Gold alloys

Dr. Dmitri Kopeliovich

• Gold
• Purity of gold alloys
• Color of gold alloys
• Applications of gold
• Chemical compositions of some gold alloys
• Properties of some gold alloys

Gold (Au) is a transition metal with an atomic number 79.
The atomic mass of gold is 196.966569 (naturally occurring isotope).
A nucleus of gold is composed of 76 protons and 118 neutrons.

Gold is found in hydrothermal quartz veins mostly in chemically uncombined metallic state. The natural gold is commonly alloyed with silver with minor amounts of platinum, copper and other metals. The natural gold-silver alloy is called electrum.
Gold occurs also in a combined form as AuTe₂ (telluride of gold). The minerals containing telluride of gold are calaverite and sylvanite.

The three most prominent properties of gold:

• Chemical inertness. In contrast to most of metals gold does not oxidize (tarnish, rust) in the atmosphere at normal temperatures. It does not react with most of acids and bases. Gold may slowly react with cyanide solutions and some halogens. Gold can also be dissolved in Aqua regia (a mixture of nitric and hydrochloric acid in the proportion 1:3). Due to its exceptional chemical stability gold is referred to noble metals together with ruthenium, rhodium, palladium, silver, osmium, iridium and platinum.
• Yellow color. Most of metals have a characteristic grey silvery color with the exceptions of copper (red) and gold (yellow). Yellow lustrous color of gold is a result of an action of the outermost (valence) electrons of the gold atoms absorbing blue and green photons. The color of gold alloys may significantly differ from yellow.
• Malleability and ductility. Gold has an extraordinary ability to be compressed (malleable) or stretched (ductile). 1 ounce of gold may be flaten out to 300 square feet (1 g - to 1 square meter).

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Pure gold is very soft (25 HV) therefore it is rarely used in an non-alloyed state.
The strength, modulus of elasticity and hardness of gold may be considerably increased by alloying with other metals (silver, copper, nickel, zinc, palladium, platinum, iron, aluminum).

Gold alloys are classified according to their purity and color.
Purity of gold alloys is measured in carats (karats).
The purity in carats (caratage) C_K may be calculated from the concentration in % C_% by the formula:

C_K = 0.24*C_%

For example the caratage of an alloy containing 75% of gold is C_K = 18K
The caratage of gold alloys may vary between 9K to 24K (pure gold).

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• Yellow gold is a gold alloyed with silver and copper. Yellow gold alloys have a good malleability. The alloys are stronger and less expensive than yellow pure gold.
• White gold is a very popular type of jewellery gold alloys. White gold may be obtained by alloying various metals:
  • Palladium alloyed white gold. 20-25% of palladium impart a white color to the alloy. In contrast to nickel alloyed alloys Au-Pd alloys do not irritate skin. However they are heavier and more expensive.
  • Nickel alloyed white gold has an appearance alike to platinum. It is less expensive than Au-Pd but some people are allergic to nickel.
  • Alternative white gold alloys do not contain expensive palladium and they are also nickel free. Platinum, silver, iron, zinc, cobalt, chromium may serve as alloying elements in such alloys. To improve the cosmetic appearance of the jewellery made of the alloys they are plated with rhodium.
• Red gold is a result of alloying with copper. The higher copper content the deeper is the resulting red color varying from pink to red.
• Green gold alloys contain silver and may also contain copper and cadmium.
• Purple gold is obtained by alloying with aluminum (~20%).

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• Jewellry;
• Coinage;
• As an investment;
• Electrical contacts;
• The isotope Au-198 is used in medical treatments;
• Dentistry;
• Solder and Brazing filler;
• As a metal for Electroplating and electroforming;
• Chemical equipment;
• Infrared reflectors.

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(Materials Data)

• Gold
• Gold-platinum alloy

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• Steels and cast irons
• Aluminum alloys
• Copper alloys
• Magnesium alloys
• Nickel alloys
• Titanium alloys
• Silver alloys
• Platinum alloys

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