Corrosion and rancidity

Corrosion

When a metal is slowly eaten away by substances around it, such as air, water or an acid, the process is corrosion. Corrosion is a chemical reaction. It is an oxidation.

• Rusting of iron. Iron becomes covered with a reddish brown, flaky solid, the rust.
• Tarnishing of silver. Silver becomes black, because it reacts with sulphur in the air to make silver sulphide.
• Copper gets a green coating, because it reacts with moist air and carbon dioxide.

What does iron need to rust?

Three nails are kept in three test tubes for some days.

1. Air and water. The nail rusts.
2. Boiled water with a layer of oil. Boiling takes out the air, and the oil stops new air from entering. The nail does not rust.
3. Dry air. Calcium chloride takes up the water vapour. The nail does not rust.

So iron rusts only when both oxygen and water are present. Rust is hydrated iron oxide, $\text{Fe}_2\text{O}_3 \cdot x\text{H}_2\text{O}$.

Loss from corrosion

Corrosion damages bridges, ships, railings, cars and iron gates. A large part of the iron that is made is used only to replace the iron that has rusted.

How to prevent rusting

The metal must be kept away from air and water.

• Painting. Paint covers the surface.
• Oiling and greasing the moving parts of machines.
• Galvanising. The iron is coated with a thin layer of zinc. Zinc is more reactive, so even if the coating is scratched, the zinc corrodes first and the iron is safe.
• Alloying. Iron is mixed with other metals. Stainless steel is iron with chromium and nickel. It does not rust.
• Coating with another metal, such as chromium or tin.

You should find that no rust forms in dry air, whatever else you change, and that salt doubles the rate.

Rancidity

When fats and oils are oxidised in air, they become rancid. The food smells and tastes bad. Chips, butter and oil kept for a long time show it.

How to prevent rancidity:

• Add antioxidants, which are substances that stop oxidation, to foods that have fats and oils.
• Keep the food in airtight containers.
• Keep the food in a refrigerator, where the reactions are slow.
• Flush the packet of chips with nitrogen gas, which does not react with the food, in place of air.

Practise with new numbers

Try it in Python

This program uses the data of the nail experiment. It finds the rate of rusting.

Remember these

• Corrosion is the slow eating away of a metal. Rusting needs both oxygen and water.
• Prevent rusting by painting, oiling, galvanising or alloying.
• Rancidity is the oxidation of fats and oils. Use antioxidants, airtight packs, cold, or nitrogen.

Check yourself

Try each one on your own first. If you are stuck, open a hint. Then check with Show the answer.

1. In the nail experiment, why is the water boiled, and why is oil poured on top?

2. Why do cars rust faster in towns near the sea?

3. Give two ways to keep the chain of a bicycle from rusting.

4. Why does butter left out of the fridge for many days start to smell bad?