Electroplating

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Under Electroplating (also electroplating or electrodeposition called) electrochemical deposition of metal precipitation, ie coatings on substrates (objects), respectively. Most frequently in practice it is about coating processes for metallic coatings on substrate bodies in an electrolytic bath. In contrast, the use of electroforming is rarely used. In addition to the actual separation, pre- and post-treatment also play a decisive role in the final layer properties. The history of electroplating, as electroplating technology is colloquially known, goes back to the Italian doctor Luigi Galvani , who discovered electroplating named after him on November 6th, 1780 . The application of low-frequency ( "galvanic") current in medicine, also called galvanization galvanotherapy not heard the herein described electroplating.

Galvanic copper plating of a metal (Me) in a copper sulphate bath

In electroplating, electricity is passed through an electrolytic bath. The metal to be applied (e.g. copper or nickel ) is often located on the positive pole ( anode ), and the object to be coated on the negative pole ( cathode ). By means of an electric current, metal deposited on the workpiece is obtained from dissolved metal ions by reduction . Depending on the process, the anode dissolves at the same time, or the metal ions are deposited from solutions with a finite amount of substance. The object to be refined is coated evenly with the metal on all sides. The longer the object is in the bathroom and the higher the electrical current, the thicker the metal layer (e.g. copper layer) becomes. As a rule, the substrate must be pretreated before electroplating .

Strictly speaking, a distinction is made between electroplating (also known as electroforming ), the electrolytic production of metallic objects on the one hand, and electroplating , the production of metallic coatings on the other. The term Galvanostegie has almost completely been replaced by the general term Galvanotechnik . However, since fewer and fewer equestrian statues were needed, electroplating was also somewhat forgotten, but experienced a small renaissance in connection with microsystem technology , namely as micro- electroforming , also known as lithographic-galvanic molding technology . Electroplating is also used in mold construction for injection molding of plastics .

history

The voltaic column enabled the first attempts at electroplating.

The term electroplating goes back to the Italian doctor and naturalist Luigi Galvani (1737–1798), who noticed in 1789 during experiments with frogs' legs that they twitched when they touched two electrodes made of different connected metals. Alessandro Volta realized that the effect was caused by the different metals. In combination with an electrolyte, these generate an electrical voltage that is discharged in the animal muscle. He then built his battery , which played an important role in the establishment of electrical engineering. It was probably the first battery at all, even if there are suspicions that people would have built batteries thousands of years ago: Certain clay vessels that were found near Baghdad and in which a copper cylinder with an iron rod was inserted were interpreted as the first batteries. The electrolyte used is unknown. They are dated to around 2000 BC. And are commonly referred to as the "Battery of Baghdad" . Today one doubts, however, that it was really a battery. Accordingly, it is also questionable whether electroplating was possible in ancient times. The assumption that in antiquity the gilding of objects with the help of electroplating techniques has therefore not been proven.

Historic electroplating company, around 1900

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 solutions containing cyanide . He used the method in the company he founded with his cousin. In 1865, when Elkington died, it employed almost a thousand workers and was then a leading electroplating company. From the middle of the 19th century, life-size statues were produced by electroplating. A general distinction is made in electroplating between electroplating , which describes the production of electroplating , and electroplating (term rarely used today), which describes the application of metallic coatings. The relatively non-toxic application of metallic coatings led to the peeling of the fire gilding or silver plating, which is extremely harmful to health due to the mercury used and its vapors.

Galvanic applications

A general distinction is made between decorative and functional electroplating technology. The former is mainly used to beautify 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 as well as the gold plating of jewelry and cutlery .

The functional electroplating is used for corrosion protection , the wear protection , the catalysis , the improvement of electrical conductivity and the reduction of frictional forces . The ductility and deformability of workpieces can also be improved by electrodeposited coatings. Here are some examples:

Due to their wear resistance and their good sliding properties, hard chrome layers can also be used as a coating for hydraulic cylinders or for dip tubes in suspension forks. The final properties of these components are significantly better after coating than z. B. that of their basic materials.

