General

Types of Metal Plating

Introduction

This article will take an in-depth look at types of metal plating.

The article will bring more detail on topics such as:

  • Principle of Metal Plating
  • Types of Metal Plating Based on the Metal Used
  • Applications and Benefits of Metal Plating
  • And Much More…

Chapter 1: Principle of Metal Plating

This chapter will discuss what metal plating is, its types of processes, and the materials used.

What is Metal Plating?

Metal plating is a process where a thin layer of metal coating is applied on the surface, or on the substrate of a metal part, product, or component. The process of metal plating can consist of electroplating, which makes a deposition of metal ions on the surface of the substrate by making use of an electric current.

Types of Metal Plating

Electroless plating applies metal ions on the workpiece as well without making use of any electrodes. Despite the process each possesses many benefits with the main purpose of improving the resistance to corrosion of a metal.

Metal Electroplating

Even though metal plating can give protection to a part or component from environmental factors that can potentially damage it, it also improves the appearance of the part. There are different types of coatings which can either be industrial or commercial.

Commercial decorative coating is utilized on items that are used every day to provide them with improved appearance and strength. Most commercial coatings are used on certain tools, silverware, and jewelry. Industrial coatings are applied to metal parts to give them endurance and to improve their wear resistance so that they can withstand harsh conditions. This helps in protecting and strengthening machine parts and truck parts.

Metal Plating

Coatings are also capable of improving the solderability and strength with reduced friction to reduce potential wear. By applying a coating to a metal, the conductivity of the metal is altered or its ability to conduct an electric current. Metals that are coated can be painted and magnetic properties are improved.

Metal Plating Processes

The various metal plating processes include:

Electroplating Process

Electroplating is a process where electrodeposition is utilized to coat the workpiece in a thin layer of metal. Controlled electrolysis is utilized by engineers for the transfer of the desired metal coating from the anode (the part that contains the metal that will be made use of as the plating) to the cathode (the part that is to be plated). Both the cathode and the anode are immersed in an electrolyte chemical bath and a continuous electrical charge is applied.

Electroplating

The electric current causes the movement of negatively charged ions (anions) to the anode and the movement of positively charged ions (cations) to the cathode, forming a thin layer around or plating the desired workpiece in an even metal coating. By the process of electroplating, a substrate material is taken and encapsulated in a thin shell of metal like copper or nickel.

The process of electroplating is most commonly used on other metals, due to the basic requirement that the substrate is a conductive material. There are other less common autocatalytic pre-coating processes that have been developed that produce a conductive interface that is ultra-thin, allowing different metals, most notably nickel and copper alloys, to get plated on parts made of plastic.

Electroplating vs. Electroforming

Both processes of electroplating and electroforming are carried out using electrodeposition. The major difference between these two processes is that electroforming makes use of a mold that is then removed after the forming of a part.

Electroforming

The process of electroforming is utilized for the creation of solid metal pieces, while electroplating is utilized for the covering of an existing workpiece, which is made from a material that is different in metal.

Materials Used in Electroplating

A single metal can be electroplated onto an object, or a combination of metals. Many manufacturers opt for layering metals like nickel and copper to increase the strength and conductivity. The most commonly used materials in the process of electroplating are listed below:

  • Brass
  • Cadmium
  • Chromium
  • Copper
  • Gold
  • Iron
  • Nickel
  • Silver
  • Titanium
  • Zinc

Almost any material can be used in the making of substrates, from stainless steel and other different types of metals to plastics. Artisans have organic materials that are electroplated, like soft fabric ribbons and flowers. It must be noted that substrates that are non-conductive, like plastic, glass, and wood must be made conductive prior to their electroplating process. This can be achieved by coating a substrate that is non-conductive in a layer of conductive spray or paint.

Electroless Plating

Electroless plating is a metal plating method by chemical instead of electrical means, in which the workpiece that is to be metal plated is immersed in a reducing agent that, when certain materials catalyze it, turns metal ions to metal that deposit around the workpiece. The electroless plating process has found its major expansion in the area of plastic materials, like in the plating of printed electronic circuits. Many various consumer goods are metal plated using this process for the creation of attractive and durable surfaces.

Electroless Plating

Nickel plating is the most commonly used electroless plating technique, although copper, gold, and silver layers can also be applied in a similar manner. The process of electroless plating is also called auto-catalytic plating or chemical plating.

Electroless plating is a method of metal plating, that is non-galvanic, that includes several reactions that are simultaneous, in an aqueous solution. The reactions occur with no use of external power supply.

