We continue to review some of the most important materials in heat treatment and metallurgy.
Zinc (chemical Symbol: Zn)
Zinc is a bluish silvery, brittle and hard metal most commonly used as an alloying element and for galvanic corrosion resistance.
Brass, an alloy of copper and zinc, was used as early as the third millennium B.C., well before its discovery as a metal. Brass artifacts have been dated from Babylonia and Assyria from 3000 B.C. and from Palestine dating from 1400-1000 B.C. It was later used in India, as evident from the refining waste left by a large-scale zinc smelter at Zawar, in Rajasthan, dating to the 13th century.
Zinc was produced in Zawar by reducing calamine (zinc carbonate, ZnCO3) with organic substances such as wool. Zinc refining in China was carried out on a large scale by the 1500s. An East India Company ship sank off the coast of Sweden in 1745 while carrying a cargo of Chinese zinc pieces. Samples recovered later were analyzed and determined to be high-purity zinc.
In 1668, Flemish metallurgist P. Moras de Respour reported the extraction of metallic zinc from zinc oxide, but credit for the discovery is generally given to German chemist Andreas Marggraf in 1746 because he was the first to recognize it as a new metal. It was named after the German word Zinke.
Work by Luigi Galvani and Alessandro Volta uncovered the electrochemical properties of zinc by 1800. This led to its most common use today: to provide a corrosion-resistant coating for iron and steel. Zinc oxidizes more readily than iron or steel and thus will attract almost all oxidizing elements until it completely corrodes away, thereby serving as a sacrificial metal. A protective surface layer of oxide and carbonate forms as the zinc corrodes, remaining even if the zinc layer is scratched or penetrated. The zinc is applied electrochemically or as molten zinc by hot-dip galvanizing or spraying. Galvanization is used on automotive components, chain-link fencing, structural beams for construction, light posts, metal roofs, heat exchangers and many others.
Zinc is currently the fourth-most widely used metal in the world after iron, aluminum and copper, and its consumption has grown exponentially in recent years (Fig. 1). After galvanizing, the second most common use of zinc is as an alloy in combination with copper to form brass and with other metals. Zinc lozenges are shown to reduce the severity and longevity of the common cold. Zinc oxide is used as an ingredient in diaper-rash cream, and zinc pyrithione is a main ingredient in dandruff shampoo.
Here are a few scientific and engineering facts about zinc.
- Atomic number: 30
- Element category: transition metal, alternatively considered a post-transition metal
- Standard atomic weight: 65.38
- Melting point: 419.53°C (787.15°F)
- Density near R.T.: 7.14 g/cm3
- Crystal structure: hexagonal close-packed (hcp)
- Speed of sound at R.T.: 3850 m/s
- Thermal conductivity: 116 W/(m·K)
- Electrical resistivity at R.T.: 59.0 nΩ·m
- Young's modulus: 108 GPa