Corrosion of metals Paper 2 Flashcards
What happens to metals when they are in contact with air?
Metals can oxidise in air. They react with oxygen and form metal oxides.
When does corrosion happen?
Corrosion happens when a metal continues to oxidise. The metal becomes weaker over time, and eventually all of it may become metal oxide.
What is rusting?
Rusting is a specific example of corrosion, which occurs when iron or steel reacts with oxygen and water
What is the formula for iron reacting with O2 and H2O?
iron + oxygen + water → hydrated iron(III) oxide
Hydrated iron(III) oxide is the orange-brown substance seen on the surface of rusty objects.
What is needed for iron to rust?
Both oxygen and water are needed for rusting to happen.
How can rusting be prevented?
Rusting can be prevented by creating a physical barrier to oxygen and water. Ways to do this include:
- painting
- oiling and greasing
- coating with plastic
Different methods are used depending on the situation.
What is sacrificial protection?
Iron can be protected from rusting if it is in contact with a more reactive metal, such as zinc. The more reactive metal oxidises more readily than iron, so it ‘sacrifices’ itself while the iron does not rust.
Once the sacrificial metal has corroded away, it can simply be replaced.
Three nails are left in contact with air and water for a few days. A nail wrapped in magnesium does not rust. A nail alone rusts but a nail wrapped in copper rusts more. Explain these observations.
Magnesium is more reactive than iron. It oxidase’s more readily than iron, so the nail does not rust.
Iron is more reactive than copper. This means it oxidase’s more readily than copper, so it rusts faster than the nail alone.
What is meant by galvanising?
When iron is coated in zinc, the process is called galvanising. The zinc layer stops oxygen and water reaching the iron. Zinc is more reactive than iron, so it also acts as a sacrificial metal. This protection works, even if the zinc layer is scratched.
The inside of a steel food can is electroplated with tin. Tin is less reactive than iron. It provides a physical barrier to oxygen and water, stopping the can rusting. Explain why the inside of the can rusts very quickly if the layer of tin is broken.
Oxygen and water can reach the steel when the layer is broken. Iron is more reactive than tin, so it oxidises more readily than tin. The damaged can rusts faster than if the tin was not there at all.
What is electroplating?
Electroplating involves using electrolysis to put a thin layer of a metal on the object:
the cathode is the iron or steel object
the anode is the plating metal
the electrolyte contains ions of the plating metal
For example, steel cutlery can be electroplated with silver using a silver anode and silver nitrate solution. Electroplating improves the corrosion resistance of metal objects. It also improves their appearance and may be used to produce gold-plated jewellery.