Electroplating in practice

Industrial electroplating plant for the production of printed circuit boards

Electroplating may be integrated into the production process of a metalworking company ( operating electroplating ) or, as a service provider - that is, by production of commissioned works ( Electroplating ) - act. In a broader sense, anodizing systems and other (mostly current-driven ) processes are also referred to as electroplating .

Electroplating systems are usually a very long series of tanks in which the various process steps take place one after the other. Modern systems are controlled more or less completely automatically . They are served by surface technicians. The previous designation “ electroplater ” was replaced by the professional designation “surface coater”. There is a choice between the processes of "piece electroplating" (goods carriers with goods are cycled through individual basins), "bulk electroplating" (bulk material in rotating drums is transported through different basins) and "continuous electroplating" (permanent passage of components through a system without individual "cycles “) Differentiated.

Economic importance of electroplating

The number of electroplating plants in the EU was given in 2005 at around 18,000. According to the industry association, the number of employees across Europe will be around 440,000 in 2020. In Switzerland, around 80 electroplating companies are organized in the Swissgalvanic industry association. According to an industry analysis from 2006, around 49,000 people were working in electroplating technology in around 2,100 companies in Germany at that time. The operations included around 50 specialist suppliers for electroplating chemicals, 550 service electroplating shops from the craft sector and 1,500 industrial contract and in-house electroplating shops. The total turnover of these companies in Germany was estimated at almost 6 billion euros at the time, in 2020 it was given as 8.3 billion euros with a workforce of around 60,000. An estimate was also given that the galvanic protective layers would prevent corrosion damage and thus costs of 150 billion euros per year.

Base material

Nowadays, all common metal base materials as well as most non-conductive polymers (plastics) and ceramics can be coated in the laboratory.

In plastic electroplating, only two common methods of polymer coating have become established on an industrial scale. Direct metallization according to the so-called Futuron process , as well as the conventional process sequence via activated electroless metallization as the first metallic process stage (layer sequence: pre-nickel, bright copper, bright nickel, chrome) are found here especially in the decorative segment. In the automotive industry, high quality features and manufacturers' demands force you to deposit up to four different nickel layers in a composite in order to achieve optimal durability, function and appearance.

shine

The quality of a workpiece is often determined by its gloss . The physiological impression of the gloss of metallic layers cannot easily be determined with physical measurement methods (degree of reflection or the like). Human perception can deviate from these physically determined quantities. It is particularly important for decorative applications. Special brighteners are used in the various processes to achieve a high gloss. It must be ensured that a high gloss can change other physical properties (e.g. electrical conductivity, hardness, solderability) of a layer.

Metal coatings can give objects shine and an impressive appearance. So z. B. Cutlery made of cheap metal can be coated with a more expensive metal. For example, to silver-plate a nickel spoon , the spoon is first cleaned and then connected to the negative pole of a voltage source. The spoon is then the cathode. A silver rod serves as the anode. Both electrodes are immersed in a silver nitrate solution. After the voltage is applied, silver atoms are released into the solution as silver ions by releasing electrons. These ions are attracted to the cathode and are deposited on the spoon, taking up electrons. The nickel spoon is coated with a thin layer of silver. The reaction equations are:

Anode: Ag → Ag + + e -
Cathode: Ag + + e - → Ag

Leveling

If a base material is rough, the surface can be leveled by a suitable selection of the galvanic process. The technically better term for leveling is the term micro- spreadability . This property is used, for example, in bearings , rollers or decorative applications (see also gloss).

When leveling, a distinction must be made between geometric leveling (possible leveling due to the geometry of the unevenness) and between real leveling. In the case of real leveling, more material is deposited in deeper places “valleys” than in the raised places “mountains”. The leveling can be improved by adding additives, so-called "levelers".

