|
|
|
|
|
Broadly speaking, bronzes are copper alloys in which the major alloying element is not zinc or nickel. Originally "bronze" described alloys with tin as the only or principal alloying element. Today, the term is generally used not by itself but with a modifying adjective. Brasses are the most numerous and the most widely used of the copper alloys because of their low cost, easy or inexpensive fabrication and machining and their relative resistance to aggressive environments. They are, however, generally inferior in strength to bronzes and must not be used in environments that cause dezincification. A modern and comprehensive document on the subject is the second edition of the classic CORROSION BASICS textbook. Some excerpts of that document are used here.
Tin bronzes were among the first alloys developed by ancient metalworkers more than 4000 years ago. The addition of tin to copper produces an alloy that is harder and more wear- and corrosion resistant than either of the pure metals. The discovery of bronze brought immense benefits to the early societies, and the period from 2000 BC to 1000 BC is familiarly known as the Bronze Age.
Today, bronze is the generic term for a family of copper alloys in which the principal alloying element is neither zinc (which forms brasses) nor nickel (copper-nickels, also called cupronickels). The alloys' names reveal their basic composition, e.g., aluminum bronze, nickel-aluminum bronze, silicon bronze, etc., although any of these alloys may contain several additional alloying elements to imbue specific properties. Like the copper-tin bronzes, the alloys have a light golden color, high corrosion resistance and excellent mechanical properties.
Tin bronzes containing more than 10% tin are generally harder and more corrosion resistant than brass and up to 20% tin is often added. So-called phosphor bronzes additionally contain small amounts of phosphorus to further increase hardness and wear resistance; hence, they are often specified for sliding contacts and connectors. Phosphorus also increases the fluidity, thereby enhancing castability. Lead is sometimes added to act as an "internal" lubricant, and leaded bronzes are widely used in bearings that must withstand prolonged sliding action.
The presence of lead also makes the alloy easier to machine. Since ancient times, bronze has been the preferred material for casting statues and other decorative artifacts because it reproduces every detail of the mold and because its high corrosion resistance ensures that the statue will last. (Few artists intentionally erect temporary statues.) With time, atmospheric corrosion causes the metal's surface to take on a brown or green color known as a patina. The patina enhances the decorative appearance of the statue and protects the underlying metal from further corrosion.
Silicon bronzes are alloyed with 1-3% of silicon; about 1% of iron, nickel, manganese or tin can also be present. Although silicon bronze presents some fabrication difficulties, it has excellent chemical resistance and becomes stronger when worked, e.g. by rolling. Silicon bronzes are used for pumps, boilers, marine hardware and, chemical vessels, and one alloy in this family is preferred by sculptors (because of its color and fluidity) for welding electrodes used to join sections of bronze statuary.
