Plating is the general name of surface-covering techniques in which a metal is coated onto a solid surface. Plating is indispensable for the manufacture of computers, mobile phones, and electronic devices as well as for surface treatment techniques. Moreover, it is a key technology for the development of new machines. It is also used for decoration, typically to provide a silver or gold exterior. Thin-film deposition techniques have accomplished plating on scales as small as the width of an atom, so it is appropriate to call some plating applications nanotechnology.

There are several plating methods. For example, in one method, a solid surface is covered with a metal sheet and then plated by heating. Other plating techniques include vapor deposition under vacuum, sputtering, and methods using vacuum conditions or gas. Recently, however, only plating techniques using a liquid tend to be called "plating". Metallizing refers to the process of coating metal on non-metallic objects.

Electroplating

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Electroplating is the process by which a metal in its ionic form is supplied with electrons to form a non-ionic coating on a desired substrate. The most common system involves: a chemical solution which contains the ionic form of the metal, an anode (positively charged) which may consist of the metal being plated (a soluble anode) or an insoluble anode (usually carbon, platinum, titanium, lead, or steel), and finally, a cathode (negatively charged) where electrons are supplied to produce a film of non-ionic metal.

Electroless plating

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Electroless plating, also known as chemical or auto-catalytic plating, is a non-galvanic type of plating method that involves several simultaneous reactions in an aqueous solution, which occur without the use of external electrical power. The reaction is accomplished when hydrogen is released by a reducing agent, normally sodium hypophosphite, and oxidized thus producing a negative charge on the surface of the part. The most common electroless plating method is electroless nickel plating. See: Electroless nickel plating

Specific cases

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Gold plating

Gold plating is a method of depositing a thin layer of gold on the surface of other metal, most often copper or silver.

Gold plating is often used in electronics, to provide a corrosion-resistant electrically conductive layer on copper, typically in electrical connectors and printed circuit boards. With direct gold-on-copper plating, the copper atoms have the tendency to diffuse through the gold layer, causing tarnishing of its surface and formation of an oxide/sulfide layer. A layer of a suitable barrier metal, usually nickel, has therefore to be deposited on the copper substrate, forming a copper-nickel-gold sandwich.

Silver plating

For less demanding applications in electronics, silver is often used as a cheaper replacement for gold. Care should be used for parts exposed to high humidity environments. When the silver layer is porous or contains cracks, the copper undergoes rapid galvanic corrosion, flaking off the plating and exposing the copper itself; a process known as red plague.

Palladium plating

Palladium plating is used in some special applications instead of gold plating. Due to migration of copper atoms, the nickel layer is required here as well.

Rhodium plating

Rhodium plating is occasionally used on silver or copper and its alloys. A barrier layer of nickel is usually deposited on silver first, though in this case it is not to prevent migration of silver through rhodium, but to prevent contamination of the rhodium bath with silver and copper, which slightly dissolve in the sulfuric acid, usually present in the bath composition.

Chrome plating

Chrome plating is a finishing treatment utilizing the electrolytic deposition of chromium. The most common form of chrome plating is the thin, decorative bright chrome, which is typically a 10 µm layer over an underlying nickel plate. It imparts a mirror-like finish to items such as metal furniture frames and automotive trim. Thicker deposits, up to 1000 µm, are called hard chrome and are used in industrial equipment to reduce friction and wear and to restore the dimensions of equipment that have experienced wear.

Zinc plating

Zinc coatings prevent oxidation of the protected metal by forming a barrier, and by acting as a sacrificial anode if this barrier is damaged. Zinc oxide is a fine white dust that (in contrast to iron oxide) does not cause a breakdown of the substrate's surface integrity as it is formed. Indeed the zinc oxide, if undisturbed, can act as a barrier to further oxidation, in a way similar to the protection afforded to aluminium and stainless steels by their oxide layers.

See also:

• Hot-dip galvanizing
• Corrugated galvanised iron

Tin plating

- on the use of tin to protect iron or steel. The tin plating process is used extensively to protect both ferrous and nonferrous surfaces. Tin is a useful metal for the food processing industry since it is non-toxic, ductile and corrosion resistant. The excellent ductility of tin allows a tin coated base metal sheet to be formed into a variety of shapes without damage to the surface tin layer. It provides sacrificial protection for copper, nickel and other non-ferrous metals, but not for steel.

Tin is also widely used in the electronics industry because of its ability to protect the base metal from oxidation thus preserving its solderability. In electronic applications, lead may be added to prevent the growth of metallic "whiskers" in compression stressed deposits, which would otherwise cause electrical shorting

Alloy plating

In some cases it is desirable to co-deposit two or more metals resulting in an electroplated alloy deposit. Depending on the alloy system, an electroplated alloy may be solid solution strengthened or precipitation hardened by heat treatment to improve the plating's physical and chemical properties. Nickel-Cobalt is a common electroplated alloy.

Composite plating

Metal matrix composite plating can be manufactured when a substrate is plated in a bath containing a suspension of ceramic particles. Careful selection of the size and composition of the particles can fine tune the deposit for wear resistance, high temperature performance, or mechanical strength. Tungsten Carbide, Silicon carbide, Chromium Carbide, and Aluminum Oxide (alumina) are commonly used in composite electroplating.

References

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Electrochemistry Encylopedia

Coatings | Metal plating | Thin film deposition | Industrial processes