Electroplating construction

A workpiece is designed for electroplating by taking certain principles into account, which favor the planned electroplating process and avoid possible problems. The need for galvanic-compatible construction is based, for example, on the formation of field lines in the electric field and the associated different rapid (inhomogeneous) deposition of the material.

  • Through holes are cheaper than blind holes. Depending on the diameter and depth, the latter can hinder or prevent the penetration and leakage of the process fluids (air bubbles). Delayed leakage of liquids from the blind holes complicates the flushing processes and can lead to subsequent corrosion.
  • Rounded contours are cheaper than sharp-edged outer and inner angles: Increased separation (up to the formation of burrs or buds) on sharp outer edges. Reduced or no deposition at sharp interior angles.
  • A continuous V-seam is cheaper than an overlap joint or a spot-welded connection: If two surfaces are not welded tightly, the liquids are "held" in the gap by means of capillary action. The layer is destroyed again by these liquids when it dries. The same applies to flanges and riveted connections.
  • Faraday cage : In the case of a completely closed workpiece with openings that are too small, no electrical field can arise in the workpiece. Only purely chemical processes work in this area. In an electrochemical process, the depth of penetration is usually equivalent to the opening; i.e., for a pipe with an inside diameter of 2 cm, a coating to the depth of 2 cm in the pipe is achieved.
  • Material selection: Steels with a high carbon content can impair the adhesion of the layer. With high-strength steel there is a risk of embrittlement. Combinations of different materials on a workpiece can lead to problems, e.g. B. if there are different indications and a mutual contraindication in the pretreatment.

The design and choice of materials have a major impact on a subsequent electroplating process in terms of potential problems and economic efficiency. For this reason, an interdisciplinary working method should be chosen from the outset for new designs.

Aftercare

Post-treatment processes, for example for galvanized goods, can include the application of a conversion layer (by phosphating or chromating ), as well as painting .

Strip electroplating

In the case of strip electroplating, a metal strip with previously punched out parts that are still attached to one another is drawn continuously through all the necessary baths.

The advantages of strip electroplating are:

  • the layer thickness varies only slightly.
  • the parts do not have to be contacted or hung individually.
  • with insoluble anodes, a very fast, high-quality coating is possible (e.g. over 1 µm / s for silver)
  • By covering with straps it is possible to coat only individual strips from the tape (selective coating). This means that less coating material is used.

Strip electroplating is used, among other things, for gold-plating electrical contacts and for coating semiconductor contact substrates (see chip bonding ).

Strip electroplating systems are usually less harmful to health than other electroplating systems, as the systems are usually completely covered and provided with air extractors and no manual work is necessary. This keeps toxic gases and vapors away from the surrounding space.

Dental technology

With the electroplating in the can Dental restorations are manufactured, which consists of thin gold cap and is veneered with ceramic. The electrochemical process produces self-supporting metal frames from gold. In the Auro-Galvano-Crown-Process (AGC), a gold layer of approx. 200 µm is deposited on the tooth stumps prepared with silver powder in the dental laboratory. The frameworks have a purity of 99.99% gold. Electroplating is suitable for the production of single crowns , denture bases , ceramic veneered partial crowns and inlay fillings (inlays / onlays), telescopic crowns , dental bridges for replacing a tooth and dental implant suprastructures.

Electrochemical processes

Electroplating is an electrolysis in which chemical processes (redox reactions; see chemical reaction ) take place under the influence of electric current. The metal deposition that occurs during electroplating is a reversal of metal dissolution. For example, the processes involved in galvanic zinc plating can be viewed as the reverse of the dissolution of a zinc electrode that takes place in many galvanic cells, e.g. B. in the Daniell element or in the zinc-carbon cell .

