuit boards and copper interconnects in integrated circuits. It is also used to purify metals such as copper.
The aforementioned electroplating of metals uses an electroreduction process (that is, a negative or cathodic current is on the working electrode). The term "electroplating" is also used occasionally for processes that occur under electro-oxidation (i.e positive or anodic current on the working electrode), although such processes are more commonly referred to as anodizing rather than electroplating. One such example is the formation of silver chloride on silver wire in chloride solutions to make silver/silver-chloride (AgCl) electrodes.
Electropolishing, a process that uses an electric current to selectively remove the outermost layer from the surface of a metal object, is the reverse of the process of electroplating.[1]
Throwing power is an important parameter that provides a measure of the uniformity of electroplating current, and consequently the uniformity of the electroplated metal thickness, on regions of the part that are near to the anode compared to regions that are far from it. It depends mostly on the composition and temperature of the electroplating solution, as well as on the operating current density.[2] A higher throwing power of the plating bath results in a more uniform coating.[3]
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Simplified diagram for electroplating copper (orange) on a conductive object (the cathode, "Me", gray). The electrolyte is a solution of copper sulfate,CuSO
4
Cu
2+
The electrolyte in the electrolytic plating cell should contain positive ions (cations) of the metal to be deposited. These cations are reduced at the cathode to the metal in the zero valence state. For example, the electrolyte for copper electroplating can be a solution of copper(II) sulfate, which dissociates into Cu2+ cations and SO2&#;
4 anions. At the cathode, the Cu2+ is reduced to metallic copper by gaining two electrons.
When the anode is made of the metal that is intended for coating onto the cathode, the opposite reaction may occur at the anode, turning it into dissolved cations. For example, copper would be oxidized at the anode to Cu2+ by losing two electrons. In this case, the rate at which the anode is dissolved will equal the rate at which the cathode is plated, and thus the ions in the electrolyte bath are continuously replenished by the anode. The net result is the effective transfer of metal from the anode to the cathode.
The anode may instead be made of a material that resists electrochemical oxidation, such as lead or carbon. Oxygen, hydrogen peroxide, and some other byproducts are then produced at the anode instead. In this case, ions of the metal to be plated must be replenished (continuously or periodically) in the bath as they are drawn out of the solution.[5]
The plating is most commonly a single metallic element, not an alloy. However, some alloys can be electrodeposited, notably brass and solder. Plated "alloys" are not "true alloys" (solid solutions), but rather they are tiny crystals of the elemental metals being plated. In the case of plated solder, it is sometimes deemed necessary to have a true alloy, and the plated solder is melted to allow the tin and lead to combine into a true alloy. The true alloy is more corrosion-resistant than the as-plated mixture.
Many plating baths include cyanides of other metals (such as potassium cyanide) in addition to cyanides of the metal to be deposited. These free cyanides facilitate anode corrosion, help to maintain a constant metal ion level, and contribute to conductivity. Additionally, non-metal chemicals such as carbonates and phosphates may be added to increase conductivity.
When plating is not desired on certain areas of the substrate, stop-offs are applied to prevent the bath from coming in contact with the substrate. Typical stop-offs include tape, foil, lacquers, and waxes.[6]
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Initially, a special plating deposit called a strike or flash may be used to form a very thin (typically less than 0.1 μm thick) plating with high quality and good adherence to the substrate. This serves as a foundation for subsequent plating processes. A strike uses a high current density and a bath with a low ion concentration. The process is slow, so more efficient plating processes are used once the desired strike thickness is obtained.
The striking method is also used in combination with the plating of different metals. If it is desirable to plate one type of deposit onto a metal to improve corrosion resistance but this metal has inherently poor adhesion to the substrate, then a strike can be first deposited that is compatible with both. One example of this situation is the poor adhesion of electrolytic nickel on zinc alloys, in which case a copper strike is used, which has good adherence to both.[5]
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The pulse electroplating or pulse electrodeposition (PED) process involves the swift alternating of the electrical potential or current between two different values, resulting in a series of pulses of equal amplitude, duration, and polarity, separated by zero current. By changing the pulse amplitude and width, it is possible to change the deposited film's composition and thickness.[7]
The experimental parameters of pulse electroplating usually consist of peak current/potential, duty cycle, frequency, and effective current/potential. Peak current/potential is the maximum setting of electroplating current or potential. Duty cycle is the effective portion of time in a certain electroplating period with the current or potential applied. The effective current/potential is calculated by multiplying the duty cycle and peak value of the current or potential. Pulse electroplating could help to improve the quality of electroplated film and release the internal stress built up during fast deposition. A combination of the short duty cycle and high frequency could decrease surface cracks. However, in order to maintain the constant effective current or potential, a high-performance power supply may be required to provide high current/potential and a fast switch. Another common problem of pulse electroplating is that the anode material could get plated and contaminated during the reverse electroplating, especially for a high-cost, inert electrode such as platinum.
