There are many different types of metal plating and in it’s simplest definition, metal plating is a thin covering of metal on top of another metal. This type of metal plating has been used for hundreds of years and is a critical part of modern technology. The reasons for metal plating are as varied as the types of metals used and the processes of plating:
- Improves corrosion resistance.
- To harden the surface
- Improve paint adhesion
- Improve the wearability of the part
- Reduces friction
- Can alter the conductivity of the material
- As a shield against radiation
- Improve the look of cheaper metals in jewelry
There are many metals used in plating:
- Zinc Iron
- Black Nickle
- Zinc Nickle
Types of metal plating
There are many different types of metal plating and the process of electroplating is probably the most well known. It involves passing an electric current through an electrolyte solution. Two terminals called electrodes are dipped into the electrolyte solution, which connects them into a powered circuit.
The electrodes are the Cathode (the object being coated) and the Anode (the metal being used as the coating. When the electricity flows through the circuit the electrolyte splits some of the metal atoms from the anode, which are then deposited in a thin layer on top of one of the cathode.
Many types of metals can be electroplated in this process; gold, silver, tin, zinc, copper, chrome, nickel, platinum and lead.
In the diagram below; copper (the anode) is being used to coat another metal (the cathode). The electrolyte being used is copper sulphate.
When choosing a process for metal plating, one would opt for electroplating. However, in certain cases, there is an alternative; Electroless Plating. Electroless plating is a simpler, more cost-effective alternative to electroplating. Also known as autocatalytic plating, this method is a way of plating without using an external power source.
The process itself involves plating the part in an aqueous solution and depositing nicking. This creates a catalytic reduction of nickel ions to plates the part, as this is a purely chemical process it does not require electricity or extra machinery.
Electroless plating is suited to parts for which a very hard surface, better resistant to corrosion is required. This makes this process best suited to the oil or marine industries. Parts such as pumps or valves which are subject to corrosive agents will typically be best suited to being electroless plated.
This is the process of applying adhering layers of nobler metals to another metal’s surface by dipping the part into a solution of nobler metal ions. So when metal parts made from a material such as copper are put into the electrolyte, the nobler metal ions will coat the parts the copper releases it’s electrons.
Also known as metal replacement or dip plating. Just like electroless plating, there is no external current, it is a chemical process. Unlike electroless plating; once the part is completely coated, the deposition of metals is halted. Immersion coating alters the metal’s surface to improve wear and corrosion electrical resistance and electrical conductivity. It can also alter the appearance and reflectivity and bonding capabilities of the coated part.
Also known as case hardening, carburizing is a heat treating process which 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.
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, the 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
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, erosion and wear. The coating can also change the appearance and electrical properties of the part.