What is Metal Plating?
Metal Plating is a thin layer of metal that has been added to the outside of a material. It is a surface covering process by which a metal is deposited on a conductive surface. Plating has been done for hundreds of years; it is also critical for modern technology.
Plating is used to decorate objects, for corrosion inhibition, to improve solderability, to harden, to improve wearability, to reduce friction, to improve paint adhesion, to alter conductivity, to improve IR reflectivity, for radiation shielding, and for other purposes. Jewelry typically uses plating to give a silver or gold finish.
The thin-film deposition has plated objects as small as an atom, therefore plating finds uses in nanotechnology.
There are several plating methods and many variations. In one method, a solid surface is covered with a metal sheet, and then heat and pressure are applied to fuse them. Other plating techniques include electroplating, vapor deposition under vacuum, and sputter deposition. Recently, plating often refers to using liquids. Metallizing refers to coating metal on non-metallic objects.
Benefits of Metal plating
Metal plating provides many benefits to products made from metal and other materials. This is mostly achieved through electroplating, which requires an electric current, or through electroless plating, which is in an autocatalytic chemical process. These techniques, as well as a few others, result in one or several of the following benefits:
- Improved corrosion resistance
- Decorative appeal
- Increased solderability
- Enhanced hardness
- Reduced friction
- Altered conductivity
- Enhanced paint adhesion
- Material deposition
- Increased magnetism
Types of Metal plating
There are many different types of metal plating as described Below:
- Electroless Plating
- Immersion Plating
- Physical Vapor Deposition
- Plasma Spray Coating
Electroplating is the most common method of plating. Electroplating uses an electrical current to dissolve positively charged metal particles (ions) in a chemical solution. The positively charged metal ions are attracted to the material to be plated, which is the negatively charged side of the circuit.
The part or product to be plated is then placed in this solution, and the dissolved metal particles are drawn to the surface of the material. Electroplating results in a smooth, even, and rapid coating for the material that is plated, effectively changing the surface of the material.
There are a number of different steps and processes that can be involved in electroplating, including cleaning, striking, electrochemical deposition, pulse electroplating, and brush electroplating.
Electroplating is used to provide a protective coating, a decorative appearance, or to change the properties of a material for engineering. Electroplating improves the chemical, physical, and mechanical properties of the workpiece which affects the way it performs when machined.
Plating the workpiece can be used to build it up from a smaller size, make it easier to machine, and increase solderability, conductivity, or reflectivity.
2. Electroless Plating
Electroless plating is so-called because it is a plating method that does not use external electric power. Electroless plating involves a chemical reaction that induces metal atom reduction.
In other words, the solution of metal ions (particles) when mixed with a reducing agent is converted into a metal solid when they come in contact with the catalyzing metal (which triggers the reaction). This results in the metal being plated with a solid layer of the plating metal.
Electroless or autocatalytic plating is suitable for diverse sizes and shapes of materials and doesn’t require external electricity or plating baths, which reduces costs. However, electroless plating is slower, can’t create thick plates, and is more difficult to control than electroplating.
The most common method of autocatalytic plating is electroless nickel plating. However, plating in silver, gold, and copper can also be applied with this technique.
Effects of electroless plating on the end product include protecting the base metal from corrosion, increasing the size of the workpiece, and altering solderability, reflectivity, and conductivity.
3. Immersion Plating
Immersion plating involves immersing one metal into a solution of metal ions from a more noble metal. The ions from the nobler metal are more stable, and so there is a natural ‘pull’ to displace the surface metal ions from the less noble metal with a thin layer of the nobler metal ions.
Immersion plating is a slower process, and can only be used for plating fewer noble metals with nobler metals. Nobler metals are metals that are chemically inert. For example, gold, platinum, or silver.
Immersion plating results in only a thin coverage of plating, after which point the plating process will stop. Immersion plating also seems to be of poorer adhesion quality, where the plating doesn’t ‘stick’ as firmly to the base metal.
The effects of immersion plating on the end product include improved corrosion resistance, altered electrical conductivity, changed appearance, greater hardness, torque tolerance, and modified bonding capabilities.
