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

Noble Metals (Chemistry) 483

is used in spinnerets through which cellulose acetate and 80Pt–20Pd are used as anodes in ship hull cathodic protec-
rayon fibers are extruded. This alloy is being replaced by tion because of platinum’s corrosion resistance in salt wa-
90Pt–10Rh in some applications. ter, as well as to protect pipelines. Electrodes of 90Pt–10Ir
The alloys are used in thermocouples for use with liq- are used in several electrochemical processes. The metal
uid helium and for temperatures as high as approximately is used to line autoclaves for ethyl chloride production at
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1300 C. Gold coatings make excellent reflectors for use approximately 1000 C, as well as to line a container for
in solar energy collection, radiant heating and drying de- mixtures of phosphates and fluorides up to 500 C. Alloys
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vices using infrared radiation, thermal barrier windows, containing 10–40% Rh are used as furnace heater coils for
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and as a protective coating for space craft and space suits. continuous operation. With 40% rhodium, a 1500–1800 C
furnace temperature can be maintained in an oxidizing at-
mosphere. Platinum and Pt–Rh alloys have thermal expan-
2. Silver
sion coefficients very similar to those of high quality glass
Silver is used as a coating on mirrors because of its good (e.g., optical glass) and along with their corrosion resis-
reflectance properties. This application has potential in tance are very useful for making glass–metal seals. These
solar energy collection. The coating is produced by pre- seals are important when good temperature resistance and
treating the glass with SnCl 2 followed by AgNO 3 or an ductility is required (e.g., camera lenses, eyeglasses, and
ammonia complex with a reducing agent such as sugar, television tubes). The Pt–Rh–Ru alloys are used in strain
Rochelle salt, or formaldehyde. Silver is also coated over gages. Platinum with 4–10% Ru is used in aircraft mag-
copper, nickel, or carbon steel for use in the food, drug, netos and aircraft spark plug electrodes.
and chemical industries in which a high degree of product
purity is required in a corrosive environment. These coated
products are used to concentrate chemicals where flavor 4. Palladium
and stability problems may exist. Hormones and vitamins Palladium is often substituted for the more expensive plat-
often are prepared in this type of container. Evaporating inum. Conversion of urea to melamine at 275–595 Cis
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pans are used to concentrate high-purity KOH, NaOH, and done in palladium-lined equipment; in this application,
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organicacids.Attemperaturesuptoapproximately325 C, platinum is attacked rapidly. Palladium is also used in trays
silver tubes have been used as condensers for 70% HF(aq) for firing phosphors in oxidizing environments. A hard al-
and HF vapors during UF 6 preparation. Fluorophospho- loy, 60Pd–40Cu, is used when metal contact is involved
ric acid has been prepared in a silver-lined reactor for (e.g., springs and brushes made from the same material).
corrosion protection from the phosphoric anhydride and Palladium and Pd–Ag alloys are used as septa to purify
anhydrousHF.Steelbearingswithapalladiumcoatingfol- hydrogen gas.
lowed by a silver coat are used as engine bearings because
of their good lubrication properties, moderate hardness,
5. Iridium
good thermal conductivity, and low solubility in iron.
Iridium is usually used in alloys; the pure metal is difficult
to forge or fabricate. Iridium is most commonly used with
3. Platinum
platinum to impart additional corrosion resistance. An ex-
Platinum is used in high-temperature measurement. This ample of its use is the Pt–Ir crucible. Thermocouples of
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application requires high-purity (99.999%) platinum, iridium and Ir–Rh can be used up to 2000 C. An Ir–W al-
which is sometimes designated as Pt67. The resistivity of loy is used for springs that must function at high operating
platinum can change considerably depending on the im- temperatures. Iridium can be used with mercury because
purity identity and its concentration. Platinum/platinum– it will not be wet.
rhodium can be used as thermocouples at 1500 to 1850 C
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depending on the rhodium content. Platinum–rhodium 6. Rhodium
spinneretts are used in fiberglass production. Platinum
and Pt–Ir alloys are used for crucibles and to line melt Rhodium is used mainly in platinum alloys; the rhodium
furnaces used for high-purity glass (e.g., manufactur- imparts additional corrosion resistance and good high-
ing optical glass). Platinum–iridium crucibles are used temperature characteristics. A rhodium coating on steel
at temperatures up to 2100 C in the process to grow or brass is effective for corrosion prevention, especially
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yttrium–aluminum–garnet (YAG) crystals, which are used in sea water. The coating must be very thin to justify its
to project laser and maser beams. Platinum with approxi- cost. Rhodium has very high optical reflectance and has
mately 3.5% rhodium is used in crucibles and heated high- been used in optical equipment, especially where tarnish
pressure vessels. Electrodes composed of platinum or might be a problem. Sometimes, it is coated on to silver