Electroplating

Encyclopedia : E : EL : ELE : Electroplating

Electroplating is the coating of an electrically conductive object with a layer of metal using electrical current. The result is a thin, smooth, even coat of metal on the object.

Contents

1 Explanation
2 Process
3 Industrial Use
4 History
5 See also
6 External links

Explanation

The process used in electroplating is called electrodeposition and is analogous to a galvanic cell acting in reverse. . The item to be coated is placed into a container containing a solution of one or more metal salts. The item is connected to an electrical circuit, forming the cathode (negative) of the circuit while an electrode typically of the same metal to be plated forms the anode (positive). When an electrical current is passed through the circuit, metal ions in the solution are attracted to the item. The result is a layer of metal on the item. However, considerable skill and craft-technique is required to ensure an evenly-coated finished product.

Process

Silver electroplating. The anode is a silver bar and the cathode is an iron spoon.

The anode and cathode in the electroplating cell are connected to an external supply of direct current, a battery, or more commonly a rectifier. The anode is connected to the positive terminal of the supply, and the cathode (article to be "plated") is connected to the negative terminal. When the external power supply is switched on, the metal at the anode is oxidized from the 0 valency state to form cations with a positive charge. These cations associate with the anions in the solution. The cations are reduced at the cathode to deposit in the metallic, 0 valency state. Example: In an acid solution Cu is oxidized to Cu²⁺ by losing two electrons. The Cu²⁺ associates with the anion SO₄^2- in the solution to form copper sulphate. At the cathode, the Cu²⁺ is reduced to metallic Cu by gaining two electrons.

A closely-related process is brush electroplating, in which localized areas or entire items are plated using a brush saturated with plating solution. The brush, typically a stainless-steel body wrapped with a cloth material that both holds the plating solution and prevents direct contact with the item being plated, is connected to the positive side of a low voltage direct-current power source, and the item to be plated connected to the negative. The operator dips the brush in plating solution then applies it to the item, moving the brush continually to get an even distribution of the plating material. The brush acts as the anode, but typically does not contribute any plating material, although sometimes the brush is made from or contains the plating material in order to extend the life of the plating solution.

Brush electroplating has several advantages over tank plating, including portability, ability to plate items that for some reason can't be tank plated (one application was the plating of portions of very large decorative support columns in a building restoration), low or no masking requirements, and comparatively low plating solution volume requirements. Disadvantages compared to tank plating can include greater operator involvement (tank plating can frequently be done with minimal attention), and inability to achieve as great a plate thickness.

The plating is most commonly a single metallic element, not an alloy. However, some alloys can be electrodeposited, notably brass and solder.

Industrial Use

Example of Gold Plating Cell

Electroplating is used in many industries for functional and/or decorative purposes.

Some well known examples are chrome-plating of steel parts on automobiles. Steel bumpers become more corrosion-resistant when they have been electroplated with first nickel and then chromium.

Hard chromium is used in services where frictional wear must be minimum, such as hydraulic pistons and camshaft bearing diameters.

Plain steel or aluminum parts in light fixtures become beautiful when they are electroplated with nickel and then decorative chromium or brass.

Nickel, in the form of nickel sulfamate, is used to restore dimensions on worn parts, and as an under plate for hard chrome. The nickel sulfamate bath is unsuitable for decorative work.

Steel bolts last much longer because they are sold with a coating of zinc or cadmium that have been applied by electroplating. These electroplating and conversion coatings provides a double protection system for steel components. Virtually all types of steel can be protected including castings. Newly developed electrolytes and process methods are able to provide greatly increased corrosion prevention and brilliant finishes. Specially developed processes produce improved metal distribution over complex shapes. Alloy co-deposits offer extra performance.

Passivation processes (better known as conversion coatings) are usually applied to zinc and cadmium deposits to improve component life. These coatings used to be based on hexavalent chromium chemistry providing enhanced surface corrosion resistance but these have recently been superseded by trivalent chromium chemistry on both health and environmental grounds. In addition to the well known chromic conversion, there are blue, olive and black variants available to meet modern requirements.

Electroplating can be used to silver plate copper or brass electrical connectors, since silver tarnishes much more slowly and has a higher conductivity than those metals. The benefit of the silver is lower surface electrical resistance resulting in a more efficient electrical connection. Silver plating is also popular for RF connectors because radio frequency current flows primarily on the surface of its conductor; the connector will thus have the strength of brass and the conductivity of silver.

Low force/low voltage separable connectors used in telecommunications switchgear, computers, and other electronic devices are typically plated with gold or palladium over a barrier layer of nickel. The tail ends of these connectors, which are usually joined to the device by soldering, are plated with a tin/lead alloy, or pure tin.

History

Nickel Plating

Modern electrochemistry was invented by Italian chemist Luigi V. Brugnatelli in 1805. Brugnatelli used his colleague Alessandro Volta's invention of five years earlier, the voltaic pile, to facilitate the first electrodeposition. Unfortunately, Brugnatelli's inventions were repressed by the French Academy of Sciences and did not become used in general industry for the following thirty years.

By 1839, scientists in Britain and Russia had independently devised metal deposition processes similar to Brugnatelli's for the copper electroplating of printing press plates. Soon after, John Wright of Birmingham, England discovered that potassium cyanide was a suitable electrolyte for gold and silver electroplating. Wright's associates, George Elkington and Henry Elkington were awarded the first patents for electroplating in 1840. These two then founded the electroplating industry in Birmingham England from where it spread around the world.

As the science of electrochemistry grew, its relationship to the electroplating process became understood and other types of non-decorative metal electroplating processes were developed. Commercial electroplating of nickel, brass, tin, and zinc were developed by the 1850's. Electroplating baths and equipment based on the patents of the Elkingtons were scaled up to accommodate the plating of numerous large scale objects and for specific manufacturing and engineering applications.

The plating industry received a big boost from the advent of the development of electric generators in the late 1800s. With the higher currents available metal machine components, hardware, and automotive parts requiring corrosion protection and enhanced wear properties, along with better appearance, could be processed in bulk.

The two World Wars and the growing aviation industry gave impetus to further developments and refinements including such processes as, hard chromium plating, bronze alloy plating, sulfamate nickel plating, along with numerous other plating processes. Plating equipment evolved from manually operated tar-lined wooden tanks to automated equipment, capable of processing thousands of pounds per hour of parts.

One of American physicist Richard Feynman's first projects was to develop technology for electroplating metal onto plastic. Feynman successfully developed this technology, allowing his employer to keep commercial promises he had made but could not have fulfilled otherwise.

Electroplating is one of the three processes that form the LIGA-process used to manufacture MEMS devices.

External links