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Encyclopedia of Physical Science and Technology EN010b-481 July 14, 2001 18:45

480 Noble Metals (Chemistry)

materials (approximately 260 C) have been introduced, 4. Palladium
◦
which make it practical to consider firing fiberglass and
Electrolytic plating of palladium uses the salts
high-temperature plastics. Other plating methods are hot
Pd(NH 3 ) 2 (NO 2 ) 2 palladium P salt; Na 2 [PdCl 4 ]; and
dipping, chemical vapor deposition sputtering, and vac-
[Pd(NH 3 ) 4 ]X 2 , where X is Cl or NO 2 . Palladium is con-
uum evaporation. Multiple metal coatings are used for
sidered as a viable substitute for gold in many plating
special purposes. These alloy coatings require more care
operations because it is less dense and is less expensive
to produce, but similar results cannot be obtained from
than gold. Two plated alloys 80Pd–20Ni and 50Pd–50Ni
single metal coatings.
havebeenusedinprintedcircuits,connectors,andcontacts
because of the cost benefit over pure gold or palladium.
1. Gold [Pd(NH 3 ) 4 ] 2+ salts with hydrazine are used for electroless
plating. Liquid palladium solutions are available which in-
Gold was deposited by coating the article with a Au–Hg
corporate such compounds as dichloro(di-n-butylsul-fide)
alloy and then heating it to remove the mercury. Currently,
palladium(II).
electrolytic gold plating is done using [Au(CN) 2 ] or,toa
−
lesser extent, [Au(SO 3 ) 3 ] , usually as the potassium salt.
3−
Baths have been formulated using acidic, neutral, and ba- 5. Iridium
sic solutions to which agents are added to tie up impurities
Methods to plate iridium are known but have not been of
and to brighten, harden, and control the metal grain size.
much importance.
Gold alloys have been prepared with antimony, tin, nickel,
copper, silver, indium, and cobalt. Many of these coatings
6. Rhodium
were developed for use in the jewelry and electronics in-
dustry. Electroless plating uses a palladium catalyst with Rhodium is the most commonly plated PGM. It is noted
reducing agents such as hydrazine or hypophosphite. Once for its brilliant finish, tarnish resistance, reflectivity, and
the gold starts to deposit, it becomes the catalyst for further hardness. If the coating is to tin, lead, zinc, cadmium, alu-
plating. The organometallic golds known as liquid golds minum, iron, or steel, a precoating of silver or nickel is
can be prepared by reacting AuCl 3 with sulfurized Venice required to prevent faulty adhesion. Rhodium can be di-
turpentine to yield an ink or paint. Recently, the liquid rectly deposited on silver, copper, nickel, brass, phosphor-
golds have been composed of alkyl and aralkyl mercapto bronze, and some copper alloys. Electrolytic deposition of
gold compounds. If oxidizable metals, such as bismuth, rhodium uses an acid solution and rhodium sulfate, sulfa-
tin, or vanadium, are present in the paint, they can form a mate, fluoroborate, or phosphate salts. The exact species
good bond with glass, porcelain, or metal. Gold leaf (23K present in the electrolyte are not known. Vapor-phase
gold) is also used for coating. plating involves rhodium fluorocarbon-β-diketonate com-
plexes. Rhodium plating is used for searchlight reflectors,
2. Silver heat concentrators in infrared ovens, cutlery, jewelry, and
electrical applications. The maximum thickness usually
Fortheelectrochemicalplatingofsilver,a99.9%silveran-
recommended is approximately 0.002 in., and the mini-
ode is used with an electrolyte composed of [Ag(CN) 2 ] −
mum is 0.00002 in. to protect against tarnish. Thick coat-
and other required additives. Silver should not come into
ings of rhodium have a tendency to crack.
direct contact with steel or severe corrosion can occur.
Steel parts can be protected by first coating with copper or
nickel. Silver coating is commonly used for flatware, hol- 7. Osmium
loware, jewelry, electrical, and mechanical applications. Methods for plating osmium have been described, but due
Chemical methods are available to apply silver as a coat- to the tendency of osmium to oxidize to toxic OsO 4 , they
ing to plastic or glass. have remained laboratory curiosities.

3. Platinum
8. Ruthenium
Platinum electroplating is done on a commercial scale.
Laboratory-scale ruthenium electroplating solutions in-
The plating solutions are usually based on H 2 PtCl 6 and
volving complexes such as Na 2 [Ru(NO)(NO 2 ) 4 OH] and
its salts. Liquid platinum, platinum sulforesinates, and
(NH 4 ) 2 [Ru(NO)Cl 5 ] have been developed.
alkylmercaptides in organic solvents have been used to
coat platinum on glass and ceramics. Platinum can also
B. Brazing and Soldering
be coated on metallic and nonmetallic articles by the va-
por deposition of Pt(CO) 2 Cl 2 or Pt(acetyl-acetonate) 2 fol- Alloys used for soldering and brazing have two dif-
lowed by firing. ferent functions. Solders are used to make mechanical,