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 cover the definition of metal plating, the various processes involved, and the materials utilized in the process.
Metal plating involves applying a thin layer of metal onto the surface or substrate of a metal part, product, or component. This process can include electroplating, where metal ions are deposited onto the substrate's surface through the use of an electric current.
Electroless plating deposits metal ions onto the workpiece without using electrodes. Both this method and others are designed to enhance the metal's resistance to corrosion, each offering various benefits.
While metal plating can protect a part or component from environmental damage, it also enhances its appearance. There are various types of coatings, categorized as either industrial or commercial.
Commercial decorative coatings are applied to everyday items to enhance their appearance and durability. These coatings are commonly used on tools, silverware, and jewelry. On the other hand, industrial coatings are applied to metal parts to increase their toughness and wear resistance, allowing them to endure harsh conditions and better protect and strengthen machine and truck components.
Coatings can enhance solderability and strength while reducing friction to minimize wear. Applying a coating to metal can also affect its conductivity, potentially altering its ability to conduct electric current. Additionally, coated metals can be painted, and their magnetic properties may be improved.
The various metal plating processes are:
Electroplating involves using electrodeposition to apply a thin metal coating to the workpiece. Engineers control the process through electrolysis, transferring the metal coating from the anode (which holds the metal to be plated) to the cathode (the workpiece being plated). Both the anode and cathode are immersed in an electrolyte bath, with a continuous electrical current applied to facilitate the coating process.
The electric current drives negatively charged ions (anions) toward the anode and positively charged ions (cations) toward the cathode, creating a uniform metal layer on the workpiece. This electroplating process results in the substrate being enveloped in a thin metal coating, such as copper or nickel.
Electroplating is predominantly applied to metals, as the substrate must be conductive. However, there are some less common autocatalytic pre-coating techniques that generate an ultra-thin conductive layer, enabling the plating of various metals, including nickel and copper alloys, onto plastic components.
Both electroplating and electroforming involve electrodeposition. The key difference between the two processes is that electroforming uses a mold, which is removed once the part has been formed.
Electroforming is used to create solid metal pieces, whereas electroplating is used to coat an existing workpiece made from a different metal material.
A single metal or a combination of metals can be electroplated onto an object. Many manufacturers choose to layer metals such as nickel and copper to enhance strength and conductivity. Below are some of the most commonly used materials in the electroplating process:
Almost any material can serve as a substrate, including stainless steel, various metals, and plastics. Even organic materials, such as soft fabric ribbons and flowers, can be electroplated. It is important to note that non-conductive substrates, such as plastic, glass, and wood, must be made conductive before electroplating. This can be achieved by applying a conductive spray or paint to the substrate.
Electroless plating is a technique for applying metal coatings using chemical reactions instead of electrical currents. This method involves immersing the workpiece in a solution containing a reducing agent and a catalyst, which together convert metal ions into a metal deposit on the surface. It is commonly used for coating plastics, such as those found in printed electronics, and enhances both the durability and appearance of various consumer items.
Unlike electroplating, which relies on electrical currents, electroless plating operates through chemical reactions in a solution without the need for electricity. In this process, the reducing agent, often sodium hypophosphite, releases hydrogen, leading to oxidation and the creation of a negative charge on the workpiece. This technique enables even deposition of metal on complex shapes, including those with holes and irregular surfaces, which can be difficult to plate using traditional electroplating methods.
This method is especially valuable for making non-conductive materials, such as plastics, conductive. By applying a conductive metal layer, electroless plating prepares these surfaces for subsequent electroplating or other applications requiring electrical conductivity.
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 plating technique that does not rely on galvanic action and involves multiple concurrent reactions in a liquid solution. This process is carried out without an external electrical power source.
The process is driven by a reducing agent that releases hydrogen, such as sodium hypophosphite, which becomes oxidized and creates a negative charge on the surface of the workpiece.
