4.8 Corrosion

Question 1

Describe how iron corrodes (redox reaction (2)

Answer 1

• If iron comes into contact with air and water, after a whole, it will corrode. This limits the lifetime of iron objects. Corrosion only happens when the iron’s in contact with both oxygen (from the air) and water.
• Corrosion of iron is a redox reaction. Iron loses electrons when it reacts with oxygen. Each Fe atom loses three electrons to become Fe3+, so iron’s oxidised. Simultaneously, oxygen gains electrons when it reacts with iron. Each O atom gains two electrons to become O2-. Oxygen’s reduced.

Question 2

Describe the experiment that can show water and oxygen are needed for iron to corrode (3)

Answer 2

• If you put an iron nail in a boiling tube with just water, it won’t rust. (The water is boiled to remove oxygen and oil is used on the top to stop air getting in).
• If you put an iron nail in a boiling tube with just air, it won’t corrode. (Calcium chloride can be used to remove any water vapour from the air).
• If you put an iron nail in a boiling tube with air and water, it will corrode.

Question 3

Which part of metals corrode?

Answer 3

The surface metal

Question 4

What happens when iron rusts?

Answer 4

The rust that forms on the surface gradually flakes off, revealing new metal underneath. This means that the next layer can also rust and overtime that layer will also fall away. So if left long enough, the entire chunk of iron can break down until there’s nothing left.

Question 5

Why does only the surface layer of aluminium corrode?

Answer 5

This is because when the surface layer of aluminium corrodes, to form aluminium oxide, they effectively form a protective layer around the piece of aluminium, which protects the rest of the metal from the environment.

Question 6

Why should we find ways to protect metals from corrosion?

Answer 6

It is good for people using metallic objects (as they’ll last longer) and for the life cycle assessments of these objects.

Question 7

What are the barrier methods of preventing corrosion? (3)

Answer 7

• Painting/Coating with plastic: ideal for big and small objects alike. It can be decorative too.
• Electroplating: this uses electrolysis to reduce metal ions onto the metallic object. It can be used to coat the object with a layer of metal that won’t corrode.
• Oiling/Greasing: this has to be used when moving parts are involved, like on bike chains.

Question 8

What are the sacrificial methods of preventing corrosion? (2)

Answer 8

• Sacrificial method: this involves adding a more reactive metal, like aluminium or zinc, to the metal. This means that if the object is exposed to oxygen, the more reactive metal will be oxidised instead of the metal and so the metal won’t rust. For example, big blocks of magnesium can be bolted onto steel ships or underground iron pipes.
• Galvanising: an object can be galvanised by spraying it with a coating of zinc. The zinc layer is firstly protective, but if it’s scratched, the zinc around the site of the scratch works as a sacrificial method as it is more reactive.

Question 9

What is the definition of corrosion?

Answer 9

Corrosion is the process by which metals are slowly broken down by reacting with substances in their environment.

Question 10

Rusting is a special type of corrosion. It refers to the corrosion of which metal?

Answer 10

Iron

Question 11

What is the word equation for rusting?

Answer 11

Iron + oxygen + water -> hydrated iron (III) oxide