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Noble Metals (Chemistry) 469

7. Osmium Alloys below 14 karats are susceptible to stress corrosion
cracking.
Osmium melts at 3045 C. It is a hard, heavy element that
◦
Common gold alloys, including Au–Ag, are soft and
is easily oxidized in air to toxic OsO 4 . The ease with which
malleable. Gold–copper alloys are harder, more fusible,
this oxide is formed demands caution. Osmium is difficult
and have higher tensile strength than does pure gold.
to work with because of its hardness at low temperatures
Iron alloys of gold have a lower melting point than does
and the possible formation of OsO 4 at elevated temper-
pure gold, and the iron-rich phase is ferromagnetic. Gold–
atures. It is very resistant to rubbing wear, resistant to
platinum alloys have good corrosion resistance and better
H 2 SO 4 and 40% HF at ambient temperatures, and 36%
mechanical properties than does gold itself. By varying
HCl at 100 C but is easily attacked by oxygen, HNO 3 ,
◦
the ratio of gold, silver, and copper, the green, yellow,
and aqua regia.
and red golds can be produced; Au–Ni–Cu(Zn) alloys are
white golds. Gold has been alloyed with elements of the
8. Ruthenium main group, transition group, and some of the lanthanide
elements.
Ruthenium is one of the hardest elements known. It has
Zinc is commonly added to gold jewelry alloys to deox-
good corrosion resistance and is not attacked by common
idize, lighten the color, decrease the hardening that may
acids, aqua regia up to 100 C, H 2 SO 4 up to 500 C, or
◦
◦
occur on air cooling, and lower the melting point. Frac-
most molten metals in the absence of oxygen. It is slowly
tional percentages of iridium or rhodium together with
attacked by alkaline hypochlorites and is more rapidly
ruthenium reduce the grain size of the alloy, which will
attacked by chlorine and Na 2 O 2 . Ruthenium cannot be
impart increased strength, hardness, and toughness to the
cold worked and is hot worked only with difficulty.
alloy.
Aside from the structured alloys, gold has been stud-
D. Alloys ied in several glassy metal, amorphous forms. Widely
Alloys, multicomponent elemental compositions in solid referenced are Au 4 Si, which was the first commercially
solution, exist in the substitutional or interstitial forms de- produced glass ribbon, La 80 Au 20 , which has supercon-
pending on the relative size of the elements. Substitutional ducting properties, Co 62 Au 38 ,Fe 60 Au 40 , and Au–Ge–Si.
alloys form when the components have similar radii so that These glassy systems are produced by rapidly cooling a
one element can easily be replaced by another without liquid system to a solid while maintaining the disorder
disrupting the overall lattice. If the radii vary by approx- of the liquid. The glasses crystallize to an ordered struc-
imately 15% or more, the resulting lattice structure may ture on heating. The amorphous alloys generally are mag-
be different from that of the elements because of packing netically soft, mechanically strong, show high electrical
preferences. In general, the more ordered the alloy, the resistivities and superconductivity as a type II or “dirty”
more ductile, malleable, and better conducting it is com- superconductor, corrosion resistant because of the lack of
pared to a disordered structure. If the alloying element is structural defects and grain boundaries, and will become
small enough, it will not disrupt the structure, but it will brittle on mild heating.
fit into the interstitial positions in the lattice to give the
material different properties. Some elements capable of
2. Silver
forming this type of structure are hydrogen, boron, carbon,
and nitrogen. By varying the size, electronic structure, and Silver alloys with a substantial number of metals such
ratio of the alloying elements, a series of materials with as aluminum, silicon, nickel, copper, zinc, gallium, ger-
useful properties can be fabricated. This section addresses manium, cadmium, indium, tin, paltinum, gold, mercury,
the most commonly used precious metal alloys and the tellurium, and lead are known. The most common Ag–
properties that make them important. Cu alloy is sterling silver, 92.5 Ag–7.5 Cu, known for its
wear resistance and hardness. As the copper content is
increased, the melting point, electrical conductivity, and
1. Gold
thermal conductivity are lowered. The oxygen content in
Gold alloys are used in a wide variety of applications. the alloy is decreased by the addition of a trace amount
Most alloys with greater than 50% gold are resistant to of phosphorous. In alloys such as Ag–Mg, the magne-
tarnish and corrosion. The scale for rating gold alloys sium is oxidized on heating, which serves to harden the
used in jewelry fabrication is the karatage scale: pure gold alloy to more than double its original strength. In general,
is 24 karat (or 1000 fine). The karat rating relates to the trace amounts of oxidizable impurities harden the alloy
amount of gold in an alloy and does not relay any informa- and restrain grain growth. Tarnish resistant silver and sil-
tion concerning the makeup of the remainder of the alloy. ver alloys can be obtained by either coating with nickel
For example, a 12-karat gold item will have 50% gold. then rhodium or by adding 50% palladium or 70% gold to