Electroless plating ensures that all parts of an object are uniformly coated with metal ions, providing an even deposit inside cavities, along edges, and on irregular surfaces, which can be challenging to plate evenly using electroplating methods.
This technique is also used to apply a conductive layer to non-conductive objects, enabling them to be subsequently coated through electroplating.
Immersion plating involves applying a layer of more noble metals to a base metal by immersing it in a solution containing ions of the noble metal. This process triggers a replacement reaction, resulting in the deposition of a metallic coating onto the base metal. During this process, a metal with a lower oxidation potential typically displaces a metal ion with a higher oxidation potential from the solution.
This technique is utilized to enhance electrical properties and improve the adhesion of coatings or organic layers to the substrate. Known as dip plating or metal replacement, immersion plating differs from electroplating as it does not require an external current. The method operates on the principle that when a less noble metal, such as copper, is submerged in an electrolyte solution containing ions of a more noble metal, the less noble metal will dissolve.
This dissolution process releases electrons, allowing the more noble metals to deposit onto the substrate. Unlike electroless plating, the deposition stops once the substrate is fully covered with the noble metal. Immersion plating is performed at elevated temperatures, with gold immersion typically occurring between 80°C and 90°C, and silver immersion between 50°C and 60°C.
Both electroplating and electroless plating enhance a component’s durability and corrosion resistance while improving its appearance, such as in jewelry or other consumer products. The key distinction between these methods lies in the use of electric current: electroplating employs an electric current, whereas electroless plating does not.
In electroplating, a power source like a rectifier or battery supplies electric current to a component submerged in a chemical solution. This current changes the chemical state of the metals, resulting in a hard, durable coating on the component’s surface. This method is more intricate than electroless plating, necessitates exceptionally clean conditions, involves potentially hazardous equipment, and may require multiple layers to achieve the desired coating thickness.
Electroless plating, on the other hand, is less complex. The process starts with cleaning the component using chemical agents to remove contaminants and oils. The component is then immersed in a solution with anti-oxidation chemicals. This method produces a plated component with excellent corrosion and friction resistance.
Electroless nickel plating is simpler as it does not involve complex filtration systems or additional equipment, and since it does not use electricity, it eliminates the risk of electrical hazards.
This chapter will cover the various metal plating types according to the specific metals employed.
This method involves applying an alloy treatment to enhance the hardness and durability of a metal or plastic. Electroless nickel plating is less complex compared to electroplating. Unlike electroplating, it does not require the flow of electric current through the chemical bath to initiate the plating process.
Instead, the metal surface undergoes a sequence of autocatalytic reactions and cleaning, a process refined through electro-coatings. The following are the stages involved in electroless nickel plating:
Electroless nickel plating can be deposited at rates ranging from 5 microns per hour to 25 microns per hour. As the process is continuous and self-sustaining, it can achieve virtually limitless thickness. However, as the thickness increases, minor imperfections become more noticeable. Depending on the specific needs, one of five distinct coating options may be applied through electro-coating.
Zinc is a cost-effective material used to apply a galvanized coating to various metal substrates. It can be applied through different methods, including electroplating, the sherardizing process, dipping in a molten bath, and spraying. In the cold or electrolytic process, the item to be plated serves as the cathode in a bath with an electrolyte solution of soluble zinc salts, while metallic zinc acts as the anode.
This process yields a highly ductile coating of pure zinc, with precise control over its uniformity and thickness. The sherardizing method is specifically used for coating small hardware items such as nails and screws. In this method, the items are combined with zinc dust in a barrel and heated to approximately 500°F.
Inside the barrel, the parts are tumbled, resulting in a coating composition of 10% iron and 90% zinc. For larger items, molten zinc can be applied through dipping or manual coating methods. Occasionally, a small amount of aluminum is added to the bath to enhance fluidity and improve the coating on irregularly shaped objects.
This type of plating is frequently applied to various automotive parts. It is preferred by aircraft manufacturers for its sacrificial protection properties and natural lubricity, making it ideal for components that are frequently removed and reinstalled. Additionally, this metal plating is well-suited for marine environments, as it performs effectively against both saltwater and freshwater exposure.
