Metals and alloys

You should be able to:

■ Explain why metal alloys are often used in place of the metals.

■ Relate the properties of the metals (aluminium, lead, iron) and their alloys to their uses.

■ Investigate the conditions necessary for the corrosion of metals.

QUICK POINTS

■ Mixtures of metals are called alloys.

■ Alloys are usually harder, stronger and more corrosion-resistant.

■ Metals corrode when their surface reacts with oxygen and moisture in the air, forming an oxide of the metal.

METAL ALLOYS

Alloys allow for changes in the physical properties that would not be achieved by using the pure metal alone. For instance, increasing strength, resistance to corrosion, and lower melting point. Common alloys are steel, solder, brass, bronze and Duralumin.

Alloys are made by mixing metals with another element, usually another metal.The layers of a pure metal can slide if force is applied. When another element or metal is added, this causes distortion of the layers which reduces or prevents sliding of the layers. This causes an alloy to be stronger than the pure metal.

USES OF ALLOYS

Iron can be mixed with carbon and other elements to form steel. Stainless steel is iron alloyed with carbon, chromium and nickel. Stainless steel is corrosion-resistant and used to make cutlery, sinks and cooking utensils.

Aluminium forms the alloys Duralumin and magnalium. Duralumin contains aluminium alloyed with magnesium, copper and manganese. This makes it light and strong, suitable for making aeroplane parts and bullet trains.

Bronze contains copper alloyed with tin. This makes it hard, strong and corrosion-resistant, and is used to make statues, monuments, medals and artistic materials.

Brass is copper alloyed with zinc. It is used to make musical instruments and kitchenware.

QUICK CHECK

How are the properties of a metal changed on becoming an alloy?

CORROSION OF METALS

Corrosion of metals occurs when they react with oxygen and moisture in air. The corrosion of iron is called rust, which is a form of iron (III) oxide (hydrated). Acid rain and salts speed up the corrosion process.

Ways of preventing rusting include any method that prevents its contact with water or air, such as painting, coating with oil or grease or with a metal.

In galvanising, iron is coated with zinc, which provides a protective coating of zinc oxide.

In tin coating, the steel cans are coated with a thin layer of tin and covered by plastic to prevent corrosion. Tin reacts more slowly than iron and so protects the metal.

QUICK CHECK

How can the corrosion of metals be prevented?