Corrosion Control

Why metals rust

Corrosion is what happens when a metal reacts with its surroundings and is slowly eaten away. For iron and steel, that corrosion is called rust - and it costs the world a fortune every year. The good news: rusting only happens under certain conditions, so we can prevent it. This module covers what corrosion is, how to stop rust, and why mixing metals into alloys makes them stronger.

Corrosion words

Four terms unlock the topic.

Corrosion protection terms

What rust needs

What two substances must both be present for iron to rust?

• Both oxygen and water
• Only oxygen
• Only water
• Oxygen and carbon dioxide

How to stop rust

Because rust needs oxygen AND water, we prevent it by keeping one or both away, or by a clever trick: - Barrier methods - paint, oil, grease or plastic keep out oxygen and water.\n- Sacrificial protection - attach a more reactive metal (zinc or magnesium); it corrodes instead of the iron. - Galvanising - coating iron with zinc does both: a barrier AND sacrificial protection.\n- Electroplating - a thin metal layer for appearance and corrosion resistance.

Preventing rust

Select the TWO methods that would help prevent iron from rusting.

• Painting the iron
• Galvanising it with zinc
• Leaving it in salty water
• Keeping it permanently wet

Two ways to protect

Barrier methods and sacrificial protection both stop rust, but they work differently - and one keeps working even when scratched.

Match each metal to its use

Why sacrifice a metal?

Why does sacrificial protection stop the iron from rusting?

• A more reactive metal is oxidised in preference to the iron
• It makes the iron heavier so water runs off
• It simply paints over the surface of the iron
• It removes all the water from the air

Corrosion in a paragraph

Corrosion is the _____ of a metal. Iron rusts only when both _____ and _____ are present. Coating iron with zinc gives _____ protection, because zinc is more _____ than iron.

Why alloys are stronger

Put the reasoning for why an alloy is stronger than a pure metal into a logical order.

• A pure metal has layers of atoms that are all the same size
• These regular layers can slide over each other easily
• In an alloy, atoms of a different size are mixed in
• The different sizes disrupt the layers so they cannot slide easily
• The alloy is therefore harder and stronger

Choose the protection

Three real situations. Pick the best corrosion-control choice each time.

• An iron ship's hull is underwater, where paint easily gets scratched. Which protection keeps working even when scratched?
• You want a simple, cheap barrier method to protect a garden gate. What do you use?
• A steel can is coated with a thin layer of tin for appearance and corrosion resistance. What is this process called?

Your turn: explain

Explain how iron rusts, and describe TWO different ways to prevent it, saying how each one works.

• State that rusting is oxidation and needs both oxygen and water
• Describe a barrier method and how it works (keeps out oxygen and water)
• Describe sacrificial protection and how it works (a more reactive metal is oxidised first)
• Note one advantage of sacrificial protection over a barrier (it works even when scratched)

Rust needs both

The marks here hinge on precision. Rust is the oxidation of iron and needs both oxygen and water - stating just one loses the point. Prevention works by removing a condition (barrier methods) or by sacrificial protection, where a more reactive metal is oxidised in preference. And for alloys, always give the particle reason: different-sized atoms disrupt the regular layers so they cannot slide, making the alloy harder and stronger than the pure metal.