Electrodeposition precious metals

In view of the high cost, when tarnish resistance of the surface is the only requirement it is customary to use the thinnest possible coatings of rhodium (0-000 25-0-000 5 mm). Since rhodium deposits in this thickness range, like thin electrodeposits of other metals, show significant porosity, readily corrodible metals, e.g. steel, zinc-base alloys, etc. must be provided with an undercoating deposit, usually of silver or nickel, which is sufficiently thick to provide a fairly high level of protection to the basis metal even before the final precious metal deposit is applied, and, in this way, to prevent accelerated electrochemical corrosion at pores in the rhodium deposit. 

Once the initial conductive metallic film has been prepared as described virtually any coating suitable for electrodeposition can be applied to it. In practice the most usual system is nickel-chromium ( chrome plating ), which— provided application is as it should be—gives excellent resistance to abrasion, to corrosion, and generally is very durable. For certain types of work, coatings of precious metal (especially gold) are popular. 

Anode mud — During the anodic dissolution of an impure electrode (e.g., in an - electrorefining or - electrodeposition process) a residue may form which is called anode mud or anode slime. It contains elements which are insoluble under the conditions of the - electrodissolution process and thus sediment near the anode. Usually the anode mud contains precious metals (Ag, Au, Pt, etc.) which are recovered from it. 

Glassy carbon felt electrodes modified by electrodeposited poly(pyrrole-viologen) films containing electroprecipitated microparticles of precious metals like Pt, Pd, Rh or Ru have been shown to be suitable for the electrocatalytical hydrogenation of several organic substrates including benzonitrile, in acidic aqueous solution. Pd exhibited the highest current efficiency and yields for benzylamine formation when compared with Pt and Rh. 

An important category of the surface finishing industry is plating. This includes the process of electroplating which provides metals with surface coatings (usually by electrodeposition) for corrosion protection, wear resistance, improved electrical and mechanical properties, etc. Ferrous and non-ferrous materials can be electroplated with copper, nickel, chromium, zinc, lead, cadmium, etc. They can also be electroplated with precious metals such as gold and silver. 

MorrissQf, R. J., "Porosity and Galvanic Corrosion in Precious Metal Electrodeposits, Electrochemical Techniques for Corrosion Engineering, R. Baboian, Ed., NACE, Houston, TX, 1986, p. 167. 

The future of cathodic metal removal seems to be fruitful, since it is a mature technology, and there is a wide variety of cell designs commercially available nowadays. A promising field of application is recovery of precious metals from, e.g., spent catalysts and printed circuit boards, in which cell design and cell potential are usually not critical due to the value of the metal. Selective metal electrodeposition from a mixture of different ions is still a challenge, especially when the electrodeposition overpotentials are very close. 

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