Zinc, Wide Usage of a Modest Metal
One of the Earliest Metals to have Appeared in Human History
Oct 3, 2008
Brass, an alloy of zinc and copper, was used in Egypt, three thousand years ago, and later by the Romans.
Pure zinc was first made in India, and employed in Chinese coinage from the end of the fourteenth century. It arrived in the west in 1605, and European extraction began in the 1740s. Annual production now exceeds seven million tonnes.
Galvanising Uses Zinc to Protect Steel from Rust
Half the world's zinc is used for rust prevention. Steel, the most important metal in modern society, corrodes in damp air. Painting retards corrosion, but on large structures like bridges, is a never-ending job. If the paint is chipped, rusting accelerates.
Steel is galvanized by dipping it in molten zinc. The zinc layer prevents air and water from attacking the iron, and continues its protection even if it becomes broken. This is called cathodic protection, occurring because an electric cell is created between the metals, resulting in zinc corroding in preference to iron. Galvanized steel structures survive many times as long as painted ones, and require less maintenance.
Industrial Die Casting Using Zinc Alloys
When alloyed with 4% aluminium and sometimes magnesium and copper, zinc is ideal for die casting. Components can be made more rapidly and accurately than those of other alloys. They are strong, durable and corrosion resistant, and can be decorated with a variety of finishes. One third of zinc die castings are used in the motor industry, to make door handles, locks, carburettors, fuel pumps etc. Other uses include power tools, engine parts and metal toys.
Zinc Compounds: Zinc Oxide, Zinc Stearate, Zinc Sulphide
Zinc oxide is a white powder formed by burning zinc in air. It is used as a pigment in paints. Unlike white lead, it is not discoloured by sulphur pollutants in air. Its use has been largely superseded by titanium dioxide, the whitest substance known, but it is still found in some paints as it prevents attack of the oils by mildew, and absorbs ultra-violet light which causes breakdown of paint.
Over 10,000 tonnes per month of zinc oxide are used in the rubber industry. In vulcanization, or hardening, the rubber is combined with sulphur, a process which takes several hours. With zinc oxide as an activator, this time is reduced to minutes.
Zinc stearate, a slippery, soap-like powder, prevents unvulcanized rubber sheets from sticking. Its lubricant properties make it important in plastics manufacture and in cosmetics. It comprises some 5% of the mass of face powder and prevents loss of perfume from talc.
Zinc sulphide provides the fluorescent dots on a television screen. Zinc carbonate, mixed with iron oxide to give it a pink colour, forms the basis of calamine lotion, a soothing treatment for sunburn and skin ailments.
Zinc in Living Organisms
The abundance of zinc in the body is almost as high as that of iron, but unlike iron, which concentrates mainly in blood haemoglobin, zinc is more widespread. It is indispensable to all forms of life, and ailments resulting from its deficiency are more common than those from any other trace element.
Lack of dietary zinc leads to skin disorders, retarded growth, foetal malformations, decreased immune response and behavioural problems. Zinc is also essential to storage, in the pancreas, of insulin, which controls blood sugar levels and prevents diabetes.
If crops lack zinc, they suffer discoloration, loss of foliage and low yield. Cattle show bone and joint disorders, stunted growth, and low fertility and milk yields. Thousands of tonnes of zinc compounds are used annually as additives to fertilizers and animal feed.
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