Plating

Plating is the general name for surface-covering techniques in which a metal is deposited onto a conductive surface. Plating is indispensable as a corrosion inhibitor for the manufacture of computers, mobile phones, and electronic devices as well as other uses such as solderability, hardness, wearability, friction loss, paint adhesion, conductivity, shielding, etc. Moreover, it is a key technology for the development of new machines. It is also used for decoration, for example in jewelry, 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 heat and pressure are applied to fuse them together (a version of this technique is called Sheffield plate). Other plating techniques include vapor deposition under vacuum, sputter deposition, 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.

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Electroplating

Main article: Electroplating

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

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

Gold plating

Main article: 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.

Metals may also be coated with gold for ornamental purposes, using a number of different processes usually referred to as gilding.

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 underlying copper undergoes rapid galvanic corrosion, flaking off the bell end crust the plating and exposing the copper itself; a process known as red plague.

Historically, silver plate was used to provide a cheaper version of items that might otherwise be made of silver, including cutlery and candlesticks. The earliest kind was Old Sheffield Plate, but in the 19th century new methods of production (including electroplating) were introduced: see Sheffield Plate.

Another method that can be used to apply a thin layer of silver to several objects, such as glass, is the Tollen's Test method, which usually is prepared as follows. Using this method the final reaction can occur by placing Tollen's Reagent in a glass and then adding Glucose/Dextrose and shaking the bottle to perform the reaction.

AgNO₃ + KOH -> AgOH + KNO₃

AgOH + 2NH₃ -> [Ag(NH₃)₂]¹⁺ + [OH]^1- (Note: See Tollen's Reagent)

[Ag(NH₃)₂]¹⁺ + [OH]^1- + Aldehyde(Usually Glucose/Dextrose) -> Ag + 2NH₃ + H₂O

Rhodium plating

Rhodium plating is occasionally used on white gold, 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

Main article: 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. When plating on Iron or Steel, an underlying plating of Copper allows the Nickel to adhere. The pores (tiny holes) in the Nickel and Chromium layers also promote corrosion resistance. Bright Chrome 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.

The traditional solution used for industrial hard chrome plating is made up of about 250 g/l of Cr0₃ and about 2.5 g/l of S0₄^-. In solution, the chrome exists as chromic acid, known as hexavalent chromium. A high current is used, in part to stabilize a thin layer of chromium(+2) at the surface of the plated work. Acid Chrome has poor throwing power, fine details or holes are further away and receive less current resulting in poor plating. As such reasonable precautions should be taken to minimize exposure of Cr⁶⁺ to people and the environment.

Zinc plating

Main article: Galvanization

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 aluminum and stainless steels by their oxide layers.

See also:

• Hot-dip galvanizing
• Corrugated galvanised iron

Tin plating

Main article: tinplate

- 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.

See also

• Gilding
• Sheffield plate
• Via (electronics)

References

Electrochemistry Encyclopedia