Also known as case hardening, carburizing is a heat-treating process that produces a wear-resistant surface whilst maintaining the strength of the core. Usually applied to low carbon steel after machining, as well as high, allow gears, bearings, etc.
Carburizing is suited to complex shapes of lower-cost materials which can be machined easily to give a very hard surface. The process involves heating the part in either a pit furnace or a sealed atmosphere furnace.
Then carburizing gases (usually carbon monoxide but also sodium cyanide and barium carbonate) are introduced at temperature, with the heat and temperature affecting the depth of carbon diffusion. The part is then either slow cooled for quenching later or quenched directly in oil.
5. Physical Vapor Deposition (PVD)
PVD is a family of coating processes in which thin films are deposited on the substrate. In the physical vapor deposition process, solid coating material such as titanium, chromium, or aluminum is evaporated by heat or by bombardment with ions.
During the process, a reactive gas such as nitrogen is introduced, forming a compound with the metal vapor and depositing on the metal’s surface as a very thin coating. This results in an extremely strong bond between the coating and the metal part.
Some of the advantages of PVD are; very hard and corrosion-resistant surface, high-temperature resistance, and good impact strength.
Ideal for a wide range of applications:
- Cutting Tools
- Thin films such as window tint, food packaging
6. Plasma Spray Coating
Plasma spray coating is one of the lesser-known types of metal plating. In this plating process, also known as thermal spraying, molten or heat-softened material is sprayed onto a surface to provide the coating.
The coating material is injected into a very high temperature plasma flame (up to 10,000 K in heat), it is rapidly heated and then accelerated to a high velocity onto the surface of the part and rapidly cools to form a coating on the part’s surface.
The process produces a coating, usually to structural materials, to provide protection against very high temperatures, for example in exhaust heat management. It also provides resistance to corrosion and wears. The coating can also change the appearance and electrical properties of the part.
Metals used in plating
Zinc is an inexpensive material that is used to provide a galvanized coating on many metal substrates. In addition to being electroplated, the element is applied by way of the Sherardizing process, by molten bath dipping, and by spraying.
In the electrolytic, or cold process, the article to be plated is set up as a cathode in an electrolytic bath of soluble zinc salts together with an anode of metallic zinc. The process produces a highly ductile coating of pure zinc whose thickness and uniformity can be precisely controlled.
The Sherardizing process is used to coat small hardware items such as screws and nails. Items are loaded in a barrel together with zinc dust and heated to approximately 500F. The parts are tumbled in the barrel producing a coating that is about 90% zinc and 10% iron.
Molten zinc may also be applied by dipping or by a manual coating of larger items. Sometimes a small amount of aluminum is added to the bath to improve fluidity and improve the coating of odd shapes.
Likewise, a small percentage of tin in the bath aids in achieving a uniform coating of the substrate and an improved finish. The hot-dip process produces a layer of zinc-iron alloy adjacent to the base metal which can be somewhat brittle and affect the adhesion of the outer layers.
Metal spraying, or metalizing, uses a flame to melt metal powders or wire and impinge them upon a substrate surface, producing a mechanical bond between the coating and the base metal. The surface of the base metal needs to be somewhat rough for the mechanical bonding to take place but fairly thick coatings can be applied in this manner.
The coatings can be porous too, but because zinc is anodic to iron and steel, this does not affect the coating’s ability to fend off corrosion. The porous nature of the sprayed metal also makes it good at holding paint.
Cadmium plating was at one time used as a substitute for zinc and often plated on to miscellaneous automotive items. Aircraft manufacturers specified it for its sacrificial protection characteristics and its natural lubricity for components that were frequently removed and reinstalled.
It was particularly suited to marine environments where it holds up well against fresh and saltwater. Due to safety concerns, its use as a plating material has diminished over the years though it is still available. Many aerospace manufacturers have turned to zinc-nickel alloy plating.
Chromium plating often serves a merely decorative purpose but it also fosters heightened corrosion resistance and hardness, making it useful for industrial applications where wear is a concern. Here it is referred to as hard chrome plating, and it is sometimes used to restore tolerances on worn parts.
