Lesson 4

Read the text: Non-ferrous metals

Non-ferrous metals are generally more expensive per pound than the ferrous metals, and are used only when some characteristic not possessed by iron or steel is essential or desirable in application. Some of these characteristics are: high electrical and heat conductivity, high corrosion resistance, non-magnetic qualities, light weight and the ease of fabrication.

Some of the non-ferrous metals possess a high electrical conductivity. Silver is the best conductor of the electric current, the conductivity of copper and aluminum is a little lower than that of silver, but they are used for electric equipment such as wire, cables, etc., while silver cannot find application in these cases because of its very high cost.

High resistance to corrosion is another useful property of many non-ferrous metals. Some of them, such as gold and platinum, do not corrode at all because they cannot combine with the oxygen in the air. Copper and tin also have a low ability of combining with oxygen, which is the cause of their high resistance to corrosion. But aluminum and zinc can easily combine with the oxygen of the air, yet we know that they are highly resistant to corrosion. How can it be? The fact is that these metals form a protective coating which prevents the oxidation of the surface of the metal. This coating, which is a thin film of metal oxide on the surface, is formed immediately when the metal comes in contact with the air, thus protecting it from corrosion.

In many cases it is very important to have a light weight metal as, for example, in building airplanes. Aluminum and magnesium are such metals. Pure aluminum has little use in industry because it is too soft for most applications, but it is widely used as a base for alloys possessing good mechanical properties and very light weight. Magnesium is very light, too, but it has almost no resistance to corrosion and therefore it may be used only in the form of magnesium alloys.

Some applications require very ductile metals, that is, metals that can be easily hammered in different shapes. Some nonferrous metals, such as lead and tin, possess very high ductility.

Lead

Lead is a very heavy, bluish grey metal. Lead is found in the form of lead ore. It is the softest metal in general use. Lead is so soft that it can be easily cut with a knife. When cut, it is very bright, but this is not long: as soon as the air comes in contact with the cut, a dark film covers the surface of lead. This dark film, however, prevents oxidation of lead, thus giving it good resistance to corrosion. Water and air have little effect upon lead and therefore it is used for special lead pipes and for covering electric cables in order to protect from oxidation. Lead is highly resistant to corrosion, but its strength is so low that it must be supported by a core of some other material whenever it is subjected to stress of any magnitude. It is extensively employed as a lining for acid tanks, as piping and as a coating for electrical cables. It is also employed as the principle constituent of much antifriction bearing metal lining as well as in other alloys.

Aluminum, a silvery white metal is found in clay; about 8 per cent of the earth is aluminum. It is soft and can be cast into many forms. Aluminum melts at 1,218 degrees Fahrenheit and it can be drawn into very fine wire or hammered or rolled into thin sheets, some only 0,00025 inch thick. Aluminum is very light in weight, does not rust and is used for automobile and airplane parts and in other cases where light-weight metal is necessary. Aluminum is a light-weight metal principally employed in alloy form in engineering applications, since its strength in the pure state is comparatively low.

An aluminum alloy containing about 8% copper is used for sand-moulded castings, and is stronger than pure aluminum but possesses less ductility. Zinc alloys of aluminum are used for die-castings, and have greater strength and ductility than copper alloy die-castings, but have less resistance to corrosion and heat. The addition from 5% to 12% silicon to aluminum furnishes excellent casting qualities.

Copper is the oldest metal known to man. This very useful tough reddish-brown metal is found in the form of copper ore and is produced in the form of wire, bars, plate and sheets. Copper is second best conductor of electricity, silver is the best of them. Copper is used for electric, telephone, and telegraph wires and cables as well as for water heaters, pipes, etc. Copper may be easily rolled and drawn into wire because it possesses high ductility. It possesses a high resistance to atmospheric effects and chemical corrosion, has good mechanical properties, and not a very high cost. Cast copper has low hardness, but its hardness and strength may be largely increased by some cold-working operations. By heating and slow cooling copper may be softened again.

Tin is a corrosion-resistant metal that is used in pure form as a protecting coating for iron and steel plate.

1. Match the left part with the right:

1. Gold and platinum do not corrode	a) as a lining for acid tanks.
2. Lead is extensively employed	b) for tubing, pipes, tanks, and electrical contacts and wiring.
3. Tin is used in pure form	c) because they cannot combine with the oxygen in the air.
4. Copper is used	d) as a protective coating for iron and steel plate.

2. Complete the sentences with the suggested words: resistance, metals, light, property, alloy

High_______to corrosion is another useful_______of many non-ferrous_______. Magnesium is very______and may be used only in the form of magnesium_______.