Other factors that affect the pulse electroplating include temperature, anode-to-cathode gap, and stirring. Sometimes, pulse electroplating can be performed in a heated electroplating bath to increase the deposition rate, since the rate of most chemical reactions increases exponentially with temperature per the Arrhenius law. The anode-to-cathode gap is related to the current distribution between anode and cathode. A small gap-to-sample-area ratio may cause uneven distribution of current and affect the surface topology of the plated sample. Stirring may increase the transfer/diffusion rate of metal ions from the bulk solution to the electrode surface. The ideal stirring setting varies for different metal electroplating processes.
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A closely-related process is brush electroplating, in which localized areas or entire items are plated using a brush saturated with plating solution. The brush, typically a graphite body wrapped with an absorbent cloth material that both holds the plating solution and prevents direct contact with the item being plated, is connected to the anode of a low-voltage and 3-4 ampere direct-current power source, and the item to be plated (the cathode) is grounded. The operator dips the brush in plating solution and then applies it to the item, moving the brush continually to get an even distribution of the plating material.
Brush electroplating has several advantages over tank plating, including portability, the ability to plate items that for some reason cannot be tank plated (one application was the plating of portions of very large decorative support columns in a building restoration), low or no masking requirements, and comparatively low plating solution volume requirements. Mainly used industrially for part repair, worn bearing surfaces getting a nickel or silver deposit. With technological advancement deposits up to .025" have been achieved and retained uniformity. Disadvantages compared to tank plating can include greater operator involvement (tank plating can frequently be done with minimal attention and the solutions used are often toxic), and the inconsistency in achieving as great a plate thickness.
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This technique of electroplating is one of the most common used in the industry for large numbers of small objects. The objects are placed in a barrel-shaped non-conductive cage and then immersed in a chemical bath containing dissolved ions of the metal that is to be plated onto them. The barrel is then rotated, and electrical currents are run through the various pieces in the barrel, which complete circuits as they touch one another. The result is a very uniform and efficient plating process, though the finish on the end products will likely suffer from abrasion during the plating process. It is unsuitable for highly ornamental or precisely engineered items.[8]
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Cleanliness is essential to successful electroplating, since molecular layers of oil can prevent adhesion of the coating. ASTM B322 is a standard guide for cleaning metals prior to electroplating. Cleaning includes solvent cleaning, hot alkaline detergent cleaning, electrocleaning, ultrasonic cleaning and acid treatment. The most common industrial test for cleanliness is the waterbreak test, in which the surface is thoroughly rinsed and held vertical. Hydrophobic contaminants such as oils cause the water to bead and break up, allowing the water to drain rapidly. Perfectly clean metal surfaces are hydrophilic and will retain an unbroken sheet of water that does not bead up or drain off. ASTM F22 describes a version of this test. This test does not detect hydrophilic contaminants, but electroplating can displace these easily, since the solutions are water-based. Surfactants such as soap reduce the sensitivity of the test and must be thoroughly rinsed off.
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Throwing power (or macro throwing power) is an important parameter that provides a measure of the uniformity of electroplating current, and consequently the uniformity of the electroplated metal thickness, on regions of the part that are near the anode compared to regions that are far from it. It depends mostly on the composition and temperature of the electroplating solution.[2] Micro throwing power refers to the extent to which a process can fill or coat small recesses such as through-holes.[9] Throwing power can be characterized by the dimensionless Wagner number:
Wa = R T κ F L α | i | , {\displaystyle {\text{Wa}}={\frac {RT\kappa }{FL\alpha |i|}},}
where R is the universal gas constant, T is the operating temperature, κ is the ionic conductivity of the plating solution, F is the Faraday constant, L is the equivalent size of the plated object, α is the transfer coefficient, and i the surface-averaged total (including hydrogen evolution) current density. The Wagner number quantifies the ratio of kinetic to ohmic resistances. A higher Wagner number produces a more uniform deposition. This can be achieved in practice by decreasing the size (L) of the plated object, reducing the current density |i|, adding chemicals that lower α (make the electric current less sensitive to voltage), and raising the solution conductivity (e.g. by adding acid). Concurrent hydrogen evolution usually improves the uniformity of electroplating by increasing |i|; however, this effect can be offset by blockage due to hydrogen bubbles and hydroxide deposits.[10]
The Wagner number is rather difficult to measure accurately; therefore, other related parameters, that are easier to obtain experimentally with standard cells, are usually used instead. These parameters are derived from two ratios: the ratio M = m1 / m2 of the plating thickness of a specified region of the cathode "close" to the anode to the thickness of a region "far" from the cathode and the ratio L = x2 / x1 of the distances of these regions through the electrolyte to the anode. In a Haring-Blum cell, for example, L = 5 for its two independent cathodes, and a cell yielding plating thickness ratio of M = 6 has Harring-Blum throwing power 100% × (L &#; M) / L = &#;20%.[9] Other conventions include the Heatley throwing power 100% × (L &#; M) / (L &#; 1), and Field throwing power 100% × (L &#; M) / (L + M &#; 2).[11] A more uniform thickness is obtained by making the throwing power larger (less negative) according to any of these definitions.
