Electroplating is the coating of an object with a metal. It is done by immersing the object and a bar of the metal in a solution containing the metal ions. Electric current is then applied; the positive goes to the metal and the negative goes to the object. The metal bar dissolves in the solution and plates out on the object, forming a thin but durable coating of metal. It is often used to gold-plate objects for decoration or to stop corrosion. Normally the metal becomes fragile, and is only used for display.
Electroplating
History
The history of electroplating, as it is colloquially known, goes back to the Italian physician Luigi Galvani (1737-1798), who discovered galvanism, named after him, at the end of the 18th century when, during experiments with frogs' legs, he noticed that they twitched when touched with two electrodes made of different interconnected metals.
Alessandro Volta realized that the effect was caused by the various metals. These, in combination with an electrolyte, produce an electrical voltage that discharges in the animal muscle. He then built his battery, the voltaic column, which played an important role in the foundation of electrical engineering. It was probably the first battery ever made, although there is speculation that humans built batteries thousands of years ago: Certain clay vessels found near Baghdad, in which a copper cylinder with an iron rod was embedded, have been interpreted as the first batteries. The electrolyte used is unknown. They are dated to about 2000 BC and are usually referred to as the "Baghdad battery". Today, however, it is doubted that it was really a battery. Accordingly, it is also questionable whether electroplating was already possible in ancient times. The assumption that the gilding of objects with the help of electroplating techniques was known in antiquity is therefore not proven.
The first documented galvanic gilding took place in 1805 by a student of Volta. In 1840, the English entrepreneur George Richards Elkington received a patent for a process for galvanic silvering with cyanide-containing solutions. He used the process in the company he founded with his cousin. It employed nearly a thousand workers in 1865, when Elkington died, and was considered the leading electroplating company at the time. From the middle of the 19th century, life-size statues such as equestrian statues were produced by electroplating, for example the famous statue of the Great Elector in Berlin, which was based on designs by Andreas Schlüter.
The relatively non-toxic application of metallic coatings has largely replaced the technique of fire gilding or silvering, which is harmful to health due to the mercury used in this process.
Galvanic applications
A distinction is sometimes made between decorative and functional electroplating. The former is mainly used to embellish objects and must have certain minimum technical properties for this purpose. Examples of decorative electroplating are plastic electroplating, the chrome plating of tubular steel furniture, fittings and motorcycles, and the gold plating of jewellery and cutlery.
Functional electroplating is used for corrosion protection, wear protection, catalysis, improvement of electrical conductivity and reduction of friction forces. The ductility and formability of workpieces can also be improved by electrodeposited coatings. The following are some examples:
- Galvanizing of screws (corrosion protection)
- Coating of machine parts with hard chrome (wear protection)
- Coating with metallic catalysts, mostly containing nickel or platinum, for the chemical industry or fuel cells (catalysis)
- Gold and silver plating of electrical contacts (electrical conductivity)
- Lead-tin-copper coatings for plain bearings (friction reduction)
- Copper plating during wire drawing (improvement of formability)
Due to their wear resistance and good sliding properties, hard chrome coatings can also be used as coatings for hydraulic cylinders or for immersion tubes in suspension forks. The end properties of these components after coating are considerably better than those of their base materials, for example.