What is Metal Plating?- Definition, Types, & Benefits

What is Metal Plating?

Metal Plating is the process of adding a thin layer of metal to the outer surface of a material. It is a surface coating process that deposits metal onto a conductive surface. Plating has occurred for hundreds of year and it is essential to modernizing.

Plating can be used for decorative, corrosion resistance, solderability improvements, to make it harder, wearability, reduce friction, improve paint adhesion, modify conductivity, improve IR reflectivity, radiation shielding, etc. Jewelry is often plated to get a silver or gold finish.

Thin-film deposition is known to have plated objects the size of an atom, therefore their application in nanotechnology.

Plating can take on many forms, there are numerous methods and even more variations. In one instance, a solid surface, X, is covered by a metal sheet, Y, and fused with heat and pressure.

There is also electroplating, vapor deposition in vacuum, sputter deposition among many processes. Recently often plating is associated with the use of liquids. Metallizing is when something that is NOT metal gets a metallic coating.

Benefits of Metal plating

Metal plating offers numerous advantages that apply to any products made out of metal or other materials. Electroplating conducted with an electric current or electroless plating which is in an autocatalytic chemical process are the most common methods for applying metal plating.

There are a number of processes for metal plating other than electroplating or electroless plating. All these processes give one or more 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:

• Electroplating
• Electroless Plating
• Immersion Plating
• Carburizing
• Physical Vapor Deposition
• Plasma Spray Coating

1. Electroplating

Electroplating is the most utilized process for plating using an electrical current to dissolve positively charged metal ions in a chemical solution. The plating process occurs when the positively charged ions are attracted to the material to be plated, also called the cathode, which is the negative half of the circuit.

The part or product is put into the solution, and the dissolved metal ions are attracted to the newly plated surface. Electroplating produces a smooth, full, and fast plating for any item being plated to a completed surface; effectively changing the surface material.

There are a number of different procedures and steps that might be involved when electroplating: cleaning, strike, electrochemical, deposition, pulse electroplating, and brush electroplating.

Effects:

Electroplating is an applied process used to create a protective coating, provide a decorative finish, or alter the properties of a material for engineering purposes. Electroplating enhances the workpieces chemical, physical, and mechanical properties which impact how it will perform as it is machined.

Plating a workpiece can allow it to build from a smaller size, make machining easier, and enhance solderability, conductivity, or reflectivity.

2. Electroless Plating

Electroless plating is called electroless because the plating process does not rely on outside electric power. Electroless plating is a chemical process in which a metal atom is reduced.

In other words, the solution of metal ions (particles) when combined with a reducing agent creates a metal solid upon contact with the metal (which initiates the reaction) on the surface, where both are used for support and to create a solid layer of the plating metal (the two elements create a plating effect).

Effects

Electroless or autocatalytic plating is applicable to all shapes and sizes of materials as it does not require an associated electrical supply and plating baths, which result in decreased costs. However, electroless plating is slower than electroplating, and cannot produce as thick a plate, as well can be more difficult to control than electroplating.

Electroless nickel plating is the most common form of autocatalytic plating, but plating in silver, gold, and copper with this method can also be used.

Effects of electroless plating on an end product include protective coating against the base metal corroding, increasing the base metal size, and altering solderability, reflectivity, and conductivity.

3. Immersion Plating

Immersion plating results from soaking or immersing one metal into a solution of metal ions from a more noble metal. The stable ions from the nobler metal ion will naturally ‘pull’ to displace any surface metal ions from the less noble surface with a thin layer of the more noble metal ions.

Furthermore, immersion plating is slower than other plating processes, and can only be produced by plating fewer noble metals with nobler metals. Nobler metals are metals that do not chemically react. Examples of nobler metals are gold, platinum, or silver.

Effects

Immersion plating can result in only a thin deposit of plating, after which the plating process will cease. Immersion plating also appears to be poorer in adhesion quality, where the plating doesn’t adhere as well to the base metal.

The effects of immersion plating on to the end product can include increased corrosion resistance, modified electrical conductivity, changes in appearance, increased hardness, increased torque tolerance, and changed bonding capabilities.

4. Carburizing

Also referred to as case hardening, carburizing is a heat treat process that results in a wear resistant surface while keeping the core strong. Normally applied to low carbon steel after machining, as well as high, alloy gears, bearings, etc.

Carburizing is good for complex shapes of low cost materials that can be machined easily to get a hard surface. The process consists of heating the part in either a pit furnace or sealed atmosphere furnace.