Due to safety concerns, the use of this type of plating has decreased over the years, although it remains available. Many aircraft manufacturers now prefer zinc-nickel alloy plating as an alternative to cadmium plating.
This type of plating is primarily used for decorative purposes but also offers significant resistance to corrosion and hardness, making it suitable for industrial applications where wear resistance is important. It is occasionally employed to restore tolerances on worn components. Chromium plating is commonly applied over nickel in the manufacturing of steel furniture and automotive trim.
Chrome plating is an electroplating process that uses a chromic acid solution known as hexavalent chromium. For industrial applications, trivalent chromium baths containing chromium chloride or chromium sulfate are also used as an alternative.
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 cost stability of aluminum and its growing range of applications make it a desirable plating material. However, as a less noble metal, aluminum forms an oxide layer easily, which restricts its use in plating. Additionally, aluminum plating can be limited by inconsistencies in grain structure formation and the weak adhesion to the base metal.
To plate aluminum, methods such as immersion, electroplating, and electroless plating are employed. The primary challenge with aluminum plating is its oxide layer, which can hinder adhesion. To mitigate this issue, a zinc immersion film is applied, with zincation being the predominant technique used to prevent oxide formation.
Copper plating is widely chosen for applications that demand cost-effectiveness and excellent conductivity. It's commonly used in electronics, such as printed circuit boards, due to its affordability and high plating efficiency. The combination of low material costs and effective plating makes copper a popular choice for various applications.
Copper plating processes can be categorized into three types: acid, alkaline, and mild alkaline. Alkaline processes offer better throwing power but necessitate lower current densities and heightened safety measures. Monitoring these levels is crucial, as health inspectors have identified links between cyanide and alkaline copper baths.
Gold is highly valued for its resistance to oxidation and excellent electrical conductivity. Gold plating, which differs from gilding as it does not use gold foil, is an effective method for imparting these properties to metals such as silver and copper. This process is commonly used for decorating jewelry and enhancing the conductivity of electronic components, like electrical connectors.
Tarnishing can be a concern when gold plating copper, but this can be effectively addressed by applying a nickel strike before the gold layer. Factors such as immersion time and the ideal bath composition should be considered, taking into account the hardness and purity of the gold used.
Silver is a popular choice for plating due to its excellent thermal and electrical conductivity, which surpasses even gold. It is widely used in applications requiring efficient power transmission and low power connectors. Its lubricating properties also make it ideal for high-temperature anti-galling applications, particularly in bearings and fasteners.
Silver plating on copper creates an effective surface for heat and electricity transfer. The technique has been in use for over 200 years, initially for plating switch gears and other electrical components. With the rise of electric vehicles (EVs), silver plating has become more prevalent.
In the silver plating process, multiple layers of silver are applied to ensure durability, as silver can wear off with continuous use. Besides its practical uses, silver plating also offers decorative appeal while maintaining electrical conductivity. It is a cost-effective alternative to gold, palladium, and rhodium, though its susceptibility to galvanic corrosion and humidity limits its applications.
Silver plating is also employed on stainless steel components such as nuts, fasteners, slip rings, thrust washers, and bushings. This plating enhances lubricity at high temperatures and provides anti-galling and anti-seizing properties. However, the oxide layer on stainless steel can complicate the plating process, requiring methods to remove the oxide before applying the silver layer.
One minor drawback of silver is that it tends to tarnish over time, which some might view as a flaw. Unlike other tarnishing processes that involve the formation of an oxide layer, silver tarnish occurs when it reacts with hydrogen sulfide or sulfur, creating a layer of silver sulfide. This tarnish can conduct electricity but can be easily cleaned by simply wiping it off.
Additionally, silver plating may not be ideal for environments with high humidity. In such conditions, silver is prone to issues like flaking and cracking, which could expose the underlying material.
