Manufacturing: Surface Finishing

Electroplating

The process used in electroplating is called electrodeposition. 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 assure an evenly-coated finished product. This process is analogous to a galvanic cell acting in reverse.

A more detailed description of the electrodeposition process follows: 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 valence 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 valence state. Example: In an acid solution Cu is oxidized to Cu++ by losing two electrons. The Cu++ associates with the anion SO4 2- in the solution to form copper sulfate. At the cathode, the Cu++ is reduced to metallic Cu by gaining two electrons.The plating is most commonly a single metallic element, not an alloy. However, some alloys can be electrodeposited, notably brass and solder.

Use in Manufacturing

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.

• Steel camshafts resist wear better when they have been electroplated with chromium.
  
  
• Plain steel or aluminum parts in light fixtures become beautiful when they are electroplated with nickel and then decorative chromium or brass.
  
  
• Steel bolts last much longer because they are sold with a coating of zinc that has been applied by electroplating. Zinc electroplating and passivation 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.

In addition to the well known yellow full passivation, there are blue, olive and black variants available to meet modern requirements. Modern electrolytes can produce brilliant chrome like finishes. Specially developed processes produce improved metal distribution over complex shapes. Alloy zinc deposits offer extra performance.

Passivation processes (also known as conversion coatings) are usually applied to zinc deposits to improve component life. These coatings used to be based on hexavalent chromium chemistry providing unique surface corrosion resistance which will withstand even the most extreme conditions over prolonged periods but have recently been superseded by trivalent chromium chemistry on both health and environmental grounds.

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

Modern electroplating 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.

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.

On June 28, 1988, four workers at an electroplating plant in Auburn, Indiana were asphyxiated by hydrogen cyanide gas produced when muriatic acid was mixed with zinc cyanide in a cleaning operation. A fifth victim died two days later.