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470 Noble Metals (Chemistry)

platinum to improve hardness and mechanical strength.
the silver. The glassy metal alloys consisting of Cu 50 Ag 50
and Pd 90 Ag 10 have been characterized. Examples of palladium glassy alloys are Pd 80 Si 20 , which
has high strength and good ductility, and Pd 90 Ag 10 .
3. Platinum
5. Iridium
Platinum has been used frequently as the predominant
Iridium is most often used as the minor component in alloy
metal in alloy mixtures. In general, sulfur attacks alloyed
mixtures, often with less than 20% iridium. Above 20%
platinum more readily than it does the unalloyed metal.
iridium, the alloy loses ductility and becomes difficult to
Platinum hardness can be increased in the general order
process. Common alloys are Ir–Os, osmiridium. Iridium
tungsten>nickel>ruthenium>osmium>copper>gold
containing a small percentage of ruthenium melts several
> iridium > rhodium > palladium, with the last yielding
hundred degrees higher than iridium. Iridium is used to in-
the softest alloy. The electrical resistivity can also be
crease palladium and platinum hardness and corrosion re-
related to the alloying element with tungsten > copper >
sistance. The intermetallic compounds Ti 3 r, ThIr 2 ,Nb 2 Ir,
ruthenium > osmium > nickel > iridium > palladium >
and Th 7 Ir 3 have been shown to possess superconducting
rhodium, in order of decreasing resistivity.
properties. Iridium–tungsten alloys have increased tensile
Some of the most common platinum alloys involve
strengths, especially at high temperatures. Glassy metal
Pt–Pd. Where palladium comprises 25% of the alloy, it
alloy systems such as Ta 55 Ir 45 and Nb 55 Ir 45 are known.
performs similarly to platinum. The resistance to HNO 3
decreases as the palladium content increases. Platinum– 6. Rhodium
iridium alloys of approximately 10% iridium have good
Rhodium is most commonly alloyed with platinum and
tarnish and corrosion resistance; resistance to aqua regia
palladium to improve corrosion resistance and mechan-
increases as the iridium content increases, and workabil-
ical properties at high temperatures. Rhodium improves
ity decreases. Platinum–iridium use is not recommended
the corrosion resistance of chromium and titanium to
above 800 C in air because a black oxide film will form.
◦
nonoxidizing acids. Alloys of titanium, approximately 3%
Commercial Pt–Rh alloys contain 5–40% rhodium but
max, increase the tensile strength of the rhodium. The al-
most commonly contain 10% rhodium. These alloys are
loy Ni 63 Rh 37 shows better corrosion resistance than does
preferred to platinum in high-temperature oxidizing con-
14-karat gold. Rhodium does not form a continuous se-
ditions, and they can be hot or cold worked. Above 40%,
ries of solutions with osmium or ruthenium because they
rhodium fabrication can be difficult. The Pt–Ru alloy is
crystallize in different structures. The rhodium-containing
similar to the Pt–Ir alloy except that less ruthenium is re-
amorphous alloys Ta 55 Rh 45 ,Nb 60 Rh 40 ,Nb 55 Rh 45 , and
quired for similar properties. For example, 5% ruthenium
Mo 82 Rh 18 have been characterized.
will act similarly to 10% iridium. The ruthenium alloy is
not suitable for high-temperature service under strong ox- 7. Osmium
idizing conditions because ruthenium can volatilize as the
Osmium is used mostly as a hardening agent in alloys, but
oxide; Pt–Rh–Ru alloys have high tensile strengths, hard-
this application has limited use because of formation of
ness, good corrosion resistance, weldability, and stability
the toxic OsO 4 . The known alloys osmiridium, found nat-
in electrical uses. These alloys are sensitive to impurities.
urally, and 85Os–15Pt are extremely hard and are used in
They are very slowly attacked by aqua regia. Platinum–
some specialty applications where hardness is of primary
cobalt alloys with approximately 50 at. % cobalt make
importance.
excellent permanent magnets for use in many specialty ap-
plications; Pt–Ni and Pt–W are alloys that are known for 8. Ruthenium
both their hardness and strength. At high temperatures un-
Ruthenium is used most often as a hardening agent for
der oxidizing conditions, selective oxidation of the nickel
platinum and palladium. The major alloy is Pt–Ru with
and tungsten occurs.
10–15% ruthenium. Special alloys with 30–70% ruthe-
nium are used in applications in which resistance to severe
4. Palladium
wear and corrosion is required. Ruthenium is replacing
Common palladium alloys include Pd–Ag with 1–60% iridium as a hardening agent because it is more effective
Pd; this material is tarnish resistant at greater than 50% and less expensive. The glassy metal alloy Mo x Ru 1−x is
Pd. The alloys are attacked by HNO 3 and cyanide but known to have superconducting properties.
are resistant to HCl except in the presence of oxidizing
agents; Pd–Ag–Cu, Pd–Ag–Au, and Pd–Au alloys also
E. Chemistry
show good corrosion resistance; and Pd–Cu, Pd–Ni, and
Pd–Ru alloys are harder than palladium. Rhodium and The noble metals, while being relatively unreactive in the
ruthenium are often used together with palladium as with metallic state, have a rich and varied chemistry. In general,