Carburizing gases (typically carbon monoxide, sodium cyanide and barium carbonate are introduced to temperature, while the heat and temperature affects the depth of carbon diffusion. The part is then either cooled slowly for quenching later or quenched directly in oil.

5. Physical Vapor Deposition (PVD)

PVD technology is a family of coating processes where thin films are deposited onto the substrate. In PVD the coating material is solid i.e. titanium, chromium, aluminum, etc. The coating material is evaporated by heat or bombardment with ions.

While the alloy or coating material is evaporated, the coating is formed when a reactive gas, such as nitrogen, is introduced into the chamber.

The nitrogen, for example, will form a compound with the vapor deposited on the substrate and create a very thin coating on the already thin metal coating. The benefit is; you are creating an extremely robust bond between the coating and the metal part.

Benefits of PVD include very hard surface, corrosion resistant, heat resistant, and impact strength.

Suitable for numerous applications:

• Aerospace
• Automotive
• Cutting Tools
• Medical
• Firearms
• Optics
• 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. Plasma spray coating is also referred to as thermal spraying, but the end-use and the benefit of the plating process is to deliver a coating onto a substrate through some sort of mechanical means.

The coating material is injected into a very high temperature plasma flame (with temperatures up to 10,000 K in heat), as it is rapidly heated and then accelerated at a high velocity to the surface of the part and rapidly cools to form a coating on the part’s surface.

This process results in a coating (most of the application of the process is done to structural materials) that provides protection from very high temperatures, for applications such as exhaust heat management.

There is also protection from corrosion and wear with this process. Furthermore, the coating can also be altered to improve appearance or electrical properties.

Metals used in plating

Zinc Plating

Zinc is an inexpensive metal that is used to create a galvanizing coating on a variety of metal substrates. Zinc can be applied in a few different ways such as using electroplating, using the sherardizing process, molten dipping, and spraying.

In the electrolytic, aka the cold process, the object being coated is configured as a cathode, in a tank called an electrolytic bath. The bath held soluble zinc salts and a metallic zinc anode. This does produce a highly ductile coating of pure zinc, with uniformity in coating thickness, and the coating properties.

The Sherardizing process, is used when coating small hardware components, such as screws and nails. The items are placed in a barrel with zinc dust and heated to 500F.

The items are tumbled in the barrel in the process, and therefore provides an outer coating which consists of approximately 90% zinc and 10% of the surface material, or iron.

There are different methods of applying molten zinc, can either be done by dunking the item into a liquid bath of bonding agent, or for larger items in adhere coating by hand.

In the case of hot dip or dunking, aluminum and zinc coatings have been used to further enhance or improve the flow of the coating fluidity and flowing on selected shapes.

In a hot-dip, some may also include tin in zinc bath enhance the hasten the paint coating properties, as well as mash the conditions of coating surface.

Hot-dip may also produce an iron-zinc layer next to the base metal, which is typically brittle and can deteriorate the adhesion bond of the upper coating layer.

Metal spray, or metalizing should be considered a flame process whereby melted metal powders &/or wire, are heated and then sent under pressure into a stream which impinges the surface of any substrate metal object to create a mechanical bond with the base-coated metal article.

To really get cold-welded mechanical bond from the melted coating metal powders, they should always be applied to coarseness surface, to further obtain thickness of coating quite robust considerably as well.

Sprayed coatings can be porous however, it is important to remember that zinc is anodic to iron and steel, so the porosity of the coating is not an issue with the coatings ability to protect the substrate. In fact, the porosity of sprayed metal coating can aid with better paint adhesion.

Cadmium Plating

Cadmium plating was once used as an alternative to zinc plating and was very commonly plated onto a variety of automotive components. Cadmium plating was specified by aircraft manufacturers for its sacrificial protection properties and natural lubricity for components that are installed and removed many times.

It was very well suited to marine environments, where it holds up well against both fresh and seawater. In recent years, due to safety concerns about cadmium, its use as a plating material has declined, but cadmium is still available. Most Aerospace manufacturers have used zinc-nickel alloy plating.

Chrome Plating

Chromium plating is typically decorative in nature, but it also provides additional corrosion resistance and hardness which are valuable in industrial situations where wear is an issue. In these instances it is referred to as hard chrome plating, and is even done occasionally to restore tolerances on worn parts.

