The depositing of a metal, most usually silver or nickel, on another metal by electrolysis. The object to be plated is made the cathode; the metal to be deposited is derived from the anode. The plating may be intended for decoration, or to provide resistance to corrosion.
Electroplating involves the coating of an electrically conductive object with a layer of metal using electrical current.
The process used in electroplating is called electrodeposition and is analogous to a galvanic or electrochemical cell acting in reverse. The part to be plated is the cathode of the circuit while anode is made of the metal to be plated on the part. A rectifier supplies a direct current to the cathode causing the metal ions in solution to lose their charge and plate out on the cathode.
On the contrary, other electroplating processes use a nonconsumable anode such as lead. In these situations ions of the metal to be plated must be periodically replenished in the bath as the plate forms out of the solution.
Process
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. The result is the effective transfer of Cu from the anode source to a plate covering the cathode.
The plating is most commonly a single metallic element, not an alloy.
Many plating baths include cyanides of other metals (e.g., potassium cyanide) in addition to cyanides of the metal to be deposited.
When plating is not desired on certain areas, stop-offs are applied to prevent the bath from coming in contact with the substrate.
Strike
Initially, a special plating deposit called a "strike" may be used to form a very thin (typically less than 0.5 mm thick) plating with high quality and good adherence to the substrate. This serves as a foundation for subsequent plating processes. The process is slow, so more efficient plating processes are used once the desired strike thickness is obtained.
The striking method is also used in combination with the plating of different metals. If it is desirable to plate one type of deposit onto a metal to improve corrosion resistance but this metal has inherently poor adhesion to the substrate, a strike can be first deposited that is compatible with both.
Current density
The current density (amperage of the electroplating current divided by the surface area of the part) in this process strongly influences the deposition rate, plating adherence, and plating quality. The higher the current density, the faster the deposition rate will be, although there is a practical limit enforced by poor adhesion and plating quality when the deposition rate is too high.
While most plating cells use a continuous direct current, some employ a cycle of 8-15 seconds on followed by 1-3 seconds off. In order to deal with the uneven plating rates that result from high current densities, the current is even sometimes reversed, causing some of the plating from the thicker sections to go back into solution.
Brush electroplating
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.
Industrial Use
Electroplating is used in many industries for functional and/or decorative purposes.
Plain steel or aluminum parts in light fixtures glisten when they are electroplated with nickel and then decorative with 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.
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. Silver plating is also popular for RF connectors because radio frequency current flows primarily on the surface of its conductor;
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.
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.
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. 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.
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.
Electroplating is one of the three processes that form the LIGA-process used to manufacture MEMS devices.
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