Activity: Corrosion.

Read the following information about corrosion and fill in all the gaps with one word from the box. Some words can be used more than once:

Oxidation is the most common and well-known form of corrosion. It occurs when a metal (aluminium or steel) is subjected to a gas containing oxygen. Hence most parts will suffer this condition since the atmosphere is made up of 21% oxygen.

When a metal like aluminium is subjected to oxygen, a chemical reaction takes place on the surface of the metal. Two aluminium atoms join three oxygen atoms to form aluminium oxide (Al₂O₃). If the metal is steel two atoms of iron react with three oxygen atoms to form the oxide (Fe₂O₃).

There is one significant difference between aluminium oxide and iron oxide: aluminium oxide forms a thin layer on the surface slowing the oxidation process dramatically since the layer seals the metal from the oxygen. Iron however forms a porous layer or oxide on its surface which can easily be penetrated by more oxygen atoms; hence the metal will oxide until it is no more.

The best way to protect against oxidation is simply to prevent the oxygen atoms contacting the metal. Covering the surface with a substance that creates an airtight seal often does this.

Aluminium can be protected by the formation of an oxidation layer preventing more corrosion. Pure aluminium will not oxide, however it is very weak. By rolling a thin layer of pure aluminium on a stronger aluminium we get the benefits of the structural strength and the corrosion protection of the pure aluminium.

alloy               chromium                   faster             first               flake              less

            oil                   oxidation                   oxygen                       paint  

reactive         rusting                       aluminium                  brass              corrosion       green              lead                            magnesium                 malleable            oxide

oxidise           piping             rapidly                       reactivity                  shiny              stored

            structural                              toxic              weakness                   sacrificial

stainless                    water                         zinc

1. Iron (or steel) unfortunately corrodes ………………… than most other transition metals and readily does so in the presence of both ………………… (in the air) and ………………… to form an iron …………………. . Iron corrosion is called ………………… and the chemical change with oxygen is called an ………………………… reaction.

2. Iron and steel (an ………………… of iron) are most easily protected by …………………, which is the cheapest method of prevention and provides a barrier between the metal and the air/water. Moving parts on machines can be protected by a water repellent ………………… or grease layer.

3. Corrosion can be prevented by connecting iron to a more …..……………… metal (e.G. zinc or magnesium). This is called …………….……… protection. By mixing iron with other metals such as ………………… you can make a non-rusting alloy called ………………… steel. Coating iron or steel with a ………………… layer is called “galvanising”. The zinc preferentially corrodes first to form a ………………… oxide layer that does’n ………………… off. However if a less reactive metal is attached, the iron then rusts ………………… as it becomes the most ………………… metal.

4. Aluminium does not ………………… (corrode) as fast as its position in the metal ………………… series would suggest. A thin protective layer of ………………… ………………… forms on the surface, and acts as a barrier to oxygen and water and prevents further ………………… . Aluminium is a useful ………………… metal. It can be made harder, stronger and stiffer by mixing it with small amounts of other metals (e.g. ………………… ) to make alloys.

5. Copper and lead are both used in roofing situations because both are …………………… (easily shaped) and neither is very reactive. The compounds formed do not ………………… away as easily as rust does from iron. Lead corrodes to a white ………………… oxide or carbonate and copper corrodes to form a basic ………………… carbonate. Both metals have beeb used for piping, but these days lead (used by the Romans) is considered too ………………… . The stronger and harder ………………… is still used for ………………… . Copper is mixed with zinc to make the alloy …………………, which is hard wearing and doesn’ t readily corrode. This alloy is used for door knobs and other domestic fittings.

6. The group 1 Alkali Metals are bright and ………………… when freshly cut, BUT ………………… tarnish and corrode in air. So they need to be ………………… under ………………… to reduce corrosion. Apart from their structural ……..……………… they would hardly be used for any outside.