Parameters that describe cell performance such as throwing power are measured in small test cells of various designs that aim to reproduce conditions similar to those found in the production plating bath.[9]
Haring&#;Blum cell[
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Haring&#;Blum cellThe Haring&#;Blum cell is used to determine the macro throwing power of a plating bath. The cell consists of two parallel cathodes with a fixed anode in the middle. The cathodes are at distances from the anode in the ratio of 1:5. The macro throwing power is calculated from the thickness of plating at the two cathodes when a direct current is passed for a specific period of time. The cell is fabricated out of perspex or glass.[12][13]
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A zinc solution tested in a Hull cellThe Hull cell is a type of test cell used to semi-quantitatively check the condition of an electroplating bath. It measures useable current density range, optimization of additive concentration, recognition of impurity effects, and indication of macro throwing power capability.[14] The Hull cell replicates the plating bath on a lab scale. It is filled with a sample of the plating solution and an appropriate anode which is connected to a rectifier. The "work" is replaced with a Hull cell test panel that will be plated to show the "health" of the bath.
The Hull cell is a trapezoidal container that holds 267 milliliters of a plating bath solution. This shape allows one to place the test panel on an angle to the anode. As a result, the deposit is plated at a range current densities along its length, which can be measured with a Hull cell ruler. The solution volume allows for a semi-quantitative measurement of additive concentration: 1 gram addition to 267 mL is equivalent to 0.5 oz/gal in the plating tank.[15]
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Electroplating changes the chemical, physical, and mechanical properties of the workpiece. An example of a chemical change is when nickel plating improves corrosion resistance. An example of a physical change is a change in the outward appearance. An example of a mechanical change is a change in tensile strength or surface hardness, which is a required attribute in the tooling industry.[16] Electroplating of acid gold on underlying copper- or nickel-plated circuits reduces contact resistance as well as surface hardness. Copper-plated areas of mild steel act as a mask if case-hardening of such areas are not desired. Tin-plated steel is chromium-plated to prevent dulling of the surface due to oxidation of tin.
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There are a number of alternative processes to produce metallic coatings on solid substrates that do not involve electrolytic reduction:
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Electroplating was invented by Italian chemist Luigi Valentino Brugnatelli in . Brugnatelli used his colleague Alessandro Volta's invention of five years earlier, the voltaic pile, to facilitate the first electrodeposition. Brugnatelli's inventions were suppressed by the French Academy of Sciences and did not become used in general industry for the following thirty years. By , scientists in Britain and Russia had independently devised metal-deposition processes similar to Brugnatelli's for the copper electroplating of printing press plates.
Research from the s had theorized that electroplating might have been performed in the Parthian Empire using a device resembling a Baghdad Battery, but this has since been refuted; the items were fire-gilded using mercury.[17]
Boris Jacobi in Russia not only rediscovered galvanoplastics, but developed electrotyping and galvanoplastic sculpture. Galvanoplastics quickly came into fashion in Russia, with such people as inventor Peter Bagration, scientist Heinrich Lenz, and science-fiction author Vladimir Odoyevsky all contributing to further development of the technology. Among the most notorious cases of electroplating usage in mid-19th century Russia were the gigantic galvanoplastic sculptures of St. Isaac's Cathedral in Saint Petersburg and gold-electroplated dome of the Cathedral of Christ the Saviour in Moscow, the third tallest Orthodox church in the world.[18]
Nickel platingSoon after, John Wright of Birmingham, England discovered that potassium cyanide was a suitable electrolyte for gold and silver electroplating. Wright's associates, George Elkington and Henry Elkington were awarded the first patents for electroplating in . These two then founded the electroplating industry in Birmingham from where it spread around the world. The Woolrich Electrical Generator of , now in Thinktank, Birmingham Science Museum, is the earliest electrical generator used in industry.[19] It was used by Elkingtons.[20][21][22]
The Norddeutsche Affinerie in Hamburg was the first modern electroplating plant starting its production in .[23]
As the science of electrochemistry grew, its relationship to electroplating became understood and other types of non-decorative metal electroplating were developed. Commercial electroplating of nickel, brass, tin, and zinc were developed by the s. Electroplating baths and equipment based on the patents of the Elkingtons were scaled up to accommodate the plating of numerous large-scale objects and for specific manufacturing and engineering applications.
The plating industry received a big boost with the advent of the development of electric generators in the late 19th century. With the higher currents available, metal machine components, hardware, and automotive parts requiring corrosion protection and enhanced wear properties, along with better appearance, could be processed in bulk.
The two World Wars and the growing aviation industry gave impetus to further developments and refinements, including such processes as hard chromium plating, bronze alloy plating, sulfamate nickel plating, and numerous other plating processes. Plating equipment evolved from manually-operated tar-lined wooden tanks to automated equipment capable of processing thousands of kilograms per hour of parts.
One of the American physicist Richard Feynman's first projects was to develop technology for electroplating metal onto plastic. Feynman developed the original idea of his friend into a successful invention, allowing his employer (and friend) to keep commercial promises he had made but could not have fulfilled otherwise.[24]
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