Gold electrolyte
Gold electrolytes are electrically conductive, in practice mostly aqueous solutions of gold salts, which play an important role in gilding and in the production of pure gold on an industrial scale in refineries . Most modern gold electrolytes are based on the cyano complex of monovalent gold in the form of potassium dicyanoaurate (I) . Rarely also be electrolytes based on Kaliumtetracyanoaurat (III) or of the Sulfitokomplexes used.
The different gold electrolytes can be divided into the following groups:
Alkaline-cyanide-containing gold electrolytes
These electrolytes are among the oldest gold electrolytes. The gold is contained as potassium dicyanoaurate. Baths containing cyanide place special demands on occupational safety (handling of toxic substances) and the professional detoxification or disposal of waste water. Potassium dicyanoaurate (I) K [Au (CN) 2 ] is a commercially available product - it contains 67–68% gold. Alkaline-cyanide-containing gold electrolytes can be operated with a) gold anodes , b) gold-copper anodes or c) platinum- coated titanium anodes. a) and b) are soluble in the electrolyte and must be topped up regularly, c) are insoluble. It can be gold layers deposited, but also gold copper - and gold silver - alloys .
Neutral gold electrolytes
The gold is also contained as potassium dicyanoaurate, but the electrolyte does not contain any free cyanide. There are insoluble anodes used ( platinum iertes titanium). Such electrolytes can be used to deposit relatively hard gold-copper layers and fine gold layers for electronics .
(weak) acidic gold electrolytes
The gold is contained as potassium dicyanoaurate. A typical electrolyte also contains cobalt or nickel and citric acid . Anodes: Platinized titanium or rust-resistant steel . Shiny, hard gold alloy layers can be deposited, which often have a low ductility .
Electrolytes based on potassium tetracyanoaurate (strongly acidic electrolytes)
The gold is contained in the form of trivalent potassium tetracyanoaurate (III) (so-called strongly acidic electrolytes). Such electrolytes are well suited for base materials that are otherwise difficult to coat, such as stainless steel . Precipitation has high internal stresses. Because this type of electrolyte has a low level of efficiency , usually only a thin layer is deposited as an adhesion promoter , and the layer is reinforced with another type of electrolyte based on monovalent gold.
Gold sulfito electrolytes
The gold is contained as alkali gold sulfite. Such electrolytes have good throwing power ; H. the layer thicknesses are relatively uniform at different points on the workpiece . In addition, the gold layers have a high ductility. Pure gold and various gold alloy layers can be deposited.
Galvanic gold plating as a high-tech process
Because of the high cost of gold and special requirements of the electronics and semiconductor -Industrie when were galvanic gilding special procedures beyond the usual electroplating process (barrel and rack technology) developed beyond: As part of the continuous electroplating of a strip, the electrolytes were toward optimized for high-speed separation. Due to the high gold content in the electrolyte, increased working temperatures and extremely strong electrolyte movements (powerful pumps and special nozzles), deposition rates of up to 2 µm / s can be achieved. The technique of selective deposition makes an important contribution to saving gold . A metal strip is not completely coated, but only where it is necessary - the areas where no coating is required are covered in special electroplating cells, for example with rubber bands. Another possibility for the economical creation of certain surface properties is the intelligent combination of two or more pure metal and alloy layers - and this in connection with the selective technology.