In the production of steel furniture, auto trims, etc. chromium is usually plated over nickel. Nickel is usually plated over copper, so once again three layers of two metals are used to minimize atmospheric corrosion of the steel.

These layers effectively provide no anodic protection as they exclude the moisture and air that would cause corrosion, so must be applied appropriately for adequate corrosion protection.

Chrome plating is an electroplating process. The plating solution consists largely of a chromic acid, typically hexavalent chromium. Industrial processes may also utilize trivalent chromium baths similarly based on chromium sulfate or chromium chloride.

In some instances chromate may be applied over zinc plating to both protect the zinc and sometimes achieve aesthetic changes as for example green or black zinc plating.

Nickel Plating

Nickel is a commonly used plating metal, particularly because it is useful for electroless plating. Nickel plating is most often used to provide decorative and wear resistant finishes for household items, such as doorknobs, cutlery, and shower fittings.

Nickel deposits can readily ennoble copper and aluminum, but can also be deposited onto many substrates, and can be used for a base plating for chromium.

When plating in electroless nickel, a nickel phosphorous alloy is used. The electrophoretic nickel phosphorous solution contains anywhere between 2 and 14% phosphorous.

Higher phosphorous content generally enhances hardness and corrosion resistance, whereas lower phosphorous content provides greater solderability and magnetism.

Copper Plating

Copper is also a popular plating metal for conductivity and cost reasons. Copper is almost always used as a strike coating pretreatment before additional metal plating, a function described above.

It also has, therefore, a large plating efficiency and inexpensive plating costs. Copper is one of the less expensive metals to plate because of its very high efficiency and low material costs, making it a popular plating metal for electronic components such as printed circuit boards as well.

There are three types of copper plating processes – alkaline, mildly alkaline and acid. Alkaline copper baths provide better throwing power but require lower current densities and require higher safety standards due to the reference to health hazards related to cyanide levels measured in the alkaline copper baths by the health inspectors.

Gold Plating

Gold is highly prized for its superior resistance to oxidation and its high electrical conduction. One of the easiest ways to impart these properties onto other metals such as copper and silver is by gold plating (laquer gold instead of gold leaf as in gilding).

Gold plating is typically used to decorate jewelry, and for other uses, to improve electronic parts, such as electrical connectors, conductivity, etc.

When gold plating copper, tarnishing is a concern, which is easily mitigated by a nickel strike prior to deposition. Other things to consider concerning hardness / purity of gold in relation to bath mixture, and immersion time.

Silver Plating

Silver, like gold, is used as a plating material in different applications to obtain a decorative look and increase conductivity. In general, silver is usually a more affordable choice for plating applications because it tends to be less expensive than gold and can plate copper substrates successfully.

There are issues which will impair silver plating as a solution such as humidity and galvanic corrosion. Specifically, silver plating is ineffective for an application if that application had high humidities because silver could crack and flake and expose the base substrate.

Tin Plating

Tin-plated steel is a historical food and beverage packaging substrate. As well as having good corrosion resistance, tin is non-toxic (in its pure metallic form), and provides a coating to allow the steel to form easily (due to the lubricity that tin provides) and be welded and soldered easily.

A passivation process coats the tin plate in a food-grade oil which improves lacquer adhesion. It is possible to specify tin plate sheets with two different tin thicknesses on each face to better match the environment of the inside and outside surfaces of the container.

There are other packaging applications that are manufactured with tin plate, not limited to paint cans and grease tins. Tin plate is made almost exclusively by the hot-dip process. Tin plating is applied in the manufacture of electrical parts.

There is another tin-based plating product, terneplate, where historically, people used a tin-lead alloy on steel as a corrosion inhibitor. If it was painted, that metal would last 90 years with the correct maintenance, although that would be favourable for looking at metal roofs.

Now, with no lead and tin is electroplated onto stainless steel, we have created a corrosion resistant roof that has the potential to develop a softened patina. This material could last, at minimum, double the desired longevity of copper roofs.

Rhodium Plating

Rhodium is a type of platinum (Pt) that provides tarnish resistance, scratches protection, and a shiny, white luster. Rhodium plating is common in jewelry manufacturing, primarily in cases of plating white gold. Rhodium plating is also commonly done on silver, platinum, and copper.

One negative aspect of rhodium plating is that, due to its soft nature, the rhodium will wear off in areas where it is subject to high levels of wear. This may lead to to the eventual discoloration of some amount of the underlying metal and, most likely, a second plating job in a few years.

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