In the experimental setup and in real operation of the electrolysis, a direct voltage is applied to two electrodes , which are thereby polarized. There arise cathode and anode . If they are in a conductive liquid, electricity flows. Liquids, e.g. B. Acids, alkalis or water with dissolved salts conduct electricity. These substances are also called electrolytes. When a voltage is applied, the positive cations move to the cathode (hence its name) and the negative anions to the anode. Oxidation takes place at the anode. Cations that get close enough to the cathode accept electrons from the cathode. So they are reduced and are deposited as metal on the cathode.

Soluble anodes

Soluble anodes are only common in silver, copper or nickel baths. The appropriate electrolyte is added to the bath; B. silver cyanide and potassium cyanide . In this case, the anode consists of fine silver sheet. During the electrolysis, the cathode is covered with a pure silver layer, and the anode dissolves.

In all soluble anodes, the metal (especially copper, silver or nickel, in the laboratory also zinc or gold) gives off electrons and is oxidized. In the ideal case, the same number of ions (e.g. Ag + for silver) would go into solution at the anode as are consumed at the cathode by reduction. In this ideal case, the concentration of the solution would remain unchanged during the electrolysis as long as the metal on the anode is not yet completely dissolved.

Insoluble anodes

As always in electroplating, the metal ions are reduced to metal at the cathode. However, an insoluble anode such as stainless steel is used. The cations that form the metal of the coating after the reaction are in the bath, e.g. B. in the gold plating bath as gold chloride . If a voltage is applied, the circuit is closed by the oxidation of the anions in the electrolyte. With chloride solutions, chlorine is produced , otherwise oxygen is produced . The process continues until the concentration of metal ions (the gold ions in the example) in the bath is too small. Therefore, metal salt must be added again and again to maintain the process.

Calculation of sales

As with any electrolysis, the amount of substance converted is calculated using the equation that follows from Faraday's laws and is given there . As a rule, the current yield must be taken into account.

quality control

The quality assurance occupies a very high place in electroplating. This includes the constant analysis of bath parameters, such as acid and metal content, control of the appearance and color of the layers, layer thickness measurements using X-ray fluorescence , ultrasound , eddy current methods, detachment methods, but also checking the raw material.

The following can also be checked: surface roughness , hardness , adhesive strength and ductility of the layer, surface defects (e.g. pores, cracks) and testing of corrosion resistance by means of salt spray test , condensation water climate , Corrodkote test , CASS test ( acetic acid salt solution).

The electrochemical properties of the electrolytes can be assessed using practical tests (e.g. Hull cell ) or comparative measurements ( Haring-Blum cell or cyclic voltammetry ).

The quality and the final properties of a coating depend, among other things, on the following parameters:

  • Current density
  • PH value
  • Bath temperature
  • Amount of metal ions in the bathroom
  • Degree of contamination of the bath (metal particles in the bath lead to more inclusions in the coating the stronger the current is)
  • Duration of electroplating
  • Distance between cathode and anode
  • Degreasing the workpiece
  • Purity of water; it is demineralized water necessary for the attachment (mixing) of the bath.
  • Size ratio between anode and cathode, the rule of thumb anode: at least twice as large surface as the cathode is used

Others

Other important points within electroplating are wastewater treatment and the associated environmental protection, instruction in handling hazardous chemicals and working in the laboratory. The thickness of the resulting metal coating varies depending on the application: decorative layers (e.g. gold or bright chrome) are often less than 1  micrometer (µm) thick, while functional layers are significantly thicker (zinc or nickel as corrosion protection around 10 µm, hard chrome or nickel as mechanically functional layers (e.g. in hydraulic cylinders ) usually 100–500 µm).

In electroplating, employees can be exposed to hazardous substances . As part of the risk assessment , the hazardous substances occurring at the workplace must be determined and suitable protective measures must be defined. DGUV Information 213-716 of the German Social Accident Insurance specifies procedures and protective measures in electroplating so that compliance with substances with occupational exposure limits is ensured. The state of the art is documented for materials without AGW.

List of electroplating processes (overview)

These technically similar electroplating processes are limited to special substrates or coating materials and were given their specific names.