Tin is a malleable, silver-white metal known for its good electrical conductivity, resistance to oxidation, and durability against corrosion. Due to its non-toxic and non-carcinogenic properties, it is commonly used in food storage and preparation. Historically, tin has been integral in making bronze tools and early cooking vessels.
Tinning involves applying a thin coating of tin to a metal surface to enhance its soldering properties, conductivity, and corrosion resistance, while also improving its visual appeal. Tin’s excellent electrical conductivity makes it ideal for coating electrical components such as terminals, battery connectors, and copper bus bars used in electrical systems.
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, a member of the platinum group, is renowned for its tarnish resistance, scratch durability, and brilliant white sheen. It is frequently used in jewelry making, particularly for plating white gold. Rhodium plating is also applied over base metals such as copper, silver, and platinum to enhance their appearance and durability.
A drawback of rhodium plating is that, over time and with frequent use, the rhodium layer may wear off. This wear can result in discoloration of the underlying metal. Consequently, reapplying a fresh layer of rhodium may be needed after a few years to restore its appearance.
Metal deposition on plastics through electroplating and electroless plating is employed to provide additional protection to the plastic material for further manufacturing steps or to make it electrically conductive. This process is commonly used across various industries to enhance the appearance of plastic components and achieve a smooth, uniform surface. Thermoplastics, which become pliable when heated, are particularly suitable for this type of plating.
Acrylonitrile butadiene styrene (ABS) is one of the most frequently plated plastics due to its robustness, durability, and cost-effectiveness. Although ABS is prone to environmental damage and has limited mechanical strength, plating can address these limitations. Research into plating methods for ABS led to significant advancements, including the development of chemicals that prepare ABS for plating and eventually paved the way for plating other types of plastics.
Successful plastic plating hinges on meticulous preparation, starting with etching the plastic in a chromic acid solution to enhance surface adhesion. Once the chromic acid is thoroughly rinsed off, the plastic is treated with a palladium and tin salt solution to prime it for the plating process. Copper or nickel is then deposited using an electroless plating method, with the palladium and tin salts acting as a catalyst. Following the completion of electroless plating, the plastic is prepared for conventional electroplating.
| Plastics That can be Metal Plated | |
|---|---|
| Thermoplastics | Thermosets |
| Acrylic | Araldite |
| Acrylonitrile Butadiene Styrene (ABS) | Bakelite |
| Nylon | Epoxy |
| Polylactic Acid (PLA) | Faturan |
| Polybenzimidazole (PBI) | Furan Resin |
| Polycarbonate (PC) | Melamine Resin |
| Polyether Sulfone (PES) | Novolak |
| Polyoxymethylene (POM) | Phenol formaldehyde resin |
| Polyether ether Ketone (PEEK) | Polybenzoxazine |
| Polyetherimide (PEI) | Polyester |
| Polyethylene (PE) | Polyester resin |
| Polyphenylene oxide (PPO) | Polyhexahydrotrianzine |
| Polyphenylene sulfide (PPS) | Polyisocyanurate |
| Polypropylene (PP) | Silicone |
| Polystyrene (PS) | Urea-formaldehyde |
| Polyvinyl Chloride (PVC) | Vinyl ester resin |
| Polyvinylidene Fluoride (PVDF) | |
| Teflon (PTFE) | |
| Polyimide | |
This chapter will explore the various uses and advantages of metal plating.
The applications of metal plating are:
Many airplane parts undergo electroplating to apply a sacrificial coating that extends their service life by reducing corrosion rates. Aircraft components, which experience significant temperature fluctuations and environmental exposure, benefit from an extra metal layer added to the underlying metal substrate for enhanced protection.
This approach is intended to preserve the functionality of parts by minimizing wear and tear. In the aerospace industry, many fasteners and steel bolts are coated with chromium through electroplating to ensure durability and reliability.
Biodegradable objects such as branches, flowers, and insects are frequently transformed into long-lasting art pieces through metal plating. This technique enhances and preserves the fine details of items that would otherwise decompose. Additionally, digital designers sometimes use electroplating to create durable sculptures.