Chromium is most often plated over nickel in the production of steel furniture, automotive trims, etc. Nickel itself is usually plated over copper, and the combination of these three element layers only protects the underlying metal from corrosion by excluding air and moisture; that is, there is no anodic action. Thus, the plating must be properly applied to achieve suitable corrosion protection.
Chrome plating is an electroplating process that most often involves the use of a chromic acid known as hexavalent chromium. Trivalent chromium baths, which consist largely of chromium sulfate or chromium chloride, are another option for industrial purposes.
Chromate is sometimes applied over zinc plating to protect the zinc and, in some cases, change the color of the metal, as, for instance, green or black zinc plating.
Nickel is a popular plating metal, especially because it is useful in electroless plating. Nickel plating often coats household products such as doorknobs, cutlery, and shower fixtures for enhanced decoration and wear resistance.
Nickel plates commonly bond with copper and aluminum, but also work on a wide variety of metals and serve as underlying platings for chromium.
In electroless plating, a nickel phosphorous alloy is used. The percentage of phosphorous in the solution can vary between 2 and 14%. Higher levels of phosphorous enhance hardness and corrosion resistance. Lower levels of phosphorous allow higher solderability and magnetism.
Copper is another popular plating metal for applications that require high conductivity and cost-efficiency. Copper plating often serves as a strike coating pretreatment for subsequent metal platings, as discussed above.
It is also a popular plating metal for electronic components such as printed circuit boards. High-plating efficiency and low material cost make copper one of the less expensive metals to plate with.
There are three types of copper plating processes alkaline, mildly alkaline, and acid. Higher alkaline levels deliver superior throwing power but require lower current densities and enhanced safety precautions. Health inspectors have linked cyanide in alkaline copper baths to certain health hazards, so it is important to monitor these levels.
Gold is prized for its high resistance to oxidation and electrical conductivity. Gold plating, which differs from gilding in that the gold is not a foil, is one of the simplest ways to impart these characteristics on metals such as copper and silver. The process is often used for jewelry decoration and for improving the conductivity of electronics parts such as electrical connectors.
When gold plating copper, tarnishing is an issue and can most easily be resolved by preceding deposition with a nickel strike. Also, consider the hardness and purity of the gold when determining factors such as optimal bath mixture and length of immersion.
Like gold, silver is used in plating applications that call for decorative appeal and improved electrical conductivity. In general, silver serves as a more cost-effective plating solution because it is cheaper than gold and plates copper well.
Issues that may limit silver plating as a viable plating solution include humidity and galvanic corrosion. Specifically, silver plating does not work well for applications that are subjected to high humidity because silver is prone to cracking and flaking, which may eventually expose the base substrate.
Tin-plated steel has long been used for the packaging of foods and beverages. In addition to providing corrosion resistance, tin is non-toxic and provides a coating that helps the steel to form readily (by virtue of the lubricity the tin provides) and welds and solders easily.
A passivation process coats the tin plate with food-grade oil, which also improves the adhesion of lacquers. Tin plate sheets can be specified with different thicknesses of tin on each face to better suit the needs of the inside and outside container surfaces to their environments.
Tin plate is used in other packaging applications as well, from paint cans to grease tins. Tin plate is almost invariably manufactured using the hot-dip process. Tin plating is also used in the making of electronic components.
Another tin-based plating, known as terneplate, traditionally used a tin-lead alloy as a corrosion inhibitor over steel. Painted, the metal could last 90 years if regularly maintained, making it ideal for use for tin roofs.
Today, the lead has been eliminated and the tin is applied over stainless steel to create corrosion-resistant roofing that develops a softened patina. The material can last twice as long as copper roofing.
Rhodium is a type of platinum that provides tarnish resistance, scratch resistance, and a shiny, white lustrous appearance. Rhodium plating is also common in jewelry production, especially in situations where white gold requires plating. Silver, platinum, and copper are also popular base metals for rhodium plating.
One downside of rhodium plating is that the protective barrier of rhodium will eventually wear away in applications that are subjected to high levels of wear. This can eventually lead to discoloration, and will likely require a second round of plating after a few years.