Precious metals may seem unlikely as engineering materials, but the same expensive metals used for coinage and jewelry also satisfy applications requiring the ultimate in corrosion resistance or electrical conductivity. Three subgroups make up the entire family of precious metals: silver and silver alloys; gold and gold alloys; and the platinum metals, which are platinum, palladium, rhodium, ruthenium, iridium, and osmium. The six platinum metals are so grouped because they occur naturally in the same ore.
Most precious metals are available as sheet, tape, foil, wire, tubing, gauze, discs, electrodes, cathodes, crucibles, catalysts, and salts or solutions for plating and coating. All precious metals are nearly completely corrosion resistant; platinum metals withstand service up to 3,200°F without any evidence of erosion or corrosion.
Gold: An extremely soft, ductile metal, gold undergoes very little work hardening. A gram of pure gold can be worked into leaf covering 6 ft^2 and only 0.0000033 in. thick. Because pure gold is too soft for large, freestanding components, it is used chiefly for linings or electrodeposits and is often alloyed with other metals such as copper or nickel to increase strength or hardness.
Gold is extremely inert. It is not attacked by nitric, hydrochloric, or sulfuric acid, but is dissolved by aqua regia and is attacked by sodium and potassium cyanide plus oxygen.
Silver: The least costly of the precious-metal group, silver is also very malleable, ductile, and corrosion resistant. Because it is not attacked by alkaline solutions, it is used to contain caustic soda and potash in all concentrations. Silver has the highest thermal and electrical conductivity of all metals.
Alloyed with copper, and sometimes with zinc, silver is used in high-melting temperature solders. These silver solders are used where more than ordinary joint strength -- or sometimes, electric conductivity -- is required.
Platinum: A silver-white metal, platinum is extremely malleable, ductile, and corrosion resistant. When heated to redness, it softens and is easily worked. It is nearly nonoxidizable and is soluble only in liquids that generate free chlorine such as aqua regia. Because platinum is inert and stable, even at high temperatures, the metal is used for high-temperature handling of high-purity chemicals and laboratory materials. Other applications include electrical contacts, resistance wire, thermocouples, and standard weights. In gauze form, platinum is used as a catalyst in air-pollution control systems.
Palladium, iridium, and rhodium: These metals resemble and behave like platinum. All can be worked but, because they work harden, require annealing between forming operations. Rhodium and iridium are more brittle than palladium, however, and must be forged or swaged above 1,472°F. Iridium cannot be cold rolled at all.
Palladium, which is harder and lighter than platinum, is used as an electrically conductive coating. It is easily applied to printed-circuit boards and various types of contacts. Palladium is often alloyed with silver or copper.
Iridium has the best corrosion resistance of all metals. Because of its excellent resistance to attack by leaded fuels, iridium is used for spark-plug electrodes in aircraft engines and in similar applications where extreme reliability is required. It is also used as an alloying agent for increasing the corrosion resistance and hardness of platinum. Iridium-tungsten alloys are used for springs operating at temperatures as high as 800°C.
Rhodium has the highest electrical and thermal conductivity of the platinum-metals group and is the hardest. It has high reflectivity, which makes it ideal for mirrors and reflectors in high-temperature or highly corrosive applications. It is also used with platinum in thermocouple wire.
Ruthenium and osmium: Although they worked successfully and have no structural applications, these metals are used as alloying elements to increase hardness and electrical resistivity of platinum or other metals in this group. Typical applications for these alloys include electrical contacts and wire for electronic products.