Designers can create 3D substrates using a desktop 3D printer and then apply electroplating in materials such as gold, silver, or copper to achieve the desired finish. This combination of 3D printing and electroplating results in cost-effective and straightforward manufacturing while maintaining the intended appearance of the final piece.
Electroplating is widely used in the automotive sector, with many leading car manufacturers employing it to produce chrome bumpers and various metal components. Additionally, it is used to craft custom parts for prototype vehicles.
Vehicle customization and restoration shops frequently use electroplating to apply chrome, nickel, and other finishes to various motorcycle and car components.
Electroplating plays a significant role in the jewelry industry. Jewelry designers and manufacturers use this technique to improve the durability, visual appeal, and color of various pieces, including bracelets, pendants, rings, and other items.
Jewelry labeled as silver or gold plated is often the result of electroplating. Different metals are combined to achieve distinctive finishes. For example, copper, silver, and gold are frequently blended to produce the popular rose gold hue.
Electroplating is utilized to add durable coatings to various dental and medical devices. Gold plating is particularly common for making dental inlays and supporting various dental procedures.
Implanted components, such as screws, plates, and artificial joints, are often electroplated to enhance their corrosion resistance and compatibility with pre-insertion sterilization. Additionally, surgical instruments and medical tools, including radiological equipment and forceps, are frequently subjected to electroplating.
Electroplating is commonly used to coat various solar and electrical components to enhance their conductivity. This process is routinely applied to the contacts of solar cells and various types of antennas. Wires, for instance, are often electroplated with nickel, silver, or other metals to improve their performance.
Gold plating is frequently employed, often in combination with other metals, to boost the durability of components. Gold is particularly valued for its ability to extend the lifespan of parts due to its excellent conductivity, malleability, and resistance to oxidation.
Traditional manufacturing methods for producing custom or low-volume metal parts can be both time-consuming and expensive, especially for prototyping. To address these challenges, engineers often combine electroplating with 3D printing to provide a more efficient and cost-effective solution.
Antennas require electrical conductivity to effectively transmit radio waves. While 3D-printed parts do not conduct electricity, they provide exceptional design flexibility and possess advantageous thermal and mechanical properties. By combining these 3D-printed components with electroplating, desired conductivity can be achieved. This approach offers an excellent solution for custom antennas used in research and development across defense, education, medicine, and automotive sectors.
The benefits of metal plating are:
Electroplating offers several advantages, including enhanced strength, conductivity, and longevity of components. Various professionals, such as manufacturers, artists, and engineers, leverage these benefits in different ways. Engineers often use electroplating to boost the durability and robustness of their designs.
Applying a metal coating, such as nickel or copper, can significantly increase the tensile strength of parts. This metallic layer enhances resistance to environmental factors like UV exposure, chemical contact, and corrosion, making the components more resilient in various applications.
Artists utilize electroplating to preserve natural elements prone to decay, such as leaves, transforming them into durable art pieces. In the medical field, electroplating is employed to create implants that resist corrosion and can be effectively sterilized.
Electroplating is also effective for adding decorative metal finishes to consumer products, figurines, sculptures, and art pieces. Additionally, manufacturers often choose electroplating for substrates to produce lightweight parts that are more cost-effective and easier to transport.
Moreover, electroplating enhances conductivity. Since metals are inherently conductive, electroplating improves the performance of electrical components, antennas, and other parts by increasing their electrical conductivity.
Despite its many advantages, electroplating comes with challenges, including hazardous conditions and process complexity. Workers involved in electroplating may be at risk of exposure to hexavalent chromium if safety measures are not strictly followed. Adequate ventilation is crucial to protect workers in these environments.
Structural plating is particularly challenging, requiring multiple baths, extended plating times, and precise metal compatibility. Due to the associated risks and the level of expertise needed, many designers and engineers choose to collaborate with specialized third-party electroplating manufacturers.
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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