The reaction is achieved when a reducing agent releases hydrogen, usually sodium hypophosphite, and is oxidized, thus a negative charge is produced on the part’s surface.

Electroless plating allows the bathing of all parts of the object by a consistent metal ion concentration, and evenly deposits metal inside holes, along the edges, and over objects that are irregularly shaped which are difficult to metal plate evenly by the process of electroplating.

The application of electroless plating is also done for the deposition of a conductive surface on an object that is non-conductive to allow it to be metal coated by the process of electroplating.

Immersion Plating

Immersion plating adheres layers of nobler metals to the surface of another metal by dipping in an ion solution of nobler metal for the production of a replacement reaction. It causes a metallic coating to be deposited on a base metal from solutions containing the coating metal. In this process, there is typical displacement of one metal by metal ion that has lower oxidation potential levels, relative to the displaced metal ion.

Immersion Plating

This process is also used for improving electrical properties and also for the enhancement of adhesive coatings or organic coatings bonding to the substrate. Immersion plating is also called dip plating or metal replacement. Immersion plating is different from different electroplating processes in the sense that no external current is present. It follows the principle that when metal components like copper are immersed into an electrolyte having ions of nobler metal, the component or metal that is less noble will undergo dissolution.

This results in the electrons being released, which allows metals that are highly noble to settle down. Unlike the process of electroless plating, the metals being deposited are halted once the object being plated is totally coated with higher nobility metals. This type of metal plating occurs at temperatures that are high such as in gold immersion occurs at 80°C to 90°C whereas in silver immersion takes place at 50°C to 60°C.

Electroplating vs. Electroless Plating

Both techniques improve the toughness of a component and its resistance to corrosion. Both techniques are also capable of making a part more attractive such as when plating jewelry or other parts viewed directly by consumers, as opposed to those parts that are more likely situated within a huge machine or utilized on the floor of a factory.

The main difference between these two processes of metal plating is the use, or not of an electric current during the plating process. Electroplating utilizes a power source like a rectifier or battery, which provides an electric current to a component in a chemical solution. This alters the chemical composition and deposits a hard and durable metal coating onto the component’s surface. Electroplating is a process that is more complex than electroless plating, requiring clean conditions, and utilizing equipment that is potentially hazardous and in some cases needing multiple applications to obtain the plating thickness that is desired.

Electroless plating is a process that is much less complex. In the process of electroless plating, the component is cleaned first with chemical cleansers for the removal of corrosive elements and oils, then is dipped in the aqueous solution and anti-oxidation chemicals are added. The resulting component is plated and exhibits high resistance to corrosion and friction.

Electroless nickel plating has no requirements of complex filtration or other equipment, and because of the absence of electricity, there is no risk of accidents that are electricity related.

Chapter 2: Types of Metal Plating by Metal

This chapter will discuss the different types of metal plating based on the metal used.

Electroless Nickel Plating

This is a form of treatment by means of an alloy designed for the increase of hardness and resistance of a metal or plastic. The process of electroless nickel plating is simpler than that of its counterpart electroplating. In this process, no passing of electric current is required through the solution in the chemical bath in order to start the plating process.

Electroless Nickel Plating

Rather, the metal surface is put through a series of autocatalytic reactions and cleaning, which has been perfected by electro-coatings. Below are the stages of electroless nickel plating:

  1. Pre-Treatment

The first step is the treatment of the surface, whereby it is cleaned with a series of chemicals for the removal of oils and grease. Thorough cleaning is required in the preparation of the component in order for it to be plated properly. The cleaning of each component is done meticulously based on the material’s surface.

  1. Surface Prepare

Once the substrate has been cleaned, it is activated by means of a proprietary solution or acid etch, preparing the surface so that nickel-phosphorus can be deposited on it.

  1. Rendering

After the end of the plating process, the electroless process of nickel renders more resistance to friction and corrosion to the component.

Electroless Nickel Plating Thicknesses

The deposition of electroless nickel plating can be done at a rate of 5 microns per hour up to 25 microns per hour. Since the process is continuous and builds on itself, the process provides a thickness that is essentially limitless. However, as the thickness increases, minute imperfections become clearly seen. Depending on the specific requirements, one of five different coating options is applied by electro-coating.

Zinc Plating

Zinc is a material that is inexpensive and it is utilized to provide a galvanized coating on different metal substrates. In addition to being electroplated, the application of the element is done by a way of the sherardizing process, by dipping in a molten bath and by spraying. In the cold or electrolytic process, the part to be plated is set up as the cathode in a bath containing an electrolyte of soluble zinc salts together with metallic zinc as the anode.