The processes that are carried out in electroplating plants, but are not galvanic processes themselves, include the creation of conversion layers by phosphating or chromating as well as burnishing and the electroless generation of metal layers using chemical processes or reducing agents, see external electroless (especially chemical nickel ) . The coloring of metals and the anodic oxidation of aluminum during anodizing are also not galvanic processes in the sense of metal deposition.

Galvanic electrolytes

Related procedures

The electrophoretic deposition (immersion coating) is as electroplating a process generated with the aid of a direct current, a coating. However, this coating is not metallic, and it is created by the change in pH value when water decomposes.

literature

Web links

Commons : Electroplating  - collection of pictures, videos and audio files

Individual evidence

  1. ↑ Nasser Kasnani: Electroplating - Principles, methods and practices of a key technology . 2., revised. u, exp. Hanser, Munich / Vienna 2009, ISBN 978-3-446-41738-0 , pp. 16 .
  2. ↑ Wolfgang Miehle: Joint and spinal rheumatism. Eular Verlag, Basel 1987, ISBN 3-7177-0133-9 , p. 174.
  3. ↑ Harald Eggebrecht: The original stream. On the trail of the secret. In: Süddeutsche Zeitung SZ.de. Süddeutsche Zeitung Digitale Medien GmbH, July 29, 2016, accessed on April 16, 2020 .
  4. ↑ Hans-Gert Bachmann, Günter Bachmann: Surface gold plating : old and new techniques . In: Chemistry in Our Time . tape 23 , no. 2 , April 1989, pp. 46-49 , doi : 10.1002 / ciuz.19890230203 .
  5. ↑ British bronze sculpture founders and plaster figure makers, 1800-1980 - E - National Portrait Gallery. In: National Portrait Gallery. National Portrait Gallery, accessed April 16, 2020 .
  6. ^ Nasser Kanani: Electroplating: Basics, Process, Practice . 2., revised. u, exp. Hanser, Munich 2009, ISBN 978-3-446-41738-0 , pp. 24 .
  7. ^ Nasser Kanani: Electroplating: Basics, Process, Practice . 2., revised. u, exp. Hanser, Munich 2009, ISBN 978-3-446-41738-0 , pp. 12-14 .
  8. ↑ Integrated pollution prevention and control. (PDF) Leaflet on the best available techniques for surface treatment of metals and plastics. In: BVT information sheets and implementing decisions. Federal Environment Agency UBA, September 2005, accessed on April 16, 2020 .
  9. ^ A b Zentralverband Oberflächentechnik eV Voice of electroplating and surface technology. Zentralverband Oberflächentechnik eV ZVO, accessed on April 16, 2020 .
  10. ↑ Swissgalvanic - members. Retrieved April 16, 2020 .
  11. ↑ a b c d sofia, Ökopool, ZVO: Industry analysis of German electroplating and surface technology. (PDF) 2006, accessed April 16, 2020 .
  12. ↑ Systemadmin_Environment: Galvanic surface coating. In: Federal Environment Agency> Topics> Economy | Consumption> Industries> Manufacture and processing of metals. Federal Environment Agency, January 29, 2013, accessed on April 16, 2020 .
  13. ^ Nasser Kanani: Electroplating: Basics, Process, Practice . 2., revised. u, exp. Hanser, Munich 2009, ISBN 978-3-446-41738-0 , pp. 127-130 .
  14. ↑ Gabriele Dietrichs, Paul Rosenhain: Electroforming: Bio-Aesthetics in Restorative Dentistry . Verlag Neuer Merkur GmbH, 1995, ISBN 978-3-921280-99-7 , p. 25– ( limited preview in Google Book search).
  15. ↑ German statutory accident insurance e. V. (DGUV): Electroplating and anodizing - Recommendations for risk assessment by the accident insurance carriers (EGU) according to the Ordinance on Hazardous Substances. Retrieved October 15, 2019 .