Corrosion (AQA GCSE Chemistry): Revision Notes
Corrosion
What is corrosion?
Corrosion happens when materials break down. This occurs because they react with chemicals in the environment. The main culprits are oxygen and water.
When metals corrode, they slowly get damaged and weakened. This is a big problem for things like bridges, cars, and buildings, leading to costly repairs and potential safety hazards.
Corrosion affects virtually all metals to some degree, but the rate and extent of corrosion varies greatly depending on the metal type and environmental conditions. This is why understanding and preventing corrosion is crucial in engineering and construction.
Rusting - a type of corrosion
Rusting is what we call it when iron or steel corrodes. You've probably seen rusty nails or old cars with orange-brown patches - that's rust!
The distinctive orange-brown colour of rust is actually iron oxide (Fe₂O₃), which forms when iron reacts with oxygen and water. This reddish coating is quite different from the original silver-gray colour of iron and steel.
What does iron need to rust?
Scientists did an experiment to find out exactly what iron needs to rust. This controlled experiment helps us understand the essential conditions for rusting to occur.
Controlled Experiment: Testing Rusting Conditions
Scientists used three test tubes with iron nails to test different conditions:
Step 1: Set up three different conditions
- Tube 1: Just air (no water) - no rusting happened
- Tube 2: Just water with no air (boiled water) - no rusting happened
- Tube 3: Both air and water - rusting occurred
Step 2: Observe and record results Only the nail exposed to both air and water showed signs of rust formation.
Conclusion: Iron needs both air and water to rust. Take away either one and rusting stops.
Key finding: Iron needs both air and water to rust. This is why controlling either moisture or oxygen exposure can prevent rusting completely.
This is why aluminium doesn't corrode easily - it has a natural protective coating that keeps air and water away from the metal underneath.
How to prevent corrosion
There are two main approaches to stopping metals from corroding, each working on different principles to protect the underlying metal.
Method 1: Keep air and water away
This method works by creating a physical barrier between the metal and the corrosive environment:
- Grease - put it on bicycle chains to repel water and create a protective layer
- Paint - covers iron bridges and gates, forming a sealed coating that blocks air and moisture
- Electroplating - cover objects with unreactive metals using electricity. For example, steel food cans are covered with tin to prevent both corrosion and contamination
The success of barrier methods depends on maintaining an intact protective layer. Any scratches or gaps in the protection can allow corrosion to start at those vulnerable points.
Method 2: Use a more reactive metal
Sometimes we deliberately add a more reactive metal. This metal corrodes instead of the iron, protecting it through chemical means rather than physical barriers.
Sacrificial protection: Magnesium blocks are attached to steel ships. The magnesium is more reactive than iron, so it corrodes first. This saves the steel ship from rusting by sacrificing the magnesium instead.
Galvanising: This means covering iron with zinc. Even if the zinc gets scratched, it's still more reactive than iron. So the zinc corrodes instead of the iron underneath, providing continuous protection even when damaged.
For sacrificial protection to work effectively, the protecting metal must be higher in the reactivity series than the metal being protected. Using a less reactive metal would provide no protection at all.
The reactivity series helps us choose
Understanding the reactivity series is essential for choosing the right metals for protection. Metals higher up in the reactivity series corrode more easily:
- Sodium (very reactive)
- Magnesium
- Aluminium
- Zinc
- Iron
- Copper (less reactive)
For sacrificial protection to work, we need a metal that's more reactive than the one we want to protect. This ensures the protecting metal will always corrode preferentially.
Key Points to Remember:
- Corrosion is when metals break down by reacting with oxygen and water
- Iron needs both air and water to rust - remove either one and rusting stops completely
- Prevent corrosion by keeping air and water away (paint, grease, electroplating)
- Or use sacrificial protection with more reactive metals (magnesium blocks, zinc coating)
- The reactivity series tells us which metals will corrode first and guides our choice of protective metals
- Understanding these principles helps engineers design longer-lasting structures and products