Zinc Plating

This process produces a coating of pure zinc that is highly ductile, whose uniformity and thickness can be controlled precisely. The sherardizing process is utilized for the coating of small hardware items like nails and screws. The items are loaded together with zinc dust in a barrel and heated to a temperature of approximately 500℉.

The parts are then tumbled inside the barrel for the production of a coating that consists of 10% iron and 90% zinc. The application of molten zinc may also be done by dipping or by manual coating of large items. The addition of a small amount of aluminum to the bath is done sometimes for the improvement of fluidity and to improve the coating of shapes that are odd.

Cadmium Plating

This type of plating is often used on miscellaneous automotive items. The manufacturers of aircrafts specify it for its sacrificial protection characteristics and also its lubricity that is natural, for components that are removed frequently and reinstalled. This type of metal plating is suited to marine environments where it holds up well against salt and freshwater.

Cadmium Plating

Because of safety reasons, the use of this type of plating has diminished over the years even though it is still available. Multiple manufacturers of aircrafts now use zinc-nickel alloy plating as a substitute for cadmium plating.

Chrome Plating

This type of plating is only used in decorative purposes but it also exhibits high resistance to corrosion and hardness, making it useful for industrial applications where there is a concern of wear. It is used sometimes for the restoration of tolerances on worn parts. Chromium is plated mostly over nickel in the production of furniture made from steel and automotive trims.

Chrome Plating

The process of chrome plating is an electroplating process that involves the utilization of a chromic acid called hexavalent chromium. For industrial purposes, trivalent chromium baths which consist of chromium chloride or chromium sulfate are another option.

Copper Plating

Copper plating is another metal that is popular for plating for applications that require cost efficiency and high conductivity. Copper plating is a popular plating metal for components used in the electronics field, like printed circuit boards. Copper is made one of the less expensive plating metals because of its low material cost and high-plating efficiency.

Copper Plating

There are three types of process utilized in copper plating which are: acid, alkaline, and mild alkaline. Higher levels of alkaline provide superior throwing power but they require lower densities of current and safety precautions that are increased. It is important that these levels are monitored because cyanide has been linked in alkaline copper baths by health inspectors.

Gold Plating

Gold has a high resistance to oxidation and also electrical conductivity, therefore it is prized. Gold plating, which is different from gliding because gold is not a foil, is one of the easiest ways of imparting these characteristics on metals like silver and copper. The process is mostly utilized for the decoration of jewelry and for the improvement of the conductivity of the parts used in electronics like electrical connectors.

Gold Plating Cup

When copper is gold plated, tarnishing can be an issue, which can be easily resolved by preceding the deposition with a strike of nickel. When determining factors like length of immersion and optimal bath mixture, consider the hardness and purity of gold.

Silver Plating

Silver is utilized for plating applications that require decorative appeal and enhanced electrical conductivity. Generally, silver provides a plating solution that is more cost effective because it has a low cost price than gold and plates copper very well. Silver plating may be limited by issues like galvanic corrosion and humidity.

Silver Plating

Specifically, silver plating is not a good solution to applications that are subjected to humid conditions because silver is more susceptible to flaking and cracking, which may expose the base substrate.

Tin Plating

For the packaging of beverages and foods, tin plated steel is often used. On top of offering resistance to corrosion, tin is non-toxic and has a coating that helps in the forming of steel, soldering, and welding. The tin plate is coated with food-grade oil by a passivation process, which also increases the lacquer’s adhesion. There can be different specifications of thicknesses of tin plate sheets on each face for the better suiting of the needs of the outside and inside surfaces of containers to their environments.

Tin Plating

Tin plating is utilized in other packaging applications like paint cans and grease tins. Tin plate is manufactured almost invariably by making use of the hot-dip process. In the making of electronic components, tin plating is also utilized.

Rhodium Plating

Rhodium is a type of platinum that offers resistance to tarnish, resistance to scratch, and a shiny lustrous white appearance. This type of plating is commonly utilized in the production of jewelry, especially in situations that require the plating of white gold. Copper, silver, and platinum are also popular base metals that are used in rhodium plating.

Rhodium Plating

The disadvantage of rhodium plating is the eventual wearing of the protective barrier of rhodium in applications that involve high wear levels. This will eventually lead to discoloration and a second round of plating after a few years may be necessary.

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